WHITE WINE PRODUCTION A schematic diagram showing important steps in white wine production is presented in Fig. 1. The illustration points to the basic steps, along with some variations, to be followed in order to produce various styles of white wines. Commercially, white wines are produced in several styles, and stylistic considerations are often guided by market forces. However, wineries making smaller lots should experiment with making both traditional as well as newer styles of wine. WHITE WINE STYLES White wines are made in various styles. The two main categories are: • Dry white wines. • Sweet white wines with varying degrees of sweetness. The dry white wine category includes three main styles: Standard whites - This style includes generic, easy to drink, everyday table wine. It is vinous, somewhat fruity, crisp and clean. Examples include jug wines. Varietal whites - These include quality varietal wines. The wines are well balanced and display prominent varietal characters. These wines are often consumed with a meal. Chardonnay, Sauvignon blanc, Seyval, Vidal, and some dry Riesling can be grouped in this category. Premium whites with complex flavor-These wines have good body plus complex and well-integrated aromas. These include premium varietal or blended, high-priced wines. Wines in this category include white Burgundies, and barrel fermented and ‘sur lie’ aged Chardonnay. Among sweet white wines, there are many styles based on the degree of sweetness and the techniques of production. The main styles are: Fresh, fruity and sweet - This style of wine includes white wines with varying level of sweetness. They are fruity and good general purpose wines. Examples are generic sweet wines and varietal whites such as white Riesling. Late harvest - These wines are usually sweeter, have more ripe fruit aromas, and a fuller mouthfeel. They are made from grapes harvested later than the usual harvest time. Examples would be late harvest style Riesling, Seyval and Vidal. Late harvest with noble rot – The wines are made from grapes infected with noble rot. They are sweet and have complex and unique flavors. An example is the Sauternes of France. Many Rieslings of Germany and other varietal wines made from Botrytis infected grapes. Ice wines - These wines are made from overripe frozen grapes. The wines are generally very sweet and delicious. Examples include the ice wines of Germany and Canada made from white Riesling grapes. The soil, climate, and viticultural practices all influence the composition and quality of fruit, and particularly, the varietal expression. The fruit composition, to a large extent, dictates wine style. Therefore, a winemaker’s job is to determine the style of wine that can best be made from a given fruit or to obtain the fruit with the most suitable composition to make a certain style of wine.
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WHITE WINE PRODUCTION
A schematic diagram showing important steps in white wine production is presented in Fig. 1. The illustration points to the basic steps, along with some variations, to be followed in order to produce various styles of white wines.
Commercially, white wines are produced in several styles, and stylistic considerations are often guided by market forces. However, wineries making smaller lots should experiment with making both traditional as well as newer styles of wine.
WHITE WINE STYLES
White wines are made in various styles. The two main categories are:• Dry white wines.• Sweet white wines with varying degrees of sweetness.The dry white wine category includes three main styles: Standard whites - This style includes generic, easy to drink, everyday table wine. It is
vinous, somewhat fruity, crisp and clean. Examples include jug wines. Varietal whites - These include quality varietal wines. The wines are well balanced and
display prominent varietal characters. These wines are often consumed with a meal. Chardonnay, Sauvignon blanc, Seyval, Vidal, and some dry Riesling can be grouped in this category.
Premium whites with complex flavor-These wines have good body plus complex and well-integrated aromas. These include premium varietal or blended, high-priced wines. Wines in this category include white Burgundies, and barrel fermented and ‘sur lie’ aged Chardonnay.
Among sweet white wines, there are many styles based on the degree of sweetness and the techniques of production. The main styles are:
Fresh, fruity and sweet - This style of wine includes white wines with varying level of sweetness. They are fruity and good general purpose wines. Examples are generic sweet wines and varietal whites such as white Riesling.
Late harvest - These wines are usually sweeter, have more ripe fruit aromas, and a fuller mouthfeel. They are made from grapes harvested later than the usual harvest time. Examples would be late harvest style Riesling, Seyval and Vidal.
Late harvest with noble rot – The wines are made from grapes infected with noble rot. They are sweet and have complex and unique flavors. An example is the Sauternes of France.
Many Rieslings of Germany and other varietal wines made from Botrytis infected grapes.
Ice wines - These wines are made from overripe frozen grapes. The wines are generally very sweet and delicious. Examples include the ice wines of Germany and Canada made from white Riesling grapes.
The soil, climate, and viticultural practices all influence the composition and quality of fruit, and particularly, the varietal expression. The fruit composition, to a large extent, dictates wine style. Therefore, a winemaker’s job is to determine the style of wine that can best be made from a given fruit or to obtain the fruit with the most suitable composition to make a certain style of wine.
Figure 1. White Wine Production Steps
RAW MATERIALS (GRAPES)
VARIETIES
A top quality premium varietal wine grape is the most desirable raw material for making good wine. Grape variety plays a key role in determining wine quality. Many varieties are used in white winemaking, but some varieties are better suited than others for making high quality wines. Vinifera grape varieties are commonly used for making wines, however, in regions where Vinifera grapes are not commercially grown, French hybrids and Native American grape varieties are used for white wine production.
In the Vinifera grape variety group, Chardonnay, Sauvignon blanc, Semillon, Riesling, Gewürztraminer, and Muscat canelli are considered distinctive and premium white wine varieties. Chenin blanc, and French Columbard make less distinctive wines and are generally used to make good quality everyday drinking wines. Chardonnay, Semillon and Sauvignon blanc are commonly used to make various styles of dry white wine; whereas, Riesling, Gewürztraminer, and Muscat canelli are generally used for fruity and sweeter styles of wine.
Among the non-Vinifera group, French hybrid varieties such as Seyval, Vidal and Vignoles make fine white wines. Other varieties such as Aurora and Villard blanc seem to make less distinctive wines. In recent years, some new varieties have been released that make high quality white wines. Varieties in this category includes Cayuga White, Chardonel and Traminette. Generally, varieties such as Vignoles, Cayuga White and Traminette are fruitier and made into sweeter styles while others with delicate varietal characters are vinified into dry white wines.
Native American varieties such as Catawba, Niagara and Delawave have been used for making white wines. These varieties have strong fruity aromas and are commonly made into sweeter styles of white wines.
Varieties other than the ones discussed above can also be made into wine depending on availability and quality. Winemakers should feel free to experiment with them.
COMPOSITION AND QUALITY
In addition to the premium grape variety, composition and quality of the grapes is important to making superior wine. The grape contains many constituents. The important ones are briefly described here.
Physical composition: The estimated physical composition of a grape cluster in terms of percent by weight is:
• stems - 2 to 6%• seeds - 0 to 5%• skins - 5 to 12%• pulp - 70 to 80%.Stems are rich in tannins and minerals. Seeds contain high levels of tannin. Skin mostly
contains pigment, aroma compounds and phenolic substances. The juice consists of sugars, acids, phenolic compounds, aromatic substances and many more chemical constituents.
Chemical composition: Grapes are rich in sugars. The main sugars include glucose andfructose. Their concentration in ripe fruit generally varies between 150 to 250 grams per liter. Sugars are obviously responsible for the sweet taste.
Next to sugars, organic acids are the most abundant solids present in the grape juice. Principal organic acids include tartaric, malic and a small amount of citric. Their concentration usually ranges from four to twelve grams per liter of must. They contribute to the tart taste. The amount and kinds of organic acids have an important bearing on juice/wine pH, which in turn affects the color, flavor and stability.
Grapes also contain significant amount of phenolic compounds. They contribute to the wine’s color and flavor. They are also involved in juice/wine oxidation and wine maturation and aging. The concentration of phenolic substances generally ranges from 100 to 250 milligrams per liter in white wines.
In addition to substances mentioned above, there are many other compounds present in grapes. These compounds occur in small concentrations but are very important constituents. These include aroma bearing compounds, nitrogenous compounds, pectic substances and inorganic constituents.
To make good table wine, the various constituents should be present in proper balance. When they are not in balance, must adjustments become necessary in order to produce good wines. Table 1. lists the desirable sugar, acid and pH levels to make table wines.
In addition to the parameters mentioned, the grapes should also have well developed color and varietal aromas. The sugar and acid values given above are suited for making dry table wines. For making other types or styles of wine, the desired composition in terms of sugar, acid and pH would be different.
PROCESSING GRAPES
STEMMING AND CRUSHING
This process involves the removal of stems and a gentle crushing of the berries to release the juice. Various types of crushers/destemmers are available to perform this function. Without the equipment, the process of crushing can be hard and laborious. For serious winemakers, this equipment is essential. During stemming and crushing, handle the fruit gently and avoid undue maceration of stems, skin and seeds. In some cases, the stems are not removed and the whole clusters are pressed. This method is followed in the Champaigne region for the production of sparkling wines. The whole cluster pressing yields high quality juice. This means lower amounts of phenolic compounds and suspended solids.
When making sweet wines from Botrytis infected grapes, whole cluster pressing (without crushing) is recommended. This minimizes the extraction of undesirable rot constituents, destructive oxidizing enzymes, and the dextran polymers, which contributes to clarification problems. Also, shriveled berries are not easily separated from the stem and, thus, can be lost along with the stem giving poor yield.
PRESSING
After stemming and crushing the crushed fruit, called must, is pressed to separate juice from skins and seeds. Commercial operations dealing with larger lots use membrane or tank presses. These yield high quality juice. Basket presses are commonly used for small-scale production. Ratchet type basket presses are traditional and have been in use for a long time. Nowadays, winemakers prefer to use a vertical basket press equipped with a bladder. These presses are faster than the ratchet type presses. The vertical basket presses are available in various capacities ranging from 35 to 85 gallons. During the operation, the basket is lined with a fiberglass or stainless steel screen and loaded with the must. In hard-to-press varieties, a small amount of rice hulls are added to facilitate the dejuicing.
As the basket is loaded, some juice will begin to drain. This is called free-run juice, which is low in phenolic compounds and suspended solids. Winemakers may choose to use only free-run juice to make distinct premium varietal wines. To obtain the remaining juice, gently apply pressure by inflating the bladder (in bladder type press) or driving the press head down (ratchet type press). Generally 30 to 40 psi pressure is sufficient to obtain a good yield. However in some cases higher pressure may be required. When using a bladder press, follow the manufacture’s recommendation to determine the maximum pressure that can be used without damaging the bladder. Depending on the type of press, it may take 45 minutes to two and a half hours to complete one press cycle. During each cycle, the pressure is released and the must is stirred several times and pressed again to release the juice. Typically the juice yield will vary between 140 to 160 gallons per ton.
SKIN CONTACT
Some skin contact (i.e., contact of grape skins with juice) inevitably occurs between crushing and pressing operations. Generally skin contact time is minimized by pressing the must immediately after crushing. The reasons for short skin contact include reducing the extraction of phenolic substances that can contribute to the harshness and bitterness in wine, lowering the risk of fermentation by indigenous yeasts and minimizing browning of must, particularly if no sulfur dioxide is added.
