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Why is it so? Sous Vide & shelf life, Pressure Cookers & tende

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#1 Ozcook

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Posted 11 December 2012 - 01:21 PM

I have 2 questions that have been bugging me:

(1) Sous-Vide and shelf life.
If I buy some vacuum packed meat (tenderloin, lamb shanks, whatever) that has a use-by date 2 months ahead and I sous-vide the meat to pasteurization, quick chill it in an ice bath and put it in the refrigerator, all the sous-vide books I have read say the meat should be consumed within a week or frozen. Why should the use-by date of the meat be so drastically shortened compared to the raw meat I started with when both are vacuum packed?

(2) Temperature and tenderness
My recollection is that meat cooked to a temp above about 185F starts to lose moisture (and tenderness) as the water is forced out of the meat, which is why it is not a good idea to make a stew at a rolling boil (212F). That being so, why do stews made in a pressure cooker at 250F come out tender? Applying the stated principle, the stew should be like concrete. Why is it so?

#2 Baselerd

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Posted 11 December 2012 - 01:43 PM

Not a complete answer to your question, but I know that the vacuum packaging used for the meat you buy at the store has stacked laminates, each with different purposes. Some of these materials are employed due to their extremely low gas permeability, which enables the long shelf life. A lot of these materials are unsafe to heat however, and thus are not used in the Sous Vide safe bags.

#3 gdenby

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Posted 11 December 2012 - 03:48 PM

Here's what I can offer.

Cooked food is less resistant to the growth of pathogens than uncooked foods. Ordinary cooking should wipe out all but 1 pathogen out of a million. But that still means even vac-pak'd foods will start to spoil if not held below freezing. Don't know how or why uncooked packaged foods last so long.

Meat begins to cook above about 120F. The protein shrinks and water loss is inevitable. The higher the temperature, the greater the contraction and disintegration. Don't know that boiling a stew makes the meat much drier, but there is a chance some flavor will be lost. As far as I know, meat that is overcooked becomes gummy, either boiled or pressured cooked. The primary difference I've experienced is that tho' sometimes chewy, pressure cooked meats are very flavorful from Maillard reactions, and all the collagen connective tissue has turned to gelatin. The mouth feel is not dry, but slippery.

#4 HowardLi

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Posted 11 December 2012 - 09:31 PM

1) Commercial vacuum packing tends to be done in harder vacuum than is ordinarily achievable in a home kitchen. A FoodSaver, for example, still leaves behind quite a bit of air and therefore oxygen.

2) Who knows?

#5 pazzaglia

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Posted 11 December 2012 - 11:20 PM

2) There is almost no evaporation in pressure cooking (2-3.5% vs 30% at no pressure according to my measurements) and the meat preserves ALL of its juices.

Ciao,
L

hip pressure cooking - making pressure cooking hip, one recipe at a time!


#6 Lisa Shock

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Posted 11 December 2012 - 11:42 PM

2) Most textbooks cover this in detail. Braises are done with certain types of collagen-rich meats which would be tough if cooked with other methods. During the first part of the cook, the meat IS tough and dry. But, you cook it beyond that point until the collagen breaks down and you get a distinct structural change. A pressure cooker speeds things up and hold in some moisture. That said, you pretty much have to cook some cuts of beef far longer than others to break down tough connective tissue. If you were to cook low collagen cuts, like the filets, as a braise it wouldn't work. And, if you cook some stew meat like a steak, it's going to be really tough.

#7 EnriqueB

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Posted 12 December 2012 - 02:21 AM

2) There is almost no evaporation in pressure cooking (2-3.5% vs 30% at no pressure according to my measurements) and the meat preserves ALL of its juices.

Ciao,
L

Sorry Laura, it's true there's almost no evaporation, but what do you mean than the meat preserves all its juices? It does not, according both to theory and to my experience in pressure cooking. It may loose its juices at a different rate than a traditional stew, due to the different temperature/time profile, but give the meat enough time in the pressure cooker and almost all its juices will be drained.... As Lisa Shock said, the meat is not dry due to collagen converting into gelatin, as far as I know.

