Dairy Products (Encyclopedia of Food & Culture)
DAIRY PRODUCTS. Dairy products are derived from milk, the secretion of the mammary glands of mammals, usually cows (bovine), sheep, goats, buffalo, mare, camel, or yak. Most dairy products originate from bovine milk and, to a lesser extent, sheep and goat milk. As milk contains approximately 80 to 90 percent water, it is prone to undesirable microbial growth with concomitant product deterioration. To prevent this problem from occurring, and to ensure a longer shelf life, milk is processed to form different products such as ice cream, cheese, milk powders, yogurt, butter, lactose, and anhydrous milk fat (also known as butteroil). Milk can be separated into a cream fraction and a skim milk fraction by a centrifugation technique called separation. This process concentrates the fat present in the milk into the cream phase, leaving a skim or partially skimmed phase with much lower fat content. Typical fat and water contents of selected dairy products are shown in Table 1. The speed of centrifugation can be adjusted to yield different fat content in cream. Milk processing applies different preservation techniques to allow for longer storage of dairy products. Milk powders are produced by concentration to remove some of the water, followed by atomization into a fine mist and drying at high temperatures. Heat and dehydration (water removal) are employed to give a long shelf life for milk powders. Ice cream is a dairy product preserved by the action of freezing. Yogurt and cheese are both fermented products. A bacterial culture is used to inoculate milk, for which the primary function is to lower the pH from 6.7 (typical for fresh bovine milk) to 4.2 for yogurt and in the range 4.6 to 6.0 for most cheese varieties. The bacterial cultures also assist in breaking down proteins and fats in the milk product to develop some of the flavor. The preserving function of added bacterial culture is to compete with unwanted pathogens for nutrients. Thus cheese and yogurt are preserved by dehydration, acidification, and competition with pathogens for survival in the product.
Heat Treatment of Milk
For food safety reasons, milk is often heat-treated prior to consumption or further processing. The most common heat treatments are holding at 162°F (72°C) for 15 seconds, called pasteurization, or at 145°F (63°C) for 30 minutes, called batch pasteurization. Both of these treatments have similar effects on killing undesirable microorganisms in milk. The treatment is sufficient to destroy two indicator organisms, Mycobacterium tuberculosis and Coxiella burnetti.
Another common heat treatment is to hold milk at 284°F (140°C) under pressure for 2 to 3 seconds, producing ultra high temperature (UHT) milk. This milk is essentially sterile and can be packaged in cardboard containers and stored at room temperature for up to six months with little microbial-induced deterioration. UHT milk is more commonly consumed in Europe than in Australia, New Zealand, or North America.
An alternative to preserving milk by heat pasteurization is high-pressure processing, where pressures of around 30000 MPa are employed to rupture the membranes of pathogens and denature enzymes that cause deterioration
|Average composition of selected dairy products (grams per 100 grams of milk)|
|Skim milk powder||3||1||35||52||1.3||1.3|
|Whole milk powder||2||27||26||38||1||0.9|
in milk quality. This process has not been adopted to any significant commercial extent as the milk must be processed in batch quantities rather than in a continuous fashion as in a pasteurizer. Gamma irradiation of milk for the purpose of preservation is not practiced as it tends to produce off-flavors. Ultraviolet light can be used to pasteurize milk, and has the additional benefit of increasing the amount of available vitamin D. Bactofugation is sometimes employed to remove bacteria in milk by a process of centrifugation. Hydrogen peroxide can also be used to improve poor-quality milk. This preservative can be removed by heating milk to 12231°F (505°C), whereupon the enzyme catalase present in milk destroys the added hydrogen peroxide.
Milk contains proteins, fat, water, lactose, inorganic salts, and other minor organic material such as phospholipids, organic acids, enzymes, hormones, vitamins, nucleotides, amines, amino acids, alcohols, aldehydes, ketones, and esters. (A complete set of detailed compositional tables can be found in Noble P. Wong et al., Fundamentals of Dairy Chemistry, pp. 18.) An understanding of how the major constituents are structurally arranged in milk is necessary to predict how milk processing conditions affect flavor, texture, and the keeping qualities of dairy products.
The gross composition of milk varies according to the species of mammal, the breed of the cow (especially for bovine milk), and the stage of lactation. Note the relatively low amount of casein in human milk, rendering it difficult to make cheese. The breed of cow will affect the level of fat and protein. Holstein cows are common in dairies in North America, Australia, and New Zealand.
An important advancement in milk compositional study was the development of a rapid test for milk fat by Stephen M. Babcock of the University of Wisconsin in 1890. This procedure allowed for accurate marketing of milk, resulted in a more consistent quality of dairy products, and facilitated the development of better farming practices to optimize fat content in milk.
Milk composition and volume are affected by the season, particularly in milder climates such as Australia and New Zealand where cows are pasture-fed all year round. Milk production drops during the winter months of May to August in the Southern Hemisphere, to a level such that there is little excess milk available for dairy processing beyond that of market consumer milk. In the colder climates found in Canada and the north of the United States and Europe, cows are fed on silage during the winter months. This produces a more uniform supply of milk and often results in paler and less yellow-colored dairy products, especially butter and cheese, due to the lower levels of beta-carotene in silage feed. Fat and total milk solids (fat, protein, and minerals) are lower in summer months compared to winter in both hemispheres. The effect of pasture quality and quantity on milk composition is a complex issue. Feed quality is affected by the level of roughage, fat, protein, energy level, and the fatty acid profile.
