MANY OF US have seen the entertaining television commerical in which 100-year-old Georgian Russians are seated tasting a refreshing cup of plain Dannon yogurt. Is yogurt indeed the secret to their good health and long life? Perhaps.

At least that is what Elie Metchnikoff, a Russian-born biologist working in France, discovered in 1908. According to Metchnikoff's report, "Prolongation of Life," Bulgarians who lived to 100 years and older ate about seven pounds of plain yogurt per day. Whereas we use oil, mayonnaise, cream, whole milk and sour cream in our cooking, they always used yogurt. Metchnikoff isolated the two types of bacteria in yogurt that ferment the milk sugar into lactic acid and give yogurt its flavor and aroma. Metchnikoff then discovered that both types of bacteria made large amounts of vitamin B, which killed the harmful bacteria in the intestines, and seemed to help considerably in maintaining good health.

Legend has it that yogurt was discovered 6,000 years ago. A camel merchant, traveling with his herd, concubines and servants from Ur to Eridu, put milk in a leather bag made from the stomach of a sheep. When the sun went down at the end of the day, he settled his herd and prepared to enjoy drinking a cool glass of milk. To his surprise he found a custardy, slightly acidic milk product. He further discovered that this drink mixed with water quenched his thirst.

Bacteria contained in the bag had combined with the milk as a result of the body warmth of the camel he was riding and the heat of the sun. At nightfall, when the desert temperature dropped dramatically, the milk coddled and stopped the action of the bacteria.

Whether this particular story is true is not important. This is more or less the way yogurt was probably discovered somewhere in the Middle East or the Balkans. Soon the nomads learned they could make yogurt by inoculating fresh milk with a small amount of already prepared yogurt.

In the ancient world yogurt was known as far north as Scandinavia (where it was called Taetta ) and as far east as China. The Mongol hordes who swept across Asia and most of Europe mixed their yogurt with fresh blood taken a little at a time from the mares they either rode or led.

Dahi (yogurt) was considered food fit for the gods thousands of years ago in India, even though other sour foods were considered dangerous and forbidden.

Galen, the wonder-working Greek physician and writer of the Second Century, prescribed yogurt for his overindulgent patients to cure bilious and burning stomachs.

And in 679 A.D., some of the leading doctors of Greece, Persia, Syria, Arabia and India wrote a treatise stating that yogurt was good for strengthening the stomach and for refreshing and regulating the intestinal tract.

Yogurt was introduced to the United States by Turkish immigrants in the 1870s.

The oldest manufacturer of yogurt in this country is Colombo of Methuen, Mass. Sarkis and Rose Colombosian came to this country fleeing the Armenian massacre of 1917. By 1929 they had started the "Wild Rose Dairy," and used horse-drawn wagons to deliver milk from their farm in Lawrence, Mass. With the extra milk Rose made yogurt on a wood stove for the family. Soon the local Syrian, Greek and Armenian communities asked for the yogurt. Word spread, and by the early '30s the Colombosians were supplying other ethnic pockets, such as the Syrian community on Brooklyn's Atlantic Avenue.

Meanwhile, in Europe a Spanish businessman named Isaac Carasso became fascinated with Metchnikoff's investigations. Gathering cultures from Bulgaria and the Pasteur Institute, Carasso began to manufacture yogurt for sale through local pharmacies. Slowly the business expanded to France, where his son Daniel took over the operation of the business -- thus the name Danone.

During the World War II Carasso immigrated to the United States, buying a small yogurt factory that had been supplying the enclaves of Turks, Arabs and Greeks in metropolitan New York. With Danoe, anglicized to Dannon, and the taking on of a new partner named Joe Metzger, who marketed yogurt as a food rather than a medicine, the health food got its new lease on life. In 1979, 567 million pounds or 6 cups of yogurt per capita were consumed in this country. That was up from 190 million pounds 10 years earlier.

Now, everyone is into the yogurt act. Dannon and Colombo have been joined by Breyers, Bisson, Erevan, Dutch Yogurt, Yoplait, La Yogurt, Weight Watchers, and Sweet 'n' Low, plus all the house-brand yogurts carried by the supermarket chains. You can find such items as Dannon's carobcoated Boysenberry frozen lowfat yogurt on a stick; Natural Nectar's raspberry incredible eatable with raspberry, honey, and frozen yogurt between a granola cookie; Beautiful Day's frozen yogurt on a stick sweetened with fructose and honey. For the first time yogurt is mentioned as one of the daily food requirements recommended in the U.S. Dietary Guidelines.

And yogurt, which was once low in calories, isn't always anymore. A cup of plain yogurt has 150 calories. A cup with fruit ranges from 230 to 270; flavored yogurts have about 200 calories. While one scoop of mango sherbert in a sugar cone has 132 calories, frozen yogurt on a stick has 130 to 140 calories. A chocolate fudge cone has 229 calories and frozen yogurt in a cup with fruit 210.

