As a result of experience alone, improvements in the development of the "home made" starter took place. By careful selection of clean milk, and the natural fermentation of this under carefully controlled conditions, as well as the control of the temperature of the cream during the ripening, improvement in the technique of cream ripening gradually developed. More and more attention was given to the preparation of the starter, and its propagation from day to day, under conditions which would prevent its deterioration. This method of utilizing naturally fermented milk or cream was gradually extended, until it became almost universal in the larger butter-producing districts. In 1890 a more refined and scientific process was introduced by the Danish bacteriologist, Storch. Recognizing the fact that butter flavor was attributable to the development of the bacteria present in the ripening cream, he conceived the idea of isolating the various types of organisms found in milk and testing them as to their effect on the quality of flavor. Selection was then made of the most favorable flavor-producing types, and these were propagated in suitable culture media, such as skim milk, which was rendered more or less perfectly sterile by pasteurization or sterilization. Under such conditions the addition of a selected ferment could be made to the fresh cream, and so control the type of fermentation which occurred therein. An essential requisite in any organism used for this purpose must be the ability to produce relatively large amounts of acid rapidly at ordinary ripening temperatures, and also to form sufficient quantities of the proper flavor-producing substances to impart a suitable flavor to the butter fat. Such starters are known as pure culture or commercial starters, and are prepared in both liquid and dry form. At present they are used to a greater or less extent in all of the leading dairy districts. Liquid starters consist of a mass of sterile nutrient medium, milk or beef broth, inoculated with the pure culture. The dry starters are made by adding liquid cultures, containing the growing bacteria, to some absorbing material, such as milk sugar, milk powder, or starch, the whole mass being dried at low temperatures, so as not to injure the bacteria. Under such conditions the bacteria, exist in a dormant state, and are protected from their own by-products, to which they would be exposed if maintained in liquid cultures. The keeping quality, therefore, of dry cultures, is much better than that of liquid cultures.