Freeze dryer vs dehydrator

How each one works

A dehydrator blows air, warmed to a set temperature, across sliced food for hours until most of the water has gone. Dried apple, tomatoes, herbs, mushrooms and fruit leather are the usual things to make.

A freeze dryer freezes the food first, then pumps the air out of its chamber. In a vacuum the ice goes straight from solid to vapour, what Utah State University Extension calls "the direct transition of a solid to a gas", and gentle heat speeds it along. Whole meals, fruit and dairy can be dried, then brought back with water. Drew Smith at Backpacking Light found his freeze-dried meals "rehydrated in 5-20 minutes in cold water".

Which keeps food longer?

Freeze-dried food, by the makers' claims, because it holds less water. But the figures you'll see are the makers' best cases:

• Harvest Right says freeze-dried food keeps "up to 25 years or more when properly packaged and stored". Utah State University Extension says "no real data exists on the shelf-life of home freeze-dried products".
• For home-dried food from a dehydrator, the National Center for Home Food Preservation recommends "4 months to 1 year", depending on the food and how cool it's kept; vegetables keep about half as long as fruit. Excalibur's own chart gives 3-5 years for most fruit and 8-10 years for most vegetables in airtight containers below 21°C, which is far longer than the NCHFP's advice.

For food you can count on in an emergency, shop-bought tins and dried food with printed dates are simpler: a store cupboard that sees you through.

What each does badly

Freeze dryers

• Fat and sugar. Harvest Right says plain butter, peanut butter and solid chocolate don't freeze-dry well on their own, and advises against honey, syrup and jam: "Freeze drying removes water, but oil remains." Utah State says thick meat or casseroles can take "more than 2 days".
• It doesn't cook anything. Utah State says freeze drying doesn't kill germs, "It preserves them", so raw meat and eggs still need cooking, before drying or after.
• Noise and upkeep. Drew Smith found that the vacuum pump "is noisy", and drained water and topped up oil after each run on his older machine. Harvest Right wants clear space round the side vents, and a room at 50-75°F (10-24°C).
• Size and price. It's a heavy machine that runs for 13 to 40 hours a batch, and costs about eight times the dearest dehydrator here.

Dehydrators

• Shorter keeping time, by the NCHFP's guidance above, because more water stays in the food: it gives about 20% left in dried fruit and about 10% in vegetables.
• Slices, not meals. Food has to be cut thin: Excalibur's drying times are for quarter-inch apple slices.

Packing it to keep

Utah State recommends packing freeze-dried food straight away, in opaque "Thick Mylar bags (5–7 mil)" with oxygen absorbers, stored cool, dark and dry. Its buying guide adds that vacuum packing and oxygen absorbers are only for food that's properly dry: "vacuum packaging is not a food safety process itself". For dehydrated food the NCHFP says to cool it fully, then pack it in clean, dry, insect-proof containers such as jars, kept in "cool, dry, dark areas".

The machines

Is a freeze dryer worth it?

Only if you'll use it often: a big garden, an allotment or orchard, or meals for walking and camping. Electricity alone is about £5.53 a batch at today's price cap, on one owner's measurement, before bags, oxygen absorbers and pump oil. For an occasional surplus of fruit and herbs, a dehydrator at a fraction of the price is the sensible start. More on both: freeze dryers in the UK and dehydrators and vacuum sealers.

Sources: Iowa State University Extension, Utah State University Extension, the National Center for Home Food Preservation, Harvest Right, Excalibur (Performance series manual), and the hands-on reports linked above.