Background
Milk is a nutritive beverage obtained from various animals and consumed by humans. Most milk is obtained from dairy cows, although milk from goats, water buffalo, and reindeer is also used in various parts of the world. In many industrialised countries, raw cow’s milk is processed before it is consumed. During processing, the fat content of the milk is adjusted, various vitamins are added, and potentially harmful bacteria are killed. In addition to being consumed as a beverage, milk is also used to make butter, cream, yoghurt, cheese, and a variety of other products.
History
The use of milk as a beverage probably began with the domestication of animals. Goats and sheep were domesticated in the area now known as Iran and Afghanistan in about 9000 B.C., and by about 7000 B.C. cattle were being herded in what is now Türkiye and parts of Africa. The method for making cheese from milk was known to the ancient Greeks and Romans, and the use of milk and milk products spread throughout Europe in the following centuries.
Cattle were first brought to the United States in the 1600s by some of the earliest colonists. Prior to the American Revolution, most dairy products were consumed on the farm where they were produced. By about 1790, population centres such as Boston, New York, and Philadelphia had grown sufficiently to become an attractive market for large-scale dairy operations. To meet the increased demand, farmers began importing breeds of cattle better suited to milk production. The first Holstein-Friesians were imported in 1795, the first Ayrshires in 1822, and the first Guernseys in 1830.
With the development of the dairy industry in the United States, a variety of machines for processing milk were also developed. In 1856, Gail Borden patented a method for making condensed milk by heating it in a partial vacuum. Not only did his method remove much of the water so the milk could be stored in a smaller volume, but it also protected the milk from germs in the air. Borden opened a condensed milk plant and cannery in Wassaic, New York, in 1861. During the Civil War, his condensed milk was used by Union troops and its popularity spread.
In 1863, Louis Pasteur of France developed a method of heating wine to kill the microorganisms that cause wine to turn into vinegar. Later, this method of killing harmful bacteria was adapted to a number of food products and became known as pasteurisation. The first milk processing plant in the United States to install pasteurising equipment was the Sheffield Farms Dairy in Bloomfield, New Jersey, which imported a German-made pasteuriser in 1891. Many dairy operators opposed pasteurisation as an unnecessary expense, and it wasn’t until 1908 that Chicago became the first major city to require pasteurised milk. New York and Philadelphia followed in 1914, and by 1917 most major cities had enacted laws requiring that all milk be pasteurised.
One of the first glass milk bottles was patented in 1884 by Dr Henry Thatcher, after seeing a milkman making deliveries from an open bucket into which a child’s filthy rag doll had accidentally fallen. By 1889, his Thatcher’s Common Sense Milk Jar had become an industry standard. It was sealed with a waxed paper disc pressed into a groove inside the bottle’s neck. The milk bottle, and the regular morning arrival of the milkman, remained a part of American life until the 1950s, when waxed paper cartons of milk began appearing in markets.
In 1990, annual milk production in the United States was about 148 billion lb (67.5 billion kg)—equivalent to about 17.2 billion U.S. gallons (65.1 billion litres). About 37% of this was consumed as fluid milk and cream, 32% was converted into various cheeses, 17% was made into butter, and 8% was used to make ice cream and other frozen desserts. The remainder was sold as dry milk, canned milk, and other milk products.
Types of Milk
There are many different types of milk. Some depend on the amount of milk fat present in the finished product. Others depend on the type of processing involved. Still others depend on the type of dairy cow that produced the milk.
The U.S. Food and Drug Administration (FDA) establishes standards for different types of milk and milk products. Some states use these standards, while others have their own. Prior to 1998, federal standards required that fluid milk sold as whole milk must have no less than 3.25% milk fat, low-fat milk must have 0.5–2.0% milk fat, and skimmed milk must have less than 0.5% milk fat. Starting in 1998, the FDA required that milk with 2% milk fat be labelled “reduced-fat” because it did not meet the new definition of low-fat products as having less than 3 g of fat per serving. Milk with 1% milk fat could still be labelled “low-fat” because it did meet the definition. As a comparison, light cream has no less than 18% milk fat, and double/heavy cream has no less than 36% milk fat.
Other types of milk are based on processing. Pasteurised milk has been heated to kill potentially harmful bacteria. Homogenised milk has had the milk fat globules reduced in size and uniformly dispersed to prevent them rising to the top as cream. Vitamin-fortified milk has various vitamins added. Most milk sold in U.S. markets is pasteurised, homogenised, and vitamin-fortified.
