Milk and milk products cover a very wide range of raw materials and manufactured products. No attempt is made in this chapter to deal with all of them. Rather, the properties and processing of fluid milk and some of the more common products manufactured from it, such as specialty milks, ice cream, and cheese, are discussed. Butter and margarine are considered in Chapter 16 on fats and oils.
Milk is a unique substance in that it is both consumed as fluid milk with minimal processing and it is the raw material used to manufacture a wide variety of products. Milk also has unique nutritional properties that make it an especially important food, particularly for the young. It is commonly pasteurized and homogenized and its composition is very close to what it was when taken from the cow. Milk can be separated into butterfat, casein and other milk proteins, and lactose. These are sold and used as products in their own right; or they may be further processed into butter, cheese, ice cream, and other well-known dairy products. Similarly, the milk may be modified by condensing, drying, favoring, fortifying, demineralizing, and still other treatments. Whole milk or its components may be combined in various proportions for incorporation into numerous manufactured food products, such as milk chocolate, bread, cakes, sausage, confectionery items, soups, and many other food products not primarily of dairy origin.
FLUID MILK AND SOME ITS DERIVATIVES
Milk is the normal secretion of the mammary glands of all mammals. Its purpose is to nourish the young of the species. The nutritional needs of species vary and so it is not surprising that the milk from different mammals differs in composition.
The principal constituents of milk –fat, protein (primarily casein), milk sugar or lactose, and the minerals of milk, which collectively are referred to as ash –vary not only in amounts among the different animals’ species but also, with the exception of lactose, somewhat in chemical, physical, and biological properties. Thus, the fatty acids of goat milk fat have different milting points, susceptibility to oxidation, and flavor characteristics than those of cow’s milk. Similarly, milk protein of various species may differ with respect to heat sensitivity, nutritional properties, and ability to produce allergic reactions in other species.
This high degree of variability among the milks of different animals becomes especially important in processing operations. The conditions for condensing, drying, cheese-making, and so on that are optimum for cow’s milk may not be satisfactory when applied to a dairy in India because the composition of the milk is different. In the remainder of this chapter, unless otherwise indicated, discussion will apply to the milk from cows.
Even the milk from cow will vary in compositions, depending on many factors. These include the breed, animal to animal variability, age, stage of lactation, season of the year, the feed, time of milking, period of time between milking, the physiological condition of the cow, including whether it is calm or excited, whether it is receiving drugs, and so on. All of these factors also affect the quality of the milk. Because of these sources of variation, seldom do literature values on the compositions of milk agree exactly. Nevertheless, it is useful to remember the approximate composition of cow’s milk, since most commercial milk supplies contain the mixed milk from several farms and variations tend to average out. The approximate compositions of milk in Table 13.2 is on such a mixed milk. All of the solids in milk (total solids) amounts to approximately 13%. The terms solids nonfat or milk solids nonfat (MSNF) refer to total solids minus the fat, in this vase 9%. Milk solids nonfat also are referred to as serum solids. The market price of milk purchased in bulk generally is based on its fat content and to a lesser extent on its solids-nonfat content. These solids of milk further determine the approximate yields of other dairy products that can be manufacture from the milk (Table 13.3).
The most important single factor governing the composition of cow’s milk is the breed of the cow. The principal milk producing breed are the Ayrshire, Brown Swiss, Guernsey, Holstein, and Jersey. Holsteins generally produce the most milk, but Guernsey and Jerseys produce milk with the highest fat contents (around 5%).
Legal Standards
Milk is the most legally controlled of all food commodities in the United States and many other countries. The minimum standard for fat is regulated by low in each state in the United States with values ranging from 3.0% to 3.8%. regulations also cover total solids, which for most states in the United States range from 11.2% to 12.25%. there also are standards of composition and regulations against conditions that would constitute adulteration for milk and all important milk products. In the case of fluid milk, federal standards include minima of 3.25% fat and 8.25% MSNF.
Each state and many cities in the United States regulate veterinary inspections on farms and specific sanitary requirements throughout the entire chain of milk handling and milk processing. This is essential to protect health since improperly handled milk can be a source of serious disease.
Milk has been referred to as a human’s most nearly perfect food from a nutritional standpoint. Its wholesomeness and acceptability further depend on the strictest sanitary control, and the sanitary practices employed by the dairy industry have for many years been the guide to the entire food industry.
“The milk equivalent of ice cream per 3.8 liters (1 gal) is 6.8 kg (15 lb). Plant reports indicate that 81.24% of the butterfat in in ice cream is from milk and cream. Thus the milk equivalent of the milk and cream is about 5.4 kg (12 lb).
Milk is also highly regulated with respect to pricing structure and permissible marketing practices. An example of the former is that the same supply of milk often will be priced according to the end use. Thus, a supplier generally must charge more for milk going into fluid whole milk channels than for milk to be used for manufactured products such as cheese or butter, even when the same milk source is used for both purposes. Control over marketing practices has included laws against standardizing high and low fat milks by the addition of butterfat or skim milk although the final blend may be well above the legal minimum fat content.
Many dairy pricing and marketing laws were originally established to protect the interests of producers and processors in a given region. Often, with time, such regulations tend to restrict rather than help a particular segment of the food industry. This has been particularly true in the dairy industry, especially as nondairy or partial dairy substitute products such as margarine, certain coffee whiteners, and synthetic milks have grown in importance.
Milk Production Practices
The under of the cow produces milk from components withdrawn from the animal’s bloodstream. The milking operation stimulates release of hormones, which in turn act on muscles in the udder causing let-down of milk into the four teat canals. Hand milking in the United States is largely a thing of the past. Milking machines working on a vacuum from the milking machine cups through pipes leading to a bulk holding tank in another room.
This tank is provided with refrigeration to quickly cool the milk to 4.4C or lower to control bacterial growth. Milk secreted by a healthy udder is sterile but quickly becomes contaminated with microorganisms from the external body of the cow and from milk-handling equipment. Milk should not be held in the cold tank on the farm more than 2 days before it is transported to a milk receiving station or milk processing plant. Often it is transported the same day it is produced.
Cooled milk is most commonly transported in bulk tank trucks from the farm to the processing plant. The milk is pumped into insulated stainless steel tanks holding up to 6600 gal. the tank truck driver records the volume of milk collected and removes a small sample for later analysis of fat and total solids on which to base price to the farmer, and for microbiological tests.
The milk, maintained cold in the tank-trucks, may go directly to a milk processing plant or milk may be brought to a central receiving station where it is pooled. Here high-fat milk may be blended with low-fat milk before it is shipped to a processing plant. Such a blending of natural milks is legal even in states where standardization by the addition of butterfat or skim is not permitted.
Quality Control Tests
Upon receipt of milk at a processing plant, several inspections and tests may be run to control the quality of the incoming product. Some of these tests may have been done at an earlier receiving station. These tests commonly include determination of fat and total solids by chemical or physical analyses; estimation of sediment by forcing milk through filter pads and noting the residue left on the pad, determination of bacterial counts, especially total count, coliform count, and yeast and mold count; determination of freezing point as an index to possible water pickup; and evaluation of milk flavor. Tests for cells from the cow are sometimes conducted as an indicator of possible infection in the cow’s udder. Under special circumstances, tests for detection of antibiotic residues from treated cows and for pesticide residues that may get into the milk from the feed or from other farm use also may be made.