In some cases, winemakers allow skin contact and delay the pressing. The purpose of deliberate skin contact is to extract desirable flavor compounds. Usually skin contact for eight to twenty four hours at about 10C can yield beneficial results. Skin contact should not be practiced at higher temperatures and particularly in the case of moldy and rotten fruit. It should be noted that all varieties might not benefit from skin contact treatment, therefore, only through trials should one determine the merits of this procedure.
In some aromatic varieties such as Riesling, Gewürztraminer, Muscat, Vignoles, Vidal and Cayuga White, must treatment with glycosidase enzyme may enhance the aroma in the resulting wine. Many variables influence enzyme treatment, therefore trials should be conducted before resorting to a certain procedure.
Processing of grapes, such as crushing and pressing at relatively cooler temperatures, is necessary to produce good white wine. Commercial wineries have access to must chillers to cool the must. For home winemakers or small scale commercial producers, this option is not feasible. Therefore, cooling the grapes before processing is suggested. Furthermore, the grapes should be processed quickly and the juice should be chilled before any further treatment.
PREPARING JUICE FOR FERMENTATION
JUICE TREATMENT
SULFUR DIOXIDE ADDITION
Freshly pressed juice tends to oxidize and become brown. The reaction is catalyzed by the enzyme polyphenol oxidase. At warmer temperatures, this enzymatic activity is enhanced, therefore processing grapes at a cooler temperature is recommended. Sulfur dioxide is added to inhibit the activity of the enzyme and protect the juice from browning. Another benefit of adding sulfur dioxide is to inhibit the activity of harmful microorganisms such as bacteria and some indigenous yeasts which, if not controlled, can contribute to off odors in the resulting wine. The dose of sulfur dioxide used to control microbial activity and oxidation ranges from low levels such as 30 to 50 milligrams per liter to higher levels of 75 to 100 milligrams per liter.
For clean and sound fruit, the lower dose of 30 to 50 milligrams per liter is recommended. In case of spoilage or rot, a higher dose of 70 to 100 milligrams per liter may be needed. In order to minimize the use of sulfur dioxide in wine, some winemakers prefer no sulfur dioxide addition to the must before fermentation. However, there is considerable evidence to suggest that the addition of small amounts of sulfur dioxide (30 to 50 milligrams per liter) generally gives a better quality white wine.
To determine the sulfur dioxide additions to a given volume of must, see Table 2. To add sulfur dioxide (SO2 ) to the must, a salt known as potassium metabisulfite is often used. The chemical contains about 58% SO2.
Table 2. The amount of K-metabisulfite required to attain various levels of SO2
When dealing with smaller lots, it may be difficult to measure a small amount of chemical unless one has a good balance. In such a situation, it may be advantageous to weigh a larger amount of chemical to make a stock solution for the necessary SO2 additions. During the crush, SO2 additions are done more frequently. Using a stock solution of potassium metabisulfite can be a convenient and easy way to handle SO2 additions. The SO2 solution loses its strength over time, therefore, fresh stock solution should be made on a weekly basis.
The strong SO2 solution should be handled with care. Using protective eye glasses and a gas mask is suggested.
PREPARING AND USING AN SO2 STOCK SOLUTION
Make a 6.6% stock solution of potassium metabisulfite by weighing 66 grams of potassium metabisulfite in enough distilled water to make the volume a liter. Label the solution and store in a cool dry place. One ml of this 6.6% stock solution, when added to 1 gallon of must, will yield 10 ppm SO2 . A small, graduated cylinder can be used to measure the proper amount of SO2 solution required for a given batch of wine. After adding the solution, the must should be stirred to mix the SO2.
Table 3. can be used to determine the amount of stock solution (6.6%) needed to achieve various levels of SO2 for different must volumes.
Table 3. ML of 6.6% potassium metabisulfite solution required to attain various concentrations of SO2 (ppm)
To make a 6.6% stock solution of potassium metabisulfite, weigh out 66 grams and dissolve it in distilled water. Then add enough distilled water to make the volume a liter. Label the solution and store in a cool dry place.
One ml of this 6.6% stock solution, when added to 1 gallon of must will yield 10 ppm SO2 . A small, graduated cylinder can be used to measure the proper amount of SO2 solution required for a given batch of wine. After adding the solution, the must should be stirred to mix the SO2.
ADDING PECTIC ENZYME
Commercial pectic enzymes are often added to the must to facilitate juice extraction and clarification. Pectin is a cell wall constituent that naturally occurs in grapes. The enzyme hydrolyzes the pectin and thus enhances the release of juice. Generally the yield of free-run juice is increased. The enzyme also helps to prevent pectic haze and contributes to the natural settling of juice. The enzyme activity is influenced by temperature, pH, and other factors. To obtain best results, closely follow manufacturer’s directions.
JUICE CLARIFICATION
Freshly squeezed juice contains many non-soluble solids, largely pulp particles which contribute to juice turbidity. To make fresh, fruity and clean white wines, the level of these suspended solids is reduced to about one to two percent and relatively clear juice is used for
making wine. To achieve clarification, sulfited juice should be stored at a cooler temperature (4 to 10 ºC) for twelve to forty-eight hours. During storage, the suspended solids settle at the bottom (due to gravity) and form a sediment. The clear juice fraction above the sediment is racked off leaving the sediment behind.
The clear juice can be removed by siphoning or pumping. For siphoning, use 1/4” diameter tubing. A siphon tube consists of a 1/4” hard plastic tubing, attached to a flexible polyvinyl tubing. The stiff end of the tube is immersed in the wine at just about the sediment level. The wine flow is started by applying suction on the other end of the siphon tube. A pinchcock can be used to stop and start the flow. When using a pump, a larger diameter tubing may be required. In either procedure, clear juice should be removed without disturbing the sediment.
Commercial wineries can use a centrifuge and lees filter to clarify the juice, but the equipment is expensive and not adaptable to small-scale wine production. Settling the juice naturally may take a longer time, but can accomplish comparable results.
MUST ADJUSTMENTS
SUGAR ADJUSTMENT
During fermentation, the sugar in the must is converted into alcohol and carbon dioxide. Theoretically, 180 grams of sugar yields 92 grams of ethanol and 88 grams of carbondioxide. In practice, however, the alcohol yield is less than the theoretical value. Generally, ethanol yield is about 47% on a weight basis. A common way to estimate the potential alcohol yield is to multiply the ºBrix value by a factor of 0.55. It should be noted that using a factor of 0.55 will give an estimated alcohol yield. The actual alcohol yield will vary depending on variety, climate and fermentation conditions.
ºBrix x 0.55 = % alcohol by volume
Table 4. shows the potential alcohol by volume resulting from fermentation of the must at various ºBrix levels.
Table 4. Potential alcohol yield based on oBrix
The alcohol content in table wines ranges from nine to thirteen percent with a range of 11.5 to 12.5 being more common. To obtain this alcohol level, the sugar content in the must
needs to be 21 to 22 oBrix. Many grapes are harvested at 21 to 23 oBrix and do not need sugar adjustment to produce dry table wines. When the grapes contain insufficient amount at harvest, then sugar addition is needed.
Generally dry cane sugar is used for raising the must sugar content but sugar syrup or concentrate can also be used.
USING DRY SUGAR
To calculate the amount of sugar needed to raise the Brix to 22o, use the following formula:
S = 0.125 (B-A)S = Amount of sugar needed to raise the Brix to 22o
B = Desired Brix 21 or 22o
A = Brix reading of the sugar deficient must.0.125 = A factor to determine the pounds of sugar to be added to a gallon of must to
raise sugar by 1oBrix.Example: A must with 18 oBrix sugar needs to be adjusted to 22 oBrix. Calculate the
amount of sugar needed.Using the formula, S = 0.125 (B-A)S = 0.125 (22-18)S = 0.125 x 4S = 0,5Answer: 0,5 pounds of sugar per gallon of must will be needed to raise the sugar level
from 18 to 22 oBrix.
USING CONCENTRATE Concentrates are often produced at 68 to 72 oBrix. They can be used to raise the sugar
level in sugar deficient must. The amount of concentrate to be used can be calculated using the Pearson’s square technique. This technique is explained in the blending section. An example is given below.
Example: Calculate the amount of 68o concentrate needed to increase the sugar content in a must from 18 to 22 oBrix.
Figure 2. Pearson’s Square
Answer: Adding 1 part (68 oBrix) concentrate to 11.5 parts (18 oBrix) must will yield must with 22 oBrix.
So far methods to raise sugar content have been discussed above. Sometimes the grapes may contain excessively higher sugar levels at harvest. Under such a condition the must can be diluted to a desired ºBrix by adding water. Another option is to use high sugar grapes to produce wine with residual sugar by stopping fermentation at a predetermined alcohol and sugar level.
ADJUSTING ACIDITY
Acidity is an important wine quality parameter. Acids contribute to the tart taste and also influence wine pH. The wine pH, and thus indirectly acidity, has a profound influence on alcoholic and malolactic fermentation, the wine’s color, physical stability and effectiveness of chemicals such as sulfur dioxide and sorbic acid.
For white wine production, acidity in the range of seven to nine grams per liter titratable acidity is considered sufficient. Sometimes the grapes at harvest can be deficient in acidity. In such a case acid addition is required. On the other hand, grapes may have high acid level at harvest. Such a must would require deacidification for producing balanced white wine.
ACID ADDITION
Must acidity can be increased by acid addition. The choice of acids depends on several factors but generally tartaric acid is used. Tartaric acid occurs naturally and is a stronger acid than malic or citric acids. It is biologically more stable than the other two acids mentioned. When tartaric acid is added to the must, it reacts with potassium and forms potassium bitartrate salts. Precipitation of this salt following fermentation and cold stabilization results in a loss of acidity. To compensate for this loss, a slightly higher amount of tartaric acid should be added to reach the desired level of acidity. Depending on the wine pH (below 3.55 to 3.60), loss of bitartrate salt also causes reduction in the pH, which is generally beneficial to wine quality.
ACID REDUCTION
In high acid must, deacidification can be achieved in several ways. These include physical methods, biological methods, and chemical methods.
Physical methods - Certain varieties of grapes, such as Concord or Catawba grown in the Eastern part of the U.S. contain low sugar and high acid at harvest. When using these grapes for winemaking, the must is often ameliorated with sugar syrup to dilute and lower acidity. Dry sugar in the amount of 2.35 pounds per gallon of water is needed to make a sugar syrup with 22 oBrix. For commercial wineries, the amount of ameliorating material authorized is 35% of the resulting volumes. For noncommercial producers such as home winemakers, these regulations don’t apply. Generally the must is ameliorated in the range of 10 to 15% of the resulting volume. The aim of amelioration should be to produce a well-balanced wine, not to increase the volume at the expense of wine quality.
One other physical method of acid reduction is to blend high acid must with low acid must to achieve the desired acid balance. However, this may not be feasible if low acid must is unavailable.
Biological methods - Yeasts and bacteria can be employed to reduce acidity. Many commercial yeast strains can metabolize malic acid but some strains are more efficient than others. For example, a strain called 71B can degrade 20 to 40% malic acid depending on the conditions of fermentation.
