Pasta with meatballs reheats well, so I don’t know why the food would have to be thrown out.
You ought to be willing to eat it while “enduring” a little lower quality level in order to both use the food and keep your fast, but honestly it will probably be better if it were refrigerated overnight.
Having said that, let us say that refusing the food would greatly embarrass your host or that you know the host would go to a great deal of trouble the host really does not have time for in order to accommodate you at the last minute. In that case, I’d say kindness to the host would argue in favor of substituting a different penance. That is in keeping with the rule, “I desire mercy, not sacrifice., and knowledge of God rather than burnt offerings.” (Hos. 6:6)
I agree with those who say that your mom offered you a labor of love and you ought to accept it and substitute an alternative penance. (I hope she did not have to do a dish after dinner was done, lol…)