DAIRY PRODUCTS

Foods made from milk. This entry discusses milk composition and the processing techniques by which dairy products are made. Some of these dairy products include fluid milk, cheese, frozen desserts, butter, dry milk, yogurt, condensed milk products, and by-products.

Introduction

Since the beginning of recorded history, humans have used the milk of mammals as a food source. In the early days, each family depended on its own animals for milk. Later, dairy farms and processing plants, known as creameries, were developed close to cities to pasteurize, package, and distribute milk. Dairy products in rural America consisted at first only of raw milk, cream, butter, and cheese. With the development of the dairy industry came improved processing and packaging methods and refrigeration.

Milk Composition and Quality

Cow’s milk consists of water (87 percent), fat (3.9 percent), protein (3.2 percent), lactose (4.6 percent), and minerals (0.7 percent). These constituents vary with breed of cow, feed, stage of lactation, health and age of the animal, and environmental conditions.

The delicate, buttery flavor of dairy products is due in large part to the fat content. Milk protein, composed of two major fractions, caseins and whey proteins, is important in human nutrition. Lactose, or milk sugar, is essential for early brain development and gives milk its slightly sweet taste. Milk is an excellent source of calcium, phosphorus, and riboflavin, but a poor source of vitamin D. Consequently, vitamin D-fortified milk has been sold since the 1920s to prevent rickets in children. Because of milk’s highly regarded nutritional status, it is often characterized as “nature’s most nearly perfect food.”

Prior to 1900, raw cow’s milk generally was cooled only before consumption. Since then scientists have shown that diseases may be spread through milk that becomes contaminated by infected animals or by humans who are carriers of infectious diseases. Thus, to safeguard its quality, essentially all milk is pasteurized. Pasteurization is the process by which milk is heated to a certain temperature for a time to destroy all pathogens and most other bacteria.

Dairy products are subjected to a variety of laboratory tests to ensure public safety and meet composition standards. Mastitis is an inflammation of the cow’s udder that results in the presence of somatic cells in milk. A high level of somatic cells (more than 750,000/milliliter [ml]) in milk is considered abnormal and should not be offered for human consumption. Improper treatment of mastitis with antibiotics may result in such medicines getting into the milk supply. Their presence is determined by a variety of sensitive and rapid tests that ensure a safe milk supply.

Dairy laboratories test both the raw milk and finished products for components such as fat and protein according to federal and state standards, which specify the level of fat required in certain dairy products. These tests are described in detail by Cunniff (1995) and are the basis for determining the purchase price of milk from the farmer.

Processing

Milk from the cow at approximately 34°C is cooled rapidly to 4.4°C or below to maintain quality. Cooled raw milk is hauled by tanker to dairy plants, where it is processed into milk products.

Separation. Centrifugal cream separators, introduced in 1890, use the force of gravity to separate the fat (cream) from the milk. Cream is added back to the milk stream to yield the desired fat content, or is used in other dairy products.

Homogenization. This process involves breaking the fat globules into small particles that form a stable emulsion in the milk. The fat globules do not rise by gravity to form a cream line. Today, most fluid milk products are homogenized.

A homogenizer is a high-pressure positive pump in which milk is forced through small passages under high pressure (14 to 17 MegaPascals [Mpa]) at velocities of approximately 180–245 meters/second (m/sec). The fat globules are broken up as a result of a combination of factors—shearing, impingement, distention, and cavitation. The fat globules in raw milk (1 to 15 [µm] in diameter) are reduced to 1 to 2 µm.

Pasteurization. The process of pasteurization, named after the French scientist Louis Pasteur, involves heating milk to kill pathogenic and most other organisms and to inactivate certain enzymes without greatly altering the flavor and nutritional content. The basic regulations are included in the Grade A Pasteurized Milk Ordinance, which has been adapted by most local and state jurisdictions.

Pasteurization may be done by batch or continuous-flow processes. In the batch process, each particle of milk must be heated to at least 62.8°C and held continuously at or above this temperature for at least 30 minutes. In the continuous process, the milk is heated to at least 71.7°C for at least 15 seconds. The latter is known as high-temperature, short-time pasteurization. Other continuous pasteurization processes using higher temperatures and shorter times, called ultrahigh temperature (UHT), are commercially employed. Following pasteurization, the product is cooled quickly to 7°C or less to prevent a cooked flavor.

Packaging. Milk packaging started when glass milk bottles were filled by hand and later by mechanical fillers. Plastic-coated paper milk cartons were introduced in 1932 and plastic milk containers in 1964. Milk packaging has progressed from the quart glass bottle to the half-gallon paper carton to the gallon plastic jug of today. Approximately 75 percent of fluid milk sold comes in such packages.

Aseptic packaging has developed in conjunction with high-temperature processing and has continued to make sterile milk and milk products a commercial reality worldwide. In the United States, UHT systems currently are processing fruit juices and some cream and ice cream mixes.

Dairy Products

Fluid milk products include milk, reduced-fat milks, and cream. Fluid milk contains 3.25 percent fat as defined by the Code of Federal Regulations (1995). Also available are a range of reduced-fat milks such as 2.0 percent, 1.5 percent, 1.0 percent, and skim milk. Creams are defined as products that contain not less than 18 percent milk fat, such as whipping cream (36–40 percent milk fat) and table, coffee, or light cream (18–30 percent milk fat). Half-and-half, an alternative to cream, is a mixture of cream and milk (10.5–12 percent milk fat).

