Fermentation can be started in three ways:
Firstly, so called spontaneous fermentation. Many raw materials already contain the microorganisms necessary to start the fermentation – for instance, the cabbage leaves used for sauerkraut production already contain all the lactic acid bacteria necessary to produce the acid to put the “sauer” in sauerkraut. The only thing needed to start the process is to compress the leaves to create an anaerobic environment and to add some salt to avoid growth of unwanted microorganisms.
Secondly, by so called back slopping – the use of some leftover material from a previous successful batch to start a new batch. For example, adding a bit of left over yogurt to a fresh batch of milk to produce a new batch of yogurt.
Thirdly, the use of a starter cultures - to avoid failed batches and eliminate contamination risks the use of commercially produced starters or starter cultures (basically stabilized pure preparations of microorganisms) is now common practice in in the dairy industry.
The term fermentation technology is now widely used with respect to any large scale production of microorganism for instance in the production of probiotics.