Knowing what happens inside a jar makes canning less mysterious, and it explains why the rules are so strict. Here is how heat, acidity and a vacuum seal work together to keep food safe on the shelf. For an overview of canning and its two methods, start with our introduction to canning.
Why food spoils
Fresh food doesn't last because of four things: microorganisms (molds, yeasts and bacteria) growing on and in it, enzymes in the food itself that keep breaking it down, reactions with oxygen, and moisture loss. Canning deals with all of them at once: it heats the food to destroy microorganisms and enzymes, drives out air, and seals the jar so nothing gets back in.
The basic idea is more than 200 years old. In France in the early 1800s, Nicolas Appert found that food sealed in glass bottles and heated would keep for months, and he published his method in 1810. The science behind why it works came later, but the principle has barely changed.
The jar, the lid and the band
Home canning uses glass Mason-type jars, which can be reused for many years if they are not chipped or cracked, and two-piece lids. The flat lid has a ring of sealing compound around its edge. It should be new each time, because the compound is designed to seal once. The screw band holds the lid in place during processing and can be reused as long as it is not bent or rusty.
What happens during heating
Filled jars go into the canner with their lids on. As they heat, the food expands and gives off steam, which pushes air out of the jar from under the lid. At the same time, the heat works its way to the center of the jar, killing microorganisms and stopping enzymes.
How hot the center of the jar needs to get, and for how long, depends on the food:
- Acid foods, such as most fruits, jams, pickles and tomatoes with added lemon juice or citric acid, can be processed in boiling water at 212°F. Their acidity stops Clostridium botulinum, the bacterium behind botulism, from growing.
- Low-acid foods, such as vegetables, meat, poultry and seafood, have to reach about 240°F in a pressure canner, because botulism spores survive boiling water.
Plums, for example, are an acid fruit and go in a boiling water canner. Most tomatoes sit just on the edge of the acid line, so tested recipes add lemon juice or citric acid, and then they too can be processed in boiling water.
What happens as the jars cool
When the jars come out of the canner and cool, the steam inside condenses back into liquid. That leaves a partial vacuum, which pulls the lid down tight against the rim and seals it. You often hear it happen as a small "ping."
Let the jars cool undisturbed for 12 to 24 hours, then check each one: a sealed lid curves slightly down in the center and doesn't move when pressed. Any jar that didn't seal can be refrigerated and eaten soon, or reprocessed within 24 hours. Once the jars are sealed, take off the screw bands, wipe the jars and store them in a cool, dark place.
A good seal keeps air and microorganisms out, but it can't make up for processing that was too short or not hot enough. That's why the time and temperature matter so much.
Why tested recipes matter
A canning recipe is not like other recipes. Its processing time was worked out in a laboratory by measuring how fast heat reaches the center of a jar of that particular food, packed in that particular way. Add more onions, thicken a sauce, use a bigger jar or pack the food more tightly, and the heat travels differently, so the tested time no longer applies.
That's why it's important to use a research-tested recipe and follow it exactly: the jar size, headspace, processing time and, for pressure canning, the pressure. Recipes also need adjusting for altitude, since water boils at a lower temperature the higher you go. Many tested recipes let you adjust small amounts of dried herbs and spices to taste, but never the proportions of the main ingredients.
Tested recipes and step-by-step procedures are published by the National Center for Home Food Preservation at the University of Georgia, based on the USDA Complete Guide to Home Canning. Your state's Cooperative Extension office can help with questions about recipes and altitude.
Last reviewed November 2026.
Get your canning supplies ready
Gather everything before you start, so you don't have to stop halfway through a batch.
Canner. A boiling water canner for acid foods, a pressure canner for low-acid foods, or both. A pressure canner is not the same as a pressure cooker.
Jars. Mason-type canning jars in the size your recipe calls for, regular or wide-mouth. Processing times are tested for specific jar sizes, so don't swap in a bigger one. Check rims for chips and cracks.
Lids and bands. New flat lids for every batch, and screw bands that are clean and not bent or rusty.
Tools. A rack for the bottom of the canner, a jar lifter, a wide-mouth funnel, a headspace tool or ruler, a nonmetallic spatula for releasing air bubbles, clean cloths for wiping rims, and a timer.
From your kitchen. Large pots, a cutting board, sharp knives, measuring cups and spoons and, for recipes that give weights, a kitchen scale.
A tested recipe. From a reliable source such as the National Center for Home Food Preservation, with the adjustments for your altitude.