Water bath or pressure: the one number that decides it
Sources last opened 2026-08-26.
The short version
One number decides it. Food with a pH of 4.6 or lower can be processed in boiling water. Food above 4.6 cannot, at any length of time, and needs a pressure canner [F-1.1].
The reason is temperature, not the equipment. Boiling water tops out at the temperature of boiling water. A pressure canner reaches 240° to 250°F, and that is the range the guide says low-acid food must be sterilised at [F-1.2].
So the question to ask about a food is never "how long?" It is "which side of 4.6 is this?" — and the tested recipe you follow has already answered it by telling you which canner to use.
The question everyone actually asks, and the question that answers it
"Do I need a pressure canner?" is a question about equipment, and it cannot be answered by looking at equipment. It is answered by looking at the food.
Every food you might put in a jar sits on one side of a pH of 4.6 [F-1.1]. That number is not a rule of thumb, a preference or a manufacturer's position. It is printed in the USDA Complete Guide to Home Canning, 2015 revision, on printed page 1-8, and the whole of home canning is arranged around it.
One page further on, at printed page 1-9, the guide prints the same idea as a full-page figure: a pH scale split at 4.6, with the acid foods — the ones at 4.6 or lower — marked "Use Water Bath and/or pressure canner", and the low-acid foods — the ones above 4.6 — marked "Must use pressure canner" [F-1.1]. Read each label against the scale rather than against the other one: acid is the low-pH side, low-acid is the high-pH side. If you have ever seen that graphic reproduced without the sentence it illustrates, this is the sentence.
The tomatoes-and-figs sentence, which is where most of the confusion lives
Tomatoes are on the list of exceptions inside the exception, and the guide names a second food that almost nobody quotes alongside them.
Some tomatoes "are now known to have pH values slightly above 4.6", and figs also sit "slightly above 4.6" [F-1.5]. Both must be acidified with lemon juice or citric acid to be canned as acid foods, and once properly acidified they can be processed in a boiling water canner [F-1.5].
Three things are worth pulling out of that.
The guide does not say tomatoes are low-acid. It says some are above the line, which is why the instruction is to acidify every batch rather than to guess about the batch in front of you.
Acidifying is a tested quantity, not a splash. The amounts are published, they differ by jar size, and they are specified as bottled lemon juice rather than fresh for a reason worth understanding before you substitute [F-5.3].
And figs are the forgotten half of the sentence. If you have been given fig preserves advice that never mentioned acidification, it was working from half of a sentence in the guide.
What this does not decide
Which canner your food needs is one decision. It is not the whole of a safe batch, and I want to be straight about the limits of a page like this one.
The 4.6 line tells you which piece of equipment. It does not tell you the process time, the pressure, the headspace or the pack style, and those come from a tested recipe for that specific food in that specific jar size, adjusted for your altitude. This site publishes none of them, links to the sources that do, and will keep saying so.
It also does not tell you that a jar is safe afterwards. A sealed lid proves a vacuum formed inside the jar. It says nothing at all about the temperature the food reached before the lid pulled down, which is why open-kettle jars seal perfectly well and are still not processed food.
How to use the number tomorrow
You will meet a recipe on a page that carries no citation, and you will want to know whether to trust it. Three questions, in this order.
Which pile is the food in? Fresh vegetables, meat, poultry, seafood and milk are above 4.6. Fruits, pickles, sauerkraut, jams and jellies are at or below it. Mixtures are above it unless the recipe was tested with acid in it [F-1.1].
Does the recipe's canner match the pile? A boiling-water process for anything from the first list is the single most common dangerous instruction in this subject, and it usually appears on a page written by somebody who cans a lot and has never been shown the pH sentence.
Where did the schedule come from? A tested process names its source. If the page cannot tell you which document its times came from, you are being asked to trust the writer rather than the research, and you have no way to check it.
