Ward against maggots, CrAn 5, R:Touch, D:Ring, T:Circle
This simple spell is often cast on the outer rim of a circular container to keep all Animal-based foodstuff inside from spoiling. Typically used to preserve meat, fish, eggs or butter, with an Aquam requisite works on milk as well. Note that foodstuff added after the spell is cast does not benefit, but one can remove foodstuff a little at a time without affecting what is left inside the unbroken circle. As usual, this spell can be cast at one level less at D:Conc, and boosted to D:Ring (or D:Sun, but that requires renewal twice a day) with an appropriate version of Maintaining the Demanding spell.
Base 2 (preserve an Animal corpse from decay - from the same CrCo guideline, considering that both CrCo and CrAn provide +3 to recovery rolls at this level). +1 Touch, +2 Ring, +0 Circle
How much is this worth in savings? This depends a lot on how much food the covenant derives from animal sources: covenants with significant flocks or herds, or fishing operations, can save a lot on provisions, particularly because meat output is highly seasonal (a significant portion of the flock is slaughtered before winter, to save on fodder). I'd say this can easily save 5% of the provisions cost, all the way up to 20% and more for a covenant relying heavily on meat and fish. Magical preservation, possibly with other Forms, could also turn into a (probably Trivial, or at most Lesser) source of Income; a notable example in the Roman Tribunal is the well-known Altamura bread, known to keep fresh for up to a month.