Drying and Dehydrating: How to Preserve Fruit, Vegetables and Herbs

Three bowls of damsons on the worktop, two kilos of tomatoes in the fridge, and just as many still waiting out on the bed. That is the moment when gardeners start ambushing their neighbours. Yet the harvest needs only one thing: get the water out and it keeps.

Drying is the oldest form of preserving and the least demanding. No sugar, no vinegar, no preserving pan. Just warm air, until mould and bacteria have nothing left to work with. Fruit ends up at about a fifth of its fresh weight, vegetables at only a tenth, and the whole store fits into jars instead of a chest freezer.

The difference between apple rings that keep for a year and a batch that turns furry after six weeks comes down to two things: the temperature while you dry, and the residual moisture afterwards. In order, then: which produce wants which heat, how to work with an oven or a dehydrator, and why the screw top jar is the real quality control.

What actually happens when you dry produce

Fresh fruit is 80 to 90 per cent water, vegetables often more than 90. What decides whether it keeps, though, is not how much water is left but how much of it is freely available. Bacteria give up first, yeasts follow, moulds hold out the longest. Because the sugar in fruit binds water, dried fruit is already stable at 20 per cent residual moisture, while sugarless vegetables have to come down to around 10 per cent.

For that to happen at all, you need air exchange more than you need heat: a closed oven at 70 degrees Celsius is a steam chamber, an open one at 50 degrees actually dries.

Drying too hot costs you twice. The aroma leaves with the steam, and the surface hardens: the piece looks finished on the outside while water sits inside. Batches like that go mouldy in the jar.

Temperatures by produce

The range from 35 to 70 degrees divides up neatly: the finer the aroma, the cooler it stays.

Pre-treatment: lemon water and blanching

Cut apples turn brown before the dehydrator is even warm. That is not spoilage, it is enzyme activity, but it does cost vitamin C. A litre of cold water with the juice of one lemon is enough: soak the slices for three to five minutes, drain them, lay them out. A teaspoon of ascorbic acid per litre does the same job. Worth doing for apple, pear, quince, aubergine and pale stone fruit such as apricot, peach and nectarine, whose cut faces otherwise go leather brown. With dark stone fruit like damson or morello cherry you can skip it, since the browning does not show. One note on aubergine while we are here: dried, it is a cooking ingredient rather than a snack, so soak it and cook it before eating.

Blanching is almost exclusively a vegetable job: two to four minutes in boiling water, straight into iced water, then drained well. That stops the enzymes and shortens the drying time. Worth it for carrots, peas and kohlrabi, pointless for tomatoes and onions. Beetroot needs more than a quick blanch: boil 3 mm slices for five to seven minutes, or cook the whole roots until soft, then peel and slice. Otherwise the slices stay hard and taste raw and earthy. Herbs are best not washed at all. And only sound produce goes on the tray: bruises and rot do not dry away, they concentrate.

1. Cut it evenly

   Fruit 4 to 5 mm, vegetables 3 mm, herbs left on the whole sprig. A mandoline holds the thickness steadier than your hand does.

2. Lay it out without contact

   The pieces must not touch. Wherever two slices lie against each other, a damp spot stays behind.

3. Turn it halfway through

   Flip the thicker pieces and swap the trays around. Every machine has one shelf that runs warmer.

The oven with the door ajar

For a handful of apple rings the oven will do: fan setting, 50 to 70 degrees, trays lined with baking paper. The gap is what matters: a wooden spoon handle wedged across the door holds it one to two centimetres open, so the damp air can escape. Without that gap you are not drying, you are steaming. Swap the trays every two hours.

That makes the oven a tool for short batches, not for the 12 to 20 hours a tray of stone fruit takes. Do not leave it running unattended or overnight, because the open door is hot and within reach of children and pets. And for herbs it usually will not do at all: very few domestic ovens go below 50 degrees.

Bend test and snap test

No clock can tell you when something is done, but your hands can. Take a piece off the tray, let it cool for two minutes, then test it. Warm produce always feels softer than it really is.

Bend test for fruit. An apple ring is done when it bends like firm leather and gives up no juice when you squeeze it. If it snaps, it stayed in too long.

Snap test for vegetables. Carrot slices, celeriac and leek have to break cleanly. Any springiness left means it is not there yet.

Rustle test for herbs. The leaves crumble between your fingers and the stem cracks instead of bending.

With stone fruit the skin deceives you, because it is ready long before the flesh is. Cut a damson half across: if it glistens damp inside, the batch goes back on.

Residual moisture, the sweat test and storage

No batch of dried fruit carries the same moisture throughout: edges drier than the middle, the bottom tray drier than the top. That evens itself out if you give the store time. Conditioning is the name of the step, and almost everybody skips it.

1. Let it cool

   At least an hour, open, in a bowl. Anything jarred while still warm will sweat.

2. Fill screw top jars loosely

   Only two thirds full, then close them. The air in the jar is deliberate, because it spreads the moisture between the pieces.

3. Leave it for seven to ten days

   At room temperature, shaken hard once a day. That is how long dry edges and damp cores take to equalise. Then hold the jar up to the light.