Bacterial counts play a major role in the sanitary quality of milk on which grades are largely based. Generally, fluid whole milk for consumer use, also referred to as market milk, has higher standards placed on it than milk which will be used for manufacturing purposes. The Grade a Pasteurized Milk Ordinance Recommendations of the U.S Public Health Service/Food and Drug Administration provides an excellent guide to the setting of microbiological and sanitary standards, and many cities and states have adopted or patterned their milk regulation after this code. Among various milk products recognized by this ordinance are Grade. A Raw Milk for pasteurization, which may not exceed a bacterial plate count of 100.000/ml on milk from individual producers or 300,000/ml on commingled (blended) milk, and Grade a Pasteurized Milk which may not exceed a total bacterial count of 20.000/ml or a coliform count of 10/ml. these bacterial counts are among many other requirements that have gone into establishing grades.
As for flavor, much milk is received that is not of top quality. Milk may acquire off flavors from cows eating unusual feeds, by absorption of odors and flavors from unclean barns and excessive bacterial multiplication, by the action of the natural milk lipase enzymes breaking down fat, and by oxidation, which often is caused by the milk coming into contact with traces of copper or iron in valves, pipes, or other milk-handling equipment. As little as 1 part of copper in 10 million parts of milk can cause oxidized flavors that vary in degree and are described as metallic, cardboard, oily, fishy, and so on. For this reason, iron and copper must be kept form coming into direct contact with product or with cleaning water that can contaminate equipment surfaces contacting product; the metal of choice in milk-handling operations is stainless steel.
They type of off-flavor in milk is generally a good clue to its cause, as are defects uncovered by other quality control tests. Defects are reported to farmers with suggested methods of correction. Acceptable milk is now ready for processing.
Milk Processing and Sequence
The first step in processing milk may be a further blending of different batches to a specified fat content. All the while the milk is held cold, preferably at 4.4C.
Clarification
The milk is next passed through a centrifugal clarifier (Fig 13.1) to remove sediment, body cells from the udder, and some bacteria. Removal of these impurities in the clarifier is facilitated by distributing the milk in thin layers over conical disks which revolve at high speed. Since the milk is in thin layers, these impurities, which differ in density from the liquid milk, need travel only a very short distance under the influence of centrifugal force to be removed from the milk.
Clarification is by no means intended to rid the milk completely of bacteria, and the clarifier was not designed for this purpose. A special machine known as a Bactofuge, operating under much greater centrifugal force, has been designed for a high degree of bacterial removal. But even such machines fail to remove all bacterial from milk and could not be depended on to remove all pathogens.
Figure 13.1
The clarified milk is now ready for pasteurization if it is to be processed as market milk.
Pasteurization
The aim of pasteurizing milk is to rid the milk of ay disease-producing organisms it may contain and to reduce substantially the total bacterial count for improved keeping quality. Pasteurization also destroys lipase and other natural milk enzymes. Pasteurization temperatures and times for many years were selected to ensure destruction of Mycobacterium tuberculosis, the highly heat-resistant non-spore-forming bacterium that can transmit tuberculosis to humans. A treatment of 62C (143 F) for 30 min, or its equivalent, was employed. In more recent years it was discovered that the organism causing Q fever, Coxiella Brunetti was slightly more resistant than the tuberculosis organism and required a treatment of 63C (145 F) for 30 min, or its equivalent, to ensure its destruction.
The two accepted methods for milk pasteurization today are:
The batch (holding) method of heating every particle of milk to not less than 63 C and holding at this temperature for not less than 30 min and:
The high-temperature-short-time (HTST) method of heating every particle of milk to not less than 72 C (161 F) and holding for not less than 15 sec.
Pasteurized milk is not sterile and so it must be quickly cooled following pasteurization to prevent multiplication of surviving bacteria. Pasteurization at these temperatures does not produce and objectionable cooked flavor in milk and has no important effect on the nutritional value of milk. Although slight vitamin destruction may occur, this is easily made up by other foods in a normal diet.
Batch pasteurization is carried out in heated vats provided with an agitator to ensure uniform heating, a cover to prevent contamination during the holding period, and a recording thermometer to trace a permanent record of the time-temperature treatment. But batch pasteurization of milk has largely been replaced with HTST pasteurization.
HTST pasteurization requires the more complex system described in Chapter 8, with its heating plates, holding tube, flow diversion valve, and time –temperature recording charts. Such a HTST system is shown in Fig. 13.2, which also includes such additional equipment as a vacuum chamber to the left, to remove volatile off-flavors from the pasteurized milk.
All pasteurization equipment must be of approved design, and milk inspection visit often to check on proper equipment operation.
Raw milk contains several enzymes. One of considerable importance in public health work is alkaline phosphatase. This enzyme has heat-destruction characteristics that closely approximate the time –temperature exposures of proper pasteurization. Therefore, if alkaline phosphatase activity beyond a certain level is found in pasteurized milk.
Figure 13.2
It is evidence of inadequate processing. This enzyme has the ability to liberate phenol from phenol phosphoric acid compounds. Free phenol gives a deep blue color with certain organic compounds. This is the basis for the phosphatase test. In one from of the phosphatase test, disodium phenyl phosphate is the source of phenol and 2., dichloroqui none-chloramine is the indicator reagent. Milk is incubated with the disodium phenyl phosphate and then the indicator reagent is added. A blue color indicates improper pasteurization or recontamination with unpasteurized product.
Homogenization
After pasteurization, milk may be homogenized, or homogenization may come just before the pasteurization step if the milk has first been warmed to melt the butterfat.
Milk and cream have countless fat globules that vary from about 0.1 to 20µm in diameter. These fat globules have a tendency to gather into clumps and rise due to them lighter density than skim milk. Skim milk from which the cream has been removed has virtually no fat globules. The purpose of homogenization is to subdivide the fat globules and clumps to such small size that they will no longer rise to the top of the milk as a distinct layer in the time before the milk is normally consumed. This is an advantage since it makes the milk more uniform and prevents the cream from rising to the top of a container. In addition, subdivision and uniform dispersion of the fat gives homogenized milk a richer taste and a whiter-appearing color, as well as greater whitening power when added to coffee, than the same milk not homogenized.
In one type of homogenizer valve assembly (Fig.13.3), large fat globules in milk entering at the bottom are sheared as they are pumped under pressure through a tortuous path. They emerge at the top about one-tenth of their original diameter.
Homogenization and cooling of the milk is followed by packaging in paper cartons, plastic jugs, or other types of packages. These are then delivered in refrigerated trucks to retain and other outlets.
Related Milk Products
The processing sequence just described is basic to the production of fluid milk (i.e., market milk). However, slight departures or additional steps in the sequence are employed to produce a number of closely related milk products.
Vitamin D Milk
Milk normally contains vitamin D, but the amount varies with the cow’s diet, and with her exposure the sunlight. Since the diet of many children is deficient in vitamin D, it has become common practice to add the vitamin to milk. Milk can be increased in vitamin D activity by irradiating the milk with ultraviolet light, which, in effect, converts the milk sterol, 7-dehydrocholestrol, into vitamin D3. But the vitamin D level that can be produced this way is somewhat limited. More practical has been the addition of a vitamin D concentrate to milk at a level to bring the potency up to 400 units of vitamin D per quart. Most of the milk consumed in the United States contains added vitamin D. it is generally added before pasteurization.
Figure; 13.3
Multivitamin Mineral Milk
Most nutritionists oppose the indiscriminate fortifying of general-purpose foods since normal dietary variety tends to supply all needed nutrients and excesses of certain nutrients can be harmful. Nevertheless, fortified milks have been available in some areas in past years.
Vitamins and minerals have been added to give each quart the formerly recommended minimum daily requirements of vitamin A, vitamin D, thiamin, riboflavin, niacin, iron, and iodine. Vitamin C was not commonly added since it is quickly destroyed during milk processing and normal storage. Such fortified milks represented but a small fraction of total milk production, and the new standards of identity for fluid milk products do no permit these multivitamin mineral milks.