Cheese. Cheese making is based on the coagulation of casein from milk to produce curds and whey. Casein is precipitated by acidification, which can be accomplished by adding bacteria that produce lactic acid from lactose. There are over 400 cheese varieties; the composition of many is listed in Wong et al. (1988). Over 32 percent of the total milk supply in the United States is used to make cheese. The most popular cheeses are cheddar and Italian varieties. Cheddar cheese is made by inoculating pasteurized milk with a lactic acid culture and rennet to coagulate casein. The coagulated milk is cut into cubes and cooked to remove whey. The whey is drained, and the curd cubes are allowed to knit closely together by the cheddaring process. At the end of this process, the curd is milled into smaller cubes and salted. The salted cheese is pressed overnight for further whey removal and aged up to a year for flavor development. Other cheese varieties use different cultures and cooking times. In the case of mozzarella, heating the cheese curds develops the stringiness seen on pizza.

Frozen desserts are popular in the United States, especially ice cream, which was first sold in New York City in 1777. Ice cream consists of milk fat (8–20 percent) and nonfat milk solids (8–15 percent), with a total solids content of 36–43 percent. Dairy ingredients include milk; cream; butter; and condensed whole, nonfat, or dry milk. Sweeteners include a blend of cane or beet sugar and corn syrup solids. Stabilizers that improve the body of ice cream include gelatin, sodium alginate, sodium carboxymethyl cellulose, pectin, and guar gum. Emulsifiers such as lecithin, monoglycerides, diglycerides, and polysorbates incorporate air and improve the whipping properties. A mixture of these ingredients is pumped to a freezer, which whips the mix to incorporate air and freezes it to ice cream.

Other frozen desserts include frozen yogurt, sherbet, and mellorine-type products, parfaits, ice cream puddings, novelties, and water ice products. New reduced-low-, and nonfat products and products containing low-calorie sweeteners are also on the market. Frozen desserts use 9.7 percent of the U.S. milk supply.

Butter contains over 80 percent milk fat with not more than 16 percent moisture. It was originally manufactured by churning farm-separated raw cream and had a relatively short shelf-life. Today a continuous operation with automatic controls is common. Per capita butter consumption has remained steady at 4 pounds for the past 20 years, using about 8 percent of the U.S. milk supply.

Dry milk is made by drum- or spray-drying to preserve milk in times of surplus. Approximately 5 percent of the milk supply is used for this product. Drying is preceded by concentrating milk in an evaporator. Drying takes place on a heated drum or by spraying milk under high pressure into a large stainless steel dryer, where it contacts heated air at approximately 200°C and evaporates to produce milk powder. Dry whole milk must be vacuum- or gas-packed to maintain quality during storage. Dry milk is a concentrated source of protein and lactose used in other manufactured food products. The moisture content for nonfat dry milk is 5 percent or less for standard grade and less than 3 percent for dry whole milk.

Yogurt is a fermented milk product that is increasingly consumed in the United States. Milk is fermented with Lactobacillus delbrueckii bulgaricus and Streptococcus thermophilus organisms that produce lactic acid and the characteristic yogurt flavor. Milk with 1–5 percent fat and 11–14 percent solid-nonfat is heated to about 82°C and kept at that temperature for 30 minutes, homogenized, cooled to 43–46°C, and inoculated with a 2 percent mixture of yogurt cultures. It is incubated at 43°C for three hours in a vat or in the final container and cooled and held at 4.4°C or lower. Fruit-flavored yogurts are common; 30 to 50 grams of fruit are placed in the bottom of the carton (sundae style) or mixed with the yogurt (Swiss style).

Condensed Milk Products. Evaporated milk contains at least 6.5 percent milk fat, 23 percent total milk solids, and 16.5 percent milk solids-nonfat. It is produced by condensing milk in a vacuum evaporator, packaging it in cans, and sterilizing it at 116–118°C for 15–20 minutes. It is subsequently cooled to room temperature within 15 minutes. Vitamins A and D and stabilizing salts, such as sodium citrate and disodium phosphate, may be added prior to sterilizing.

Sweetened condensed milk contains from 43 to 45 percent sugar, at least 8.5 percent milk fat, and 28 percent total milk solids. Condensed milk products are used widely in the manufacture of ice cream, baked goods, confectionery, and other food products.

By-products resulting from the separation or alteration of milk components are also of value. Lactose or milk sugar has about one-sixth the sweetening strength of sucrose and is used in infant formula, other processed foods, and pharmaceutical products. Casein is used to fortify flour, bread, and cereals and for glues and microbiological media. Many nondairy products such as coffee creamers, toppings, and icings use casein. Whey, the by-product of cheese making, is used widely as a dried or concentrated ingredient in other food products for its nutritive and functional value.

By John G. Parsons in "Encyclopedia of Rural America - The Land and People", Gary A. Goreham (Editor), ABC-CLIO, Inc.USA, 1997, excerpts p.177-179. Adapted and illustrated to be posted by Leopoldo Costa.