If memory serves, the equipment lists listed meals as something like the below:
Light meal, 3% chance of illness.
Normal meal, 2% chance of illness.
Heavy meal, 1% chance of illness.
Week?s rations, Normal spoilage.
Trail rations, 1 week. Preserved.
Normal Spoilage would equate to: 0% first day, with +1%/day after the first.
Preserved always depended upon preservation. Even the Neanderthals had fairly good preservation techniques such smoking/drying meats, vegetables, and fruits for the long haul. We used the rule of 0% first week, with +1%/week after the first.
We used the "Chance of Illness ? 3" to indicate the "Chance of Food Poisoning."
Maybe lame, but that is what my wife and I used.
rmfr