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Newer Knowledge of Dairy Foods
Milk
Protecting the Quality of Milk and Other Dairy Foods
Milk's quality relates to its chemical, microbiological, physical, and organoleptic properties, as well as to its safety. Cow's milk is among the most perishable of all foods, due to its fluid form and excellent nutritive composition. As it comes from the cow, milk provides an ideal medium for bacterial growth. To protect milk's quality, this food is handled under rigid sanitary conditions, resulting in low bacterial count, good flavor and appearance, satisfactory keeping quality, high nutritive value, and freedom from disease-producing organisms and foreign constituents. Today, less than 1% of all disease outbreaks due to infected foods and contaminated water involve milk and fluid milk products. The responsibility for ensuring milk's quality is shared by public health officials, the dairy industry, and consumers.
- Grade A Pasteurized Milk Ordinance (PMO). Milk and other dairy foods are among the most highly regulated and monitored food products in the U.S. The PMO is one of the most effective instruments for protecting the quality of Grade A milk ( Table 24: PDF 7k ). It is essentially a set of requirements for product safety, milk hauling, sanitation, equipment, and labeling specified by the U.S. Public Health Service (USPHS)-Food and Drug Administration (FDA). The extensive requirements cover milk from production at the farm to shipment from the processor. The National Conference on Interstate Milk Shippers has a key role in setting standards and regulations for the PMO. The PMO is voluntarily adopted by states and other jurisdictions. To maintain or improve milk's quality, the PMO is revised periodically as technological advances in processes, equipment, and research are made.
- Grade A Milk. More than 95% of all milk produced and sold to consumers as milk or milk products in the U.S. conforms to Grade A requirements as defined in the PMO. Other milk is classified as manufacturing grade (Grade B), or reject (Grade C). Grade A milk is obtained from dairy farms that meet the sanitation requirements of the PMO or its equivalent as enforced by local and state authorities. The milk must be obtained from cows tested and found free of disease and disease-producing organisms. The raw milk is cooled immediately to the specified legal temperature and maintained at no higher temperature from the completion of milking until processing at the dairy plant that also conforms with state and local sanitation requirements. Raw milk delivered to the dairy plant is tested for temperature, sediment, microbial load, somatic cell count (count of the mixture of dead epithelial cells and leukocytes that migrate into milk from the udder), off-flavors, and antibiotic residues. Likewise, after milk and milk products are pasteurized at the processing plant, they are routinely tested for bacterial counts (e.g., standard plate counts, coliforms), drug residues, and proper pasteurization (i.e., phosphatase test). Information on these tests can be found in the PMO and in Chandan (1997). After processing, milk is cooled again to the specified legal temperature and maintained at no higher temperature until sold.
- Protecting Against Unintentional Microconstituents. To protect against microconstituents such as pesticides and antibiotic residues, milk is routinely sampled on a state-by-state basis and tested following procedures outlined in the PMO. FDA and other federal agencies such as the Environmental Protection Agency (EPA) also conduct surveys and other monitoring activities to determine compliance with the provisions of the PMO. For example, a 1997 FDA survey of 628 samples of milk/dairy products/eggs found that 97% of these foods were free of pesticide residues and none contained residues in amounts over tolerance levels.
- rBST (Recombinant Bovine Somatotropin). The dairy industry extensively tests new procedures and technologies (e.g., genetic engineering) prior to their widespread adoption to ensure that they are safe for both cows and milk consumers. An example of a relatively new technology is the milk-producing hormone rBST, a naturally occurring growth hormone in cows, used to increase the efficiency of milk production. Prior to its 1993 approval by the FDA for use in cows, numerous independent tests of rBST were carried out by the FDA, state agricultural departments, and independent scientists. There is no chemical difference between the milk from rBST-treated cows and milk from cows that have not been treated with rBST. The FDA and other scientific groups have concluded that dairy foods from rBST-treated cows are safe for human consumption.
