Microwave freeze drying of fruits and vegetables
Microwaves are electromagnetic waves in the frequency range from 300 MHz to 300 GHz. Within the electromagnetic spectrum, they are concluded between radio frequency at lower frequencies and infrared and visible light at higher frequencies. Thus, microwaves are classified as non-ionizing radiation. The microwave frequency range is also used for telecommunications such as mobile phones and radar transmissions. To prevent problems with interference, special frequency bands (called ISM frequencies) are reserved for industrial, scientific and medical applications. In accordance with these international regulations, the frequencies of 2450 MHz and 915 MHz are mainly used for industrial microwave devices.
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In addition to the regulations regarding ISM frequencies, there are additional safety regulations:
(A) Threshold value for maximum human microwave exposure
(B) Threshold value for maximum radiation from industrial microwave systems
For more information on the international safety standard, readers can refer to the safety standard IEC60519-6.
Limitations on the rate of heat transfer in conventionally conducted freeze drying operations have led to attempts to provide internal heat generation using microwave energy. In principle, faster drying times can be achieved with microwaves as they generate heat directly inside the product.
In simple terms, a microwave freeze dryer is a conventional freeze dryer with the added option of using microwaves in a drying chamber. The entire drying process takes place under vacuum conditions from 0.5 to 2.5 mbar by sublimation. Microwave freeze-dried products are similar in quality to conventional freeze-dried products. But compared to a conventional freeze drying system, which dries layer by layer, starting from the outside, the microwave system generates heat inside the product itself, so that freeze-drying occurs throughout the product.
Unlike the product itself, water has very low dielectric losses at temperatures below -10 ° C when frozen. Therefore, when using microwave freeze drying, the energy will be mainly absorbed by the organic molecules of the product. The dielectric loss of water can be neglected during the first phase of microwave freeze drying when the penetration depth is significant and energy can be transferred due to the dielectric loss of the organic product. For this reason, microwave freeze drying is much more efficient and faster than conventional freeze drying. Experiments and numerical predictions show that drying costs can be reduced using microwave freeze drying.
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Microwave freeze-drying offers the following benefits:
- Bulk microwave heating deep in the product layer, since the frozen water is almost transparent to microwaves;
- Fast energy dissipation throughout the material;
- Automatic adaptation of the absorbed microwave energy to the dielectric properties of products changing during the drying process;
- More efficient drying during periods of falling speed with low energy consumption
In conventional freeze drying, the heat gradient acts from outside to inside. This limitation of heat transfer results in long drying times, especially when water freezes inside fruits or vegetables, while their surface layer, already dry, becomes more insulating and limiting heat transfer. The biggest advantage of using microwaves is that energy is generated throughout the entire volume within the depth of penetration. When water is frozen at temperatures below -5 ° C, the penetration depth of microwaves becomes even higher, and they can penetrate deep into the mass of vegetables, whole fruits, as well as whole loads of products with a volume of up to 20-40 cm. Sublimation occurs in the entire volume of vegetables and fruit. This advantage has a huge impact on increasing the drying speed.
Рicture 1. Breakdown voltage versus atmosphere.
The design and manufacture of microwave freeze dryers is challenging as design errors can subsequently lead to the formation of plasma in the drying chamber. Plasma usually occurs when the electric field strength in the vacuum chamber exceeds the breakdown voltage. Ionization of the residual gases present in the vacuum chamber results in the appearance of plasma light, which causes burns on the surface of the product. This phenomenon causes significant energy losses and excessive heating of the dry zone of materials, seriously damaging the final product. The breakdown voltage threshold is vacuum dependent. The minimum is within the vacuum pressure range used in conventional freeze drying. Therefore, in order to avoid plasma discharges, it is necessary to optimize the pressure in the chamber, the process parameters and the microwave power, to ensure a good connection between the microwave generator and the vacuum chamber containing the product. To avoid the formation of plasma, the design of the vacuum chamber itself is also important.
Typical times for conventional freeze drying are 20 to 60 hours, depending on the type and amount of dried vegetables or fruits. In the final stages of drying, the product temperature rises above 0 ° C, because when only a small amount of moisture remains inside the product, the boiling point loses its influence on the product temperature. Temperatures from 0 ° C to 50 ° C are possible. When the final drying time is long, as with conventional freeze drying, the product is exposed to elevated temperatures for several hours. In addition, the thermal conductivity of the product becomes very low and it takes a long time to transfer heat to the center of the product. This is a problem, especially with whole fruits such as strawberries, raspberries, blueberries, etc. Due to excessive heating during this drying stage, volatile substances such as flavors, vitamins and antioxidants are attacked and broken down. The end product may still have excellent texture and good dehydration properties, but it has lost most of its vitamins, antioxidants and flavors. But with a total duration of only a few hours in an industrial microwave oven, exposure of the product at temperatures above 20 ° C is much shorter, which allows it to retain most of its vitamins, antioxidants and flavors.