Lactic acid bacteria are more commonly used for deacidification. These organisms convert malic acid to lactic acid and thus reduce acidity. Acid reduction by bacteria is often accompanied by changes in the wine’s aroma, which may or may not be desirable.
Chemical methods - The must acidity can also be reduced by chemical means. Commonly used chemicals for deacidification include potassium carbonate, potassium bicarbonate, and calcium carbonate.
Potassium carbonate and bicarbonate - Potassium carbonate and bicarbonate can both be used, however potassium bicarbonate should be preferred for deacidification. Treatment with potassium bicarbonate results in the formation of carbonic acid and potassium bitartrate. Carbonic acid dissociates into carbon dioxide and water and the carbon dioxide is lost. Potassium bitartrate is poorly soluble and it precipitates. The acid reduction occurs due to both neutralization and precipitation. Along with deacidification the treatment also causes an increase in must/wine pH. For sound enological reasons the must and wine preferably should maintain a value below 3.6 pH. Potassium bicarbonate treatment, therefore, should be used in moderately high acid (eight to ten grams per liter titratable acidity) and low pH (3.0 to 3.1) must, where acid can be reduced to a desired level without raising pH to an unacceptable value. The amount of carbonates needed for deacidification should be determined by conducting a laboratory trial. As a rule of thumb, 0.62 grams per liter of potassium carbonate and 0.9 grams per liter potassium bicarbonate can be used to lower the acidity by one gram per liter or 0.1%. Chill the wine following the treatment to facilitate bitartrate precipitation.
Calcium carbonate – Calcium carbonate can also be used to deacidify high acid (more than 9 grams per liter titratable acidity) must/wine. Treatment with calcium carbonate yields carbonic acid, calcium tartrate, and calcium malate. Calcium tartrate is less soluble than calcium malate. As a result, most of the calcium tartrate will precipitate over time but a fair amount of calcium malate can remain in the wine. This could lead to a calcium instability problem if sufficient tartrate is not available. For acid reduction, add 0.67 grams per liter of calcium carbonate to reduce titratable acidity by 0.1%. Calcium carbonate treatment is often suggested for achieving substantial acid reduction. However, it can cause calcium instability and also can
impart a chalky or salty taste to the wine. As always, when dealing with chemical treatments, a laboratory trial should be carried out before administering the treatment to the entire lot of wine.
Other deacidification methods such as the Acidex treatment or ion exchange can also be used, but these methods are more practical under commercial winemaking operations.
ALCOHOLIC FERMENTATION
THE PROCESS
Fermentation is the key process that transforms juice (must) into wine. It is carried out by the yeast under anaerobic conditions. During fermentation the yeast utilizes sugar and produces alcohol (ethanol), carbon dioxide, and many other constituents. The reaction is typically described by the following equation.
Figure 3. Fermentation Reaction
According to the equation, 180g of sugar (glucose) yields 92 g of ethanol and 88 g of carbon dioxide. Most of the ethanol is retained in the wine while most of the CO2 is lost. The reaction also generates heat, which if not dissipated, will raise the temperature of the fermenting must to an undesirably high level. Theoretically, the ethanol yield from the fermentation of glucose should be 51.1% by weight of the glucose present. But in practice the alcohol yield is slightly lower than the theoretical value. It is usually about 47 to 48% of the weight of the sugar fermented. The important reason for the lower alcohol yield is that some of the sugar is used in the production of new yeast cells and many other constituents.
As shown in the equation above, a copious amount of CO2 is also generated during the fermentation. For example, a complete fermentation of must containing 210 g/L of sugar at 20 ºC would generate about 56 liters of CO2 per liter of must. Such a large volume of CO2 gas, if not removed, can be very dangerous to cellar workers. Therefore, it is important to conduct fermentation in a well-ventilated area.
Heat is another by product of fermentation. It is estimated that if the heat produced during fermentation is not removed, it will raise the must temperature by 2.3 °F for each degree drop in °Brix. For example, fermentation of a 21 °B must will generate enough heat to raise must temperature by 48.3 °F, assuming the heat is not removed. This means that if the beginning temperature is 60 °F, and all the heat is retained then the final must temperature would be expected to be around 108.3 °F. In practice, however, a significant amount of heat is lost and the fermentation temperature usually does not reach such a dangerously high level. But it should be clear that unless the fermentation temperature is properly controlled it could reach to an unacceptably high level.
YEAST GROWTH CYCLE
Alcoholic fermentation is closely linked to the growth and metabolic activity of yeast. In batch fermentations such as winemaking, the yeast growth cycle is characterized by four principal phases. Following inoculation, the yeast cells go through an adjustment period. The growth is temporarily suspended and this phase is referred to as the lag phase. If the conditions for yeast growth are not favorable, then the lag phase continues and the fermentation is delayed. Generally, in a must inoculated with a pure culture of active dry yeast, the lag phase is short. In the second phase called the log phase, the yeast cell growth resumes, the cells multiply rapidly (exponentially), and the population density can reach as much as 2x108 (200 million )cells/ml. In the next phase, called the stationary phase, the population levels off. The nutrient level continues to decrease, and toxic metabolic end products accumulate. In this phase the yeast’s fermentation activity continues. In the fourth and final phase, also known as the decline phase, the yeast population progressively declines. The fermentation slows down and then comes to an end.
As mentioned earlier, the sugar utilization during fermentation is closely associated with yeast growth and metabolic activity. Typically about half of the must sugar is consumed by the yeast in the growth phase and the bulk of the remaining sugar is used up during the stationary phase. An important point to remember here is that a healthy growth of yeast cells is essential to obtain a complete and trouble-free fermentation.
FACTORS AFFECTING FERMENTATION
Many factors influence the fermentation. The important factors include temperature, sugar, ethanol, nitrogen, and oxygen.
TEMPERATURE
Temperature is an important factor affecting fermentation. Most fermentations are conducted in the range of 50 to 86 °F. In this range increasing temperature increases the rate of fermentation. At a higher temperature, in the range of 86 to 95 °F, the fermentation becomes sluggish, and at over 90 °F, it can stop prematurely. The lethal effect of the higher temperature is enhanced by ethanol concentration. At higher ethanol levels, as observed towards the end of fermentation, the yeast cells become more sensitive to an increase in fermentation temperature. In addition to the speed of fermentation, the temperature also affects the formation and retention of certain by-products. Generally, wines produced at low fermentation temperatures (50 to 60°F) tend to have higher alcohol content and fresh and fruitier aroma. Red wines are fermented at slightly higher temperatures (71 to 86 °F) to facilitate the extraction of color and other skin constituents.
SUGAR
Sugar is the principal substrate for fermentation. Glucose and fructose are the predominant sugars in grapes and at harvest; they are generally present in equal amounts. Most wine yeasts preferentially ferment glucose. Therefore, in sweet wines with residual sugar, the amount of fructose is usually higher than glucose. From a practical standpoint this is beneficial since fructose tastes considerably sweeter than glucose at a given concentration. Five carbon sugars such as arabinose and rhamnose, naturally occurring in grapes, are not fermented by wine yeast.
Must with 21 to 24 °Brix is often used for dry table wine production. The must sugar level at harvest is substantially higher for sweet wines such as late harvest or ice wine. At
higher concentrations, such as over 30 °Brix, sugar has a retarding effect on fermentation. The fermentation is slower and with the formation of alcohol and other by-products, it ceases before all the sugar is utilized.
ETHANOL
Ethanol is an important by-product of fermentation. Table wines usually contain 11 to 14% alcohol by volume. Although the yeast can produce such high alcohol levels, its growth is suppressed at a much lower concentration of ethanol. In general alcohol has an inhibitory effect on yeast growth and the toxic effect is enhanced with increasing temperature. Yeast strains vary in their ability to tolerate alcohol. Many indigenous yeast strains have low alcohol tolerance and therefore, participate only in an earlier stage of fermentation. As the fermentation proceeds and alcohol accumulates, the alcohol tolerant strains dominate the fermentation.
NITROGEN
Nitrogen is an essential nutrient for the growth and development of wine yeast. Free amino acids and the ammonium ion constitute the principle nitrogen bearing compounds used by the yeast during fermentation. Generally these compounds are present in sufficient amount in the must to support a healthy yeast growth. But when they are deficient, many fermentation problems are encountered. To ensure a complete and trouble-free fermentation, many winemakers add diammonium phosphate to the must as a source of nitrogen. To get the most benefit, add DAP to the must at the beginning of the fermentation.
AERATION
Although fermentation is an anaerobic process, aeration and, thus, oxygen exposure, has a stimulating influence on fermentation. Aeration promotes the formation of survival factors, which are required by the yeast for building a large and healthy population. Commercially produced active dry wine yeasts are rich in survival factors and therefore, should be preferred for conducting an alcoholic fermentation.
SULFUR DIOXIDE
Sulfur dioxide is an antiseptic agent. It inhibits the activity of many undesirable microorganisms such as bacteria and yeast. In winemaking, sulfur dioxide in the range of 30 to 50 ppm free, is often added to the must before fermentation. At this level SO2 seems to be more toxic to the bacteria and some strains of indigenous yeast than the wine yeast. The yeast also produces some SO2 during fermentation. The amount formed is strain dependent.
CONDUCTING FERMENTATION
After making sugar and acid adjustment to the clarified and sulfited must, it is ready for fermentation. The choice of fermenter is an important consideration. Fermenters made of stainless steel, wood, plastic or glass are commonly used. The ability to control temperature and ease of cleaning should be the guides for choosing the fermenter. Closed top fermenters are used for white wine production. For small batches, five-gallon carboys work fairly well. Small plastic containers, wood barrels, and small stainless steel tanks can also be used. The fermenters should be filled with must to about 70 to 75% capacity. This will prevent vigorously fermenting juice from spilling over in case of excess foaming.
The fermenter should be fitted with a fermentation lock to keep the air out, while allowing CO2 to escape. Inoculate the must with a properly rehydrated, selected strain of wine yeast. For yeast rehydration, and inoculation, follow the procedure given below.
REHYDRATION AND INOCULATION
During the production of active dry wine yeast, the moisture content of the yeast cell is reduced from about 70% to about 8%. In order to resume the fermentative capacity, the moisture content of the dried yeast cell needs to be restored. A precise rehydration procedure must be followed to obtain good viability and sound fermentation. A general guideline for rehydrating a dry wine yeast is given below.
1. Use 104 ºF warm water2. The amount of water should be five to ten times the weight of the yeast3. Slowly add dry yeast to water4. Allow yeast to remain in warm water for 15 to 20 minutes5. Inoculate must with rehydrated yeastDo not add dry yeast directly to the must. This results in poor dispersion, incomplete
rehydration, and poor cell viability. Also warm water and not juice should be used for rehydration and avoid excessive stirring.