#8 pazzaglia

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Posted 12 December 2012 - 02:52 AM

Enrique, thanks for the opportunity to clarify. I assumed ozcook intended a stew cooked "until it's done".

In pressure cooking, the meat's and cooking liquids do not evaporate, as with oven roasting or non-pressure boiling. When cooked the appropriate amount of time, the juices remain in the meat and the pressure prevents ANY evaporation. In an over-cooked situation, as Enrique pointed out, even though juices may move out of the meat itself the meat's juices still remain -- but in the PRESSURE COOKER!

Enrique, you may have noticed, too, that if you open a pressure cooker with the Normal or Cold-water Quick release and leave the meat uncovered ALL of the super-heated juices quickly evaporate in a fast and aromatic plume of vapor - leaving the meat tough and dry BECAUSE its been pressure cooked!

Ciao,

L

hip pressure cooking - making pressure cooking hip, one recipe at a time!


#9 cookalong

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Posted 12 December 2012 - 05:40 AM

How come store-bought meat has such long shelf life when it's vacuum packed? Is there no risks of botulism? From what I know, botulism is the reason why you don't store sous vide food for more than a couple of days in the fridge.

#10 rotuts

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Posted 12 December 2012 - 08:52 AM

you can store SV foods that have been pasteurized in a very cold refrig for quite some time: 30 - 90 days is says in some places but i mean a cold cold refrig. 33 - 34 degrees.

#11 HowardLi

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Posted 12 December 2012 - 10:56 AM


2) There is almost no evaporation in pressure cooking (2-3.5% vs 30% at no pressure according to my measurements) and the meat preserves ALL of its juices.

Ciao,
L

Sorry Laura, it's true there's almost no evaporation, but what do you mean than the meat preserves all its juices? It does not, according both to theory and to my experience in pressure cooking. It may loose its juices at a different rate than a traditional stew, due to the different temperature/time profile, but give the meat enough time in the pressure cooker and almost all its juices will be drained.... As Lisa Shock said, the meat is not dry due to collagen converting into gelatin, as far as I know.

Perhaps it is that much less liquid is used in a pressure cooker, and therefore the concentration of the gelatin is much higher than in a typical braise after the collagen hydrolyzes. So, when you eat it, you get more of the juicy mouth feel because there's more gelatin.

Maybe one could save a regular braise simply by adding gelatin to the juices?

#12 Lisa Shock

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Posted 12 December 2012 - 09:53 PM

No one here is claiming that the pressure cooked meat is juicier than a regular braise. Go back and look at the original questions.

The pressure cooker is faster, but, not necessarily better for a braise.

And no, adding gelatin to a braising liquid won't help anything -unless a cut of meat is lacking in collagen, like trying to braise a t-bone.

#13 inductioncook

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Posted 06 January 2013 - 09:37 PM

Ozcook is asking VERY good questions, and we don't really have answers here. A braise cooked without pressure at 95-100 degrees C will get dry and fairly tough, because although some of the collagen has gelatinized, collagen also will have contracted, toughening the meat. But this doesn't happen in cooking below the boiling point at 120C or 250F in a pressure cooker. It's a very complicated topic and I don't think anyone in the world really knows the answer right now. In some cases, the pressure cooker does a better job than a regular braise. Why is that and when does it happen? My anecdotal observation is that the pressure cooker is better for lamb stew (such as navarin d'agneau) and equal or not as good for veal (blanquette de veau). And pazzaglia says that the results in a pressure cooker depend in large measure on how the pressure is released: now that is a very interesting concept.

#14 inductioncook

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Posted 06 January 2013 - 09:42 PM

Some of the complexities are discussed by nathanm and douglasbaldwin here: http://forums.egulle...equipment-2011/

#15 FeChef

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Posted 07 January 2013 - 10:25 AM

(1) most of the vacuum packed meats have a solution of natural juices and preservatives in them, extending the shelf life. Once you let oxygen into them you reduce that shelf life date.





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