Low-Sodium Milk
People with high blood pressure or edema may be on a restricted sodium diet. Low-sodium milk, available in a number of cities, is prepared by passing the milk through an ion exchange resin that replaces sodium with potassium. Low-sodium milk generally contains about 3 -10 mg of sodium per 100 ml, whereas untreated milk contains about 50 mg/100 ml.
Soft-Curd Milk
The casein of milk coagulates and forms curd when acted on by enzymes and acid of the stomach. This curd may be harder or softer depending on the amount of casein and calcium in the milk and other factors. Human breast milk form a soft fluffy curd; pasteurized cow’s milk forms a harder more compact curd. Soft-curd milk may be easier to digest by infants and young children. Various treatments are known for producing soft curd milk. These include heat treatments comparable to those used in producing evaporated milk, removal of some calcium by ion exchange, treatment of the milk with enzymes, and other methods. Soft curd milks are commercially available.
Human milk is more easily digested than cow’s milk, which is substantially higher in protein and ash and lower in sugar than human milk. To correct these differences, in preparing infant formulas it is common practice to dilute cow’s milk with water and to add sugar, making it more like human milk.
Low-Lactose Milk
A surprisingly large number of people suffer from a condition known as lactose intolerance. Normally, humans hydrolyze lactose into its two monosaccharides, glucose and galactose, which are then readily absorbed in the small intestine. Some individuals, however, produce low levels of the enzyme lactase, responsible for this hydrolysis. The disaccharide lactose is not easily absorbed in the small intestine and passes on to the colon where it produces fluid accumulation by osmotic action and undergoes microbial fermentation causing intestinal distress. One way to overcome this problem is to treat the milk with the enzyme lactase in the course of its processing. Another is to add the enzyme to regulate milk in the home and hold the milk in the refrigerator for a prescribed time to give the enzyme an opportunity to work. Manufacturers are providing lactase for this purpose.
Sterile Milk
Milk may be sterilized rather than pasteurized by using more sever heat treatments. If the temperature is sufficiently high, the time may be very short, preventing cooked flavor and color change. A typical heat treatment is of the order of 150C for 2-3 sec. the milk is sometimes referred to as ultrahigh-temperature (UHT) processed milk. A system for heating and cooling such milk is shown in Fig. 13.4. in the past, sterile milk found its greatest use where refrigeration was not always available. The potential energy savings from elimination of refrigeration requirements is focusing greater attention on this type of product today.
Evaporated Milk
Evaporated milk is the most widely used form of concentrated milk. It is concentrated to an approximately 2.25 times the solids of normal whole milk. To produce evaporated milk, raw whole milk generally is clarified, concentrated, fortified with vitamin D (to give 400 units per 0.946 liter when the evaporated milk is diluted with an equal volume of water), homogenized, filled into cans, sterilized in the cans in large continuous pressure reports at temperatures of about 118C for 15 min, and cooled.
Figure; 13.4
This heat treatment gives evaporated milk its characteristic caramelized cooked color and flavor.
Whole milk also is being concentrated approximately 2:1 and 3:1 and sterilized evaporated milk and sterile 3:1 concentrate without cooked color and flavor. When concentrated milks are heated, the proteins have a tendency to gel and thicken on storage. Various pre warming treatments, addition of stabilizing phosphates and other salts permitted in specified low levels, and, more recently, membrane treatment of the milk to slightly modify its composition may minimize this problem, but its complete elimination is rarely achieved.
Sweetened Condensed Milk
Unlike evaporated milk, sweetened condensed milk is not sterilized, but multiplication of bacterial present in this product is prevented by the preservative action of sugar. The product is made from pasteurized milk that is concentrated and then supplemented with sucrose. Concentration and sugar addition are adjusted to give a sugar concentration of about 63% in the water of the final product. Preservation of milk with sugar has largely given way to milk solids is convenient in food manufacture, and large amounts of sweetened condensed milk are today used by the baking, ice cream, and confectionery industries. The different steps involved in the manufacture of evaporated milk and sweetened condensed milk are outlined in Fig 13.5.
Figure: 13.5
Whole Milk
Whole milk is dehydrated to about 97% solids principally by spray drying and vacuuming. The drying operation is quite efficient, but on storage, the whole milk product acquires off-flavors, frequently of an oxidized character. How to completely prevent off-flavors is still not known, and this is why dried whole milk of beverage quality. Whenever, large quantities of dried whole milk are used in the manufacture of other products where storage flavor is not as apparent.
Separation of Milk
The products discussed to this point result largely from processing whole milk that has not been separated into its components. But milk can be readily separated into its two principal fractions, cream and skim milk. The separation is made in a centrifugal cream separator, which looks and functions quite like a milk clarifier (Fig. 13.1) but has separate discharge nozzles for cream and skim milk. The cream separator bowl rotates at a speed of several thousand revolutions per minute. Milk enters the top center of the bowl. The skim milk having a heavier density than the whole milk or cream is driven by centrifugal force to the outside of the bowl while the lighter cream moves toward the center of the bowl. The machine can be adjusted to separate cream over a wide range of fat contents for different uses.
Skim milk may be used directly as a beverage or it may be concentrated or dried for use in manufactured foods and animal feeds. Similarly, cream may be used directly, or it may be frozen, concentrated, dried, or further separated to produce butter oil and serum solids. All of these forms are used in manufactured foods.
Low-Fat Milk
Skim milk, which may contain 0.5% fat or less, has been consumed as a beverage for many years. Other low-fat milks may contain 1% or 2% fat. Because fat-soluble vitamins are removed when the fat is separated, the new federal standards for skim milk and low-fat milk require addition of vitamin A. vitamin D also may be added, but it is optional.
Milk Substitutes
The prices of milk, cream, and other dairy products are largely determined by their milk fat (butterfat) contents. Butterfat for many years has sold for about five times the price of common vegetable fats and oils. As is discussed in Chapter 16, current technology can modify many fats and oils of vegetable, marine, or animal origin to perform nearly interchangeably in numerous food applications. This, of course, is the basis for the margarine industry.
Recently, various substitutes for dairy products other than butter have become commercially important. These have included vegetable fat frozen desserts (ice cream substitutes), vegetable fat coffee whiteners, and vegetable fat whipped toppings. Vegetable fat milk substitutes also have appeared. When these milk substitutes are made by combining nondairy fats or oils with certain classes of milk solids, the resulting products are referred to as filled milks. The term imitation milk, on the other hand, has been used to describe products that resemble milk but contain neither milk fat nor other important dairy ingredients. Use of the term imitation, with its negative connotation, is no longer legal requirements for these types of products provided they meet certain nutritional requirements and are labeled so as not to be misleading. Their compositions from a nutritional standpoint are of the greatest significance in view of the important role milk has in the diets of persons of all ages.
Today it is possible to manufacture fluid milk substitutes of considerable quality. Generally, such products are made from skim milk or reconstituted skim milk powder plus coconut fat or some other vegetable fat. Additional ingredients such as monoglyceride and Diglycerides emulsifiers, carotene (for color), vitamin D, and other substances commonly are added. The products are pasteurized, homogenized, packaged, and marketed quite like market milk. These filled milks may not be called milk but go under such names as Melloream (Mellorine is the name of a vegetable fat ice cream), protein drink, and other trade, names.
