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A milkman adds a very small amount of baking soda to fresh milk.(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?(b) Why does this milk take a long time to set as curd?

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The pH of the milk is 6 as it is slightly acidic in nature because of the presence of lactic acid.

It is to prevent the milk from getting sour due to the development of lactic acid that the milkman adds a small amount of baking soda to fresh milk.
Baking soda helps in adjusting the pH of the milk by making it slightly alkaline in nature.

It takes a long time for the milk having a pinch of baking soda in it to set into curd because the lactic acid produced by the action of bacteria is first consumed in the neutralization of the pH and then the pH reduces to the point where milk can turn into curd.

Final Answer:

Adding a small amount of baking soda to milk disturbs the pH of fresh milk making it slightly alkaline and hence making it difficult to turn into curd or prevent souring.

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