In 1908, a Russian scientist named Ilya Metchnikoff won a Nobel Prize for his research on how white blood cells fight infection. He was head of the Pasteur Institute at the time, working on an overall study of longevity.

Metchnikoff wanted to live as long as possible. In pursuit of that goal, he came upon a report that Bulgarians had a longer average life-span than others, and immediately began a prolonged study of the Balkan people. He eventually came to the conclusion [as yet scientifically unproven] that the fountain of youth was filled with yogurt.

The theory was based on his discovery of bacteria in yogurt that kill or inhibit some of the harmful organisms present in the large intestine. This same bacterium -- Lactobacillus bulgaricus -- acts on various vitamins, including riboflavin and niacin, and makes them more available to our bodies. Moreover, yogurt's protein value is higher than milk; it is digested twice as fast; and it has a cleansing effect on the digestive tract.

So it is not surprising that yogurt has become popular. And for those who want to make it at home, these are the best procedures and tools:

The process begins by heating milk [regular, evaporated or skim] until it starts to boil, thereby killing existing bacteria. Do not use an aluminum pot. A sauce pan of enamel-clad steel, enamel-clad cast iron or stainless steel is preferable; but a glass double-boiler will do the best job. The surface will not interact with the milk and you will be able to control the heat and prevent the milk from being scorched. If a skin forms on the milk, be sure to take it off.

The milk is then allowed to cool to 115 degrees. A "starter" of about two teaspoons of "live" yogurt is stirred into every 8 ounces of the milk. The mixture is set into some type of incubator that will keep the temperature steady at the optimum point of 115 degrees for four to 12 hours, depending on how sharp you like your yogurt to taste. [The longer, the sharper.]

This incubator can be an electric or non-electric yogurt-maker, or it can be an overn, electric skillet or wide-mouthed thermos jar.

Contempra's electric yogurt-maker has four variations. The standard model NYM-1 has a base which holds six 8-ounce glass containers -- covered with individual plastic lids -- and keeps them at the proper temperature. It costs $13.95.

The automatic model NYM-2T, at $22.50, comes with the valuable addition of a five-to-10-hour adjustable timer, and shuts off automatically when the time has elapsed. The Thriftee model TY-66 is a no-frills unit, but still gets you there on time; $11.95.

All three of these models have overall dimensions of 10-by-5-by-12-inches, making them convenient to store.

Finally, Comtempra makes the Big Batch model, yielding two quarts in one cylinder 14 1/2 inches high and 6 inches across. After the yogurt is made, the entire unit can go right into the refrigerator for storage; $14.95.

The Big Batch and NYM-1 have dial devices which you mark to remind you when the yogurt is finished. The problem is that there is no alarm or system which will call attention to the unit when the time is up. Consequently, the automatic model NYM-2T is highly preferable.

Salton manufactures two electric yogurt-makers. The basic model GM5 produces a quart in five milk-glass cups, each with its own plastic lid. The unit is thermostatically controlled, comes with a thermostatic

Salton's GM10, described as "family-sized," produces two liters in five glass cups. It comes with the thermometer spoon and a marvelous device for turning yogurt into an ideal low-calorie and smooth-textured substitute for cream cheese; $22.50.

Terraillon's new yogurt-marker is compact, thermostatically controlled, extremely attractive, and will hold six 6-ounce cups. Terrailon is made in Annemasse, France, by the same company that produces the award-winning kitchen scales; $30.

Non-electric yogurt makers are basically elaborate thermos bottles which do an acceptable job of holding the milk mixture at a proper temperature.

Since temperature control is the essential element, these thermos containers are a midway point between the electrics and the oven. The most successful design is the Yo-Gourmet model for about $25.

But your oven will do the job, if it will hold a roasting pan of water at a constant 115 degrees. Put milk and starter mixture into a series of clean glass jars, and set them into water, making sure that the milk level in the jars and water level in the pan are even. The same thing can be done with an electric skillet.

Whatever system you use, the benefits are much the same:

By controlling the time period, you will be able to make yogurt to your precise taste. You can make it from skim milk, thereby reducing the fat and calorie content. It will be free of additives and preservatives. And if you make it from reconstituted dry milk, the cost will be about 9 cents per cup -- a considerable saving over the 45 to 75 cents per cup for commercial varieties.

My household uses approximately seven cups of yogurt per week. By making it from reconstituted dry milk, I was able to save almost $150 during a year. CAPTION: Picture, Salton yogurt maker