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Separation in Food Processing and Milk Processing

Nov 22, 2014






It is the set of methods and techniques used to transform raw ingredients into food or to transform food into other forms for consumption by humans or animals either in the home or by the food processing industry. Food processing typically takes clean, harvested crops or butchered animal products and uses these to produce attractive, marketable and often long shelf-life food products. Similar processes are used to produce animal feed. Food processing dates back to the prehistoric ages when crude processing incorporated slaughtering, fermenting, sun drying, preserving with salt, and various types of cooking (such as roasting, smoking, steaming, and oven baking). Salt-preservation was especially common for foods that constituted warrior and sailors' diets, up until the introduction of canning methods. This holds true except for lettuce. Evidence for the existence of these methods exists in the writings of the ancient Greek , Chaldean, Egyptian and Roman civilizations as well as archaeological evidence from Europe, North and South America and Asia. These tried and tested processing techniques remained essentially the same until the advent of the industrial revolution. Modern food processing technology in the 19th and 20th century was largely developed to serve military needs. In 1809 Nicolas Apart invented a vacuum bottling technique that would supply food for French troops, and this contributed to the development of tinning and then canning by Peter Durand in 1810. Although initially expensive and somewhat hazardous due to the lead used in cans, canned goods would later become a staple around the world. Pasteurization, discovered by Louis Pasteur in 1862, was a significant advance in ensuring the micro-biological safety of food. In the 20th century, World War II, the space race and the rising consumer society in developed countries (including the United States) contributed to the growth of food processing with such advances as spray drying, juice concentrates, freeze drying and the introduction of artificial sweeteners, colouring agents, and preservatives such as sodium benzoate. In the late 20th century products such as dried instant soups, reconstituted fruits and juices, and self-cooking meals such as MRE food ration were developed. In Western Europe and North America, the second half of the 20th century witnessed a rise in the pursuit of convenience; food processors especially marketed their products to middle-class working wives and mothers. Frozen foods (often credited to Clarence Birdseye) found their success in sales of juice concentrates and "TV dinners". Processors utilised the perceived value of time to appeal to the post-war population, and this same appeal contributes to the success of convenience foods today.

Separations are vital to all areas of the food processing industry. Separations usually aim to remove specific components in order to increase the added value of the products, which may be the extracted component, the residue or both. Purposes include cleaning, sorting and grading operations, extraction and purification of fractions such as sugar solutions or vegetable oils, recovery of valuable components such as enzymes or flavour compounds, or removal of undesirable components such as microorganisms, agricultural residues or radionuclides. Operations range from separation of large food units, such as fruits and vegetables measuring many centimetres, down to separation of molecules or ions measured in nanometres. Separation processes always make use of some physical or chemical difference between the separated fractions; examples are size, shape, colour, density, solubility, electrical charge and volatility. The separation rate is dependent on the magnitude of the driving force and may be governed by a number of physical principles involving concepts of mass transfer and heat transfer. Rates of chemical reaction and physical processes are virtually always temperaturedependent, such that separation rate will increase with temperature. However, high temperatures give rise to degradation reactions in foods, producing changes in colour, flavour and texture, loss of nutritional quality, protein degradation, etc. Thus a balance must be struck between rate of separation and quality of the product. Separations may be classified according to the nature of the materials being separated, and a brief overview is given below.

Solid foods include fruits, vegetables, cereals, legumes, animal products (carcasses, joints, minced meat, fish fillets and shellfish) and various powders and granules. Their separation has been reviewed by Lewis and can be subdivided as follows.

Particle size may be exploited to separate powders or larger units using sieves or other screen designs. Air classification can be achieved using differences in aerodynamic properties to clean or fractionate particulate materials in the dry state. Controlled air streams will cause some particles to be fluidised in an air stream depending on the terminal velocity, which in turn is related primarily to size, but also to shape and density. Also in the dry state, particles can be separated on the basis of photometric (colour), magnetic or electrostatic properties. By suspending particles in a liquid, particles may be separated by settlement on the basis of a combination of size and density differences. Buoyancy differences can be exploited to separate products from heavy materials such as stones or rotten fruit in flotation washing, while surface properties can be used to separate peas from weed seeds in froth flotation.

Plant materials often contain valuable components within their structure. In the case of oils or juices, these may be separated from the bulk structure by expression, which involves the application of pressure. Alternatively, components may be removed from solids by extraction, which utilises the differential solubilities of extracted components in a second medium. Water may be used to extract sugar, coffee, fruit and vegetable juices, etc. Organic solvents are necessary in some cases, e.g. hexane for oil extraction. Supercritical CO2 may be used to extract volatile materials such as in the decaffeination of coffee. A combination of expression and extraction is used to remove 99% of the oil from oilseeds.

Liquid foods include aqueous or oil based materials, and frequently contains solids either in true solution or dispersed as colloids or emulsions.

