Question

Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer.

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.

Complete step by step solution: We all have studied that on the pH scale, 7 is the neutral point. 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 have taken place. We all know milk is liquid while the curd is a bit solid. Further, the curd is sour while milk is not. Sourness is a property associated with acids. For example- Vinegar, the sirka that we all use in our homes is sour in nature. So, what actually happens is that during inoculation 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 during the change of milk into curd. The curd has a pH in the 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.