: Describe how bacterial fermentation causes milk to sour.
Bacterial fermentation is responsible for causing milk to sour. When milk is left at room temperature, bacteria naturally present in the environment, as well as bacteria that may be introduced through unclean utensils or contaminated hands, can start to multiply and grow in the milk. These bacteria primarily belong to the group of lactic acid bacteria, such as Lactobacillus and Streptococcus.
The process begins when the lactic acid bacteria feed on the lactose, a sugar present in milk. Through a series of enzymatic reactions, the bacteria break down lactose into two simpler sugars, glucose and galactose. This process is called lactose fermentation.
Once the bacteria have broken down the lactose, they start converting the glucose and galactose into lactic acid. This conversion is achieved through the bacteria's metabolic processes, specifically the process of glycolysis. In glycolysis, glucose is converted into pyruvate, which is further metabolized into lactic acid. The accumulation of lactic acid in the milk causes the pH to decrease, making the milk more acidic.
As the lactic acid concentration increases, the milk becomes sour in taste and its texture may change, becoming curdled or clumpy. The increased acidity also inhibits the growth of other microorganisms that may spoil the milk differently, helping to preserve it in a sour state for some time.
To summarize, bacterial fermentation in milk involves the breakdown of lactose into glucose and galactose, followed by the conversion of these sugars into lactic acid. This metabolic process leads to a decrease in pH, causing the milk to sour.