One important factor in milk quality that has important consequences in dairy processing is the health of the cow. Mastitis is an infection of the udder that results in high somatic cell counts in the milk. Immunoglobulin levels are higher in mastitic milk, whereas fat, lactose, casein, and whey protein levels are lower. Treatment for mastitis requires antibiotics; however, this has the undesirable effect of killing introduced bacterial cultures in the manufacture of cheese and yogurt. Milk processing factories rigorously and routinely check milk samples from individual farms for antibiotics, as well as for levels of fat, whey proteins, caseins, urea, and lactose. Milk containing antibiotics is discarded and a penalty fine may be imposed on the farmer. Routine testing also allows correct payment to be made to the farmer, usually based on the amount of casein plus fat. In addition, due to the variable composition of milk, testing allows batches of milk to be standardized to a particular casein or fat content depending upon the type of dairy product to be made.
Milk Fat Globules
The fat in milk is emulsified with a membrane material consisting primarily of proteins, phospholipids, and enzymes. Other minor components of the membrane include glycoproteins, phospholipids, carotenoids (including beta-carotene), and sterols (including cholesterol). There are approximately 15 trillion fat globules in one liter of milk, with a size in the range of 0.1 to 20 micrometers. The membrane surface serves to protect the fat from undesirable oxidation. Fat globules will still cream given sufficient time. The creaming effect is accentuated by the agglutinin reaction, which results in fat globules clustering and rising rapidly to the surface to form a cream layer.
Approximately 98 percent of the fat in milk exists in the form of a triacylglyceride, where three fatty acids are attached to a glycerol molecule. The fatty acids comprise CH2 methylene groups linked together to form a chain of varying carbon lengths. Milk fat has a significant proportion of fatty acids with a length of four carbon atoms (C4), known as butyric acid, which causes the sharp, acidic taste in some cheese varieties. This flavor is often described as rancid. Other major fatty acid components of the triacylglyceride include myristic acid (C14), palmitic acid (C16), stearic acid (C18), and oleic acid, which also contains 18 carbon atoms but differs from stearic acid in that it contains one unsaturated C=C bond.
Due to the many possible arrangements of different length fatty acids, the melting point of milk fat is not as sharply defined as for pure compounds, but rather extends over a wide range of temperatures. Milk fat is entirely solidified at 0°F (0°C), and has melted completely at 104°F (40°C). At refrigeration temperature milk fat is between 40 and 50 percent solid, despite the apparent solid appearance at this temperature.
Milk fat, when solidified, crystallizes into three main forms (alpha, beta, and betapolymorphs) depending upon the rate of cooling. Tempering of a food product containing milk fat is a process of careful adjustment of the temperature at different heating and cooling rates. This process promotes the formation of a particular polymorphic structure with desirable texture. Tempering is used in butter manufacture.
Unsaturated Milk Fat
Butters, spreads, and margarines that are high in unsaturated fatty acid content are believed to protect against heart disease. Margarines usually contain a higher level than butter. The unsaturated C=C bond in oleic acid (and other unsaturated acids) can exist in two forms, the cis form where hydrogen atoms are on the same side as the C=C bond, and the trans form where the hydrogen atoms are on either side. The cis form is more common in nature and believed to offer better protection against heart disease.
Conversion of a polyunsaturated liquid oil to a higher melting point mono-unsaturated solid fat (such as in margarine manufacture) can take place by the process of partial chemical hydrogenation, where hydrogen atoms are added to some of the C=C bonds to form saturated C=C bonds. The process of hydrogenation requires the initial removal of polar lipids such as phospholipids, and heating the oil at 320°28°F (160°20°C) under a pressure of 20 atmospheres with a 0.01.2 percent nickel catalyst. Consumption of trans mono-unsaturated fatty acids has been associated with heart disease (see Beardsley, p. 34).
The degree of unsaturation is measured by the iodine test, where the number of grams of iodine reacting with the C=C bonds in 100 grams of oil or fat is determined. Generally, the higher the number of unsaturated bonds and the lower the carbon chain length, the lower the melting point of the fatty acid.
Milk Fat Deterioration
Deterioration of milk fat can occur by two main mechanisms: fatty acid release and oxidation of C=C bonds. Release of fatty acids occurs by cleavage (hydrolysis) of a fatty acid from a triacylglyceride molecule. The presence and release of butyric acid will give the typical rancid flavor found in blue and Italian-style cheeses. Oxidation of C=C bonds results in formation of hydroperoxide radicals that form high molecular weight compounds over time with increased viscosity and propensity for foaming. This is more of a problem with vegetable oils used for frying where extreme temperature fluctuations take place. Oxidation is promoted by the presence of oxygen at high temperatures and high water activity.
Fatty acid hydrolysis is catalyzed by an enzyme called a lipase. Milk contains 1 milligrams per liter of a native lipase called milk lipoprotein lipase. This enzyme is 90 percent inactivated under pasteurization conditions. Hydrolysis (more specifically lipolysis, in the case of oils) is exacerbated at higher pH, a temperature of around 37°C, by light, and by the degree of agitation of milk. Exposure of milk fat as a result of agitation will provide access by the milk lipoprotein lipase, and lipolysis will occur. Sufficient agitation of milk can occur during homogenization. If milk is not pasteurized prior to homogenization, enough lipolysis can take place, with release of fatty acids, to burn the throat of someone unfortunate enough to consume this milk. Needless to say, milk is always pasteurized before homogenization.
Milk obtained fresh from the cow is partially protected against lipolysis by the milk fat globule membrane, and also by the separation of milk lipoprotein lipase (largely bound to the casein micelles) from the milk fat substrate. As already stated, agitation of milk promotes lipolysis. Heating fresh unpasteurized milk to 86°F (30°C) then cooling back to refrigeration temperatures will cause rancidity to occur within twenty-four hours. This problem will occur if warm fresh morning milk is added to cooled evening milk from the previous day, then cooled back down again. In addition, spontaneous lipolysis occurs in one out of five cows after cooling milk to 598°F (15°0°C). This problem is circumvented by mixing with four to five times the volume of normal milk. Freezing and thawing of milk can also promote lipolysis. A slower rate of freezing will increase the rate of lipolysis.
