Vacuum query

Had a go with my new Anova last night. I think I must have done something wrong as, although the food was vacuumed, half-way through cooking, the bag seemed to get a lot of air in it which meant the food was steamed rather than cooked sous vide. Any idea why this would happen?

What were you cooking?

Vegetables are particularly good at doing this, but it happens with meat too. Home vacuum sealers really don’t pull a true vacuum. Also most food stuffs contain air which is not removed in the vacuum sealing process. The most likely explanation is that the air remaining in the bag has expanded giving the result you see.

However, there is a very slight possibility that there is something else going on. It would be really useful to have more information about your cook when you’re asking the community questions about what went wrong.

Welcome to the wonderful world of sous vide.

Thanks for the replies. Sorry, yes, I was cooking carrots. The vacuum seal looked pretty good but, as I say, after a while, the bag ballooned. How can I prevent this from happening? If it does happen, can I simply take the bag out and re-vacuum it?

Vegies have a big habit of doing that. Most folk add something heavy to the bag or clip something heavy to the bottom of the bag. Or else you can stand them up in a rack.

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What causes the bloating, I wonder?


Vegetables are prone to floating. On the whole they’re less dense than meat. The structure tends to have air between the cells whereas meat has moisture.

I did weight them down. I’ll be interested to see how I get on with some meat next. Interesting about the density of vegetables compared with meat!

Hm yeah. Veggies definitely have a tendency of doing that. Usually when I make carrots or any veggies that try to float, I push the food down with a wooden spatula and leave it laying there. This works for me.

If you are using a Food Saver like I am you can use my method. When I am doing carrots, I seal a couple butter knives in the bottom of the bag. I make them their own chamber. Then I seal the carrots in the upper part. Those knives keep it on the bottom.

Good tip, thanks. My only remaining concern is how the carrots get cooked properly if there is a lot of air in the bag. I understood that sous vide works because there is only a thin layer of plastic between the food and the water. If there is air in the bag, surely it is steaming rather than cooking ‘sous vide’. In that case, at the very least an adjustment needs to be made for the time it is in the pot, surely. Certainly my carrots were very much al dente. Perhaps a solution would be to put them in a ziplock bag instead and leave an inch or so undone so that air can escape. What do you think?

I use the Food Saver. While there is sometimes a little air in the bag after vacuum sealing, it isn’t enough to give me any problems. My carrots always come out excellent.

I use Food Saver too. The bag seemed reasonably free of air when it was sealed - certainly no obvious pockets. I’ll just have to try again!

Try to leave some space between the veggies so that the bag can fill the voids where the air is. I have a chamber vacuum, which eliminates that problem.

James, you didn’t do anything wrong. That wasn’t “air” you failed to evacuate.

Due to the higher temperatures employed to cook vegetables they usually produce gases that are trapped in the bag as they cook. Just tumble the bags around several times to ensure even cooking. You can also weigh them down with a rack or plate to keep them submerged, but still tumble them.

I understood that sous vide works because there is only a thin layer of plastic between the food and the water. If there is air in the bag, surely it is steaming rather than cooking ‘sous vide’.

At 183°F / 83.9°C you’d have to be at the top of a mountain in the Andes, say, to have them steamed (4,700 metres or 15,400 feet). Having the plastic sack in contact with the item being cooked is because “air” is a poor conductor of heat.