Discrete solids may be removed from liquids using a number of principles. Conventional filtration is the removal of suspended particles on the basis of particle size using a porous membrane or septum, composed of wire mesh, ceramics or textiles. A variety of pore sizes and geometric shapes are available and the driving force can be gravity, upstream pressure (pumping), downstream pressure (vacuum) or centrifugal force. Using smaller pore sizes, microfiltration, ultrafiltration and related membrane processes can be used to fractionate solids in true solution. Density and particle size determine the rate of settlement of dispersed solids in a liquid, according to Stokes Law. Settlement due to gravity is very slow, but is widely used in water and effluent treatment. Centrifugation subjects the dispersed particles to forces greatly exceeding gravity which dramatically increases the rate of separation and is widely used for clarifying liquid food products. A range of geometries for batch and continuous processing are available.

Centrifugation is again used to separate immiscible liquids of different densities. The major applications are cream separation and the dewatering of oils during refining.

Differences in solubility can be exploited by contacting a liquid with a solvent which preferentially extracts the component(s) of interest from a mixture. For example, organic solvents could be used to extract oil soluble components, such as flavour compounds, from an aqueous medium.

An alternative approach is to induce a phase change within the liquid, such that components are separated on the basis of their freezing or boiling points. Crystallisation is the conversion of a liquid into a solid plus liquid state by cooling or evaporation. The desired fraction, solid or liquid, can then be collected by filtration or centrifugation. Alternatively, evaporation is used to remove solvent or other volatile materials by vaporisation. In heat-sensitive foods, this is usually carried out at reduced operating pressures and hence reduced temperature, frequently in the range 4090oC. Reverse osmosis is an alternative to evaporation in which pressure rather than heat is the driving force. Ion exchange and electrodialysis are used to separate dissolved components in liquids, depending on their electrostatic charge.

These separations are not common in food processing. Removal of solids suspended in gases is required in spray drying and pneumatic conveying and is achieved by filter cloths, bag filters or cyclones. Another possibility is wet scrubbing to remove suspended solids on the basis of solubility in a solvent.

The separation by sedimentation of two immiscible liquids, or of a liquid and a solid, depends on the effects of gravity on the components. Sometimes this separation may be very slow because the specific gravities of the components may not be very different, or because of forces holding the components in association, for example as occur in emulsions. Also, under circumstances when sedimentation does occur there may not be a clear demarcation between the components but rather a merging of the layers. For example, if whole milk is allowed to stand, the cream will rise to the top and there is eventually a clean separation between the cream and the skim milk. However, this takes a long time, of the order of one day, and so it is suitable, perhaps, for the farm kitchen but not for the factory. Much greater forces can be obtained by introducing centrifugal action, in a centrifuge. Gravity still acts and the net force is a combination of the centrifugal force with gravity as in the cyclone. Because in most industrial centrifuges, the centrifugal forces imposed are so much greater than gravity, the effects of gravity can usually be neglected in the analysis of the separation. There are many applications of separation by centrifugation in food processing some example includes separation of cream from milk and refining of vegetable oils. Centrifugal separation is also employed in brewing industries, processing of fish protein concentrate, and removal of cellular materials from fruits juices. Centrifugal filtration is also employed in the food industries for separation of crystalline products (lactose, sucrose) from crystal slurry. In this case, centrifugal force causes the solid particles to form a filter cake on a screen in a rotating basket. Rotation of the basket allows washing and drying of the separated slurry following centrifugation.

Schematic representation of a centrifugal separator

Ultrafiltration is a high-level filtration system for the treatment of water and other liquids. Liquids are forced through a membrane with extremely fine pores to filter colloids and molecules between 0.002 and 0.1 microns in size. Ultrafiltration is capable of removing solids, bacteria and viruses. It is a variety of membrane filtration in which hydrostatic pressure forces a liquid against a semipermeable membrane. Suspended solids and solutes of high molecular weight are retained, while water and low molecular weight solutes pass through the membrane. This separation process is used in industry and research for purifying and concentrating macromolecular (103 - 106 Da) solutions, especially protein solutions. Ultrafiltration is not fundamentally different from microfiltration, nanofiltration or gas separation, except in terms of the size of the molecules it retains. Ultrafiltration is applied in cross-flow or dead-end mode and separation in ultrafiltration undergoes concentration polarization. Harvesting and processing seafood is an important source of income for residents of the nation's coastal states. Over five billion pounds of seafood are landed annually i n the United States, and two million pounds are landed in North Carolina annually. Water is needed for processing the harvest into marketable products, the annual water consumption by the

seafood processing industries is estimated to be 2.5 billion gallons; most of which i s discharged as seafood processing wastewaters. The heart of an ultrafiltration process is the semipermeable membrane which i s usually sandwiched between supporting media for its strength. There are several configurations of packing the membrane material in commercially available ultrafiltration units such as the spiral wound, the hollow fine fibre, the plate and frame, and the single tube membrane. The membrane filter cartridge used in this experiment is a combination of the hollow fine fibre and single tube configurations. The cartridge consists of numerous parallel turbines each having an inner diameter of 0.043 inches. The turbines are bundle together with nylon net and are contained i n 25-inch long clear plastics hell with bores of the fibres exposed. As raw wastewaters passes through the bores of the turbines under pressure, water permeates through the membrane into the void sealed between the turbines and the plastics hell. Two permeate ports are provided on the plastics hell for collecting the permeate and also for back flushing the membrane.

