This site does not publish home canning recipes. Preserving instructions are deliberately out of scope here: they are a discipline with tested processes, and the right place to get one is the body that tested it.
This page exists for a different reason: a specific piece of reasoning is repeated constantly in the shelf-stable-food corner of the internet, it sounds authoritative, and it is wrong. It goes:
Tomatoes are acidic, so tomato sauce can be safely water-bath canned.
The first half is roughly true. The second half does not follow from it, and the gap between them is where botulism lives.
The only number that matters
Home canning divides all food at pH 4.6. In NCHFP’s definition, a food with a pH above 4.6 is low-acid, and one at 4.6 or below is an acid food (NCHFP, Ensuring Safe Canned Foods).
The line is there because Clostridium botulinum cannot grow below it. Acid foods have enough acidity either to stop it growing or to destroy it faster when heated. Above 4.6, there is nothing stopping it, and a sealed jar is an anaerobic environment, exactly what the organism wants. (The same mechanism, in a different container, is what makes garlic in oil dangerous.)
This is a threshold, not a gradient. A food at pH 4.5 and a food at pH 4.7 are not “similar” for canning purposes. They require different equipment.
Where tomatoes actually sit
Straddling it. Per NCHFP, tomatoes can naturally have a pH above 4.6, up to 4.8, which puts some of them, some of the time, in low-acid territory.
Not most. Not reliably. Some. And you cannot tell which by tasting, because perceived sourness tracks the sugar-to-acid balance rather than pH, and a jar that tastes sharp can still be above the line.
So “tomatoes are acidic” is a statement about the average and the safety question is about the outlier. Averages do not help you here: you are not canning the average tomato, you are canning the ones in front of you.
Faced with this, the USDA had two options, require a pressure canner for every tomato product, or require acid. It chose acid, so that with acid added, tomatoes can be handled as foods below pH 4.6 for home canning. Acidification is not a flavour step or an optional extra. It is the thing that makes the process valid.
What that means in practice
The NCHFP quantities:
| Acid | Per pint | Per quart |
|---|---|---|
| Bottled lemon juice | 1 tablespoon | 2 tablespoons |
| Citric acid | ¼ teaspoon | ½ teaspoon |
| Vinegar (5% acidity) | 2 tablespoons | 4 tablespoons |
Four points that get lost when this is summarised by other people:
It applies to pressure canning too. This is the one that surprises people. Acidification is required for both boiling-water and pressure processing of tomatoes, because the pressure processes for tomato products are calculated as equivalent heat treatments to the boiling-water ones, not as low-acid processes. Using a pressure canner does not buy you out of adding the acid.
Bottled lemon juice, not fresh. The directions specify bottled juice from concentrate. Bottled is standardised; a fresh lemon is whatever that lemon happened to be.
Do not leave it out to protect the flavour. NCHFP is direct: if the acid makes it taste too sharp, add sugar to balance it, but never leave the acid out. Sugar changes how it tastes without changing the pH.
Some tomatoes cannot be canned at all: NCHFP says not to can tomatoes from vines that are dead or have been killed by frost.
A trap for UK readers
Those are US measures. A US quart is about 946 ml and a US pint about 473 ml, while a UK pint is 568 ml. British preserving jars are sold in millilitres and in sizes that match neither.
Acid per jar is a ratio to volume, so a UK cook reading “1 tablespoon per pint” and applying it to a 568 ml jar is under-acidifying by around 20%. This is precisely the sort of quiet arithmetic error that produces a jar which is nearly safe, and “nearly” is not a category that exists here. Work from the volume your jars actually hold, and check it rather than assuming.
Why water-bath canning is the wrong tool for low-acid food
Because water boils at 100°C and that is not hot enough. Botulinum spores are heat-resistant; NCHFP gives the temperature needed to destroy them reliably as 240 to 250°F, which a pressure canner reaches at 10 to 15 PSI.
That is 116–121°C, and you cannot get there in an open pot at any altitude in Britain. NCHFP puts the alternative in the starkest possible terms, processing low-acid food in boiling water would take 7 to 11 hours, against 20 to 100 minutes in a pressure canner. That figure is worth carrying around. A recipe telling you to water-bath a low-acid food for 45 minutes is not cutting a corner by a small margin; it is out by an order of magnitude.
What to take from this
- pH 4.6 is a hard line, and the food’s reputation is not evidence about which side of it a particular batch falls on.
- Acidify every jar of tomatoes, whichever canner you use.
- Follow a tested process from a body that tested it: NCHFP, or the USDA Complete Guide to Home Canning. Not a blog, and not this page.
- Be suspicious of reasoning that starts from an ingredient’s character rather than from a measurement. “Tomatoes are acidic”, “vinegar preserves things”, “sugar preserves things” are all directionally true and none of them is a process.
For everyone who just wants good food from a cupboard rather than a preserving hobby: tinned tomatoes are already this, done properly, at scale, for less than it costs you to do it. They are a genuinely excellent ingredient and there is no compromise involved in using them.