How does freeze drying work?
Freeze drying is a process that removes water from food and other materials. The process starts by first freezing the food, then placing it under a vacuum. The vacuum environment allows the ice to sublime, or transform from solid to vapor. Freeze drying is also known as Lyophilization or Cryodesiccation.
Freeze drying has four stages:
- Pretreatment - Food can be prepared before freeze drying it. This is done to change the drying process, or the final dried product. Pretreatment can be as simple as slicing the food to certain thicknesses. Or by adding citric acids to maintain the appearance of the final dried foods. Some recipes need to be modified for freeze drying. Ingredient flavors might change from either the drying, or the reconstituting processes.
- Freezing - Food is cooled below and to left of its triple point to make sure sublimation will happen during the drying stages.
- Primary drying - This stage removes the free ice by applying a small amount of heat. The heat provides the energy needed to sublime the free ice without thawing the food. Primary drying removes about 95% of the moisture.
- Desorption [Secondary drying] - Bonded water molecules are released during this phase. It happens by increasing the heat. This stage actually starts before the sublimation/primary drying is complete.
This final stage brings the moisture content to between 1% and 4%.
How does freeze drying preserve food?
What makes freeze dried food last longer?
Microorganisms and enzymes cause food to spoil, ripen, or rot. Freeze drying removes the water from food, so the microorganisms and enzymes can’t reproduce or react to degrade the food. Oxygen absorbers further preserve the food from their own enzymes that would cause it to ripen or spoil.
How long will freeze dried food last?
Freeze dried food can last between 20-30 years. The shelf life depends on how it is stored. Both the container and the surrounding temperature affect the shelf life. Cooler temperatures and non-porous containers will preserve the food longer.
The 4 stages — see the section above.
The triple point of a substance is the pressure and temperature that solid, liquid, and gas can exist at the same time (in equilibrium).
Sublimation is when a substance passes from a solid to a gas. Sublimation can be sped up by cooling a substance and placing it in a vacuum to bring its temperature and pressure below the triple point. When the temperature is increased with the vacuum, the phase shift is from a solid to a gas.
Freeze drying can reduce the moisture composition of food down to between 1-4%.
Changes that happen:
- Freeze speed can affect the cell walls of the material — results in a “mushier” reconstituted product
- Some foods will brown, or otherwise change in appearance — e.g. carrots lose some of their orange color
- The obvious benefit: water content and mass decrease
Not much for chemical changes, thus the benefit to freeze drying.
The history of freeze drying
For the first instances of freeze drying, people used their natural environment to their advantage.
Peruvian Incas made what they called chuño from drying potatoes and charqui from dried beef. They used the freezing night temperatures and low atmospheric pressures of the high Andes.
Inca 'colca' on the mountain side
Across the valley, at the center of the photo, you can see the 'colca', where the Inca stored their surplus food. The cold wind helped to preserve their food supply. In the Sacred Valley Cuzco, Peru
Inca city Machu Picchu
The high altitude and cold temperature of Machu Picchu made it possible to semi-freeze dry food. They made both chuño and charqui.
Eskimos, Buddhist Monks, and the Vikings were also known to freeze dry some of their foods.
Here is a timeline view of the freeze drying technology advancements compiled from many sources:
|Freeze drying using nature:|
|13th century||Peruvian Inca |
|Freeze drying with a chemical pump/vacuum:|
|1890|| Altman - drying tissue   
The documentation of his research was “rediscovered” 40 after the fact, so he is not often attributed to the discovery
|1905||Benedict and Manning - drying animal tissue  |
|1906||Arsène d'Arsonval and Frédéric Bordas |
|Freeze drying with a mechanical pump/vacuum:|
|1909||Shackell - first use of a mechanical vacuum  
Independently “rediscovered” because Altman’s work was not known 
|1911||Downey Harris and Shackle - to preserve a live rabies virus |
|1927||Tival - first U.S. patent to mention drying frozen food under a vacuum |
|1934||Elser - patent of dryer using dry ice |
|1935||Flosdorf and Greaves - first commercial freeze dryer 
This allowed for the transportation of plasma to troops during WW2  
|1938||Many online sources mention1938 as the date for Nestle freeze drying coffee, but the Nestle site claims the 1965 date. |
|1965||Coffee freeze dried by Nestle - start of freeze dried food at scale |
|1966||36 total U.S. freeze drying patents |
|1968||Astronaut ice cream made |
|1980s||Flowers freeze dried|
|2013||Patent issued for Harvest Right home freeze dryer |
 - http://www.lyotechnology.com/fd-milestones.html
 - https://en.wikipedia.org/wiki/Freeze-drying
 - https://www.ncbi.nlm.nih.gov/pubmed/801137
 - http://tofflon.fanyacdn.com/imglibs/files/the%20history%20of%20freeze%20drying%20(2nd%20issue).pdf
 - https://www.google.com/patents/US9459044
 - http://www.nestle.com/media/newsandfeatures/nescafe-75-years
Different ways freeze drying is done
With time, food in your freezer will lose moisture through sublimation. The best example of this it to look at an ice cube tray that has been in the freezer for a long time. Notice the ice cubes shrink? Also, see the History section - Peruvian Incas, Eskimos, Buddhist Monks, and the Vikings used their local climate.