But to Middle Easterners, where it all began, plain yogurt is essential to good eating. Tahn , an Armenian thirst-quenching drink, is made from yogurt thinned with water. Yogurt with garlic and lemon makes a salad dressing or a soup filled with cucumbers. Yogurt can be eaten as a side dish with meat or can be incorporated into the sauce for a delicious lamb dish. Yogurt is a low-calorie substitute for sour cream in dips and adds a pleasant taste to the pound cake that follows:

Fresh milk and a fast cooling process are necessary to good yogurt.

Yogurt advocates cannot guarantee that you will live to the age of 100, but they can put you on a refreshing pathway in that direction. Try making your own yogurt -- it is unbelievably easy -- or use the commercial varieties for the following dishes. GEORGE EDER'S YOGURT RECIPE (1 quart) 1 quart whole milk 1 teaspoon yogurt for starter

Using a saucepan bring the milk to scalding, just before boiling. Remove from heat. When the milk temperature drops to 100-110 degrees, remove the scum, add the starter and mix well. Pour into 4-6 individual custard dishes. Cover and place in a warm oven that has been preheated to 200 degrees and then turned off. Let stand overnight, or 8 hours.

Next morning immediately place in the refrigerator to set for 8 more hours before serving.

Note: Contrary to what one might think the more starter used the more watery the yogurt, so don't add too much. Also, remember to reserve a bit of the yogurt for your next batch. FRUIT YOGURT POUND CAKE 2 1/4 cups all-purpose flour 2 cups sugar 1/2 teaspoon salt 1/2 teaspoon baking soda 1 teaspoon grated lemon peel 1 teaspoon vanilla 1 cup soft margarine or butter 1 cup pineapple-orange-fruit yogurt (or your favorite fruit flavor) 3 eggs (3/4 cup)

Combine in a large mixer bowl and blend at low speed all the ingredients. Beat three minutes at medium speed and pour into the greased pan. Bake in a 325 degree oven 60 to 70 minutes until cake springs back when touched. Cool upright in pan in 15 minutes. Remove and cool completely. Serve as is or with a glaze made from 1 cup confectioners sugar and 1 to 2 tablespoons lemon juice. LAMB SHANKS WITH YOGURT (6 servings) 6 lamb shanks 1 clove garlic, thinly sliced 1 teaspoon salt 1/4 teaspoon pepper 2 tablespoons vegetable oil 1 can (4-ounces) mushroom pieces, with liquid 1/2 cup finely chopped onion 1 cup dry white wine 2 tablespoons flour, diluted with a little water 1/2 teaspoon dill weed or 1 1/2 teaspoons snipped fresh dill 1 teaspoon paprika 1 cup yogurt

Make a slit next to the bone in each shank and insert a silver of garlic. Sprinkle with salt and pepper and brown in a skillet on all sides in the oil. Add mushrooms, onions and wine, and simmer 1 1/2 hours or until the meat is tender.

Remove shanks to heated platter. Combine flour mixture with the pan drippings, stirring until thickened. Reduce heat and stir in the yogurt, dill and paprika. CUCUMBER-YOGURT SALAD (4 to 6 serving) 4 small cucumbers 1 teaspoon salt 2 cloves minced garlic 2 tablespoons chopped fresh dill Juice of 1/2 lemon 1 cup plain yogurt 1 tablespoon chopped fresh mint leaves

Peel the cucumbers if the skin is waxy. Otherwise, draw tines of a fork lengthwise all around. Slice the cucumbers paper thin and sprinkle with salt. Let sit for 1/2 hour and squeeze out liquid. Combine the garlic, dill, lemon, and yogurt. Add to the cucumbers and mix well. Add more yogurt if you want a dressing. Chill well and serve sprinkled with fresh mint.

NOTE: You can also add fresh tomatoes, green pepper or onions to this salad. COLD YOGURT SOUP WITH CUCUMBER AND RAISINS (6 to 8 servings) 3 cups plain yogurt 1 1/2 cups cold water 6 ice cubes 3/4 cup lowfat milk 2 hard-boiled eggs, chopped 2 small diced cucumbers 3/4 cup raisins 2 teaspoons salt 1/4 teaspoon freshly ground pepper 1 tablespoon freshly chopped mint, basil or dill or to taste 2 teaspoons chopped chives or to taste

Combine the yogurt, water, ice cubes, lowfat milk, eggs, cucumbers, raisins, and salt and pepper to taste. A large glass or ceramic bowl is best for this. aCover and refrigerate overnight. Before serving, sprinkle each portion with chopped mint or dill and chives. TAHN -- YOGURT DRINK (1 serving) 1/2 cup yogurt 1/2 cup water with ice cubes

Dilute the yogurt with water and ice cubes to make it thin enough to drink.

Chill and serve as a drink with meals.