boiling it. The exudation is rich in natural yeasts which bring about its rapid fermentation to produce an intoxicating alcoholic drink.
Arrack is obtained by distillation of fermented toddy. If allowed to ferment for more than 24 hours, toddy becomes unpalatable as
alcohol is changed to vinegar.
Microbes are also used to ferment fish, soyabean and bamboo shoots to make foods. A most popular type of fermented food is produced
from soyabeans called tempeh kedele. It is important source of vitamin B 12. Tofu and sofu are also obtained from soyabeans.microbes in industrial products
Fermentative activity of microbes is used industrially to obtain a number of products. The two common ones are beverages and
antibiotics. The container, where fermentation is carried out, is called fermentor or bioreactor.
Fermented beverages
Yeast species used in alcoholic fermentation are Saccharomyces cerevisiae (brewer’s yeast), S. ellipsoidens (wine yeast), S. sake (sake
yeast) and S. pireformis (ginger beer/ale yeast).
The nutrient medium is barley malt for beer, fermented rye malt for gin, fermented rice for sake, cashew-apple for fenny, potato for
vodka, fermented cereals for whisky, fermented molasses for rum and fermented juices for wines and brandy.
baker’s and brewer’s yeasts
The yeasts are basically of two types-baker’s yeast and brewer’s yeast. The selected strains of Saccharomyces cerevisiae, used in
baking industry to bring about desired changes in flavour and texture of the baked products, are sold as baker’s yeast. They are
available in the form of powder or cakes. The yeasts, which are used in the brewing industry for the preparation of alcoholic drinks,
are collectively called brewer’s yeast.Different kinds of alcoholic beverages are produced by using different brewer’s yeast and the fermenting medium. Beer and wine are
undistilled beverages whereas whisky, rum, gin, brandy, vodka and fenny are distilled beverages. Distilled beverages are called
‘hard liquors’ as they contain higher percentage of alcohol.
Beer is an undistilled product of grain-mesh fermentation brought about by Saccharomyces cerevisiae and
S. carlsbergensis. Wine is an undistilled product of fruit-juice fermentation brought about by Saccharomyces ellipsoideus.
Fermentation can be done using three different methods:
(i) Batch culture : In the batch culture, the conditions are set up and not changed from outside once the fermentation starts;
for example, no more nutrients are added. This is why the process is described as a closed system. The process is stopped once
sufficient product has been formed. The contents of the fermenter are then removed, the product isolated, the microorganism
discarded and the fermenter is cleaned and set up for a fresh batch.
(ii) Continuous culture : Continuous culture involves continuous long-term operation over many weeks, during which nutrient
medium is added as fast as it is used, and the overflow is harvested. Although this is closer to the situation in natural
environments, such as the gut, continuous culture has found only limited application. However, it is used for the production
of single cell protein where a large biomass of cells is required.
(iii) Fed batch culture : This is a compromise between the two systems. With fed-batch culture more control of the process
is involved than with batch culture. The period of growth is extended by adding nutrients at low concentrations during the
fermentation, and not adding all the nutrients at the beginning. One advantage of this is that the rate of growth can be
regulated to keep pace with the rate at which oxygen can be supplied. The process is commonly used to produce yeast cells
for the baking industry.
antibiotics
The term ‘antibiotics’ was coined by Waksman (1942). Antibiotics (Fr. anti = against;
bios = life) are chemical substances secreted by some microorganisms which inhibit the
growth and development of other microbes. Antibiotics are obtained from eubacteria,
fungi and actinomycetes. Most of them are produced by actinomycetes, specially the genus
Streptomyces.
The study of antibiotics began by the discovery of penicillin in 1929, when Alexander
Fleming proved that the filtrate of a broth culture of Penicillium notatum has antibacterial
properties in relation to Gram-positive bacteria. In 1940, Ernest Chain and Howard Florey
commercially extracted a relatively stable preparation of penicillin. The chemical was extensively