Animal grease is not suitable as a lubricant because it soon becomes "rancid," that is, it gives off a disagreeable odor and forms acids. Careful experiments show also that the changes which take place in grease and other organic substances when exposed to warm, moist air are caused by small living plants or organ-isms. When these minute organisms alight upon certain vegetable and animal substances, they grow vigorously, and live on the material. As the result of their action, a chemical change takes place. In the case of starch or sugar this change is called fermentation; in the case of fat, rancidity; and in the case of proteids (compounds of nitrogen, carbon, oxygen, and hydrogen), putrefaction. These living organisms are called microbes, germs, and bacteria.
All the changes that take place in milk, such as souring, becoming tainted, etc., are due to bacteria. Cream, as it is obtained from milk, contains bacteria in large quantities, and as these organisms grow they produce the ripening effect which gives flavor to the butter. Certain species of bacteria carry disease and produce undesirable effects upon the flavor of the cream and butter. To counteract such harmful changes, growths of special protective bacteria called cultures are introduced into the butter for the purpose of preserving its flavor. Some bacteria are very harmful as they produce disease in both the human body and in other substances, but others are extremely useful in industry, as they produce desirable chemical changes and assist in converting raw materials into finished products. Such a beneficial change is produced by bacteria in the case of tanning.