Air in vac bag.

In general, as the meat cooks, especially for a longer time, it can release some gases from the cooking process. As long as the bag stays submerged it is normally just fine. But if it smells bad it might be more of an issue.

The 'air' can be one of three things:

1 normal air
2 water vapour from water in the food which evaporates during the cooking
3 other waste gaseous products from microbial activity

My limited experience is that 1 and 2 are generally small amounts and in the case of 2 will pretty much disappear if the food is cooled in an ice bath (the water vapour condenses). 3 tends to puff the bags up and as Jason says mostly associated with longer cooking times.

Provided I'm confident that the bag has been submerged during the whole of its cooking and no power cuts, I tend to go by the unscientific and not always reliable method of smell, so if it smells off (not always easy to decide with meat) I tend to bin it as a precaution.

I'm still a relative newcomer to sous vide but I've only had to throw one package away because it smelled off and that was ox tail after a 4 day cook.

Thanks for the replies, I got busy with other projects and didn't get back here till now. I have had the air develop while cooking but the one my wife defrosted had the air before putting it into the water bath. I just got uncomfortable and gave that one to our dog, he enjoyed it.

GeneK