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Tomato
The tomato is the exception that every canning guide has to spell out. It reads as an acid fruit, tastes acid, and is not reliably acid enough. Modern varieties bred for sweetness, over-ripe fruit and fruit from a dead vine all drift upward past pH 4.6, which is why every published home-canning process for tomatoes now requires acid to be added to the jar regardless of the recipe.
The season
Greenhouse tomatoes from July, outdoor and polytunnel fruit in August and September. The season ends abruptly with the first cold night, which is why every northern gardener has a box of green tomatoes in late September.
Acidity decides the method
Tomatoes sit right on the line, roughly pH 4.3–4.9 depending on variety and ripeness. This is the single most consequential fact on the page.
This is a reference guide, not a recipe authority and not medical advice. The one organism the safety rules here are written against is Clostridium botulinum, and the one way to be protected against it is a tested recipe followed exactly — substitutions of the acid, the sugar or the salt are not cosmetic. Where the national food authority publishes advice for a specific food (Baltic fish, wild mushrooms), that advice outranks this page.
Method by method
| Method | Verdict | The note |
|---|---|---|
| Freezing | Good | Freeze whole and raw in bags. The skins slip off under warm water as they thaw — no blanching needed. |
| Drying | Good | Halved, salted lightly, dried at 57–63 °C until leathery. Store airtight; keep in oil only in the fridge. |
| Fermenting | Good | Firm green or barely ripe tomatoes in a 3.5–5 % brine, exactly like cucumbers. |
| Pickling in vinegar | Good | Whole green or small red tomatoes in a tested vinegar brine — a Baltic and Russian classic for the end-of-season green fruit. |
| Jam and sugar preserves | OK | Green tomato jam is a real preserve, and it needs plenty of lemon for both set and safety. |
| Water-bath canning | Best | Only with added acid: 2 tablespoons of bottled lemon juice or half a teaspoon of citric acid per quart (about one litre) jar. |
| Pressure canning | OK | Necessary once the jar has enough low-acid company — meat, plenty of onion, peppers — that it is no longer a tomato product. |
| Curing and salting | Poor | Not a tradition worth reviving. |
| Smoking | OK | Smoked and then dried tomatoes are excellent, but it is a flavour step, not preservation. |
| Root cellaring | OK | Green tomatoes wrapped in paper at 10–13 °C ripen slowly over several weeks. |
The classic preparation: Passata and whole tomatoes in jars
Blanch the tomatoes for thirty seconds, drop them in cold water and slip the skins. For passata, cook down and pass through a sieve or mill; for whole tomatoes, pack them raw or hot into jars. Then, before the lid goes on, add the acid — two tablespoons of bottled lemon juice or half a teaspoon of citric acid per quart-sized jar, halved for a pint jar. Bottled lemon juice is specified in every published process because its acidity is standardised, and a fresh lemon's is not. Add salt to taste if you want it; salt is flavour here and does nothing for safety. Then process in a boiling-water canner for the time your tested recipe gives for that jar size.
Preparation that matters
Use fruit that is ripe but sound. Do not can tomatoes from a frost-killed or dead vine, and do not can bruised or mouldy fruit — both drift toward the wrong side of the pH line and neither improves in a jar. Green tomatoes are a different product with a different acidity and belong in a green-tomato recipe, pickled or fermented, rather than being mixed into a red batch. For drying, small plum varieties with dense flesh and few seeds work far better than a big salad tomato.
What the preserve is worth
Tomatoes are the one place in this whole reference where processing makes the nutrition better rather than worse. Heat breaks down the cell walls and converts lycopene into a form the body absorbs more readily, so cooked and canned tomatoes deliver more usable lycopene than raw ones. Vitamin C goes the other way and falls with heat and time. Frozen raw tomatoes keep more vitamin C than canned ones and are the better choice if the destination is a sauce anyway.
The ways this goes wrong
- Skipping the acid because the recipe is old. Pre-1990s recipes predate the current guidance.
- Adding a generous amount of onion, garlic or peppers to a tomato sauce and then water-bath canning it — those additions raise the pH and the jar now needs a pressure canner.
- Canning tomatoes from a dead or frost-hit vine.
- Storing home-dried tomatoes in oil at room temperature, which is the garlic-in-oil hazard again.
Safety rules, temperatures, blanching times, headspace and the pH 4.6 line: USDA Complete Guide to Home Canning and the National Center for Home Food Preservation guidance (US Government / land-grant publications, public domain). Acidity classes: the FDA's published approximate-pH tables. Jam composition: Directive 2001/113/EC. Fish freezing before raw or lightly cured use: Regulation (EC) No 853/2004, Annex III. Checked 2026-09-06.
Sources for this food
- USDA Complete Guide to Home Canning, tomatoes and tomato products (public domain)
- FDA approximate pH of foods and food products
Frequently asked questions
Why do canned tomatoes need lemon juice or citric acid?
Because tomatoes are borderline at pH 4.6 and many varieties fall on the unsafe side of it. The published figure is 2 tablespoons of bottled lemon juice or ½ teaspoon of citric acid per quart jar, halved for a pint. It does not make the tomatoes taste sour.
Can I freeze tomatoes whole?
Yes, and it is the easiest preserving there is: wash, dry, bag, freeze. The skins split and slip off as they thaw, and the flesh goes straight into a sauce.
Is it safe to can my own tomato sauce with onions and garlic?
Not in a boiling-water canner unless the recipe was tested with those quantities. Onion, garlic and peppers are low-acid and change the pH of the jar. Freeze such a sauce instead, or use a tested pressure-canning process.
Related pages
All foods compared
The full table, season by season, with the classic preparation for each.
Nutrition figures
The published USDA row for this food, per 100 grams.
When to sow and harvest it
The sowing calendar entry for this crop in this climate.
How long it keeps fresh
Fridge, freezer and pantry days for this food before it is preserved.
From the garden to the log
TempoLife counts the berries you picked and the jam you ate from them, so the season's work shows up where it belongs.