Fluid Milk Production
This page describes the individual operations that are used in producing beverage milk. Topics covered are the legal Fluid Milk Definitions, Standardization, Pasteurization, Homogenization, Vitamin Fortification, and Specialty Milk Beverages such as lactose-reduced and protein fortified.
Fluid milk is an industry term for milk processed for beverage use.
Milk, as defined by the U.S. Code of Federal Regulations (CFR), 21 CFR 131.110, is: “the lacteal secretion, practically free from colostrum, obtained from the complete milking of one or more healthy cows. Milk that is in its final package form for beverage use shall have been pasteurized or ultrapasteurized, and shall contain not less than 8.25% solids and not less than 3.25% milk fat. Milk may have been adjusted by separating part of the milkfat therefrom, or by adding thereto cream, dry whole milk, skim milk, or nonfat dry milk. Milk may be homogenized.” Milk solids are the non-water components of milk – protein , lactose, and minerals. Sometimes the combination of protein, lactose and minerals is called the solids not fat content, and when the fat is included it is called total solids content.
Although the CFR states that milk is obtained from cows, the production of milk from other dairy animals in the U.S. (goats, sheep, and water buffalo) also is covered in the Grade A Pasteurized Milk Ordinance (PMO).
The fat content of milk varies with species (cow, sheep, goat, water buffalo), animal breed, feed, stage of lactation, and other factors. In order to provide the consumer with a consistent product, most milk in the U.S. is standardized.
To achieve standardization, milk is processed through centrifugal separators to create a skim portion and a cream portion of the milk. Separation produces a skim portion that is less than 0.01% fat and a cream portion that is usually 40% fat, although the desired fat content of the cream portion can be controlled by changing settings on the separator. The cream portion is then added back to the skim portion to yield the desired fat content for the product. Common products are whole milk (3.25% fat), 2% and 1% fat milk, and skim milk (< 0.1% fat).
The majority of U.S. fluid milk is pasteurized using a high temperature short time (HTST) continuous process of at least 161°F (71.6°C) for 15 seconds. These conditions provide fresh tasting milk that meets the requirements for consumer safety. Higher heat processes, such as ultrapasteurization or aseptic processing , are used to extend the shelf life of refrigerated products or allow for storage at room temperature, respectively, but may impart a cooked flavor to the milk.
The fat in milk is secreted by the cow in globules of non-uniform size, ranging from 0.20 to 2.0 µm. The non-uniform size of the globules causes them to float, or cream, to the top of the container. Milk that is not homogenized is sometimes referred to as “creamline” milk. Pasteurized milk does not necessarily need to be homogenized. However, homogenized milk should be pasteurized to inactivate native enzymes that deteriorate fat (lipases) and cause rancidity, which results in off-flavors and reduced shelf life in milk.
The purpose of homogenization is to reduce the milk fat globules size to less than 1.0 µm which allows them to stay evenly distributed in milk. Homogenization is a high pressure process that forces milk at a high velocity through a small orifice to break up the globules. The result of homogenization is the creation of many more fat globules of a smaller size. The native milk fat globules are covered in a protein membrane that stabilizes the fat phase in the aqueous (water) phase of milk. Although the milk fat globule membrane is disrupted during the homogenization process, it spontaneously migrates back to the fat globules after homogenization. The new globules created during homogenization are spontaneously coated by proteins in the skim phase from the original milk fat globules.
Whole milk is considered a good source of vitamin A. Vitamin A is a fat soluble vitamin that is found in the fat phase of milk. The vitamin A content that occurs naturally in 2%, 1% and skim milk is less than in whole milk because of the lower fat levels. Nutritional concerns about consumption of lower fat milk in the late 1970s led to the required fortification of vitamin A in lower fat milks. To achieve the nutritional equivalence of whole milk, lower fat milks should be fortified to 300 IU vitamin A per 8 oz serving. The FDA encourages fortification to a level of 500 IU of vitamin A per 8 oz serving, which is 10 % of the recommended daily allowance (RDA).
Vitamin D is a fat soluble vitamin that occurs naturally in milk but at low levels. Because milk is not considered an important natural source of vitamin D in the diet, vitamin D fortification is voluntary. Fortification of milk with vitamin D began in the U.S. in the 1930s to reduce the incidence of rickets in children. Although rickets is not currently a major concern in the U.S., adequate vitamin D is necessary for human health. Vitamin D helps with calcium absorption, has an important role in bone health, and has a protective effect in cancer. Milk may be fortified with vitamin D to a level of 100 IU per 8 oz serving, which is 25% of the RDA.
The dairy industry has developed specialty fluid milk beverages to meet the diverse nutritional needs of consumers. Lactose-reduced and lactose-free milk, and acidophilus milk were developed for people with lactose intolerance (maldigestion). Lactose-reduced and lactose-free milk are processed, prior to packaging, with the lactase enzyme to separate lactose into its component sugars, glucose and galactose. Acidophilus milk contains Lactobacillus acidophilus, a probiotic lactic acid bacterium that is beneficial to human health. The Lactobacillus acidophilus bacteria use lactose for an energy source and reduce the amount of lactose present in milk. They also make the lactase enzyme which assists humans with lactose digestion in the small intestine.
Specialty milk beverages are available that are tailored to specific segments of the population. There are milk beverages with added plant sterols aimed at helping to improve cholesterol levels and others that are fortified with protein and calcium designed for adults. There are carbohydrate-reduced and vitamin fortified milk beverages for people watching their weight. Milk beverages targeted for teen athletes are protein fortified and fat-reduced. Milk beverages designed for children are calcium fortified, fat-reduced and flavored. The flavored milks compete with soft drinks for children's attention and come in a wide range of flavors from the traditional chocolate and strawberry to milks flavored like their favorite candy bar or ice cream.