Why does milk tastes sweet while curd tastes sour?

Milk tastes sweet primarily due to the presence of lactose, a natural sugar. When milk is fermented to make curd, bacteria convert lactose into lactic acid, which gives curd its sour taste. This fermentation process not only changes the flavor but also alters the texture and nutritional profile of the milk. Thus, the sweetness of milk and the sourness of curd result from different microbial activities and chemical compositions.