Question

Fresh milk has a pH of 6. When it changes to curd (yogurt), will its pH value increase or decrease? Why?

Answer 1

1. During the formation of curd lactose $\left({\mathrm{C}}_{12}{\mathrm{H}}_{22}{\mathrm{O}}_{11}\right)$ from milk is converted into lactic acid $\left({\mathrm{C}}_3{\mathrm{H}}_6{\mathrm{O}}_3\right)$.
2. While lactose is neutral, lactic acid is an acid.
3. For acidic solution pH value is less than 7, for the basic solution pH is more than 7, and for neutral solution pH value is 7.
4. So as more lactose is converted to lactic acid curd becomes more acidic.
5. More acidic the solution, the lesser the pH.

Therefore, when the milk of pH = 6 converts to curd its pH will decrease because of the production of more lactic acid.