April 2008

April 2008

Wine is an alcoholic drink that isformed from the fermentation ofgrapes or grape juice. The word winecomes from the Latin vinum referringto both ‘wine’ and ‘vine’. Drinks thatare wine-like can also be made fromother fruits or from fl owers or grains.When substances other than grapesare used to make ‘wine’, a referenceto the substance is made in the name,for example lime wine. This is becauseby itself the term ‘wine’ is defi nedtechnically and legally as the beveragederived from the fermentation ofgrapes or grape juice. The word ‘wine’by itself always means grape wine.

THE HISTORY OF WINEThe earliest known evidence of a fermented wine-like drink is from the Chinesevillage of Jiahu, dated from 9000 years ago (7000 BC). This rice wine wasdiscovered by chemically analysing the contents of sixteen buried jars.The fi rst wine made from grapes appears to have been made between 8000and 4500 BC during the Neolithic period in the Middle East. Various civilisationsfrom this time contributed to making wine popular and improving the technologyinvolved in the process.In Australia, grape growing to produce wine began soon after the arrival ofEuropean settlers, with grape vines among the cargo of the First Fleet of 1788.Governor Phillip brought vines from Rio de Janeiro and the Cape of Good Hopeand planted them at Farm Cove in 1788.This wine production waslargely unsuccessful due to thelack of suitable vine types untilGregory Blaxland (more famousfor crossing the Blue Mountainswith Lawson and Wentworth)arrived in Sydney in 1806. Hebrought a number of varieties ofvines and developed practices toensure the vines were resistantto disease. Shortly afterwards,James Busby, who had studiedviticulture in France, arrivedin New South Wales in 1824with a collection of cuttingsand obtained a land grant of2000 acres in the Hunter Valley.

New settlements around the Australian coast at this time ledto the spread of vine planting in Western Australia, Victoria and South

Australia.The increased immigration to Australia in the 1840s led to a rapid and successfulexpansion of the industry for a variety of reasons. Some examples include:

GRAPE JUICE AND WINE—A COMPARISON

What are the main differences?

The main differences between the composition of grape juice and wine involvethe amounts of sugar and ethanol. Compared with the starting solution of grapejuice, the wine contains far less sugar and more ethanol (alcohol).This is dueto the process of fermentation. Fermentation in wine is a process that involves amicro-organism breaking down a molecule without using oxygen to produce energy.The end product of fermentation is an organic molecule, in this case ethanol.

THE ROLE OF YEAST IN WINE-MAKING

The micro-organism involved in the fermentation ofgrape juice to wine is yeast. Yeast is a microscopic,single-celled fungal organism. The yeast isresponsible for providing enzymes that assist thesugar molecules to break down. An enzyme is acatalyst that helps a reaction take place withoutundergoing permanent change in the reaction.The yeast breaks down the sugar moleculeto obtain energy. There are many differentforms of sugars, which are collectively classedas carbohydrates. Simple sugars, like glucose andfructose (found in grapes), are the monomers ofmore complex carbohydrates including starchand cellulose. Glucose and fructose have the same

formula (C6H12O6).

This image, formed by using an electron microscope, showsa number of unicellular yeast cell

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Types of respirationThe breakdown of sugars can occur completely if oxygen is available and partiallyif oxygen is unavailable. The process of breaking down an organic molecule forenergy use by an organism is respiration. Respiration with oxygen is aerobic andwithout oxygen is anaerobic respiration.The quantity of energy made available through aerobic respiration is muchgreater than for respiration without oxygen. This can be seen when the energyreleased is shown with the respective equations:

Humans and most other organisms respire aerobically. Some organisms,including yeast, are able to respire aerobically and anaerobically when insufficientoxygen is available. For this reason they are described as facultative organisms.When yeast respires anaerobically, the process is called fermentation and resultsin the product required by wine-makers, namely ethanol.

Place 5 g of yeast in the Erlenmeyer flask. Add 10 g of sugar, then 40 mL of warmwater to the 50 mL Erlenmeyer flask. Swirl the flask to ensure the contents are mixed.Cover the flask tightly with a balloon.Record the circumference of the balloon when it is in position. Record thecircumference again after 5, 10, 15, 20, 25 and 30 minutes.

Fermentation, yeast and tolerance limitsAn investigation into fermentation reveals the process is highly dependent on thefunctioning of yeast cells. In particular, the cells play an important role due to theirenzyme production.The yeast has optimal temperatures and pH ranges for working. Outside ofthese ranges (tolerance limits) the yeasts may die, for example if the yeasts becometoo hot. This is a real problem for wine-makers as the fermentation reaction isexothermic. This means that heat is released during the process. If the fermentationcontainer is insulated and heat cannot escape, the temperature will rise. If thetemperature of the fermenting juice rises above or falls below the temperature rangetolerated by yeast, then the organism will die.Yeast will also die if the alcohol (its waste product) concentration goes overa certain percentage. This is because the alcohol remains in the fermenting liquid.Yeast generally can tolerate 10–15% alcohol, but some especially cultured strainsof yeast have been developed to tolerate up to 21%. The other by-product, CO2,bubbles through the liquid and dissipates into the air.

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ASSESSING THE GRAPES

Grape-growers carefully monitor their vine crop (Vitis vinifera) all year. They checkthat the plants are free of pests and sample the soil. The soil and climate have a largeinfl uence on the quality of the grape produced. They also help determine what typeof grape variety is selected for growth in certain areas. The soil and climate in theQueensland Granite Belt appears to be best suited to growing white wine varietieslike chardonnay, semillon, sauvignon blanc and verdelho.Following each harvest, vines are cut back as fl owers will develop only on newbuds. The fl owering occurs by the end of October. Pollen from the stamen (the malepart of the fl ower) transfers to the stigma (the female part) and fertilisation takesplace. During the next few months there is a period of growth where the grapeberries start to develop from the fl ower ovary. They begin as small, hard, greenberries and slowly start to soften, get bigger and take on some colour. At this stagethey contain large amounts of acid. They do not contain sugar, which is essentialfor fermentation, or any fl avour.The grape berries then start to ripen. This period is known as veraison.The concentration of acid found in the grape berries starts to decrease and theconcentration of sugar starts to increase. The colour of the berries, due to moleculescalled anthocyanins, also begins to develop and deepen, and the berries start tosoften.

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