Table of Contents
Introduction
The presence of water on Earth is one of the most essential factors for sustaining human life. However, moisture sometimes acts like a double-edged sword, leading to the spoilage of food, medicines, and more. Throughout history, humans have sought ways to properly preserve perishable foods and other spoilable materials. Methods such as using salt or drying products in sunlight have been employed, each with its own drawbacks.
With technological advancements, industrial and household refrigerators were introduced, but they couldn’t adequately preserve the quality of frozen materials over extended periods. In the mid-20th century, freeze-drying, also known as lyophilization, revolutionized the storage and freezing of food and pharmaceuticals. If you’ve been curious about these machines and how they work, join us. Rest assured, many of your questions will be answered by the end of this note.
What Freeze Drying is
Freeze-drying, or lyophilization, is a process where, after freezing the product, the water present in it is removed, and the product is placed under vacuum. Subsequently, the ice directly transforms from a solid to vapor (sublimation). This process is composed of three distinct and interdependent phases: freezing, primary drying (sublimation), and secondary drying (desorption).
Lyophilization leverages the scientific principle of sublimation, meaning the direct transition of a solid to a gas, by removing ice (solid) from frozen food and converting it into water vapor (gas). Using sublimation, the product retains its texture, flavor, and essential nutrients, provided it is rehydrated appropriately.
Freeze-drying can be divided into two simple processes: freezing and drying under vacuum. Initially, products are frozen to approximately -40 degrees Celsius. After freezing, the products enter the vacuum drying stage. Air and water vapor (gas) are removed from the processing chamber using a vacuum pump. This action involves eliminating the water vapor that was previously inside the products. When these two stages are executed correctly, the product becomes adequately dry, allowing for safe storage at room temperature.
Components of a Freeze Dryer
- Main Chamber: This part, made of stainless steel, is polished and insulated, with a structure suitable for loading products classified inside.
- Condenser: Comprised of pipes, it can be located inside or outside the main chamber. Its primary function is to absorb the water vapor produced during the sublimation of products. During the primary drying stage, the condenser temperature must always be about 20 degrees Celsius lower than the products.
- Shelf Fluids (Silicone Oil): The amount of heat energy required during the primary and secondary drying stages is regulated by an external heat exchanger. Silicone oil circulates through the device with the help of an oil pump.
- Cooling System: This part of the freeze dryer uses liquid nitrogen or compressors to cool the shelves and condenser. It provides the energy needed to freeze the product.
- Vacuum System: This section utilizes a two-stage vacuum pump to reduce the pressure inside the chamber to 50 to 100 microbars, lowering the air pressure inside.
- Control System: The control system regulates controlled values for temperature, pressure, and the working time of the shelves, depending on the product or process being performed. A freeze dryer can work for several hours or even days, depending on the product.
How a Freeze Dryer Works
The fundamental principle in freeze dryers is sublimation, the direct transition from a solid to a gas. Similar to evaporation, sublimation occurs when a molecule gains enough energy to break free from the molecules around it. When the molecules have enough energy to break free but the conditions are not suitable for forming a proper liquid, the ice (solid) transforms into water vapor (gas). This is precisely what a freeze dryer accomplishes.
A typical device includes a freeze dryer chamber with several shelves connected to heating units, a condenser freeze unit linked to a cooling system, and a vacuum pump. In most devices, before the freezing process begins, the products to be dried are placed on the shelves. When the main chamber door is closed, and the freezing process starts, the compressors are activated to lower the temperature in the chamber. The frozen substance acts as a solidified material that, on a molecular level, separates water from anything around it.
Then, the vacuum pump is turned on to remove air from the chamber and reduce the pressure to below 0.06 atmospheres. The heating units apply a small amount of heat to the shelves, causing a change in the ice’s phase. Because the pressure is very low, the ice directly transforms into water vapor. The water vapor exits the freeze dryer chamber and passes through the condenser pipes. The water vapor, just as water compresses into ice on a cold day, condenses into solid ice on the condenser pipes.
This process continues for several hours or even days, gradually drying the materials. The process is lengthy because excessive warming of the materials can significantly alter their chemical and physical structure. Additionally, accelerating the sublimation process can generate more water vapor in a given period, potentially causing hydration and reducing the final product’s quality.
When the materials are adequately dried, they are vacuum-sealed in special packaging. If the packaging remains intact, the products can be stored for years without issues, without the need for preservatives, at room temperature. To use them, a simple rehydration is required. If everything works correctly, the materials will go through all stages almost completely and without damage.
Applications of Freeze Dryers
The use of freeze dryers has fewer undesirable effects on products compared to other drying methods. For example, it does not cause shrinkage or hardening of the product, and rehydrated products are easily usable. Frozen food products usually do not lose their taste, aroma, and color, which is an excellent feature for evaluating the performance of freeze dryers. However, it’s crucial to note that during sublimation, water is not the only substance that evaporates; the disappearance of alcohol and citric acid present in the materials can have undesirable effects.
Hydrating a product creates microscopic pores in it. These pores are formed by ice crystals that sublimate and remain in place during the subsequent sublimation. This is especially important for pharmaceutical purposes. Freeze-drying can also be used to extend the shelf life of some medications for several years. In summary, let’s briefly mention a few applications of freeze dryers.