The sentence the entire decision rests on
Low-acid foods have pH values higher than 4.6. They include red meats, seafood, poultry, milk, and all fresh vegetables except for most tomatoes. Most mixtures of low-acid and acid foods also have pH values above 4.6 unless their recipes include enough lemon juice, citric acid, or vinegar to make them acid foods. Acid foods have a pH of 4.6 or lower. They include fruits, pickles, sauerkraut, jams, jellies, marmalades, and fruit butters.
USDA Complete Guide to Home Canning, 2015 revision, p. 1-8
The link above opens the document itself, so a printed page number leads somewhere. What this document is, and who maintains it.
Opened and read by this site on 2026-08-26.
Read it as three lists and one trapdoor.
List one, low-acid, above 4.6: red meats, seafood, poultry, milk, and every fresh vegetable except most tomatoes. Pressure canner.
List two, acid, at or below 4.6: fruits, pickles, sauerkraut, jams, jellies, marmalades, fruit butters. Boiling water.
The trapdoor is the sentence in the middle. A mixture of low-acid and acid food is above 4.6 unless the recipe includes enough lemon juice, citric acid or vinegar to bring it down. Note the word "enough". Adding some acid does not acidify a food; adding a tested quantity does, and only a tested recipe knows what that quantity is. This is why a soup with tomatoes in it is a pressure-canner food, and why a salsa is only a boiling-water food if you follow the salsa recipe exactly.
Note also what the passage does not do. It gives no times, no pressures and no exceptions for being careful. It sorts food. Everything else in canning follows from which pile a food lands in.
Why 4.6 and not some other number: the two temperatures
Botulinum spores are very hard to destroy at boiling-water temperatures; the higher the canner temperature, the more easily they are destroyed. Therefore, all low-acid foods should be sterilized at temperatures of 240° to 250°F, attainable with pressure canners operated at 10 to 15 PSIG. PSIG means pounds per square inch of pressure as measured by gauge.
USDA Complete Guide to Home Canning, 2015 revision, p. 1-8
The link above opens the document itself, so a printed page number leads somewhere. What this document is, and who maintains it.
Opened and read by this site on 2026-08-26.
Clostridium botulinum is a bacterium that survives in food as a spore. The spore is the problem: it is not killed by boiling water in any practical time, and inside a sealed jar of low-acid food it has exactly the conditions it needs to wake up.
Acid is what stands in its way. The guide lists the conditions the spores need before they grow into cells and produce toxin, and the first one on the list is a moist, low-acid food [F-1.4]. An acid food is not one. Take that condition away and the boiling water is left dealing with moulds, yeasts and ordinary bacteria, which it handles easily.
Above 4.6 there is no acid doing that work, so the heat has to do all of it. The guide gives the range that does: 240° to 250°F, reached with a pressure canner at 10 to 15 PSIG. A pot of boiling water does not reach it, cannot be made to reach it, and gets further from it the higher you live.
And the mechanism, stated plainly, because most pages get it backwards
Pressure does not destroy microorganisms, but high temperatures applied for an adequate period of time do kill microorganisms. The success of destroying all microorganisms capable of growing in canned food is based on the temperature obtained in pure steam, free of air, at sea level. At sea level, a canner operated at a gauge pressure of 10.5 lbs provides an internal temperature of 240°F.
USDA Complete Guide to Home Canning, 2015 revision, p. 1-19
The link above opens the document itself, so a printed page number leads somewhere. What this document is, and who maintains it.
Opened and read by this site on 2026-08-26.
The pressure is not the point. The pressure is how you get the temperature.
Trapped steam raises the boiling point of the water in the canner, and a higher boiling point is a higher temperature. The gauge reading is a proxy for that temperature — at sea level, a gauge pressure of 10.5 lbs corresponds to 240°F inside [F-1.8].
Two consequences follow, and both matter. Air mixed in with the steam breaks the relationship, because a canner full of air and steam is cooler than a canner full of pure steam at the same gauge reading. And altitude breaks it too, which is why the same food gets a different pressure in Denver than in Miami.
