Q.Name the bacterium responsible for converting milk into curd (dahi), and explain briefly how its activity changes both the flavour and the nutritional value of milk.

[!TLDR] Lactobacillus (a lactic-acid bacterium) ferments milk's lactose into lactic acid, coagulating its proteins into curd.

[!ANSWER] The bacterium is Lactobacillus. It ferments lactose in milk into lactic acid; the rising acidity coagulates casein, setting the milk as curd, and also increases the curd's vitamin B12 content compared with the starting milk.

Lactobacillus (a lactic-acid bacterium, LAB) is responsible for converting milk into curd. When a small starter of existing curd -- carrying a live LAB population -- is added to warm fresh milk, the bacteria multiply rapidly and ferment the milk sugar lactose into lactic acid. As lactic acid accumulates, the pH of the milk falls; once it drops sufficiently, the milk proteins (mainly casein) can no longer remain in their normal dissolved state and instead coagulate, setting the liquid milk into the semi-solid gel we know as curd. Along with the change in texture and the characteristic sour flavour, this bacterial fermentation also improves the food's nutritional value, since Lactobacillus synthesises vitamin B12 during growth.

[!ANSWER] Lactobacillus ferments milk lactose into lactic acid, whose acidity coagulates milk proteins into curd while also enriching the food's vitamin B12 content.

Identify the specific bacterial genus involved, then trace the causal chain: sugar fermented → acid produced → pH falls → protein coagulates → texture changes; separately note the nutritional side-effect (vitamin synthesis).

Students often just say 'bacteria turn milk into curd' without naming Lactobacillus or explaining why the milk sets -- the coagulation is caused by falling pH, not by the bacteria directly.