Why do milkman add baking soda to milk before distribution?

It is bacteria which spoil the milk by converting lactose present in milk to lactic acid. That is, it is the acid which curdles milk. In hot weather the bacterial growth is fast and milk curdles faster. So the vendors add baking soda which is an alkali to neutralise the lactic acid so that the milk doesn't curdle. Doing so ofcourse is illegal.