Microfiltration is a filtration process which removes contaminants from a fluid (liquid & gas) by passage through a micro porous membrane. A typical microfiltration membrane pore size range is 0.1 to 10 micrometres (m). Microfiltration (MF) designates a membrane separation process similar to UF but with even larger membrane pore size allowing particles in the range of 0.2 to 2 micrometres to pass through. The pressure used is generally lower than that of UF process. The membrane configuration is usually cross-flow. MF is used in the dairy industry for making low-heat sterile milk as proteins may pass through but bacteria do not. Please click above link for a schematic diagram of this membrane processes. Microfiltration is fundamentally different from reverse osmosis and Nano filtration because those systems use pressure as a means of forcing water to go from low pressure to high pressure. Microfiltration can use a pressurized system but it does not need to include pressure. Microfiltration is the process of filtration with a micrometre sized filter. The filters can be in a submerged configuration or a pressure vessel configuration. They can be hollow fibres, flat sheet, tubular, spiral wound, hollow fine fibre or track etched. These filters are porous and allow water, monovalent species (Na+, Cl-), dissolved organic matter, small colloids and viruses through but do not allow particles, sediment, algae or large bacteria through. Microfiltration systems are designed to remove suspended solids up down to 0.1 micrometres in size, in a feed solution with up to 2-3% in concentration and are very suitable for use in place of traditional clarifiers or as a pre-filter to water recycling/recovery reverse osmosis system.

Reverse osmosis (RO) designates a membrane separation process, driven by a pressure gradient, in which the membrane separates the solvent (generally water) from other components of a solution. It is a filtration method that removes many types of large molecules and ions from solutions by applying pressure to the solution when it is on one side of a selective membrane. The result is that the solute is retained on the pressurized side

of the membrane and the pure solvent is allowed to pass to the other side. To be "selective," this membrane should not allow large molecules or ions through the pores (holes), but should allow smaller components of the solution (such as the solvent) to pass freely. Reverse osmosis is most commonly known for its use in drinking water purification from seawater, removing the salt and other substances from the water molecules. This is the reverse of the normal osmosis process, in which the solvent naturally moves from an area of low solute concentration, through a membrane, to an area of high solute concentration. The movement of a pure solvent to equalize solute concentrations on each side of a membrane generates a pressure and this is the "osmotic pressure." Applying an external pressure to reverse the natural flow of pure solvent, thus, is reverse osmosis. The process is similar to membrane filtration. However, there are key differences between reverse osmosis and filtration. The predominant removal mechanism in membrane filtration is straining, or size exclusion, so the process can theoretically achieve perfect exclusion of particles regardless of operational parameters such as influent pressure and concentration. Reverse osmosis, however, involves a diffusive mechanism so that separation efficiency is dependent on solute concentration, pressure, and water flux rate. Formally, reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a semipermeable membrane to a region of low solute concentration by applying a pressure in excess of the osmotic pressure. The membranes used for reverse osmosis have a dense barrier layer in the polymer matrix where most separation occurs. In most cases, the membrane is designed to allow only water to pass through this dense layer, while preventing the passage of solutes (such as salt ions). This process requires that a high pressure be exerted on the high concentration side of the membrane, usually 217 bar (30250 psi) for fresh and brackish water, and 4070 bar (600 1000 psi) for seawater, which has around 24 bar (350 psi) natural osmotic pressure that must be overcome. This process is best known for its use in desalination (removing the salt from sea water to get fresh water), but since the early 1970s it has also been used to purify fresh water for medical, industrial, and domestic applications. Osmosis describes how solvent moves between two solutions separated by a semipermeable membrane to reduce concentration differences between the solutions. When two solutions with different concentrations of a solute are mixed, the total amount of solutes in the two solutions will be equally distributed in the total amount of solvent from the two solutions. Instead of mixing the two solutions together, they can be put in two compartments where they are separated from each other by a semipermeable membrane. The semipermeable membrane does not allow the solutes to move from one compartment to the other, but allows the solvent to move. Since equilibrium cannot be achieved by the movement of solutes from the compartment with high solute concentration to the one with low solute concentration, it is instead achieved by the movement of the solvent from areas of low solute concentration to areas of high solute concentration. When the solvent moves away from low concentration areas, it causes these areas to become more concentrated. On the other side, when the solvent moves into areas of high concentration, solute concentration will decrease. This process is termed osmosis. The tendency for solvent to flow through the membrane can be expressed as "osmotic pressure", since it is analogous to flow caused by a pressure differential. Osmosis is an example of diffusion.

In reverse osmosis, in a similar setup as that in osmosis, pressure is applied to the compartment with high concentration. In this case, there are two forces influencing the movement of water: the pressure caused by the difference in solute concentration between the two compartments (the osmotic pressure) and the externally applied pressure.

