Destiny of Unused food after flight
The crew eats the food, no doubt. Just joking. The unused food is off loaded from the airplane into catering trucks and is discarded. It seems a bit sad but the reality is, any food that is leftover from a flight is considered “contaminated” so is discarded. This includes meals that passengers didn’t finish as well as entire carts full of food that were never used.   


  So what is being done to help eliminate unnecessary waste? With
  limited space on flights, some airlines have started to make small
  changes by only loading 95% of the required meals on each flight with
  the expectation that some passengers will opt not to eat. Other
  airlines have started charging around $10 a meal, recognizing that
  less food is consumed when meals are only available for purchase
  rather then complimentary. In the past, some airlines have even
  donated their unused food to shelters and charities to help make a
  difference. Unfortunately due to incidents where food poisoning
  occurred, the liability became much too high and is no longer
  practiced by airlines. (Source)