However, the freeze drying process can be sped up:
- Freeze the food to a temperature below water’s triple point
- Taste, structure, and appearance are more likely to preserve by freezing food
- Freeze drying does not cook the food
- The food can be frozen with:
- Mechanical refrigeration (the most common today)
- Other methods:
- Dry ice in liquid Methanol
- Liquid nitrogen
- Use a vacuum to bring the surrounding pressure below water’s triple point.
- Mechanical vacuum pump (the most common today)
- Chemical - Displacement of the air with ethyl ether
Freeze drying was first used at a large scale in the 40s during WW2 for blood plasma and penicillin transportation. It became more popular for drying food during the 60s. [source]
Normal commercial applications:
- Pharmaceuticals — this is where freeze drying got its start at scale
- Space — obvious weight advantages (in spite of its name, "astronaut ice cream" never made it to space source)
- Milk & other dairy products — Other benefits: retain color, flavor, nutrients, and consistency source
- Hiking meals — who wants to pack extra water weight?
- Food storage — 20+ year shelf life
Freeze drying has historically been too expensive for home use. Entry-level freeze drying units started at $30,000 and were for commercial use.
The only "reasonable" option to have your own freeze dried food was to buy it.
Today, that is changing.
Some have tested a jar vacuum with silica beads to partially freeze dry food. This is a fun video. Realize that jar vacuum sealers do not accomplish enough of a vacuum to completely freeze dry. The cellular collapse is a good indication of the incomplete freeze drying.
Others have built their own freeze dryers. This guy has given every step to build your own. As you can see from the video, it required a lot of mechanical expertise to build one.
While freeze drying itself is an old technology, at-home freeze drying is opening the technology to everybody. Harvest Right has built the first freeze dryer designed for the home consumer. Instead of the prohibitive $30,000 price tag for a freeze dryer, every-day consumers can now get their own home freeze dryers starting at about $2400.
Home freeze drying is still in its early stages. But the ability to create your own recipes has many advantages. You also have have the cost-saving advantage of saving your leftovers. Harvest Right has opened freeze drying to an exciting new audience. Many will enjoy doing their own home food preservation.
What are the advantages & disadvantages of freeze drying?
- Great snacks
- Little to no chemical changes to food
- Little physical changes to food
- Nutritional value is almost completely maintained - it is healthy
- Long-term storage — 20+ years
- Lightweight food
- No refrigeration required
- Water required for reconstitution (except for simple snacks)
- Slow process — average cycle is 24+ hours
- Some dislike the dry, styrofoam texture
- Not all foods can be freeze dried
- Airtight containers are required for long-term storage
- No space savings — the food's cellular structure is mostly retained
- Sometimes, normal dehydration is better
- Production cost
- Custom recipes
- Great food variety
- Allergy awareness
- Self reliance
- Knowledge & novelty
- Food quality
Could be good or bad:
- Learning curve
- Learning how different food types dry
- Cost (when comparing other food preservation methods)
- Power consumption
- Freeze drying is one of the most energy-intensive methods to preserve food [source]
- Machine maintenance
- Noise [source]
- Space / Footprint
- Commercial vendors can flash freeze products before placing it in the freeze dryer
- Flash freezing creates smaller ice crystals, which causes less damage to the cell structures
What to avoid
Low eutectic point foods
The eutectic point refers to the melting or freezing temperature of something. Items that need lower temperatures will not easily freeze dry because they don't easily freeze.
