Why does curd set faster in summer than in winter?

The process of converting whole milk to cheese (curds) is a chemical reaction. Often biological catalysts (like the enzyme Rennet) are used to make the process happen faster.

Like any chemical reaction, the warmer the constituents are, the faster the reaction will occur.

Temperature is actually a measure of molecular activity: the warmer something is, the higher the level of molecular motion. When things are cooled to very low temperatures (such as 0 Kelvin, or "Absolute Zero"), molecular motion basically stops. This occurs around -460'F, or -273' C.

So summer, being generally warmer, will tend to favor chemical reactions proceding more quickly.

If too much heat is added--especially to biological compounds, like enzymes--you can actually "kill" the chemical reaction by denaturing the proteins involved.

This may allow other, unfavorable reactions to occur.