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Fermentation is the process of controlling microbes (bacteria, yeast, and moulds) to modify food, producing a desired product.

Often we talk about the negative issues caused by microorganisms in food such as food spoilage. But they can be used as efficient live factories to produce health beneficial food. Without some microorganisms, production of certain food items may not be possible or may be costly. Microbes are cheap resources that consist of numerous enzymes which can convert complex chemical structures into simple digestible molecules with a high efficiency. These microorganisms use the nutrients in the food as the substrate to produce energy and other required precursors for their growth in which a fermented food results. There are numerous examples for fermented food.

Fish sauce
In production of fish sauce, uneviscerated fish is mixed with salt and placed in fermented tanks to allow liquefaction for about six months. The collected liquid is further ripened for few more months. Halophillic microbes are involved in this fermentation process. Streptococcus, Micrococcus and Bacillus species predominate. This product is dark coloured with a distinct aroma.

Sauerkraut
This refers to fermented cabbage. Normal microflora in cabbage is involved in the fermentation process under anaerobic conditions. Leuconostoc mesenteroides and Lactobacillus plantarum is involved. Temperature is a crucial factor in the control of fermentation. If the temperature is below 21 degrees Celcius, Lactobacilli outgrow. L. mesenteroides require a lower temperature below 21 0C. Acidity created by Lactobacilli prevent the growth of L. mesenteroides.
Pickels
Pickels consist of vegetables like cucumber, onions, chilies etc. Lactic acid bacteria such as Leuconostoc mesenteroides, P. cerevisiae, L. brevis, L. plantarum are involved in the fermentation process These bacteria also take part in fermentation of olives.

Soy sauce
In production of soy sauce, a mixture of soybean and wheat flour is inoculated with Aspergillus oryzae and Aspergillus soyae. These fungi digest complex starch and produce sugars which facilitate the growth of bacteria. Anaerobic bacteria carry out fermentation to produce soy sauce.

Beer and Ale
Malted beverages are produced by brewing. Mainly the yeasts are involved in the process. Yeasts convert fermentable sugars to ethanol and carbon dioxide. As yeasts do not produce enough amylases to hydrolyze starch in barley grains, they are germinated prior to brewing. Hops which are added for bitterness have an inhibitory effect on gram positive bacteria. Saccharomyces carlsbergensis is the principle organism used. This species is subjected to various genetic modifications to increase the efficiency of the fermentation. In addition to ethanol and carbon dioxide, yeasts produce a small amount of glycerol, acetic acid and aromatic esters. Ale is a top fermented beverage with Saccharomyces cerevisiae.

Wine
Wine is made from grape juice in large scale. Yests; Saccharomyces cerevisiae var. ellipsoideus is the culture used in wine fermentation. High temperature is not suitable for this fermentation as yeasts die while low temperature allows the growth of lactic acid bacteria.

Environmental factors such as acidity, pH, oxygen level, moisture level, temperature, sugar content are important for this fermentation processes. In most food commodities, acidity is developed by microorganisms. This developed acidity is important for preserving the food. Proteolytic action may bring down the pH to a higher value. Some yeasts produce alkaline by products such as ammonia in their regular metabolism which is encouraged in the production of limburger cheese.

Alcohol, which is a byproduct of many fermentation pathways also have a preservative action. Alcohol content produced will depend on the sugar content, type of yeast involved in fermentation, temperature and the oxygen level. Most yeast can’t tolerate high alcohol levels.
Temperature has a direct effect on microbial fermentation which in turn affects the final quality of the product. Sauerkraut production is an ideal example to show the importance of temperature in fermentation process.
Microorganisms have different oxygen requirements for their growth and fermentation. In wine production, yeasts grow best under aerobic conditions. In baking, anaerobic conditions favour the quality of the final product. Vinegar production involves anaerobic as well as aerobic fermentation.