Fractionation is a separation process in which a certain quantity of a mixture (solid, liquid, solute, suspension or isotope) is divided up in a number of smaller quantities (fractions) in which the composition changes according to a gradient. Fractions are collected based on differences in a specific property of the individual components. A common trait in fractionations is the need to find an optimum between the amount of fractions collected and the desired purity in each fraction. Fractionation makes it possible to isolate more than two components in a mixture in a single run. This property sets it apart from other separation techniques. Fractionation is widely employed in many branches of science and technology. Mixtures of liquids and gases are separated by fractional distillation by difference in boiling point. Fractionation is widely employed in many branches of science and technology. Mixtures of liquids and gases are separated by fractional distillation by difference in boiling point. Fractionation of components also takes place in column chromatography by a difference in affinity between stationary phase and the mobile phase. In fractional crystallization and fractional freezing, chemical substances are fractionated based on difference in solubility at a given temperature. In cell fractionation, cell components are separated by difference in mass. Fractionation is also used for culinary purposes, as coconut oil, palm oil, and palm kernel oils are fractionated to produce oils of different viscosities, that may be used for different purposes. These oils typically use fractional crystallization (separation by solubility at temperatures) for the separation process instead of distillation.

A conical flask is used as a receiving flask. Here the distillation head and fractionating column are combined in one piece.

Plasma proteins are separated by using the inherent differences of each protein. Fractionation involves changing the conditions of the pooled plasma (e.g., the temperature or the acidity) so that proteins that are normally dissolved in the plasma fluid become insoluble, forming large clumps, called precipitate. The insoluble protein can be collected by centrifugation. One of the very effective ways for carrying out this process is the addition of alcohol to the plasma pool while simultaneously cooling the pool. This process is sometimes called cold alcohol fractionation or ethanol fractionation. It was described by and bears the eponym of Dr Edwin J. Cohn. This procedure is carried out in a series of steps so that a single pool of plasma yields several different protein products, such as albumin and immune globulin. Human serum albumin prepared by this process is used in some vaccines, for treating burn victims, and other medical applications.

Distillation is a method of separating mixtures based on differences in their volatilities in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction. Commercially, distillation has a number of applications. It is used to separate crude oil into more fractions for specific uses such as transport, power generation and heating. Water is distilled to remove impurities, such as salt from seawater. Air is distilled to separate its componentsnotably oxygen, nitrogen, and argonfor industrial use. Distillation of fermented solutions has been used since ancient times to produce distilled beverages with higher alcohol content. The premises where distillation is carried out, especially distillation of alcohol are known as a distillery. The application of distillation can roughly be divided in four groups: laboratory scale, industrial distillation, distillation of herbs for perfumery and medicinal (herbal distillate), and food processing. The latter two are distinctively different from the former two in that in the processing of beverages, the distillation is not used as a true purification method but more to transfer all volatiles from the source materials to the distillate. The main difference between laboratory scale distillation and industrial distillation is that laboratory scale distillation is often performed batch-wise, whereas industrial distillation often occurs continuously. In batch distillation, the composition of the source material, the vapours of the distilling compounds and the distillate change during the distillation. In batch distillation, a still is charged (supplied) with a batch of feed mixture, which is then separated into its component fractions which are collected sequentially from most volatile to less volatile, with the bottoms (remaining least or non-volatile fraction) removed at the end. The still can then be recharged and the process repeated. In continuous distillation, the source materials, vapours, and distillate are kept at a constant composition by carefully replenishing the source material and removing fractions from both vapour and liquid in the system. This results in a better control of the separation process.

Distillation column

This technology is routinely utilized for partial or complete demineralization of the water supply, softening, and dealkalization, or it can be customized for selective removal of a specific contaminant (for example, denitratization). In simplest terms, ion exchange involves using a selective resin to exchange a less desirable ion with a more desirable ion. Of course, a great deal of chemical research goes into the development of these selective resin materials, but the functional outcome remains straightforward. For example, softening resins are often employed to remove hardness (calcium and magnesium) from the water entering boilers and heat exchangers. In this application, the hardness ions are not wanted. The softening resin (for example, sodium zeolite clay) is charged with active and replaceable sodium ions. When the hard water passes across the softening bed, the resin has selectivity for calcium and magnesium, so it replaces them for sodium. The result is that the water exiting the softener is virtually free of calcium and magnesium (since they were replaced by sodium) and is safe to use in boilers and other equipment, since it will no longer have the tendency to form scale. To supplement the major treatment systems mentioned above, the carbonated beverage producer often utilizes a host of other support technologies, including activated carbon filtration (to remove organic contaminants and chlorine), sand filtration (to remove particulates), and primary and secondary disinfection (using chlorine, ozone, ultraviolet, heat, or a combination). By the time the treated water is finished, it is microbially and chemically safe, clear, colourless, and ready to be used for syrup and beverage production.