4. Read the sweat test

   If it mists up inside or droplets hang from the lid, there was too much water in there: back into the dehydrator for one to two hours at 50 degrees. If the glass stays clear, your store is finished.

Finished dried fruit sits at 13 to 20 per cent residual moisture, dried vegetables and herbs at around 10. You cannot measure that percentage at home, so the sweat test takes the place of an instrument. It matters most for fruit and tomatoes; dried vegetables and herbs go all the way to brittle anyway and barely need conditioning at all.

Store it dark and cool, ideally at 10 to 18 degrees. Dried fruit then keeps for about a year, dried vegetables roughly half as long because they pick humidity back up faster, and herbs six months to a year. Write the date on the lid.

What does not dry well

Anything with a high fat content is out: avocado, olives and oily seeds turn rancid, because fat does not dry, it oxidises. Nuts are cured at room temperature, not run through a dehydrator.

Watery salad crops like lettuce, cucumber and radish give you paper without flavour. Strawberries disappoint plenty of people too, because they go tough rather than leathery. For soft, juicy berries, bottling or making jelly is usually the better route, with raspberries and blueberries as the exception, since those can go on the tray whole. Basil is the best known failure of all, because its aroma sits in volatile oils. Freezing it or turning it into pesto gets you further, and the same goes for chives.

Garlic and onions dry well, but they hand their smell to everything else that is in the machine at the same time. Dry onion rings in the morning and apple rings in the afternoon, and you will have onion apples by the evening.

A word on the obvious next step: putting dried tomatoes and garlic into oil is preserving with a risk attached. Under oil there is no oxygen, and that is exactly what Clostridium botulinum likes. The German Federal Institute for Risk Assessment advises against keeping home made oil preserves as a store. If you do make them, acidify them, keep them in the fridge and use them up within a few days.

The drying season through the year

CW 30 to CW 42 is the densest stretch, because stone fruit, tomatoes and summer vegetables all arrive at once. We are in the middle of it, late July · CW 31. Until mid August · CW 33 the tray space belongs to damsons, apricots and the first tomatoes. Early summer apples such as Apple ‘White Transparent’ are ripe right now but dry badly, because they lack the structure. Herbs are at their most aromatic just before they flower.

Late August to late September · CW 35 to CW 39 brings the first pome fruit and the elderberries. Wait until October and the umbels are usually picked over or cleared by the birds.

Late September to mid October · CW 40 to CW 42 brings squash, root vegetables, late keeping apples such as Apple ‘Boskoop’, which gives firm rings, and rosehips, which you halve, deseed and dry at 40 degrees. Here you stay cooler than with other fruit for once, because the vitamin C otherwise goes with the heat.

Drying alongside the other methods

No single method does everything. Bottling holds texture and flavour better, fermenting is no use for fruit, and for potatoes and cabbage, storing them in a cellar makes more sense. Split your harvest up: flawless carrots into the sand box, crooked ones into the dehydrator.

Anyone who notes down what they dried, when, and at what temperature has their own table by the third year, and it will fit better than any general recommendation ever could. Your oven heats differently from mine, and your Tomato ‘Berner Rose’ holds more juice than mine. In Gartenkern you can set the drying season up as a recurring task and pin a photo of each batch to its harvest date. After a few years you will be able to see exactly when your damsons were ripe.

Common questions about drying

How long does fruit need in a dehydrator?

Apple rings at 4 to 5 mm need 6 to 10 hours at 60 degrees, halved damsons 12 to 20 hours at 65 degrees, tomato slices 8 to 14 hours at 55 degrees. Trust the bend test rather than the clock: it is done when it bends like firm leather and gives up no juice.

Can I dry fruit in the oven instead of buying a machine?

Yes, on the fan setting at 50 to 70 degrees, with one condition: the oven door has to stay open a crack so the damp air can escape. A wooden spoon handle wedged across the door holds the one to two centimetres you need. Swap the trays every two hours. The drawback is consumption, at 1,500 watts and more against the 250 to 700 watts a dehydrator draws.

Why does my home dried fruit go mouldy after a few weeks?

Almost always there was still moisture in the core, and it redistributed itself inside the closed jar. It happens especially when you dry too hot, because the surface hardens and traps the water. The sweat test prevents it: pack the cooled produce loosely into screw top jars and shake them daily for seven to ten days at room temperature. If the glass mists up, the batch needs another one to two hours at 50 degrees.

What temperature should herbs be dried at?

At 35 to 40 degrees and no higher, because the warmer it gets, the faster the essential oils evaporate. In a dehydrator it takes two to four hours for fine leaved species such as thyme and oregano, four to ten for fleshy ones like mint, sage and lovage. Without a machine, the bundle works: tie eight to ten stems together loosely, hang them upside down somewhere dark and airy, and wait five to fourteen days. Strip the leaves off only once they are dry.

Do I have to blanch vegetables before drying them?

With firm root and brassica crops it is worth it: carrots, kohlrabi and peas for two to four minutes in boiling water, then into iced water and drained well. That stops the enzymes, holds the colour and shortens the drying time. Beetroot wants cooking through rather than blanching, and 3 mm slices need five to seven minutes for that. Do not blanch tomatoes, peppers, courgettes, onions or mushrooms, because they only lose substance.