In the United States, whole milk powder from lipase-treated milk is used in the manufacture of milk chocolate to impart a slight rancid flavor note. This flavor is what the U.S. consumer has come to expect in milk chocolate flavor, presumably for historical reasons when low-quality and less expensive milk may have been used in the manufacturing process. This flavor is absent in milk chocolate produced in Europe, Australia, and New Zealand.
Proteins in milk are broadly classified as either caseins or whey proteins. Caseins are mostly insoluble in water and exist in an aggregated form called a casein micelle of average size 0.2 micrometers. This is sufficiently small to scatter light and render to milk its white appearance. Whey proteins are soluble over a wide pH range and have high nutritional value.
Whey comprise the class of proteins that are soluble in milk. They are globular proteins of size 2 nanometers. The major subcomponents of bovine whey are a-lactalbumin and β-lactoglobulin. Human milk does not contain β-lactoglobulin, and a-lactalbumin is highly valued when extracted from bovine whey for use in powdered infant formula. Consumption of bovine milk or formula containing β-lactoglobulin may elicit an allergic response when consumed by very young infants. The main utility of β-lactoglobulin is as a gelling agent in food products such as comminuted meats. The presence of a-lactalbumin will impair the gel structure formed from β-lactoglobulin, hence the requirement to separate these two components from whey.
Other components of bovine whey include bovine serum albumin, immunoglobulins, lactoferrin, and lactoperoxidase. Lysozyme is an enzyme found in milk that also has antimicrobial activity. This enzyme is found in higher concentrations in milk colostrum, protecting the gut of a newborn calf against invasion by pathogenic bacteria.
Colostrum is the initial secretion of the mammary gland after birth and contains a much higher casein and whey protein concentration than milk at later stages of lactation. The level of lactose is less than in milk, and the amount of fat is slightly higher. Colostrum confers immunity to a newborn calf through the high levels of immunoglobulins, and also provides a high-quality nutritional diet.
Whey is the main by-product of cheesemaking. It has historically been considered to be of low value, and used as pig food, fertilizer for agricultural fields, or simply disposed of into the ocean. Now, whey is spray-dried to produce a powdered protein ingredient, highly valued by the food processing industry for its excellent nutritional and textural modifying properties. Whey can be further fractionated into components that are spray-dried, and these command a premium price for food and pharmaceutical applications.
Functional Properties of Whey Proteins
Functional properties of whey proteins include its water solubility, viscosity modification ability, gelation, texturization, high nutritional value, flavor-binding, foaming, emulsification, fat and water retention, and control over shrinkage in products such as yogurt and gelled foods. The high solubility and excellent nutritional properties allow whey proteins to be used to fortify high-energy sports beverages. Increasing the viscosity of fluid food products by whey protein addition will affect the sensory properties, and reduce the tendency for particulate matter (if present) to sediment. Different types of gels can be created by heating β-lactoglobulin. The pH, concentration and types of mineral salts present, and the rate of heating will affect the properties of the gel, such as opacity, elasticity, and the propensity for shrinkage with expulsion of liquid during storage (syneresis). Gels that are formed far from the isoelectric point (around pH 5.5, where there is no average net charge on the whey proteins) will tend to be less opaque, more fine stranded, and more elastic.
Texturization of whey proteins is achieved by heating to form a coagulum followed by extrusion though small diameter holes under high pressure to align the protein fibers. These can be further processed between heated rollers to remove more moisture, promote adhesion, and increase the toughness and chewiness. Textured dairy proteins have an application in forming vegetarian meat-like products.
Food products that contain oil require stabilizers to emulsify the oil and prevent excessive separation from the water phase. Both whey proteins and caseins can perform this function by homogenization to create small oil globules coated with proteins. Generally, the smaller the globules, the more stable the food product will be against creaming or oil separation. Examples of this type of food product include salad dressing and mayonnaise. Most oil and water food products are emulsified such that the oil globules are suspended in the water phase. If the oil or fat content is too high, a phase inversion takes place where water droplets are then suspended in the oil phase. In this case, whey proteins have little efficacy to act as emulsifiers. An example of a water-in-oil emulsified food product is butter.
Foam formation and stability are important attributes in food products such as ice cream, cappuccino coffee foam, meringue, marshmallow, mousse, and bread. Air bubbles are induced by rapid whipping and are lined with stabilizers to prevent rapid collapse of the foam. Whey proteins are excellent foaming agents, and their ability to unfold (denature) and cross-link at the air-water interface provides for good foam stability. Strong protein-protein interactions, such as that which occurs near the isoelectric pH of around 5.5, will promote foam stability. Free oil will cause a decrease in foam stability, which is the reason egg yolks (containing oil) are separated from egg white (containing proteins capable of stabilizing the foam) prior to whipping of the egg white. Emulsified oil, such as in milk or cream, will not affect foam stability to the same degree, as the oil is coated by the native milk fat globule membrane and does not come into direct contact with the protein foaming agents in dairy foams. However, if the native membrane is ruptured, perhaps due to enzymatic action or excessive turbulence when pumping milk during processing, the foaming ability is impaired. This is often a seasonal occurrence in some countries, creating cappuccinos with poor foams at certain times of the year when enzymatic activity is more pronounced. Special cappuccino milk is sometimes sold containing a higher proportion of added spray-dried foaming whey proteins to alleviate this problem. Further details about functional properties and emulsions are provided by Walstra in Food Chemistry (pp. 9555).
