Convective microwave-vacuum drying installation

Convective microwave-vacuum drying installation

    The process of removing moisture from materials using heat energy to evaporate moisture and removing the resulting vapor is called drying. According to this definition, drying is a diffusion process, since the transfer of moisture from the material to the environment occurs during surface evaporation of moisture and its movement from the inner layers to the surface of the material.

    Biological materials must be dried at a low temperature,  since a slight increase in it causes a sharp deterioration in their features. Choosing a temperature range for drying biological materials, it is necessary to keep in mind the following.

    For all materials with an increase in the heating temperature, the specific heat capacity increases. For water in the range from 0 ° C to 37 ° C, the heat capacity decreases, and in the range from 37 ° C to 160 ° C it increases. This explains the fact that all living organisms and biological materials exist and retain their properties in this most energetically economical range from 35 ° C to 45 ° C. Therefore, in order to maintain the properties of organic substances during their dehydration, it is desirable to work in this particular range.

    Taking into account the world experience of the developed processes of drying biological materials in leading countries, Germany and the USA, it is optimal to implement a new drying technology to the Samun convective microwave-vacuum drying unit by GreenPower, which carries out the process of moisture removal in two stages, taking into account the biological characteristics of plant raw materials:

1) Unbound moisture is removed in a vibration-convective (pseudo-boiling layer) drying unit;

2) Bound moisture is removed in a microwave-vacuum drying unit.

This technology is a combination of two methods of dehydration: conventional (convection) and dehydration in microwave – vacuum drying installation, which puts a new and innovative method on a fundamentally new level in contrast to existing methods of drying products, excactly:

  • reduce the dehydration time of products by more than 95% from days and hours to minutes;
  • products retain their natural color, taste, texture and composition of nutrients;
  • electricity consumption reduces by up to 70%, and capital costs per unit of manufactured product – by more than 80%.

In the first stage of drying, surface moisture is removed. Thermal energy is spent on heating raw materials and on evaporation of moisture. The duration of the first stage is insignificant in comparison with the second stage. Physically, the first stage ends after the removal of free moisture, which is removed by creating a suspended swirling layer or by the “suction” method. The optimal parameters of moisture removal during convective drying are the temperature of the heat carrier 70 – 120 ° С and the end of the process at a humidity level of 40-50%. The resulting layer of dried product resists both the transition of steam from the vaporization zone to the surface of the product, and the transfer of heat energy from the outside to the vaporization zone, which is excluded during the two-stage process, since the formation of a layer of the dried product is excluded due to uniform moisture removal throughout the volume of the dried material .

The second stage is the removal of bound moisture. The second stage is accompanied by an increase in the temperature of the material, and the temperature of the material tends to the temperature of the coolant, which disadvantageously affects the nutrients of the raw material. If the temperature is not reduced to a safe value, then the main nutrients are destroyed, and most importantly, biologically active substances. However, a decrease in temperature during convective drying is accompanied by an increase in the duration of the process, due to the difficulty of removing moisture from the inner layers of raw materials. It is possible to intensify the process in the second stage by slowly decreasing the pressure, thereby harshly increasing the intensity of evaporation, significantly reducing the time of the second stage.

    Since vacuum drying takes place in a hermetically closed installation, the heat transfer by convection is small. Therefore, in order to maintain a substantial intensity of drying in a vacuum, heat energy in a sufficient amount must be continuously supplied to the vaporization zone. The supply of thermal energy to the vaporization zone by supplying microwave radiation does not provoke any difficulties. The biomaterial is processed by microwaves in a vacuum environment, where heating does not take place on the surface, but throughout the entire volume of the product containing polar water molecules, since radio waves penetrate deep enough. This shortens the heating time of the food. The cellular structure inside the product remains unchanged, as a result of which the product largely retains its stable shape. Because microwave radiation heats food very quickly, the degradation of vitamins in food is reduced.

    Laboratory studies have shown that microwave radiation has a sterilizing effect against staphylococci, Escherichia coli and other microorganisms. The reason of this effect is that the temperature inside the food rises very quickly, while the proteins of the microorganisms are heated synchronous. The so-called “heatstroke” occurs, which destroys microorganisms.

    At the stage of removing residual moisture, the most important factors for high product quality are the duration of impact to the heightened temperatures and its maximum value. During the period of removal of residual moisture, the product must not be heated above 40 ° C, otherwise the destruction of vitamin C will begin, for what drying is carried out at low temperatures. With microwave-vacuum drying, the optimal moisture removal parameters are at the temperature of the heat carrier no more than 40 ° C and the increasing vacuum of 13 … 23 kPa.

    Two-stage drying is a drying process that does not destroy cells with accelerated dehumidification.

    Evaluation of the nutritional value of the resulting product, after two-stage drying, according to organoleptic, physicochemical indicators, the degree of digestibility and assimilability shows their slight differences from the original products. At the same time, polyunsaturated fatty acids, essential amino acids, vitamins, minerals and other important indicators of the nutritional value of products preserve quite well. The aroma and taste inherent the products are also well remaining. The product is processed with microwaves in a vacuum environment.