It focuses on a comparative experimental evaluation of diverse drying techniques ranging from traditional hot air drying, vacuum drying, and freeze drying to microwave vacuum drying and microwave freeze drying of fruits, vegetables, and aquatic products. Brief Introduction of Tunnel Microwave Dryer of Fruit and Vegetables. To cope with products with high BRIX values, the vacuum should be as low as possible, taking into account the economic feasibility (Mageean et al, 1991). This is a problem especially with whole fruits like strawberries, raspberries, blueberries etc. The handling of banana slices is difficult as they are mechanically very unstable and oxidate quickly. Fruits such as grapes don't require this treatment, as they are small and their peels will shrink down with them. The lab scale microwave vacuum dryer offers a freeze drying option. Risks and Hazards Caused by ELECTROSTATICS in the LAB?! The quality of microwave freeze dried products is similar to conventional freeze dried products concerning texture. Therefore by the use of microwave freeze drying energy will mainly be absorbed by the organic molecules of the product. Whole microwave freeze dried strawberries (left) which were cut after drying to demonstrate inner texture and structure (right). The final product might still offer excellent textures and good rehydration properties, but it has lost most of its vitamins, antioxidants and flavours. This limitation in transferring heat is causing the long drying time, especially when water is sublimated within fruits or vegetables while their surface layer, being already dry, is becoming more isolating and restricting for the heat transfer. Basically a faster drying time can be achieved with microwaves because they generate the heat directly inside the product volume. In Industrial- and developing countries, the food producers will gain a better position alongside the global food companies such a, if they will be able to preserve and process their products after harvest. Table 2: Drying parameters in the drying of strawberries. The quality of microwave vacuum dried products is almost similar to freeze dried products, especially when high quality texture and fast rehydration are required. Peel it if desired and core and slice it thinly. To overcome this limitation, strawberries are finely perforated on the surface; this will enable a better diffusion and a higher drying rate. Microwave Vegetable Drying Machine Introduction Microwave vegetable drying machine adopts microwave to dry fresh or seasonal vegetables, meanwhile achieving the effect of sterilization, greatly extending their shelf life. The moisture content of some fruits and vegetables including apples, pears, strawberries, bananas, and avocados, potatoes, carrots, onions, lettuce, and cabbage, was determined using a microwave oven. However in frozen condition they are good to handle and are stable enough for microwave freeze drying in a drum (see picture 15). Due to this fact, most of the microwave freezes drying processes are within the range of 1-4 hours to dry from 70-98% initial moisture to the range of 5-10%. Sublimation occurs throughout the complete volume of vegetables and fruits. The light white colour and sensitive fine structure require gentle drying conditions. The demand for high quality food will increase during the coming years. For 20-g minced samples of fruits and vegetables, these three methods yielded equivalent results, but usage of a microwave oven substantially shortened the … This is the reason why microwave freeze drying is much more efficient and faster than conventional freeze drying. This disadvantage can be avoided by microwave freeze drying where drying times from 2-4h are significantly shorter with better flavour retention. Table 2 shows the drying parameters. Process control parameters in microwave freeze drying of raspberries, Figure 15. [2,8,9,10] In recent years, microwave drying has gained popularity as an alternative drying method for a wide There is a high demand for bananas, mainly as a ingredient for cereal and snacks. Figure 3. Experiments and numerical predictions show that drying costs can be reduced by using microwave freeze drying (Wu et al, 2004; Duan et al, 2010). The IR camera can measure from -50 to 200°C. The first of four trials shown below was carried out in this lab microwave freeze dryer configuration as shown in Figure 18. Ionization of residual gases present in the vacuum chamber leads to the appearance of plasma light, causing burns on the product surface. Microwave related drying (MW-hot air; MW-vacuum; MW-freeze; MW-osmotic) combination drying is a rapid dehydration technique that can be applied to specific foods, particularly to fruits and vegetables. Strawberries are a good example to demonstrate this effect (see figure 10). So a reasonable compromise needs to be found. Drying fruit preserves the majority of the nutrients, and