Calcium Phosphate in Milk
The major mineral component in milk (and most dairy products) is calcium phosphate, an inorganic salt of low solubility in water. A high intake of calcium in the diet is believed to promote strong bone development, hence the recommendation of calcium in the diet of young children. The low solubility of calcium phosphate in water (and also in milk) would result in calcification, or boiler-scale, in the mammary gland if it were not for the unique properties of the casein micelle in solubilizing this mineral.
Each of the four main casein molecules (α-s1-casein, α-s2-casein, β-casein, and κ-casein) contain at least one phosphate group that is capable of binding to the calcium phosphate mineral complex in milk. Some twenty-five thousand of these casein molecules, with bound calcium phosphate, aggregate to form the heavily hydrated casein micelle of molecular weight 10809 Daltons. Thus calcium phosphate is rendered soluble in milk (as the casein micelle itself can exist in milk as a stable suspension) and can be considered to be the binding agent that holds the micelle together. There is some controversy over the nature of the substructure of the casein micelle. The two main competing models are described by Pieter Walstra (1999) and Carl Holt and David S. Horne (1996).
Formation of Milk Curds
Cheese and yogurt making has been in existence for thousands of years. Milk would have been carried around in the warm Middle Eastern climate in sacks made from animal skins, such as the stomachs of ruminant animals. Milk stored in a sack made from the fourth stomach of a young calf and carried around at temperatures in excess of 68°F (20°C) would eventually coagulate and separate into curds and whey given sufficient mechanical disturbance. The curds would have provided a nourishing meal and the whey a refreshing and nutritious beverage. This process has evolved into the highly scientific and mechanized approach used today in modern cheese-making plants.
The casein micelle will undergo extensive aggregation as the pH approaches the isoelectric point. For caseins in milk, this occurs at pH 4.6. This is the basis for the coagulation of milk and separation of the curds from the whey to make cheese. A comprehensive treatment on the physical chemistry of curd formation and subsequent reactions is given by Dalgleish (pp. 6900), Green and Grandison (pp. 10140), and Walstra (pp. 14191) in Patrick F. Fox, ed., Cheese: Chemistry, Physics and Microbiology.
Curds are composed of aggregated casein micelles and trapped fat globules within the protein matrix. The whey phase contains mostly water with soluble minerals, whey proteins, and lactose. Casein micelle clotting (with consequent curd formation) can also occur by addition of coagulating enzymes or 20 percent alcohol. Most cheese varieties are manufactured by enzymatic coagulation, with some formed by acid precipitation to pH 4.6. Acidification can also take place by the addition of bacterial culture, directly by addition of acids such as hydrochloric, sulfuric, or lactic acids, or a combination of bacterial culture and direct acidification. Most bacterial cultures used in fermented dairy products are classified as lactic acid bacteria, as they are capable of metabolizing lactose present in milk into lactic acid. This will lower the pH of milk and aid in the formation of a milk clot.
Casein micelles are prevented from forming a rapid milk clot in fresh milk by the presence of a hairy layer of adsorbed κ-casein molecules on the surface of the micelle. The κ-casein prevents the close approach of micelles at the natural pH of milk (6.7) by a mechanism known as steric stabilization. Both acid and alcohol addition cause a partial flattening of the κ-casein hairy layer, allowing coagulation to occur. Acid coagulation occurs via an electrostatic attraction mechanism, and alcohol coagulation by hydrophobic interaction. Casein micelles have sufficient inherent hydrophobicity to cause aggregation to occur in much the same way as hydrophobic oil droplets will coalesce, once the κ-casein layer has been flattened or removed.
Milk Clotting Enzymes
The enzyme present in the fourth stomach of a calf, chymosin, is extracted by maceration of the stomach lining in a salt solution. The purified salt solution containing chymosin is called rennet. Chymosin will cleave the κ-casein hairy layer on the casein micelle at a very specific location, between the phenylalanine and methionine amino acids at positions 105 and 106 in the primary sequence of the protein. This cleavage point is fortuitously located at the point where the κ-casein molecule extends away from the micelle surface to form the hairy layer. Chymosin is therefore capable of removing the k-casein steric stabilizing layer, allowing micelles to coagulate and form a curd, such as found in yogurt and cheese.
As the demand for chymosin is greater than the supply of calf stomachs, other enzymatic methods to coagulate milk have been investigated. An enzyme called pepsin, extracted from the stomach lining of pigs, calves, and chickens, has some efficacy; however, cheese made from this enzyme is often too soft due to excessive protein degradation. Often, rennet extracts contain a proportion of bovine or porcine (from pigs) pepsin in addition to bovine chymosin.
Enzymes extracted from fungi, bacteria, and plants can also be used to coagulate milk. Plant coagulants can be extracted from papaya, figs, pineapple, kiwi fruit, and Cynara cardunaculus (cardoons and artichokes), a thistle which in Portugal is used to make Serra cheese. Most enzymes derived from plant sources are highly proteolytic (capable of extensive degradation of proteins) and non-specific in their action on proteins. The resultant small peptides, which are the degradation products of enzymes acting on proteins, produce a soft and pasty cheese with bitter flavor unless other steps are taken to circumvent these problems. Bitterness in cheese is correlated with higher amounts of hydrophobic peptides.
An increasingly common method to produce milk coagulant for cheese and yogurt manufacture is by recombinant DNA technology. The gene for expressing chymosin is spliced into the DNA sequence of bacteria, such as Escherichia coli, which is then grown in a reaction vessel to levels that permit the extraction and subsequent purification of chymosin.
Endogenous Milk Enzymes
The main native milk enzyme that hydrolyzes proteins is plasmin. This enzyme is located in the casein micelle, and therefore concentrated in cheese during the manufacturing process. Plasmin will hydrolyze proteins during cheese ripening and contribute to texture and flavor development. It has an optimum activity at pH 7.5, hence the term alkaline protease, and at 99°F (37°C). Pasteurization has an effect of increasing the total plasmin activity, as the otherwise inactive precursor, plasminogen, is activated. Plasmin itself can be inactivated by heating at 176°F (80°C) for ten minutes.
