As scientific investigations regarding the causes of food spoilage were undertaken, they pointed the way to a wider application of methods already in use and to the discovery of new ones. Before 1860 changes in food were explained on the theory of spontaneous generation. Pasteur demonstrated that ferments, molds, and some forms of putrefaction were caused by the presence of microorganisms widely distributed in the environment. Since these microorganisms are the main cause of food spoilage, food preservation depends on rendering conditions unfavorable for their growth. Processes of preservation may be generally classified as drying, heating, refrigeration (see also frozen foods), and the use of chemicals or other particular agencies.