Sous vide is a bit of a misnomer. It means “under vacuum”. That was how it originally started but now it would really be more appropriate to call it something like precision cooking now.
Vacuum was first used as a way to remove air from around the food so that there was good contact with the water in the bath since air is a poor conductor. Now while it’s possible to use vacuum a lot of sous vide cooking is done using Ziploc freezer bags and removing air by letting water in the bath press the plastic onto the food. You can also do this kind of cooking in–you guessed it–mason or Ball jars although you’ll need to increase times just a bit since glass is a poorer conductor of heat than with the think plastic.
Note that I specifically used the Ziploc brand name. Those are recommended as being safe and holding up to the cooking temperatures and times used. Most recipes suggest gallon bags, and not ones with the slider. You want the double seal. Since I’m mostly cooking single-serving size amounts I’ve been using quart size, and a smaller water bath than is usually recommended.
Some of you might be cringing at the temperatures and the times you see. They are safe. Even the lower temperatures essentially pasteurize the food according to USDA standards. More on that later, but pasteurization is really a pretty fascinating process.
I’ve already cooked one thing in a mason jar–lentils. Cooking right in the jar makes one of the good things about sous vide even better especially in cooking for one: that jar of lentils (about 3 really hefty servings) could go in the fridge so there was no extra washing up. Awesome. I’m envisioning some other advantages to sous vide for doing single-serving cooking!