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How are microbes used in household products like curd and bread?

Class 12 BiologyNCERT chapter: Microbes in Human Welfareइसे हिंदी में पढ़ें

Exam-ready answer

Lactic acid bacteria (LAB) such as Lactobacillus convert milk into curd: they produce acids that coagulate and partially digest milk proteins, and a little curd added to fresh milk acts as the starter. Bread dough is fermented by baker's yeast, Saccharomyces cerevisiae, whose carbon dioxide makes it rise. Idli and dosa batter is fermented by bacteria.

Explanation

Microorganisms such as Lactobacillus and others, commonly called lactic acid bacteria (LAB), grow in milk and convert it into curd. A small amount of curd added to fresh milk works as an inoculum or starter, because it already contains millions of LAB. At a suitable warm temperature they multiply, and the acids they produce coagulate and partially digest the milk proteins, so the liquid milk sets into thick curd.

LAB also improve the nutritional quality of curd by increasing vitamin B12. In our stomach they are useful because they check disease-causing microbes. The dough used for making idli and dosa is also fermented by bacteria. During fermentation the microbes release carbon dioxide gas, which gets trapped in the batter and gives it a puffed-up appearance, making the food soft and light.

Bread dough is fermented using baker's yeast, Saccharomyces cerevisiae. The yeast breaks down sugars in the dough and releases carbon dioxide, which forms bubbles that make the dough rise and the bread spongy. Microbes make other foods too: toddy, a traditional drink of parts of southern India, is made by fermenting sap from palms, and the large holes in Swiss cheese come from carbon dioxide made by Propionibacterium sharmanii.

Points to remember

  1. Lactic acid bacteria (LAB), for example Lactobacillus, convert milk into curd.
  2. LAB produce acids that coagulate and partially digest milk proteins.
  3. A little curd is used as inoculum (starter) because it contains millions of LAB.
  4. LAB increase vitamin B12 in curd and check disease-causing microbes in the stomach.
  5. Bread uses baker's yeast (Saccharomyces cerevisiae); idli and dosa batter is fermented by bacteria; the puffing is due to CO2.

Exam tip

Write each scientific name in full and underline the genus and species separately in handwriting (Lactobacillus, Saccharomyces cerevisiae), and name carbon dioxide as the gas that puffs up the dough.

Questions students also ask

Why does curd set faster in summer than in winter?
Lactic acid bacteria multiply and work best at a warm temperature. In summer the milk stays warm, so the bacteria grow quickly and produce acid fast. In winter the low temperature slows their growth, so curd takes much longer to set.
How are Swiss cheese and Roquefort cheese made differently?
Both depend on microbes. The large holes in Swiss cheese are due to carbon dioxide produced by the bacterium Propionibacterium sharmanii. Roquefort cheese is ripened by growing specific fungi on it, which give it a particular flavour.
Why is curd considered more nutritious than milk?
Lactic acid bacteria in curd increase its vitamin B12 content. The acids they produce partially digest the milk proteins. In the stomach, these bacteria also help check the growth of disease-causing microbes.

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