Vacuum drying is to be done with dried food materials placed in a closed drying room. In the vacuum system, at the same time, the dry material is heated to make the moisture inside the material diffuse to the surface. Water molecules on the material surface, after overcoming the attraction between the molecules, escape to the low-pressure air in the vacuum chamber, which is removed by a vacuum pump.
Under various heating and drying methods under normal pressure, due to the heating of materials, their color, aroma, taste, and nutrients will suffer a certain degree of loss. If the method of vacuum drying is adopted, the isolation of air under negative pressure makes part of the materials prone to oxidation and other chemical changes in the drying process, which can better maintain the original characteristics and reduce the loss of quality.
Vacuum drying has a fast drying speed, high efficiency, and will not cause damage to the nutrients in products. It is mainly designed for drying materials with high heat sensitivity, easy decomposition, and easy oxidization, but it can also be filled with inert gas into the interior, especially some complex materials that can also be quickly dried.
It is worth mentioning that many vacuum drying equipment with recycling devices is not only easy to collect the important components in the material but also can recover pollutants, making it an environmentally friendly type of "green" drying.
With the rapid development of the food industry and the national attention to energy conservation and environmental protection of food equipment, coupled with the addition of consumption upgrading, people's demand for health, nutrition, and safe food is increasing, which provides a good development opportunity for the vacuum dryer.
At present, with the development of modern mechanical manufacturing technology and electrical technology, the vacuum drying equipment includes the vacuum disc continuous dryer, double cone rotary vacuum dryer, vacuum rake dryer, plate vacuum dryer, low temperature belt continuous vacuum dryer, continuous vacuum dryer, and other forms of vacuum drying equipment.
Vacuum drying technology is widely used in the dehydrated drying of fruits and vegetables, food, health care products, drugs, etc., and its drying quality is good, which is more and more favored by users. The application of this technology in grain drying is introduced below.
In northeast China, the corn harvest season is close to the severe winter, and the air humidity is not conducive to drying, so mechanical drying is widely used. The hot air temperature of the traditional dryer is mostly 50–60 °C. If the drying process is improperly controlled, a large temperature gradient will be formed inside and on the surface of the grain, and the crack rate and crushing rate of corn will be high. At this time, the vacuum drying technology highlights its advantages.
Drum type grain vacuum low temperature dryer structure, this equipment is developed by Luoyang Yuanhong vacuum drying equipment and is composed of heat tube type vacuum drying bin, rear bracket, vacuum unit, heating unit, drive device, guide pipe, roller way, front bracket, chassis, wheel, catheter, and support wheel.
After the wet grain enters the dry warehouse, close the cover and start the vacuum unit to extract the gas from the warehouse. The driving device drives the drying warehouse to rotate through the conduction axis, and the heat dissipation pipe and spiral blade in the drying bin play the role of stirring food to prevent agglomeration. The moisture generated by the heating of the wet grain in the drying chamber is connected to the vacuum unit by the exhaust outlet on the sealing cover through the guide pipe and is discharged by the vacuum unit.
The heating temperature of the thermally conductive working medium during grain vacuum drying is 50–120 °C, and the optimal setting of the vacuum drying degree in the vacuum drying warehouse is set to 96 MPa, which can realize a low temperature drying environment of 32–60 °C;
Yhe vacuum drying technology applied to dry food overcomes the high temperature drying solute loss phenomenon, avoids starch gelatinization and protein denaturation, reduces vitamin C decomposition, keeps the original physical and chemical characteristics and the color, aroma, taste, shape, and nutrients, and solves the problem of dry grain cracking and damage.
The vacuum dryer can dry various grains such as corn, rice, wheat, sorghum, soybeans, and rapeseed, as well as jujube, peanuts, sunflower seeds, and traditional Chinese medicine, with its versatility, strong adaptability, good stability, and high safety.
Grain vacuum drying has been developed in the past ten years. The research, application, and implementation of grain vacuum drying technology have attracted more and more attention from scientific and technological workers. Many scientific and technological workers and factories of production equipment and application equipment units jointly develop grain vacuum dryers.
However, in the development process of grain vacuum drying technology, there are many difficulties, which can be summed up in three aspects: the ideal heat transfer in the equipment, a large amount of water vapor in a short time, and the uniformity of the moisture content of the dried material. If the above problems are solved, grain vacuum drying will usher in large-scale industrial production.