How Freeze Drying Works

Freeze drying freezes food solid, then lowers the air pressure so far that the ice turns straight into vapor without melting. Gentle shelf heat drives it, a −40 °F (−40 °C) cold trap catches the water, and a final warm-up takes out what’s left. The food keeps its shape and most of its flavor and nutrition, and lasts for years sealed in a bag.

The three stages

The three stages of a freeze-drying batch Shelf, food and cold-trap temperatures over a typical full batch of fresh food, about 50 hours. The food takes about 8 hours to freeze, stays frozen through main drying while the shelves warm, then rises to the shelf temperature in the final dry. The cold trap holds at minus 40 degrees throughout. 1 2 3 −40°F 0°F 70°F 120°F 0 h 12 h 24 h 36 h 48 h Under vacuum · about 550 mTorr
  • Shelf
  • Food
  • Cold trap, −40 °F (−40 °C)
A full 5-Shelf batch of fresh food, illustrated. Food you froze ahead skips most of stage 1; times and temperatures vary with the food and the load.
  1. 1

    Freeze

    The compressor chills the shelves until the food on them is frozen solid. For an average load of fresh food that takes about 8 hours on the 5- and 7-Shelf, 10 on the Large and 12 on the Mega. Food you froze ahead of time in your own freezer starts drying within about 0–4 hours.

  2. 2

    Main drying

    The vacuum pump lowers the chamber to 550 mTorr by default — about 1/1,400 of normal air pressure, far below the 4,580 mTorr at which water can exist as a liquid. At that pressure ice can’t melt; it sublimes straight to vapor. The shelf heaters supply the energy sublimation needs, and the food stays frozen throughout. This is the long stage: a 5-Shelf pulls out roughly half a pound (about 0.2 kg) of water an hour.

  3. 3

    Final dry

    Once the ice is gone the shelves warm to the maximum shelf temperature — 120 °F (49 °C) by default — and hold there to drive off the last bound water: 7 hours at the factory setting, though we recommend 5, 9 or 12 hours by load size, and longer for sugary fruit and ice cream. Then let air back into the chamber slowly and bag the food straight away.

Inside the machine

Inside a home freeze dryer 1, the vacuum pump lowers the chamber pressure. 2, shelf heaters supply heat. 3, the frozen food releases water vapor. 4, the vapor freezes onto the cold trap at the bottom of the chamber. 5, the ice is melted and drained after each batch. Pump 1 2 3 4 5
  1. Vacuum pump lowers the pressure to about 550 mTorr and holds it there.
  2. Shelf heaters supply the heat that turns the ice into vapor.
  3. The food stays frozen while its ice sublimes away, so it keeps its shape.
  4. Cold trap, −40 °F (−40 °C) — at the bottom of the chamber, where the vapor freezes back into ice.
  5. Drain — melt the ice and drain it out after each batch.

The water has to go somewhere. As vapor leaves the food it travels to the coldest surface in the chamber — the cold trap at the bottom, which runs at −40 °F (−40 °C), or −35 °F (−37 °C) on the Mega — and freezes there as frost. A full 5-Shelf batch leaves up to 14 lb (6.4 kg) of ice, about 1¾ gallons (6.6 litres) of water once it melts, which you drain before the next batch. That is also why every machine has an ice capacity: load more water than the trap can hold and the batch won’t finish.

The vacuum pump never touches the food. It keeps the pressure low enough for sublimation, and the little water vapor that gets past the trap ends up in its oil — which is why pump oil needs changing: every 5 batches on our Premier pump, every 30 on the Commercial.

Freeze dried or dehydrated? A dehydrator blows warm air over food, so it shrinks, hardens and changes color, and usually keeps more of its moisture. Freeze drying takes out nearly all the water without heat damage, so food keeps its shape, rehydrates in minutes and stores far longer — at a much higher price per machine. Our dehydrator comparison covers when the cheaper machine is the right one.

Can I freeze dry it?

Search a food to see whether it works, what prep it needs, and the settings we use. Settings are from our own recipe testing; owners get all 126 recipes with the machine.