Electrodialysis is used to transport salt from one solution, the dilute, to another solution (concentrate) by applying an electric current. This is done in an electrodialysis cell providing all necessary elements for this process. The concentrate and dilute are separated by the

membranes into the two different process streams (concentrate and dilute), as shown in the figure below. An electric current is applied, moving the salt over the membranes.

Inside an electrodialysis unit, the solutions are separated by alternately arranged anion exchange membranes, permeable only for anions and cation exchange membranes, permeable only for cations. By this, the two kinds of compartments are formed, distinguishing in the membrane type facing the cathode's direction. By applying current, cations within the dilute (blue compartment set) move toward the cathode passing the cation exchange membrane facing this side and anions move towards the anode passing the anion exchange membrane. A further transport of these ions, now being in a chamber of the concentrate (red compartments), is stopped by the respective next membrane:

The membranes are separated by spacers (5) consisting of a fabric in the active area filled with the electrolyte combined with a sealing around it. The spacer net prevents the membranes from touching each other. The stacked spacers form with their holes tubes, which are arranged in a way to build two different channel systems. By this way, the concentrate and dilute circuit is built.

1. Propylene end plate 3. Electrode chamber 5. Spacer fabrics 7. Steel frames 9. Inlet concentrate cell 11. AAM 13. Inlet cathode chamber

2. Electrode 4. Spacer-sealing PVC 6. Screws 8. Inlet anode cell 10. Cation exchange membrane 12. Inlet dilute cell

Deionization removes ions as does Ion Exchange. Ion exchange deionization (DI) columns use synthetic resins similar to those used in water softeners. Typically used on water that has already been pre-filtered, Deionization systems use a two-stage process to remove virtually all ionic material remaining in water. That is why we call the process Deionization.

In Deionization two types of synthetic resins are used, one to remove positively charged ions (cations) and another to remove negatively charged ions (anions). Cation deionization (DI) resins ions remove cations, such as calcium, magnesium and sodium and replace them with the hydrogen (H+) ion. Anion deionization resins remove anions, such as chloride, sulphate and bicarbonate and replace them with the hydroxide (OH-) ion. The deionization graphic at the right shows the process. In this case a salt molecule dissolves into its cation Na+ and Anion Cl-. Deionization occurs with the Na+ replaced by the Hydrogen Ion - (H+) and the Cl- replaced by the Hydroxide Ion - (OH-). Deionization occurs as the H+ and OH- combine to produce good old H2O or water. Deionization resins have design capacities and must be regenerated and/or replaced upon exhaustion. This occurs when equilibrium between the adsorbed ions is reached. Cation deionization resins are regenerated by treatment with acid, which replenishes the sites with H+ ions. Anion deionization resins are regenerated with a strong base which replenishes (OH-) ions. Regeneration can take place off-site with regenerated exchange tank deionization tanks brought in by a service company. Please see our exchange-tank-portable-systems page for more information.

The vast majority of dissolved impurities in modern water supplies are ions such as calcium, sodium, chlorides, etc. The deionization process removes ions from water via ion exchange. Positively charged ions (cations) and negatively charged ions (anions) are exchanged for hydrogen (H+) and hydroxyl (OH-) ions, respectively, due to the resin's greater affinity for other ions. The ion exchange process occurs on the binding sites of the resin beads. Once depleted of exchange capacity, the resin bed is regenerated with concentrated acid and caustic which strips away accumulated ions through physical displacement, leaving hydrogen or hydroxyl ions in their place.

Deionizers exist in four basic forms: disposable cartridges, portable exchange tanks, automatic units, and continuous units. A two-bed system employs separate cation and anion resin beds. Mixed-bed deionizers utilize both resins in the same vessel. The highest quality water is produced by mixed-bed deionizers, while two-bed deionizers have a larger capacity. Continuous deionizers, mainly used in labs for polishing, do not require regeneration.

Water quality from deionizers varies with the type of resins used, feed water quality, flow, efficiency of regeneration, remaining capacity, etc. Because of these variables, it is critical in many DI water applications to know the precise quality. Resistivity/ conductivity are the most convenient method for testing Dl water quality. Deionized pure water is a poor electrical

conductor, having a resistivity of 18.2 million ohm-cm (18.2 mega ohms) and conductivity of 0.055 microsiemens. It is the amount of ionized substances (or salts) dissolved in the water which determines water's ability to conduct electricity. Therefore, resistivity and its inverse, conductivity, are good general purpose quality parameters. Because temperature dramatically affects the conductivity of water, conductivity measurements are internationally referenced to 25C to allow for comparisons of different samples. With typical water supplies, temperature changes the conductivity an average of 2%/C, which is relatively easy to compensate. Deionized water, however, is much more challenging to accurately measure since temperature effects can approach 10%/C! Accurate automatic temperature compensation, therefore, is the "heart' of any respectable instrument.

