How Does a Vacuum Sealer Work?
A vacuum sealer removes air from around the food to slow spoilage, then presses a heated bar across the bag to melt it shut. External sealers pull air through the bag opening; chamber sealers evacuate the whole chamber at once.
A vacuum sealer does two jobs in sequence: it removes air from around your food, then it melts the bag shut so air cannot get back in. Understanding those two steps explains almost every buying decision and every failed seal.
Step one: removing the air
Food spoils because of oxygen. Oxygen feeds the aerobic bacteria and molds that cause rot, drives the oxidation that turns fats rancid and browns cut surfaces, and lets moisture migrate, which is what creates freezer burn. Remove most of the air and you slow all of those processes at once.
An external sealer uses a small vacuum pump to draw air out through the open mouth of the bag. This is why these machines need textured or embossed bags: the tiny channels in the film give the air a path to escape. A smooth bag would collapse and seal off those channels before the air was gone.
A chamber sealer works differently. The entire bag sits inside a rigid chamber, and the pump evacuates air from the whole chamber at once. Because the pressure inside and outside the bag drops together, liquids do not get pulled toward the opening, which is why chamber machines can seal soups and marinades that would defeat an external unit. When the chamber re-pressurizes, the higher outside pressure presses the bag flat against its contents.
Step two: sealing the bag
Once the air is gone, a heated seal bar presses across the open end of the bag and melts the two layers of film together into a single weld. The machine controls how long and how hot that bar runs. Too little heat and the weld is incomplete; too much and it burns through the film. This is exactly what a "moist" setting adjusts: damp film needs a longer heat cycle to form a solid seal, so moist mode extends the sealing time.
A double seal simply lays down two parallel welds instead of one, adding a backup barrier for long freezer storage or for contents with sharp edges that might nick a single seal.
Why the vacuum is never perfect (and why that is fine)
No home sealer pulls a true, total vacuum. A small amount of residual air always remains, and that is enough for practical food storage. What you are really doing is removing enough oxygen to dramatically slow spoilage and prevent freezer burn, not creating a laboratory vacuum. This is also why vacuum sealing is not a substitute for proper refrigeration, freezing, or canning: it extends storage life within safe temperatures, it does not make food shelf-stable on its own.
The practical takeaway
If your food is dry or firm, an external sealer removes the air cheaply and seals it well. If your food is wet or liquid, the physics favor a chamber sealer, because it takes the air from around the whole bag at once instead of dragging it (and your liquid) out through one end.
Next: why won't my vacuum sealer seal? or how to choose the right one.
Frequently asked questions
Does a vacuum sealer remove all the air?
No home sealer pulls a true total vacuum, and it does not need to. Removing most of the oxygen is enough to slow spoilage and prevent freezer burn.
Is vacuum sealing the same as canning?
No. Vacuum sealing extends storage life within safe temperatures but does not make food shelf-stable. Low-acid foods still need pressure canning.