The spoilage mechanic looks great! I have a question though: do half-spoiled items stack the way science packs/ammo do? It would be quite inefficient if they didn't, but combining items can mess with the spoilage rate (two half-spoiled fruits spoil faster than a fresh one)
Edit: I suppose a stack can be stored as an array of how many 1%-100% packs is in a stack, displaying the 'sum' as how many full items you have and 'count' is used for the spoilage rate and inserters will extract the 'true' items, but maybe that's not the route the devs went for
They just avarage the same way damaged items average.