What does curd formation in Litmus Milk indicate?

Curd formation in Litmus Milk primarily indicates acid production due to lactose fermentation. When bacteria are introduced to milk, they can ferment the lactose present, resulting in the production of lactic acid. This increase in acidity causes the casein protein in milk to coagulate, leading to curd formation. This is a distinct characteristic of certain bacteria that can utilize lactose effectively, and the curd's texture and firmness can provide further insights into the specific activity of the microorganisms present.

The other options, while related to different microbial processes, do not directly explain the occurrence of curd formation. For instance, the presence of ammonia generally indicates deamination processes rather than fermentation of lactose, which does not lead to curd formation. Similarly, reduction of litmus, which refers to the change in color due to certain redox reactions, does not specifically result in curd formation but rather affects the coloration of the medium. Excessive bacterial growth could occur without leading to curd formation, depending on the specific metabolic activities of the organisms present. Therefore, acid production due to lactose fermentation is the most accurate answer in the context of curd formation in Litmus Milk.