it's great to use in baked goods and granola. Good candidates for this process are hearty, juicy fruits such as apples, mangoes and pears, although grapes would also work. Slice fruits and vegetables thinly. Once the product temperature exceeds the boiling point of -5 °C, the dielectric losses of frozen water increase significantly. However, the shorter drying time means a compromise in terms of volume and rehydration water characteristics (see figure 8 and table 1). Regarding quality and preservation of valuable ingredients the microwave freeze dryers are superior to conventional freeze drying systems. Process control parameters in microwave freeze drying of banana slices, Commercial Potential/Viability of Microwave Freeze Drying for Food Applications. Set the microwave to defrost and microwave the fruit, flipping occasionally, for 30 minutes. Increasing concerns about product quality and production costs have motivated the research to investigate and the industry to adopt combination drying technologies. The greatest advantage applying microwaves is that the energy is generated throughout the volume within the penetration depth. But compared to a conventional freeze drying system, which dries layer after layer starting from the outside, the microwave system generates heat within the product itself so that sublimation is taking place within the complete product volume. Figure 10. Due to excessive heating in this drying stage the volatiles like flavours, vitamins and antioxidants are attacked and cracked. The minimum is within the vacuum pressure range used by conventional freeze drying (see picture 2). Fig.5.microwave /vacuum drying ... the viscous natures of fruits and vegetables increase while their elasticity de- creases due to the sugar uptake. Microwave freeze dried raspberries, Figure 14. This advantage has a tremendous impact on the increase of drying speed. Figure 12. In order to cover the invest costs farmer may join into unions to share the production and drying facilities. Figure 9. The lower the vacuum, the more safe the process will be due to the safety distance to the critical temperature of -5°C. Moreover, heat conductivity of the product becomes very poor and it takes a lot of time to transfer heat into the product centre. Dehydrate at a temperature of 140 degrees Fahrenheit for anywhere between five to thirty hours. In addition to the regulations concerning ISM Frequencies, there are further safety regulations: (A)   Threshold value for maximum microwave exposure of a human, (B)   Threshold value for the maximum emission of industrial microwave systems. Microsoft Internet Explorer 6.0 does not support some functions on Chemie.DE. 3kg of frozen whole strawberries in an open drum (left) and after microwave freeze drying (right). The concept of microwave freeze drying is based on smaller footprint, higher energy efficiency and better quality in terms of volatiles compared to conventional freeze drying with long vacuum vessels and huge product loads. The nutritional components of vegetables are retained to the maximum. The occurrence of this phenomenon is responsible for considerable energy losses and excessive heating of the materials dry zone, seriously damaging the final product (Arsem and Ma, 1985; Gould and Kenyon, 1971). In general 2kW of microwave power will require about 2kW chilling power of the condenser. Douse any fruits that may turn brown (such as apples) with lemon juice diluted with water (about a tablespoon of lemon juice to a cup of water). The whole drying process proceeds under vacuum environment of 0,5 to 2,5mbar by sublimation. One more limitation is the outer peel which is working as a membrane with limited diffusion capacity. Microwave drying is based on a unique volumetric heating mode facilitated by electromagnetic radiation at 915 or 2,450 MHz. Whole strawberries are very difficult to dry with conventional freeze drying methods as drying time and energy requirement for such processes on an industrial scale are extremely high. All in-text references underlined in blue are added to the original document and are linked to publications on ResearchGate, letting you access and read them immediately. Challenges. Figure 18. For further information on the international standard for safety, readers can refer to the IEC60519-6 safety standard. Place pieces on drying racks of the dehydrator, without allowing them to overlap or touch. analytica-world.com is the industry portal of the international trade fair analytica, which is organized by Messe München. ¹ The Dehydration Ratio is measured by the water uptake of the dried product in 80°C hot for 1min in comparison to the initial weight before. Dry it thoroughly. Figure 4. Microwave vacuum drying and microwave freeze drying have one thing in common. Figure 5. Due to this fact, most of the microwave freezes drying processes are within the range of 1-4 hours to dry from 