The reason for curdling of milk is

The reason for the curdling of milk is Lactobacillus. This bacterium is a type of lactic acid bacteria that plays a key role in the fermentation of milk. When added to milk, Lactobacillus converts lactose, the natural sugar in milk, into lactic acid. The increase in lactic acid causes the pH of the milk to decrease, leading to the coagulation of proteins such as casein. This process results in the formation of curds, which are used to make curd, yogurt, and other dairy products. The action of Lactobacillus not only curdles the milk but also imparts a tangy flavor and enhances the nutritional value of the final product. The other options, Mycobacterium, Staphylococcus, and Yeast, do not contribute significantly to the curdling of milk in the context of making curd.