- Pasteurization. Pasteurization, as defined by the PMO, is the process of heating milk and fluid dairy products in properly designed and operated equipment at a sufficiently high temperature for a specified length of time to destroy disease-producing microorganisms. The process inactivates about 95% of pathogenic microorganisms. The most common time-temperatures used by processors include: 145°F for 30 minutes or 161°F for 15 seconds. The pasteurized product is then cooled quickly to 38 to 40°F to retard the growth of surviving organisms. Pasteurization is required by law for all Grade A fluid milk and milk products moved in interstate commerce for retail sale. This heat treatment has relatively little effect on milk's nutritive value.
- Ultrapasteurization. This is the process of heating fluid dairy products at or above 280°F for at least 2 seconds to destroy all pathogenic organisms. Ultrapasteurized products are packaged in an aseptic atmosphere in presterilized containers and refrigerated. These milk products have an extended shelf life (14 to 28 days at refrigerated temperatures), with little effect on their nutritive value.
- Ultrahigh Temperature. Ultrahigh temperature (UHT) milk products are heated to 275 to 302°F for 4 to 15 seconds. These products are packaged aseptically in specially designed multilayer containers. The heat treatment and packaging allow these milk products to be stored at room temperature for extended periods of time. Ultrahigh temperature processing may cause some loss of folate, vitamin C (which is already low in milk), vitamin B12 , and thiamin.
- Homogenization. Homogenization results in milk of uniform composition and palatability without removal or addition of its constituents. Milk fat occurs in milk as fat globules varying in size from 2 to 6 um. During homogenization, fat globules are mechanically reduced in size to 1 um and dispersed permanently in a fine emulsion throughout milk. Since the globules do not adhere to each other or coalesce, they do not rise and form a cream layer on the surface. Other advantages of homogenization include a richer flavor, lower curd tension and possibly increased digestibility. Although this process is optional, most market milk in the U.S. is homogenized.
- Fortification. Milk may be fortified with nutrients such as vitamin A, vitamin D, and calcium. Because the vitamin A and carotene in milk exist in the fat portion, much of the vitamin A activity is removed with the milk fat during the manufacture of reduced fat and nonfat milks. Consequently, fluid reduced-fat and nonfat milks are required to be fortified with chemically derived vitamin A (e.g., retinyl palmitate) to a level found in whole milk or 300 IU (6% Daily Value) per 8-fluid-ounce serving. However, dairy processors are encouraged to continue to fortify lowfat milks to the current level of 500 IU of vitamin A per cup (10% Daily Value) or 2,000 IU per quart. Good manufacturing practices require vitamin A to be suspended in reduced fat and nonfat milks. Vitamin A palmitate is combined with a carrier such as butteroil, evaporated milk, or glycol. Palmitic acid is the major fatty acid in palm oil. Palm oil itself is neither added to or present naturally in milk. The amount of palmitic acid added to milk is insignificant. If vitamin A is added to whole milk, other milks, or yogurt for which it is not required, the label must state this fact.
Unfortified cow's milk contains a low level of natural vitamin D. While optional, most milk marketed in the U.S. is fortified with vitamin D (vitamin D3 or 7-dehydro-cholesterol) to obtain a standardized amount of 400 IU per quart. When a food is fortified with vitamin D or a nutritional claim is made, vitamin D must be listed on the nutrition label. To help ensure that milk contains the amount of vitamin D specified on the label, milk is monitored primarily by state governments in cooperation with the FDA. The FDA recommends that the vitamin D content of milk be measured by a certified laboratory and determined to be acceptable by this regulatory agency. According to good manufacturing practices, the acceptable range allowed for vitamin D fortification of milk is not less than 100% and not more than 150% of label claims (i.e., 400-600 IU vitamin D per quart). Vitamin D fortification of milk is an important public health measure that has been largely responsible for the elimination of rickets in the U.S.
