Yes, it’s a sin to waste food. A sin is anything against the will of God, and do you really think God wants humans to waste food? That is not very responsible behavior.
You know how we pray grace before meals: “Bless us, O Lord, and these Thy gifts, which we are about to receive from Thy bounty, through Christ our Lord. Amen.”
And in the Our Father we pray: “…give us this day our daily bread…?”
What you think God would think if we just threw away the gifts we received from His bounty, the daily bread He gave us, because we were so irresponsible that we prepared or ordered more food than we needed at that meal, or because we bought more food to put in our fridge than we needed, and it spoiled?
Just browsing the Catechism, here are CCC refs that could be used to show that wasting food is a sin:
CCC 2288 Life and physical health are precious gifts entrusted to us by God. We must take reasonable care of them, **taking into account the needs of others and the common good. Concern for the health of its citizens requires that society help in the attainment of living-conditions that allow them to grow and reach maturity: food **and clothing, housing, health care, basic education, employment, and social assistance.
CCC 2290 The virtue of temperance disposes us to avoid every kind of excess: the abuse of food, alcohol, tobacco, or medicine. Those incur grave guilt who, by drunkenness or a love of speed, endanger their own and others’ safety on the road, at sea, or in the air.
**CCC 2401 **The seventh commandment forbids unjustly taking or keeping the goods of one’s neighbor and wronging him in any way with respect to his goods. It commands justice and charity in the care of earthly goods and the fruits of men’s labor. For the sake of the common good, it requires respect for the universal destination of goods and respect for the right to private property. Christian life strives to order this world’s goods to God and to fraternal charity.
CCC 2404 **"In his use of things man should regard the external goods he legitimately owns not merely as exclusive to himself but common to others also, in the sense that they can benefit others as well as himself."187 The ownership of any property makes its holder a steward of Providence, with the task of making it fruitful and communicating its benefits to others, first of all his family. **
CCC 2443 God blesses those who come to the aid of the poor and rebukes those who turn away from them: “Give to him who begs from you, do not refuse him who would borrow from you”; "you received without pay, give without pay."231 It is by what they have done for the poor that Jesus Christ will recognize his chosen ones.232 When “the poor have the good news preached to them,” it is the sign of Christ’s presence.233
**CCC 2446 ****St. John Chrysostom vigorously recalls this: “Not to enable the poor to share in our goods is to steal from them and deprive them of life. The goods we possess are not ours, but theirs.”**238 “The demands of justice must be satisfied first of all; that which is already due in justice is not to be offered as a gift of charity”:239 When we attend to the needs of those in want, we give them what is theirs, not ours. More than performing works of mercy, we are paying a debt of justice.240
**CCC 2451 **The seventh commandment enjoins the practice of justice and charity in the administration of earthly goods and the fruits of men’s labor.