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Thu, Nov. 24th, 2005, 10:52 pm
Sous Vide

There's a newfangled cooking process involving keeping vacuum-sealed foods in a lower than normal cooking temperature chamber for extended periods of time. It involves some very cool technology, but I'm unclear on why food which has been vacuum sealed cooks at a lower temperature than non-vacuum-sealed food does. Obviously the pressure internal to the seal is still a single atmosphere, because the plastic seal isn't strong enough to maintain an actual vacuum. Could someone explain this to me?

When I first read about sous vide, I assumed that a sous vide cooker had an actual vacuum chamber. Of course, that would tend to freeze dry food, but it's an interesting question what happens if you heat up freeze-dried food under a vacuum, then moisturize it after cooking. Does anybody know if that might be a viable cooking technique?

Oddly, some quick web searching seems to indicate that there aren't currently any commercial freeze dryers on the market.

Fri, Nov. 25th, 2005 07:49 am (UTC)

The temperature required for boiling water, gradually lowers as the pressure does. This could well have a similar effect on the required cooking temperature.

At least, that's my first guess.

Fri, Nov. 25th, 2005 08:28 am (UTC)

That was my first guess as well, but then I remembered that that's exactly the process freeze drying uses, so it appears that the low temperature water boiling causes drying rather than cooking. Why that is, I don't know. What I'm suggesting is using raised temperature to cook in a dry and mostly vacuum environment, which would obviously cause no browning, because there's very little oxygen to bond to, but should be able to kill all the bacteria, rendering the food safe to eat.

Fri, Nov. 25th, 2005 01:53 pm (UTC)

With freeze drying, you're removing the moisture, but if it's vacuum sealed, what moisture is sealed in, stays there.

Fri, Nov. 25th, 2005 04:55 pm (UTC)

Yes, I understand why freeze drying dries food, what I'm unsure of is why it doesn't cook food, and why vacuum sealed food will cook at a lower temperature than non-vacuum-sealed food.
(Deleted comment)

Fri, Nov. 25th, 2005 08:31 am (UTC)

But why does the food under vacuum get cooked at temperatures lower than what food not under vaccum gets cooked at?

Fri, Nov. 25th, 2005 03:53 pm (UTC)

As air density goes down, its ability to hold moisture decreases.

think of air density like a sponge, it is the surface area of a sponge that allows water to cling to the internals of a sponge.

The same principal happens with air. as air gets cooler (via thermal transfer, or air pressure change) its ability to hold water decreases, because there are less active air molicules bouncing around to keep water particals afloat.

this is why mountain tops are colder than coast lines, even though they are closer to the sun, the air is thinner and it has less capacity to keep moisture afloat.

Fri, Nov. 25th, 2005 01:22 pm (UTC)

Sounds like similar properties as microwaving and pressure cooking. Of course, why not just use microwaves and pressure cookers ... beats me.

Fri, Nov. 25th, 2005 02:42 pm (UTC)

So far as I can tell from brief research on the web, the vacuum seal does nothing to change the required cooking temperature. You can cook food at low temperatures for long periods without it being sealed. Solar ovens and straw box cooking work this way.
The seal is there for storage puposes. You can store the food without it oxydizing or becoming contaminated by bacteria. It also prevents the food from drying out during the long cooking period.

Pressure cooking is the opposite, of course, cooking at higher temperature for a shorter period.

Fri, Nov. 25th, 2005 05:07 pm (UTC)

The argument is that the sealed container keeps the flavors in until served.

The long cooking period is also key. Since the food won't dry out, you can keep it cooking longer at a lower temperature than you would otherwise need if you were fighting the time it takes to dry the food out.

Mon, Dec. 26th, 2005 10:12 pm (UTC)
derek225: Enzymatic activity is the final piece of the puzzle

My understanding is, which is bolstered primarily by reading Harold McGee's *On Food and Cooking*:

- the cooking temperature is unchanged regardless of whether the food is under vacuum

- the bag helps do two things:
1. seals in moisture, which might include sauce, and definitely includes the moisture within the meat. the central challenge in cooking meat is maintaining moistness.
2. meat cells contain digestive enzymes that act on meat as tenderizers, but function in a different role in living tissue. they have high activity at body temperature, and tenderize the meat (hence the long cooking time)

Sat, Dec. 31st, 2005 06:35 pm (UTC)
yunal: So much bullshit


For home use the main point of Sous Vide is that the meat can reach the correct core temperature without frying the outside. The "vacum" is not needed. Once the meat has the right core temperature you can put it in a frying pan and make a nice frying crust very quickly in a very hot pan.

From what I have read Sous Vide is a very _safe_ preparation method, it makes food keep longer - much like pasturization. There seems to be little talk of boutulin.


Fri, Jan. 6th, 2006 12:04 am (UTC)



Fri, Sep. 15th, 2006 05:17 am (UTC)
polyscience: sous vide

There are a few reasons for vacuum sealing for sous vide. First is that you can achieve very rapid heating of the product as water conducts 100 times faster than air. The thin bag does not create a thermal barrier. Secondly the temperature control you can achieve with water is far superior than what you could get with air or even combi steam. Cooking at a precise temperature is much different from "boil in a bag" as you can bring the product to temperature without breaking the cell structure and releasing moisture. The technique can work well for fish and vegetables. for meats it is also excellent but a secondary exposure to high heat is typically required (Maillard reaction) to release flavor and aroma.
Freeze dried food is simply removing moisture under vacuum at low temperatures. The moisture moves through sublimation skipping the liquid phase. There are lots of fun things to do with this process but I wouldn't consider it a "cooking technique". There are however several ways it can be a technique for manipulating food.