Some musts are deficient in nitrogen content and cannot support healthy yeast growth. If you suspect the must to be deficient, add about 500 mg/L of diammonium phosphate. In addition to the nitrogen supplement, add yeast nutrient to the must at the time of yeast inoculation. The temperature of the must at inoculation should preferably be about 70 to 75 °F. At this temperature, the fermentation will normally become evident in four to forty-eight hours. Once fermentation begins, slowly cool the must and ferment at 55 to 60 °F.
To control fermentation temperatures, wineries use fermenters with cooling jackets or external heat exchangers. For small batches, controlling temperatures can be difficult. One practical approach is to use a walk-in cooler with shelves that can hold a five-gallon carboy, or other small container. A large refrigerator with some modifications can be used to ferment smaller lots. Some cooling can also be achieved by spraying or sprinkling cold water on glass or steel containers. A well air conditioned room can also be used to provide cooler fermentation conditions.
Since a large amount of CO2 is released during fermentation, it is important to conduct fermentation in a well ventilated area. In order to monitor the progress of fermentation, test and record the °Brix and temperature of the must every day. At a temperature of 55 to 60 °F, it will usually take two to four weeks to finish fermentation. At the completion of fermentation, the °Brix reading will generally be negative (1 to 15 °Brix). The fermentation is considered complete when the residual sugar decreases to less than 2 g/L or 0.2 %. The procedure outlined above can be used to produce generic and varietal styles of wine.
Another option in making dry white wine is the production of barrel fermented wine with “sur lie” aging, often called the “sur lie” method. In this approach, a wine is fermented in the barrel and matured in contact with the yeast lees with periodic stirring. Since it is difficult to control the fermentation temperature in a wood barrel, it is rather important to maintain a cooler ambient temperature during barrel fermentation. Following alcoholic fermentation, malolactic fermentation is encouraged. The wine is then matured on lees for six to nine months, or even longer, if so desired. While on lees, the wine is stirred periodically to resuspend the sediment. During prolonged lees contact, the wine is enriched by the constituents released from yeast autolysis. Wine produced by the “sur lie” method is often described as having enhanced body, a creamier, richer mouthfeel, greater complexity and depth of flavor, and better integration of fruit and wood-derived components. The “sur lie” method can sometimes have a negative effect
on wine quality. Higher amounts of acetic acid, acetaldehyde, and reduced sulfur compounds can be formed. For this reason, great care should be exercised when making the “sur lie” style of dry white wine.
White wine can also be produced in a sweeter style containing some residual sugar. The degree of sweetness is, of course, a matter of personal preference. This style of sweet wine is made from well ripened fruit and the fermentation is arrested at a certain predetermined level (2 to 4%) of residual sugar. The wine is chilled to 32 to 34 °F and racked off the yeast lees to stop fermentation. It is then stored at a cooler temperature during processing to prevent refermentation.
Sweet white wine is also made from late harvest fruit that is infected with a fungus called Botrytis cinerea, commonly called noble rot. Such grapes usually have a higher sugar content (25 to 30 °Brix) and the must composition is altered by the activity of botrytis mold. It contains a powerful oxidizing enzyme laccase, which is not easily inhibited by normal (50 ppm) levels of sulfur dioxide. The must, as well as the wine made from botrytised grapes, should be carefully handled to minimize oxidation. The must also becomes deficient in vitamins and nitrogenous compounds that are needed by the yeast during fermentation. Addition of nitrogen (DAP) and vitamins is crucial for fermenting must derived from Botrytised grapes. The fermentation is generally slower due to higher sugar content (referred to as substrate inhibition); and may take six to eight weeks or longer to complete. Factors other than sugar concentration, such as temperature, must nutrient status, yeast growth, and ethanol formation will all influence the length of fermentation.
Ice wines made from late harvest frozen grapes are also made into sweet wines. The sugar content in frozen grapes may be even higher (for example, 35+ °Brix) and would, therefore, pose considerable difficulty in fermentation. Since high sugar must is slow to ferment, special attention should be paid to ensure that favorable conditions for the fermentation are maintained.
MALOLACTIC FERMENTATION
Malolactic fermentation (MLF) is also referred to as secondary fermentation. It is brought about by the activity of lactic acid bacteria which converts malic acid to lactic acid and carbon dioxide. The reaction is given below:
Figure 4. Malolactic Fermentation Reaction
Malolactic fermentation can occur naturally in a wine and it may not always be desirable. Its desirability depends on several factors such as must composition, wine style, andthe winemaker’s preference. In a high acid, low pH must, MLF can be employed to reduce acidity to a desirable level. Conversely, in a low acid, high pH must, onset of MLF can reduce acidity to unacceptably low levels.
In the production of fruitier, German-style white wines, MLF is generally not desired; however, in making more complex Burgundian style white wine, MLF is encouraged. Subjecting a wine to MLF also depends on the winemaker’s choice. It is a matter of preference and his/her view of whether a given wine will or will not benefit from secondary fermentation.
There are three major effects of MLF on wine. These are 1) acid reduction, 2) flavor modification, and 3) biological stability. A winemaker should consider these effects when making a choice regarding malolactic fermentation.
ACID REDUCTION
Conversion of malic acid to lactic acid results in the reduction of titratable acidity and an increase in wine pH. Lactic acid bacteria also metabolize citric acid, which is generally present in small amounts, and this also contributes to the lowering of wine acidity. Deacidification affects the taste of the wine, making it less tart. Thus, a winemaker can use MLF as one of the tools to reduce acidity in high acid wines. The increase in pH following MLF has a greater impact on the stability of the wine than on the taste. Generally, higher pH would make wine more prone to microbial spoilage. It is important to remember that pH also affects the color, biological stability, and effectiveness of antimicrobial agents, such as sulfur dioxide and sorbic acid. Malolactic fermentation leads to acid reduction and an increase in wine pH, but the magnitude of changes in acidity and pH are difficult to predict. Generally, one can expect a reduction in TA of 0.1 to 0.3% and a rise in pH of 0.1 to 0.3 units following malolactic fermentation.
FLAVOUR MODIFICATION
Growth and activity of lactic acid bacteria causes changes in the composition of wine. Besides the formation of lactic acids, numerous other compounds are formed. The notable flavor producing compounds include diacetyl, acetoin, and 2,3-butanediol. Diacetyl in small concentrations is considered to give complexity to wine aroma, but in higher concentrations, it can impart a distinct butter-like aroma. Malolactic wines are often described as soft (due to deacidification) and having a buttery or cheesy aroma.
In addition to the constituents mentioned above, many other compounds contribute to the complexity of aroma resulting from MLF. The strains of lactic acid bacteria play a crucial role in determining the type and concentration of various aroma compounds produced. Some spoilage causing lactic acid bacteria can produce off-flavor compounds. It is, therefore, important to choose and encourage only those strains of LAB that produce desirable changes in the aroma of the wine.
BIOLOGICAL STABILITY
It is a common belief that a malolactic fermented wine is biologically stable with respect to lactic acid bacteria. The logic behind such a notion is that once all the malic acid is consumed, the bacteria is not likely to grow due to lack of nutrients, particularly malic acid. Although this may be true in many cases, it is not good to assume that a wine is totally free from the danger of refermentation by LAB.
There are many constituents in wine that can support bacterial growth, particularly if other conditions are favorable. One of the conditions that need to be mentioned here is high pH. Depending on the initial must pH and other conditions, MLF can cause a substantial shift in wine pH (>3,8) that can make wine vulnerable to attack by spoilage causing organisms. The
addition of SO2 at high pH level is of little value, since the higher dose required to control bacteria may itself adversely affect the organoleptic properties of wine.
NATURAL VS. INDUCED MALOLACTIC FERMENTATION
MLF can occur naturally and many winemakers rely on the natural process; however, there are certain disadvantages with this approach that merit consideration. Natural MLF is unpredictable and this can cause undue delays in wine processing. Various strains of lactic acid bacteria are likely to participate and some of these organisms might produce undesirable odor compounds. The conditions that favor MLF also favor the growth of harmful microorganisms. Thus waiting for natural MLF to occur can expose wine to attack by spoilage causing organisms. For these reasons, we suggest inducing MLF by inoculating the wine with a desirable pure culture of lactic acid bacteria.
Pure cultures of lactic acid bacteria in freeze-dried form are commercially available. Some of these cultures require reactivation and considerable effort in making a starter culture. The manufacturer provides the protocol for culture preparation. There are lactic acid bacteria cultures that do not require prior reactivation and can be directly added to the wine to ensure the onset of MLF. We strongly recommend using them.
Bacterial culture can be added at the beginning, during, or after the completion of alcoholic fermentation. To determine the time of inoculation, as well as the procedure for conducting MLF, follow the manufacturers directions. However, direct addition of culture soon after the completion of alcoholic fermentation is recommended. Depending on temperature and other conditions, usually MLF can be completed in two to four weeks.
FACTORS AFFECTING MALOLACTIC FERMENTATION
Many factors influence malolactic fermentation. By manipulating these factors a winemaker can control the course of malolactic fermentation. Conditions that encourage MLF include 1) temperatures of 65 to 75 °F, 2) pH preferably above 3.2, and, 3) total SO2 less than 50 ppm. Lower alcohol (< 12% by volume) and delayed racking also favor a secondary fermentation.
Conversely, conditions such as a low storage temperature, lower pH, higher total SO2, higher ethanol, and the presences of certain inhibitory compounds will discourage MLF.
WINE CLARIFICATION
Young wine is cloudy and the cloudy (turbid) appearance is due to the presence of yeast, bacteria, fragments of grape tissue, and colloidal matter in suspension. Over time, the suspended matter settles to the bottom, forming a sediment (lees) and the wine becomes clear. The clear wine is transferred to another container and this process is called racking. By racking the wine several times a fairly clear wine can be obtained. Usually, in addition to racking, fining and filtration are often used to produce brilliantly clear wine.
RACKING
The process of racking involves the separation of clear wine from the lees (or sediment). The simple method of racking from a carboy, using a polyvinyl tubing. A carboy, or other container, with sediment is placed at a higher elevation than the receiving carboy. One end (the suction end) of the siphon tube is lowered into the wine, just above the sediment level. Care must be taken not to disturb the lees while lowering the siphon tube. Flexible tubing, when
placed in wine may move and disturb the lees. To avoid this movement, the flexible or stiff polyvinyl tube should be tied to a dowel rod, or wood stick, for support, in such a way that when the siphon tube is placed in the wine, the suction end of the tube will be just above the surface of the lees. Also note that the end of the tube is cut at an angle. This arrangement will minimize lees pickup while the wine is being transferred.
The delivery end of the siphon tube is placed close to the bottom of the receiving carboy. This permits the end of the tube to remain below the wine surface while the carboy is being filled. This method is designed to minimize wine aeration. To start the wine flow, suck the wine through the tube. When the tube is full, lower the end into the receiving container. The wine will begin to flow by gravity. A pinch cock should be used to control the flow. After all the clear wine is transferred, the sediment is discarded.
After racking, the wine should be sulfited. This allows SO2 to be mixed as the wine fills the container. It is crucial that after racking, the clear wine is stored in a completely full container.
