When the harvest comes in faster than you can eat it in high summer, you face the old gardener's question: what do you do with all the zucchini, tomatoes, beans and cherries? Canning is the answer generations before us gave. A jar of hot harvest, a rubber ring, a little patience, and you have summer on the shelf until the next one comes.
This article walks you through the basics. It is deliberately honest about one point where many old guides stay silent: not every vegetable is equally safe in a jar. Acidity decides, and with low-acid crops like beans and squash a second, careful look pays off.
Auf einen Blick
- Acidity decides safety. Acidic fruit and acidified vegetables (pH below 4.6) are safe in a water bath; low-acid vegetables are not.
- Canning and preserving mean the same thing: heating food in a jar until a vacuum seals it and makes it keep.
- Beans, peas, corn, squash, mushrooms and meat are the problem children. They only become safe through acidifying (pickling), freezing or a pressure canner.
- Best time: CW 28 to 40, when berries, tomatoes, zucchini and stone fruit ripen all at once.
- Checking is mandatory: domed lids, cloudy brine or a hissing jar belong in the bin, unopened.
Canning, preserving, putting up: what is what?
The terms cause more confusion than they need to. At heart, canning and preserving mean the same thing. You fill food into a jar, heat it and let it cool slowly. As it cools the contents contract, a partial vacuum forms inside the jar, and that pull presses the lid tight against the rim. The vacuum keeps air and new germs out.
The German name Einwecken comes simply from the Weck company, whose classic jars with glass lids, rubber rings and clamps shaped the method for over a hundred years. Putting up is often used more broadly and includes jam, chutney and pickled foods. For this article: we are talking about preserving through heat in a sealed jar.
Two things happen with the heat at the same time. First, the microorganisms that would spoil the food are killed. Second, the vacuum forms that prevents new germs from getting in. For the first to work reliably, you need to understand why some foods are easier to tame than others.
Acidity decides everything
This is the most important sentence in this article: how hot and how long you have to heat depends on the acidity of the food. The line runs at a pH value of 4.6.
Anything more acidic than pH 4.6 counts as high-acid. This includes most fruit, berries, rhubarb, foods pickled in vinegar and sauerkraut. In this acidic environment the most dangerous germs cannot multiply. Heating in a boiling water bath at around 90 to 100 degrees is entirely enough to make these foods safely shelf-stable.
Anything less acidic than pH 4.6 counts as low-acid. This includes almost all vegetables in their natural form: beans, peas, corn, carrots, squash, zucchini, mushrooms, plus meat and fish. In this mild environment one particular germ finds ideal conditions, and it is the reason for all the caution that follows.
Botulism: the invisible reason for all caution. The spores of Clostridium botulinum survive the boiling water bath at 100 degrees. In an airtight, moist, low-acid jar they germinate and produce a nerve toxin that you can neither see nor smell nor taste. Even the smallest amounts can be life-threatening. This is exactly why you must never simply water-bath can low-acid vegetables like beans, peas, corn or squash and then eat them cold from the shelf. The next section describes the safe ways.
Beans and squash: handling the problem children right
Low-acid vegetables are not forbidden, they just need the right method. You have three safe options, and all three work in everyday life.
- Acidify and pickle
The simplest way. Add enough vinegar or citric acid that the beans or squash end up in an acidic brine below pH 4.6. Classics are pickled cucumbers, beetroot in a vinegar stock or sweet-and-sour squash. From now on the vegetable counts as high-acid and is safe in a water bath.
- Freeze instead of canning
Blanch beans and peas for three minutes, cool them in cold water and freeze them. That is safer, faster and gentler on vitamins than any canning. For low-acid vegetables without a vinegar taste, the freezer is almost always the better choice.
- Use a pressure canner
Only under pressure do you reach the 121 degrees at which the spores reliably die too. A pressure cooker or a dedicated pressure canner is the only way to preserve low-acid vegetables without added acid. In North America this is standard; here the equipment is rarer, but available.
If you do have low-acid canned goods in your stores, heat them to a rolling boil for at least ten minutes before eating. The possible toxin is heat-sensitive and is destroyed by boiling, even though the spores are not. But this does not replace a safe base method, it is only the last line of defence.
Tomatoes: the honest borderline case
Tomatoes sit right on the line. Depending on variety and ripeness their pH hovers around 4.3 to 4.9, dangerously close to the 4.6 mark. Old, fleshy varieties like Tomato 'Ochsenherz' are often milder and therefore less acidic than small, pointed varieties.
That is why one simple rule applies when canning tomatoes and passata: always acidify afterwards. One tablespoon of lemon juice or a pinch of citric acid per litre pushes the pH safely below the line and costs you almost no flavour. That turns the borderline case into a safe case.
What you really need
The equipment is manageable and lasts for decades.
- Jars with an intact seal
Weck jars with a glass lid and rubber ring, or screw-top jars with twist-off lids. The ring or sealing disc must be new or undamaged; a tired rubber ring no longer seals.
- A large pot or canning boiler
Big enough that the jars stand up to three quarters in the water and do not touch each other. A cloth or rack on the bottom stops the jars from cracking.
- Thermometer and a sense of timing
A simple kitchen thermometer helps you hold the temperature. More important than a rolling boil is a calm, even heat.
- A clean kitchen
Rinse jars and lids in hot water, wipe the rims clean before sealing. A film of grease on the jar is the most common cause of leaky lids.
How to can high-acid fruit and vegetables
The method below applies to fruit, berries, rhubarb, acidified tomatoes and pickled foods. For low-acid vegetables the rule still stands: acidify or freeze first.
Prepare the jars. Rinse jars, lids and rings in hot water. Run a finger around each rim to check for chips; a tiny nick prevents the vacuum.