Grade A milk refers to milk produced under sufficiently sanitary conditions to permit its use as fluid milk. About 90% of the milk produced in the United States is Grade A. Grade B milk is produced under conditions that make it acceptable only for manufactured products such as certain cheeses, where it undergoes further processing. Certified milk is produced under exceedingly high sanitary standards and is sold at a higher price than Grade A milk.
Speciality milk includes flavoured milk, such as chocolate milk, which has had a flavouring syrup added. Other speciality milks include Golden Guernsey milk, produced by purebred Guernsey cows, and All-Jersey milk, produced by registered Jersey cows. Both command a premium price because of their higher milk fat content and creamier taste.
Concentrated milk products have varying degrees of water removed from the fluid milk. They include, in descending order of water content: evaporated milk, condensed milk, and dry milk.
Raw Materials
The average composition of cow’s milk is 87.2% water, 3.7% milk fat, 3.5% protein, 4.9% lactose, and 0.7% ash. This composition varies from cow to cow and breed to breed. For example, Jersey cows average 85.6% water and 5.15% milk fat. These figures also vary by season, animal feed, and many other factors.
Vitamin D concentrate may be added to milk at 400 international units (IU) per quart. Most low-fat and skimmed milk also has 2,000 IU of vitamin A added.
The Manufacturing Process
Collecting
Dairy cows are milked twice a day using mechanical vacuum milking machines. The raw milk flows through stainless-steel or glass pipes to a refrigerated bulk milk tank, where it is cooled to about 40°F (4.4°C).
A refrigerated bulk tank truck makes collections from dairy farms in the area within a few hours. Before pumping the milk from each farm’s tank, the driver collects a sample, checks flavour and temperature, and records the volume.
At the milk processing plant, the milk in the truck is weighed and pumped into refrigerated tanks through flexible stainless-steel or plastic hoses.
Separating
The cold raw milk passes through either a clarifier or a separator, which spins the milk through a series of conical discs. A clarifier removes debris, some bacteria, and any sediment that may be present. A separator performs the same task but also separates the heavier milk fat from the lighter milk to produce both cream and skimmed milk. Some processing plants use a standardiser-clarifier, which regulates the milk fat content by removing only the excess fat. The excess milk fat is drawn off and processed into cream or butter.
Fortifying
Vitamins A and D may be added at this time by a peristaltic pump, which automatically dispenses the correct amount of vitamin concentrate into the milk flow.
Pasteurising
The milk—whether whole, skimmed, or standardised—is piped into a pasteuriser to kill bacteria. The most common method is High-Temperature, Short-Time (HTST) pasteurisation, in which the milk is heated as it flows continuously: 161°F (72°C) for 15 seconds for whole, skimmed, and standardised milk. (Other products have different time/temperature requirements.) The hot milk passes through a pipe designed so the residence time is exactly 15 seconds. A temperature sensor at the end diverts the milk back to the inlet for reprocessing if it has fallen below the required temperature.
Homogenising
Most milk is homogenised to reduce the size of remaining milk fat globules. This prevents cream separation and ensures even distribution. The hot milk from the pasteuriser is pressurised to 2,500–3,000 psi (17,200–20,700 kPa) by a multi-cylinder piston pump and forced through very small passages in an adjustable valve. The shearing effect breaks down the fat globules to the proper size.
The milk is then quickly cooled to 40°F (4.4°C) to protect flavour.
Packaging
The milk is pumped into coated paper cartons or plastic bottles and sealed. The bottles or cartons are stamped with a sell-by date to ensure retailers do not keep milk on shelves beyond its safe storage period.
Milk cartons or bottles are placed in protective shipping containers and kept refrigerated. They are shipped to distribution warehouses in refrigerated trailers and then on to individual markets, where they are kept in refrigerated display cases.
Cleaning
To ensure sanitary conditions, the inner surfaces of the process equipment and piping are cleaned once a day. Almost all equipment and piping used in the processing plant and on farms are made from stainless steel. Highly automated clean-in-place (CIP) systems are incorporated, allowing cleaning solutions to be circulated and then flushed clean between milk intakes.
Quality Control
The U.S. FDA publishes the Grade A Milk Ordinance, which sets sanitation standards for milk production in most states and for all interstate shippers. The composition of milk and milk products is specified in Agricultural Handbook 52 (U.S. Department of Agriculture), listing both federal and state standards. Testing includes fat content, total solids, pasteurisation efficiency, the presence of antibiotics used to control cow disease, and other parameters.
The Future
The trend towards low-fat dairy products over the last 20 years is expected to continue. Sales of butter are expected to decline, while sales of low-fat yoghurt and low- or reduced-fat milk are expected to increase. Overall consumption of liquid milk is likely to rise as the population increases.