Q.What does make idlies puffy?

The yeast Saccharomyces cerevisiae carries out alcoholic fermentation of sucrose: its enzyme invertase first hydrolyses sucrose into glucose and fructose, and its enzyme zymase then ferments these sugars into ethanol...

Yeast first splits sucrose into simple sugars (invertase), then ferments them into alcohol and CO2 (zymase).

Alcoholic fermentation of sucrose is carried out by the yeast Saccharomyces cerevisiae in two enzymatic steps. First, the yeast enzyme invertase (also called sucrase) hydrolyses the disaccharide sucrose into its constituent monosaccharides, glucose and fructose. Second, the yeast enzyme complex zymase acts on these hexose sugars, fermenting them anaerobically into ethyl alcohol (ethanol) and carbon dioxide gas. This two-step microbial fermentation process is exploited commercially in the brewing and wine industries as well as in the baking industry, where the CO2 produced causes dough to rise.

Correct answer: (b) Saccharomyces

Alcoholic fermentation is carried out by the yeast Saccharomyces cerevisiae.

Alcoholic fermentation is an anaerobic microbial process in which sugars (glucose, sucrose) are converted into ethanol and carbon dioxide. It is brought about by the yeast Saccharomyces cerevisiae, which uses the enzyme complex zymase to ferment hexose sugars into ethyl alcohol and CO2. This process is exploited commercially in brewing, wine-making and the baking industry (the CO2 released causes dough/batter to rise). Lactobacillus causes lactic acid fermentation (e.g. curd), Trichoderma is used as a biocontrol fungus, and Streptomyces is an antibiotic-producing actinomycete — none of these carry out alcoholic fermentation.

Microbial fermentation of the idli batter (by bacteria such as Leuconostoc mesenteroides and yeasts) releases CO2 gas, which gets trapped as bubbles in the batter, making it rise and become light and puffy.

Fermenting microbes in idli batter release CO2 that leavens (puffs up) the batter.

Idli batter is prepared from a mixture of rice and black gram (urad dal), which is left to ferment overnight. During this fermentation, naturally present bacteria (chiefly Leuconostoc mesenteroides, which produce lactic acid and gas) along with yeasts metabolise the sugars in the batter, releasing carbon dioxide gas as a byproduct. Because the batter is viscous, the CO2 gets trapped as numerous small bubbles distributed throughout it, causing the batter's volume to increase (rise) and giving steamed idlis their characteristic light, spongy, porous texture. This same principle of gas-producing microbial fermentation underlies the puffiness of dosa and bread as well.