Freeze drying was developed during World War II as a way to keep medication for the wounded from spoiling due to uncertain refrigeration while being transported to war zones. The technology was refined and used extensively during the NASA space program, providing varieties of freeze dried food for astronauts.One of the advantages of freeze-dried food is its long shelf life.
When properly stored, many freeze-dried foods can be eaten 15 to 25 years after they have been prepared. Even after that length of time the food’s flavor and nutritional value is as good as it was the day it was preserved.Because of its long shelf life and nutrition preservation, numerous commercial companies produce freeze-dried food for families, as well as for the military. By just adding water, the food is quickly rehydrated and ready to eat.
A home freeze dryer machine is simply an appliance that removes nearly all the moisture from foods.Freeze drying is a process that removes water to preserve the shelf-life and quality of a material. In simple terms, a freeze dryer keeps things fresh longer. Through a process referred to as sublimation (when a substance transitions from a solid into a gas without going through a liquid phase) a freeze dry rapidly removes moisture to preserve the quality, flavor, aroma, and shape of a material.
How freeze drying works?
The fundamental principle in freeze-drying is sublimation, the shift from a solid directly into a gas. Just like evaporation, sublimation occurs when a molecule gains enough energy to break free from the molecules around it. Water will sublime from a solid (ice) to a gas (vapor) when the molecules have enough energy to break free but the conditions aren’t right for a liquid to form.
There are two major factors that determine what phase (solid, liquid or gas) a substance will take: heat and atmospheric pressure. For a substance to take any particular phase, the temperature and pressure must be within a certain range. Without these conditions, that phase of the substance can’t exist. The chart below shows the necessary pressure and temperature values of different phases of water.
You can see from the chart that water can take a liquid form at sea level (where pressure is equal to 1 atm) if the temperature is in between the sea level freezing point (32 degrees Fahrenheit or 0 degrees Celsius) and the sea level boiling point (212 F or 100 C). But if you increase the temperature above 32 F while keeping the atmospheric pressure below .06 atmospheres (ATM), the water is warm enough to thaw, but there isn’t enough pressure for a liquid to form. It becomes a gas.
This is exactly what a freeze-drying machine does. A typical machine consists of a freeze-drying chamber with several shelves attached to heating units, a freezing coil connected to a refrigerator compressor, and a vacuum pump.
With most machines, you place the material to be preserved onto the shelves when it is still unfrozen. When you seal the chamber and begin the process, the machine runs the compressors to lower the temperature in the chamber. The material is frozen solid, which separates the water from everything around it, on a molecular level, even though the water is still present.
Next, the machine turns on the vacuum pump to force air out of the chamber, lowering the atmospheric pressure below .06 ATM. The heating units apply a small amount of heat to the shelves, causing the ice to change phase. Since the pressure is so low, the ice turns directly into water vapor. The water vapor flows out of the freeze-drying chamber, past the freezing coil. The water vapor condenses onto the freezing coil in solid ice form, in the same way water condenses as frost on a cold day. (See How Refrigerators Work for more information on condensers and refrigeration coils.)
This continues for many hours (even days) while the material gradually dries out. The process takes so long because overheating the material can significantly change the composition and structure. Additionally, accelerating the sublimation process could produce more water vapor in a period of time then the pumping system can remove from the chamber. This could rehydrate the material somewhat, degrading its quality.
Once the material is dried sufficiently, it’s sealed in a moisture-free package, often with an oxygen-absorbing material. As long as the package is secure, the material can sit on a shelf for years and years without degrading, until it’s restored to its original form with a bit of water (a very small amount of moisture remains, so the material will eventually spoil). If everything works correctly, the material will go through the entire process almost completely unscathed!
The Science Behind Freeze Drying
What is freeze drying?
According to the FDA: “Lyophilization or freeze drying is a process in which water is removed from a product after it is frozen and placed under a vacuum, allowing the ice to change directly from solid to vapor without passing through a liquid phase. The process consists of three separate, unique, and interdependent processes; freezing, primary drying (sublimation) and secondary drying (desorption)” .
Freeze drying takes advantage of the scientific principle of “sublimation,” the direct transition of a solid to a gas, by removing ice (the solid) from frozen food as water vapor (a gas). Using sublimation, the food retains much of its original texture, flavor, and nutrition when rehydrated.Freeze drying is broken down into two simple processes: freezing and vacuum drying. Foods are first frozen to well below 0°F.
The colder the freeze the more efficient the next step will be. Once frozen, foods are subjected to vacuum drying. The air and water vapor (gas) are removed from the food processing chamber using a vacuum pump. This includes removing the water vapor that was once inside of the foods. When these two steps are properly completed, the food is dry enough to allow for safe storage at room temperature.
Is freeze drying food safe?
