First, let's take a look at a really shitty diagram.



As of right now, stockpiled food doesn't decay. This is going to change in the future. The problem is, the last time I looked in my refrigerator I happened to notice that cooked food decays, too. I think there needs to be a distinction between prepared food and preserved food, and the approach to food preparation should be volume and resource centric as opposed to emphasizing the production of specific food items. (Which may be covered in recipes later down the road.)

• Preserving food should result in basic food items that last a very long time. (Seasons to years.) The method used to preserve the food should also have a chance of adding to or subtracting from its value. Preserved foods should be both usable as ingredients and edible unprepared.
• Preparing food should result in complex food items that last a relatively short time. (A season at the absolute longest.) Obviously the end product should be at least as valuable as the constituent ingredients.
• Raw food should have the shortest shelf life of all and, if not processed within the month in Dwarf Mode, should begin rotting. This should occur regardless of whether or not it's stockpiled.

This means that if you want the heaps of purple fungus you harvest to last you through the winter, you had better at least dry them out first even if they depreciate in value from it. (On the other hand, cured, smoked, and pickled variants of preserved foods should increase in value and acquire the taste 'prefs' of the items used to make them. Descriptors like 'smokey', 'salty', 'sweet', that sort of thing.) Prepared food rations should be more enticing to your dwarfs than just the ingredients, but to keep food in the cupboard year round an emphasis should be placed on preserving it and then cooking it a little bit at a time. (Besides, it gives your farmers more work!) Cooking should also prefer fresh ingredients, to encourage getting rid of all the stuff one way or another before it goes bad.

One other suggestion is that the kitchen should have a toggle to use seasonings when available. The resulting food items would in the very least be spiced, resulting in higher quality meals and preserves. (As it stands, quarry bush leaves aren't just a spice, they're a substrate. With heaps of meat, fish, flour, fruits and vegetables of every stripe, tallow, and even cheese sitting around in my stockpile, I have actually had cooks use up all of my spices first before even touching anything that could be considered food. That means I had massive stacks of roasts made out of leaves, leaves, leaves, and more leaves.) This provides an elegant solution to the 'beer and prickle berry seed roast' problem that's been addressed with cooking. If spices, seeds, sugar, salt, and non-water liquids become classified as seasonings, they would only add value to meals made to them and catch wider preferences without actually being substantial parts of the final product.

Reasons for wanting the change? Not just to make food production harder, but to encourage planning around the fortress food supply. When you're just arriving at your site, unless you have ready access to salt and sugar, you're probably going to be drying your vegetables and smoking your meats. (If not drying those, also.) As you continue producing and preserving food, you anticipate what your fortress will need and what you'll have to throw out. When the resources become available to you, you can start producing larger volumes of higher quality preserves, and either trade them off or cook them at your fortress when fresh food is in short supply. If fertilizers and pollution become factors in the game, you'll want to try and waste as little as you can even if you produce food in abundance. (That is, if it's going to cost you something to make it and do you no good at best to keep the excess, you might as well trade it off or something before it's no good.)