Which chemical reactions occur between my tongue and the food I eat?
1) I would guess that all of the different interactions are possible except for ionic and covalent because those are much stronger than the rest and it would be difficult for the food molecules to disassociate from the taste receptors to be used again. Enzymes in saliva break down complex food molecules to smaller and simpler ones.

2) Started explaining in 1, molecules are not usually permanently bound to the taste receptor in order for the taste receptor to be used again (another molecule being able to bind to it). Some toxins form covalent bonds to receptors, such as curare. I'm not sure what kinds of receptors taste ones are but toxins target different specific receptors. Curare binds to nicotinic acetylcholine receptors, found in muscles.

3) We don't taste things unless a molecule binds to a taste receptor.