Food Fermentation: Idli and Dosa

Food Fermentation: Idli and Dosa

Idli is a popular fermented breakfast in India, especially South India. It is prepared from rice (Oryza satiua L.) and black gram (Vigna mungo). It has a spongy texture and a pleasant sour taste due to the lactic acid in it. Idli is a legume based fermented food, that is prepared by steaming a fermented batter.

Production of Idli

The ingredients, Rice and black gram are carefully washed, soaked in water separately for 5-10 hours to soften them and to facilitate decortication, then ground, mixed, and finally allowed to ferment overnight. The proportion of rice and black gram varies from place to place. The mixture is allowed to ferment overnight (20-22 hours). In the traditional system the fermentation is spontaneous and the mixture is leavened up to approximately 2 or 3 times. The fermented batter is steamed and served hot. Idli is highly nutritious, being rich in nicotinic acid, thiamine, riboflavin, and methionine.

 Microflora associated with fermentation

1. The microorganisms developing during the initial soaking of ingredients are sufficient to carry out fermentation.
2. Both bacteria and yeast play an important role in fermentation. Bacteria are responsible for acid and gas production, whereas yeast is responsible for organoleptic quality.
3. Black gram allows maximum growth of fermentation organism as it is high in nutrients that support growth of lactic acid bacteria.
4. Bacteria associated are Leuconostoc mesenteroides, Lactobacillus coryniformis, Lactobacillus delbrueckii, Lactobacillus fermentum, Lactobacillus lactis.
5. Yeast associated with fermentation are Saccharomyces cerevisiae, Torulopsis candida, Trichosporon pullulans, Debaryomyces tamarii. 
6. The organisms involved in the acidification have been identified as Streptococcus faecalis, and Pediococcus spp. The leavening is brought about by Leuconostoc mesenteroides, although the yeasts, Torulopsis candida and Trichosporon pulluloma are also found in traditional Idli. 
7. The role of lactic acid bacteria is to reduce pH of batter to level 4.4 to 4.5 and this pH is optimum for yeast activity.

Biochemical changes

1. There are two major changes and that are Leavening and acidification.  During leavening the volume of batter increases from 1 to 3 times. Acidification is due to the lactic acid produced.
2. The pH value decreases from 6 to 4.3 – 5.3. There is a slight increase in level of soluble salts and non-protein nitrogen.
3. The level of soluble nitrogen and free amino acids increases. The essential aminoacids/nutrients like choline, methionine, cysteine and vitamin C are present.
4.  Idli is a source of protein, calories, and vitamins and have enhanced nutritive value over the raw ingredients.  It contains vitamins and methionine and lack antinutritional factors since they are hydrolysed during fermentation. Fermentation eliminates and/or reduces antinutrients such as phytic acid and enzyme inhibitors and minimizes the flatulence problem.

Idli and dosa batters are both made from rice and urad dal, but they differ in texture, rice type, and thickness. Idli batter is thick and slightly coarse for soft steamed cakes, while dosa batter is thin and smooth for a crispy pancake.

Differences in Idli and Dosa batter

1. Ingredient Composition

• Idli Batter Ratio: Usually uses a 3:1 or 4:1 ratio of rice to black gram.
• Dosa Batter Ratio: Usually uses a 3:1 or 3.5:1 ratio of rice to black gram and often includes fenugreek seeds.

2. Grinding and Texture

• Idli Texture: The rice is ground to a coarse texture, while the urad dal is ground into a thick, frothy paste. The gritty rice particles provide the skeletal structure needed for the idli to rise and stay fluffy without collapsing.
• Dosa Texture: Both rice and dal are ground together or separately into a completely smooth, fine, silky paste.

3.  Consistency

• Idli Viscosity: High viscosity. The batter is kept thick. Low water content ensures that the carbon dioxide bubbles trapped during fermentation do not escape, allowing the idli to steam into a spongy cake.
• Dosa Viscosity: Low viscosity. The batter is diluted with water to a pourable, runny consistency. This allows it to be spread radially using a ladle into a paper-thin sheet on a hot tawa.

4. Fermentation and Aeration

• Idli Aeration: Requires intense aeration. The urad dal is whipped extensively during grinding to trap air. This batter is fermented until it doubles in volume. Over-fermentation makes it too sour, ruining the delicate taste of idli.
• Dosa Aeration: Requires moderate aeration. While it needs to ferment to develop lactic acid and carbon dioxide, a slightly sour, older batter is actually preferred for dosas because the high acid content and broken-down sugars caramelise better on the pan, enhancing crispiness and flavor.

 References

• Modern Industrial Microbiology and Biotechnology, Nduka Okafor, Science Publishers
• Role of Leuconostoc mesenteroides in Leavening the Batter of Idli, a Fermented Food of India,  S. K. Mukherjee, M. N. Albury, C. S. Pederson, A. G. Van Veen, And K. H. Steinkraus, Applied Microbiology, March, 1965, American Society for Microbiology, Vol. 13, No. 2
• http://www.generalmicroscience.com/microbiology/idli-fermentation-process-details/
•  Idli, An Indian Fermented Food: A Review N. R. Reddy, S. K. Sathe', M. D. Pierson And D. K. Salunkhe', Journal Of Food Quality 5 (1981) 89-101.