Question

Fresh milk has a pH of six. How do you think the pH will change as it turns into a curd? Explain your answer.

Answer 1

A scale which is used to measure the acidity and basicity of an aqueous solution is known as pH. 7 is the measurement of neutral solution above it the solution is basic and below, its acidic in nature.

Complete Step by Step Answer:
The conversion of milk into curd results in the formation of lactic acid. Therefore, the pH of this newly formed curd decreases (4.5 - 5) as compared to fresh milk.
Process of synthesis of curd from milk:
- The process of conversion of milk into curd is known as curdling (fermentation) and it is a chemical process.
- It is irreversible as the curd cannot be converted back to milk.
- The process is carried out by the bacteria Lactobacillus that is capable of digesting the lactose sugar.
- The milk consists of lactose (sugar) and casein (protein).
- The lactobacillus hydrolyzes the lactose into glucose and galactose with the help of the enzyme galactosidase. The glucose thus produced undergo glycolysis to produce pyruvic acid. The pyruvic acid further undergoes lactate fermentation to produce lactic acid.
- This lactic acid acts on the casein protein and leads to its degeneration. It destroys the tertiary and quaternary structure of the milk protein.
- Glycolysis also results in the production of ATP.
- Consequently, the globular protein changes into the fibrous structures that give the appearance of curd.
- It also imparts a tangy sour taste to the curd.

Note:
- The process of curdling can also be considered as a physical process because it led to the formation of lumpy masses due to coagulation.
- The main lactose degrading bacteria are Lactobacillus bulgaricus and Streptococcus thermophilus.
- The appropriate temperature for the formation of curd is $34^\circ \ C - 40^\circ \ C$.