The industrial production of microbial food cultures is carried out after careful selection process and under strictly controlled conditions. First, the microbiology laboratory, where the original strains are kept, prepares the inoculation material, which is a small quantity of microbes of a single (pure) strain. Then, the inoculation material is multiplied and grown either in fermenters (liquid) or on a surface (solid) under defined and monitored conditions. Grown cells of pure culture are harvested, eventually blended with other cultures and, finally, formulated (preserved) for subsequent transportation and storage.  They are sold in liquid, frozen or freeze-dried formats.
Another and traditional way of starting a food fermentation is often referred to as spontaneous fermentation. Cultures come from raw milk, i.e. milk that has not undergone any sanitation treatment or from the reuse of a fraction of the previous production (back-slopping). The composition of such cultures is complex and extremely variable. The use of such techniques is steadily decreasing in developed countries. Some countries even prohibit the back-slopping technique because of the "potential to magnify pathogen loads to very dangerous levels".