65 foods

Fruit13 foods
  • Works greatStrawberriesSlice or halve. Eat them crunchy or rehydrate for baking.
  • Works greatRaspberries & blackberriesFreeze whole. Delicate once dry, so store in rigid containers or full bags.
  • Works with prepBlueberriesPierce or halve every berry first. The skin traps moisture, so whole berries burst or shrivel.
  • Works with prepCherriesPit and halve. Whole cherries trap moisture and shrivel.
  • Works greatApples & pearsSlice thin. A quick dip in lemon water limits browning if you care about color.
  • Works greatPeaches & nectarinesSlice; peel first if you dislike the skin.
  • Works with prepBananasSlice about ¼ in (6 mm) thick. High in sugar, so plan a longer final dry — about 10 hours.
  • Works with prepMangoHigh sugar: lower the target vacuum to 350–500 mTorr and run about a 10-hour final dry.
  • Works with prepPineappleUse fresh pineapple — canned does not turn out well. High sugar, so a 10-hour final dry.
  • Works with prepGrapesBlanch first — whole raw grapes turn into raisins.
  • Works with prepWatermelon & melonVery high in sugar and water: final dry 10 hours at minimum, up to 15.
  • Works with prepStore-bought frozen fruitWorks, but bagged frozen sliced fruit tends to shrivel. Fresh fruit holds its shape better.
  • Works with prepCitrus segmentsPeel and separate the segments; cut larger ones so the moisture can escape.
Vegetables12 foods
  • Works greatCornBlanch, then cut the kernels off the cob.
  • Works greatPeasBlanch briefly before loading.
  • Works with prepGreen beansBlanch and cut into 1 in (2.5 cm) pieces.
  • Works with prepBroccoli & cauliflowerBlanch first — raw broccoli shrivels badly.
  • Works with prepCarrotsBlanch slices or dice for the best color and texture.
  • Works with prepPotatoesCook first — mashed, diced or hash browns. Raw potatoes darken.
  • Works greatTomatoesSlice or dice. Dried tomatoes powder well for sauces and soups.
  • Works greatPeppers & onionsDice; no blanching needed.
  • Works greatMushroomsSlice. Rehydrate in warm water.
  • Works with prepLeafy greensDelicate: set the max shelf temperature to 50–100 °F (10–38 °C) and the target vacuum to 300–450 mTorr.
  • Works with prepFresh herbsSame as leafy greens — low shelf heat and a lower target vacuum keep the color.
  • Works with prepSquash & pumpkinBlanch cubes or slices, or cook and purée.
Meals4 foods
  • Works greatSoups, stews & chiliCool first, then spread no more than about ¾ in (2 cm) deep.
  • Works greatCasseroles & pasta dishesCook as normal. Break long pasta into short pieces so it rehydrates evenly.
  • Works greatRice & beansCooked rice and beans rehydrate well with boiling water.
  • Works with prepVery greasy dishesSkim or blot the fat. Fat does not freeze dry, and it turns rancid in storage.
Meat & fish7 foods
  • Works greatCooked chicken & turkeyCook to 165 °F (74 °C), then dice or shred.
  • Works greatCooked ground beefBrown to 160 °F (71 °C), drain well and blot the fat before loading.
  • Works with prepRaw meat & poultryFreeze drying is not a kill step. Utah State Extension says cook it first; Minnesota Extension allows raw if you cook it to a safe temperature after rehydrating.
  • Works with prepFish & shrimpCook to 145 °F (63 °C) first.
  • Works with prepWild gameCook it to 160 °F (71 °C) before drying, as Utah State advises for all meat. Freezing doesn’t reliably kill the parasites wild game can carry (CDC), and neither does drying.
  • Works with prepBacon & fatty cutsCook it and blot away as much grease as you can. Fat does not dry, and it shortens storage life.
  • Skip itSausage in casingsThe dense casing stops moisture escaping, and the fat goes rancid.
Eggs & dairy9 foods
  • Works greatEggs, scrambledScramble and cook, then dry. Powder it in a blender for storage.
  • Works with prepRaw eggsExtension services split: Utah State says use pasteurized liquid egg or cooked scrambled egg instead; Minnesota and Penn State allow raw egg if it is cooked to 160 °F (71 °C) after rehydrating. Never eat it as is.
  • Works with prepMilk (pasteurized)Target vacuum 500–600 mTorr and a 7-hour final dry, then powder it. Pasteurized milk only.
  • Skip itRaw milk & unpasteurized dairyListeria, Salmonella and E. coli risk, and freeze drying does not remove it.
  • Works greatYogurtPipe drops onto parchment for snacks, or spread thin in the tray.
  • Works greatCheeseShred or cube. Harder cheeses work best.
  • Works greatSour cream & cottage cheeseSpread thin; rehydrates well for cooking.
  • Skip itButterAlmost pure fat — it never dries.
  • With careBreast milkPopular with parents. Use a 90 °F (32 °C) max shelf temperature and a 15-hour final dry, and weigh the milk first so you know how much water to add back. Freeze drying doesn’t kill germs — see Food safety below, and check with your pediatrician first for a premature or medically fragile baby.
Desserts & candy12 foods
  • Works with prepIce creamScoop small and keep it frozen until loading. Final dry about 12 hours.
  • Works greatCheesecake & dessertsCut into bite-size pieces.
  • Works with prepSkittlesCandy mode at 140 °F (60 °C) with about 3 hours of final dry.
  • Works with prepGummy worms & gummy bearsCandy mode at 150 °F (66 °C); usually done in under 4 hours.
  • Works with prepNerds Gummy ClustersCandy mode at 140 °F (60 °C) with about 3 hours of final dry.
  • Works with prepSaltwater taffy & Junior MintsThese start puffing near 50 °F (10 °C), so keep the shelf temperature low.
  • Works with prepCaramels & Milk DudsCandy mode at 150 °F (66 °C).
  • Works with prepJolly Rancher hard candyThe one hard candy that reliably works — candy mode at 150 °F (66 °C).
  • Skip itStarburstDoes not puff well in our tests.
  • Skip itJelly beans & Sour Patch KidsDo not freeze dry well.
  • Skip itPure chocolateMostly fat, so it melts instead of drying. Chocolate-coated candy needs low shelf heat.
  • Skip itMost hard candiesA hard shell will not expand. Jolly Rancher hard candy is the exception.
Other8 foods
  • Works greatCoffeeBrew it strong, freeze in the tray, and you have your own instant coffee.
  • Works greatSmoothiesPour into the tray or drop into portions; powder or eat as melts.
  • Skip itHoney, syrup & jamAlmost pure sugar — it stays sticky or melts rather than drying.
  • Skip itOils, mayonnaise & dressingsFat does not dry, and it goes rancid in storage.
  • Skip itPeanut butter & nut buttersToo much fat to dry, and it goes rancid.
  • Skip itAlcoholDoes not freeze solid at these temperatures, so it will not freeze dry.
  • Skip itRaw flour & cookie doughRaw flour can carry E. coli and Salmonella, and there is no kill step.
  • Works with prepPet treatsCook meats and liver first. The CDC advises against raw pet food, freeze-dried included — drying only reduces the germs.

