The manufacturers of automatic yogurt makers play on the consumer's insecurity about homemade yogurt. The existence of these gadgets does not preclude, however, the simplicity (and thrift) of making the yogurt in an empty mayonnaise jar.

There's very little mystery to yogurt making. Like other living things, yogurt does best when it's coddled and gets lots of rest. The bacteria grow best at 110 degrees--a little higher than the temperature for yeast doughs, and a temperature easily achieved in a variety of ways.

One book on yogurt recommends setting a cake rack on a heating pad, then the yogurt on the cake rack and covering the whole thing with a blanket. A single attempt at that convinces the cook never to make yogurt again. Instead, says Dr. Virginia Holsinger, research leader at the Department of Agriculture's Eastern Research Center in Philadelphia, the "pilot light in the oven is just ideal" for producing the proper amount of heat for yogurt growth. Setting the jar on a rack over warm water--in a variation on the automatic yogurt maker theme--produces satisfactory results. Another alternative is to preheat the oven to its lowest setting, turn the heat off and set the jar in the oven to incubate. Placing the jar next to the warm back of a refrigerator (near the coil) has reportedly been successful.

These latter methods keep the jar out of the way and undisturbed for the three-hour incubation period.

Proper temperature is the key to all stages of yogurt making and probably the only aspect of yogurt making that could be called tricky. Indeed, not only is making your own yogurt incredibly easy, it is inexpensive. If you are worried about fresh milk spoiling before you have time to make your yogurt, just make it entirely from dried milk. That dried-milk cooked taste that many people object to is irrelevant in yogurt making because all the milk is heated anyway, and after incubation, live bacteria give the yogurt its proper flavor. And certainly, the tools for yogurt making can be found in most kitchens.

To make yogurt, the cook needs a measuring cup, thermometer, a saucepan or double boiler, a small bowl and a clean jar with a lid in addition to milk, nonfat dried milk solids and live yogurt culture (from plain, natural yogurt found in the grocery store).

PLAIN YOGURT (1 quart) 1 quart skim milk 1/2 cup instant nonfat dried milk 3 tablespoons plain yogurt, room temperature

Place 1 cup of fluid milk in a small bowl. Add dried milk and stir until dissolved. Pour remaining fluid milk into a saucepan or double boiler (using the saucepan takes less time but you'll have be a little more careful). Add the dissolved dry milk and mix well. If using a saucepan, heat over medium heat, stirring occasionally, until milk reaches 180 to 190 degrees (the rule of thumb is 15 minutes, but that can vary). Remove from heat and set aside to cool (this goes more quickly if you set the pan in cold water). Place the yogurt in a clean jar. When the milk has cooled to 110 degrees, pour it into the jar and stir well with a whisk. Cover and incubate for 3 to 4 hours at about 110 degrees. After 3 hours, give the jar a gentle shake to see if it is firm. When it has become firm, refrigerate the jar. Chill at least 6 hours.

This recipe can be doubled and tripled; just use bigger jars. The incubation period will be the same.

The clear liquid that rests on top of the yogurt is the whey. It is nutritious and can be stirred back into the yogurt, or can be poured off to create a thicker yogurt.

To make individual fruit-flavored yogurts, use small containers and put a tablespoon or two of your favorite preserves or pastry fillings (prune, poppy seed or apricot, for instance) in the bottom of each. Add milk-starter, cover and incubate as required. To make blended style yogurt, stir the warm milk culture into the preserves and proceed as usual.

To make richer yogurts, use 2 percent or whole milk, or make the above recipe with part half-and-half or cream. This adds calories (in the form of milkfat) and reduces nutritive value, however.