Yes, if …
(1)The two sub-processes, freezing and vacuum drying are done correctly:
a.The freezing process must be quick and the vacuum process should leave only residual moisture.
b. For example, chilling foods safely is defined as reaching 41°F (refrigeration temperature) in 1-4 hours or less. Pre-refrigerated or pre-frozen foods can be placed in the freeze-dryer to minimize this concern.
c.Drying foods safely is defined as reaching “a safe residual moisture level.” To determine this at home, most Cooperative Extension resources suggest that foods should be dried to a “crisp” or “breakable”texture, although foods with high levels of sugars, such as fruits, may be flexible, but not sticky, when correctly dried (Andress, Harrison, Reynolds, and Williams, 2014).
(2)Proper safe food handling techniques were employed in the preparation of the food prior to freeze-drying. Freeze-drying does not kill bacteria.
What happens to microorganisms in the freeze-drying process?
Nothing. The microorganisms stay viable, but dormant, even under the extreme conditions of freeze drying. In fact, scientists use a laboratory version of freeze drying to preserve microorganisms for future studies because the microorganisms can be rehydrated alive for decades (see Kupletskaya & Netrusov, 2011). Therefore, when home freeze drying raw foods the microorganisms on those raw foods will remain viable, then activate upon rehydration. Food items that are traditionally cooked before eating must also be cooked beforeeating as a freeze-dried food. Examples are raw meats, raw seafood, raw eggs, and foods containing these raw ingredients.
Can freeze dried foods be safely vacuum packaged?
Yes. As long as the food is dried to a low residual moisture, vacuum packaging is safe. Remember, vacuum packaging is not a food safety process itself. In fact, removing oxygen from a package may make it more of a concern for the botulism bacteria to grow and produce toxin if there is a moist environment. Fortunately, without moisture (water) the botulism bacteria (and all bacteria, yeast, and molds) cannot grow. Therefore, it is safe to place properly dried or freeze-dried foods in vacuum packaging or in containers that also have oxygen absorber packets placed inside.
Advantages and Disadvantages
Advantages of Freeze-drying
- Easy preparation. Food is prepared for freeze-drying the same way food is prepared for regular freezing. After rinsing and removing blemishes, blanch most vegetables; pretreat fruits if necessary to prevent browning; meats can be freeze-dried cooked or raw; casseroles are typically assembled from already cooked foods. Place the product on the trays and push the button to start the machine.
- Freeze-drying can preserve foods other preservation methods cannot, such as most dairy or egg products.
- Freeze-drying can replace pressure canning of low acid foods.
- Storage. When finished, freeze-dried products are shelf-stable, lightweight, and food safe for longer other food preservation methods. Conservative food safety estimates of commercial food “canned” in metal-Mylar-type pouches is 8 to 10 years (Jahner & Nummer, 2008). This does not address the food quality after that time, only the food safety. However, no real data exists on the shelf-life of home freeze-dried products, because the company that invented and manufactured the first home freeze-dryer began sales in 2013.
- Nutrition. Nutrition labels of commercially freeze-dried broccoli, pineapple and cooked chicken chunks compare favorably to nutrient data of raw or commercially frozen products (see the USDA Food Composition Database).
- Taste. Freeze-dried products rehydrate more fully than dehydrated products, so the taste and texture are closer to fresh with a freeze-dried product than with a dehydrated product. •Cost. Home freeze-dried foods are substantially cheaper than commercially freeze-dried foods. Even including supplies and electricity costs, the commercial companies often have a mark-up of up to 85% more than a home-produced product .
Disadvantages of Freeze-Drying
During the freeze drying process, food products are flash frozen and then placed in a large vacuum chamber. Inside the vacuum chamber, temperatures reach as low as negative 50 degrees F. Minimal heat is then applied, causing ice in the food to evaporate without ever reverting back to liquid form. All moisture is thereby removed from the food product. Freeze dried foods quickly reconstitute with the application of warm water, regaining their original texture and appearance. The flavor and aroma of the food is also not affected.
One of the main disadvantages of freeze drying is the expense. The equipment need for the freeze drying process, which requires very low temperatures, can be quite costly. “It is usually carried out under vacuum, at absolute pressures that readily permit ice to change directly from solid to vapor,” according to The National Sustainable Agriculture Information Service.
Not all foods are able to withstand the freeze drying process. Freeze drying is only cost-effective for select foods like fruits and meats. Even foods that are able to endure the process can become damaged during the initial freezing. Freeze drying will preserve the color properties of foods, although this is not true in every case. Certain foods such as carrots need to be pretreated so they are not subject to color loss.
Storage problems can be another disadvantage to freeze drying foods. During the freeze drying process, foods keep their original shape and size, meaning that they may require extra storage space compared to other preserved foods. If freeze dried foods are not stored correctly, or if they are subject to humidity, they will deteriorate very rapidly. Freeze dried foods require storage in airtight, moisture-tight containers such as cans or bags. These products are also very frail, crumbling easily if not handled and stored with care.
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