Imitation milk and beverages made from casein derivatives (caseinates) or soybean protein and vegetable oils represent a still further departure from natural milk. Such products are being manufactured and sold in Asia and other regions. Carefully formulated, they can be of considerable importance nutritionally, since they generally can be produced at a lower cost than natural milk.
Milk components such as milk proteins, lactose, butterfat, and modified forms of these are increasingly being used as functional and nutritional ingredients in other manufactured foods. milk components find their way into a variety of products ranging from confections to infant formula.
Growth Hormones and Milk Production
In the 1950s it was discovered that cows would produce more milk when injected with an extract of the pituitary gland from other slaughtered cows. Subsequently, it was discovered that a small protein was responsible for this increased milk production and that this protein also affected the growth rate of young animals. The protein is now called Bovine Growth Hormone (BGH) or more technically, Bovine Somatotropins (BST). In fact, all animals have some type of similar growth hormone, including humans. At one time it was hoped that BST could be used to treat human dwarfism which results from a genetic inability to produce human growth hormone. Unfortunately, growth hormones differ among species and so hormone from one species is not active in another.
By transferring the gene responsible for making BGH from a cow to a bacterium, scientists have learned how to make an inexpensive and readily available source of BGH. When given injections of BGH, lactating cows not only produce 10-15% more milk, but they do more efficiently with regard to the amount of feed required. The dairy industry around the world is interested, of course, in increased efficiency and has begun to use BGH. In 1993, the U.S Food and Drug Administration approved the use of BGH (BST) to treat lactating cows in the United States.
This technology is not without controversy. Some worry about the safety of milk from BGH-treated cows. However, numerous scientific studies and several review panels have found no reason for concern. Others worry that BGH will cause economic disruption in the dairy industry and that it will speed the decline in the numbers of family farms in the United States.
ICE CREAM AND REALTED PRODUCTS
Ice cream was known in England in the early 1700s, but was still a rare item when served in the United States to White House guests by Dolly Madison in 1809. Today, multi cylinder continuous freezers can turn out over 1000 gal of uniformly frozen ice cream per hour. In the United States about 1 billion gal of ice cream and related products are consumed annually.
Composition of Ice Cream
Dairy ingredients in many forms used in the manufacture of ice cream and related products. These may include whole milk, skim milk, cream, frozen cream, butter, butter oil (which contains about 99% butterfat), condensed milk products, and dried milk products. Ice cream is composed of milk fat (butterfat) and milk solids-nonfat (MSNF) derived from these ingredients, plus sugar, stabilizer, emulsifier, flavoring materials, water, and air.
The mixture of these constituents, before the air is incorporated and the mixture frozen, is known as the ice cream mix. The mix composition may be made richer or leaner in fat. MSNF, and total solids, depending on market requirements; but in addition, a mix of chosen fat and MSNF composition can be formulated from various combinations of the basic dairy ingredients. In typical commercial operations the supply and cost of dairy ingredients varies throughout the year, and so the ice cream plant manager frequently adjusts mix formulas to keep the overall ice cream composition constant at the lowest possible cost.
Typical compositions of commercial ice creams and related products are listed in Table 13.4. A good average ice cream would contain about 12% milk fat, 11% MSNF, 15%, sugar, 0.2% stabilizer, 0.2% emulsifier, and a trace of vanilla. This would give 38.4% total solids and the remainder would be water. To this might be added other ingredients such as nuts, fruit, chocolate, eggs, and additional flavorings.
Deluxe and French ice cream may have 18% fat; economy ice creams, 10% fat; and ice milk products only 4% fat. Fruit-flavored sherbets usually contain less than 2% fat, and fruit ice generally contain no fat.
Milk fat is the most expensive major ingredient of ice cream, and so the higher the fat content, generally the more expensive the product. State and federal regulations covering compositions of frozen desserts are largely based on milk fat and total milk solids contents. For example, according to federal standards, plain ice cream may contain no less than 10% milk fat and 20% total milk solids, whereas fruit, nut, or chocolate ice cream may contain no less than 8% milk fat and 16% total milk solids. There are also allowances for other ingredients. Products with learner compositions may not be called ice cream. Federal standards also specify minimum compositions of ice milk and other frozen desserts. Proposals to change the standards have recently resulted in regulations permitting replacement of up 25% of the MSNF in ice cream and related products with whey solids. Standards are subject to further changes, especially as the demand for lower-fat products increases.
These compositions are based on the ice cream exclusive of air, that is, percentages are based on weight of the ice cream. But ice cream is made to contain a great deal of air and is truly a whipped product. This air, uniformly whipped into the product as small air cells, is necessary to prevent ice cream from being too cold in the mouth.
The increase in volume caused by whipping air into the mix during the freezing process is known as overrun. The usual range of overrun in ice cream is from 70% to 100%. If ice cream has 100% overrun, then it has a volume of air equal to the volume of mix that was frozen. In other words, 1 liter of mix makes 2 liters of frozen ice cream of 100% overrun. The overrun of any ice cream can be calculated from the formula:
%overrun = formula
The maximum allowable overrun also is specified in federal and state standards by defining the minimum permissible weight per volume of product.
Functions of Ingredients
Each of the major ingredients in ice cream serves specific functions and contributes particular attributes to the final product. Milk fat gives the product a rich flavor and its smooth texture and body. The fat also is a concentrated source of calories and contributes heavily to the energy value of ice cream. Milk solids-nonfat contribute to the flavor and also give body and a desirable texture to ice cream. Higher levels of MSNF also permit higher overruns without textural breakdown. Sugar not only adds sweetness to the product but lowers the freezing point of the mix so that it does not freeze solid in the freezer. The sugar may be sucrose from cane or beet sources or it may be dextrose from corn syrup, or dextrose –fructose mixtures.
The stabilizers used in ice cream are generally gums such as gelatin, gun guar, gum karaya, seaweed gums, pectin, or manufactured gums of the carboxymethyl cellulose type, which are cellulose derivatives. The stabilizers form gels with the water in the formula and thereby improve body and texture. They also give a drier product, which does not melt as rapidly or leak water. The stabilizers by binding water also help to prevent large ice crystals from forming during freezing, which would give the product a coarse texture.
Egg yolk is a good natural emulsifier due to its content of lecithin. Commercial emulsifiers are numerous and generally contain monoglyceride and Diglycerides. Emulsifiers help disperse the fat globules throughout the ice cream mix and prevent them from clumping together and churning out as butter granules during the freezing mixing operation. Emulsifiers also improve shipping properties to reach desired overrun and further help to make ice cream dry and stiff.
Flavors give ice cream variety and consumer appeal. Vanilla is still the most popular flavor, followed by chocolate, strawberry, and a very large number of fruit, nut, and other combination.
Manufacturing Procedure
The first step in preparing ice cream mix is to combine the liquid ingredients in a mixing vat and bring them to about 43C. The sugar and dry ingredients are next added to the warm mix which helps dissolve them. Gross particulates such as nuts or fruits are not added at this time since they would be disintegrated during subsequent processing. Instead, they are added during the freezing step.
Pasteurization
The mix is now pasteurized by a batch or continuous heating process. Pasteurization temperatures are higher than for plain milk since the high fat and sugar contents tend to protect bacteria from heat destruction. Common temperatures for batch pasteurization are 71 C for 30 min, and for continuous HTST pasteurization, 82C for 25 sec. expect for the higher temperature, pasteurization equipment is much the same as that used for milk.
