Before 1890 it was supposed that the flavor of fine butter depended upon certain volatile oils and acids peculiar to butter-fat. In the early nineties Professor V. Storch of the Danish Experiment Station showed, however, that it was due rather to the products of bacteria and he isolated the lactic acid bacilli which would produce such exquisite flavor even when perfectly neutral and tasteless butter-fat was churned with milk acidified or ripened with a pure culture of these bacilli. In this country Dr. H. W. Conn of Wesleyan University, Storrs, Conn., did much to advance the theory and practice of ripening cream with a pure culture starter. “Pure cultures” are produced in the bacteriological laboratory by picking out under the microscope colonies of the desired species of bacteria, planting them in a sterilized medium and allowing them to grow under the most favorable conditions and with the exclusion of all other germs. [Illustration: Streptococcus lacticus (Storch, No. 18) ] [Illustration: Bacterium lactis acidi (from Storch) ] When such a culture has reached its maximum growth it is transplanted into a larger quantity of a sterilized medium containing proper nourishment for the particular organism. In the bacteriological laboratory, where alone absolute sterility of utensils and medium, and entire exclusion of foreign infection are possible, the culture may remain pure while this inoculation and propagation are repeated over and over again. But when the propagation is carried on in the house or the dairy, for instance in preparing starters or buttermilk, such absolute cleanliness is impossible and in the long run infections will creep in from the air or from the utensils and after a while it becomes necessary to start with a new “pure culture.” How often such renewal must be resorted to depends largely upon the surroundings and the care of the operator. Usually it must be done after a week’s time, although it is surprising to find milk preparations made by the simplest processes equal in purity to those prepared with the assistance of bacteriological science and technique. This is, for instance, the case in Bulgaria, where the famous Yoghourt sour milk is prepared pure without special care and in Denmark where the country is fairly permeated with the lactic acid bacilli used in ripening the cream for the celebrated Danish butter and where careful buttermakers often maintain their starters for months or even for years without “renewal.” [Illustration: Typical lactic acid bacteria (L. A. Rogers) ] There are many different varieties of bacteria which convert sugar of milk into lactic acid, at the same time developing flavors more or less agreeable and characteristic for the various products. In the bacteriological laboratory certain species are selected which will produce the results desired for the particular purpose in view.