When racking wine from barrels, it is difficult to see the sediment at the bottom. The wine, however, can be transferred by slowly pumping from one barrel to another. This is done by maintaining the suction end of the hose below the wine surface as the wine is pumped out of the barrel containing sediment. As the wine level gradually drops close to the bottom, the lees can be seen with the help of a flashlight. At this point, the pump speed should be further slowed so the remaining clear wine above the sediment can be transferred without disturbing the lees. To minimize disturbing the sediment, the suction end of the hose can be attached to a clean stick in such a way that the end of the stick extends 3 to 4” beyond the hose opening. The hose tied to the stick can be lowered into the barrel. This arrangement will hold the end of the hose away from the bottom, above the sediment level.
Large wineries use specially designed stainless steel barrel racking tubes for transferring wine from the barrel to other containers. When handling large volumes, the fermentation is conducted in tanks equipped with racking valves that permit removal of clear wine with very little sediment disturbance.
The loss of wine as lees is greater at the first racking. The lees volume, and thus, loss of wine during racking, diminishes with subsequent racking, and the wine becomes increasingly clear. Generally a wine is racked three to four times before it achieves acceptable clarity. During wine transfer, the wine is exposed to airwhich is responsible for the wine’s oxidation. To protect wine from the adverse effects of oxidation, wine aeration should be minimized during wine transfer. Some winemakers use inert gases such as nitrogen or carbon dioxide to reduce oxygen pickup. Generally the receiving container is sparged with gas before filling it with wine. Sparging wine during transfer can remove dissolved oxygen, and thus protect wine from oxidation.
Keeping wine storage containers completely full and maintaining adequate levels of SO2 are other important steps that should be followed in order to prevent undue wine oxidation. Wines made from botrytized must are particularly susceptible to oxidation; therefore, great care must be exercised when racking late harvest style wines made from Botrytis-infected grapes. Wine oxidation is also influenced by the temperature. At lower temperatures, more oxygen is dissolved. The oxidative reaction speeds up at higher temperatures. For this reason, wine should be protected from undue aeration when racking, particularly after cold stabilization.
FINNING
Fining agents are often used to further clarify a wine, but these compounds also can influence wine stability and organoleptic properties. In the case of white wines, bentonite is commonly used to clarify and render a wine heat (protein) stable. Other fining compounds such as casein, PVPP, isinglass, gelatin, and kieselol can also be used in clarification. These fining
agents also react with phenolic compounds and therefore, will influence the sensory character of the wine.
When using fining agents, follow the manufacturer’s recommendation regarding the preparation and method of application. Determine the dose of fining material required by conducting a laboratory scale trial. To obtain the best results from fining, make sure that the fining agent is properly mixed with the wine. Improper mixing can lead to poor results.
BENTONITE TREATMENT
The amount of bentonite needed to clarify and stabilize a wine should be determined by conducting a trial.1. Prepare a 5.2% bentonite slurry by adding 5.2 grams of bentonite to enough water, making a total volume of 100 ml. Use distilled water to make the slurry. The water for hydration can be cold (for agglomerated) or not (non-agglomerated) depending on the suppliers recommendation. Mix the slurry thoroughly to allow the bentonite to properly hydrate. Usually letting it sit overnight is sufficient. 2. Using a graduated pipette, add various amount of slurry to a series of 100 ml wine samples. Table. shows the amount of slurry to be added to wine samples in order to attain various rates of bentonite.
3. Stir the samples to thoroughly mix the bentonite. Let it stand for several hours or overnight.4. Examine the samples for clarity. Use untreated wine to compare the difference. 5. The sample showing the greatest clarity with the smallest dose of bentonite should be further tested for heat stability. 6. Place clear sample in an incubator at 120 °F for 2 days. Check sample for haze. 7. Hold sample for an additional 24 hours at room temperature and observe the haze.8. The amount of bentonite that produces the clearest wine and does not become hazy upon heating and cooling is the appropriate amount that should be chosen for wine treatment.
Bentonite treatment often results in a significant amount of lees. To minimize the amount of lees and make it more compact, the wine can be counterfined with sparkalloid or silicon dioxide. Cold stabilization, following bentonite treatment, can also help in lees compaction.
FILTRATION
Wine can also be made clear by filtration. It is a mechanical operation in which a cloudy wine is passed through a filter bed that retains suspended particles. The resulting filtered wine becomes clear. There are two main types of filter beds, the depth filter and the sieve filter. Pad filters and diatomaceous earth filters are examples of depth filters. Membrane filters are examples of sieve filters.
PAD FILTERS
Pad filtration is commonly used by wineries to clarify wine. For small scale production, filters using a small pad size (8 x 8 cm) are available. The pads are made of cellulose and other material. The are made in a wide range of porosity and, depending on the clarification need, various grades of pads are used. Coarse pads with high throughput are used for initial clarification. As the wine becomes more clear, tighter pads for polishing and sterile pads for bottling the wine are used. During pad filtration, the larger particles in wine are retained on the surface due to sieve action. The smaller particles are trapped in the filter matrix due to small pore size and electrostatic adsorption. Take the following precautions when filtering a wine.
• The two sides of the filter pads are not identical. One side appears coarse and the other side has a finer texture. Follow the manufacturer’s recommendations in arranging the pads between the frames. The wine should enter the coarse side and leave the pad on the finer side.
• Make sure that the seals between the plates are tight. First, run some water through the pad; the wet pads are easy to tighten.
• Run water containing citric acid and sulfur dioxide (1% citric acid and 200 ppm SO2) through the filter. This will remove impurities that may give a filter pad taste to the wine. It will also sanitize the filter.
• Operate the filter under constant pressure.
MEMBRANE FILTERS
Membrane filters are sieve filters with very little solid holding capacity. Therefore, the wine must be very clear to pass through the membrane. They are made of cellulose esters and other polymers. They are available in various pore sizes. A membrane with 0.45 micron pore size is often used to obtain sterile filter wine. The membranes are housed in a cylindrical cartridge and can withstand high pressure. The membrane filtration is employed as a final filtration before bottling.
WINE STABILIZATION
Wine stability is an important issue in wine production. When subjected to very low or high temperatures during storage, a wine can become cloudy or hazy, and can form precipitate. To prevent this problem, winemakers stabilize the wine before bottling.
COLD STABILIZATION
During fermentation some potassium bitartrate precipitates and is removed with the lees. When a wine is cooled, the solubility of potassium bitartrate decreases and the salt precipitates to form a crystalline deposit. Several factors, such as the wine’s pH, alcohol content, and temperature influence the precipitation of bitartrate salt.
In some cases calcium tartrate can also precipitate. This happens when the calcium content of the wine is high enough to cause saturation. This salt, however, does not readily precipitate in response to low temperatures, and takes a longer time to settle out of solution.
The common procedure for cold stabilization is to store a wine at close to freezing temperature, about 24 °F, for two to three weeks. During this period, most of the insoluble salt will settle out. It is important to rack the wine while it is still cold to avoid redissolving the tartrate crystals. Cold wine is more susceptible to oxidation, therefore, care should be exercised to minimize oxidation (air exposure) during wine transfer. In cooler regions, one can take advantage of cold winter temperatures to stabilize wine.
PROTEIN INSTABILITY
Wine contains grape derived proteins. Some of the protein fractions become unstable and cause haze in wine when it is subjected to warm storage temperatures. It is important to remember that not all, but only some, proteins cause cloudiness. Only the heat sensitive unstable protein fractions need to be removed to make a white wine protein stable. The general heat stabilization procedure involves treatment of wine with bentonite. Bentonite clay particles carry a negative charge. When a wine is treated with bentonite, the negatively charged particles adsorb positively charged proteins. The bentonite protein complex settles out and unstable proteins are removed with bentonite lees.
The amount of bentonite required for protein stabilization should be determined by conducting a laboratory trial. A simple procedure is to add various doses of bentonite to a series of wine samples (4.0 oz.) and subject them to 120 °F heat for twenty-four hours. The smallest dose yielding clear wine (without haze) is the appropriate dose for wine treatment.
WİNE MATURATION AND AGINGThe process of wine maturation begins after the completion of fermentation and the first
racking. The maturation process involves a series of changes that lead to the improvement of wines’ aroma and taste. At this point it is necessary to distinguish between the term maturation and aging. The maturation process refers to the changes occurring in a wine during bulk storage. Aging implies modification in the flavor of a wine after bottling. The key difference is that a wine is periodically exposed to air (oxygen) during maturation but is stored in a bottle in the absence oxygen during aging. The purpose of maturation is to retain the desirable grape and fermentation aromas and further encourage the development of attractive and pleasant flavors. Some of the changes that occur during white wine maturation include, 1) change in color from light greenish-yellow to golden yellow, 2) gradual loss of varietal and fermentation aromas, 3) formation of new vinous aromas, and 4) the mellowing of wine possibly due to loss of acidity and harsher phenolic compounds. Most importantly, the various taste and aroma constituents integrate in a harmonious manner yielding a delicious wine.
The technique of wine maturation depends on the style of white wine produced. Fresh and fruity wines made for early consumption are matured for a relatively short duration. They are produced with minimum air exposure and storedunder cooler cellar temperature. Wines made from aromatic grape varieties such as Muscat, Gewürztraminer, and Riesling; Vignoles, Vidal and Cayuga White are processed with little maturation. In order to retain their fruity varietal character, they are bottled soon after clarification, stabilization, and blending. Sweet wines such as late harvest or ice wines are also made with a short maturation period. Alternatively some white wines are made in a different style. These would have less emphasis on simple fruitiness and more focus on flavor complexity. In this style of white wine the flavor complexity is achieved by using several techniques such as fermenting and/or storing wine in oak barrels, malolactic fermentation, and extended yeast lees contact. This style of wine
generally benefits from longer maturation and aging. Chardonnay, Sauvignon blanc, Semillon, Seyval and others are often vinified in this style.
Regardless of wine style, it is very important to exercise proper care during the course of white wine maturation. The containers must be kept full so that the air is excluded. During storage some wine is lost due to evaporation. This loss is minimal in glass and stainless steel containers, but can be significant in wood barrels. The amount of wine lost must be periodically replenished to keep air out. This practice is called topping. The presence of air (oxygen) on the wine’s surface causes oxidation and encourages the growth of harmful aerobic microorganisms such as acetic acid bacteria and film yeast. Only clean and sound wine should be used for topping. Some winemakers use inert gases, either carbon dioxide or nitrogen, as a blanket over the wine to minimize air exposure, when the wine stored in a partially full container. This practice is not very reliable and, therefore not recommended. Inert gas can be used to protect a wine from air with right equipment and proper set up. However, in small-scale wine production, such an approach will be economically prohibitive.
Maintaining adequate levels of free sulfur dioxide in wine is crucial during storage. As mentioned elsewhere, sulfur dioxide is an excellent antioxidant and antimicrobial agent. Based on the wine’s pH, 20 to 30 mg/ L of free sulfur dioxide is sufficient to protect a wine from oxidation and harmful microbes. Regardless of the size of the operation, a winemaker should have the laboratory equipment to analyze the wine for SO2 content and he should also routinely check free SO2 levels to ensure that the wine is adequately protected.
