The short version

Three methods, three different preservation logics, and only one that becomes genuinely dangerous if you improvise it.

• Purée. Forty-eight hours in the fridge. Months in the freezer. In jars kept at room temperature, only with measured acidification and a timed water bath.
• Drying. Take it to brittle, not to chewy. Dried tomatoes belong in a dry, dark cupboard, not in a jar of oil sitting on the kitchen shelf.
• Lacto-fermentation. A brine of at least 25 g of salt per litre, additive-free salt, 21 to 24 °C, and a month of patience. It gives the most interesting flavour of the three, and it is the one where the salt level is not up for negotiation.

One number governs everything else: Clostridium botulinum does not grow below pH 4.6, and tomatoes sit right on that boundary. That is why canned tomatoes behave like a low-acid vegetable rather than like a fruit.

What a tomato actually gives you

Methods later, numbers first. The figures below come from the USDA’s FoodData Central database, per 100 g of product.

Per 100 g	Raw tomato	Canned purée, no salt	Sun-dried tomato	Sun-dried in oil, drained
Energy	18 kcal	38 kcal	258 kcal	213 kcal
Fibre	1.2 g	1.9 g	12.3 g	5.8 g
Potassium	237 mg	439 mg	3430 mg	1560 mg
Sodium	5 mg	28 mg	107 mg	266 mg
Lycopene	2570 µg	21,800 µg	45,900 µg	not published
Vitamin C	13.7 mg	10.6 mg	39.2 mg	102 mg

Two things follow from that table. First, everything concentrates together: a dried tomato is not more “nutritious”, it simply holds four to five times less water, so every figure multiplies, calories included. Second, the salted canned version climbs to 202 mg of sodium per 100 g against 28 mg unsalted. Over a plate of pasta made with 200 g of sauce, that gap is close to 0.9 g of salt, against the WHO recommendation of under 5 g of salt a day.

Lycopene, neither miracle nor marketing line

Lycopene is the pigment behind the red of a ripe tomato. Two facts are well established. It concentrates during processing, as the table shows. And its absorption improves with heat and with fat present, because cooking breaks down cell walls and lycopene is fat-soluble. A 2025 review in Frontiers in Plant Science on tomato phytochemicals sets this out. So a purée simmered with a drizzle of olive oil is, on this narrow point, a better source than a raw tomato.

What that means for health needs more care. A 2026 meta-analysis in Nutrients, covering thirteen studies and 9131 participants, found that high circulating lycopene was associated with lower carotid intima-media thickness, with a pooled odds ratio of 0.70 (95% CI 0.59 to 0.84) and substantial heterogeneity between studies. The full abstract is here. These are cross-sectional observational data: they show an association, not causation, and people who eat a lot of tomatoes tend to have other favourable habits too. Making your own purée is a good idea for flavour, for waste and for salt control. It is not a cardiovascular treatment.

The rule that governs everything: pH 4.6

This is not a boilerplate warning. It changes three of the recipes on this page.

Clostridium botulinum is a bacterium whose spores are everywhere in soil and on vegetables. They survive boiling, sometimes for hours. In the absence of oxygen, at room temperature and in a low-acid environment, they germinate and produce one of the most powerful toxins known. The WHO is explicit on two points: the bacterium will not grow below pH 4.6, so the toxin does not form in acidic foods; and home-canned, home-preserved and home-fermented foods are a common source of foodborne botulism.

The tomato is the borderline case. Depending on variety, ripeness and season, its pH hovers around 4.5 and can drift above it. A very ripe tomato is less acidic than a green one. That is why the National Center for Home Food Preservation at the University of Georgia, which maintains the recommendations derived from the USDA canning guide, requires acidification for any canned tomato product stored outside the fridge.

Two consequences that get missed:

• Dunking a jar in boiling water does not sterilise it in the sense people assume. Boiling kills vegetative bacteria, not spores. A clean jar is necessary and never sufficient.
• Toxin that has already formed is destroyed by heating above 85 °C for at least five minutes. That is a reason to boil a doubtful homemade sauce before tasting it, never a reason to rescue a badly made preserve. If a jar looks wrong, bin it without tasting.

Making purée, and keeping it properly

Cooking it down

Use fleshy, low-water tomatoes. Roma, San Marzano and Cornue des Andes are the classics, but any ripe garden tomato works: reduction time matters more than variety. Discard damaged or mouldy fruit, and do not can tomatoes picked from dead or frost-killed vines, a point the American centre makes explicitly.

Skinning is optional and purely about texture: score a cross, thirty seconds in boiling water, straight into cold water, and the skin slips off. If you plan to put the sauce through a food mill, skip this step, since the mill catches skins and seeds anyway.

