Dosa or Dose is arguably one of the most popular dishes in India; it is a typical part of the South Indian diet and is gaining popularity all over the world. It is a type of pancake made from fermented batter of rice and blackgram. The literary references to dosa date back to the 1st century AD. The place of origin of dosa is debatable but traditionally accepted to be Udupi, a town in the state of Karnataka. However a popular variant, the thin crust-crispy dosa, was believed to be originated from the Indian state of Tamil Nadu.
Typical crispy dosa serving
Preparation and serving: Mixture of rice and black grams (usually 2:1) soaked in water overnight is finely ground to form a batter and a pinch of salt is added. The batter is allowed to ferment overnight and then mixed with water to get the desired consistency. The batter is then ladled onto a hot griddle greased with oil or clarified butter. It is spread out evenly with the base of a ladle to form a pancake. Typical dosa is served hot along with vegetable soup (sambar), potato curry and coconut-chilly sauce (chutney) (Fig. 1), but now a day, one can find hundreds of varieties of dosa depending upon their taste and preferences.
Leaving of the batter due to Fermentation
Nutrition: The main ingredients of dosa are rice (Oryza sativa) and blackgram (Phaseolus mungo). White rice, which is normally used for dosa, contains about 90% carbohydrates, 8 percent proteins and 2% fat. It is also a good source of calcium, magnesium, phosphorus, manganese, selenium, iron and vitamins, folic acid, thiamine and niacin.
It has low fiber content and contains pro-inflammatory omega-6 fatty acids. Black gram or Mungo bean is rich in carbohydrates (about 60%) and proteins (about 25%), It also contains about 18% of dietary fiber and is a good source of minerals, potassium, calcium, iron and vitamins, niacin, thiamine, and riboflavin. Black gram has been found to be very useful in controlling cholesterol levels.
Fermentation: Fermentation gives the characteristic texture (leavening), aroma and taste to the dosa batter along with improved digestibility and nutritional value (Fig. 2). Fermentation is the process of converting carbohydrates to alcohol or organic acids with the help of microorganisms, under oxygen free conditions. (The science of fermentation is known as zymology or zymurgy.)
The microorganisms responsible for the fermentation are naturally present in the ingredients of dosa batter, black gram and rice. Some of the fermentation bacteria/microbes are also provided by water and air. A temperature of 25°-30° C is found to be highly favorable for the microorganisms to boost the fermentation process.
Fermentation of dosa batter is carried out mainly by Lactobacillales or lactic acid bacteria (bacteria that convert milk to yogurt), recognized as lactobacillus delbrueckii, L. lactis, Strptociccus lactis, S. faecalis, Leuconostoc mesenteroides and Pedicococcuscerevisiae. Wild yeasts, recognized as Saccharomyces cerevisiae, Debaryomyces hansenii and Trichosporon beigelli, on the other hand, are found to produce flavor compounds and help in the saccharification (hydrolysis) of starch. In the early stages of fermentation, the ‘heterofermentative’ type bacteria like Leuconostoc mesenteroides (Fig. 3) are found to predominate, producing carbon dioxide and alcohol along with the lactic acid (the mucilaginous property of dosa batter helps to trap the carbon-dioxide evolved during fermentation which results into leavening of the batter).
Fig 5 : Amylose (n=5 to 600)
Fig 5 : Amylopectin (m=200 to 2000 ; n=20 to 30)
Fig 7 : Hydrolysis of starch by amylase enzymes
During the later stages of the fermentation the homofermentative lactic acid bacteria like Lactococcus lactis (Fig. 4) dominate and produce only lactic acid. Due to this batter starts turning sour over the time. Starch in the rice and black gram (or in general) contains two types of homopolysaccharides, amylose and amylopectin. Amylose is an unbranched homopolysaccharide consisting of about 5-600 glucose units, linked by α-(1→4) glycosidic bonds.
It forms a helix structure with six glucose units in each helix (Fig. 5). Amylopectin is a branched molecule formed by several glucose units ranging from several hundreds to fifty thousand in a main chain, which are joined by α-(1→4) glycosidic bonds.
