There are two ways good food goes bad: something grows on it (see microbial growth) or it starts to taste bad. If you found a 50 year old bag of navy beans, chances are you wouldn’t get sick if you cooked them up and ate them. You’d probably just get very little from them nutritionally, and they’d taste like braised cardboard. Time is the enemy of taste, and nowhere is that more apparent than with lipid oxidation.
Most “shelf-stable food” isn’t really stable. Technically, it’s “meta-stable”. We’re not just trying to be annoying when we say that. Shelf-stable food is like glass. Try tapping on a window, or for that matter, hurl yourself through it. It sure feels like a solid, but really it’s a “super-cooled liquid” that flows over time. Corn chips may look like they’d last forever on the shelf, but really, they don’t. Lipid oxidation is the first problem the chips face. In a few months, they start to taste rancid and unappealing. Even worse, byproducts of lipid oxidation encourage vitamin degradation, and other undesirable reactions.
Luckily, water activity levels affect lipid oxidation rates. Knowing where you fall on the water activity scale can help you slow down the oxidation process. Keep in mind that lipid oxidation is typically lowest around 0.4 aw, and increases whether you go up or down from that level.