Crystallization is the (natural or artificial) process of formation of solid crystals precipitating from a solution, melt or more rarely deposited directly from a gas. Crystallization is also a chemical solid-liquid separation technique, in which mass transfer of a solute from the liquid solution to a pure solid crystalline phase occurs. In the food industry, controlling crystallization is a key factor in quality as it relates to texture, with some foods requiring the promotion of crystallization and others its prevention. The crystallization process consists of two major events, nucleation and crystal growth. Nucleation is the step where the solute molecules dispersed in the solvent start to gather into clusters, on the nanometre scale (elevating solute concentration in a small region), that becomes stable under the current operating conditions. These stable clusters constitute the nuclei. However when the clusters are not stable, they redissolve. Therefore, the clusters need to reach a critical size in order to become stable nuclei. Such critical size is dictated by the operating conditions (temperature, super saturation, etc.). It is at the stage of nucleation that the atoms arrange in a defined and periodic manner that defines the crystal structure note that "crystal structure" is a special term that refers to the relative arrangement of the atoms, not the macroscopic properties of the crystal (size and shape), although those are a result of the internal crystal structure. The crystal growth is the subsequent growth of the nuclei that succeed in achieving the critical cluster size. Nucleation and growth continue to occur simultaneously while the super saturation exists. Super saturation is the driving force of the crystallization; hence the rate of nucleation and growth is driven by the existing super saturation in the solution. Depending upon the conditions, either nucleation or growth may be predominant over the other, and as a result, crystals with different sizes and shapes are obtained (control of crystal size and shape constitutes one of the main challenges in industrial manufacturing, such as for pharmaceuticals). Once the super saturation is exhausted, the solid-liquid system reaches equilibrium and the crystallization is complete, unless the operating conditions are modified from equilibrium so as to supersaturate the solution again. Many compounds have the ability to crystallize with different crystal structures, a phenomenon called polymorphism. Each polymorph is in fact a different thermodynamic solid state and crystal polymorphs of the same compound exhibit different physical properties, such as dissolution rate, shape (angles between facets and facet growth rates), melting point, etc. For this reason, polymorphism is of major importance in industrial manufacture of crystalline products.

The compositional and physicochemical characteristics of cow and buffalo milk are presented. Various technologies for the hygienic production of milk, its collection, transport and processing is described to help meet quality, safety and shelf life-related requirements of end-products. Information on ingredients used in indigenous milk products and their role in quality up gradation are also detailed in a separate subsection. All these aspects are fundamental to modernization of the traditional dairy products sector.

The Milk Processing section contains general information on operations important in milk processing. A brief discussion of milk handling from the farm to the processing plant is provided as an introduction to this section. Topics covered are:

Pasteurization is a process of heating a food, usually liquid, to a specific temperature for a definite length of time, and then cooling it immediately. This process slows microbial growth in food. The process was named after its creator, French chemist and microbiologist Louis Pasteur. The first pasteurization test was completed by Louis Pasteur and Claude Bernard on April 20 1864. The process was originally conceived as a way of preventing wine and beer from souring. Pasteurization aims to reduce the number of viable pathogens so they are unlikely to cause disease (assuming the pasteurized product is stored as indicated and consumed before its expiration date). Commercial-scale sterilization of food is not common because it adversely affects the taste and quality of the product. Certain food products, like dairy products, are superheated to ensure pathogenic microbes are destroyed.

To increase milk safety for the consumer by destroying disease causing microorganisms (pathogens) that may be present in milk. To increase keeping the quality of milk products by destroying spoilage microorganisms and enzymes that contributes to the reduced quality and shelf life of milk.

Pasteurization can be done as a batch or a continuous process. A vat pasteurizer consists of a temperature-controlled, closed vat. The milk is pumped into the vat, the milk is heated to the appropriate temperature and held at that temperature for the appropriate time and then cooled. The cooled milk is then pumped out of the vat to the rest of the processing line, for example to the bottling station or cheese vat. Batch pasteurization is still used in some smaller processing plants. The most common process used for fluid milk is the continuous process. The milk is pumped from the raw milk silo to a holding tank that feeds into the continuous pasteurization system. The milk continuously flows from the tank through a series of thin plates that heat up the milk to the appropriate temperature. The milk flow system is set up to

make sure that the milk stays at the pasteurization temperature for the appropriate time before it flows through the cooling area of the pasteurizer. The cooled milk then flows to the rest of the processing line, for example to the bottling station. There are several options for temperatures and times available for continuous processing of refrigerated fluid milk. Although processing conditions are defined for temperatures above 200F, they are rarely used because they can impart an undesirable cooked flavour to milk.

