Idli batter fermentation
How does idli batter ferment? Where does the yeast that ferments the batter come from?
ARUNA VENKATRAMAN
Coimbatore, Tamil Nadu
Fermentation of idli or dosa batter is carried out largely by lactic acid bacteria (not yeast as mentioned in your question), the same class of microorganisms that are found in ‘dahi.’
The predominating bacteria in the relevant early stages of fermentation have been found to be the ‘heterofermentative’ type like Leuconostoc mesenteroides, which produce carbon dioxide in addition to lactic acid, the acid found in dahi.
Homofermentative lactic acid bacteria produce only lactic acid. During the later stages of fermentation, the type of dominating bacteria change, but by then the batter is probably too sour. Good idli can be made only with batter fermented for a limited time.
The presence of yeasts has also been reported in the fermenting batter, but in the idli or dosa fermentation, the yeasts are more of a nuisance than helpful microbes. Anyhow, in the mildly acid environment created by lactic fermentation, growth of yeasts is probably suppressed to a great extent.
The bacteria responsible for the fermentation are naturally present on the ‘urad’ seeds, and no ‘starter’ is needed, unlike in making dahi.
Parboiled rice is needed, because the texture and taste of the idli or dosa made with plain rice is usually not satisfactory. Parboiling probably leads to partial breakdown of starch in the rice grains, changing the texture and taste.
Some people also add a handful of fenugreek seeds for soaking along with the urad.
As an aside, I might add my personal observation (not supported by definitive scientific evidence) that the dahi made in southern states of India usually seems to have quite a bit of carbon dioxide in it — ‘heterofermenative.’
It gives a lovely tingling sensation on the tongue. Dahi in the northern states is usually flat — does not have carbon dioxide in it.
The difference could be due to the type of bacteria prevalent, and also the type of milk used — ‘homogenised,’ buffalo milk, etc., as different from non-homogenised, and cow milk.
T. S. RAMAN
Retired Biochemist,
Indian Agricultural Research Institute, New Delhi
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