Pharmaceuticals and Biotechnology
Pharmaceutical companies often use freeze dryers to increase the shelf life of products such as vaccines containing live viruses and other injectable substances. By freezing the materials in a glass vial, they can easily store, transport, and later prepare it for injection by rehydrating it. Examples of frozen biological products include many vaccines such as the live attenuated virus vaccine, smallpox vaccine, and meningococcal polysaccharide vaccine groups A and C. Other frozen biological products using freeze dryers include factor VIII for hemophilia, alpha interferon, streptokinase blood clot medication, and more.
It’s interesting to note that dried powders of probiotics, often containing freeze-dried live microorganisms like lactic acid bacteria and bifidobacteria, are produced using freeze-drying.
Food Industry
The primary goal of using freeze dryers in the food industry is to extend shelf life while preserving quality. Freeze-drying in food results in the highest quality but also imposes the highest cost on the producer. For example, freeze dryers are used in storing seasonal fruits, vegetables, coffee, military rations, astronaut food, and mountaineering provisions. It’s worth noting that during the conventional drying process, strawberries lose their taste, color, and shape due to their delicate structure and high water content. However, with the help of freeze dryers, the highest possible quality in terms of taste, color, and rehydration capability can be achieved.
To view various types of food freeze dryers from Takvin Azmayesh Parseh company, click on the link below.
Samples of dried ice cream prepared by freeze dryers from the Parseh Industrial Group
Industrial Technologies
In chemical synthesis, most products are freeze-dried to have greater stability and dissolve more easily in water for subsequent use. In biological separation, freeze-drying can be used as a purification method in the final stages, effectively removing solvents. However, it should not be forgotten that due to the high cost of freeze dryers, freeze-drying is often used for heat-sensitive materials such as proteins, enzymes, microorganisms, and blood plasma. The low temperature of the freeze-drying process minimizes damage to heat-sensitive products.
In nano-technology, freeze dryers are used to purify nanotubes to prevent pressure build-up resulting from capillary forces during regular heat drying.
Taxidermy
Freeze-drying is one of the methods used in taxidermy to preserve the bodies of animals. When animals are preserved using this method, they are called freeze-dried taxidermy. Freeze-drying is commonly used to preserve hard-skinned animals, fish, reptiles, insects, and smaller mammals. Freeze dryers are also used as a means to preserve the bodies of household pets after death. Many pet owners choose freeze-drying over traditional taxidermy for preserving their pets as it inflicts less damage to the animal’s body.
Advantages and Disadvantages of Freeze Dryers
Advantages of Freeze Dryers:
- Extended Shelf Life: The low-temperature processing and conversion of water to water vapor through sublimation contribute to the prolonged preservation of food products. Proper packaging, coupled with this method, allows for the preservation of food for years.
- Maintaining Material Quality: Freeze-dried foods retain their original form and have a desirable taste and aroma.
- Rehydration (Water Reabsorption): For optimal product quality, water reabsorption must be maximized. Freeze dryers facilitate the absorption of 85% of water, placing the product in ideal conditions.
- Easy Storage and Transportation: This method allows for lightweight and easily transportable products.
Disadvantages of Freeze dryers:
- Relatively Long Processing Time: The freeze-drying process takes a considerable amount of time.
- Packaging Requirements: Freeze-dried products need to be packaged in three layers and sealed.
- Lower Production Capacity Compared to Heat Dryers: Freeze dryers generally have lower production capacity compared to heat dryers.
Types of Freeze dryers:
Contact Freeze-Dryer:
Contact freeze dryers use the contact of food materials with a heat element to provide energy for sublimation. One of the main ways these freeze dryers operate is through the contact of shelf-like structures with the products. In this process, the shelves play a crucial role, acting as heat exchangers at different stages of the freeze-drying process. The shelves are connected to a silicone oil system that removes thermal energy during freezing and supplies energy during drying.
A weakness of this type of freeze-dryer is that heat is transferred only through direct contact with the sample immediately in contact with the vapor. This issue can be minimized by maximizing the surface area of the sample in contact with the heat element, using a tray with spacers, minimal compression of the sample between two heated solid plates, or compression with a heated mesh from above and below.
Radiation Freeze-Dryer:
Radiation freeze dryers use infrared rays to heat the raw material in trays. This type of heating allows the use of simple flat trays, as the infrared source can be placed above the flat trays and radiate onto the product. Infrared radiation provides very uniform surface heating of the product. However, it should be noted that infrared has very low penetration power, so shallow trays and homogeneous materials should be used.
Microwave-Assisted Freeze-Dryer:
Microwave-assisted freeze dryers use microwave waves to penetrate the sample more deeply, accelerating the sublimation process and heat in freeze-drying. The implementation of this method is complex, as microwave waves can create an electric field capable of turning gases in the sample chamber into plasma. This plasma, potentially, can burn the sample; therefore, maintaining proper microwave power is crucial to avoid such incidents. The rate of sublimation in a product can affect the microwave impedance, and if this happens, the microwave power must change proportionally to it.
Contact Us:
For the price of freeze dryers and economic considerations, it’s important to note that freeze-drying is relatively expensive compared to other drying methods due to the use of sophisticated and up-to-date technology in manufacturing these devices. Freeze dryers are best suited for products with high economic value.
Final Note:
In this article, we tried to describe everything that a user needs to know about the freeze dryers. If you intend to purchase a certain type of freezer, you will need more serious and specialized studies in this field. In Iran, freeze dryers are produced and assembled by the knowledge-based company Takvin Azmayesh Parseh, which makes acquiring more precise information about your needs easier.