Question

it is generally observed that curd left at room temperature for a long time becomes sour in taste state the possible reason for this observation

Answer 1

HEY mate here is your answer.

Yes the curd becomes sour because at room temperature the bacteria called as lactobassilus grows rapidly which helps the curd make its test soar in nature.
You must have observed when curd is put inside a refrigerator the curd remain as it is ,hence from this we get that bacteria can grow in room temperature but not in low temperature.

Hope it helps you.

^_^

Answer 2

when  the curd is kept in the room temperature the bacteria outside it reacts with the curd and as a result it becomes sour in taste