Chop, then cook in a wide pan with olive oil and whatever aromatics you like. Width matters more than heat: evaporation depends on surface area. Allow 30 minutes for a loose sauce, an hour or more for a dense one. The useful benchmark is volumetric: reduce by about a third for thin sauce, by half for thick. Blend or mill, then season only at the end, once the concentration has settled. A pinch of sugar corrects unpleasant acidity, but it never replaces the safety acidification described below.

Three storage routes, in order of risk

In the fridge. Cool the purée at room temperature, get it cold within an hour or two, in a closed container. The UK’s Food Standards Agency puts the fridge between 0 and 5 °C and recommends eating homemade leftovers within 48 hours. The “3 to 5 days” you read everywhere is a habit, not an official recommendation.

In the freezer. For a household this is by far the simplest and safest route. At minus 18 °C the FSA describes the freezer as a pause button. Freeze in flat portions in bags, or as cubes you then tip into a sack, so you can take out what you need without thawing the rest. Purée handles freezing well precisely because it has already been blended.

In jars at room temperature. This is where precision matters, because the common version of this recipe, pouring hot purée into a jar and boiling it “for a while”, does not protect against botulism.

The American centre’s protocol, adapted from the USDA guide, comes down to four points:

1. Acidify in the jar, before filling. Per litre of purée: 2 tablespoons of bottled lemon juice, or 1/2 teaspoon of citric acid. For a 500 ml jar: 1 tablespoon, or 1/4 teaspoon. Use bottled lemon juice rather than a fresh lemon, because the acidity of fresh fruit varies and is not guaranteed. Vinegar at 5% acidity is an accepted alternative at 4 tablespoons per litre, but it marks the flavour noticeably.
2. Do not leave the acid out because the sauce tastes sharp. Add sugar to balance it if you want. The acid is a safety parameter, not a seasoning.
3. Leave about 6 mm of headspace, seal, then process in a boiling water bath with the jars submerged.
4. Follow the timing, altitude included. For standard tomato sauce in a boiling water canner: 35 minutes for 500 ml and 40 minutes for a litre below 300 m. Between 300 and 900 m, which covers most of the Swiss Plateau from Geneva to Zurich, add five minutes: 40 and 45. Between 900 and 1830 m, add ten. Water boils cooler at altitude, so the extra time compensates.

If that level of rigour feels out of proportion to the amount of tomatoes you have, freeze instead. Nobody has ever caught botulism from a bag of purée in the freezer.

Drying tomatoes

Oven or dehydrator

The principle is the same either way: remove enough water that neither moulds nor bacteria can get going.

In the oven, halve medium tomatoes lengthways, lay them skin-side down on a lined tray, oil them lightly, salt sparingly, add thyme or oregano. Bake at 90 to 100 °C with the door propped open on a wooden spoon so steam escapes. Allow 4 to 6 hours depending on size and water content, longer for very juicy fruit.

In a dehydrator, set 55 to 60 °C, arrange the halves so they do not touch, and allow 6 to 10 hours. A dehydrator uses less energy and gives a more even result, but it ties up the machine for the day. Dehydrating plant-based meals follows exactly the same principles, including the end-of-drying tests.

The dryness test that decides everything

This is the step most recipes explain badly. A vegetable is properly dried when it is brittle, not when it is pliable. The Georgia centre puts the target at roughly 10% residual moisture for vegetables, a stage at which some of them snap cleanly under a finger. A tomato still soft in the middle is not dried, it is confit, and it will go mouldy in the jar.

Let everything cool completely before packing: a warm tomato sweats, and that moisture alone is enough to start mould. Store in a clean, dry, airtight glass jar in a cool dark cupboard. Glass has a practical advantage, since condensation shows immediately if the drying was incomplete. Check the jars for the first few days, and if you see condensation, dry the whole batch again.

On shelf life, the centre’s guidance gives a range of 4 months to 1 year for dried foods, with a clear rule: the warmer the storage, the shorter the life, and vegetables keep roughly half as long as fruit. In practice, in a Swiss kitchen at 20 °C, count on a few months rather than a year.

Dried tomatoes in oil, a necessary correction

That picture of a jar of tomatoes confit in olive oil with garlic and herbs, sitting on the shelf, is bad advice. Garlic or herbs under oil combine three conditions that favour botulism: a low-acid ingredient, no oxygen under the oil layer, and room temperature. The US Centers for Disease Control and Prevention list chopped garlic in oil among documented sources of botulism, and recommend refrigerating homemade garlic or herb oils and throwing them out after four days.

The sensible household approach:

• Dried tomatoes kept dry, in a sealed jar in the cupboard: that is your long storage.
• Dried tomatoes in oil: a small jar, in the fridge, a few days, and lift them out with a clean spoon.
• Raw garlic in oil: fridge, and no more than four days.

Our guide to homemade flavoured oils works through those limits for other aromatics.

