Many investigations have been undertaken in order to both understand and improve milk. Like most foods, milk contains protein, fat, carbohydrate, and water. The protein portion of milk is of particular interest. This is because changes in proteins not only affect a food's quality, but are often used to produce certain characteristics. The two primary proteins in milk have characteristics which are very distinct from one another. Methods of preparation and processing of milk products have been specialized to take advantage of either casein, lactalbumin, or both. For example, casein is particularly susceptible to denaturation by the enzyme rennin and by low pH. The resulting coagulation is the first step in most cheese production.

The type of milk product created varies with processing. In turn, processing may have an influence on the quality of the product. In the United States, a major portion of the milk sold on the retail market is homogenized. That is, the fat particles have been reduced to micellular size. Additionally, the market for skim and low fat milks has increased in recent years, perhaps because of lower calories. A desire for an easy to store and/or transportable milk has given rise to NFDM (nonfat dry milk solids) and evaporated milk. Evaporated milk products have approximately 50% of the moisture removed. In addition, current processing and other considerations have meant the development of new and different milk products. These engineered products vary considerably in composition depending upon the particular characteristics desired.

However, the different dispersion systems may be listed as

The protein portion of milk demands attention in food preparation. Following is a table showing the principal milk proteins.

What is known about the casein macromolecule? Why is this important?

Above is a diagrammatic representation of casein. It consists of a calcium caseinate molecule made up of calcium, alpha-, beta- and kappa-casein, and some phosphate. The exact structure is not known, however, it is a large macromolecule made up of the milk casein proteins, calcium and phosphate in some type of colloid. The protein consists of approximately 82% of the total protein. This is the protein that primarily makes the clabber, curds and cheese. Certainly, most children have heard of whey. The nursery rhyme of "Little Ms. Muffet Sat On A Tuffet Eating Her Curds and Whey" is familiar. The counterpart today may be junket.

The many whey products now in use throughout the food industry take advantage of the whey proteins as well as the other by-product constituents. These products vary considerably. Generally, most whey products consist of the whey proteins, carbohydrate and minerals. The general characteristics of whey proteins are:

At one time, whey was considered to be a waste product of commercial manufacturing. Whey has been primarily the liquid portion left over after cheese production. It contained the whey proteins, water, riboflavin, and the mineral salts. As a waste product it was dumped into the rivers and streams. The environmental movement noted that BOD (biological oxygen demand) of these streams increased and it was a poor environment for fish and water life, other than algae and bacteria. The Environmental Protection Agency insisted the industry not continue to dump the waste products. With this imperative, the industry looked for alternatives. From this beginning the variety of whey products has been developed.

Denaturation Factor	Casein Protein	Whey Protein	Examples
Acid	appreciable affect	no appreciable affect
Enzyme	appreciable affect	no appreciable affect
Salts	appreciable affect	no appreciable affect
Phenolics	appreciable affect	no appreciable affect
Heat	no appreciable affect	appreciable affect

The main protein of fresh milk, casein, is that which the food preparer notices easily with acid precipitation. This is partly because casein is roughly 80% of the total milk protein. It is also partly due to the consequence of denaturing casein. In the natural clabbering of milk, Lactobacillus metabolizes the milk sugar lactose into lactic acid. The decrease in pH of fresh milk from pH 6.6 towards the isoelectric point of pH 4.6 brings about precipitation. Certainly, yogurt and cheese producers do not rely on the natural bacteria in milk but rather add bacteria and/or an acid. This information is diagrammed as follows:

Acid precipitation is the major step in the production of cheese. However, there are other instances of acid precipitation of casein which affect the quality of the food product. Some processes improve a product and some decrease its quality.

The following indicate some of the effects of acid on product quality:

	In buttermilk buttermilk serves as an acid source in milk.
	Curdling of cream when mixed with berries results from the berry acid acting upon the casein in milk.
	Historically, beginning foods courses frequently look at the influence of the ingredient mixing order of cream of tomato soup and its resulting ease of curdling. Certainly, the key interaction was that between casein milk protein and the acid in the tomatoes.
	The phenolics and acids in coffee may curdle the cream. This is especially true if the coffee is hot and the cream is acceptable but possibly just a little old.

Rennin

The formation of a milk clabber consists of both milk coagulation and milk gelation. If done improperly one gets curdled milk. Following is a summary of factors affecting coagulation and gelation.

Rennin is an enzyme which coagulates milk proteins to form a gel. It is used for making custards. Rennin comes from rennet, a salt extract from the stomach of milk fed calves. Rennin is responsible for releasing glycomacropeptides from the kappa casein of milk. Glycomacropeptides are extremely hydrophilic; this accounts for rennin stabilizing properties. Rennin cleaves the peptide bond between phenylalanine and methionine in kappa casein. This hydrolyzes the kappa casein and an insoluble gel is the result. Because milk protein is 82 percent casein, rennin proves to be a very effective thickening agent.