Storage can affect the concentration of vitamins added to milk. Vitamin D is very stable in homogenized whole milk and there is no loss of vitamin D in fortified homogenized whole milk during long periods of proper storage. However, vitamin A- and D-fortified lowfat and nonfat products are subject to decreases in vitamin A. In fluid nonfat and lowfat milks, added vitamin A gradually decreases during normal storage of milk at 40°F in the dark. However, this vitamin is rapidly destroyed when the milk is exposed to sunlight in transparent glass bottles or translucent plastic containers. This light-induced destruction of vitamin A depends on the intensity and wave length of the light and the milk source. Vitamin A destruction can be retarded by the use of amber or brown glass bottles, pigmented plastic containers formulated with specific light barriers, and colored paper cartons. Vitamin A losses of 8% to 31% were found in 2% lowfat milk exposed to 200 foot candles of fluorescent light for 24 hours in opaque plastic containers. These losses were virtually eliminated by use of pigmented containers or gold shields over fluorescent tubes.
Calcium is being added to a variety of milks and other dairy foods. Some of these new products appearing in the dairy case include a calcium-fortified, fat-free, lactose-free milk containing 500 mg calcium per cup or 67% more calcium than in 1 cup of milk; a fat-free frozen yogurt with 50% more calcium than 1 cup of milk; nonfat milk with 34% more calcium; and fat-free cottage cheese with twice the calcium. These new milk products are an option to help meet calcium needs and reduce the calcium crisis in the U.S.
- Care of Milk and Milk Products in the Home. The consumer is responsible for protecting the quality of milk and other dairy foods in the home. Open dating and proper handling of dairy products can help consumers be assured of dairy foods with a good shelf life (the length of time after processing that the product retains its quality).
Open dating is the date on milk and other dairy food containers indicating when the product was manufactured or meant to be sold by. It is used by industry to indicate the age of individual packages and does not reflect the shelf life of the product. Generally, proper care ensures freshness for a few days beyond the "sell-by date." Regulation of open dating varies among states and municipalities. To preserve the quality of milk and other dairy foods, the following practices are recommended for consumers:
- Containers. Use proper containers to protect milk from exposure to sunlight, bright daylight, and strong fluorescent light. This will prevent the development of off-flavors and reductions in light-sensitive nutrients such as riboflavin, ascorbic acid, and vitamin B6.
- Storage of milk. Refrigerate milk at 40°F or less as soon as possible after purchase. Keep milk containers closed to prevent the absorption of other food flavors in the refrigerator. An absorbed flavor changes the taste but the milk is still safe. Use milk in the order purchased. Return the milk container to the refrigerator immediately to prevent bacterial growth. Temperatures above 40°F reduce the shelf life of milk and other milk products. Never return unused milk to the original container.
- Serving milk. Serve milk cold. Serve UHT milk cold and store in the refrigerator after opening.
- Canned milks. Keep canned milk in a cool, dry place and invert cans every 2 months. Condensed and evaporated milks keep for about 1 year at room temperature. Once opened, canned milk should be transferred to a clean, opaque container and refrigerated.
- Dry milks. Store dry milk in a cool, dry place and place in an airtight container after opening. At temperatures lower than 80°F and less than 65% relative humidity, dry whole milk has a shelf life of about 6 to 9 months. Under these same conditions, nonfat dry milk has a shelf life of 12 to 18 months. Humidity causes dry milk to lump and may affect flavor and color. If such changes occur, the milk should not be consumed. Once reconstituted, dry milk should treated like any other fluid milk (i.e., it should be covered and stored in the refrigerator).
- Freezing. With the exception of butter, ice cream, frozen yogurt, and other frozen dairy desserts, freezing of most dairy foods (e.g., milk, cream, yogurt, milk puddings, soft cheeses) is not recommended. Although natural hard cheeses, semi-hard cheeses, and processed cheeses can be frozen, freezing affects their texture, causing them to become crumbly and mealy after thawing. However, they are suitable for cooking and/or for use in salads or salad dressing. If frozen, cheeses should be thawed slowly in the refrigerator for 24 hours. Cheese that has been frozen should be used as soon as possible after thawing.