70-98% initial moisture to the range of 5-10%. On the consumer side people have an increased awareness of healthy food and are willing to pay for high value food with higher prices. Of course a low vacuum also means high costs. This paper demonstrates the weakness of common drying methods applied for fruits and vegetables and the possible ways to improve the quality using different drying techniques or their combination with an emphasis on the microwave energy. Wash and dry the fruit. But Microwave freeze drying of strawberries can achieve excellent textural properties as well as a good energy and heat transfer rate. Figure 1: Position of microwaves in the electromagnetic spectrum in relation to the dielectric losses of water at 25°C. The combination of microwave and vacuum drying also has a certain potential. 2.Microwave drying equipment for fruits and vegetables has better elasticity, it could reduced viscosity, combined with vacuum drying,increased elasticity more significantly. In today’s global market place consumers find a wide selection of those mainly conventional freeze dried products like packed fruit & vegetable snacks offered by niche suppliers on the internet. The threshold value of the break down voltage is depending on the vacuum. In the final drying phase it is difficult to keep the temperatures low because with little moisture left in the product the actual temperature will not follow the evaporation temperature any more. Generally Plasma will occur if the electric field intensity in the vacuum chamber is exceeding the break down voltage. This advantage has a tremendous impact on the increase of drying speed. The vacuum level also plays a key rule. Drying of Fruits & Vegetables using Püschner Microwave Vacuum Dryer, µWaveVac 0150 Drying is the oldest method of preserving food and account for majority of processes in food industry. It is operated by LUMITOS AG, one of analytica's official sales partners. Your browser does not support JavaScript. However this limitation was solved by Püschner microwave freeze dryer designs. In contrast to the product itself water has only very low dielectric losses when in frozen condition below -10°C. A chiller provides a cooling power of about 1.5 kW at -50°C to re-condense the steam within the condenser. Texture, rehydration and flavours are the main parameters for the food processing industry. The dryer machine can be used for the dehydration of many kinds of vegetables, such as carrots, cabbage, To use all the functions on Chemie.DE please activate JavaScript. Textural properties of frozen strawberries (left), microwave freeze dried (middle) and partly freeze dried strawberries (right). Radish is widely used as a soup ingredient in food industry. Soon microwave freeze drying will penetrate into the food industry starting with applications where the drying capacities and throughput rate are in the range of 5 to 100kg/h of high value product like medical herbs, roots (Ginseng) and also fruits as well as high value soup ingredients. Due to those international regulations for industrial microwave applications mainly the frequencies 2450MHz and 915MHz are being used. Lay the fruit directly on the microwave turntable. Also 915MHz has about a 2.7x bigger penetration depth (see Figure 4). As shown in Figure 10, if there are frozen ice pieces or areas with higher water content throughout the volume of the strawberry, they have an impact on the microwave freeze drying process and quality of the product to be dried. Figure 11. For this reason moving the product while drying is also essential to provide an even energy uptake in each single product. There can also be a combined drying process of microwave freeze drying with 1mbar as main drying step and microwave vacuum drying with 20mbar for end drying. Also important to avoid plasma is the design of the vacuum chamber itself (Duan et al, 2010). Then the penetration depth increases from 1 cm to over 10-20cm at 2450MHz and to 27-54cm at 915MHz. Figure 10 shows texture of strawberries after a 3 hour microwave freeze drying process. With the mature of high-power magnetron production, microwave drying has been widely used as a clean and efficient method in the field of fruits and vegetables processing. Figure 7. Check the fruit. Figure 16. microwave drying of fruits and vegetables results in high temperature efficiency, shorter drying time and result in better product quality compared to conventional hot air ventilation drying. Food dehydrators can be pricey, and buying pre-packaged dried fruit can add up over time. Thus farmers will become more independent from seasonal prices and will be able to offer high quality dried products to the market using microwave technology with microwave freeze dryers which need smaller footprint and lower costs than conventional freeze dryers. Make sure to evenly space the fruit to allow the air to circulate around it. This technique decreases the cost of storage, packaging, and transport of fruits and vegetables. 