Step 1: Understanding the Concept:
Milk stays liquid because its main protein, casein, remains dissolved and evenly spread through the milk at milk's normal, close-to-neutral pH. Curd forms when that protein stops staying dissolved and clumps together into a semi-solid mass, which only happens once the milk's pH drops enough.
Step 2: Key Approach:
So the real question to answer first is what lowers milk's pH, since whatever organism does that is the one converting milk into curd.
Step 3: Detailed Explanation:
Lactic Acid Bacteria, a group that includes Lactobacillus species, ferment the milk sugar lactose and release lactic acid as a waste product. This lactic acid steadily lowers the milk's pH, and once the pH falls low enough, the casein protein loses the electrical charge that was keeping it dissolved and spread out, so it clumps together (coagulates) into the soft solid we call curd. Checking the other options: Propionibacterium shermanii produces propionic acid and carbon dioxide, and it is used for the characteristic holes and flavor in Swiss cheese, not for curd formation. Pseudomonas and Xanthomonas are known mainly as plant pathogens and have no established role in dairy fermentation.
Step 4: Final Answer:
Lactic Acid Bacteria acidify milk enough to coagulate its casein into curd, so option 2 is correct.