Alkaline phosphatase is another enzyme present in milk, and is inactivated by pasteurization. A test for alkaline phosphatase activity is used to determine if milk has been pasteurized, as the conditions of inactivation mirror those of pasteurization. This enzyme is preferentially adsorbed onto the surface of fat globules.
It is interesting to note that enzymes in milk are often segregated away from their respective substrates, thus preventing rapid deterioration of milk. Alkaline phosphatase reacts with the phosphate ester groups in the casein micelle, and would result in micelle disintegration if the enzyme were located there. In the same fashion, lipases are often found adsorbed into casein micelles away from the milk fat substrate.
Lactose, the Milk Sugar
Lactose, also known as milk sugar, is a disaccharide molecule comprising two simple sugars (glucose and galactose) linked together. This sugar is rarely found outside of dairy products, unless specifically added. Lactose crystals present in dairy products, particularly ice cream, can cause a sandy texture if the crystals are too large.
Lactose must first be hydrolyzed by the enzyme lactase into glucose and galactose before it can be further metabolized in the human body. If lactase is absent from the body, a common occurrence among adult Asians and Africans, digestion problems may arise after consuming milk. These problems are referred to as lactose intolerance, and for this reason, milk is not usually consumed by adults from these two racial groups. Lactose is water-soluble, therefore largely absent in high-fat dairy products such as butter, butteroil, and ghee. In aged, fermented dairy products such as cheese, all of the lactose is metabolized by lactic acid bacteria into lactic acid within the first three to four weeks, so consumption of this product will not cause lactose intolerance. Even freshly consumed cheeses such as cottage and cream cheese are low in lactose. Lactose intolerance is discussed in Wong et al., Fundamentals of Dairy Chemistry (pp. 32830).
Browning Reactions in Milk
Two types of browning reactions occur in food products, enzymatic browning and the non-enzymatic Maillard reaction. The Maillard reaction is the more relevant of the two in dairy products, and is initiated by reactions between the amine part of a protein with sugars such as lactose. This reaction is inhibited at lower moisture content, pH below 6, and lower temperature. Besides the color change in some processed dairy products, there is also a nutritional consequence to the Maillard reaction. An important amino acid in milk proteins, lysine, contains an amine group that can participate in the Maillard reaction, resulting in a loss of bioavailability of this amino acid.
Goat and Sheep Milk
It is of interest to note that the average size of fat globules in goat milk is slightly smaller (2 micrometers) than bovine milk (2.5.5 micrometers), and that the agglutinin reaction does not occur in goat milk. This latter effect is the primary reason for the scarcity of goat cream and butter on the market today, as the fat globules will not rise to the surface to the same extent as in bovine milk. Before the advent of centrifugal separators, it would not have been possible to obtain large amounts of cream from goat milk. Most goat milk is processed into cheese, rather than into yogurt or consumed fresh. A component of goat milk fat, 4-methyloctanoic acid, is responsible for the "billy-goat" flavor of goat milk cheese.
Sheep milk has almost twice as much fat and protein, and slightly more minerals (ash), than bovine milk, and contains a higher proportion of immunoglobulins so is more resistant to unwanted microbial growth. The fat in goat and sheep milk is whiter than in bovine milk due to a lower amount of b-carotene, and contains a higher proportion of the medium chain length fatty acids C6-C12 that provide a better and more rapidly utilized energy source than the more common longer chain fatty acids. The suckling period for young kids and lambs is three to six weeks, and the milking period extends for a six-month period in spring and summer.
The volume of goat and sheep milk obtained in traditional sized family-owned farms or nomadic flocks is 4000 liters per year. In contrast, commercial goat and sheep milk farms produce about 40000 liters of milk per year. By comparison, 4,000,500 liters per year are obtained from bovine cows in commercial dairies.
The milk fat globule membrane is more fragile in goat milk compared to bovine milk, therefore there is a greater propensity for development of off-flavors. Goat milk is also more liable to undergo spontaneous lipolysis at 39°F (4°C). Further information on the processing of goat and sheep milk is provided by Frank Kosikowski and Vikram V. Mistry in volume 1 of Cheese and Fermented Milk Foods (pp. 29713).
Consumer milk is often sold in the United States according to the percentage of fat: 1 percent, 2 percent, whole milk (about 3 percent), and skim (less than 0.5 percent fat). The shelf life is usually around two weeks. The milk can be fortified with vitamins A and D. Full fat milk is often simply referred to as Vitamin D milk in the United States. Frozen milk may be stored for several months before use; however, it is prone to fat separation, lipolysis, and curd formation.
Flavored milks are becoming increasingly popular as a nutritious beverage, particularly among young people. These drinks are often low in fat with added stabilizers (such as guar and carrageenan) to compensate for the loss in creaminess. An unfortunate problem with gums and stabilizers is that drinks can frequently take on a viscous and elastic texture if too much is added. A wide range of flavors are added to milk, particularly in Australia, where chocolate, coffee, caramel, strawberry, banana, and vanilla are very popular. These have never reached the same level of popularity in North America.
Yogurt probably originated in the Middle East, where goat and sheep milk was soured by the presence of naturally occurring bacteria. It is a favorite food in India, where it is unflavored and made from the milk of buffalo. Its consumption in India signifies the end of the meal. Plain yogurt is often used as a garnish in Middle Eastern meals. A discussion on yogurt manufacture is given by Kosikowski and Mistry in volume 1 of Cheese and Fermented Milk Foods (pp. 8708).
