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Is it advisable to sous vide in steam fresh bags, the type where one cooks the veggies in the microwave for 4 minutes?

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No, those bags are not completely sealed. They are designed to let steam escape, therefore they are not watertight, if you put them in your sous vide machine they'd get waterlogged.

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    What do you mean by steam? Water tight and vapour tight are different things. – Dan Gravell Jul 24 '18 at 9:49
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    All the steam fresh bags I've seen have small perforations @DanGravell, definitely big enough for water as opposed to vapor. – GdD Jul 24 '18 at 13:02
  • thanks, just interested... not a technique I've ever used! – Dan Gravell Jul 24 '18 at 15:18
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No, in addition to the valid reasons offered by @GdD sous vide relies on conduction to transfer heat from the water into the food being cooked. "Steam Fresh" bags are not vacuum sealed. The air in the bag is an insulator between the water and the food, making the heat transfer more difficult and uneven.

While vacuum sealed bags are preferable for sous vide a 'near' vacuum sealed, such as air being forced out of a ziplock bag, is functionally 'good enough'. The volume of air in a steam fresh bag would be problematic.

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    While "sous vide" directly translates to "under vacuum", vacuum sealing is frequently not necessary. Simply removing as much air as possible (as you point out) is fine. This can easily be acomplished by forcing the bag under water then sealing. A steam fresh bag might be porous enough to allow water in. There are applications where water in the bag is not a problem (eggs come to mind), but I'm unclear about how air in a submerged steam fresh bag would be an issue. – moscafj Jul 23 '18 at 15:24
  • @moscafj Air acts as an insulator so you'd have to force the air out. You are right about not needing a perfect vaccuum. I use the water pressure method with freezer bags and there's typically a little air that becomes apparent when it expands when the bag is placed in hot water. But as long as that air doesn't separate the food from the heat, it's irrelevant. – JimmyJames Jul 23 '18 at 15:50
  • @JimmyJames correct. I was responding to the last line in this answer. – moscafj Jul 23 '18 at 16:01
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    @JPhi1618 My experience with these steam-in bags (I never actually cook in them) is that the air doesn't come out readily. I'm not sure how much pressure would be required to eliminate it and how much would more would pop the bag. Getting air out of an open ziplock requires only being submerged with maybe a little guidance of the air to the top. – JimmyJames Jul 23 '18 at 16:52
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    I'm not sure where the question is here? Yes, these bags come with significant air in them, air which blocks (insulates) the the transfer of heat from the liquid environment to the destination food to be cooked. Uneven contact with the bag as gravity and the circulation of the bath would prevent an even and predictable transfer of heat. Given that the kinds of foods generally available in these stay fresh bags (mostly veggies) can be safely served raw it wouldn't necessarily be 'bad' but it would run counter to the idea of sous vide. – Cos Callis Jul 23 '18 at 17:03

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