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Frequently Asked Questions


    OBJECTIVES

    The learner will be able to --
  • match the dispersion of milk components to one of the three major dispersion systems.
  • list the major milk proteins.
  • match the casein and whey milk proteins catagories to their sensitivity to denaturation by acid, enzyme, salts, and phenolics.
  • describe specifically the reaction and factors affecting acid and/or enzyme denaturation of each major category of milk proteins.
  • explain the process of cheese preparation.
  • describe a cream foam.
  • state factors and their affect upon milk and cream foam formation and stability.

  • CONTENT

    An understanding of how milk behaves and functions is based upon knowledge of its composition and how its constituents are dispersed. The constituents of milk are dispersed as either

      colloidal dispersion
      solution
      suspension
    For example, the carbohydrate portion is present in solution, and the ash is likely also dispersed as a true solution. There is some indication that calcium phosphate may form a large micelle and have characteristics of a colloidal dispersion. The fat may be present as globules in suspension or emulsified throughout the milk. Protein is colloidally dispersed.

    Composition of Cow's Milk
  • 88% water
  • 3.3% protein
  • 3.3% fat
  • 4.7% carbohydrate
  • 0.7% ash
  • 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

    True Solution Colloids Suspension
    Sugars
    Salts
    Vitamins
    Lipids are emulsified
    Proteins are a colloidal sol.
    If not homogenized, fat is likely suspended as well.

    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:

    -molecular weight 14,000 to 1,000,000
    -compact, globular conformation
    -subject to denaturation and sulfhydryl group activities
    -subject to protein-protein interaction by disulfide interchange and calcium bonding
    -denatured form insoluble at isoelectric pt. (pH 4.5-5.0)

    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 industr y 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

    milk clabberclabbered milk with showing cut curds

    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:

    pH6.6 to pH4.6 diagram

    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. That is why buttermilk biscuits to the red velvet cakes use soda instead of the balanced baking powder.
    Curdling of cream when mixed with berries results from the berry acid acting upon the casein in milk.
    tomato soup into 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.

    Factors Affecting Milk Coagulation with Rennin

    Decreased coagulation time with direct hydrochloric acid acidification
    Decreased colloidal calcium phosphate concentration
    Increased in Ca++ activity
    Decreased coagulation time with the addition of CaCl2

    Factors Affecting Gelation

    -Dephosphorylation of the casein micelle results in formation of a soft curd.
    -Consolidation of coagulum involves interactions between Ca++ ions and phosphate groups of the casein proteins.

    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.
    Factor Conditions
    Temperature
  • 65C and up: no action
  • 40-42C - optimum
  • 10C or below: no action
  • Hydrogen ion concentration
  • pH - 5.8 optimum
  • Casein, calcium ion, calcium phosphate concentration increase
    Previous heat treatment decrease
    Other cations

    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]
    Other old and new products can be listed and defined as follows:
    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 has the enzyme added that hydrolyzes the lactose into the glucose and galactose. This is often selected by consumers who do not have the enzyme to break down the lactose sugar.
    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. 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

  • Impart a high moisture absorption capacity to the dough, causing an increase in dough viscosity and facilitating dough handling during processing.
  • Increase buffering capacity during fermentation thereby preventing rapid and excessive acidification
  • Afford better control of amylase activity
  • Improve tolerance to bromate
  • Facilitate moisture transfer during gelation of starch
  • Improve baking quality of weak flours
  • Control rate of gas emission
  • Minimize effect of overmixing
  • Enhance flavor development and crust color
  • Improve toasting characteristics
  • Strengthen crumb structure and texture
  • Act in moisture retention and retard the staling process
  • Improve nutritional quality
  • 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.
    Factor Milk Foam Cream Foam
    Temperature 4C optimum 4C optimum
    Acid
    Sugar decreased formation and stability decreased formation and stability
    Fat Generally speaking, 18% will form a soft weak unstable foam. As fat increases you get increased amount of foam and increased stability up to approximately 30%. From 30 to 36% fat it will increase stability.
    Age As cream ages [before it sours] it will form a foam easier and more stability. There is a natural "fat clumping" enzyme in cream which will help.