PROPERTIES OF RENNIN COAGULATION

Many factors contribute to the speed of rennin coagulation of milk, as with the strength of the gel created. Since rennin is an enzyme, it requires specific temperatures and pH in order to coagulate milk. Optimum temperature for coagulation is 40-42 degrees Celsius. Research shows that no coagulation occurs below 10 or above 65 degrees Celsius. If milk has been heated above 65C, the gel strength from the addition of rennin is reduced. This is because heat precipitates the lactoglobulins onto the kappa casein which interferes with gel formation. Hydrogen ion concentration (pH) is optimum at 5.8 for milk coagulation. Milk has a natural pH of 6.5. Therefore adding dilute acid to decrease pH to 5.8 increases gel strength and lowers coagulation time, Acid addition, however, is not mandatory for a sturdy gel.

Calcium and fat concentration both correlate directly to gel strength. Coagulation requires calcium, and gel strength is directly related to calcium content. The relationship between milkfat and gel strength is inversed. As milkfat content increases, the strength of the milk gel decreases.

These factors are summarized in the following table.

The formation by acid and/or enzyme processes is generally the first step in cheese production. It may vary in type of clabber by the type and composition of the milk used, source of acid and of enzyme, and environment of production. Once a clabber is formed, it may be cut into squares. It is allowed to further denature the protein through the use of heat. This denatured curd is than pressed to remove the whey. The amount of moisture pressed out depends upon the cheese products. Thus denaturation of milk protein may include the use of heat, enzyme, bacteria, or mold growth. Heat affects milk characteristics. Changes resulting from heat application include flavor alteration, decreased ease of cheese production, and destabilization of the protein macromolecule. Some of the changes in the milk proteins are described: Enzymes are proteins and are heat denatured. Casein in fresh milk is NOT heat denatured Whey proteins in fresh milk ARE denatured Beta-lactoglobulin changes contribute the "cooked" flavor Interaction of denatured whey proteins and casein micelles Conversion of ionic and soluble calcium and magnesium phosphates and citrates to colloidal phosphate Deposition of the heat-induced colloidal phosphate onto casein micelles Maillard reaction of proteins and sugars Caseinates are not profoundly affected by heat unless the milk is slightly acid. However, the whey proteins are easily heat denatured. For example, the problem with production of cream of tomato soup is that the combination of the acid in the tomato juice and the heat in the preparation process are perfect conditions for the curdling and precipitation of both the casein and whey milk proteins. Scalded milk is another excellent example of precipitation. It should be recognized that because of their size, whey proteins may precipitate onto the casein macromolecule and change its functionality. For example, with pasteurizated or heated milk, rennin production of cheese is not as effective. Heat induced changes in whey proteins include: -Cooked flavor due to exposure of sulfhydryl groups of beta-lactoglobulin -Increased heat stability -Increased heat resistance to rennet clotting due to precipitation of casein macromolecule -Improvement of bread products possibly due to enzyme denaturation -Loss of carbon dioxide -Decreased solubility of calcium phosphates. Here is a review of these changes in skim milk: Whey proteins denature Interactions occur between denatured whey proteins and casein micelles Complexing of calcium, magnesium, and other ions by milk proteins. Reduced rate of rennin coagulation of casein micelles Lower solubility of milk powder Color and flavor development by Maillard reaction In the United States, very little milk is sold unprocessed. If it is sold as raw milk, most states require it to be certified. However, certified raw milk has been indicated as a source for the outbreak of Salmonella dublin and Listeria monocytogenes. Because of this, most milk is heat processed. Pasteurization and homogenization are the two main processing procedures used for fluid milk in the United States. Pasteurization is heating milk to kill microorganisms that can cause illness in people. The hold method of pasteurization heats milk to 63C and holds it for 30 minutes before it is cooled to 7C. The high-temperature short-time pasteurization heats milk to 72C and holds it there at least 15 seconds before it is cooled to 10C. Ultrahigh temperature pasteurization heats milk rapidly to 138C and holds it for at least 2 seconds. It is then stored in a sterile container. This milk can be stored at room temperature until the sterile container is opened. Homogenization of milk prevents creaming. There is a natural tendency for milk to "cream". Homogenization forces the milk through a die (has tiny holes) which splits the fat globules into sizes less than 2 microns. This prevents coalescing and rising of the fat to the surface. There are other methods of processing of milk: evaporation of milk, formation of sweetened condensed milk, drying, fermentation and production of cheese. A review of milk products emphasizes the primary results of processing. The type of milk product created varies with processing. In turn, processing may have an influence on the quality of the product. In the United States, a major portion of the milk sold on the retail market is homogenized. That is, the fat particles have been reduced to micellular size. Additionally, the market for skim and low fat milks has increased in recent years, perhaps because of lower calories. A desire for an easy to store and/or transportable milk has given rise to NFDM (nonfat dry milk solids) and evaporated milk. Evaporated milk products have approximately 50% of the moisture removed. In addition, current processing and other considerations have meant the development of new and different milk products. These engineered products vary considerably in composition depending upon the particular characteristics desired. One of the most important aspects of producing various milk products is the quality of the milk. Various casein and whey products have become important functional ingredients in formulated food products. There are many uses and functions of milk protein. These are listed in the table below. Property Milk Protein Type Example of Food Emulsification Caseinates, WPC* Coffee Whiteners Stabilization Caseinates, WPC Whipped Toppings Aeration Caseinates, WPC Meringues Film Formation Sodium caseinate Bakery Glazes Opacity Calcium caseinate Nutritional Beverages Water Binding Caseinates, WPC Chewy Cookies Fat Binding Caseinates, WPC Processed meats Texturization Caseinates, rennet casein Imitation Cheese Thickening Caseinates Frozen desserts Heat Stability Caseinates Canned soups Gelation WPC Egg Replacers Acid Solubility WPC Fruit Beverages Flavor Development Various Caramels Browning Various Crackers *Whey Protein Concentrates Swartz, M. and C. Wong. 1985. Milk proteins: Nutritional and functional uses. Cereal Foods World 30(2): 173-176. [In Public Domain from The American Association of Cereal Chemists]

