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Microbes in Human Welfare — BIOLOGY STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 ScienceBIOLOGYMicrobes in Human Welfare2 Marks
Question
Write name of group of bacteria that converts milk into curd. Explain the process. Write one other useful activity of this type of bacteria.
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Answer

Curd: It is prepared by bacterial fermentation of milk. The bacterium taking part in the production of curd from milk is Lactobacillus lactis, commonly known as Lactic Acid Bacteria (LAB). Lactic acid bacteria include-Lactobacillus acidophilus, L. lactis and Streptooccus lactis. These bacterial strains produce lactic acid that coagulates and partially digest to milk protein below $40^{\circ} C$ temp. A small quantity of curd (starter) is added in fresh milk. Starter contains millions of lactic acid bacteria which rapidly multiply at suitable temperature, resulting in the conversion of milk into curd. Vit. $B _{12}$ is also increases, due to which nutritional quality is also improved. During this process, Lactobacillus converts lactose sugar into lactic acid (sour) while streptococcus coagulates casein protein of milk. 
Curd making Process: For the production of curd, milk is boiled and cooled upto $37^{\circ} C$  temp. Now, it is poured into curd making container and added 1-2 teaspoons of curd. Mixed well and kept at room temperature. After adding starter, curd is prepared within 5-6 hrs. If starter is added in more quantity or curd is left unused, it becomes sour. In the curd of 2-3 days old, bacterial population increases rapidlly, resulting in foul smell. The bacteria of this group are also used in preparation of yoghurt and butter milk.

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