You're standing at the counter with a colander of tomatoes, or a sack of green beans, or a pot of chili you'd love to put up, and the same question comes up every time: which method does this need?
One question answers it, and once you understand the why behind it, you won't have to guess again. Is the food high-acid or low-acid?
Why acid decides the method
The danger that matters most in a jar of home-canned food is botulism, and the bacterium behind it is ordinary. Its spores are in garden soil and on the surface of fresh vegetables. Out in the open air, they do nothing.
They wake up only when they get three things at once: no air, low acid, and time at room temperature. A sealed jar of low-acid food sitting on a shelf gives them all three. That's when they grow and make a toxin you can't see, smell, or taste.
Canning takes those conditions away in one of two ways.
Acid. The spore can't grow in a food that's acidic enough. In a high-acid jar, it stays asleep. The heat of a boiling water bath then handles the molds, yeasts, and ordinary spoilage that high-acid foods have to worry about.
Heat. A low-acid food has nothing in the jar to hold the spore in check, so the spore has to be destroyed, and it's tough. A pot of boiling water can't get hotter than boiling, no matter how high you turn the burner. A pressure canner traps steam, and under pressure the temperature climbs past the boiling point to the heat that destroys the spore. That makes it a different tool from a water bath: it reaches a temperature a water bath physically can't.
The National Center for Home Food Preservation lays out the science, and where food scientists draw the line, in Ensuring Safe Canned Foods. The rule that comes out of it is short. High-acid foods can go in a boiling water bath. Low-acid foods need a pressure canner.
Which side a food falls on
High-acid: most fruits, and the jams, jellies, and fruit butters you make from them. Most pickles, because the vinegar brings the acid. Sauerkraut. And properly acidified tomatoes, which get their own section below.
Low-acid: vegetables, like green beans, corn, carrots, potatoes, and beets. Dried beans. Meat, poultry, and fish. These are most of the savory things you want on a pantry shelf, and none of them has the acid to hold the spore in check on its own.
Mixtures: soups, stews, chili, and sauces. Anything with meat, poultry, or fish in it is a pressure canner job, full stop, and so is a soup or stew of mixed vegetables. The center notes that most mixtures of acid and low-acid foods end up on the low-acid side unless the recipe adds enough acid. A few tomato and vegetable mixtures, like salsa, have a tested water bath process for exactly that reason: the recipe adds a measured amount of acid.
Why tomatoes need added acid
Tomatoes are the crop that gets most people canning, and they sit right on the line. Plenty of tomatoes are acidic enough on their own. Some aren't, and modern varieties bred to be sweet and mild can drift to the wrong side without tasting any different. A perfectly red, perfectly sweet tomato can be exactly the one that falls short. You can't tell by tasting, and you can't tell by looking.
So there's no gamble to take. Every plain tomato product, whole, crushed, juiced, or plain sauce, gets a tested amount of bottled lemon juice or citric acid added, every time, because you have no way to know which jar needed it. I use bottled lemon juice, not fresh, because its acidity is consistent. The center's introduction to canning tomatoes gives the amounts, and it makes a point that surprises people: the acid goes in even when you pressure can those tomatoes, because the tested pressure options were developed for tomatoes with added acid.
Once you stir in onions, peppers, or meat, the jar changes. Salsa mixes acid and low-acid ingredients, and the center's answer on canning your own salsa recipe is that the specific tested recipe, and sometimes how it's prepared, determines the method. Some tomato and vegetable mixtures are tested for a water bath with added acid. Others, like a pasta sauce with onions and peppers stirred in, are tested for the pressure canner only. The recipe tells you which, and you follow it as written.
What acid can't fix
Acid works in a pickle because the recipe was tested with that exact ratio of vinegar to vegetables. That's why you never cut the vinegar to soften the bite, water down the brine, or stretch a batch with an extra handful of cucumbers. Change the herbs and spices all you like. Leave the acid where it is.
A splash of vinegar in a jar of plain vegetables is a different matter. A tested pickle like dilly beans is built on a full vinegar brine, and a splash isn't that. The center's vegetable FAQ is direct: vinegar added to fresh green beans does not change the process they need. Plain green beans are a low-acid food, and they go in the pressure canner.
When you're not sure
Sooner or later you'll have a food that doesn't fit neatly on one side. Take the cautious path and treat it as low-acid: a pressure canner, with a tested recipe written for that food. If you can't find one, don't can it. Freeze it or keep it in the refrigerator instead. The danger only runs one direction, and a low-acid food in a water bath is the mistake that can cost you everything.
The tested recipe is the real answer. One from a source like the USDA or the National Center for Home Food Preservation names the method right on the page, because the people who tested it settled the acid question in a lab so you don't have to settle it at your counter. The CDC's page on home-canned foods and botulism puts the low-acid half in one line: pressure canning is the only recommended method for low-acid foods.
One question, every time
Before any jar gets filled, ask it. High-acid or low-acid? Answer that, and the method follows. Then follow a tested recipe exactly, and check every jar the same way when it comes out. It Sealed, So It's Safe, Right? covers that part.
If you want the craft taught from zero in one book, with the why behind every rule and a method named on every recipe, that's the book I wrote: The Complete Guide to Home Canning & Preserving.
Water bath and pressure canning, dehydrating and root cellaring, taught from zero, with the why behind every rule.
About the book