Recombined milk is a product produced from milk powder which is rehydrated. GEA Liquid Processing specialises in the processing of dairy products. GEA Liquid Processing can supply components and complete processing lines perfectly suited for the production of recombined milk products. Recombined milk is produced from milk powder. Through this process, all types of milk products can be produced. Although recombined milk does not taste the same as fresh milk, it is a cheaper product. The milk powder used is produced through extracting the water in the milk to produce a powder. This has obvious advantages for the transportation of the powder over long distances. When the powder reaches its destination water is added to rehydrate it. Unlike reconstituted milk, in which the milk fat is already mixed in with the milk powder, milk fat also needs to be added to the powder and the water in the production of recombined milk. As this is a process of mixing, it is important that accurate dosing and mixing of the ingredients is observed so as to produce the best possible product. Beverage milks can also be prepared by recombining skim milk powder and butter with water. This is often done in countries where there is not enough milk production to meet the demand for beverage milk consumption. The concept is simple. Skim milk powder is dispersed in water and allowed to hydrate. Butter is then emulsified into this mixture by either blending melted butter into the liquid mixture while hot, or by dispersing solid butter into the liquid through a high shear blender device. In some cases, a non-dairy fat source may also be used. The recombined milk product is then pasteurized, homogenized and packaged as in regular milk production. The final composition is similar to that of whole milk, approximately 9% milk solids-not-fat, and either 2% or 3.4% fat. The water source must be of excellent quality. The milk powder used for recombining must be of high quality and good flavour. Care must be taken to ensure adequate blending of the ingredients to prevent aggregation or lumping of the powder. Its dispersal in water is the key to success.

Yogurt is a fermented milk product that contains the characteristic bacterial cultures Lactobacillus bulgaricus and Streptococcus thermophilus. All yogurts must contain at least 8.25% solids not fat. Full fat yogurt must contain not less than 3.25% milk fat, low-fat yogurt not more than 2% milk fat and non-fat yogurt less than 0.5% milk. The full legal definitions for yogurt, low-fat yogurt and non-fat yogurt are specified in the Standards of Identity listed in the U.S. Code of Federal Regulations (CFR). The two styles of yogurt commonly found in the grocery store are set type yogurt and swiss style yogurt. Set type yogurt is when the yogurt is packaged with the fruit on the bottom of

the cup and the yogurt on top. Swiss style yogurt is when the fruit is blended into the yogurt prior to packaging.

The main ingredient in yogurt is milk. The type of milk used depends on the type of yogurt whole milk for full fat yogurt, low-fat milk for low-fat yogurt, and skim milk for non-fat yogurt. Other dairy ingredients are allowed in yogurt to adjust the composition, such as cream to adjust the fat content, and non-fat dry milk to adjust the solids content. The solids content of yogurt is often adjusted above the 8.25% minimum to provide a better body and texture to the finished yogurt. The CFR contains a list of the permissible dairy ingredients for yogurt. Stabilizers may also be used in yogurt to improve the body and texture by increasing firmness, preventing separation of the whey (syneresis), and helping to keep the fruit uniformly mixed in the yogurt. Stabilizers used in yogurt are alginates (carrageenan), gelatines, gums (locust bean, guar), pectins, and starch. Sweeteners, flavours and fruit preparations are used in yogurt to provide variety to the consumer. A list of permissible sweeteners for yogurt is found in the CFR.

The main (starter) cultures in yogurt are Lactobacillus bulgaricus and Streptococcus thermophilus. The function of the starter cultures is to ferment lactose (milk sugar) to produce lactic acid. The increase in lactic acid decreases pH and causes the milk to clot, or form the soft gel that is characteristic of yogurt. The fermentation of lactose also produces the flavour compounds that are characteristic of yogurt. Lactobacillus bulgaricus and Streptococcus thermophilus are the only 2 cultures required by law (CFR) to be present in yogurt. Other bacterial cultures, such as Lactobacillus acidophilus, Lactobacillus subsp. casei, and Bifid-bacteria may be added to yogurt as probiotic cultures. Probiotic cultures benefit human health by improving lactose digestion, gastrointestinal function, and stimulating the immune system.

Cheese comes in many varieties. The variety determines the ingredients, processing, and characteristics of the cheese. The composition of many cheeses is defined by Standards of Identity in the U.S. Code of Federal Regulations (CFR). Cheese can be made using pasteurized or raw milk. Cheese made from raw milk imparts different flavours and texture characteristics to the finished cheese. For some cheese varieties, raw milk is given a mild heat treatment (below pasteurization) prior to cheese making to destroy some of the spoilage organisms and provide better conditions for the cheese cultures. Cheese made from raw milk must be aged for at least 60 days, to reduce the possibility of exposure to disease causing microorganisms (pathogens) that may be present in the milk. For some varieties cheese must be aged longer than 60 days.

Cheese can be broadly categorized as acid or rennet cheese, and natural or process cheeses. Acid cheeses are made by adding acid to the milk to cause the proteins to coagulate. Fresh cheeses, such as cream cheese or fresco, are made by direct acidification. Most types of cheese, such as cheddar or Swiss, use rennet (an enzyme) in addition to the starter cultures to coagulate the milk. The term natural cheese is an industry term referring to cheese that is made directly from milk. Process cheese is made using natural cheese plus other ingredients that are cooked together to change the textural and/or melting properties and increase shelf life.

The main ingredient in cheese is milk. Cheese is made using cow, goat, sheep, water buffalo or a blend of these milks. The type of coagulant used depends on the type of cheese desired. For acid cheeses, an acid source such as acetic acid (the acid in vinegar) or gluconodeltalactone (a mild food acid) is used. For rennet cheeses, calf rennet or, more commonly, rennet produced through microbial bioprocessing is used. Calcium chloride is sometimes added to the cheese to improve the coagulation properties of the milk. Flavourings may be added depending on the cheese. Some common ingredients include herbs, spices, hot and sweet peppers, horseradish, and port wine.

