Which of the following is the natural source of lactic acid?

Curd

The correct answer is option 4. Curd.

Lactic acid is a naturally occurring organic acid that is commonly found in fermented foods and beverages. It is produced through the process of lacto-fermentation, where certain types of bacteria, known as lactic acid bacteria, convert sugars, such as lactose, into lactic acid.

One of the most well-known sources of lactic acid is curd, also known as yogurt. Curd is made by fermenting milk with specific strains of lactic acid bacteria, such as Lactobacillus bulgaricus and Streptococcus thermophilus. These bacteria metabolize the lactose present in milk, breaking it down into lactic acid. As a result, curd contains significant amounts of lactic acid, which gives it its characteristic tangy flavor and contributes to its texture and consistency.

During the fermentation process, lactic acid bacteria produce lactic acid through anaerobic respiration, where sugars are converted into energy in the absence of oxygen. This process not only acidifies the environment, giving the fermented product its characteristic sour taste, but also helps to preserve the food by inhibiting the growth of harmful bacteria.

In summary, curd is a natural source of lactic acid due to the fermentation of milk by lactic acid bacteria. The presence of lactic acid in curd contributes to its flavor, texture, and preservation, making it a popular and nutritious food product consumed worldwide.