The choice of storage container is an important consideration in maturation of wine. Many home winemakers use glass carboys, and containers made of plastics and stainless steel. Glass carboys and stainless steel containers are well-suited for wine storage when properly sealed. Plastic containers are somewhat questionable. Some plastics may be permeable to oxygen and impart undesirable odors. Only high quality food grade plastic containers should be used for short-term wine storage. Ideally, stainless steel tanks and in some cases wood barrels are the most suitable containers for bulk wine storage. Wood barrels can provide desirable aroma constituents to the wine. A wine is more prone to oxidation when stored in a barrel and, therefore, requires more care during storage. Barrels are also difficult to clean and sanitize and so a special skill is required to handle them as storage containers. Many winemakers use oak chips in place of barrels to obtain wood flavors in wine. In some situations this can be a good technique to attain flavor complexity in the wine.
The temperature and humidity in the cellar have a significant impact on wine during bulk storage. The reactions occurring during maturation are accelerated with increasing cellar temperatures. In order to achieve proper flavor development, white wines should be stored at a cellar temperature of about 55 to 59 ºF. Sweet white wines would require even lower storage temperature. The cellar humidity becomes an important factor especially when wood barrels are used for storage. Evaporation of alcohol and water from wine is influenced by cellar humidity. Generally barrels should be stored at about 65% relative humidity.
BLENDING
Blending is an important step in the wine finishing operation. It is a valuable skill and an art that a serious winemaker must acquire.
PURPOSE
The object of blending is to produce a wine that is superior to the wines contributing to the blend. Usually this involves mixing of wines to modify one or more characteristics to obtain an improved product. For example a high acid and less fruity wine may be blended with very
fruity and low acid wine to produce a wine with enhanced fruit aroma and moderate acidity. Blending is also practiced to produce a consistent product. This point is important to commercial wine producers. Sometimes a wine is blended to mask a defect. In most cases this approach is not effective and it results in a larger volume of an inferior or at best a mediocre product.
PROCEDURE
A blending trial should be conducted before making the final blend. The winemaker should have a clear goal that he/she wishes to achieve through blending. The wines contributing to the blend should be critically evaluated for aroma, taste and overall quality. Their strong and weak points should be recorded. Based on their evaluation, several trial blends should be made and compared with the wines that makeup the blend. It is helpful to seek the opinion of other skilled tasters, during wine evaluation. Always check the stability of the final blend even if the individual components are stable.
CALCULATIONS
There are several ways to calculate the amount of wine required in making a blend. It is important to note that the concentration of certain constituents in a wine such as sugar, acid, alcohol, can be quantitatively measured. Their composition shows a linear relationship, and their adjustment in a blend can be easily computed. Other parameters such as intensity of aroma and taste are measured using sensory techniques and their adjustment can be achieved by conducting a blending trial.
To calculate the amount of wine in a blend, a simple technique known as the Pearson square can be used. This procedure can best be understood by using an example. Suppose a winemaker has two wines A and B, with a titratable acidity of 1.0 % and 0.6% respectively. He wishes to produce a wine with a titratable acidity level of 0.7%. What is the amount of wine A and B needed in the blend to obtain a wine with .7% TA?
As shown in Fig. 2., draw a rectangle. At the upper left hand corner place the TA value of wine A (1.0%), at the lower left hand corner write the TA value of wine B (0.6%) and in the center place the target TA value of 0.7%. Now subtract 0.7 from 1.0 (TA wine A) and place the result, at the lower left-hand (diagonally opposite) corner. Next subtract 0.6 (TA wine B) from 0.7 and note the difference at the upper right hand corner. The resulting values of 0.1 and 0.3 represent the ratio in which Wine A and B are to be blended to get the desired TA of 0.7%. From the results in this example it should be clear that the winemaker will need to mix 1 part wine A (TA 1.0%) with 3 parts of wine B (TA 0.6%) to obtain the desired TA value of 0.7%.
BOTTLING
The purpose of bottling is to package the wine for consumption at a later date. Bottling should be done with the object of preserving the wine’s original quality and encouraging further improvements through bottle aging. In order to prevent any damage to the wine’s quality during bottling the winemaker must protect it from excess aeration and microbial contamination.
PREPARING WINE FOR BOTTLING
After a wine has become brilliantly clear, is stable, reached its peak maturity and has attained balance (through blending if necessary), it is ready for bottling. Adjust free SO2 level between 30 to 40 ppm based on wine’s pH of 3.2 to 3.4. A dry white wine can be bottled without any additional treatment. However, a sweet wine would require the addition of a preservative such as sorbic acid. Taking a wine’s pH and alcohol content into consideration, e recommend adding 150 to 200 mg/L of sorbic acid as potassium sorbate. A wine can be sterile-bottled if one chooses not to use a preservative. However, for many small-scale wine producers this may not be a viable option. If a wine needs to be sweetened, then a sweetening agent such as sugar, sweet reserve (juice), or concentrate should be added just before bottling. After making all the necessary adjustments, the wine should be analyzed, tasted and the data should be recorded. If this final quality check proves satisfactory, the wine should be bottled.
BOTTLE FILLING AND CORKING
Only new and clean bottles should be used for bottling a wine. Previously used wine bottles although cheap, can be a source of unwanted contamination. If one has to use old bottles, then it is absolutely essential that they be thoroughly cleaned with soap (using a bottle brush) and sterilized by soaking in 80 ºC hot water for 20 minutes. For smaller lots of wine, such as in 5-gallon carboys, the bottles can be gravity filled by using a siphoning tube. Before filling the new bottles, they should be flushed with nitrogen or CO2 to remove dust and to sparge them with an inert gas. They may also be rinsed with 500 ppm SO2 and 1% citric acid water if sterilization is desired. Always fill the bottles from bottom up keeping the delivery end of the siphon tube below wine level in the bottle.
Another piece of bottle filling equipment suitable for smaller lots is now available. The machine is called Enolmatic Filler, and it fills bottles using the vacuum principle with minimum aeration.
For handling larger lots the bottling operation involves use of several pieces of equipment. Typically the wine is pumped from a holding tank, through a filter to the bottle filler; or to an elevated bottling tank above the filler which feeds wine to the bottle filler by gravity. The filler is equipped with two or more spouts that fill the bottles with wine using a siphon principle. It is important to remember that at each step in the wine’s movement, it is exposed to the danger of aeration and microbial contamination. It is therefore necessary to protect the wine by using inert gas and very stringent sanitary practices.
Immediately after filling, the bottles should be corked. Only the finest grade of sterile corks should be used, and the bottles should be stored upright for a couple of days before storing them on their sides. This will allow the pressure to be equalized and prevent wine leakage. As an alternative to corks, screw caps with sealable inner liner may also be used.
BOTTLE AGING
Fresh and fruity white wines produced for early consumption are stored for a short period before consumption. Full bodied, complex and rich barrel aged white wines such as Chardonnay and Sauvignon Blanc are aged for a longer period as they continue to develop a fine bottle bouquet.
RED WINE PRODUCTION
The basic procedure of red wine production is outlined in the diagram. An important point in making red wine is, that the fermenting must consists of juice skins and seeds. As a result, the composition of red wine is determined by the constituents extracted from skins and seeds in addition to those present in the juice.
RED WINE STYLES
Red wines are made into a vari-ety of styles. The stylistic differences are based on differences in wine characteristics such as grape variety, color, flavor, body, mouthfeel, and aging potential. The styles range from simple, fruity, fresh, light-colored blushes and rosés to complex, full-bodied, rich and dark-red, with long aging potential. Many factors such as a variety, soil, climate, growing conditions, and viticultural practices influence the fruit composition, and therefore, the style of wine that can be produced. In addition to fruit composition, winemaking techniques also play an important role in determining the wine style.
Figure 5. Red Wine Production
VARIETIES
Many varieties are available for red wine production. The wines are usually produced as varietals, or as blends containing several varieties. A list of commonly used red wine varieties is given in Table 5.
Varieties from the Vinifera group are most widely used for winemaking. In regions where Vinifera grapes are not grown, French hybrids, Labrusca, and other varieties are often used.
Table 5. Red Wine Varieties
Among the Vinifera group, Cabernet Sauvignon alone, or in combination with Merlot and/or Cabernet Franc is used in Premium red wine production. Pinot noir, the famous grape of Burgundy, makes excellent red wine. When grown in other parts of the world, the wine does not always attain the same level of quality as found in Burgundy.
Zinfandel, though popular for blush wine, can also make dark, full-bodied, and flavorful red wine. Syrah, the popular grape of Rhôneand Australia makes fruity wines with softer tannins.
Concord is the leading red wine variety among American grapes. Wines from these grapes have a strong flavor, which is often referred to as a “foxy” aroma. Another American red wine grape, Cynthiana/Norton, does not have the foxy aroma and can make full-bodied, dark red wines.
Among the varieties in the French hybrid category, Baco, Chambourcin, Foch, and Rougeon are commonly used for red wines. These varieties, with proper handling, make good red table wines.
Fresh grapes make the best raw material for making red wine. In a situation where fresh grapes are un-available, frozen grapes or grape concentrate can be used, particularly for making smaller lots of wine.
MATURITY AND HARVEST
The decision to harvest grapes with certain maturity parameters is guided by many factors. These include wine style, variety, and maturity criteria. Typically during the course of maturation sugars accumulate, titratable acidity declines, pH rises, color, and phenolic compounds increase and the formation of distinct varietal aroma components occurs. It would be highly desirable to have all these parameters in an ideal balance. However, in practice this can be difficult to achieve since these parameters are influenced by many factors.
Generally the fruit is harvested based on sugar (ºBrix), titratable acidity, and pH. It should be noted that for making red wine, following only these harvest criteria is not sufficient. Skin constituents such as color, tannins, and flavor strongly influence red wine character and, therefore, their level should also be evaluated when making harvest decisions. Because the skin
is fermented with the juice, the skin condition (freedom from rot) and the proportion of skins to juice (depending on berry size) are also important considerations.
Generally, the accumulation of some components such as color and tannin closely follows the accumulation of sugars. But this may not necessarily hold true for the flavor. Aroma development may follow a different pattern. In such a case, sugar measurements to determine harvest may not yield the best result. A good understanding of the fruit composition and the way it is influenced by factors such as region, climate, variety, and viticultural practices is essential in determining optimum fruit maturity, and the time of harvest.
PREFERMENTATION PROCESSING
DESTEM/CRUSH
The most common practice of handling harvested grapes is to separate the berries from the stems. This is achieved by using the machine called a stemmer/crusher. The object of destemming and crushing is to remove the stem and gently break the berry skin. Care is taken to avoid excessive skin maceration and breaking of seeds. The crushed fruit consisting of pulp, skin, and seed, called must, is transferred to a container and about 30 mg/L of free SO2 is added. The purpose of SO2 addition is to prevent the development of unwanted microbes such as indigenous yeast and harmful bacteria.