Some of those items include:
- High sugar - Sugar bonds to water, which is actually why it's good for preserving food. It's not so good for freeze drying, because it also lowers the freezing point of the food
- High salt - Salt is used to melt snow and ice. It lowers the freezing temperature of water enough to cause issues when you try to freeze dry food with very high salt levels
- Honey - Honey doesn't actually freeze; it's a supersaturated solution with only about 18% water [source], and becomes "glassy" between (−44 and −60°F)
- Here's a cool video that shows how honey becomes glassy on dry ice
- High fructose corn syrup - Like honey, high fructose corn syrup is only about 25% water and doesn't easily freeze
On a separate note, food that is not completely frozen will boil during the drying stages. Here's what strawberry puree looked like when we didn't give the cycle enough time to completely freeze:
The two middle trays didn't freeze long enough before the dry cycles.
The trays were also very full; likely close to capacity of the 4-tray freeze dryer.
The unfrozen puree boiled during the dry cycle, creating an interesting texture.
- Thick food takes much longer to dry - The food dries from the outside-in, so thick food takes much longer
- Varying thicknesses - Your cycle will take as long as the thickest item takes to dry
- Overfilling trays - If the trays are filled beyond the freeze dryer's capacity, you will need to defrost the ice buildup, and continue drying
Other common mistakes
- Raw meats - They are still raw and should not be mixed with cooked items
- Flavors mix - Even in a vacuum, flavors can transfer from one food to another
- Fruit skins - Reduce freeze drying time by puncturing the skin
- High fat - Fat reacts with oxygen and goes rancid
Companies that sell freeze dried food provide numbers that put their products in the best light. Companies that sell the freeze dryers do the same in favor of their own products. It’s basic Marketing.
For example, with a simple search on google for “home freeze drying”, the first two Ads that displayed at the time of writing this were:
The first Ad [by Harvest Right, a freeze dryer vendor] is going to show you a page with the best possible reasons to buy your own home freeze dryer.
The second Ad [by preparewise, a freeze dried food vendor] will show the benefits of buying freeze dried food vs your own freeze dryer. They use a somewhat exaggerated calculation of the Total Cost of Ownership (TCO) of a freeze dryer. But of course they do, they’re selling a product. And a good product, at that. There is a lot of time and effort that goes into creating a great recipe for freeze-dried food. That time can add value for their customers. That value is going to (and should) cost extra money.
We’re working to cut through the marketing to present the basic economics of freeze drying. We do this by showing you ALL the costs associated with owning your own freeze dryer.
Realize that you are buying two completely different products. Both have their advantages.
If your main goal is to have the recommended amount of food per person for an emergency, then you can accomplish your goal by buying the food from a commercial freeze drying company. It is “hassle-free”, and you don’t have to wait very long to be fully stocked.
If you decide to buy your own freeze dryer, you can end up with much more than the end product (the freeze dried food.) You also enable yourself to produce your food storage by yourself. You don’t have to rely on anybody else. That’s a good feeling. That being said, you will only be able to run one cycle at a time. The amount of food produced varies based on which freeze dryer you have. But it’s not an overnight process, and will take some time to build up your storage.
Sometimes it’s not just about the end product. Sometimes it’s about the experience, freedom, flexibility, and knowledge gained from the process.
These are some ideas to consider when deciding between buying freeze dried food and buying your own freeze dryer:
- Freeze drying is a healthy mix of art and science, no matter how well the science of the process is understood, food is variable
- Every new combination of food types will have a different drying process
- Large commercial freeze dryers have gone through hundreds of cycles to find the best process for each of their products
- Large operations can enjoy bulk pricing and their operation scale
- With their process and scale advantages, they cannot offer the variety and flexibility of a home freeze dryer
- We prefer a healthy mix of canning, dehydrating, and freeze drying for our food storage
- Nothing is going to beat the economy of canning or dehydrating, but freeze drying will give you a 25+ year shelf life
We've dedicated a section of familycanning.com to understanding the costs associated with owning a freeze dryer. We explore materials, maintenance, and power. We also break down how long it would take to break even with the price of the machine. The results are very promising.
See our blog post about pork chops to why the return on investment is so promising.
The basic concept is "just add water. " Just not too much...or too little.
It takes some trial and error to get the right amount of water to reconstitute freeze dried foods. Some foods (or ingredients) don't restore to their original form. Recipes may need to change to account for reconstitution differences.
We have dedicated a part of our freeze dryer recipes to reconstituting the freeze dried ingredients used in them.
We also have a cheat sheet for freeze dried ingredients used in our recipes. It's a quick reference to look up the right amount of water to reconstitute ingredients.
Other freeze drying applications
- Flowers / Bouquets
- Book / Document recovery
- Pet food
- Biological research