It was the great Russian microbiologist, Professor Ilia Metchnikoff, who developed this theory, and in 1908 won the Nobel Prize in medicine for this epoch-making work. Chance played a part in the matter—Professor Metchnikoff happened to be traveling in Bulgaria, just as I did later. But, while I was interested in youth, he was interested in centenarians. He was struck by the fact that the peasants in Bulgaria often lived to be a hundred or more and that the average life-span was eighty-seven years. Investigating their habits, Metchnikoff found that these old peasants were handicapped by the most primitive hygienic conditions, which certainly should have killed them off young. At the same time, however, he learned that the mainstay of their diet was a kind of sour milk which under analysis proved to contain an enormous amount of lactic acid microbes. These microbes, he said, prevented the multiplication of noxious bacteria which infest man's alimentary canal and consequently shorten his life.
But since there were other microbes in milk as well, the difficult task of Professor Metchnikoff was to extract only the necessary microbes and implant these in sterilized milk so as to present to the world the perfect product. The newly isolated microbe was named Bacillus bulgaricus, in honor of those old Bulgarian octogenarians.
In Biblical times there was already mention of soured milk—and all sour milk contains some Bacillus bulgaricus. In Genesis, Abraham offered his guests some soured milk and some sweet milk, as well as some veal that had just been brought in. In the Fifth Book of Moses there are a lot of foods Jehovah allowed to his people, and he made them eat plenty of soured milk. It is not recorded if Methuselah, who lived to the ripe old age of 969 years, ate soured milk; but I take it that he did, because in his time this was the mainstay of the diet.
In Egypt, since remotest antiquity, the people had enjoyed soured milk which was known as leben raib, and in Africa yogurt is known as yaourth. Actually, yogurt can be spelled in as many different ways as the name Shakespeare. It is variously referred to as yoghurt, yoghourt, yogurt, yohourt, yahourt, etc.
But there are other sour milk products which resemble yogurt very closely and which have totally different names. For instance, there is kefir, made by adding to the milk grains akin to yeast which are also called kefir. Kefir therefore is a fermented milk product, just like yogurt. It is a great favorite in the Caucasus, and if you went there and asked the natives who first thought of kefir, they would reply in all seriousness that it was first eaten by Allah. For, with kefir, as with the leaven of bread, a speck is left over in order to create a new supply.
Then there is kumiss, also a Caucasian favorite, which you can find in Siberia, too, if you are willing to go that far. It is fermented, just like yogurt and kefir, but the milk used in it is either mare's or camel's, and a further difference is that it has a very small alcoholic content. Mare's or camel's milk must be absolutely fresh to make kumiss; to this fresh milk a tiny bit of yesterday's kumiss is added, and then the stuff is left to ferment. It goes far back in history. Herodotus mentioned it, and the ancient Scythians loved it. Kumiss, just like yogurt, is supposed to have great medicinal value for the alimentary canal and, in fact, around forty years ago became the rage in Swiss sanatoriums, since it seemed to help people afflicted with severe cases of pulmonary diseases.
In Egypt yogurt is made of buffalo milk. This is the leben raib I spoke of before. Petit whey, another yogurt-like product, is made by the Arabs from the liquid portion left over from the milk used in cheese-making.
Let us get back to yogurt, and especially how to make it. This is one of the simplest things in the world. First you have to free the milk from all the cream, because fat is not good in preparing yogurt. Then you have to boil the milk, stirring it constantly to keep it from sticking. Boil it for a full five minutes and then let it cool. Now, to each quart of milk, add a tablespoon or a tablet of yogurt culture (Bacillus bulgaricus), which you may obtain from a score of yogurt plants throughout the United States or from some drug stores. And then stir well again. After this pour the yogurt into glasses or jars, covering them with cheesecloth and placing them in a cool spot (not in the refrigerator!) for six hours. At the end of this time remove the cheesecloth, leave the jars uncovered for another six hours—and there you are! Now you can put it in the refrigerator and chill it before you eat it.