- Storing cheese. Cheeses should be wrapped tightly in the original wrapping, other wrapping, or tightly covered refrigerator containers for refrigerator storage. Unopened jars of cheese spreads and cheese foods can be stored at room temperature (70°F) for about 3 months. However, once these products are opened, they should be tightly covered and refrigerated to prevent drying and an unattractive "oiling off." Grated cheeses in moisture-proof packages will keep for about 3 months at room temperature or about 12 months at a refrigerator temperature of 40°F or below.
- Dealing with mold on cheese. If mold develops on cheese, discard one-half inch of cheese on all sides of the visible mold. The exception is mold-ripened cheeses such as Roquefort and blue. In general, most molds are harmless, although some may produce toxins that may diffuse into the cheese. Undesirable mold occurs when cheeses are improperly wrapped and consequently exposed to air. Consumer-size packages of natural and processed cheese may contain mold inhibitors, such as sorbic acid (0.2 to 0.1% by weight of cheese), that increase the shelf life of these products.
- Serving cheese. With the exception of cottage cheese, which should be chilled when served, most cheeses should be served at room temperature. The amount of cheese to be used should be taken from the refrigerator and allowed to warm to room temperature for about 30 minutes prior to serving.
- Cooking with cheese. Cook cheese dishes at a low temperature for a minimum amount of time. A relatively high temperature or a prolonged period of cooking causes fat separation and protein denaturation, resulting in a stringy, rubbery product. To hasten the melting time of cheese, it may be diced, shredded, or crumbled. It is also preferable to melt cheese in the top of a double boiler over simmering water. When microwaving cheese dishes, use lower power settings (30% to 70%).
Guidelines on the storage life of milks and milk products are shown in Table 25 (PDF 11k) . A variety of factors influence the shelf life of milk and other dairy foods. For example, the storage life of cheeses varies according to their moisture content, acidity, and conditions of storage. Cheeses such as Cheddar and Swiss keep up to a month or more when stored properly, whereas very hard cheeses such as Parmesan can last longer. In general, the higher the moisture content and lower the acidity, the shorter the shelf life of cheese.
References
U.S. Department of Health and Human Services, Public Health Service, Food and Drug Administration. Grade "A" Pasteurized Milk Ordinance.1997 Revision . PHS/FDA Publication No. 229.
Chandan, R. Dairy-Based Ingredients . St. Paul, Minn.: Eagan Press, 1997.
Kosikowski, F.V., and V.V. Mistry. Cheese and Fermented Milk Foods. Volume 1: Origin and Principles . 3rd ed. Westport, Conn.: F.V. Kosikowski, 1997.
Food and Drug Administration. Pesticide Program. Residue Monitoring 1997. Washington, D.C.: Food and Drug Administration. http://www.FDA.gov
Report of the Food and Drug Administration. Review of the Safety of Recombinant Bovine Somatotropin. 1998.
Juskevich, J.C., and C.G. Guyer. Bovine growth hormone: human food safety evaluation. Science 249 : 875, 1990.
Food and Drug Administration. Animal drugs, feeds, and related products; sterile sometribove zinc suspension. Fed. Regist. 58 (Nov. 12): 59946, 1993.
International Dairy Federation. Heat-induced Changes in Milk . 2nd ed. Brussels, Belgium: International Dairy Federation, 1995.
Food and Drug Administration. Lowfat and skim milk products, lowfat and nonfat yogurt products, lowfat cottage cheese: revocation of standards of identity; food labeling, nutrient content claims for fat, fatty acids, and cholesterol content of food. Final rule. Fed. Regist. 61 (225): 58991 (Wed. Nov. 20), 1996.
National Food Safety Database. Safe Food Storage. Times and Temperatures . http://foodsafety.ifas.ufl.edu/HTML/consumer.htm
Berry, D.G. …With extra calcium. Dairy Foods 99(8) : 39, 1998.
Klausner, A. EN's guide to calcium in unexpected places. Environmental Nutr. 22(5) : 5, 1999.
Berry, D. Code dating update. Dairy Foods 100(8) : 2, 1999.
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