Homogenization
The pasteurized mix is next homogenized at the temperature it comes from the pasteurizer. A two-stage homogenizer may be used (Fig. 13.6) with the mix pumped at a pressure of 1.7 X 107 Pa (2500 psi) through the first-stage valve and 4.1X 106 Pa (600 psi) through the second-stage valve. Homogenization breaks up fat globules and fat globule clumps and, together with the added emulsifiers, prevents churching of fat into butter granules during the freezing operation. Homogenization also improves the overall body and texture of ice cream. After homogenization, the mix is cooled to 4.4 C.
Ageing the Mix
The mix is held anywhere from 3 to 24 h at a temperature of 4.4C or lower in vats. During ageing the melted fat solidifies, the gelatin or other stabilizer swells and combines with water, the milk proteins also swell with water, and the viscosity of the mix is increased. These changes lead to quicker whipping to desired overrun in the freezer, smoother ice cream body and texture, and slower ice cream melt-down. Some manufacturers of stabilizers and emulsifiers claim that through the use of their products, ageing time may be drastically reduced or even eliminated, but ageing is still employed in many ice cream plants.
Freezing
The mix is now ready to be frozen. The cold, thoroughly blended mix is pumped to a batch or continuous freezer. Continuous freezers with multiple freezing chambers (Fig. 13.7) are more common in large manufacturing operations.
Mix and air enter the freezing cylinders, which are chilled by circulating refrigerant between double walls. The main purposes of the freezing operation are to freeze the mix to about -5.5C and to beat in and subdivide air cells.
Freezing must be quick to prevent the growth of large ice crystals that would coarsen texture, and air cells must be small and evenly distributed to give a stable frozen foam. These are accomplished within the freezing chamber, which is of the scraped-surface type described in Chapter 9. The freezing chamber is provided with a special mixing element or dasher (see Fig. 5.6). The rotating dasher with its sharp scraper blades shaves the layers of frozen ice cream off the inner freezer wall as they are formed. This prevents buildup of an insulating layer, which would decrease freezing efficiency of the still colder freezer wall. The ice cream scrapings mixed into the remaining mix in the freezing cylinder also serve to seed the mix with small ice crystals, which speeds freezing of the mass. The dasher’s rods and bars also beat air into the freezing mass, much as in the shipping of cream or egg white.
Mix passing through the freezer cylinder its frozen and whipped in about 30 sec or less to a temperature of about -5.5 C. At this temperature not all of the water in frozen, and the ice cream is semisolid; in this condition it is easily pumped out of the cylinder as a continuous extrusion by the incoming unfrozen mix and the propelling action of the dasher.
The semisolid ice cream emerging from the freezer goes directly into packaging cartons or drums. The consistency of the ice cream entering the cartons is that of soft ice-cream-like products sold at roadside stands.
Various kinds of nozzles and filling attachments are available to make novelty ice cream as the product is extruded from the freezer cylinder. Thus, for example, three flavors can be pumped from three freezer cylinders through a three-compartment square nozzle to give the common vanilla, chocolate, strawberry block.
Ice Cream Hardening
Cartons of semisolid ice cream are placed in a hardening room where a temperature of about -34C is maintained. Storage in the hardening room freezes most of the remaining water and makes the ice cream stiff. When stiff, the product is ready for sale.
Physical Structure of Ice Cream
The physical structure of ice cream should be understood, since changes in physical structure are the cause of several common defects in this product. As mentioned earlier, ice cream is a foam containing cells which constitute overrun, and that the overrun will give ice cream with approximately twice the volume of the original mix. The dairy foam illustrated in Fig. 13.8 is similar to the foam in ice cream. In the frozen ice cream foam, the films of mix surround the air cells. The fat globules are dispersed within the films or layers of mix. Also within the films are the frozen ice crystals. As ice cream ages in storage, foams can shrink. In addition, weakened films of mix can collapse, and represents a serious defect. Figure 13.9 is a photomicrograph of the internal structure of ice cream with more detail. The white area marked “b” are air cells. All the rest are films of frozen mix surrounding the air cells. Within the films are ice crystals solidified fat globules, and insoluble as well as dissolved sugars, salts, proteins, and other mix constituents. If the ice crystals marked “a” become too large, as occurs when fluctuating storage temperatures permit repeated partial thawing and refreezing, the ice cream becomes coarse and icy. If there is too much lactose from excessive milk solids and it should crystallize out, the ice cream becomes grainy or sandy. In addition to foam collapse and loss of overrun from formulas low in solids, excessive shrinkage can result from partial melting at too high a freezer storage temperature. Shrinkage due to mechanical compaction also occurs when ice cream is dipped from tubs to make cones; this is called dipping loss. Other textural defects make ice cream gummy, crumbly, curdy, watery, and so on, due largely to poor mix formulations. Ice cream also may have flavor defects common to other dairy products; this include cooked flavor, oxidized flavor, or even rancidity if made from off-flavor dairy ingredients. In addition, ice cream may have a host of unnatural flavors from poor quality flavoring ingredients.
Other Frozen Desserts
Many frozen desserts other than ice cream are available. They differ in their compositions (Table 13.4) and physical characteristics. Most are manufactured with much the same equipment and in accordance with the same principles used in making ice cream. Several of these frozen desserts are simply varieties of ice cream. Thus, we have plain ice cream, fruit ice cream, with about 8-14% milk fat: deluxe ice creams with about 16-20% milk fat; French ice cream and frozen custard, which contain liberal quantities of egg yolk; parfait and spumoni, which are high in fat and generally also contain fruits and nuts; and other products.
Figure 13.8 and 13.9
Lower-fat products include “frozen shakes” and “soft ice milk” with milk fat contents from about 6% down to about 3%. These are the popular products served directly from the freezer at drive-in restaurants and other retail establishments. In most marketing areas, these products may not be called ice cream but instead are referred to as “soft serve” or by trade names.
Sherbets usually contain less than 2% milk fat and corresponding low levels of other milk solids. Sherbets, which are principally, water, sugar, and tart fruit flavorings, have low overruns of 30-40%. Ices are similar to sherbets but generally contain no dairy products and also have low overruns of 25-30%.
Many of these products and related items are loosely referred to by different names in different areas and parts of the world. Standards for several of these products, however, are quite specific. A newcomer among frozen desserts in frozen yogurt, which is made from milk or milk solids that have been fermented by lactic acid producing organisms.
Some ice cream and ice milk-type products are being manufactured with vegetable fat in place of milk fat with a cost advantage in manufacture. Federal law requires that such products be appropriately labeled if they move in interstate commerce. This is to protect dairy interests and to eliminate any question of intended deception. There are also a number of products in which all or part of the fat has been replaced with a fat substitute in an attempt to maintain the desirable texture of fat without the calories. The success of these products varies and no fat substitute has completely mimicked all properties of fat in ice cream and related products.
CHEESE
In addition to being delightful foods that contribute variety and interest to our diets, cheeses of various kinds always have been important sources of nutrients wherever milk-producing animals could be raised. Whereas today the gourmet may pay several dollars per pound for cheese, at the other extreme, in less developed regions where milk rapidly spoils because of lack of refrigeration, cheese may be a staple of diet sometimes made under the most primitive conditions.
Kinds of Cheese
Cheese may be defined as a product made from the curd of the milk of cows and other animals, the curd being obtained by the coagulation of the milk casein with an enzyme (usually rennin), an acid (usually lactic acid), and with or without further treatment of the curd by heat, pressure, salt, and ripening (fermentation) with selected microorganisms. This broad definition, however, does not cover all cheeses, since some are made from milk whey solids that remain after removal of coagulated casein. Further, vegetable fats and vegetable proteins are being used in cheese-like products.