Рicture 2. Penetration depth of a free wave of 2450 MHz and 915 MHz, penetrating into a dielectric body
The microwave frequency of 915 MHz is mainly used for large industrial installations, as magnetrons ranging from 30 to 100 kW are economically available. Also 915 MHz has about 2.7 times the penetration depth.
Рicture 3. Comparison of microwave vacuum drying and microwave freeze drying
Microwave vacuum drying and microwave freeze drying have one thing in common. It is difficult to maintain low temperatures in the final drying stage, because with a small amount of moisture in the product, the actual temperature will no longer match the evaporation temperature. The only way to avoid raising the temperature too high is to lower the energy density of the microwaves. Most foods with a high moisture content have dielectric properties ranging from ” = 1.0 to 30 at a temperature of about 25 ° C, which decreases from ” = 0.05 to 1.0 when frozen. Then the penetration depth increases from 1 cm to 10-20 cm at a frequency of 2450 MHz and to 27-54 cm at a frequency of 915 MHz.
Examples of Uses for Microwave Freeze Drying of Food
Drying root vegetables is interesting for both medical research and production, as they contain many vitamins, antioxidants and flavorings. Basically, the roots are very mechanically stable, they can be turned and stirred while drying. An example of a root with a strong aroma is horseradish. The experiment was conducted to evaluate the quality of dried horseradish in terms of appearance and taste. Horseradish was subjected to gentle freeze-drying in a microwave oven. It can be concluded that microwave freeze drying can provide better product texture as well as higher quality than conventional freeze drying.
Рicture 4. Drying parameters and snapshot of freeze-dried horseradish dried in a microwave oven
Table 1. Horseradish drying parameters
|Parameter||Data at the beginning||Summary data|
|Weight||346 g||110 g|
Drying time – 2 h
Maximum temperature – 5 ° C
Productivity is determined based on the initial moisture content of the raw material and the final moisture content of the product required by the consumer. This determines the amount of moisture to be removed. The amount of moisture to be removed is divided by the capacity of the installation.
Picture 5. Fresh fruits and their moisture content after drying
Radish is widely used as an ingredient in soups in the food industry. Texture, dehydration and taste are the main parameters for the food industry. The light white color and fine texture require gentle drying conditions. Nowadays, freeze-drying provides a product with excellent texture, in addition to avoiding large losses during processing. The drying time for this process is 20 to 50 hours. During this long drying period, the aroma of the product gradually disappears. This disadvantage can be avoided by microwave freeze drying, where the drying time of 2 to 4 hours is significantly shorter and better retains flavor. However, a shorter drying time also implies a lower dehydration ratio (see table 2).
Table 2. Comparison of drying parameters of various drying technologies for ordinary radish
40 ° С
10 ° C
40 ° С
Dehydration ratio *
* The degree of dehydration is measured by the water absorption of the dried product at 80 ° C hot for 1 minute compared to the original mass before that.
Picture 6. Microwave freeze-dried strawberries
There is a huge demand in the berry industry for high quality dried whole fruits. In conventional freeze drying, strawberries are dried mainly in slices to provide significant heat transfer from the heated shelves to the product. Whole strawberries are very difficult to dry using conventional freeze-drying methods as the drying time and energy requirements for such processes are extremely high on an industrial scale. But freeze-drying strawberries in the microwave can provide excellent textural properties as well as good energy and heat transfer rates. Below are the drying parameters. On fig. 6 shows the texture of strawberries after 3 hours freeze-drying in the microwave. 3 kg whole strawberries were dried in an open drum.
Starting weight – 3 kg. Total weight – 0.51 kg
Drying time – 3 h
Maximum temperature – 42 ° C. Applied microwave energy – 2.81 kWh
The freezing process has a great influence on the quality of the dried product. In addition, the freezing rate affects the size of the ice crystals. The faster it freezes, the smaller the crystal structure and the better the fine structure of the product will be preserved. On the picture 7 can compare the texture properties of frozen and freeze-dried strawberries dried in an industrial microwave oven
Picture 7. Whole strawberries, freeze-dried in the microwave (left), cut after drying to show their internal texture and structure (right).