Some winemakers retain a small (15 to 20%) amount of whole berries and also add a fraction of stems to the must. The stem addition is intended to extract extra tannins. In some cases, this can be beneficial, however, the stems can also contribute to harshness and loss of pigments.
COLD SOAK
In the practice of cold soak or cold maceration, the must is cooled to about 15 to 20 ºC (41 to 68 ºF) to slow down the onset of fermentation by indigenous yeast, and contact between skins and juice is promoted. The purpose of cold soaking is to encourage extraction of pigments and other phenolic compounds from skins in the absence of ethanol. The skins are soaked for one to two days and the must is pumped over or mixed to facilitate the phenolic extraction.
The cold maceration is thought to improve color, body, and mouthfeel of the resulting wine. The effectiveness of this approach will depend on variety, fruit composition and the condition of the fruit.
MUST ADJUSTMENT
Grapes are generally harvested at 22 to 24 ºBrix for red wine production. Some arieties may not have sufficient amount of sugar at harvest. For these varieties (e.g., Concord), sugar addition to the must would be necessary. Sugar addition can be done to the must at the beginning of fermentation. However, one needs to make an allowance for the volume of seeds and skins when calculating the amount of sugar needed. To circumvent this problem, some winemakers prefer to add sugar to the fermenting must after pressing and removing seeds and skins.
In low sugar, high acid American grapes such as Concord, a sugar syrup in place of dry sugar can be used. This process is also called amelioration. The advantage of this process is that while sugar content increases, the acid level decreases due to dilution. To ensure the quality of the resulting wine, the extent of amelioration within legal limits should be carefully evaluated.
ADJUSTING ACIDTY
Compared to white wines, red wines are produced with lower acidity levels. Generally a titratable acidity in the range of 6.5 to 7.5 g/L and a Ph value of 3.4 to 3.6 is preferred. If the grapes are low in acid content (e.g., less than 5 g/L) then the acidity should be raised by tartaric acid addition. It is important to bear in mind that a portion of the tartaric acid added to the must will be lost (by precipitation of potassium bitartrate) following fermentation and cold stabilization. Allowance for this acid loss should be made when determining the amount of tartaric acid addition.
Sometimes red grapes at harvest contain high acid levels (>9 g/L). To produce well-balanced wines from these grapes, a reduction in acid level may be desired. To reduce acidity, a winemaker should consider chemical as well as biological (yeast and malolactic fermentation) deacidifications.
MUST TREATMENT
The issue of SO2 addition needs some consideration. Some wine-makers do not add free SO2 to red must prior to fermentation. The rationale is to minimize SO2 levels in wine, facilitate malolactic fermentation, and maybe to achieve flavor complexity by allowing indigenous yeast to participate in alcoholic fermentation. The problem with this approach is that no SO2 addition can leave must unprotected from the activity of undesirable microorganisms such as wild yeast and spoilage-causing bacteria. The addition of a small amount (20 to 30 mg/L free SO2) of SO2 to the clean must; must with rot will need higher (75 to 100 ppm) doses. This level (20 to 30 ppm) is sufficiently high to discourage spoilage organisms but not too high to suppress malolactic fermentation, if it is so desired.
Pectolytic enzymes have been in use for white wine production. In recent years some commercial enzyme preparations have been made available for red winemaking. These enzymes are designed to promote the release of pigments, tannins, and polysaccharides in the must. For certain styles of wines, use of these enzymes may be beneficial. However, the merits of using these enzymes should be experimentally evaluated.
Adequate nutrient level is necessary to ensure sound and complete fermentation. Therefore, addition of diammonium phosphate (DAP), (a nitrogen source) and yeast nutrient containing essential vitamins, is recommended. The amount of DAP required will depend on must nitrogen status, yeast strain, and the conditions of fermentation. Generally a DAP addition in the range of 250 to 500 mg/L should be sufficient to prevent fermentation problems such as H2S formation and ensure a clean fermentation.
FERMENTATION
After making all the necessary adjustments (sugar, acid, etc.) and additions, the red must, consisting of juice, skin, and seeds is ready for fermentation. The must can be fermented in open top containers. This allows for ease in must handling, cap management, and temperature control. However, some provision should be made to keep the fruit flies away from the fermenting must. Some winemakers prefer to use fermenters with closed tops or some cover to keep fruit flies away. Smaller lots can be fermented in tubs, tanks, bins, or other containers made of plastic or stainless steel. For larger must volumes, specially designed stainless steel fermenters should be used. The fermentation should be conducted in a well-ventilated area, and provisions should be made to remove excess CO2 generated during fermentation.
YEAST
A wide selection of yeast strains is available for conducting red wine fermentation. The winemaker should choose the strain that will ferment the must efficiently and completely with very little (below sensory threshold) amounts of undesirable compounds such as acetic acid, ethyl acetate, and hydrogen sulfide. To obtain a clean and rapid fermentation, commercially produced strains of active wine yeast in dry form should be used. Dry yeast must be properly rehydrated before inoculating the must. We suggest that winemakers experiment with various strains to make proper selection.
Some winemakers use indigenous yeast strains. This practice can sometimes give good results; however, it is risky and requires a lot more skill and attention. We prefer commercially produced pure culture stains and suggest their use in red wine fermentation.
CONTROLLING FERMENTATION TEMPERATURE
The fermentation releases a significant amount of heat, which further increases the must temperature. Increased temperature enhances the rate of fermentation and also the extraction of color and phenolic compounds. Beyond a certain level (e.g., above 89 to 95 ºF) the excessively high temperature can cause stuck fermentation, promote the growth of undesirable microorganisms and contribute to the formation of off-odor compounds. Therefore, controlling temperature during fermentation is critical.
Red must is generally fermented in the temperature range of 77 to 86ºF. As the fermentation begins, the skins and seeds rise to the top and form a cap. A portion of the heat released is trapped by the cap, which leads to a higher temperature in the cap as compared to the fermenting liquid below. In order to release the trapped heat and promote extraction of skin constituents, the cap is periodically broken and the must is stirred.
In smaller lots, stirring the must can be sufficient to lower the fermentation temperature. For larger must volumes, pumping over, along with the use of cooling jackets, or must chiller may be needed to control the temperature.
CAP MANAGEMENT
With the onset of active fermentation the skins rise to the top of the fermenting liquid and form a cap. Thus the skins and juice in a fermenter are somewhat separated. In order to maximize the extraction of color and flavor from skin it is important to keep skins in close contact with the juice during fermentation. To achieve this, the cap is punched and skins and juice are mixed. For smaller lots, punching the cap twice daily is sufficient to facilitate extraction and release heat.
For larger lots, punching the cap is difficult. In such a case, the juice can be drawn and pumped over the cap. Some winemakers use a sprinkling device that sprinkles the juice on top of the skin using a pump. The object is to thoroughly moisten the cap to release the heat; thus, cooling the must and encouraging extraction of skin constituents. When using pump-over, about one volume of juice is pumped over the cap; and this is done about twice a day. Some winemakers vary the volume and frequency of pump-over and choose the best approach that suits them. Using smaller and shallower fermenters, punching the cap, and mixing the must gives good color and flavor extraction.
EXTRACTION OF SKIN CONSTITUENTS DURING MACERATION
Skin constituents have a significant influence on the quality and style of red wine. A good understanding of these components, their extraction pattern, and their evolution during maturation and aging is important in making stylistic decisions in red wine production.
The color and tannins are the two major components that are extracted from skins during fermentation. The purplish-red color of red grapes is due to the pigments known as anthocyanins. The pigments are located mostly in the outer layers of the skins. In grapes, many kinds of anthocyanins are present. They occur in both color and colorless forms. The amount of pigment in colored or colorless form is strongly influenced by the pH of the wine and also by the presence of free sulfur dioxide. Lowering the pH shifts the equilibrium towards the colored form and SO2 has a bleaching effect on the (monomeric) anthocyanins.
Tannins are complex polymeric phenols. They react with proteins, and it is this property that is used in tanning hides to make leather. They are bitter and astringent compounds with a wide range of molecular sizes. Condensation and polymerization of smaller tannin molecules leads to the formation of bigger tannin molecules such as condensed and highly condensed tannins. These large tannin polymers are less astringent and assume yellow-red to yellow-brown color. When the tannins become too large, they precipitate.
Tannins also play an important role by forming complexes with pigments, which contributes to color stability. These polymeric pigments (pigment and tannin complexes) are less sensitive to changes in pH and SO2 levels in wine. During alcoholic fermentation, both the pigments and tannins are extracted from the skin, but their pattern of extraction is slightly different.
The extraction of color is rapid at the beginning of fermentation. It reaches a peak in the first two to three days; and, then slightly declines during the remainder of fermentation. This means a short maceration time of about two to three days is sufficient to obtain good color.
Tannins and other phenolic substances are also extracted quickly at the beginning but their rate of extraction slows down as the fermentation proceeds. However, the concentration of total phenols (this includes tannins) continues to increase towards the end of fermentation.
The extraction of color and tannins is influenced by temperature, length of skin contact, and the cap management technique followed during fermentation. Increasing fermentation temperature from 20 to 30 ºC causes an increase in color (pigment) and tannin content of the resulting wine.
Various cap management options have already been mentioned. For better extraction, a thorough mixing of must is essential. Large wineries use autofermenters and rotary fermenters to facilitate good mixing. However, smaller lots can be mixed by punching the cap and mixing the must.
The length of skin contact also influences extraction. A longer contact time generally means greater extraction of skin and seed constituents into the wine.
SKIN CONTACT TIME AND PRESSING OPTIONS
The winemaker has several options in determining the length of time of skin contact during red wine fermentation. The decision is based on the wine style and the level of extraction of skin component that the winemaker wishes to have in the wine. Although a winemaker has many choices, presented are three of the widely used approaches.
Short or No Skin Contact
Red grapes are crushed and pressed, and the skins are separated immediately. The must is treated like a white wine. This approach will have very little color in the wine. To obtain slightly more color, a short skin con- tact of about 24 hours may be allowed before the must is
pressed. Wines produced in this style have a light color and a fruity aroma with some residual sugar. They are processed for early consumption. Blush and light rosé wines are the examples of this style.
A More Common Approach
Many winemakers typically ferment the must until the sugar level drops between 5 to 0 ºBrix. Depending on the conditions of fermentation, it may take three to five days to reach this level. Note that in this range (0 to 5 ºBrix), the must will contain some residual sugar and the fermentation will be expected to continue after the must is pressed and skins are removed. This approach should yield wines with good color and fruit flavor with a soft and round mouthfeel. These wines are consumed when relatively young or after a short maturation period. They would not require prolonged aging to achieve a higher quality.
Press At Dryness or After Extended Skin Contact
The must is fermented until it reaches dryness, i.e., all the fermentable sugar is used up, and then pressed. If a winemaker wishes to extract more tannins, the skin contact time is extended for one to three weeks. Generally, after the completion of fermentation, the tank is closed and the must is left undisturbed. Over time the cap sinks to the bottom and the must is then pressed. This approach is recommended for the production of full-bodied, dark, and tannic red wines. They require a long maturation and aging time before they are ready for consumption.
