=Yoghourt= or =Bulgarian Sour Milk= is prepared with a culture of bacteria originally found in Bulgaria where Metchnikoff, the late director of the Pasteur Institute of Paris, found people living to exceptional old age which he ascribed to the fact that their principal diet is sour milk of very high acidity. The theory is that a luxurious growth of lactic acid bacilli, acting as a germicide, destroys other fermentations in the lower intestines. The bacilli active in Yoghourt require a somewhat higher temperature for their best growth than the lactic acid bacilli predominant in sour cream for the finest butter, a fact which must be taken into consideration in preparing the various products. FERMENTED MILK In the preparation of Koumis, Kefir and other fermented milks of the same class, Yeast plays an important part, changing some of the milk-sugar into the alcohol which is found in these preparations in quantities up to 2%. [Illustration: Baba Vasilka, 126 years old and her son, Tudor, 101; Bulgarian peasants whose principal food has been sour milk all their lives ] =Koumis= was first made from mare’s milk by the Tartars, but is now prepared in this country from cow’s milk by the addition of sugar and yeast. As carbonic acid is developed in the process it is quite effervescent and may be regarded both as a food and a stimulant. It is sometimes prescribed by physicians in cases when the patient cannot retain other food. =Kefir= is a similar preparation originally made in the Caucasus by a fermentation started by Kefir grains which contain both yeast and various bacteria. The dry Kefir grains are soaked, first in lukewarm water, later in lukewarm milk, for several days, until they develop so much gas as to float on the top. A couple of tablespoonfuls of the grains are then added to a quart of milk which is left to ferment at 70° F. under frequent stirring or shaking. When the fermentation is fully developed the grains are strained off and the milk is bottled and kept for 1 to 2 days at 60°. The grains may be used at once again in fresh milk, and when the manufacture is finished they may be dried and kept, to be used again another time. [Illustration: Kefir grains of natural size; a, b and c dried; d, e and f soaked (Orla Jensen, Milk Bacteriology) ] ------------------------------------------------------------------------ CHAPTER III CHEESE [Illustration: Students making cheese in the University cheese factory, Madison, Wis. ]