" Almost any soft food material is safe - vegetable peelings, apple cores, rice, tea leaves, cooked meat, pastry, bread, nuts, pickles, porridge - all will pass through safely as long as the water is running. Grinding citrus skins will help to reduce foul odours which sometimes come from waste disposals as scraps of rotting food accumulate inside the unit.
Harder materials can also be ground - in fact it's beneficial to the unit to grind things like chicken bones, ice cubes, eggshells, and small fruit pips and stones. These scour the workings of the unit and help to remove the coating of gunk and grease which builds up with use. Vinegar ice cubes will have a two-fold effect because of the acid in the vinegar.
It's not recommended that you attempt to grind pips as large as an avocado stone - anything up to the size of a hazelnut (filbert) should be fine for household units, and any bones larger than a chicken thigh should be chopped up.
As a general rule, it's best to avoid putting anything other than food into a waste disposal, although paper towel shouldn't give it any problems. Allowing any kind of plastic food wrapping to get into it is asking for trouble.
Do not try to grind fibrous materials such as corn husks, artichokes, asparagus or even celery. These can seize up your waste disposal and block your drain. Coconut shell is also best avoided - partly because of the coconut hairs, and partly because it is just a little too hard for all but industrial waste disposals - and bits of shell can even be spat back out of the unit.
Uncooked meat, especially gristle and connective tissue, can be tough enough to wrap itself around the innards of a waste disposal unit, and once stuck there it will begin to decompose, filling your kitchen with the stink of rotting meat.
Don't pour fat or grease into the disposal, or indeed down any drain. It's unlikely that it will cause any harm to the unit, but fats which are solid at room temperature will inevitably block your drain and the waste disposal. Pour them into a jar which is kept in the fridge until full, then throw it away like any other piece of rubbish. "