Sous vide cooking involves sealing food in plastic bags and submerging them in water that is temperature-regulated by a sous vide machine, resulting in the most evenly-cooked food possible. More specifically, the food must be vacuum-sealed, instead of just placed inside a Zip-Loc, for best results.

Vacuum-sealing food so that no air bubbles or pockets are present whatsoever is essential to sous vide, since heat is directly transferred from the water to the food. If there are pockets of air between the food and the water, it can impede the transfer of heat, and your food ends up cooking unevenly.

There are zipper-lock and sous vide bags on the market that don't require a vacuum-sealing machine to achieve an adequate seal, but these can be expensive and difficult to find. Investing in a vacuum sealer is the best way to ensure that your food stays moist and cooks evenly while in the sous vide water bath.