Lactic acid bacteria (a 1:1 ratio of Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus) convert lactose to lactic acid in milk, lowering the pH from 6.7 to around 4.2 and giving yogurt its characteristic clean, acid taste. Often fruit and fruit flavorings are added to make yogurt into a dessert or snack product. The reduction in pH by added bacterial culture, along with refrigerated storage, contribute to the preservation of yogurt. The most common form of yogurt is a smooth viscous liquid; however, it can be frozen and served as a nutritious and refreshing yogurt-based beverage. The trend in the United States is for reduced fat (or nonfat) yogurt, more so than in Europe. The fat content in North American yogurt is typically around one percent.
To enhance the creaminess sensation of reduced or nonfat yogurt, polysaccharide stabilizers such as gelatin, pectin, or locust bean gum are often added, although they are not entirely successful at mimicking fat. These stabilizers have the additional function of reducing syneresis, the clear yellow liquid (whey) that appears on the surface of yogurt due to partial shrinkage of the casein protein network. Probiotic cultures, such as Lactobacillus species (casei and acidophilus) and Bifidobacterium species can be added to yogurt and also to milk protein-based beverages. The combination of bacterial cultures, including probiotic cultures, found in yogurt has long been believed to provide good health. This may explain the relatively large number of centenarian inhabitants of the Caucasus region in southwest Asia, who consume large amounts of yogurt.
Milk for yogurt manufacture is firstly standardized to the appropriate fat and protein level (commonly 125 percent total solids and 1 percent fat) using skim milk powder. The milk is heated at 185°F (85°C) for thirty minutes, or alternatively, 194°F (90°C) for forty to sixty seconds. The stabilizers are next added, followed by homogenization at 7 MPa and cooling to 8613°F (30°5°C). Bacterial cultures are added, fruit puree is mixed in if desired, and the milk is allowed to set for sixteen hours to form a coagulum before refrigerating to 39°F (4°C) and packaged for consumption.
Homogenization of milk prior to yogurt manufacture increases the viscosity of yogurt and inhibits the formation of syneresis during refrigerated storage. The shelf-life of yogurt at 39°F (4°C) is from thirty to sixty days. The two main types of yogurt are stirred and set. In set yogurt, milk is allowed to coagulate to form the yogurt network structure without mechanical disturbance. In stirred yogurt, the coagulated milk is stirred while cooling down, then the fruit puree is added if required.
Yogurt cheese is prepared as for other types of yogurt in the initial stages of manufacture, except that Lactococcus lactis bacterial culture and rennet are mixed with the milk one hour into the setting period, and a cheese similar to cream cheese is manufactured.
Cream and Sour Cream
Cream is produced by separating milk into a skim phase (with less than 0.5 percent fat) and a cream phase. The fat content of the cream increases with the speed of the separator. A separator consists of concentric stainless steel cones rotating at high speed. Fat in cream can vary from as little as 102 percent in half-and-half, popular in the United States, to 300 percent in table cream and whipping cream. Plastic cream can contain up to 80 percent fat. Off-flavors can be removed from milk and cream by the process of vacreation, where steam is injected into the product and removed, along with the unwanted flavors, under a partial vacuum.
Sour cream is produced by lactose acid bacteria fermentation or direct addition of mineral acids to cream. Fresh cream is first standardized to 20 percent fat, homogenized at 20 MPa and 160°F (71°C), pasteurized at 165°F (74°C) for thirty minutes, cooled, and then inoculated with bacterial culture. Rennet can be added to produce a firmer product. The fermenting cream is incubated for sixteen hours at 72°F (22°C) until a pH of 4.5 is reached, then cooled and packaged. The shelf life is three to four weeks. Condiments such as blue cheese, seafood, and onion can be added to sour cream to produce a dip.
Ultra High Temperature Milk
Ultra high temperature (UHT) milk is produced by heat-treating milk at 284°F (140°C) for two to three seconds, essentially sterilizing the milk. It is packaged aseptically and stored at room temperature for up to six months. The two main methods of producing UHT milk are direct steam injection, and indirect heating by passing milk over a stainless steel surface that is heated by high pressure steam. UHT milk is characterized by a cooked flavor that dissipates over time and which is then replaced by a stale oxidized flavor that develops during storage. Another potential problem with UHT milk is that it is susceptible to gelation and sedimentation. The chemical and physical mechanisms for this are unclear.
Some work has been done using a spinning cone column to eliminate undesirable flavors from UHT milk. This technology utilizes a series of rapidly rotating concentric cones through which milk or cream can be passed down. Low temperature steam under a partial vacuum is passed up in the counter-current direction, which strips flavor components from the dairy product. The steam can be condensed and the flavor compounds distilled and kept for later use. This process can be used to remove feed-related off-flavors from milk, the oxidized flavor from UHT milk, or to strip desirable buttery flavors from cream for subsequent addition into low-fat dairy products.
Buttermilk, Butteroil, and Ghee
Buttermilk is a by-product of churning in butter manufacture. It is low in fat (about 1 percent), and rich in the phospholipid and protein components at the milk fat globule membrane layer. Milk fermented with lactic acid bacteria and subsequently separated into cream can be used in the manufacture of cultured or ripened butter. Cultured buttermilk is derived from cultured butter.
Butteroil is produced by centrifuging liquid butter to a fat content in excess of 99 percent, the remainder being mostly water. This product is solidified at room temperature. A higher grade, at more than 99.8 percent fat, is called anhydrous milk fat.
Ghee is a product similar in composition to butteroil and used for confectionery manufacture and for cooking in India, and in Egypt where it is called samma, or samn. Buffalo milk is boiled for one hour, the curd skimmed off, and churned for thirty minutes to form a butter. This is heated and strained to yield ghee, a clear oil with a characteristic cooked odor and flavor.