Fill with headspace. Fill in the prepared harvest and leave about two centimetres of air to the rim. That is the room the expansion needs. Jars filled to the brim push the lid back open.
Clean the rim and seal. Wipe the rim with a clean, damp cloth. Fit ring and lid, clamp Weck jars firmly or screw twist-off lids only hand-tight.
Heat in a water bath. Set the jars into the pot on a cloth, fill with lukewarm water up to three quarters of the jar height. Heat slowly to 90 degrees for fruit and hold that temperature for 25 to 30 minutes. Berries and soft fruit need 25 minutes, firm stone fruit closer to 30.
Let them cool slowly. Take the jars out and stand them on a wooden board or cloth, never on cold stone. Let them cool undisturbed. As they cool you often hear a quiet pop: that is the vacuum drawing the lid down.
Check the next day. The lid must curve firmly inward and must not press in. With Weck jars, take off the clamps and lift the jar carefully by the lid: if it holds, the vacuum is set.
Jam: the friendliest way in
If canning feels like too much technique, start with jam. Here sugar and fruit acid work for you. The high sugar content binds water, the fruit acid keeps the pH low, and hot filling does the rest. Strawberries in June, currants and cherries in July, plums and apples from August: every wave of fruit can be rescued into a jar with almost no risk.
Bring the fruit to a boil with preserving sugar per the packet instructions, do the set test on a cold plate and fill the boiling-hot mass close to the rim into clean jars. Seal at once and, if you like, stand the jars upside down for five minutes. The hot filling sterilises lid and rim by itself; with jam you do not need an extra water bath.
What is sour and sweet enough almost keeps itself. Everything else needs your head.
Old gardener's rule
Shelf life, storage and checking
Properly canned, high-acid fruit stored cool and dark keeps for a year and more, often until the next harvest. Label every jar with contents and date; a strip of tape and a marker will do. Store cool between 10 and 18 degrees, dark and dry. Light bleaches the colour out, warmth speeds up spoilage.
A cool, dark cellar is the best pantry. If you do not have one, a cupboard on the coldest, darkest wall of your home will do too. How to carry the rest of your harvest through the winter is covered in Storing the Harvest: Cellar, Sand Box and Clamp.
Check your jars before every opening. These signs mean: get rid of it, unopened, into the bin.
- Domed lid
If the lid bulges outward or springs back when you press it, gases are at work inside the jar. The vacuum is lost, the contents spoiled.
- Hissing or fizzing
If it hisses unusually strongly when opened or bubbles rise, that is a warning sign. With high-acid fruit a slight hiss can be normal; when in doubt, throw it out.
- Cloudy brine or mould
Liquid that has turned cloudy, streaks or nests of mould on the surface do not belong in the jar. Skimming does not help either, the jar is done.
- Foul smell
If it smells sharp, sour-rotten or yeasty when opened, trust your nose and eat none of it.
Important: botulism precisely does not give itself away through smell or appearance. That is why the safe base method for low-acid vegetables matters so much; a sensory check only catches the obvious spoilage, not the invisible toxin.
Where to start this summer
Start small. One batch of strawberry jam in June, a few jars of pickled cucumbers in July, acidified passata from the tomato glut in August. If beans and squash are dear to you, pickle them or freeze them rather than risk them. That way your stores grow with your experience, and every jar on the shelf is a safe one.
If you would rather use the oven dry, Drying and Dehydrating Fruit and Vegetables covers the second great way to carry the harvest through the winter without refrigeration.
Häufige Fragen
Which vegetables can I not simply water-bath can?
All low-acid vegetables with a pH above 4.6: beans, peas, corn, carrots, squash, zucchini, mushrooms, plus meat and fish. In a boiling water bath at 100 degrees the spores of Clostridium botulinum survive and can, in an airtight, low-acid environment, form a dangerous nerve toxin. Such vegetables only become safe through acidifying (pickling with vinegar or citric acid below pH 4.6), through freezing, or through a pressure canner that reaches 121 degrees.
What is the difference between canning and preserving?
There is no real difference. Both terms mean the same procedure: fill food into a jar, heat it, and let a vacuum form as it cools that seals the lid tight. The German name Einwecken comes from the Weck company, whose jars with glass lids, rubber rings and clamps shaped the method. Putting up is often used more broadly and includes jam and pickled foods.
Do I have to acidify tomatoes when canning?
Yes, it is advisable. With a pH of around 4.3 to 4.9, tomatoes sit right on the critical line of 4.6, and fleshy, mild varieties are often less acidic. Add one tablespoon of lemon juice or a good pinch of citric acid per litre. That pushes the pH safely below the line, costs almost no flavour and turns the borderline case into a safe one.
How do I tell that a canned jar has spoiled?
A domed or springy lid, cloudy or streaky liquid, mould on the surface, strong hissing when opened, or a foul, yeasty smell are clear warning signs. Such a jar belongs in the bin, unopened; skimming does not help. Important to know: botulism does not give itself away through smell or appearance. That is why a sensory check never replaces the safe base method for low-acid vegetables.
When is the best time to can?
The main season runs from CW 28 to CW 40, that is from mid-July to early October. In these weeks berries, stone fruit, tomatoes, zucchini and squash often ripen at the same time and faster than you can use them fresh. Start with jam and pickled foods when the first glut comes, and work your way from week to week through whatever is ripe right now.
Quellen
Rechtlicher Hinweis
This article summarises general basics of food preservation and does not replace food-law or medical advice. Canning low-acid foods carries a real health risk from botulism. When in doubt, throw a jar out rather than risk it, and follow the recommendations of the German Federal Institute for Risk Assessment.
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