Cheese-making is an old process and still retains aspects of an art even when practiced in the most modern plants. Part of this is due to the natural variation common to milk and the imperfect controllability of microbial populations. The basic cheese types evolved as products of different types of milk, regional environmental conditions, accidents, and gradual improvements by trial and error. There are over 800 names of cheeses, but many of the names describe similar products made in different localities or in different sizes and shapes. Of these, however, there are basically only about 18 distinct types of natural cheeses, reflecting the different processes by which they are made. These include brick, Camembert, Cheddar, cottage, cream, Edam, Gouda, hand, Limburger, Neufchatel, Parmesan, Provolone, Romano, Roquefort, sapsago, Swiss, Trappist, and whey cheeses.
The way some of the multiplicity of subtypes and different names has arisen can be illustrated by Cheddar cheese. In Fig. 13.10 are shown types of cheese hoops in which Cheddars may be pressed, giving rise to different sizes and shapes. The cheeses then get such names as Longhorns, Picnics, Daises, Twins, and so on. But they all are of the Cheddar type.
Classification by Texture and Kind of Ripening
A useful means of classifying the types and important varieties of cheese is indicated in Table 13.5. It is based largely on the textural properties of the cheeses and the primary kind of ripening. Thus, there are hard cheeses, semi hard cheeses, and soft cheeses, depending on their moisture content; and they may be ripened by bacteria or molds, or they may be unripening. The bacterial my produce gas, and so for eyes as in the case of Swiss cheese, or they may not produce gas as in the case of Cheddar and so on eyes are formed.
Figure 13.10
Table 13.5 Classification of Cheese
Soft
Un-ripened:
Low Fat –cottage, pot, bakers.
High fat –cream, Neufchatel (as made in United States).
Ripened: Bell Pease, brie, Camembert, cooked, hand, Neufchatel (as made in France).
SEMISOFT
Ripened principally by bacteria: brick, Munster,
Ripened by bacteria and surface microorganisms; Limburger, Port du Salut, Trappist.
Ripened principally by blue mold in interior; Roquefort, Gorgonzola, Blue, stilton,
Wensleydale.
HARD
Ripened by bacteria, without eyes: Cheddar, Granular, Caciocavallo.
Ripened by bacteria, with eyes: Swiss, Emmentaler, Gruyere.
VERY HARD (grating):
Ripened by bacteria: Asiago old, Parmesan, Romano, sapsago, Spalen.
PROCESS CHEESE
Pasteurized, cold –pack, related products.
WHEY CHEESE
Mysost, Primost, Ricotta.
SOURCE: Sanders (1953).
Among the soft and semisoft cheeses, Limburger is ripened primarily bacteria, and Camembert by a mold; cottage cheese is not ripened.
The classification is extended to include “process” cheeses, which are essentially melted or blended forms of the above cheeses, and whey cheeses, which are made from the whey remaining after coagulation and removal of the casein. Whey cheeses are high in beta-lactogtobulin and alpha-lactalbumin, the second and third principal proteins in amount in milk. These are not coagulated by rennin or by the acid in most cheese-making processes and so they remain soluble in the whey. However, they can be easily coagulated from the whey as curds by heating.
All of the major types of cheese can fit into a classification such as this. The approximate percentage compositions of several of these cheeses are given in Table 13.6.
Cheddar Cheese –Curd-making and Subsequent Operations
All cheese types begin with curd-making and then involve various manipulations of the curd or whey. Illustrative of the cheese-making process is the preparation of Cheddar, the most popular cheese in the United States, Canada, and England.
Milk contains fat, proteins (principally casein, less beta-lacto globulin and still less alpha-lactalbumin), lactose, minerals, and water. When acid/or the enzymes rennin are added to milk, the casein coagulates, trapping much of the fat, some of the lactose, and some of the water and minerals in the coagulant. This is the curd. The remaining liquid which contains dissolved lactose, proteins, minerals, and other minor constituents is the whey. In making Cheddar cheese, curd is formed under controlled conditions of temperature, acidity, and rennin concentration. This gives curd of the desired moisture content and texture for subsequent processing.
Cheese curd can be made from raw or pasteurized milk. When it is made from raw milk, the FDA requires that the finished cheese be ripened for 60 days or more as safeguard against pathogens; such storage under the acid conditions of the cheese inhibits the common disease-producing organisms that could be present in the milk. But most Cheddar cheese is made from pasteurized milk, since pasteurization also destroys most spoilage types of organisms and undesirable milk enzymes and gives better control over subsequent fermentation of the curd.
Setting the Milk
The pasteurized whole milk is added to a vat and brought to about 31C; a lactic acid-producing starter culture of Streptococcus lactis is added at a level of about 1.0% based on the milk. At this point, natural color may be added to the stirred milk is the Cheddar is to be of the orange-colored type. After about 30 min, a mildly acidic condition of about 0.2% acidity (calculated as lactic acid) will exist and the rennin enzyme in the form of a dilute solution is added. The commercial rennin preparation is known as rennet. Whereas the name rennin (also called chymosin) refers to the pure enzyme, commercial rennet, obtained from the fourth stomach of the calf, contains rennin and small amounts of other materials. Rennin link enzymes also are produced by selected microorganisms and are available to the cheese maker as commercial microbial rennet. Recently, rennin has been produced from microorganisms which have been genetically altered to produce the animal enzyme. The mild acidity improves the coagulating ability of the rennin.
Stirring is now stopped and the milk is allowed to set. In about 30 more minutes a uniform custard like curd forms throughout the vat. Acid continues to be formed, as it will throughout the curd-making operation. The combination of rennin and acid forms a curd with a desirable elastic texture which, when subsequently heated or pressed, will shrink and squeeze out much of the trapped whey.
Cutting the Curd
The next step after setting is cutting the curd. This is done with curd knives that are made up of wires strung across a frame. One knife has the wires going vertically and the other horizontally. By drawing the knives through the length of the vat and then back and forth with the width of the vat, the curd is cut into small cubes. In the case of Cheddar, these are 0.25-0.5 in. on a side. The smaller the cube is, the greater the surface area, and so the quicker and more complete is the removal of whey from the cubes, which can lead to a drier cheese. The curd for different types of cheese, therefore, is cut into different size cubes.
Cooking
After cutting, which may take only 5 -10 min, the cubes are gently agitated and the jacketed vat is heated with steam to raise the temperature of the curds and whey. The temperature is bought to about 38C over a 30-min period and held at 38C for about 45 min longer. This is known as “cooking.”
Cooking at 38C further helps to squeeze they whey form the curd cubes. Heat increases the rate of acid production and makes the curd cubes shrink. Both help expel the whey and toughen the curd cubes, which now take on a more rounded cottage-cheese-like form. During cooking, the curds continue to be gently agitated.
Draining whey and Matting Curd
Agitation of the curds is stopped and they are permitted to settle. The whey is drained from the cheese vat and curds are trenched along the sides of the vat to further facilitate whey drainage. After all of the whey has been drained, the curds are allowed to mat for about 15 min. during matting, the individual curd pieces-fuse together to from a continuous rubbery slab (Fig 13.11)
The process of matting and subsequent handling of matted curd is known as a cheddaring and it unique to the production of the Cheddar type of cheese. Cheddaring involves cutting the matted curd into blocks, turning the blocks at 15-mi intervals, and then pilling the blocks one another, two or three deep. The purposes of cheddaring are to allow acid formation to continue and to squeeze whey from the curd. The purposes of cheddaring are to allow acid formation to continue and to squeeze whey from the curd. The weight of the blocks on one another is a milked form of pressure. During cheddaring, the vat is maintained warm. The cheddaring operation of stacking and turning the blocks goes on for about 2 h or until the whey coming from the blocks reaches 0.5-0.6% acid.
