(Note: this is a continuation of this question: Sous vide effectiveness without using a vacuum sealer?)
What is the scientific impact of trapping small amounts of residual air in a pocket above the water bath? This assumes that there is still a very strong thermal coupling between the water bath and sous vide item.
In the photo below, you can see at least a 2-inch gap between the air pockets above water level and the actual short ribs. The food does not float.
From a heat transfer perspective, I can't imagine that a vacuum sealed bag can perform significantly better—certainly not relative to a 48hr cook time.
Is there (scientifically) something fundamental about vacuum pressure with sous vide, or is it only continuous, stable heat-transfer? What kind of "oxidation causing off flavors or promote spoilage" is possible in the described scenario?
Is there a possibility of bacterial contamination?
What would happen if I cut a small hole in the bag above the water level and let the trapped air escape--as long as the food does not float and water does not leak in?