Hello scientists:

If I put a fresh head of lettuce in a vacuum jar, will it dry out?

excon

Depending on just how hard of a vacuum you are talking about - much of the water in the lettuce would boil out and turn to vapor. It's also possible that some of the water remaining in or on the lettuce would turn to ice (yes, it's possible to have both ice and boiling water at the same time, if the pressure is low enough). In any event your lettuce would not turn out so great.

Hello e:

Thanks, but I don't know where freezing or boiling comes in to play. I'm just talking about a home vacuum machine. I suppose fresh lettuce wasn't a good example...

Let's talk about jerky... It's dried meat, but it has SOME residual moisture. If I put it in one of the vacuum jars that came with the machine, will it totally dry out in the vacuum, or will it retain whatever moisture it has?

Ahhh... I should have asked it that way in the first place.

excon

I mentioned freezing or boiling because that's what happens to water in a vacuum. You mentioned a jar, so I figured you were talking about putting food into a vacuum chamber (perhaps your'e doing strange experiments in your underground laboratory - like let's see what happens if we put the pet gerbel into a vacuum chamber). But the commercially available vacuum sealers for storing food typically use a pouch that gets sealed around the food, or they use very small containers. There is no place for the water vapor to go, so it doesn't boil off. From the ads I've seen it seems that they work well for many types of food.

Excon, liquid water notwithstanding, I think putting food in vacuum would be pretty much the same as putting it in a really dry atmosphere (such as in a more traditional food dehydrator). Any liquid water present at the surface of the food would boil off quite quickly, but otherwise the water within the food would slowly diffuse out. All else being equal (e.g. temperature), the speed of diffusion in that case should be inversely proportional to the partial pressure of water in the atmosphere. In other words, from the water molecules' point of view, the ability to evaporate only depends on the concentration of water vapor present in the atmosphere. The presence of oxygen, nitrogen, etc. is irrelevant. Total vacuum is indistinguishable from completely dry air.

Hence leaving something like jerky in vacuum long term would likely cause it to dry out completely, just like leaving it in a food dehydrator for a long, long time. That being said, however, a vacuum jar is self-limiting in this respect because it has a finite volume. As the water evaporates from the food, the pressure within the jar increases until an equilibrium state is reached. The smaller the jar, the more quickly that equilibrium would be reached and the less dehydration would result. Hence a container with almost zero excess volume, such as a vacuum-sealed bag, barely dehydrates the food at all. But sticking a head of lettuce in a 100-gallon bell jar would probably make it look more like a brussel sprout. :)