Which one of the following microbes causes acidification and curding of milk ?

The acidification and curding of milk are primarily caused by Lactic Acid Bacillus, commonly known as Lactobacillus. These bacteria feed on the lactose (milk sugar) and ferment it to produce lactic acid [1]. The production of lactic acid increases the acidity of the milk, which causes the milk proteins to coagulate and form curd [1]. This process also gives the curd its characteristic sour taste [2]. While other microorganisms like Saccharomyces cerevisiae (yeast) are used in bread making and can coexist with lactic acid bacteria in certain dairy products like kefir, they do not typically cause the primary curding of milk. Other options like Clostridium botulinum and Vibrio cholerae are pathogenic bacteria associated with food poisoning and cholera, respectively, rather than beneficial dairy fermentation.

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
2. [2] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > Table 2.3 Some naturally occurring acids > p. 28