And then doesn't eat it, or takes a bite or two and then that's it, what do you do?

The girls seem to be wasting a lot lately, but it's stuff they've asked for. I try saving it but they won't eat it later on then it goes bad (banana or Apple turns brown and they won't touch it). It's getting frustrating because even if I tell the two of them they have to split something it still usually goes to waste. Any tips?