Freeze drying occurs in three phases:
- Freezing
- Primary Drying (Sublimation)
- Secondary Drying (Adsorption)
Proper freeze drying can reduce drying times by 30%
Freezing Phase
Freezing is the most critical phase of freeze drying, and there are many methods for it. Freezing can be done in a freezer, a chilled bath (shell freezer) or on a shelf in the freeze dryer. Cooling the material below its triple point ensures that sublimation, rather than melting, will occur. This preserves its physical form.
Freeze drying is easiest to accomplish using large ice crystals, which can be produced by slow freezing or annealing. However, with biological materials, when crystals are too large they may break the cell walls, and that leads to less-than-ideal freeze drying results. To prevent this, the freezing is done rapidly. For materials that tend to precipitate, annealing can be used. This process involves fast freezing, then raising the product temperature to allow the crystals to grow.
Primary Drying (Sublimation) Phase
Freeze drying’s second phase is primary drying (sublimation), in which the pressure is lowered and heat is added to the material in order for the water to sublimate. The vacuum speeds sublimation. The cold condenser provides a surface for the water vapor to adhere and solidify. The condenser also protects the vacuum pump from the water vapor.About 95% of the water in the material is removed in this phase. Primary drying can be a slow process. Too much heat can alter the structure of the material.
Secondary Drying (Adsorption) Phase
Freeze drying’s final phase is secondary drying (adsorption), during which the ionically-bound water molecules are removed.By raising the temperature higher than in the primary drying phase, the bonds are broken between the material and the water molecules.Freeze dried materials retain a porous structure.After the freeze drying process is complete, the vacuum can be broken with an inert gas before the material is sealed.Most materials can be dried to 1-5% residual moisture.
FAQs
Freeze drying works by freezing the material, then reducing the pressure and adding heat to allow the frozen water in the material to change directly to a vapor (sublimate).
How does the freeze drying process work? ›
Freeze-drying, also known as lyophilization, is a process in which water in the form of ice under low pressure is removed from a material by sublimation. This process has found many applications for the production of high quality food and pharmaceuticals.
What foods cannot be freeze-dried? ›
Here are some of the few foods that aren't suitable for freeze drying:
- Fatty meats.
- Meat containing bones.
- Butter.
- Pure chocolate.
- Coffee beans.
- Honey.
- Jams and jellies.
- Mayonnaise.
The freeze dryer's vacuum speeds sublimation. The freeze dryer's cold condenser provides a surface for the water vapor to adhere and solidify. The condenser also protects the vacuum pump from the water vapor. About 95% of the water in the material is removed in this phase.
How is freeze-dried candy made? ›
The process includes a few steps.
- Freezing. When the sweets are placed on the trays of the freeze-drying machine, the machine will start lowering the temperature inside the chamber. ...
- Pressure reduction. Once these low temperatures are achieved, the machine will start creating a vacuum inside the chamber.
- Dehydration.
The freeze-drying process removes up to 99% moisture, resulting in preserved food that can last for 25 years or more. Dehydrating, on the other hand, only removes 85 to 95% moisture, resulting in a shelf life of several months to a year.
Can you freeze dry food at home? ›
You can freeze dry almost anything if you properly freeze dry the food, it can be good for up to 25 years. The flavor and nutritional value of the produce you harvest will remain and the process is fairly easy to do.
Can bacteria grow on freeze-dried food? ›
Preparing freeze-dried foods
It is important to note that, when rehydrated, any bacteria that were present on the raw food product prior to freeze-drying may become active again. The process of freeze-drying does not kill bacteria but rather slows bacterial growth by reducing its ability to access water.
What is the problem with freeze-drying? ›
Problems To Avoid During Freeze Drying
Condenser overload caused by too much vapor hitting the condenser. Vapor choking – the vapor is produced at a rate faster than it can get through the vapor port, the port between the product chamber and the condenser, creating an increase in chamber pressure.
How bad is freeze-dried food? ›
Does Freeze-Drying Change the Nutritional Value of Food? Freeze-drying doesn't affect the calorie count, the fiber content, or the overall amounts of minerals. It can make a difference with a few water soluble vitamins, such as folate and Vitamin C. But, most of the nutritional value will be maintained.
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4 more rowsYes, it is possible to freeze-dry your food without a machine using methods such as a home freezer, dry ice, or a vacuum chamber. However, results may vary, and these methods may not produce the same quality and efficiency as a dedicated home freeze-dryer.
How long does a freeze dryer last? ›
While it is true that some freeze dryers continue to lyophilize effectively for decades, the average life span of a freeze dryer in today's laboratory environment is approximately 10 to 15 years. Do you want to know the secrets to getting that much life out of your freeze dryer?
What candy Cannot be freeze-dried? ›
Candies with low moisture that don't seem to freeze-dry well include jelly beans, peppermint patties, and Oreos. Interestingly, Oreos are well-known in some circles to be better when left out for about a week to stale.
Is it healthier to eat freeze dried candy? ›
In general, freeze-dried candy contains the same amount of nutrients as regular candy, with less moisture.
Is freeze-dried food healthy? ›
It is a healthy food preservation method, as it retains most of the beneficial plant compounds and nutrients in foods, as well as their color, taste, and appearance. However, it doesn't kill harmful bacteria. Therefore, you should thoroughly cook freeze-dried raw foods like meat or eggs.
Why does stuff expand when freeze-dried? ›
There's a lot of fun science behind the process. To break it down in layman's terms, it expands due to the pressure in the machine. Because most candy is considered semi-solid, when the sublimation occurs and the moisture gasifies, there is nothing to keep it from blowing up like a balloon as the water dissipates.
What are the best foods to freeze-dry? ›
Let's dive a little deeper into some of the common food products you can freeze dry.
- Fruits. They are great to eat even without rehydrating them. ...
- Vegetables. The restaurant industry often uses freeze-dried vegetables. ...
- Meat. ...
- Dairy products and eggs. ...
- Drinks. ...
- Desserts and candy. ...
- Pasta and rice. ...
- Spices and herbs.
Freezing is the most critical phase of freeze drying, and there are many methods for it. Freezing can be done in a freezer, a chilled bath (shell freezer) or on a shelf in the freeze dryer. Cooling the material below its triple point ensures that sublimation, rather than melting, will occur. This preserves its physical form.