Lateral chains of about 20-30 glucose units are linked to the main chain by a α-(1→6) glycosidic bond. Glucose units on the lateral chain are linked again, joined with themselves by α-(1→4) glycosidic bonds. These branches provide a spongy fiber-like structure to the amylopectin and prevent the formation of a helical structure (Fig. 6). During the fermentation process the starch content of the dosa batter is hydrolysed (broken down) yielding maltotriose and maltose from amylose, or maltose, glucose and limit dextrin from amylopectin, with the help of amylases, the calcium metalloenzymes (Fig. 7). These amylase enzymes (mainly α-amylase, β-amylase and γ-amylase) are provided by the microorganisms (yeasts and bacteria) naturally present in the ingredients of dosa batter and the atmosphere.
The glucose, and other sugar molecules, then undergo a splitting process called glycolysis, a multistep metabolic pathway which involves a sequence of about ten enzyme-catalyzed reactions. Glycolysis can occur either in presence or absence of oxygen. Glycolysis is carried on in two slightly different ways, depending on the microorganisms (enzymes) and conditions involved in the process. In Embden–Meyerhof–Parnas pathway (homolactic process), glucose is gradually split into two molecules of pyruvate (3 carbon sugar) and yields two molecules of ATP (free energy containing molecule) and two "high energy" electron carrying molecules of NADH.
Scheme 1 : Homolactic fermentation of glucose
Scheme 2 : Homolactic fermentation of glucose
The phosphorylation and transport of glucose usually occurs by an ATP-dependent hexose kinase or a phosphoenolypyruvate (PEP) sugar phosphotransferase system (PTS). In a concomitant step, the 2 electrons that were added to NAD+ in the glycolysis are once again removed from NADH and added back to the pyruvate molecule, regenerating NAD+ and producing lactic acid (or lactate) (Scheme 1). In the phosphoketolase pathway (hetero lactic process), Glucose molecule is converted into glucose-6-phosphate which then dehydrogenates to give 6-phosphogluconate, upon subsequent decarboxylation it yields pentose-5-phosphate and one molecule of CO2. Pentose-5-phosphate is cleaved into glyceraldehyde phosphate (GAP) and acetyl phosphate. GAP is further metabolized to lactate as in homofermentation, with the acetyl phosphate reduced to ethanol via acetyl-CoA and acetaldehyde intermediates. End-products (CO2, lactate and ethanol) are produced in equimolar quantities (Scheme 2).
Benefits of fermentation: Fermentation process helps to break down the anti-nutrients like phytic acid present in rice and blackgram (phytic acid is known to block the mineral absorption and cause deficiencies). The mocroorganisms involved in the fermentation also produce useful substances like vitamins, folic acid, riboflavin, niacin, thiamin, biotin, vitamin K and some free amino acids as well as some antibiotic and anti-carcinogenic substances, hence increasing the total nutritional value of the dosa. Since dosa batter is predigested by bacteria, it is easier to digest. The lactic acid formed during the fermentation process, along with the various enzymes, aid the digestion of food, especially protein digestion. Lactic acid formed during fermentation not only preserves the food but also promotes the growth of a healthy intestinal flora. Lactobacilli are known to promote digestive health by inhibiting bacteria like Shigella, Salmonella and E.coli.
Role of water: Water is important to maintain the consistency (swelling and gelatinization) of the dosa batter. It acts as a solvent medium for sugars and other substrtates of fermentation process and also for enzymes. Water also acts as a source for the various microorganisms and minerals needed for the fermentation process.
Pinch of salt: Presence of salt helps to controll the fermentation process. It slows down the yeast action and helps the liberation of yeasts enzymes and other useful substances into the batter via osmosis. Since the lactic acid bacteria can tolerate high salt concentrations, the presence of salt gives them an advantage over other less tolerant species and allows the lactic acid fermenters to start the metabolism, which produces lactic acid, which further inhibits the growth of non-desirable organisms. Further addition of salt increases the mineral value of the dosa and the taste.
References and sources
- Wikipedia and other online sources.
- Future of Idlis': A Scientific Assessment (A Study on South India’s Most Preferred Breakfast Cereal) - Elizabeth Yorke, Department of Culinary Arts, Manipal University, Karnataka.
- Toder’s Online Text Book of Bacteriology.