As shown in Picture 7, If there are frozen pieces of ice or areas with a higher water content in the entire volume of strawberries, they will affect the freeze-drying process in the microwave and the quality of the dried product. Due to the uneven distribution of the electromagnetic field and due to differences in product consistency, some parts of the strawberry can absorb a higher energy density and increase the temperature. When the product temperature exceeds the boiling point of -5 ° C, the dielectric loss of the frozen water increases significantly. As a consequence, the temperature will rise and the frozen water will become liquid, causing pinpoint high absorption of microwave energy. Water immediately evaporates in the bulk of the product, forming bubbles that destroy the original structure. Strawberries are a good example to demonstrate this. Therefore, there is a requirement for uniform product properties such as size, shape and frozen state, as well as for microwave energy to be as uniform as possible in order to ensure an even transfer of microwave energy into the product. For this reason, product movement during drying is also important to ensure that energy is absorbed evenly by each individual product. The vacuum level also plays a key role. The lower the vacuum, the safer the process will be, thanks to the safety distance to the critical temperature of -5 ° C. Of course, low vacuum also means high costs. So you need to find a reasonable compromise. It should be borne in mind that sugar content also has a huge effect and increases the boiling point. To cope with products with high BRIX values, the vacuum must be as low as possible, taking into account economic feasibility. Another limitation is the outer casing, which acts as a diffusion-limited membrane. Evaporation through this membrane is limited and reduces moisture diffusion and mass transfer. To overcome this limitation, the surface of the strawberry is finely perforated; this will provide better spread and faster drying rates.
Рicture 8. Freeze-dried banana
Bananas are in high demand, mainly as an ingredient in cereals and snacks. Drying banana slices is difficult because they are mechanically very unstable and oxidize quickly. However, when frozen, they are easy to handle and stable enough for microwave freeze drying in a drum. There can also be a combined freeze drying process with a 1 mbar microwave as the main drying step and vacuum drying with a 20 mbar microwave for the final drying. During vacuum drying at 20 mbar, a blistering effect can be achieved, increasing the product volume by 2-5 times.
Commercial Potential / Viability of Microwave Freeze Dryer for Food
In the coming years, the demand for quality food will increase. The quality of microwave-dried products under vacuum is almost the same as freeze-dried products, especially when high-quality texture and fast dehydration are required. Microwave freeze dryers are superior to conventional freeze drying systems in terms of quality and preservation of valuable ingredients. In industrialized and developing countries, food producers will gain a better position than global food companies, for example, if they can store and process their food after harvest. Thus, farmers will become more independent from seasonal prices and will be able to bring high quality dried produce to the market using microwave technology with microwave freeze dryers, which require less floor space and lower costs than conventional freeze dryers.
On the consumer side, people are more aware of healthy food and are willing to pay higher prices for high-value foods. In today’s global marketplace, consumers find a wide variety of mostly conventional freeze-dried foods such as packaged fruit and vegetable snacks offered by niche suppliers on the Internet. You may also notice that regular supermarkets are more likely to offer freeze-dried snacks as specialty items. The demand for such products is constantly growing. While traditional freeze drying technology will come under more pressure due to increased energy costs and such a rise in steel prices, it will be partially replaced by faster, more compact and more efficient microwave freeze dryers.
Picture 9. Vacuum microwave oven 3-12 kW with 4 chambers “Samun”
Strawberry and raspberry case studies were conducted in an experimental 6 kW / 2450 MHz microwave freeze dryer. The largest commercial microwave freeze dryer is shown in Figure 10 below.
Рicture10. 24 KW Industrial Microwave Freeze Dryer
The use of microwave freeze drying has already begun in the high-end ceramic, pharmaceutical and chemical industries. Microwave freeze drying will soon enter the food industry, starting with areas where drying and production rates range from 5 to 100 kg / h for high value products such as herbs, roots (ginseng), as well as fruits and high value products. , ingredients for soup.
The concept of microwave freeze drying is based on a smaller footprint, higher energy efficiency and better quality in terms of volatiles compared to traditional freeze drying with long vacuum vessels and huge product loads. The use of microwave freeze drying in industrial drying systems is generating growing interest, especially because of its high energy efficiency and potential for results.
In the 21st century, consumers have an increased awareness of healthy ingredients and the aesthetic appeal of processed foods and are willing to pay higher prices for high quality food. With the understanding of the medical importance of vitamins, antioxidants and flavors, microwave freeze-dried products can be ranked higher as they retain much more valuable nutrients than conventional freeze-dried products
What’s more, due to rising energy and processing costs and rising steel prices, traditional freeze drying technology will come under more and more pressure and will soon be partially replaced by faster, more compact and more efficient microwave freeze drying machines. dryers.