Which of the following is a recognized souring agent in milk?

Lactic acid is a recognized souring agent in milk. This natural organic acid plays a crucial role in the fermentation process of milk products. When milk is exposed to lactic acid bacteria, these microorganisms consume lactose (the sugar found in milk) and convert it into lactic acid. This process lowers the pH of the milk, leading to the characteristic sour taste associated with fermented dairy products like yogurt and cheese.

The presence of lactic acid not only contributes to flavor but also has implications for food safety and preservation, as the lower pH can inhibit the growth of spoilage and pathogenic bacteria. This process of acidification is essential in dairy production, promoting a desirable texture and flavor profile in many fermented products.

In contrast, while formic acid, citric acid, and carbonic acid are also acids with various applications, they do not specifically serve as the primary souring agent in milk fermentation. Formic acid is used in agriculture and industry, citric acid is commonly found in citrus fruits and often used as a preservative or flavoring in food products, and carbonic acid is formed from dissolved carbon dioxide in beverages but does not play the same role in the souring of milk.