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Food Fermentation - Fermentation of food results in the production of organic acids, alcohols, esters etc., which not only help in preserving the food but may also generate distinctive new rood products. The fermentation may be by yeast, bacteria molds, or by a combination of these organisms.
Food products such as bread, beer and wine are produced using yeast, while -both yeast and bacteria are, involved in the production of vinegar. Bacteria are involved in the production of fermented milks, while molds are important in the production of oriental foods such as soy sauce and other soybean products.
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These organisms by virtue of their growth bring about desirable changes in the food composition and at the same time bring about a certain degree of preservation. For example in fermented pickles, the end product of fermentation namely lactic acid, serves as a preservative. Fermented pickles, therefore, do not need the addition of any preservative.
In bread making, microorganisms are involved in gas production which helps in the production of bread with a porous structure and also in the production of flavouring substances. During the preparation of dough, the yeast ferments the sugars to produce CO2 and alcohol.
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Instead of the bread yeast (bakers yeast) other gas forming yeasts such as wild yeasts and heterolactic acid bacteria have also been used. During the leavening process there is little growth that occurs but fermentation begins as soon as dough is mixed and continues till the temperature of the oven, inactivates the enzymes.
Addition of a large number of yeast cells can hasten the fermentation process and discourage growth. During fermentation, conditioning of, the dough which results from the action on gluten by proteolytic enzymes in the flour by the added yeast or from the malt and a reduction in pH takes place. Sometimes dough conditioners are also added to stimulate yeast, growth.
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The main objective of the baker during leavening is to have enough gas produced and to have the dough in such a condition, that it will hold the gas. Yeasts are also reported to contribute to, the flavour of bread through products released during the fermentation such as alcohols, acids, aldehydes, esters etc. If enough time is given for growth of bacteria before baking they may also and to the flavour.
During baking, the temperature inside the loaf does not reach 100 but the beat serves to kill the yeast, inactivate their enzymes and allows expansion of gas present to give the right structure to the bread.Beer is the principal fermented malt beverage produced world over from malt, hops, yeast and malt adjuncts. The production of malt wort, and fermentation bas been dealt earlier.
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In Japan, an alcoholic beverage called sake with an alcohol content of 14-17 per cent is produced using a. combination of a mold and yeast. Production of this involves, the, preparation of a starter (Koji) derived by growing Aspergillus oryzae on soaked and steamed rice mash. This is then mixed with more rice mash where the starch is converted into sugar by the enzymes in the koji and a mixture of yeasts is added to carry out, the alcoholic fermentation. The liquor is filtered from the fermented mash after 10-14 days and distilled.
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Microorganisms causing wine and beer spoilage are chiefly wild yeasts, molds and bacteria. Film yeasts can oxidize alcohol to water and carbon dioxide. Bacteria such - as Acetobacter convert alcohol into acetic acid. Also, bacteria such as Lactobacillus and Leuconostoc, oxidize the wine and reduce the pH or make the wine cloudy.
Pickles may be prepared with partial, complete or without fermentation. The chief types of fermented; pickles, are salted pickles, (in which sodium chloride is used) and unsalted pickles. When fermentation is desired, this is carried out mostly by lactic acid bacteria and yeast such as Pediococcus. Sometimes Pseudomonas, Flavobacterium, Bacillus may also grow and cause spoilage.
In low salt pickles other bacteria such as coliforms, Leuconostoc mesentroides etc., may also grow. Film yeast$ of the genera, Candida, Debaryomyces and Endomycopsis have been found to grow in pickles. In addition to salt, oriental pickles contain spices and oils, which are antibacterial and also add flavour to the pickles.
Coffee, tea and cocoa are also produced by partial fermentation. Tea can be either fermented (black), unfermented (green) or semi fermented. In the preparation of black tea, the leaves are allowed to undergo auto fermentation and no microorganisms are involved. The growth of Rhizopus, Penicillium and Aspergillus during the fermentation may cause spoilage.
In coffee, either the berries are allowed to dry or after removal of the outer skin, are soaked in water and allowed to ferment. During this process pectinolytic bacteria, (mostly coliform) degrade the pectin which is followed by a lactic fermentation by Leuconostoc mesentroides, Lactobacillus brevis, L. plantarum and S. faecalis.
After the pulp and residue have been washed, the beans are dried. Sometimes, due to an uncontrolled fermentation, an off flavoured coffee may be produced and deterioration of the beans can occur during drying.
In cocoa fermentation, yeasts such as Candida krusei and lactic bacteria grow and remove both the adhering pulp from the bean, kill the embryo as well as produce an aroma, flavour and colour. The temperature during fermentation of several days may rise upto 44.60°C. Molds and actinomycetes have been reported to cause spoilage of beans during curing.
In the orient, a variety of foods are prepared using microorganisms. In India for example, preparation of a product from rice and black gram called idli involves an overnight fermentation with Leuconostoc mesentroides, S. faecalis and Pediococcus cerevisia. The fermented product is cooked by steaming. Dosa from fermented rice flour and rabri from pearl millet is also prepared by fermentation.
In Japan, soya sauce is prepared using Aspergillus oryzae. In the initial stages this organism produces a variety of enzymes which breakdown soy protein and starch. In the subsequent lactic fermentation, lactic bacteria produce lactic acid. More acid production Pediococcus halophilus and alcoholic fermentation by Saccharomyces rouxi and Zygosaccharomyces soyae occurs.
An Indonesian food called Temphe is also prepared from soybean. Soybeans are soaked at 25°C, seed coats are then removed and the split beans are cooked in water for 20 minutes. These are then cooled and inoculated with spores of Rhizopus sp. (R. arrhizus or R .oryzae). The mash is packed into plastic containers or rolled in banana leaves and incubated at 32°C for 20 hours to allow mycelial growth.
The product is then sliced, dipped in salt water and fried in fat before it is consumed.
A variety of Japanase and Chinese foods such as miso, angkbak, soybean cheese etc. are also prepared by the use of fungi.
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