Quite a simple suggestion really, A food buff that can be produced from the provision house that costs 4x a basket and lasts 4x as long and takes 4x as long to produce, but also uses saltpetre.
Great Feast
480 fish
240 bread
80 sausage
40 saltpetre each
Why saltpetre?
Well at the moment its probably the least used resources that people have stockpiled since you get loads from adventures and scouts, but it never actually gets used (who makes cannons?)
It is used as a meat preservative. - so not just a random extra ingredient.