Seal-a-meal type bags work fine as long as you get a good seal. Store brand bags generally do not. Ziploc bags maybe maybe not. You'd be surprised how many bags look like they have good seals on every side and turn out to have a small pinhole somewhere. But if you have one of those vacuum food sealers you can pretty much make a decent sealed bag out of most anything. And because you're holding the food for a long time at relatively low temperatures, (121 to 164 for most of the things I cook) they never get close to boiling so you don't have moisture escaping as steam.
Low temperature means that you must cook for a long time for two reasons:
1) to kill off any germs (this is a time/temp thing. Most industrial processes use very high temps for very short periods - but you can accomplish the same at a lower temp for a longer period. Ever wonder why canned tuna doesn't taste as good as pouched tuna? Its not just "the can" - its also that it takes much longer to get the tuna in the center of the can to the desired temp to kill germs tan it does in that nice flat, thin, pouch - and the tuna stops cooking after faster, too!
2) to gelatinize proteins (collagens) that normally don't start breaking down until much higher temperature (and then do so much more quickly).
Low temperature means that you must cook for a long time for two reasons:
1) to kill off any germs (this is a time/temp thing. Most industrial processes use very high temps for very short periods - but you can accomplish the same at a lower temp for a longer period. Ever wonder why canned tuna doesn't taste as good as pouched tuna? Its not just "the can" - its also that it takes much longer to get the tuna in the center of the can to the desired temp to kill germs tan it does in that nice flat, thin, pouch - and the tuna stops cooking after faster, too!
2) to gelatinize proteins (collagens) that normally don't start breaking down until much higher temperature (and then do so much more quickly).
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