Where does the sour taste come from in foods such as chesee and yogurt?

The sour taste in fermented dairy products like cheese and yogurt comes from the production of lactic acid by bacteria during the fermentation process. Here's how it happens:

1. Starter Cultures: Cheese and yogurt are made using starter cultures, which are specific strains of bacteria added to milk to initiate fermentation. These starter cultures typically contain lactic acid bacteria (LAB), such as Lactococcus lactis, Lactobacillus acidophilus, and Streptococcus thermophilus.

2. Lactic Acid Production: During fermentation, LAB consume the lactose (milk sugar) present in milk and convert it into lactic acid as a byproduct. This process is known as lactic acid fermentation.

3. pH Drop: As lactic acid accumulates, it lowers the pH of the milk, making it more acidic. This drop in pH causes the proteins in the milk, particularly casein, to coagulate or curdle, resulting in the formation of a gel-like structure in yogurt and cheese curds in cheesemaking.

4. Sour Flavor Development: The increased concentration of lactic acid imparts a sour flavor to the fermented dairy product. The amount of sourness can vary depending on the type of LAB used, the fermentation temperature, and the duration of fermentation.

5. Flavor Complexity: In addition to lactic acid, other flavor compounds can contribute to the sour taste in cheese and yogurt. These compounds include acetic acid, propionic acid, and butyric acid, which are produced by different LAB strains during fermentation. These acids and other flavor compounds add complexity and depth to the overall flavor profile of fermented dairy products.

The controlled fermentation process allows for the development of desirable flavors, textures, and aromas in cheese and yogurt, making them popular and widely enjoyed dairy products around the world.