Question

when we chew grain we feel sweet why

Answer 1

when we chew some grain we feel sweet because most of the grains contain carbohydrates which when mix with the saliva tastes sweet

Answer 2

grains contain some amount of carbohydrates vichare initially polysaccharide that is sugar having multiple units..... but as we chew grains.. then polysaccharide breakdown due to the action of saliva and turns into monosaccharide. that's why feel sweet.

note that polysaccharide is tasteless and monosaccharide having single unit is actually sweet in taste....

and I hope this answer helps you.