You are allowed to use food for any purpose. Just don’t take a chance of it becoming disgusting and ruined. Two exceptions:
You are allowed to ruin food for medicinal purposes.
If it is the derech to use the food in such a way, it is permitted. (e.g. smearing oil). (ShA 1 – MB 2,3,4,5)
Do not throw bread even if it won’t become ruined. Other foods (e.g. nuts) can be thrown, as long as they won’t become dirty or ruined. (ShA 1 – MB 9)
If you see food disgracefully laying on the ground, you must lift it up unless it is a whole loaf. (MB 11)
Do not feed human food to animals and birds unless there is nothing else available to feed them. This hetter supports the minhag ha’olam to feed bread to the birds. (MB 11)
Do not sit on a collapsible container of food, if the food will become squashed and ruined. (ShA 2)
You may use your bread as a spoon to eat porridge provided that you eat the bread afterwards. Otherwise, it’s wasting since no one else will eat that bread. Some people are particular not to use bread as a spoon unless they are eating a small bite of bread each time. (ShA 3 – MB 15,16,17)
When throwing food in front of the chosson when he gets his aliya, be sure it is not soft food that gets ruined. (MB 21)