What won’t freeze dry

Almost any food with water in it freeze dries. Three things don’t:

  • Fat. Butter, oils, mayonnaise, peanut butter and pure chocolate don’t dry, and fat goes rancid in storage. Fatty meat works only if you cook it and blot away the grease.
  • Pure sugar. Honey, syrup and jam stay sticky or melt instead of drying.
  • Alcohol. It doesn’t freeze solid at these temperatures, so there is no ice to sublime.

Extension services in Arizona and Idaho, Ohio, Indiana (Purdue) and Utah all name the same list.

Food safety

Freeze drying is not a kill step. It doesn’t heat food enough to kill bacteria — it preserves whatever goes in, and the germs become active again when water comes back. That matters most for raw meat, poultry, seafood and eggs: they need cooking before they go in, or after they are rehydrated.

  • Raw meat, poultry, seafood and eggs: the extension services disagree. Utah State says cook them first — meat to 160 °F (71 °C), poultry to 165 °F (74 °C), seafood to 145 °F (63 °C) — and use pasteurized liquid egg or cooked scrambled egg. Minnesota, Penn State and Arizona allow raw, as long as it is cooked to the USDA safe temperature after rehydrating. Either way, never eat raw freeze-dried meat or egg as it comes out of the bag.
  • Never freeze dry raw milk or other unpasteurized dairy, or raw flour and cookie dough.
  • Before loading, don’t leave food out for more than 2 hours (1 hour above 90 °F, or 32 °C), cool cooked food quickly in shallow containers, wash produce, and blanch most vegetables.
  • Make sure it is dry. The machine finishing means the program ended, not that the food is dry. Snap a large piece: it should be crisp all the way through, with no dark, cool or soft spot in the middle. If one piece is wet, the whole tray goes back in for more final dry time. The surest check is weight — weigh a tray, run another couple of hours, weigh again, and when it stops getting lighter it is done.
  • Only seal food that is completely dry. Properly freeze-dried food reaches a water activity of about 0.3 or lower, far too dry for bacteria. Damp food sealed with an oxygen absorber is the risky case: Utah State warns that taking the oxygen out of a moist package can make botulism more of a concern, not less.

