Dairy Products

Foods made from milk. This article 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 byproducts.

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, are 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. Because raw milk is a poor source of vitamin D, 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 on the farm generally was cooled only before consumption. Since then scientists have shown that diseases may be spread through milk if it becomes contaminated by infected animals or by humans who are carriers of infectious diseases. Thus, to safeguard its quality, essentially all milk now 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 tests in the laboratory to assure public safety and to 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/ml) in milk is considered abnormal and should not be offered for human consumption. Improper treatment of mastitis with antibiotics may result in such compounds getting into the milk supply. The presence of these chemicals is determined by a variety of sensitive and rapid tests that insure a safe milk supply.

Dairy laboratories tests 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 Latimer and Horwitz (2007) and Wehr (2004), and are the basis to determine the purchase price of milk from the farmer.

Processing

Milk at approximately 37°C from the cow 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 Mpa) at velocities of approximately 180 to 245 m/s. The fat globules are broken up as a result of a combination of factors such as shearing, impingement, distention and cavitation. The fat globules in raw milk (1 to 15 micrometers in diameter) are reduced to 1 to 2 micrometers.

Membrane Processing. Reverse osmosis, nanofiltration, ultrafiltration and microfiltration comprise membrane processes that are used in various applications. All processes generate two streams: concentrate and permeate. Reverse osmosis helps concentrate milk or whey by filtering water out. Nanofiltration is used to remove water, smaller ions and lactose. Ultrafiltration separates out water solubles such as minerals and lactose, and microfiltration is capable of removing bacteria and separate caseins and whey proteins from milk or whey. Thus, when milk is microfiltered, the permeate comprises bacteria-free milk. With certain types of membranes the permeate will comprise whey proteins and small milk components and the concentrate consists of casein. These processes pump milk or whey under pressure across the surface of a membrane to help achieve separation. The resulting products have unique characteristics that can be employed in other dairy and food products.

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 adopted 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 cooked flavor.

Packaging. In the early days, milk packaging started with hand and later with mechanical fillers, for glass milk bottles. 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, and represents approximately 75 percent of fluid milk sold.

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 U.S., 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 (2007). Also available are a range of fat-reduced milks such as 2-, 1.5-, and 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 to 40 percent), table, coffee, or light cream (18 to 30 percent), half and half, a mixture of cream and milk (10.5 to 12 percent).

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 Kosikowski and Mistry (1997). Over 32 percent of the total milk supply in the U.S. 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. 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 U.S., especially ice cream, which was sold first in New York City in 1777. Ice cream consists of milk fat (8 to 20 percent) and nonfat milk solids (8 to 15 percent) with a total solids content of 36 to 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 manufactured originally 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 4.0 lbs. 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 under high pressure into a large stainless steel dryer where it contacts heated air at approximately 200°C and evaporates the moisture 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 increasing rapidly in consumption in the U.S. Milk is fermented with Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus organisms that produce lactic acid and the characteristic yogurt flavor. Milk with 1 to 5 percent fat and 11 to 14 percent solid-nonfat is heated to about 82°C and held for 30 minutes, homogenized, cooled to 43 to 46°C, and inoculated with 2 percent culture. 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 less. 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, and 23 percent total milk solids and 16.5 percent milk solids not fat. It is produced by condensing milk in a vacuum evaporator, packaging in cans, and sterilizing at 116 to 118°C for 15 to 20 minutes. It subsequently is 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 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, bakery, confectionery and other food products.

Other products resulting from 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 coproduct of cheese making, is used widely as a dried or concentrated product in other food products for its nutritive and functional value. Sweet buttermilk, produced in the butter making process, is converted to a powder and used in the baking industry.

— John G. Parsons and Vikram V. Mistry

See also

• Agricultural Programs; Agri/Food System; Dairy Farming; Food Safety; Livestock Industry; Markets; Marketing; Nutrition; Policy, Food References
• Fuquay, John W., Patrick F. Fox, and Hubert Roginski, eds. Encyclopedia of Dairy Science (4 volumes), 1st ed. London: Academic Press, 2002.
• Kosikowski, Frank V., and Vikram V. Mistry. Cheese and Fermented Milk Foods (2 volumes), 3rd ed. Great Falls, VA: F.V. Kosikowski, LLC, 1997.
• Latimer, George W., Jr. and William Horwitz, eds. Official Methods of Analysis of AOAC International. 18th ed. Arlington, VA: Association of Official Analytical Chemists, 2007.
• National Archives and Records Administration. Code of Federal Regulations. Parts 100–169, April 1. Washington, DC: Office of the Federal Register, National Archives and Records Administration, 2007.
• U.S. Department of Health and Human Services. Milk Pasteurization Controls and Tests, 2nd ed. Rockville, MD: U.S. Department of Health and Human Services, Public Health Service, Food and Drug Administration, 1986.
• U.S. Department of Health and Human Services. Grade A Pasteurized Milk Ordinance. Washington, DC: Public Health Service and Food and Drug Administration, 2005.
• Wehr, Michael, ed. Standard Methods for the Examination of Dairy Products, 17th ed. Washington, DC: American Public Health Association, 2004.