Other Milk Products
Kefir is an alcoholic, carbonated milk-based beverage popular in eastern and central Europe. Yeast and bacteria convert lactose into lactic acid, carbon dioxide gas, and alcohol (ethanol) overnight at ambient temperature. The levels of lactic acid and alcohol are around 1 percent each. Koumiss from central Asia is similar to Kefir but made from mare's milk, and has a higher alcohol content of 2.5 percent. Other nonalcoholic fermented milk beverages include acidophilus milk, and Bulgarian buttermilk with a comparatively high lactic acid content of 2 percent.
Evaporated and Condensed Milk
Concentrated milk can be produced by partial evaporation of water to yield either evaporated or condensed milk. The removal of water is done by heating under reduced pressure to avoid excessive heat damage to proteins. Evaporated milk is sterilized and aseptically packaged, whereas condensed milk does not have this extra heat treatment step. Variations of condensed milk include sweetened, where 18 percent sugar is added to milk before evaporation, and condensed skim milk.
Liquid whey is an orange-colored liquid comprising about 5 percent lactose, 0.7 percent whey proteins, and 1 percent minerals. The disposal of whey has always been an issue in the manufacture of cheese due to the large volumes produced. Approximately 90 percent of the volume of milk is removed as whey when making cheese. The high biological oxygen demand of whey prevents disposal into lakes and streams, where it can deprive fish of oxygen.
Whey proteins are very nutritious and have a high protein efficiency ratio (ratio of weight gained to weight of protein consumed). Fruit juices can be fortified with around 6 percent acid whey powder to increase the nutritional value without adversely affecting flavor and color. Whey powder is highly water-soluble and adds vitamins, minerals, and high-quality nutritional proteins to other food products.
Liquid whey can be manufactured into cheese by adding milk or cream and then concentrating by evaporation. Cheese made by this method includes Brunost, Mysost, Gjetost, and Primost, which are popular in Scandinavian countries. The last two of these cheeses are made from goat whey. Ricotta is a whey-based cheese made by heating acidified bovine milk.
Whey Protein Powder
The two main types of whey are acid and sweet whey. Acid whey is obtained by addition of acids such as lactic, phosphoric, hydrochloric, sulfuric, and acetic to skim milk, reducing the pH from 6.7 to 4.6 and causing the casein to precipitate (which is then removed). Milk has a very high buffering capacity, requiring large quantities of acid to reduce the pH to this level. Acid whey contains very little lactic acid, as curds and whey are separated without the fermentation step.
Sweet whey originates from the cheese manufacturing process. Curds and whey are separated at a pH of approximately 6.2 in cheddar cheese manufacture, therefore sweet whey from this cheese is less acidic than acid whey. Sweet whey from cheesemaking contains around 0.5 percent fat. Whey from cottage cheese contains little fat, and around 0.4 percent lactic acid as a greater amount of fermentation of lactose to lactic acid takes place before the curds and whey are separated.
Sweet whey contains an additional protein fragment, the glycomacropeptide (GMP), which originates from the surface of the casein micelle as a consequence of enzymatic hydrolysis during the milk clotting reaction. The presence of GMP affects the textural functionality of whey powder when used as a food ingredient. GMP does not contain the amino acid phenylalanine, which can cause brain damage in children suffering from the disease phenylketonuria, an inability to metabolize this amino acid. There has been some research done on the extraction of GMP from sweet whey for use as a protein source for phenylketonuriacs.
Fractionation of whey into components with very specific nutritional, textural (functional), and pharmaceutical properties is a burgeoning field of research with the potential of large profit margins for the dairy industry. Components of whey that inhibit microbial growth include lactoferrin, lactoperoxidase, lysozyme, and various immunoglobulins. Lactoferrin binds iron necessary for microbial growth. These antimicrobial agents have the potential for use as "natural" preservatives in food products.
Sweet whey from cheesemaking is first clarified to remove casein particulate matter, then separated to remove most of the fat. The whey is initially concentrated to around 400 percent solids in a vacuum evaporator before homogenization and drying. Most drying is done using a spray-dryer where the concentrated viscous whey is passed through an atomizer and allowed to fall through a chamber with countercurrent heated air to produce a dispersible and nonhygroscopic powder of around 10 percent moisture. Spray-dryers can produce as much as twenty metric tons per hour of powder. Further processing can take place on a fluidized bed dryer where heated air is passed up through a vibrating layer of powder to further reduce the moisture to around 4 percent. A lecithin mixture can also be sprayed onto the powder to promote dissolution in water, a procedure known as instantization. Other types of dryers that are less commonly used include drum and roller dryers and freeze dryers. Roller dryers result in a much more irregularly shaped powder particle than spray-dryers.
Typical composition of whey powder is shown in Table 1. These powders are called whey protein concentrates (WPC), and suffixed with a number to indicate the percentage of protein, for example, WPC 80 contains 80 percent protein. Generally, the higher the protein content, the higher the price that the powder commands in the market. The solubility of powders can be enhanced by collecting the smallest particles in a spray-dryer cyclone and mixing back with the partially dried powder at the top of the dryer. This creates an agglomerated particle with a surface containing many crevices, thus increasing the total surface area of a particle and increasing the solubility.
Higher protein levels in excess of 90 percent can be achieved by a combination of ion-exchange chromatography, electrodialysis to remove minerals, evaporation, ultrafiltration, lactose crystallization, and filtration. Whey protein isolates (WPI) are produced by ion-exchange and have a typical protein content of 93 percent. These isolates command a higher market price than WPC powders. The amount of fat in WPI is much lower than WPC, at around 0.5 percent. Ingredient applications for WPC and WPI include fortifying beverages such as high energy sports drinks, infant formulas, salad dressings, ice cream, custards, reformed meats, yogurt, surimi, bakery, and dessert products (as an egg replacement).