3kg of complete strawberries were dried in an open drum. The application of microwave freeze drying in industrial drying systems is of growing interest, particularly because of the high energy efficiency and achievement potential. Basically roots are mechanically very stable and can be tumbled and mixed during drying. The limitations on heat transfer rates in conventionally conducted freeze drying operations have led to attempts to provide internal heat generation by the use of microwave power (Burke and Decareau, 1964; Copson, 1962). The most important thing is that you are eating your fruits and veggies! It can be concluded that microwave freeze drying can produce a better product texture as well as higher quality compared to conventional freeze drying. In the end drying stages the product temperature rises above 0°C, because when only little moisture is left inside the product the boiling point looses its influence on the product temperature. The Horse radish was subjected to gentle microwave freeze drying. 24kW Production scale microwave freeze dryer. Due to the uneven distribution of the electromagnetic field and due to differences in the product consistency, some parts of the strawberry may absorb higher energy densities and will rise in temperature. The power of the microwave was in the range of 115 to 285W and a pressure range of 6.523.5 KPa, the mushroom was sliced to a thickness of 614 mm. In conventional freeze drying, strawberries are mainly dried in slices in order to maintain a significant heat transfer and mass transfer from the heated plates / shelves into the product. The length of dehydration will depend largely on the type of food you are drying. The case studies with strawberries and raspberries were carried out in a 6kW/2450MHz microwave pilot scale freeze dryer. Though fruit can be dried in an oven or food dehydrator, drying fruit in the microwave is a quick alternative. Within the electromagnetic spectrum (shown in Figure 1) they are embedded between the radio frequency range at lower frequencies and infrared and visible light at higher frequencies. Moreover, due to the increase of energy cost and processing cost and also increasing steel prices the conventional freeze drying technology will get more and more under pressure and will be partly replaced by faster, smaller and more efficient microwave freeze dryers in the time to come. As the above mentioned temperatures are based on normal tap water, it should be considered that the sugar content has also a huge impact and will increase the boiling point. Figure 19. Drying Parameters and snapshot of microwave freeze dried horse radish. During the vacuum drying at 20mbar a puffing effect can be achieved, extending the product volume by factor 2-5 (see picture 16). Follow these five steps to have a delicious dried snack! Microwaves are electromagnetic waves within a frequency band of 300 MHz to 300 GHz. Comparison between microwave vacuum drying and microwave freeze drying. Tunel microwave dryer is a dryer which uses microwave to reduce the moisture from raw material Unlike other external heating drying, microwave dryer for spice can dry the raw material from internal and external at the same time. Microwave freeze drying offers the following advantages: In conventional freeze drying, the heat gradient is working from outside to inside. The faster it will freeze, the smaller the crystal structure and the better fine product structures will be kept. dried fruit food hacks fruit kitchen hacks If you don’t have an appliance dedicated to dehydrating your favourite healthy snack, the defrost setting on your microwave can work in a pinch. Frozen banana slices in a drum. Microwave (MW)-related (MW-assisted or MW-enhanced) combination drying is a rapid dehydration technique that can be applied to specific foods, particularly to fruits and vegetables. Microwave frequency of 915MHz is mainly used for big industrial installations because magnetrons are economically available from 30kW up to 100kW. Therefore there is a requirement for homogeneous product properties such as size, shape and frozen condition as well as for the microwave energy to be as uniform as possible to enable a uniform microwave energy transfer into the product . 2kW/2450MHz microwave freeze dryer. With the understanding of the medical significance of vitamins, antioxidants and flavour, microwave freeze dried products can gain a superior position as they retain much more valuable nutrients than conventional freeze dried products. Our advertising formats support you in getting your message across to a clearly defined target audience. Table 1: Comparison of drying parameters of various drying techniques for drying of common radish. The Raspberries were microwave freeze dried in an open drum, Figure 13. 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