Breast milk

Many parents freeze dry breast milk to save freezer space and make a stash easier to store and travel with. It dries well with the right settings: a 90 °F (32 °C) maximum shelf temperature and a 15-hour final dry. For frozen pouches, soak the pouch in water to release the slab; after drying, break the milk up with a fork and check for ice inside.

Two things matter for safety. Freeze drying removes water but doesn’t kill bacteria, so the powder is only as clean as the milk you start with: pump and store it hygienically, freeze it promptly, use clean trays, and seal the powder airtight with an oxygen absorber as soon as it is dry. And it has to go back to the right strength: weigh the milk before drying and add back the same weight of water, using water that is safe for your baby. No US agency or pediatric body has published guidance on freeze-drying breast milk, and some health authorities advise against doing it at home — Alberta Health Services’ dietitians don’t recommend freeze-dried human milk, and a Nebraska Extension (University of Nebraska–Lincoln) food-safety specialist recommends frozen storage instead — because germs such as Cronobacter are most dangerous to newborns and to premature or medically fragile babies. If your baby was born early or has a medical condition, ask your pediatrician before using it.

Selling what you make

Rules for selling freeze-dried food vary sharply by state, and several treat candy and fruit differently. Our state-by-state guide has the details and the sources.

Storing it

  • Bag it right away. Freeze-dried food starts pulling moisture from the air the moment the door opens. Seal it in mylar bags (5–7 mil, with no clear window) with an oxygen absorber, using an impulse sealer — or in glass jars with an absorber for food you’ll use soon. Vacuum-sealer bags are good for two to three years at most, because air and moisture slowly pass through plastic.
  • Keep it cool, dark and dry — below 75 °F (24 °C), ideally below 60 °F (16 °C). Storage life roughly halves for every 10–11 °F (6 °C) warmer.
  • How long it lasts. The familiar “25 years” comes from manufacturers and from commercially packed food. For food dried at home, Utah State Extension recommends using it within 5–10 years; there is little data beyond that.
  • Once opened, reseal with a fresh oxygen absorber. Rehydrated food is perishable again: refrigerate it and use it within 3–4 days, like leftovers, and reheat cooked food to 165 °F (74 °C).
  • Label every bag with what it is, the date, and whether it went in raw or cooked.

More detail in our guide to packaging freeze-dried food for long-term storage.

Running the machine safely

  • Get the power right. The Small plugs into a regular household outlet. On 110 V, give the 5- and 7-Shelf a dedicated 15 or 20 A outlet, and the Large and Mega a 20 A circuit with a NEMA 5-20 outlet.
  • Let it breathe. On the 5-Shelf, 7-Shelf, Large and Mega, leave at least 12 in (30 cm) of space on both sides. Air comes in on the left and leaves on the right, so keep heat sources away from the left side. The Small needs 12 in (30 cm) clear on its right side.
  • Look after the pump. Keep it somewhere between 50 and 75 °F (10 and 24 °C) with clean air, drain the oil mist filter after each batch, and change the oil on schedule — every 5 batches on the Premier pump, every 30 on the Commercial.
  • Let the air back in slowly when a batch finishes, with the vacuum release valve (the drain valve on the Small). Letting air in fast can collapse delicate food or blow it off the trays.
  • Read the screen, not just the beep. The machine beeps for errors as well as when it’s done.

Keep going

What size freeze dryer do I need?Match your weekly amount to a machineFreeze dryer cost calculatorWhat a batch costs, and when it pays offIs a freeze dryer worth it?An honest answer for your householdStay Fresh owners’ groupTips, recipes and settings from other owners

Sources

Don't leave without an exclusive offer from us!

By signing up you agree to receive emails from Stay Fresh. Unsubscribe anytime. Privacy policy