Lactose in milk powder is in the form of a concentrated amorphous (non-crystalline) glass that is very hygroscopic (high propensity for absorbing water). Milk powders that are high in lactose can potentially aggregate and lose the ability to flow freely. To avoid this problem, concentrated liquid whey is held at refrigeration temperatures for twenty-four hours prior to drying to ensure that lactose crystallizes into the stable and non-hygroscopic α-hydrate polymorph to reduce the propensity for powder caking. The concentrated whey is injected into the spray-dryer at less than 126°F (52°C) to prevent solubilization of the α-hydrate crystals.
Lactose can be converted to galactose and glucose by the enzyme lactase. This procedure can be utilized to manufacture a low-lactose whey powder, which is sweeter, but more likely to undergo Maillard browning reactions than regular whey powder. Lactase can also be encapsulated and added to milk. Once the milk is consumed, the capsules break open, releasing the lactase, thus preventing the occurrence of lactose-intolerance symptoms.
The degree of heat-treatment of milk powders is quantified by the whey protein nitrogen index (WPNI), a measure of the quantity of whey proteins that are not denatured. Excessive heat during spray-drying causes a large degree of protein denaturation with subsequent loss of solubility of the powder. Low heat powder has a WPNI greater than 6 milligrams of nitrogen per gram, medium heat between 1.5 and 6 milligrams of nitrogen per gram, and high heat less than 1.5 milligrams of nitrogen per gram of powder. The degree of heating of powder during manufacture has implications for solubility, heat stability, viscosity, and flavor of the dairy product containing whey powder. High-heat powder will give high viscosity when added to yogurt, good heat stability in reconstituted evaporated milk, and intense flavor when used in milk chocolate, but poor clotting properties for recombined milk used in cheese manufacture.
Buttermilk contains many surface-active components that function as emulsifiers. By making changes to the butter manufacturing process, different kinds of buttermilk powder (BMP) can be produced with various functional properties when used as a food ingredient. There is some scientific evidence that the components of the milk fat globule membrane layer are essential for development of cheese flavor during the ripening period. If this is so, BMP could find a use in fortifying low fat cheese to improve the flavor. Other applications for BMP include emulsifying fat in salad dressing, bakery products, ice cream, dips, and spreads.
The composition of BMP is not as tightly controlled as for other milk powders, as it is often thought of as a secondary product coming out of the primary process of butter manufacture. Volumes produced of this powder are generally much lower than for other powders, so their full potential has not yet been fully explored. BMP comprises mostly lactose with around 10 percent fat. About one-fifth of the fat comprises phospholipids, the highly surface-active components that are used as emulsifiers.
Casein and Caseinate Powders
Casein powder is produced by isoelectric precipitation of milk using mineral acids, lactic acid bacteria fermentation, or enzymatic coagulation by chymosin (rennet). New Zealand is the world's largest producer of casein powder. A combination of acid and chymosin is used to prepare a low-viscosity casein for use in the paper industry to bind pigments. Precipitated casein curds are washed in water to remove residual lactose, whey proteins, and minerals, then pressed with rollers, dried (using a fluid bed dryer), and ground into a powder. Attrition drying occurs when casein curd is ground and dried by exposure to hot air concurrently. These particles are highly irregular in shape and more soluble in water.
Applications of casein powder include paper coating, adhesives, water-based paints, food ingredients, and animal feed. Food ingredient applications include beer and wine clarification, protein fortification of food, and texturized simulated meat products. Casein hydrolyzates are formed by partial acid hydrolysis of casein to improve the flavor of soups, dried meat products, crackers, and snack foods.
Caseinate is produced from acid casein by increasing the pH toward neutrality to dissolve the precipitated casein. Most commonly, sodium and calcium hydroxides are used to prepare sodium and calcium caseinate, respectively. Caseinates can be spray-dried after reconstituting in water to around 205 percent total solid material. The ingredient application for dried caseinates includes sausages, coffee whitener, ice cream and dairy-based desserts, soups, crackers, and sauces.
Other Milk Powders
Other types of milk powder include skim milk powder (SMP), whole milk powder (WMP), cream powder, lactalbumin, colostrum, cheese powder, and milk protein concentrate. WMP contains around 30 percent fat, whereas SMP contains less than 1.5 percent. Lactalbumin powder is formed by heat-induced precipitation of cheese whey followed by drying. This powder is insoluble in water.
Cheese powder is produced from highly flavored cheeses such as cheddar and parmesan, by grinding, adding water and emulsifying salts to form a viscous suspension, followed by pasteurization, homogenization, and spray drying. This product quickly deteriorates after several months. The major ingredient application is for use as a cheese flavoring in food products.
Cream can also be spray dried to form a high fat (40-75 percent) powder. The amount of free fat in milk powders is of importance and affects dispersibility in water. This free fat is usually located on the surface and within crevices on the powder particle.
Co-precipitates are produced by heating skim milk to 185°F (85°C), whereupon the whey proteins denature and bind to the casein micelles. This complex is precipitated by acid and processed to a powder using the same procedure as casein powder production.
Milk is an important food from a nutritional perspective, largely due to the presence of proteins and calcium. The high water content and only slightly acidic pH render this food susceptible to microbiological spoilage. The dairy processing industry has developed to circumvent the spoilage issue through production of products with low moisture and higher acidity. This allows the development of a dairy products export market, as milk products can now be shipped to distant lands without compromising quality and safety. The huge variety of dairy products, most notably illustrated by the seemingly unending array of cheeses, is a testimony to the potential of milk for continued development of nutritious and tasty milk-based foods.
See also Butter; Cheese; Curds; Ice Cream; Lactation; Milk, Human; Pasteur, Louis.
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