Question

how does the piece of roti or some cooked rice change after it has been chewed for some time ?

Answer 1

We know that roti and cooked rice contain starch. When we put a few drops of iodine on them, they turn-blue black which show the presence of starch.

When we chew the rice/roti, the saliva present in our mouth acts on them and converts the starch to sugar. So when we test them after chewing, they usually don't turn blue-black.