Question

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

Answer 1

pH range for solution as given on pH scale should be known. If pH is greater than $7$ , the solution is acidic. If pH is less than $7$ , the solution is basic. If the pH is equal to $7$ , the solution is neutral. Curd is acidic in nature. The pH value of milk is $6$ .

Complete step-by-step answer: Let us first see how milk is converted into curd.
Naturally milk is slightly acidic in nature but when milk is inoculated with lactic acid bacteria, they convert milk sugar lactose into lactic acid. Lactic acid acts on globular milk protein casein, causes its denaturation and conversion to fibrous state. The protein fibres form a reticulum which holds milk fat. This changes milk into curd or yoghurt.
Denaturation is basically a biological process of modifying the molecular structure of protein. It involves breaking of linkages or we can say bonds within a molecule for the formation of highly ordered structure of the protein in its actual or native state.
Curd is a product of milk after we process it. When milk is converted into curd by the action of the bacteria the medium becomes more acidic thus there is change in the pH value. The pH value decreases as milk (slightly basic) is converted into curd (more acidic). This conversion is an acidic reaction.
Thus we can conclude that milk has pH value equal to $6$ .When milk is converted into curd the medium becomes more acidic.
Hence we can conclude with this statement that pH value decreases when milk is converted into curd.

Note: Keep in mind the pH range of solutions. Do not get confused in the pH value of milk and curd. Both are acidic just the difference is milk is slightly acidic while as curd is more acidic. Pure water is known to have a pH value of $7$ .