Souring of milk involves a chemical change due to the conversion of lactose (a sugar) into lactic acid by bacteria present in milk. This process turns the milk into a yogurt-like substance with a sour taste. The chemical reaction that occurs during this process is known as lactic acid fermentation. Here's a closer look at the chemical change that takes place:

1. Initial state: Fresh milk primarily consists of water, lactose, proteins (casein and whey), and other components.

2. Introduction of bacteria: Milk contains bacteria that are capable of breaking down lactose. Common bacteria involved in this process include Lactobacillus, Streptococcus, and Bifidobacterium.

3. Lactose breakdown: The bacteria in milk produce enzymes called lactase or beta-galactosidase, which break down the lactose present in milk.

4. Formation of lactic acid: During lactose breakdown, the enzymes split the lactose molecule into its component sugars: glucose and galactose. These sugars are then further metabolized by the bacteria, leading to the formation of lactic acid.

5. pH reduction: The presence of lactic acid in milk causes a reduction in its pH level, making the milk more acidic. The drop in pH causes the proteins in milk, mainly casein, to coagulate and form a gel-like substance. This coagulation results in the thickening of the milk and gives it a yogurt-like texture.

6. Flavor and aroma: The production of lactic acid also results in a distinct sour flavor and aroma associated with sour milk.

Overall, the souring of milk involves the chemical breakdown of lactose into lactic acid by bacterial enzymes. This process alters the composition, pH, and sensory properties of the milk, transforming it into a sour and thickened substance.