Lacto-fermenting tomatoes

What actually happens in the jar

Lactic acid bacteria living naturally on the vegetables consume the sugars and produce lactic acid. The pH falls, and once it drops below 4.6 the environment turns hostile to C. botulinum and to most spoilage organisms. Salt does the rest: it holds back unwanted flora during the first few days, before the acidity is established, and it keeps the texture firm.

So the risk window is at the start, not at the end. That is the opposite of most people’s intuition, and it explains why the salt level and the temperature of the first days matter more than anything else.

Salt, without hand-waving

Three corrections against the recipes in circulation.

The amount. For the top-up brine in a vegetable ferment, the Georgia centre specifies about one and a half tablespoons of salt per quart of water, close to 27 g per litre. A brine of 25 to 30 g per litre is a sound benchmark for whole tomatoes. The common instruction that you can drop to 10 g per litre should be ignored: it puts the ferment in a zone where unwanted flora can take hold before acidification does.

The type of salt. The centre is explicit here too: use canning or pickling salt, with no additives. Iodised salt darkens the product. Flake salts vary in density, so measuring by volume goes wrong. Anti-caking agents cloud the brine. Grey sea salt and Himalayan pink salt, so often recommended on principle, have no demonstrated advantage here: they are sodium chloride with impurities. Fine non-iodised cooking salt without anti-caking agent does the job perfectly.

Do not cut it back. The centre’s general rule is blunt: do not attempt to make fermented foods by cutting back on the salt a recipe calls for. It is a safety and texture parameter, not a taste dial.

Step by step

1. The container. A clean glass jar, washed in hot soapy water and rinsed well. No need to boil it, and boiling would not sterilise it anyway.
2. The brine. 25 to 30 g of non-iodised salt per litre of water, stirred until fully dissolved. Heavily chlorinated tap water can slow the start: leave it uncovered overnight or use filtered water.
3. The tomatoes. Wash them and set aside anything split or blemished. Small whole cherry tomatoes give the best result, because intact skin limits collapse. Prick each one once with a needle so the brine can get in. Large tomatoes, cut up, turn into acidic mush fairly quickly and disappoint.
4. Filling. Add garlic, dill, a bay leaf, peppercorns or chilli. Fill leaving 3 to 4 cm of headspace, cover completely with brine, and hold everything under the liquid with a glass weight or a small saucer. Anything above the surface will go mouldy, without exception.
5. Fermenting. At 21 to 24 °C, the range recommended for vegetable lactic fermentation. Below 16 °C fermentation may not start at all; above 24 °C the product softens. Recipes that say 18 to 22 °C aim too low. Keep it out of direct light. If the jar is airtight with no airlock, open it briefly each day during the first week: gas production is real and can pop a seal.
6. Timing. Cherry tomatoes are pleasant after one to two weeks, properly sour and stable after three to four. Taste with a clean spoon. When it suits you, move the jar to the fridge, where activity slows sharply and it keeps for months.

Quick diagnosis

• A flat white film on the surface. Usually kahm yeast. Harmless, but bad for flavour. Skim it off and check that everything is submerged.
• Fuzzy, coloured, raised mould. Not the same thing at all. On a brine ferment of fragile fruit like tomatoes, the prudent move is to discard the jar rather than scrape the surface.
• Cloudy brine and bubbles. Normal in the first days, and a sign the ferment has started.
• Putrid smell, slimy brine, collapsed tomatoes. Throw it out.
• Nothing at all after five days. Too cold, or the water is too chlorinated. Move the jar somewhere warmer.

For more on the method, see our article on homemade lacto-fermentation and the one on sauerkraut, kimchi and water kefir, which works through the same parameters on other vegetables.

Probiotics: what can honestly be claimed

An unpasteurised lacto-fermented tomato contains live lactic acid bacteria. That is not enough to make it a probiotic product in any regulatory or clinical sense. The NHS points out that evidence is thin for most claims, that these products are not regulated like medicines, and that there is no guarantee about which strains are present or whether they survive as far as the gut. For a homemade jar, with no analysis, the uncertainty is larger still.

There is also the salt to account for. A tomato that has sat in a 25 g per litre brine absorbs some of it. On a single garnish portion that stays marginal; on a plate stacking pickles, salty cheese and olives it adds up quickly against the 5 g per day limit. If you already cook without added salt, treat ferments as a condiment rather than as a side vegetable.

Choosing the right method

	Purée	Drying	Lacto-fermentation
Hands-on time	1 to 2 h	20 min plus 4 to 10 h of machine time	20 min plus 2 to 4 weeks of waiting
Storage	48 h chilled, months frozen	a few months dry	several months refrigerated
Critical point	acidification if stored at room temperature	going all the way to brittle	salt level and temperature
Large batches	yes	moderately, the machine limits you	yes
Result	round, concentrated	intense, sweet-savoury	sour, sharp, genuinely different

If you have ten kilos of tomatoes and two hours, frozen purée solves the problem. If you have two kilos and want an ingredient that changes your winter cooking, fermentation earns its keep. Sensible anti-waste practice usually means combining all three by fruit condition: the ripest to purée, the firmest to ferment, the middling ones to dry.

