Question

curd turns sour more easily in summers than in winter. why?

Answer 1

because bacteria grows faster in summer than in winter due to the favorable conditions and as the number of bacteria increases it makes the curd sour.

Answer 2

Lactobacillus bacteria (micro organism) turns milk into yogurt (curd).  The sugar (sucrose) in milk is converted (fermented) into lactic acid during the fermentation process.  

In summers the Lactobacillus microbes are more active, develop more.  So the fermentation process accelerates.  Since lactic acid tastes sour, the curd tastes sour.  More lactic acid  means more sour.