Home canning acidity — why "tomatoes are acidic" is no longer enough

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. From NCHFP:

Low-acid foods have pH values higher than 4.6… Acid foods have a pH of 4.6 or lower.

— NCHFP, Ensuring Safe Canned Foods

The line is there because Clostridium botulinum cannot grow below it. Acid foods “contain enough acidity to block their growth, or destroy them more rapidly 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 tomatoes can be “treated as foods with pH below 4.6 for home canning purposes.” 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:

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: you “can add sugar to balance the acidic taste, but do not leave acid out.” Sugar changes how it tastes without changing the pH.

Some tomatoes cannot be canned at all: “Do not can tomatoes from dead or frost-killed vines.”

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; destroying them reliably needs:

temperatures of 240° to 250°F, attainable with pressure canners operated 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:

from 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.