lactic acid

Lactic acid is a mild acid made when certain bacteria (like Lactobacillus) break down sugars.

• In milk, Lactobacillus uses the milk sugar lactose and produces lactic acid as a product.
• Lactic acid makes milk turn into curd by causing milk proteins (mainly casein) to clump together and set into a thicker, semi-solid form.
• It also gives curd its sour taste and helps develop its typical smell and flavour.
• Warm conditions help because the bacteria grow and work faster, so lactic acid forms more quickly.
• Lactic acid is not only in curd—similar “souring” happens in other fermented foods like idli/dosa batter when helpful bacteria produce acids.

A useful way to think of it: lactic acid is the “sour-making” substance that shows the bacteria have changed the food into a new, more stable form.