This is also why "it reached 11 psi" is not by itself an argument that food was processed. The number on a gauge only tells you the temperature if the vessel is behaving like the vessel the process was developed in.
What happens if you try to make up the difference with time
At temperatures of 240° to 250°F, the time needed to destroy bacteria in low-acid canned food ranges from 20 to 100 minutes. The exact time depends on the kind of food being canned, the way it is packed into jars, and the size of jars. The time needed to safely process low-acid foods in a boiling-water canner ranges from 7 to 11 hours; the time needed to process acid foods in boiling water varies from 5 to 85 minutes.
USDA Complete Guide to Home Canning, 2015 revision, p. 1-8
The link above opens the document itself, so a printed page number leads somewhere. What this document is, and who maintains it.
Opened and read by this site on 2026-08-26.
This is the only place the tested source itself puts a number on the boiling-water route for low-acid food, and the number is 7 to 11 hours [F-1.3].
Set it beside the pressure figure in the same sentence: 20 to 100 minutes at 240° to 250°F [F-1.3]. Same food. Same jars. The gap is not a safety margin somebody added for caution; it is what the arithmetic of heat destruction produces when you take away 30 degrees.
So when somebody says they water-bathed green beans for three hours and nothing happened, the guide's own answer is that three hours was never near the figure. It is not a close call that went their way. It is less than half of the low end.
Note what this passage does not say. It does not offer 7 to 11 hours as an alternative method. No tested recipe uses it. It is stated to show you the size of the difference, and the guide gives no boiling-water process for low-acid food at all.
What people ask next
My recipe has both — meat and tomatoes, or vegetables in a vinegar brine. Which is it?
Follow the tested recipe's own instruction, and do not reason your way to a different answer. The guide's rule is that mixtures of low-acid and acid food are above 4.6 unless the recipe contains enough acid to bring them down [F-1.1]. "Enough" is a laboratory finding about that specific combination of ingredients at those specific proportions. Vegetables in a tested pickle recipe are an acid food; the same vegetables in a soup are not.
Can I water bath low-acid food if I do it for much longer?
No, and the guide's own figures are the clearest way to see why. The boiling-water route for low-acid food runs to 7 to 11 hours, against 20 to 100 minutes under pressure [F-1.3]. Nobody publishes a tested 9-hour boiling-water process, quality would not survive it, and the jars would need topping up throughout. The distinction the guide draws is not about patience.
I have a pressure canner. Do I still need a boiling water canner?
Often not. Several tested procedures for acid foods give both a boiling-water and a pressure option, and the guide treats them as equal processes with different time-and-temperature combinations. What you cannot do is drop a preparation step because you chose the pressure option — for tomatoes, the acidification is still required even when you pressure can [F-5.2].
Where does a steam canner fit?
On the boiling-water side, because it reaches the same temperature and no higher. It is research-tested for acid foods under a specific set of conditions, and the 2015 printed guide has not caught up with that research. That story is worth reading in full before you buy one: steam canning, what changed and when.
Two pages I read didn't mention pH at all. Were they wrong?
Incomplete rather than wrong, which is harder to spot. Two of the pages that rank highest for this exact question never print the number that sorts the food. A full-text search of one finds no "4.6", no "botulinum", no "USDA" and no "NCHFP"; the other gives the two temperatures correctly and contains no "pH" anywhere on the page [F-5.5]. A reader can finish either one knowing that low-acid food needs pressure and still have no way to decide whether the thing in front of them is low-acid. That is the gap this page exists to close, and it is a statement about those pages, not about the people who wrote them.
Read these yourself
USDA Complete Guide to Home Canning, 2015 revision — NCHFP's landing page for the document
nchfp.uga.edu
NCHFP — Recommended canners
nchfp.uga.edu
NCHFP — Canning tomatoes: introduction
nchfp.uga.edu
Penn State Extension — Acidifying canned products for safety
extension.psu.edu