Question

what pushes the food to alimentary canal, then how do astronauts digest?

Answer 1

Food is pushed down by the movement of the wall of the food pipe (known as peristalsis). The movement of food in oesophagus or food pipe is totally independent of gravity there is nothing to do with gravitational force. Astronauts mainly take liquid diet for better digestion, it was eaten through a straw to prevent floating.