Cultures for cheese making are called lactic acid bacteria (LAB) because their primary source of energy is the lactose in milk and their primary metabolic product is lactic acid. There is a wide variety of bacterial cultures available that provide distinct flavour and textural characteristics to cheeses. Starter cultures are used early in the cheese making process to assist with coagulation by lowering the pH prior to rennet addition. The metabolism of the starter cultures contribute desirable flavour compounds, and help prevent the growth of spoilage organisms and pathogens. Typical starter bacteria include Lactococcus subsp. Adjunct cultures are used to provide or enhance the characteristic flavours and textures of cheese. Common adjunct cultures added during manufacture include Lactobacillus casei and Lactobacillus plantarum for flavour in Cheddar cheese, or the use of Propionibacterium freudenreichii for eye formation in Swiss. Adjunct cultures can also be used as a smear for washing the outside of the formed cheese, such as the use of Brevibacterium linens of gruyere, brick and limburger cheeses. Yeasts and molds are used in some cheeses to provide the characteristic colours and flavours of some cheese varieties. Torula yeast is used in the smear for the ripening of brick and limberger cheese. Examples of molds include Penicillium camemberti in camembert and brie, and Penicillium roqueforti in blue cheeses.

Ice cream is a frozen blend of a sweetened cream mixture and air, with added flavourings. A wide variety of ingredients are allowed in ice cream, but the minimum amounts of milk fat, milk solids (protein + lactose + minerals), and air. Ice cream must contain at least 10% milk fat, and at least 20% total milk solids, and may contain safe and suitable sweeteners, emulsifiers and stabilizers, and flavouring materials. The finished ice cream must weigh at least 4.5 pounds per gallon and there must be at least 1.6 pounds of total solids (fat + protein + lactose + minerals + added sugar) per gallon, thus limiting the maximum amount of air (called overrun) that can be incorporated into ice cream. There are well-defined labelling requirements for the types of flavours used (natural and/or artificial) and for the presence of egg yolks in the finished product (ice cream can be called custard or "French" if the content of egg yolks is at least 1.4%). Ice cream may also be labelled as reduced fat (25% less fat than the reference ice cream), light (50% less fat than the reference), low-fat (less than 3 g fat/serving), or non-fat (less than 0.5 g fat/serving). Ice cream is sold as hard ice cream or soft serve. After the freezing process only a portion of the water is actually in a frozen state. Soft ice cream is served directly from the freezer where only a small amount of the water has been frozen. Hard ice cream is packaged from the freezer and then goes through a hardening process that freezes more of the water in the mix.

There is a wide range of ingredients and formulations (recipes) that can be used in ice cream. The basic types of ingredients and their functions are briefly described below. For a more detailed explanation of ingredient function see literature references by Marshall et al. (2003) Milk fat provides creaminess and richness to ice cream and contributes to its melting characteristics. The minimum fat content is 10% and premium ice creams can contain as much as 16% milk fat. Sources of milk fat include milk, cream, and butter. The total milk solids component of ice cream includes both the fat and other solids. The other milk solids consist of the protein and lactose in milk and ranges from 9 to 12% in ice cream. The non-fat solids play an important role in the body and texture of ice cream by stabilizing the air that is incorporated during the freezing process. Sources of non-fat solids include milk, cream, condensed milk, evaporated milk, dry milk, and whey. Sweeteners are used to provide the characteristic sweetness of ice cream. Sweeteners also lower the freezing point of the mix to allow some water to reaming unfrozen at serving temperatures. A lower freezing point makes ice cream easier to scoop and eat, although the addition of too much sugar can make the product too soft. Sweeteners used include sugar (sucrose) and corn syrups. Stabilizers are proteins or carbohydrates used in ice cream to add viscosity and control ice crystallization. Over time during frozen storage small ice crystals naturally migrate together and form larger ice crystals. Stabilizers help to keep the small crystals isolated and prevent the growth of large crystals, which causes ice cream to be coarse, icy and unpleasant to eat. Stabilizers used include alginates (carrageenan), gums (locust bean, guar), and gelatines.

Emulsifiers are used to help keep the milk fat evenly dispersed in the ice cream during freezing and storage. A good distribution of fat helps stabilize the air incorporated into the ice cream and provide a smooth product. Emulsifiers used in ice cream include egg yolks and mono- and diglycerides. A wide range of flavourings are used in ice cream. Flavourings include natural and artificial flavours, fruit, nuts, and bulky inclusions such as chocolate chunks and candies.

The production of beverage milks combines the unit operations of clarification, separation (for the production of lower fat milks), pasteurization, and homogenization. The process is simple, as indicated in the flow chart. While the fat content of most raw milk is 4% or higher, the fat content in most beverage milks has been reduced to 3.4%. Lower fat alternatives, such as 2% fat, 1% fat, or skim milk (

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