Vacuum Sealing Fails with Sous Vide

[Bobonli]

This is probably a basic physics question but I couldn't find anything on point with a general Google search and there are plenty of smart people here who also cook.

If I take my vacuum sealed food out of the fridge and put it directly into my sous vide, should the temperature change cause the vacuum/pressure inside the bag to change such that the bag is no longer a vacuum?

The deets: I seasoned a roast, and vacuum sealed it with my industrial grade FoodSaver. The bag shrunk etc before the seal applied. I checked the seal and it was intact. I put the meat in the fridge overnight. Next afternoon, removed the apparently intact package directly from the fridge and dropped it into a 139.5 sous vide, whereupon the bag developed a noticeable air bubble inside it and the roast would not submerge. I cut open the bag and used another bag to re-seal the meat. Story ended happily but I cannot understand how the bag failed. This has happened several times and each time the bag passed an inspection when I did the original seal. Only seems to happen when going from fridge directly into the water bath so I'm wondering if it's the temperature change that is causing this problem.

Only thing I can learn online is that some people think FS is junk because the seals often fail microscopically.

[htwoopup]

I am not a scientist nor do I play one on TV, and I did remarkably poorly in those subjects as a youngun'.

So, this is a scientific wild ass guess...

You know how water expands when it is frozen?

The plastic bag is pliable. So as the meat expands as it chills the bag stretches a little.

Then when the meat comes up to temperature it shrinks a little which makes the bag like a pair of too tight pants after you lose a pound. Plus the material is stretchier when warmed. And thus there is loose material/ bubble.

As I said, that thought prolly is not worth much but is what me and a bourbon have decided.

[Mark Walberg]

The relevant formula is PV=nRT, where temp is in Kelvins.
In this case, the pressure doesn't change. Only the volume changes with temp.
Increase from fridge (say about 35 degrees F) and 139.5 degrees F is just over 100 degrees F hotter.
THat is about 55 degrees Kelvin hotter. So, about 350/275 is roughly 1.3.
So, the air in the bag should increase volume only 1.3 times with that increase in temp.

[gt6267a]

We have a similar setup with FoodSaver and sous vide. We go fridge or freezer to warm water without bubble expansion issues. The FoodSaver is not a perfect vacuum. There are tiny bubbles but nothing so large as to create substantial submersion issues. There is some technique to getting a good vacuum and not trapping air. A guess as to the problem: air is trapped inside the bag in one or more places before sealing. You don’t see the quantity of gas until its submerged. Have you tried submerging right after sealing?

If we use Ziplock sometimes there are more bubbles or if FoodSaver bags have a few, I find a tiny bit of float is resolvable: Bag clip the FoodSaver / Ziplock bag to the pot / sous-vide vessel. If there happens to be a slight buoyancy due to a tiny air bubble or bag movement due to the swirling water of the sous vide, this minimal action is resolvable by clipping the bag. One addition thought here, I do this in a way to ensure the food is completely submerged and not touching the vessel wall. Its easier to do than describe. Try it, you'll quickly figure out how this might work for you.

[giordana93]

a little late, but I've had a failure or two with another brand, and I don't think it has anything to do with pv=nRT because that formula is for gases and if the vacuum process worked in the first place, there shouldn't be any left inside the bag. What I think happened (in my case at least) was that some liquid was pulled up from the meat during the vacuuming and the seal was pretty good, but like a pin hole leak in a tube you can't find: just enough to let some air in. Also, it is true that one needs to massage the meat and bag during the vacuum process to make sure any pockets of air are expelled before the final seal. Lastly, I've taken to adding a second seal (or opting for the double seal on the device menu) to ensure an intact seal. But I know one time for sure there was a wrinkle/liquid at the edge of seal which led to seam failure. I would think that an olive oil marinade could make an imperfect seal if it contaminated the area. Long story: make sure you wipe the inside of the bag clean before sealing, massage air out as vacuum is applied, and double seal or double bag for extra precaution. And don't try to make the edge too close to the meat: you need that extra inch or so.

[mg2ride]

I personally think it is a PV+nRT thing. To test it simple but the bag back into the fridge after dropping in the hot water. If it returns to it's pre Sous Vide shape, it's a PV=nRT thing, if it doesn't the seal failed.

[Dmoeny]

Also probably a dissolved gas thing too, and the gas law is only part of the explanation.