Figure; 13.11
Milling and Salting
The curd locks or slabs are now ready for the milling and slating operations. The rubbery slabs of cheddar curd are passed through a mill that cuts the blocks into small pieces. The milled pieces are spread out over the floor of the vat and sprinkled with salt. The amount of salt is about 2.5% of curd weight. The salt and curd are stirred to uniformly distribute the salt. The purposes of the salt are threefold: to further draw whey out of the curd by osmosis; to inhibit proteolytic and other types of spoilage organisms that might otherwise grow in later stages of the cheese making operation; and to add flavor to the final cheese.
Pressing
The milled and salted curd pieces are placed in hoops fitted with cheesecloth and the hoops are placed in a hydraulic press. Pressing at about 1.4X 105 Pa (20 psi) continues overnight. Pressing determines the final moisture that the finished cheese will have. The more moisture or whey retained in the cheese from the press, the more acidity that can be fermented from it. This, in turn, affects the final texture of the cheese and what microorganisms can grow during the subsequent ripening period. And, of course, pressing determines the final shape of the cheese.
Curing or Ripening
After overnight pressing, the cheese is removed from the hoop and placed in a cool room at about 16C and 60% RH for 3 or 4 days. This causes mild surface drying and forms a slight rind. To prevent mold from growing on the surface of the cheese, the cheese block or wheel is vacuum packaged in flexible film or dipped in hot paraffin. The film or coating, in addition to preventing surface molding, prevents the cheese from excessive drying out during the long ripening or ageing period. The cheese is boxed and placed in the curing room for ripening. The curing or ripening room generally will be at about 2C and 85% RH.
Ripening is continued for at least 60 days whether the cheese milk was raw or pasteurized. For peak flavor, ripening may be continued for 12 months or longer. During this period, bacteria in the cheese and enzymes in the rennet preparation modify the cheese texture, flavor, and color by continuing to ferment residual lactose and other organic compounds into acids and aroma compounds, by partial hydrolysis of the milk fat and further breakdown of fatty acids, and by mild proteolysis of the protein. In the case of Cheddar, these changes are comparatively mild because of the types of organisms present (primarily lactic acid types) and the relatively low moisture content. The flavor is correspondingly milk when compared, for example, to Roquefort of Limburger cheese.
Advanced Process
As pointed out, there is much hand labor in conventional Cheddar-making, and so considerable research has been directed to the development of mechanized and continuous cheese-making process. Several mechanized schemes have been devised over the past 30 or so years, including the Ched-O-Matic process in the United States and various other processes in Europe and Australia. Most of these processes retain the classical steps of conventional Cheddar production and replace hand operations with mechanical equivalents.
An important departure from conventional cheese-making has involved cold milk renneting. In conventional cheese-making, rennet is added to warm milk and the system is permitted to set and form the curd. If rennet is added to cold milk, the casein is altered but he milks remains a liquid. If such milk is then heated to 32C instantaneous gelling occurs. It is thus possible to work with a liquid system, which is more easily pumped, metered, and so on, and then generate coagulated curd continuously by passing the liquid through a heat exchanger.
More recent advances utilize the treatment of milk by reverse osmosis and ultrafiltration. Not only do these treatments concentrate milk solids for efficient further processing but by careful choice of membranes the ratios of retained milk solids to whey can be altered. Thus, lacto globulin can be retained with the cheese solids rather than being lost to the whey, improving cheese yield and nutritional values; likewise, lactose levels can be reduced when less acidity is desired. The automated ultrafiltration system shown in Fig. 13.12 contains numerous membrane cartridges connected in series and provided with recirculation loops for progressive separation and concentration of milk solids. Such practices are increasingly influencing cheese-making methods and cheese properties.
Figure 13.12
Cottage Cheese
Cottage cheese is an example of a low-fat, soft cheese, generally coagulated with lactic acid rather than rennin. The curd is left in particulate from, that is, is not pressed. Further, it is not aged or ripened. Starting with pasteurized skim milk rather than whole milk, the curd-forming operations for low-fat cottage cheese have similarities to the early stages of Cheddar-making. The steps are as follows:
1. Pasteurized skim milk is warmed in a vat to 22C.
2. A lactic starter (at about 1% level) is added to produce acid. In addition to S. lactis, the starter usually contains Leuconosoc citrovorum, a flavor-producing bacterium.
3. The vat is set and fermented for about 14 h (long-set method).
4. The coagulated milk is cut into small cubes.
5. The curd cubes are cooked for about 90 min with stirring and the temperature is gradually increased to 50C.
6. After cooking, the whey is drained and the curd is washed with cold water to remove excess whey and limit acidity.
7. The curd is trenched to drain all water.
8. The curd may now be mildly salted, as a mild preservative measure and for flavor.
9. The curd also may be blended with sweet or soured cream to give 2% or 4% fat. The product is then called creamed cottage cheese.
Cottage cheese is packaged as loose curd particles and undergoes no further processing. It is highly perishable and must be kept refrigerated.
The principal variations in cottage-cheese-making have to do with the length of fermentation time in the vat. The 14-h holding time at 22C is known as the long-set methods. By using a larger amount of lactic starter (about 6%) and a temperature of 32C, the proper degree of acidity for coagulation and curd cutting can be reached in 5h; this is known as the short-set method. Another variation employs low levels of rennet plus the starter for milk coagulation.
Swiss Cheese
Like Cheddar, Swiss is a hard-type cheese. However, it is characterized by the formation of large holes, or eyes, and a sweet nutty flavor, which result from the activities of an organism known as Propionibacterium sherimanii. This organism follows the lactic acid organisms and further ferments lactic acid (nor in the form of lactate) to propionic acid and carbon dioxide. The propionic acid contributes to the nutty flavor, and the carbon dioxide gas collects in pockets within the ripening curd and forms holes or eyes.
Swiss cheese, also known as Emmental cheese, generally is made from raw milk. A multiple-organism starter containing lactic acid organisms, including Lactobacillus bulgaricus and heat-tolerant Streptococcus thermophiles, which produces lactic acid through the rather high cooking temperatures (about 53C) that the curd reaches during processing, is added. The starter also may contain the eye-forming Propionibacterium or this may come in with the raw milk. Following an initial period of lactic acid fermentation, rennet is added to the kettle to coagulate the milk. The curd is cut with a harp like wire knife into rice-size particles. The curds and whey are now heated and cooked at about 53 C for about 1 h.
Unlike Cheddar-making, at this point the stirred heated curd is allowed to settle, a cloth with a fitted steel strip edge is slid under the curd, and the entire curd mass is hoisted from the kettle to drain (Fig. 13.13).
The entire curd from a kettle is placed in a single large hoop in which it is pressed for 1 day to begin forming a rind. The cheese wheel, which may weigh more than 90 kg, is removed from the hoop and placed in a large brine tank at about 10C. it floats in this brine for about 3 days and its top is periodically salted. The salt removes still more water from the cheese surfaces than could be removed by pressing and thus produces the heavy protective rind.
The cheese is next removed from the brine to a warm ripening room maintained at about 21C and 85% RH. The cheese remains here for about 5 weeks during which time the eyes are formed by the growth of the Propionibacterium. As the eyes are formed, the cheese becomes somewhat rounded (Fig 13.14). The opening of the eyes also changes the sound of the cheese when it is thumped with the finger. After about 5 weeks, the cheese is moved to a colder curing room at about 7C. Here it remains from 4 to 12 months to develop the full sweet nutty flavor associated with Swiss cheese.