	The history of powdered milks is long and fraught with some ups and downs. During World War II, drum-dried heated milk powder became a mainstay to the war effort. Unfortunately, that milk had problems with dispersing and solubility. Thus, a number of companies worked hard to develop the product that is available today, spray dried milk powder. This product is readily available on the retail market in nonfat dry milk form. One must recognize that there are a multitude of other milk products available also.
	Evaporated milk. Evaporated milk has approximately 60% of the water removed.
Condensed milk is available in the market place.	Condensed milk not only has half the water removed from whole milk but also has approximately 44% refined can or corn sugar added. Federal standards require 8.5% milk fat and not less than 20% total milk solids. The milk is canned after heating and cooling.
	Lactase milk
	Cheese is the processed product from fluid milk. All cheese preparations follow these basic processes Reproduced with Permission from OSU of Savonen, Carol. 1993 Winter/Spring. The Cheese Squeeze. Oregon's Agricultural Progress 39(2-3): 12. Check out the resources for specific information on cheese processing and cheeses. Three steps of cheese production -proteolysis -coagulation -gelation
Swartz, M. and C. Wong. 1985. Milk proteins: Nutritional and functional uses. Cereal Foods World 30(2): 173-176. [In Public Domain from The American Association of Cereal Chemists	Acid Casein is made by adjusting pH to approximately 4.6 by mineral acid addition or lactic fermentation. The resulting casein curd is isolated, washed, and dried. It is low in calcium and phosphate ions. Rennet Casein is made by inoculation with a rennet enzyme preparation. The protein then coagulates and the casein curd is isolated, washed, and dried. Rennet casein has a high pH (7.1), and a calcium-phosphate complex remains with the casein components during isolation. Caseinates are made by solubilizing casein with selected alkalis and/or sequestering agents, and drying the resulting solution. The properties of these caseinates will vary by selection of neutralizing agents. Whey Proteins exist in several different forms. They may be used as edible films for microencapsulation of flavors, for maintenance of batter coatings, for translucent films or gel products. Whey Protein Isolate (WPI) will contribute essential amino acids to a product. It can be added to beverages and clear sport drinks. The low pH of carbonated sport beverages is particularly useful for dispersal where it is useful in high acid foods. Whey proteins stay in solution below pH 4.6, whereas egg or soy proteins fall out of solution. Whey Protein Concentrates (WPC) are separated from whey by ultrafiltration to remove the lactose and soluble ions and leave the proteins. Typically WPCs are soluble over a wide pH range and gel when heated. These may be used to replace egg white to form foams in baked product formulations. They can improve foaming performance. Lactalbumins are heat sensitive proteins. When whey is heated to roughly 90C, lactalbumins precipitate and can be recovered by centrifugation. It is insoluble over a wide range of pH, and is relatively inert. Lactalbumins forms the scum on the top of heated milk. Swartz, M. and C. Wong. 1985. Milk proteins: Nutritional and functional uses. Cereal Foods World 30(2): 173-176. [In Public Domain from The American Association of Cereal Chemists

Advantages of the above products may be

Cheese variety is impacted by the type and/or portion of milk used, method of coagulation, process after coagulation, and storage characteristics. The cheese maker may vary the characteristic of cheeses simply by affecting varying the pH. The interrelationship of pH and cheese texture has been substantiated. The many research projects have shown this relationship; however, the representation used by LaBell (1997)is useful.

Oregon Agricultural Progress has published visuals of the cheese production process at an Oregon Cheese Processing Plant. These are available for those interested.

A cream foam is made up of gas surrounded by liquid with protein and fat serving to stabilize it.

aseptic packaging
amphoteric elements
viscosityod.orst.edu/glossary/v/ chymosin
clabber
coagulate
coagulation
colloidal dispersion
conglomerate
density
emulsion
emulsifier
enzyme
enzymatic reactions
foam
gel
homogenize
homogenization
hydrolysis
pasteurization
pasteurize
pathogenic microorganisms
protease
proteinase
rennet
saturated fatty acids
sterol
surface tension
triglyceride
viscosity