<P> In a batch process, all the ingredients are combined and the reactions proceed without any further input . Batch fermentation has been used for millennia to make bread and alcoholic beverages, and it is still a common method, especially when the process is not well understood . However, it can be expensive because the fermentor must be sterilized using high pressure steam between batches . Strictly speaking, there is often addition of small quantities of chemicals to control the pH or suppress foaming . </P> <P> Batch fermentation goes through a series of phases . There is a lag phase in which cells adjust to their environment; then a phase in which exponential growth occurs . Once many of the nutrients have been consumed, the growth slows and becomes non-exponential, but production of secondary metabolites (including commercially important antibiotics and enzymes) accelerates . This continues through a stationary phase after most of the nutrients have been consumed, and then the cells die . </P> <P> Fed - batch fermentation is a variation of batch fermentation where some of the ingredients are added during the fermentation . This allows greater control over the stages of the process . In particular, production of secondary metabolites can be increased by adding a limited quantity of nutrients during the non-exponential growth phase . Fed - batch operations are often sandwiched between batch operations . </P> <P> The high cost of sterilizing the fermentor between batches can be avoided using various open fermentation approaches that are able to resist contamination . One is to use a naturally evolved mixed culture . This is particularly favored in wastewater treatment, since mixed populations can adapt to a wide variety of wastes . Thermophilic bacteria can produce lactic acid at temperatures of around 50 degrees Celsius, sufficient to discourage microbial contamination; and ethanol has been produced at a temperature of 70 ° C . This is just below its boiling point (78 ° C), making it easy to extract . Halophilic bacteria can produce bioplastics in hypersaline conditions . Solid - state fermentation adds a small amount of water to a solid substrate; it is widely used in the food industry to produce flavors, enzymes and organic acids . </P>

All fermentation pathways have the following in common