|MadSci Network: Agricultural Sciences|
The process starts when the cow gets old enough to be bred and produce a calf. During her pregnancy, the cow's body gets the udder ready to produce milk. The first milk after the calf is born is pretty strange, syrupy looking stuff called "colostrum" — it is loaded with antibodies to protect the newborn calf against diseases. After a few days, the milk looks like something people would drink: white, with white or yellow cream at the top, depending on the breed of the cow and what she has been eating.
Cows are milked by machines nowadays. When the cow arrives at the milking parlor, her udder is washed with warm water or disinfectant solution, to clean it and to stimulate a hormone response that helps push the milk out, called "let-down." The milker squeezes a little milk out of each teat and examines it for signs of udder inflammation before attaching the milking machine. The milking machine alternately squeezes and releases all four teats simultaneously, and the milk that comes out of the bottom ends of the teats is conducted by vacuum through pipes to a holding tank. There is usually a viewing glass at the cow's end of the piping, so that the milker can see that the milk looks normal. Cows that are sick or have infected udders are milked last, and the milk is discarded. Cows are milked twice (sometimes three times) per day; they need to be bred so that they produce a calf about once a year, to keep them producing milk. They are usually given a few weeks' rest from milking before the next calf is due, to improve their productivity in the following year.
Milk in the bulk holding tank is mixed and cooled. It is taken away, at least every couple of days, by a truck with a stainless steel tank. It goes to a processing facility (sometimes called a "creamery"), where it is tested for antibiotics, sampled for butterfat testing, and then unloaded into a huge tank called a "silo," where it is refrigerated awaiting processing. Milk for people to drink (as opposed to drying, cheesemaking, etc.) is clarified to remove impurities such as cells from the cow's udder, standardized to the desired butterfat content (skim milk, 1% low fat, 2% reduced fat, or 3.5% whole milk, in the U.S.), homogenized so that the cream stays mixed in rather than rising to the top, and then pasteurized.
The most common kind of Pasteurization is at 72C for 15 seconds; it kills disease agents that might be present, but does not sterilize the milk, which must still be refrigerated so it doesn't spoil. Some milk is treated at "ultrahigh" temperatures (above boiling) briefly; this kills almost all microorganisms, so that the milk does not have to be refrigerated. UHT milk has a somewhat cooked flavor that wants getting used to. Pasteurized milk is filled into sterile containers and distributed by refrigerated trucks to your neighborhood food shop. If it has been carefully handled all along, it may be good to drink for another two weeks or so in a properly cooled home refrigerator. Some bacteria that survived pasteurization will eventually spoil the milk even in the fridge, but milk left out or kept in a fridge that is not cold enough will go "off" or sour in a day or a few days, depending on the actual temperature.
It is important that milk be Pasteurized, for drinking, drying, or processing into cheese, butter, yogurt, etc. There is always a certain risk that the cow's udder was infected with bacteria that can cause human illness. As some of these illnesses can be quite severe, Pasteurization has become an important measure for protecting public health.
Try the links in the MadSci Library for more information on Agricultural Sciences.