Everyone used to make his own yogurt, says Manfred Kroger, ex-yogurt manufacturer, yogurt philosopher and professor of food science at Pennsylvania State University. In fact, in eastern Europe and the Middle East, if the family yogurt culture died, it would be a "minor tragedy," Kroger adds.

Nowadays, glitzy packaging, tight schedules and modern technology have made it easy to reach for the plethora of commercial containers at the supermarket. But there's "no need to fear home technology," says Kroger. Making yogurt at home -- without the use of a yogurt machine -- is just as easy as baking a cake.

And it certainly is inexpensive, too, as yogurt can be recycled continually into new batches. (Kroger says that a few years ago an elderly couple from York, Pa., visited him with a batch of yogurt made from a culture that had been in the family since the 1930s.)

Making yogurt at home simply involves inoculating milk with active cultures (as found in plain yogurt) and then incubating it for three to four hours at approximately 110 degrees. During the incubation period, the bacteria multiply every 20 minutes -- "by the billions," according to Kroger. The bacteria produce lactic acid, which causes the milk protein to coagulate and thicken, forming yogurt.

The only really tricky part of the procedure is making sure the inoculated milk is maintained at the proper temperature for the proper amount of time. According to Kroger, methods for keeping it at the proper temperature have varied over the years. In the old days, "every grandmother from eastern Europe" used to put the inoculated milk in a box surrounded with crumbled newspaper, then wedged it between two feather pillows.

Now, methods vary from hanging a lit light bulb in the oven with the inoculated milk to placing it in a Styrofoam cooler aside a container of hot water. One popular procedure is placing the milk over the pilot light of a gas range.

The easiest method, recommends Kroger, is preheating the oven on its lowest setting, turning off the heat and placing the inoculated milk in the oven for its three- to four-hour incubation.

Kroger has other recommendations for making successful yogurt:

*Use a yogurt for inoculation that has active cultures. Pick a plain yogurt with the latest pull date to ensure freshness. And don't use fruit yogurt to inoculate; it does not incubate well.

Do not stir the milk during the incubation process. You'll be cutting into the forming gel.

*Yogurt gets more and more sour as it incubates since more bacteria are growing. So if the yogurt is refrigerated too late in the incubation process (the colder temperatures halt the bacteria growth), it may get too acidic.

*If your yogurt does not thicken, it means either: 1) the temperature was above 120 degrees (the bacteria were killed), 2) the temperature was below 95 degrees (the bacteria didn't grow), or 3) an inactive culture was used.

*Yogurt will stay palatable for up to a month. After that, it may be better used in cooking since it gets more and more sour as it ages. As long as there are no visible signs of spoilage that may have been caused by cross contamination or a dirty container, yogurt should remain safe to eat "almost indefinitely," says Kroger. PLAIN LOWFAT YOGURT (Makes 1 quart)

1 quart skim milk

1/2 cup instant nonfat dried milk

3 tablespoons plain yogurt at room temperature

Place 1 cup of the skim milk in a bowl. Add dried milk and stir until dissolved. Pour remaining skim milk in the top of a double boiler and add the dissolved dry milk. Mix well. Cook over medium heat until milk reaches 180 degrees on a thermometer.

Remove from heat and cool by placing top of double boiler with milk into a bowl of cold water, stirring periodically. Cool until temperature of milk reaches 110 degrees.

To inoculate, whisk the yogurt into the lukewarm milk until thoroughly combined. Pour into a clean glass jar (such as a mayonnaise or juice container) and incubate for 3 or 4 hours at 110 degrees.

To incubate, preheat oven to its lowest setting, then turn heat off. Place covered jar of inoculated milk into oven. A candy or meat thermometer should also be placed in the oven to better monitor temperature.

After 3 hours, give the jar a gentle shake to see if it is firm. Do not stir or agitate. You may also want to taste a spoonful of yogurt to make sure it is mildly tart. When it is firm, refrigerate for at least 6 hours, preferably overnight.

Whey may form on the top of the yogurt; it is not harmful to eat, but may be poured off if you prefer.

For a richer yogurt with more fat and calories, whole milk may be substituted in the above recipe. Yogurt Cheese

To make yogurt cheese, place homemade yogurt into a colander or funnel lined with filter paper or cheesecloth. Place a pan or bowl underneath the colander or funnel. Refrigerate overnight, allowing the whey to ooze through the mesh. The finished cheese will resemble a low-fat cream cheese.