Question

What is the change in pH values of milk when it changes into curd? Explain.

Answer 1

Hint: Curd is sour in nature. It contains lactic acid which is acidic in nature. So, the pH will change on the side of acids. The milk has a pH of 6. On converting milk to curd, the pH decreases.


Complete step by step answer:
First let us see what is pH and what is the range.
The pH can be defined as the negative logarithm of ${H^ + }$ ion concentration. The pH scale is from 7 to 14. The point on the pH scale 7 is the neutral point.
The pH below 7 comes in acidic range and the pH above 7 is alkaline range.
The milk has a pH of 6 which means it is slightly acidic in nature. When the milk changes into curd, there are several physical to chemical changes which take place. We all know milk is liquid while the curd is a bit solid. Further, curd is sour while milk is not. Sourness is a property associated with acids.
During inoculation of curd in milk, we put Streptococcus lactis and Lactobacillus bacteria in milk. These bacteria multiply and release some organic acids which lower the pH as milk changes into curd. So, the pH decreases when the milk changes into curd. The curd has a pH in range of 4.5 to 4.8. The increase in acidity or we can say the decrease in pH value lead to tangling in milk proteins changing them into solid masses.

Note: The NaOH can be used to neutralize lactic acid to delay curdling. Milk contains the protein lactose. The curd formation is an indication that the lactobacilli present are converting lactose into lactic acid.