    GLOSSARY

    • aseptic packaging: free from disease-producing microorganisms; filling a container that has been previously sterilized without recontaminating either product or container is an aseptic process
    • amphoteric elements: or compounds which act either as an acid or base (gain or lose electrons) depending upon the medium they are in.
    • viscosity: term used of liquids to define their resistance to flow (i.e. the internal friction).
    • chymosin: an enzyme from the stomach that clots milk; previously called rennin
    • clabber: is a milk gel generally formed by either acid or enzyme denaturation
    • coagulation: to form a clot, a semisolid mass, or a gel, after initial denaturation of a protein; to produce a firm mass or gel by denaturation of protein molecules followed by formation of new crosslinks.
    • colloidal dispersion: state of subdivision of dispersed particles; intermediate between very small particles in true solution and large particles in suspensior; usually refers to the state of subdivision of dispersed particles; intermediate between very small particles in true solution and large particles in suspension.
    • conglomerate: multiple crystals or particles cemented together.
    • density: or weight per unit volume; potatoes with higher density or specific gravity are heavier for their size; the weight (in vacuum) per until volume at a specific temperature.
    • emulsion: the dispersion of one liquid in another with which it is usually immiscible, for example, oil in water
    • emulsifier: a substance that acts as a bridge at the interface between two immiscible liquids and allows the formation of an emulsion; a substance that aids in producing a fine division of fat globules; in ice cream, it also stabilizes the dispersion of air in the foam structure.
    • enzyme: catalyst produced by living cells. Composed of protein and destroyed by heat and protein coagulation; responsible for most of reactions carried out in living tissues. Some are composed of two parts: the apoenzyme, the protein which is inactive alone; and its prosthetic group, a small non-protein molecule normally derived from a vitamin.
    • enzymatic reactions:
    • foam: those that are catalyzed by enzymes, which are special proteins produced by living cells; a catalyst changes the rate of a reaction without itself undergoing permanent change.
    • gel: colloidal dispersion that shows some rigidity and will, when unmolded, keep the shape of the container in which it had been placed; a semi-rigid structure at room temperature. This dispersion does show some rigidity or moldability.
    • homogenize: to subdivide particles, usually fat globules, into very small uniform-sized pieces
    • homogenization: Emulsions usually consist of a suspension of globules of varying size. Homogenization reduces these globules to a smaller and approximately equal size. In homogenized milk the smaller globules adsorb more of the milk protein, whish is a stabilizer, and the cream does not rise to the top.
      of whole milk a process in which whole milk is forced, under pressure, through very small openings, dividing the fat globules into very tiny particles
    • hydrolysis: a chemical reaction in which a linkage between subunits of a large molecule is broken; a molecule of water enters the reaction and becomes part of the end products
    • pasteurization: Heating of a specific food enough to destroy the most heat-resistant pathogenic or disease-causing microorganism known to be associated with that food.
    • pasteurize: to treat with mold heat to destroy pathogens-but not all microorganisms-present in a food product.
    • pathogenic microorganisms: microbes capable of causing disease
    • protease, proteinase: an enzyme that hydrolyzes protein to smaller fragments, eventually producing amino acids is an enzyme that brings about the clotting of milk and is used in the manufacture of cheese. A specific protease is rennin or rennate.
    • rennet: an enzyme used to make cheese. Rennet is extracted from the lining of calves' stomachs. New technologies have enabled the removal of the specific gene that produces rennet and have reproduced it in bacteria. This allows the production of rennet through a fermentation process, eliminating the need for extracts from calves' stomachs.
    • saturated fatty acids: Saturated fats are those in which all carbons contain a hydrogen, and therefore, no double bonds exist. In general, fats that contain a majority of saturated fatty acids are solid at room temperature, although some solid vegetable shortenings are up to 75 percent unsaturated. Some common fatty acids in foods include palmitic, stearic and myristic acids. Saturated fatty acids are more stable than unsaturated fatty acids because of their chemical structure.
    • sterol: alcohols derived from the steroids. Include cholesterol, widely distributed in animal tissue, including brain and egg yolk; coprosterol in feces; ergosterol in yeast, which is the precursor for the synthetic vitamin D2; and sitosterol and stigmasterol in plants.
    • surface tension: is the tension or force at the surface of a liquid that produces a resistance to spreading or dispersing; due to the attraction of the liquid molecules for each other.
    • triglyceride: neutral fat molecule made up of three fatty acids joined to one glycerol molecule through a special chemical linkage called an ester. A type of lipid consisting chemically of one molecule of glycerol combined with three fatty acids
    • viscosity: term used of liquids to define their resistance to flow (i.e. the internal friction).



    Updated: Wednesday, May 23, 2012.