Pasteurizing milk has been an industry-wide standard in the U.S. since the 1940s. Pasteurized milk offers many advantages, even though the full, rich, sturdy flavor of fresh raw milk suffers. It is against the law in almost all of the U.S. to sell raw milk or transport it across state lines.

The milk producers must pasteurize, ultra-pasteurize or ultra-high-temperature process milk before they sell it. Raw milk is high in bacteria that reduce its useable shelflife to half or a third compared to pasteurized milk. Even if it is free of bacteria, the active enzymes in raw milk would rapidly spoil it and produce sharp off flavors.

To pasteurize milk, the processor heats it slowly without boiling (boiled milk develops an unnatural cooked flavor). It takes 15 seconds to pasteurize milk at 160°F (72°C), 30 minutes at 144°F (63°C). Pasteurization destroys all pathogenic bacteria, yeasts and molds. But it destroys only 95 to 99 percent of nonpathogenic bacteria, so it is not as sterile as when it left the cow’s udder.

In practice that means the bacteria count is harmlessly low, but if the kids leave the carton out on the kitchen counter overnight, those few bacteria grow into a real problem. Pasteurization also deactivates those milk enzymes that cause rapid spoiling.

Homogenization is another process all U.S. milk undergoes to keep the tiny fat particles from congregating at the surface (they are the lightest, so slowly they rise), that would turn into heavy cream. The homogenizing process is simple. The processor pumps the milk through tiny orifices under high pressure to reduce the size of the fat globule from very small to microscopic (less than 2 millimicrons). This makes it physically impossible for them to clump together and rise to the surface.

A relatively new process allows you to store milk without refrigerating it. In ultrapasteurization, they flash-heat milk to 280°F (139°C) for 2 to 4 seconds. Ultrahigh-temperature (UHT) processing is the same, but the processor also packages the milk in sterilized, sealed paper cartons for a shelflife of many months without refrigeration. It comes in handy for camping, for emergency supplies and for times when you are out of milk in the middle of a baking project.

In an experiment food scientists put a labeled carton of UHT milk on a shelf for long-term storage. They opened the carton 2½ years later and compared the flavor with one in a fresh carton. They found no detectable difference in flavor.

Milk has to be pasteurized before it is homogenized. If they homogenized raw milk, the tiny fat globules became easy target for the disabling enzymes, and the milk would turn sour in hours.

The dairy industry has also perfected milk in another form-powdered (dehydrated). That is what they do with the extra milk they cannot sell fresh. Powdered milk keeps well on the shelf for years. Much of it finds its way to developing countries that have a shortage of fresh milk, but it is popular with domestic food processors and commercial bakeries, too. Powdered milk is always non-fat because the fat would oxidize and turn rancid with storage.

You will be surprised to learn that dehydrated milk is not a modern invention. Nomads in the steppes of northern Asia made sun-dried milk at least 1500 years ago. Their diet was predominantly dairy, but milk was both too bulky and too perishable to transport on horseback, so they dried it in the sun and carried the powder in leather pouches. At meal-time they reconstituted it with fresh water from creeks or springs-they had instant milk.

In America, you can buy dairy products in grocery stores, delis, even gas stations, in all their many forms. All these choices break into two main categories:

¨ 1. Uncultured products-butter, cream, half-and-half and ice cream.

¨ 2. Cultured products-yogurt, frozen yogurt, sour cream, buttermilk and cheese.