After cooking a few meals sous vide using the zip loc bag method of sealing the food (instead of a vacuum sealer), I've noticed that the longer the meat is held at a temperature, the less juicy it is. When I tried 72 hour short ribs, it came out tasting like a dry pot roast...not good at all and very disappointing. One article on the internet seems to have come to the same conclusion. http://www.beyondsalmon.com/2011/06/sous-vide-experiment-temperature-vs.html
I know that professional chefs do NOT have this problem as they often recommend long cooking times such as 24-72 hours, and the only reason I can think of is that they use a vacuum sealer as opposed to zip loc bags. I hypothesize that either the pressure or absence of air keeps the fluids in the meat, rather than leaking out into the bag. Can anyone with experience using both the zip loc and the vacuum sealer methods of cooking sous vide confirm my hypothesis? Thanks.
I don't think the packaging method makes any material difference to the pressure measured at the meat's surface which will be atmospheric pressure plus a few inches of water.
Its much more likely to be the quality of meat, any pre-cooked treatment ie brining, and temperature. As LeeW says there does seem to be a trade off between juiciness and tenderness in terms of temp/time, all other things being equal.