In this season, with most of the nation's farmland under a blanket of snow, we reap the greatest benefits of our many methods of food preservation. Even so, there are many arguments for and against food processing. On one extreme, we have those who maintain that all foods should be eaten fresh with little or no processing. At the other end, there are those who defend the most extreme type of manipulation of our foods.

We stand somewhere in the middle. The best foods are fresh, eaten soon after picking and with the minimal amount of processing. But we also reorganize that food preservation makes it possible for us to have the variety of foods throughout the year that are needed to assure nutritional health.

As late as the 1940s, for example, there were children in states like Vermont who showed blood levels of vitamin C that were on the borderline of scurvy after a long winter without many fruits and vegetables. A fresh orange or tangerine was a treasured "stuffer" for Christmas stockings. While it's hard to imagine much delight from a can of frozen orange juice, our plentiful supply of frozen and canned foods does insure a healthy winter for most Americans.

Foods are preserved in a number of ways. Some methods, such as drying, smoking or packing in salt, brine or sugar, have been used for centuries. Others, such as canning or freezing, are relatively new. But every method of food preservation has the same aim: to prevent spoilage so that the food is tasty, looks good and retains as many of its original nutrients as possible.

Generally, foods are preserved in one of four ways: heating, cooling, drying or fermenting. Some methods use a combination of these. Let's look at what's involved in each:

Cooling. Refrigeration is the best method for short-term preservation. Most disease-causing organisms cannot grow at temperatures below zero. Since foods that are kept cool but still above freezing will continue to ripen slowly, they are shipped in refrigerated trucks and trains so that they arrive at the market with the taste, texture and nutritional value almost as if they were freshly packed.

For long-term preservation, foods must be solidly frozen. Actually, this is done in two steps: blanching or heating the foods to temperatures just under boiling to inactivate enzymes that change color, flavor or nutritional value and to kill many microorganisms, and then freezing. The freezing slows down the growth of organisms and prevents spoilage. To keep nutrient loss to a minimum, frozen foods should be stored at 0 degrees F. Before use, thawing is best done in the refrigerator rather than on the counter at room temperature.

Drying or dehydration. This is one of the oldest methods of food preservation known and is now use mostly for things like dried milk, instant coffee and dried fruits, soup mixes and other "instant" foods. Although there are various ways of drying foods, in general they all involve using heat to remove the water that all living organisms require for growth and reproduction.

Heating. The most common form of preservation using heat is canning, which has been used in this country since 1819. Basically, canning involves heating foods to a high enough temperature to inactivate microorganisms or their spores, and sealing out air, preventing spoilage. New fast-canning methods help minimize the loss of nutrients, but to get all the water-soluble vitamins, such as vitamin C, the liquid from canned fruits and vegetables should be used.

Fermentation. This method of food preservation involves altering the food's chemical enivronment. For example, the growth of microorganisms can be prevented by setting up a partial metabolism of a food to make it more stable. This usually changes the nature of the original food, giving us, for example, cheese instead of milk, or beer instead of hops. Another way is to add a chemical to the food, such as the salt for salt pork, the brine in pickles or the sugar in jams and jellies. These chemicals act by "binding" the water and making it unavailable to microorganisms. The addition of other chemicals, such as citric and acetic acids, slows the processes necessary for bacteria and other microorganisms to grow.

In many instances, foods that are picked at their peak of ripeness and immediately and properly preserved are actually more nutritious than those that are picked while still green and then shipped long distances or stored under less-than-ideal conditions.