Shalom and thank you for your question! It is certainly important to avoid wasting food, as we can see by the Torah law of ‘Bal tashchit’, meaning ‘You shall not destroy.’
There is a discussion amongst the Halachic authorities regarding the matter of freezing food.
One opinion says that freezing food would be transgressing the prohibition against building on Shabbat, apparently since the frozen food becomes a solid mass. (Building is one of the thirty-nine forbidden ‘labours ‘ on Shabbat which stem from the thirty-nine labours involved in building the Sanctuary in the desert.)
Other options maintain that freezing food is not transgressing the prohibition against building, for two reasons:
1. There is no gathering of stones or other solid materials to form one structure, rather a liquid is being transformed to a solid. This opinion is supported by the fact that we do not make cheese from milk on Shabbat due to the prohibition against selecting – ‘borer,’ (bad from good,) and not due to the prohibition against building. (Another example of selecting would be picking rotten fruit out of a bowl. One should rather take the good fruit out and discard the rest.)
2. The formation of ice cannot be considered building, because it can only remain ice in the conditions of the freezer. Therefore it has no permanence.
There are authorities who maintain that freezing food transgresses ‘nolad’, giving birth – in other words forming a new substance (It is not forbidden to give birth on Shabbat:)) since ice is a ‘new’ substance. This is based on the logic that the opposite action is forbidden – putting something like ice or solid fat somewhere warm specifically in order to melt it. This has to do with the prohibition against using fruit juice that was secreted naturally, not by human intervention, on Shabbat. (Juicy fruit on the tree or in a vessel ‘leaked’ their goodness.) This is so that we don’t come to squeeze them purposely on Shabbat.
(We are allowed to squeeze lemons for example on to a salad, or oranges on to grated carrots, if it is just to flavor them and not to get the juice for itself.) In the case of freezing food we don’t have the concern that we have created a new substance.
Many authorities permit freezing food based on those who permit thawing it.
If it is done in a usual manner and not wrapped in some unusual way, it is permitted.
