Why rate of starch digested was lower per minute at 60 degrees than rate of 40 degrees

The rate of starch digested was lower per minute at 60 degrees than the rate of 40 degrees because the enzymes responsible for starch digestion (Amylases), have a maximum activity at 40 degrees itself.
After 40 degrees, the enzyme activity starts deteriorating, and thus, the rate of starch digestion also decrease.

The answer is texture