How does vaccuum-sealed freeze dried food expire overtime?

Question

Like if we completely remove air and 90% of moisture, and it lasts years to begin with, how does it eventually go bad? How do bacteria that can survive such conditions just appear in a sealed package with no exposure to elements, and well, why does it take years for that bacteria to build instead specifically instead of less time?

In: 8

Answer 1

Oftentimes it’s not so much the bacteria that’s spoiling the food, but enzymes in the food breaking things down.

I’m not saying there aren’t some forms of bacteria that can still live and multiply in freezing temperatures, but on the grand scale of things, there are basically no active bacteria in there. (Same with fungi, don’t forget about them!)

*Enzymes*, on the other hand, nonliving molecular machines that can’t really “die”, *can* still work at those temperatures. They’ll probably be extremely slowed down, but most of them aren’t super bothered by low temperatures. They can be permanently destroyed by high temperatures (which is basically what cooking *is*), but not freezing temps. So they’ll slowly do their job, breaking down foods to the point they become inedible.

There are also other explanations for why a food item in the freezer has gone bad. Like, if your frozen item is located near the door of the freezer, and you’re in and out of there often, it’s quite possible that the surface of the food is slightly thawing out every time you open it and put new, not-cold things in it, and then re-freezing. Each time it thaws, bacteria and fungi that are deactivated by the cold wake up and enzymes slowed down by the freezing speed up. They work a little bit, and then hunker back down when they re-freeze. Over many cycles, they can cause the food to spoil.

Modern freezers also have de-icing features where they intentionally *heat up* the walls on the inside to just above freezing to melt frost buildup. Without a feature like that, you’d have to manually de-ice your freezer every so often. But with a feature like that, any food shoved up close to the wall will also defrost slightly every time it happens.