Transcending
The
Dr
JS Pai explains the microbial activities and procedures that facilitate
the fermentation of dairy products, bakery and beverages
Fermentation
of food began aeons ago when ripe fruits fell from the trees and upon
contamination with yeast, naturally present on it, caused conversion
of sugars to ethanol. Through observation of this natural process humans
learnt the art of preparing alcoholic beverages. This art became a science
only after Louis Pasteur discovered the presence of yeast, which brought
about the formation of wine from fruit juices.
Similar
natural fermentations were also observed in case of bread dough. In
Middle Eastern countries, in ancient times, unleavened bread like a
roti used to be prepared. Chance contamination by yeast granted the
leavening effect making the dough rise, which in turn made it light,
and upon baking a soft textured bread-like product was formed. Most
of these fermentations occurred either naturally or by accident and
produced certain product characteristics which were unique and quite
desirable.
However,
these kinds of fermentations were conducted without the knowledge that
there were microorganisms involved.
Microbes
used in fermentation
These
microorganisms are microscopic in size, although collectively grown
aggregates such as mould on bread can be perceived through the naked
eye. There are three types of microorganisms that are involved in food
fermentation, namely, bacteria, yeast and mould. Bacteria are the tiniest
and they have sizes that may be as small as one or two microns (10-3
mm). Yeasts are bigger than bacteria and moulds are generally the largest.
Bacteria
are normally used in fermentation of milk and milk products. Lactic
acid bacteria like Streptococcus lactis (Lactococcus lactis) and Lactobacillus
bulgaricus while fermenting produce lactic acid, which sours the milk.
Curd and yoghurt are formed under controlled fermentation conditions.
Yeasts such as Saccharomyces cerevisiae and Sellipsoideus are used commonly
in alcoholic fermentations although many traditional fermentation products
like idli, dosa and dhokla batter undergo fermentation due to both bacteria
and yeast. Moulds like Aspergillus niger and Penicillium spp usually
are undesirable in foods and are extremely useful in the pharma industry
wherein they are used for preparing antibiotics. Soya fermented products
and cheese fermentations commonly use fermentation with moulds.
Dairy
products
Fermentation
of milk also started early and there are a large number of traditional
products which are now being produced scientifically. Milk has a large
number of naturally occurring micro-organisms, which enter the milk
from the surroundings immediately after milking. They produce lactic
acid and if allowed to grow they spoil the milk due toproduction
of excess acid and other undesirable chemicals including various kinds
of toxic substances.
Milk
is fermented with mixed natural lactic acid bacteria for making curd.
It is heated to destroy the bacteria present in the sample of milk which
in addition to desirable lactic acid bacteria may also contain undesirable
lactic acid and other bacteria as well as yeast and mould. Then it is
inoculated with culture and fermentation is carried out at the right
temperature. Lactic acid bacteria rapidly produces acid which at about
pH 4.6 causes the coagulation of casein, making curd. Further production
of acid not only increases sourness but also increases the compactness
of the curd.
Yoghurt
normally is prepared using pure culture. It may have only a single species
or sometimes more. The selection of species is based on their ability
for rapid production of lactic acid and development of flavour. In order
to avoid any uncertainty in fermentation due to already existing bacteria
in milk, all the microbes are destroyed by heating before inoculation.
The inoculums are selected for their rapid growth so even if the heated
milk has some bacteria remaining after heating, the inoculated bacteria
quickly overtakes them. Use of pure starter culture ensures uniform
results batch after batch giving consistent production of yoghurt with
same acidity, texture and flavour.
Another
common dairy product prepared after fermentation is butter. When cream
is separated from milk by centrifuging with the help of cream separator,
skimmed milk is separated from the cream. The cream is then fermented
to produce acidity and flavour using lactic acid bacteria. Commonly
two types are added simultaneously, Streptococcus lactis or Streptococcus
cremoris for rapid production of lactic acid and Leuconostoc citrovorum
for flavour generation. Quick formation of acidity prevents growth of
undesirable bacterial activity and citrate present in milk is converted
to diacetyl, which gives butter its flavour. The soured cream is churned
to separate butter from the buttermilk.
Cheese
is produced by first curdling the milk using lactic starter culture
such as S lactis, S cremoris, L bulgaricus. The enzyme rennet which
is obtained from a calves stomach or now from genetically modified microorganisms,
is then added to the fermented milk. Action of both acid and enzyme
coagulates the milk. This is then drained to remove whey and firm up
the curd. The unripe curd is used to make soft cheese like cottage or
cream cheese, whereas ripening of various degrees is undertaken for
a large variety of cheeses like Cheddar, Roquefort, Camembert, Edam,
Swiss and Blue, using either bacteria or mould.
India
has been a leader in milk production in recent years with over 80 million
tonnes production and there is a need to prepare value added product
such as cheese, yoghurt, butter and buttermilk. There has been a greater
production of these products not only for domestic consumption but also
export. There is a very good scope for some traditional products, prepared
using fermentation, such as paneer, shrikhand, and many others which
also have excellent export potential. Although machinery is available
for western products most of the Indian traditional products are still
prepared using small scale and manual operations.
There
is a need for development of machinery for these. Even the machinery
for fluid milk processing was earlier available only for large scale
whereas most of the processing units are small or cottage scale. Machinery
manufacturers have been producing smaller scale machinery useful for
this scale also. Equipment such as cream separators, homogenisers and
HTST pasteurisers are available in smaller scale also. Only the aseptic
processing machinery is still beyond the reach of smaller scale.
Bakery
products
The
same yeast that ferments beer, Saccharomyces cerevisiae, also ferments
bread dough. Although bread can be made without fermentation by yeast,
using chemical leavening agents, yeast fermentation provides an excellent
flavour. Water and yeast are added to refined wheat flour (maida). Yeast
cannot use starch so it has to......
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