Which one of the following is responsible for converting milk into curd ?

The conversion of milk into curd is a biological process driven by specific microorganisms, primarily bacteria. The most prominent bacterium involved is Lactobacillus [1][t1]. These bacteria feed on lactose, the natural sugar found in milk, and ferment it to produce lactic acid [c1][t3]. The resulting increase in acidity causes milk proteins, specifically casein, to coagulate and thicken into a solid mass known as curd [t5][t8]. This fermentation process also gives curd its characteristic sour taste [c1][c3]. While fungi like yeast are used in baking and viruses are microscopic parasites, neither is responsible for the natural curdling of milk [t1][t2]. Lactic acid bacteria (LAB) are considered beneficial microorganisms that not only transform milk but also improve its nutritional value and aid in human digestion [t7][t8].

Sources

1. [1] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 2: The Invisible Living World: Beyond Our Naked Eye > Table 2.4: Testing for curd formation using milk in different conditions > p. 22