Figure: 13.13 and 13.14
In judging the quality of Swiss cheese much emphasis is given to the size, shape, and gloss of the eyes (Fig. 13.15). This is not for appearance alone. Proper eye formation is an index of several other quality factors. For example, if the acidity is not properly controlled to produce a chewy elastic texture, then the curd would not be able to stretch and form the eye under the pressure of the generated carbon dioxide. Thus, excessive acid gives a brittle cued which forms cracks rather than eyes. So the eyes also are an index of texture. Similarly, the same organism that forms eyes produces the propionic acid necessary for the sweet nutty flavor. Good eye formation indicates active fermentation by this organism and well-developed flavor.
Blue-Veined Cheeses
Blue-veined cheeses are characterized by a semisoft texture and blue mold growing throughout the curd. There are four well-known varieties of blue-veined cheeses. Three are made from cow’s milk: blue cheese, made in Denmark, the United States, and other countries; stilton, made in England; and Gorgonzola, made in Italy. The fourth and perhaps the most famous blue-veined cheese is Roquefort, which is made from sheep’s milk and is made in the Roquefort region of France.
All of the blue-veined cheeses acquire the characteristic blue marbling by having their curd inoculated with the blue-green mold Penicillium Roquefort prior to being hooped and pressed. Mold growth is encouraged during the ripening period which can be from 3 to 10 months at cool, moist, cave like, conditions of about 4 C and 90% RH.
Figure 13.15
Molds are aerobic and so generally grow on the surface of cheeses and other foods. to permit the mold to grow throughout the cheese mass, it is common practice to pierce the pressed cheese when it is placed in the curing room. This allows air to penetrate the cheese and support mold growth throughout the mass. The blue-green color is from spores of the mold; in Fig. 13.16 the darkened lines where molds growth is heavy are visible along the pierced air channels. Penicillium Roquefort not only produces the mottled blue color but is an active splitter of milk fat. This gives rise to free fatty acids and ketones, which contribute to the sharp, peppery flavor of blue-veined cheeses.
Camembert Cheese
Another mold-ripened cheese is Camembert, which, like Roquefort, originated in France. However, this cheese is characterized by a soft cream-colored curd and a white felt like mold growth that covers its entire surface (Fig. 13.17). The mold is Penicillium camembert, and it is inoculated onto the pressed cheese curd after removal from the hoop by spraying a must of mold spores onto the cheese surfaces. Ripening, as in the case of Roquefort, is under damp conditions at about 7C and 95% RH. But ripening time is only about 3 weeks. Penicillium camembert is highly proteolytic and breaks down the curd protein, from the surface inward, to the texture of soft butter. If proteolysis goes too far because of prolonged storage in the supermarket or the home, the cheese develops a strong ammonia Cal odor.
Limburger Cheese
Limburger is a semisoft cheese which, like Camembert, is ripened from the surface inward with a characteristic proteolytic decomposition. However, the major ripening agent is a surface bacterium called Brevibacterium linens.
Process Cheese
All of the cheeses described thus far are referred to as natural cheeses; that is, they are produced through a series of natural curd-making and ripening operations (cottage cheese is unripen). Process cheese is the name given to cheese made by mixing or grinding different lots of natural cheeses together and then melting them into uniform mass.
This is done in part because different lots of natural cheese vary in moisture acidity, texture, flavor, age, and other characteristic. A highly acidic cheese, for example, can be blended with a bland cheese to yield a more acceptable product. However, process cheeses have become so popular in their own right that they have necessitated plants for the production of natural cheese solely intended for conversion to process cheese.
Figure 13.17
In making process cheese, the mixed lots are melted together by heating to about 71C. This also pasteurizes the cheese. Emulsifiers such as sodium citrate and disodium phosphate are added to prevent fat separation and to add smoothness to the texture. The hot melted cheese is then filled into cartons and allowed to cool and solidify. The best known process cheese is the popular American cheese made from blended and melted Cheddar.
Process cheese may be used to prepare process cheese foods and spreads by mixing in additional dairy ingredients, fruits, vegetable, meats, and so on. The designations “Process Cheese Food” and “Process Cheese Spread” may, be used only when the final products meet minimal federal standards with respect to fat and solids. Thus, “Process Cheese Food” must contain no less than 23% fat and no more than 44% moisture.
Cheese Substitutes
Various cheese substitutes, also referred to as cheese analogs, imitation cheese, and so on, are increasingly entering the marketplace (Fig. 13.18). They commonly have all or some of the milk fat replaced with vegetable fat and vegetable protein. The incentives for developing such products are lower cost, ready availability of substitute ingredients, changing consumer tastes, and real or perceived health benefits.
Figure; 13.18
Newer products for which there is demand include cheese substitutes with reduced levels of fat, cholesterol, and sodium.
Other Dairy Products
There are several other related dairy foods that enjoy popularity. Junket dessert is sweet milk plus flavor that is coagulated with rennet to a custard like consistency. Cultured buttermilk is pasteurized skim milk (or partially-skimmed milk) mildly coagulated with lactic acid culture (containing Leuconostoc bacteria for flavor) and consumed as a beverage. At one time, sour buttermilk was the liquid drained from the butter churn and allowed to ferment naturally, but today’s cultured buttermilk is the result of a well-controlled process. Sour cream is fresh pasteurized cream mildly coagulated with lactic acid culture plus Leuconostoc flavor bacteria. It is higher in fat and heavier in consistency than culture buttermilk.
Acidophilus milk is pasteurized milk or low-fat milk inoculated with Lactobacillus acidophilus, which is believed by some to provide health benefits by favorably altering the microflora of the intestinal tract. In the past, the popularity of this product was limited by the flavor developed during fermentation. A more recent product has over-come this by adding the live organisms to pasteurized milk and refrigeration to prevent subsequent fermentation and flavor development.
Yogurt is pasteurized milk or low-fat milk coagulated to a custard like consistency with a mixed lactic acid culture containing Lactobacillus bulgaricus and Streptococcus thermophiles. It is most often flavored with fruit preserves or other ingredients but is also consumed unflavored.
REDUCED FAT DAIRY PRODUCTS
Although dairy products are nutritionally important in the diet, especially of the young, there has been a desire to reduce not only calories but also saturated fat and cholesterol in the diet. Therefore, a number of reduced-fat and low-fat dairy like products have appeared in recent years. Most of these products substitute nonfat food ingredients for all or a portion of the animal fat in dairy products. This is different than products in which vegetable fat replaces animal fat. Often the fat substituents have calories themselves, but because the caloric density of fat is more than twice that of proteins or carbohydrates, replacement on an equal-weight basis results in an overall reduction in calories. One popular category of product is low-fat ice cream which has the eating characteristics of full-fat ice cream with less fat.
A wide variety of food ingredients have been used as fat replacers but all have the common property that they are designed to replace the functional characteristics of fat. The most important characteristic of fat is its texture or mouthfeel. As pointed our earlier, fat gives lubricity to foods, including dairy products such as ice cream. One fat substitute is made of proteins which have been processed into extremely small particles. When these particles are suspended in water (or ice milk) they impart a creamy texture to the fluid which closely mimics suspended fat particles. This creaminess is perceived in the mouth as similar to fat in such products as ice cream.
Other fat replacers are carbohydrate based. These food ingredients often bind large amounts of water and act as thickening agents. Like the particles of proteins, this thickening can be perceived by the mouth as resulting from the inclusion of fat.
Further improvements in fat replacers can be expected. One product that has been waiting for FDA approval is a fat which is non absorbable. This food ingredient has the functional properties of fat in foods but is not digestible or absorbable by humans and, thus, does not contribute to calories or fat intake. Whether or not this material will be approved remains to be seen.
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