Oh wise chemistry gurus...can we get some advice please?

Jo Ann and I were discussing the freezing process in Larry's thread below, and whether it was a drying process or one that adds moisture - can you please clarify for us?

Thanks! Celia

Freezing dries. Freezer burn is the moisture sucked out of the product.
Think freeze-dried. That's the process discovered by Peruvians for storing potatoes.

Neither of the above. Freezing itself does not dry or add moisture, it merely converts water from liquid form to solid form. The rest depends on the humdity. Freeze drying works because the humdity is very low. Freezer burn happens because air contacts food, the food dries out just like clothing does when you put laundry on a clothesline. The air in a freezer with auto-defrost is particularly dry, so freezer burn is a bigger problem than it is freezers that lack that feature. Properly wrapped food neither loses nor gains moisture during the freezing process. It just gets hard

Regarding flour (or anything for that matter), if something is cold, like in the freezer, when exposed to warmer and more humid air, it will attract and absorb moisture. Think of your glasses fogging up when you come in from outside, where it's cold out and walk into the kitchen where a pot of pasta is boiling on the stove. If the flour is frozen and not exposed to air and then if it is allowed to warm up to room temperature without exposure to air, it will not absorb moisture. But if you open the container while it is cold, it will absorb moisture.

Regarding flour (or anything for that matter), if something is cold, like in the freezer, when exposed to warmer and more humid air, it will attract and absorb moisture. Think of your glasses fogging up when you come in from outside, where it's cold out and walk into the kitchen where a pot of pasta is boiling on the stove. If the flour is frozen and not exposed to air and then if it is allowed to warm up to room temperature without exposure to air, it will not absorb moisture. But if you open the container while it is cold, it will absorb moisture.

But, every package of flour that I have ever bought is in packaging that is pourous. It occurs to me that the contents of the package (in this instance, flour) if placed in the freezer, then thawed, the contents will end up with some moisture content and thereby be more prone to mold, etc., wouldn't it?

I buy a lot of whole grain flours. I take them out of their paper bags, put them in gallon size zipping freezer bags, label them with the contents, and put them in the freezer.

(My issue is not mold, or creepy crawlies, but rancidity.)

Neither of the above. Freezing itself does not dry or add moisture, it merely converts water from liquid form to solid form. The rest depends on the humdity. Freeze drying works because the humdity is very low. Freezer burn happens because air contacts food, the food dries out just like clothing does when you put laundry on a clothesline. The air in a freezer with auto-defrost is particularly dry, so freezer burn is a bigger problem than it is freezers that lack that feature. Properly wrapped food neither loses nor gains moisture during the freezing process. It just gets hard

Cold air holds less water, ergo, low humidity. Frozen water sublimates. Food dries out. Vacuum packing eliminates that.

But, every package of flour that I have ever bought is in packaging that is pourous. It occurs to me that the contents of the package (in this instance, flour) if placed in the freezer, then thawed, the contents will end up with some moisture content and thereby be more prone to mold, etc., wouldn't it?

Jo Ann, the trick would be to package the flour in something that's not porous before putting it the freezer. A zip lock bag, for example, wouldn't be perfect but it would probably work pretty well. When defrosting, you'd want to give the flour time to come to room temperature before opening the plastic bag. Then the contents won't attract moisture. I do the same thing when freezing bread - unless I'm immediately rewarming in the oven, I always thaw bread before unwrapping. Otherwise, water condenses on the crust.

Cold air holds less water, ergo, low humidity. Frozen water sublimates. Food dries out. Vacuum packing eliminates that.

Larry, we may be agreeing violently. It's true that cold air holds less water per volume of air than warm air does but as I said before, it all depends on the relative humidity, no matter what the temperature. If the relative humidity is 100% (in other words, the air won't hold any more moisture at that temperature), then nothing will dry out. If I hang laundry out to dry at 25 F and 100% relative humidity (as it often would be here in western WA), it won't dry. If I hang the same laundry out at the same temperature in Montana and the relative humidity is 50%, it will dry, even if the water freezes first and, as you say, sublimates. There is nothing inherently desiccating about the freezing process per se, otherwise it would happen whether the food were wrapped or not.

Cold air holds less water, ergo, low humidity. Frozen water sublimates. Food dries out. Vacuum packing eliminates that.

Larry, we may be agreeing violently. It's true that cold air holds less water per volume of air than warm air does but as I said before, it all depends on the relative humidity, no matter what the temperature. If the relative humidity is 100% (in other words, the air won't hold any more moisture at that temperature), then nothing will dry out. If I hang laundry out to dry at 25 F and 100% relative humidity (as it often would be here in western WA), it won't dry. If I hang the same laundry out at the same temperature in Montana and the relative humidity is 50%, it will dry, even if the water freezes first and, as you say, sublimates. There is nothing inherently desiccating about the freezing process per se, otherwise it would happen whether the food were wrapped or not.

While I agree with almost everything you wrote, Mark, there is one other consideration to consider: the freezing process involves a phase transition, and often you can get phase separation during freezing. The classic example is sea ice in Arctic: when seawater freezes, the ice produced is free of salt (fresh water) and the seawater beneath it is saltier than seawater at the equator. Many foods are emulsified and will form microcystalline water that won't accumulate in ice crystals, but some will indeed form ice crystals that won't reintegrate even when the food is returned to room temperature.

Mark Lipton

Can't we all just get along? I think we all agree that food in the freezer gets drier, not moister.

While I agree with almost everything you wrote, Mark, there is one other consideration to consider: the freezing process involves a phase transition, and often you can get phase separation during freezing. The classic example is sea ice in Arctic: when seawater freezes, the ice produced is free of salt (fresh water) and the seawater beneath it is saltier than seawater at the equator. Many foods are emulsified and will form microcystalline water that won't accumulate in ice crystals, but some will indeed form ice crystals that won't reintegrate even when the food is returned to room temperature.

Mark Lipton

Agreed, Mark. There's also what happens to meat. While not what you technically might call a phase transition, you get ice crystals puncturing cell walls and causing the meat to lose water when it's defrosted.