Untold Sin, in Wasting Food?

Yes, it is wrong to waste food. Even non-Christians believe this too since it goes against a natural inclination toward global solidarity. The Church indirectly talks about our responsibility to help provide for those around us.

“The works of mercy are charitable actions by which we come to the aid of our neighbour in his spiritual and bodily necessities. Instructing, advising, consoling, comforting are spiritual works of mercy, as are forgiving and bearing wrongs patiently. The corporal works of mercy consist especially in feeding the hungry, sheltering the homeless, clothing the naked, visiting the sick and imprisoned, and burying the dead. Among all these, giving alms to the poor is one of the chief witnesses to fraternal charity: it is also a work of justice pleasing to God: He who has two coats, let him share with him who has none and he who has food must do likewise. But give for alms those things which are within; and behold, everything is clean for you. If a brother or sister is ill-clad and in lack of daily food, and one of you says to them, “Go in peace, be warmed and filled,” without giving them the things needed for the body, what does it profit?”
CCC 2447

A lot of restaurants and families waste a great deal of food that could have easily fed other people. So as Catholics we cannot be casual about such things. One of the first things is to ensure that we order or cook what we can consume. And if they remain to preserve them for future consumption or give them to someone else who doesn’t have any. There are programs where donations of non-perishable foods can be made for those in need or cash gifts to support those who are starving. These are ways we can fulfil our obligation to give to the poor. Remember that this is the standard Jesus suggests we shall be judged with. (Matthew 25).