Describe how bacterial fermentation causes milk to sour.

Bacterial fermentation is a common process that occurs in many food products, including milk. In the case of milk, the process of bacterial fermentation leads to its souring.

To understand how bacterial fermentation causes milk to sour, we first need to understand the microorganisms involved. The main bacteria responsible for milk fermentation are lactic acid bacteria, particularly species of Lactobacillus and Streptococcus. These bacteria convert the lactose, which is the primary sugar present in milk, into lactic acid through a series of biochemical reactions.

Here's a step-by-step explanation of how bacterial fermentation leads to the souring of milk:

1. Introduction of bacteria: Bacteria can find their way into milk from various sources, such as the udders of cows or through contamination during milk handling and processing.

2. Activation of bacteria: Once the bacteria are present in the milk, they begin to multiply rapidly, especially when the environmental conditions are favorable, like when the milk is kept at room temperature.

3. Lactose metabolism: Lactic acid bacteria have an enzyme called lactase, which can break down lactose into its two component sugars, glucose and galactose.

4. Fermentation process: The bacteria utilize the glucose and galactose as an energy source through a process called fermentation. This fermentation process involves the conversion of these sugars into lactic acid.

5. Acidification of milk: As the bacteria consume the lactose and produce lactic acid, the pH of the milk starts to decrease. The accumulation of lactic acid leads to an increase in the acidity of the milk.

6. Souring of milk: As the pH decreases, the milk gradually changes in taste and smell, becoming sour. This change is due to the presence of lactic acid, which imparts a tangy flavor and contributes to the sourness of the milk.

It's worth noting that the rate at which bacterial fermentation occurs and milk sours depends on various factors, such as the temperature, the initial bacterial load, and the specific bacterial strains present. Refrigeration can slow down the fermentation process and extend the shelf life of milk.

In summary, bacterial fermentation in milk occurs when lactic acid bacteria utilize lactose as an energy source, converting it into lactic acid. The accumulation of lactic acid leads to an increase in acidity, causing the milk to sour.