Using them in the kitchen

• Pasta with purée. Soften garlic in olive oil, add the purée, olives and capers, reduce for five minutes, finish with basil. A spoonful of nutritional yeast adds a savoury note without adding salt.
• Dried tomato spread. Blend 60 g of dried tomatoes, rehydrated for twenty minutes in hot water, with 200 g of cooked chickpeas, a garlic clove, lemon juice and two tablespoons of tahini. Keeps two days in the fridge.
• Fermented tomato salad. Rocket, halved lacto-fermented tomatoes, sunflower seeds, and a little of their brine in the dressing instead of vinegar. Do not add salt.
• Thin tart. Shortcrust, a layer of thick purée, dried tomatoes, herbs, plant cream, 200 °C until the pastry is golden.
• Quinoa bowl. Warm quinoa, roasted chickpeas, avocado, fermented tomatoes, tahini-lemon dressing.
• Pizza. Use thick purée rather than loose, or the base goes soggy. Add a few dried tomato petals after baking for intensity.

These bases combine well with the other summer sauces made in batches.

Ten mistakes that keep coming back

1. Jarring purée with no added acid and leaving it in a cupboard.
2. Using fresh lemon juice instead of bottled juice to acidify a preserve.
3. Leaving the acid out because the sauce tastes sharp, instead of adding sugar.
4. Assuming a jar dunked in boiling water is sterile.
5. Ignoring altitude in boiling water processing times.
6. Stopping the drying at the chewy stage.
7. Packing dried tomatoes while they are still warm.
8. Storing dried tomatoes or garlic under oil at room temperature.
9. Dropping fermentation salt below 20 g per litre.
10. Fermenting at 18 °C and wondering why nothing moves, then parking the jar next to a radiator in summer and getting mush.

Adapting to your situation

• High blood pressure or a low-salt diet. Favour frozen purée with no added salt, which leaves you in full control. Treat ferments as a condiment.
• Pregnancy, weakened immunity, older adults. Unpasteurised ferments and home preserves demand a rigour you would rather not have to assume. Freezing is the route with no trade-offs.
• A small kitchen with no freezer. Drying takes the least space. Otherwise, make purée in quantities matching two days of eating.
• A productive garden. Spread the load: freeze as the harvest comes in, and group the acidified jarring into a single day when you will follow the timings without improvising. If you are starting from your own plants, our article on spring sowing will help you pick varieties suited to preserving.

Common questions

Can you lacto-ferment green tomatoes? Yes, and they often hold their texture better than ripe ones, since they start firmer and more acidic. The brine protocol is identical.

Can homemade purée be frozen in glass jars? Only in jars designed for freezing, with straight sides, filled three quarters full. Purée expands as it freezes. A flat bag is safer and takes less space.

Do you need to remove the seeds for purée? Only for texture. A food mill does that work for you. Nutritionally there is no reason to take them out.

How long do lacto-fermented tomatoes keep in the fridge? Several months, as long as they stay submerged in their brine and you never put a dirty spoon in the jar. Texture softens over time and the flavour keeps developing.

Can you sun-dry tomatoes the Mediterranean way? Technically yes, but sun drying needs several consecutive days of strong heat and dry air, plus protection from insects. The Swiss climate rarely obliges, and drying too slowly gives mould time to establish.

What should you do with a jar whose lid has bulged? Throw it away unopened and untasted. A domed lid, an off smell, liquid spurting on opening: these are warning signals, and tasting to check is never an option.

Is added sugar in purée a problem? A teaspoon per litre to correct acidity is negligible on the plate. It is not comparable to commercial sauces, which often carry tens of grams per jar. Read the labels rather than refusing sugar on principle.

Sources

• National Center for Home Food Preservation, University of Georgia: canning tomatoes introduction, tomato acidification directions, standard tomato sauce, packaging and storing dried foods, general information on fermenting and the sauerkraut recipe.
• World Health Organization: botulism, sodium reduction, healthy diet.
• Centers for Disease Control and Prevention: botulism prevention.
• NHS: probiotics.
• Food Standards Agency, GOV.UK: how to chill, freeze and defrost food safely.
• USDA FoodData Central: nutrient values.
• Meta-analysis of lycopene and carotid intima-media thickness, Nutrients, 2026: abstract.
• Review of tomato phytochemicals, Frontiers in Plant Science, 2025: full text.
• Swiss Federal Food Safety and Veterinary Office: dietary recommendations for adults.