Question

Why does the food give a bad taste and smell after the expiry date?

Answer 1

It depends on the food item from one to another.

Bananas, on the other hand, specifically tasted better upon/after expiry. Most people mistaken the black patches on the banana as an indication that the banana has turned bad.

This is not the case, as a totally blackened bananas, as long as it hasn't any mold covering them, will uncover the sweetest, mellowest banana taste you have ever tasted. Especially Cavendish.
It depends. I have had food that was about to expire in 2 weeks taste awful. I have had food taste really good at expiration and after. I've been told that it really means taste best before the date but can be eaten a year after. I like milk the day after expiration just before its going to turn. It has a taste more like sweet fresh cow's milk. At least to me. The next day it is completely sour.

Answer 2

The food gives a bad taste and smell after the expiry date due to the microbial contamination.

Explanation:

• The expiry date given in all the food articles refers to the date beyond which the food item must not be consumed.
• The food naturally starts to the get decayed after expiry date because, the preservative which was added to the food loses its ability and cannot inhibit the growth of microorganism.
• This leads to rapid growth of microorganism due to which the taste and smell of the food gets changed.  
• Usually microorganisms such as yeast, fungi and bacteria has vital role in food contamination.