Carbonic Maceration
Carbonic maceration, also called whole berry fermentation, consists of fermenting whole berries, without crushing, in a CO2 saturated atmosphere. In this method, the tank, or any other container containing CO2 is filled with whole clusters. Some winemakers place a small amount (about 5 to 10%) of fermenting must in the bottom of the tank, which generates CO2. The idea is to surround all the fruit with CO2 and create an anaerobic atmosphere. The tank is sealed after it is loaded with the fruit. Under anaerobic conditions, partial fermentation begins within the cells. This fermentation is caused by the cell’s own enzymes, (without yeast). The fermentation produces a small amount of alcohol (about 1.5 to 2.5%), and brings about many changes in the must composition.
The fermentation is carried out for about eight to ten days. The temperature is held near 95 ºF. Following this whole berry fermentation, the clusters are removed and pressed. The partially fermented juice is inoculated with wine yeast and the fermentation is complete. Wines so produced are softer due to lower phenolics and reduced acidity and have a characteristic fermentation aroma. These wines are clarified, stabilized, finished, and offered for consumption within a few months of the vintage.
Pressing
A decision to press the must is made according to the desired wine style, when an optimum amount of color, flavor, tannins, and other constituents are extracted. Generally the juice is drained or pumped, the cap is transferred to the press and the must is then pressed. Following pressing, the young wine is placed in containers and is allowed to finish alcoholic (if unfermented sugar remains) and malolactic fermentation.
MALOLACTIC FERMENTATION
Red wines are often subjected to malolactic fermentation (MLF). The object is to reduce the acidity and achieve flavor complexity. The wine also achieves some degree of biological stability, but it is important to realize that MLF wines are not necessarily stable and that bacterial activity can occur if the conditions become favorable.
In the traditional approach, the malolactic fermentation is allowed to occur naturally. This practice, however, is risky because the MLF remains uncontrolled and the storage conditions favoring MLF are also conducive to microbial spoilage. It is, therefore, prudent to use a selected pure culture of ML bacteria for conducting MLF. The lactic acid bacteria culture is commercially available in two freeze-dried forms. In one case, the culture requires reactivation and propagation before addition to the must. In recent years, another form of freeze-dried culture for direct addition (without the need for reactivation) has been developed and is commercially available. We suggest the use of this freeze-dried, direct addition culture form for conducting MLF. To use the culture, follow the supplier’s directions.
The time of inoculation is an important consideration in conducting MLF. Some winemakers inoculate the must during alcoholic fermentation. This approach may have some benefits, but we think that the risk outweighs the benefits. Therefore, we recommend conducting MLF after the completion of alcoholic or primary fermentation. This approach is also suitable for using a freezedried culture, especially designed for direct addition to the must.
Proper inoculation is an important step in conducting MLF. We strongly recommend following the supplier’s instruction is strongly recommended to obtain good results.
WINE CLARIFICATION
Young red wine is cloudy. The turbidity is caused by particles that remain in suspension. The particulate matter includes grape fragments, crystalline compounds (potassium bitartrate), colloidal compounds and microorganisms such as yeast and bacteria. During storage, many of the particles slowly settle to the bottom leaving the wine relatively clear. To achieve greater clarity, i.e., to make wine brilliantly clear, wine is subjected to treatments such as racking, fining, and filtration.
RACKING
After the alcoholic and malolactic fermentation, the wine is racked off the lees. Generally, the amount of sediment is greater in the first racking and its volume decreases in subsequent rackings. The procedure for racking red wine is similar to the one described in the white wine racking section, with one exception. In white wine racking, the wine should be protected from undue aeration; whereas, in red wine racking, limited air exposure, particularly in the first racking, is desirable. Controlled air exposure during wine transfer is beneficial to the aging of red wine. It also allows for the removal of off-odors (such as hydrogen sulfide) that may have developed during fermentation. Racking wine three to four times a year should yield fairly clear wine. It is important to add appropriate amounts of free SO2 after each racking.
FINNING
Red wine can be fined to achieve greater clarity. However, the fining agents also tend to influence the flavor of the wine. Red wines are rich in pigments and phenolic compounds such as tannins that contribute to a harsh and astringent taste. Proteinaceous fining agents such as gelatin and egg white are often used to lower the tannin level, soften the wine, and enhance
clarity. The choice of a fining agent and the amount of fining material needed should be determined by conducting a fining trial and blind tasting the wines.
For egg white fining, separate the egg white from the yolk and mix it with some water. The solution will be cloudy, but adding a pinch of salt should make it clear. The egg white solution should be slowly added, without foaming, to the wine while gently stirring. Generally five to eight egg whites per barrel (50 gallons) are used for fining.
Gelatin for fining is commercially available in liquid and/or dry powder (leaf) form. For convenience and better results, using low bloom liquid gelatin is recommended. The amount of gelatin used in fining should be based on the supplier’s recommendation.
FILTRATION
Filtration is another option that can be used alone or in combination with fining agents. A wide variety of filters are available to small-scale wine producers. A small plate and frame or cartridge filter can be used to filter and clarify the wine. The filters come in various pore sizes. A polish or sterile grade filter pad often gives satisfactory results.
STABILIZATION
The practice of stabilization refers to the treatment of wine to prevent cloudiness and formation of sediment in the bottle. Red wines are rich in tannin contents, which carry a negative charge. The tannins interact with positively charged proteins which leads to agglomeration and settling of the tannin-protein complex. Due to the removal of proteins in this manner, the problem of proteinaceous haze in red wines is not a serious one. Some winemakers use a small dose of bentonite to clarify the wine, which also helps in protein stability. However, winemakers generally do not treat wine for protein instability unless a test warrants it.
The precipitation of bitartrate in the bottle can be a serious fault. Therefore, red wine is stabilized to prevent this problem.
One approach is to hold the wine at 28 to 35ºF for two to three weeks and remove the precipitated potassium bitartrate by filtering the cold wine. Some winemakers feel this treatment to be too harsh and prefer to stabilize wine by chilling or seeding with bitartrate crystals at much higher temperatures, such as in the range of 41 to 50 ºF. The rationale behind this approach is that the red wines are stored and consumed at warmer temperatures than white wines, and therefore subjecting these wines to severe low temperatures is not necessary.
Deep red and high tannic wines generally throw sediment during long bottle aging. The sediment primarily consists of pigment polymers and some bitartrates. Such a sediment is not perceived as faulty and wine is simply decanted before consumption.
MATURATION, AGING
The process of maturation and aging involves a series of changes that lead to the improvement in the appearance, color, taste, and flavor of a wine. Red wine color is due to the presence of anthocyanin pigments, which occur in monomeric and polymeric forms. Young wines have higher levels of monomeric anthocyanin pigments in various colored and colorless forms. The proportion of colored and colorless types is pH dependent. In the range of wine pH, the lower the pH, the greater the concentration of pigments in red form. Therefore, to produce young red wines of attractive color, the winemaker should strive for a lower wine pH. The monomeric pigments are also susceptible to sulfur dioxide, which causes bleaching. This reaction, however, is reversible and loss of SO2 can restore original color. This point is important to remember when sulfiting young red wines. As the wine matures, the monomeric
pigments are polymerized and the color becomes more stable. It is then less responsive to changes in pH and SO2 levels.
Another important phenolic compound in red wines is tannins. Their structure is complex and they result from oxidative and non-oxidative polymerization reactions involving many other compounds. They contribute bitterness and astringency to wine.
During maturation, some of the tannins are lost due to precipitation, while others undergo reactions that diminish astringency and increase suppleness in red wine.
The flavor of the wine becomes complex as fruit, fermentation, and oak-derived aromas become integrated.
The processing technique and the duration of maturation depends on the style of red wine. Rosé, light red, and nouveau style wines, destined for early consumption, are matured and aged for relatively short periods. The appeal of these wines is their youth and fruit-derived aromas. They are simple wines and delicious to taste when young. Medium to full-bodied, deep-colored and high tannin wines require prolonged maturation periods before they become drinkable. During maturation and aging, their tannins become soft and the complex flavors become integrated, resulting in balanced and harmonious wines.
Generally, the containers used for red wine storage include stainless steel tanks and wooden barrels. For home winemakers and others dealing with smaller lots, glass carboys may be more suitable. In glass and steel containers, there is no loss of wine due to evaporation. However in wood barrels, usually 2 to 5% of the wine is lost due to evaporation. Because of this loss, it is necessary to top the barrels with wine to keep them completely full.
The operation of topping and filling the barrel exposes wine to air which results in limited oxidation. A certain amount of air exposure (oxidation) is considered necessary for the maturation of red wine.
It is commonly believed that wine can breathe through wood, and, therefore, to facilitate the oxidation, wood barrels should be used as a container of choice for maturation. It has been demonstrated that a sealed and airtight barrel (wet staves) does not allow air to enter the barrel, and oxidation of wine occurs when the barrels are opened during topping and filling operations.
Fruity and young wines are generally matured in steel tanks (or used wood barrels) and are usually made without oak character. Medium and full-bodied premium reds are commonly matured in wood barrels. The barrels used include a mix of new and used barrels. In this approach, the winemaker is aiming for flavor complexity, including oak character. The length of wood maturation depends on grape variety, wine style, kind of barrel, winemaker’s preference, and consumer choice.
Winemakers have a wide range of choices in selecting barrels based on species of oak, geographic origin, toast levels, and method of barrel production. For a small-scale producer, the use of a 50-gallon wood barrel may or may not be practical. Barrels are generally expensive, require space for storage (full and empty), and are difficult to clean and sanitize. Many winemakers use oak chips as an alternative to oak barrels.
In smaller lots where the use of a wine barrel is not practical or possible, oak chips can be used to obtain oak character in the wine. Both American and French oak chips are available in various sizes and grades. The usual rate ranges between 10 to 15 lbs/1000 gal (4,5 to 6.8 g/gal), and the contact time between oak chips and wine varies between one to three weeks. Some winemakers prefer to use a higher dose in a portion of the wine and then blend it back with the untreated portion to obtain a desired level of oakiness in wine. It is desirable to conduct a trial to determine the optimum quantity and length of contact time.
MAINTAINING PROPER SO2
Managing proper SO2 levels in red wines is critical. The amount of SO2 used should be low enough to permit some oxidation, but high enough to control spoilage-causing microorganisms. This can be a difficult exercise, particularly if the wine pH is high (3.6 and more). This is because at a higher pH, the higher dose of SO2 necessary to control microorganisms can adversely affect the taste and flavor of the wine. To avoid the need of using excess levels of SO2, the winemaker should attempt to keep the wine pH lower, reduce microbial load by filtration before prolonged storage, and conduct all cellar operations under stringent hygienic conditions. The SO2 levels should be periodically checked and adjusted to the proper level.
BOTTLING
The procedure for bottling red wine is quite similar to bottling white wine. The reader should refer to the section on bottling Vineyard and Vintage View Vol. 15(1) pp. 7-8.
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