I found this old post when Googling water loss in vacuum packing.
I wanted to add to the info. My wife has recently completed a masters research study about the effects of packing atmosphere on sliced salmon.
It was noted that the vacuum bag packing had magnitudes greater moisture loss than air, CO2, or CO2 50% / N2 50% at atmospheric pressure in ridged sealed containers.
We struggled to understand why this is the case, because we thought the flexible bag should be exerting full atmospheric pressure on the contents, just like air does. However, we realised the pressure inside the bag must be lower than atmosphere, because the bag is wrapped tightly on the contents.
If the internal and external pressure were equal, the bag would only be loosely held against the product and you could move it easily, even though there may be no air inside. This is not the case.
The reason is due to the subtly inflexible nature of the bag material. The bag plastic wants to be in a relaxed state, but the low pressure inside (and higher pressure outside) forces it out of shape. The resistance of the bag material to being pressed and stretched around the food is what causes the internal pressure to be lower than atmosphere. If the bag were perfectly flexible and conformed to the contents with no resistance, the internal pressure would exactly equal the external pressure.
As time passes, moisture in the food will be drawn out and absorbed gas will evaporate to fill the bag and allow it to return to a relaxed state.
Anyway I hope that helps share the knowledge.
