What is the function of the vacuum? How near perfect does the vacuum need to be?
It helps with removing all of the air from the bag, which is what you want when cooking sous vide.
But why do you need to remove the air from the bag? Is it only to keep the bag and its contents submerged? Or is there more to it?
I believe the vacuum works as a better conduit to transfer heat. Thermodynamics. Air is a poor conductor of heat as evidenced by an oven and the time it takes to cook at 350. Compare that oven with the time it takes to deep fry at 350. If you have air in the bag the water has to heat the air which then has to heat the food which is ineffecient. Hope I got that somewhat right.
In reverse, consider that you can survive all day in 60 degree air temp but in 60 degree water you will suffer hypothermia in a short time due to the conductive properties of water. Dang I sound smart! LOL
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whybuynew1 was close. The reason that you want all of the air out is that itās a poor conductor of heat. Water works so much better (more efficiently) because itās much denser. That heat energy is better and (in the case of the APC) more precisely delivered to your food. Precision that our ovens that rely on air canāt deliver.
This is why the French restaurants started utilizing it decades ago.
Itās also why we have the gradients in meat when we produce with more traditional methods (eg. grills). We have to get the air much hotter to cook our meat in a reasonable amount of timeā¦so the outside is more well done than the inner core (presuming you donāt go for āhockey puckā well done that is) 
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Thank you very much. That was a very clear explanation, and now I understand the process well.
Hi. Just one more thing. With the air out the juices stay more in the meat. Try experimenting. Cook two steaks or pork chops or 2 of whatever and cook one with all the air out and one with the air kinda out. You will see a big difference. Have fun
The Princess and Fischer are absolutely correct.
Your precisely cooked food in the absence (mostly) of air and at a significantly lower temperature prevents the substantial evaporation experienced in conventional cooking.
The efficiency of heat transfer means proteins can be cooked to the point of denaturing without going over and tightening of the protein strands. The result is soft, melt in the mouth juiciness.
I knew it worked great, but now I understand why it works.
As for āhow near perfect does it have to be?āā¦
Iām still a novice sous vide cooker, but so far Iāve only been using the immersion technique for expelling air from the ziplock bags I use to cook in. This does not create a perfect vacuum, and I can still see air bubbles in the bags, but Iām very impressed with the results.
Perhaps an actual vacuum sealing machine would give better results, but for the price, Iām very happy to stick with my ziplock bags.
A few bubbles will make no difference whatsoever. A vacuum sealer is convenient, but the results wonāt be any better.
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It doesnāt have to be ānear perfect.ā Besides, a lot of ingredients will release a bit of air as they cook.
Ned, the added benefit of a vacuum sealer is in their sealing capability. Leaky bags will compromise your cooking. As long as you use quality freezer grade bags with strong seals you will do just fine. Zip-Loc is the highest rated brand.
We have had a vacuum sealer for many years. In past, did use Zip-lock freezer bags for freezing. When purchasing various meats (Costco or other great deals?) needed to freeze the excess so no freezer burns. Now the sealer will do double duty.
I will concur that the vacuum sealer is great for freezing, Have been using one for way longer than Iāve had sous vide. I also use them to make curing meats simpler. Theyāre really quite useful, but you donāt actually need one for sous vide cooking.
This is true.