Improvement of drying technology of plums by combination and ultra-high frequency methods

. The article presents the developed innovative vacuum-sublimation drying installation and technology for drying fruits and vegetables. Technical instructions and technical and technological parameters have been developed. The purpose of the scientific work is to improve the technology of freeze drying. In this article, based on studying the process of freeze-drying vegetables and analyzing the main parameters of its control, drying equipment has been created. It has been shown that during storage and use of fruits and vegetables, an increase in the level of organoleptic indicators (shape, size, taste, color and smell) has been achieved in comparison with analogues. The value of the research work lies in the fact that a technology for freeze-drying vegetables using ultra-high frequency energy transfer has been developed and a device has been created. The study discussed issues related to compliance with regulations and rules.


Introduction
Globally, the demand for sublimation-dried fruits and vegetables rich in natural vitamins, micro-and macroelements is increasing year by year.At the same time, the reforms carried out to reduce the period of high-quality, low-temperature and deep vacuum drying, evaporation of initial moisture by freezing, simplification of continuous sublimation drying technology, and improvement of its productivity are of urgent importance.
In the global food industry, scientific research is being conducted on drying fruits and vegetables based on high-quality and energy-saving technology [1].In this regard, special attention is paid to the development and testing of technologies for the production of highquality products by drying at low temperature and deep vacuum, and the acceleration of the drying process with pretreatment in the high-frequency electromagnetic range.At the same time, scientific and research work is being carried out on improving the efficiency of the process of sublimation drying of agricultural products, evaporation of initial moisture with preliminary treatment in the ultra-high frequency electromagnetic range, optimization of lowstage and continuous drying technologies.
In the process of sublimation drying, ultra-high frequency waves were used and the advantages of this technology were shown.Most foods, including fruits and vegetables, are usually dried by traditional methods such as salting, reboiling, and sun drying for 2-3 days.
Since this process lasts for a long time, a certain part of many biologically active substances, proteins, and carbohydrates lose their originality.In addition, traditional sun-dried fruits can take up to 2-3 days for the process of returning to their natural state, rehydration.This indicates the low quality of food products [2,3].
In sublimation drying technology, the main process is carried out in a cylindrical drying chamber.The chamber has a desublimator and a vacuum pump is connected.At the bottom, the chamber is connected to the unloading auger through a vacuum valve.Product drying process mode is monitored and controlled.In the process of drying, the samples are carried out in a deep vacuum, due to the evaporation of moisture.UHF waves are successfully used as a heat source in the food industry due to the rapid heating of materials.For example, it can be used as a heat source in the primary processing of the product, which shortens the drying process and allows to reduce the consumption of electricity.
Short-term use of UHF electromagnetic waves in drying technology can reduce the moisture content of the product by 25-30%.This, in turn, reduces the duration of the drying process.
The task of improving the drying technology of food raw materials is to control the processes that take place in the material to be dried in order to obtain a high-quality product with certain organoleptic, physico-chemical and structural-mechanical indicators [4 ,5].
Different methods of drying food products are known, and they depend on the method of providing energy during drying.One of the most common methods of drying products is convective drying.It is based on the transfer of heat to the product being dried using a heated drying medium -air or a steam-gas mixture.With this drying method, due to the thermal energy delivered to the product, moisture evaporates, and the transfer of moisture vapor is carried out by the drying agent (hot air or steam) [6].
The task of improving the technology of drying raw food products is to develop an algorithm and methodology for managing processes in the material to be dried in order to obtain a high-quality product with certain organoleptic, physico-chemical and structuralmechanical indicators, and consists of introduction.
Currently, our country does not have enough effective technologies and special equipment for drying raw vegetables at low temperatures.During the drying process, the nonenzymatic loss of sugar in vegetables occurs due to the formation of melonoidin and caramelization reactions [7,8].

Materials and methods
In sublimation drying technology, treatment of products in waves in the UHF electromagnetic range is carried out before the process of freezing them [9,10].Products at room temperature are treated with short-term UHF electromagnetic waves for 2-3 minutes, and then they are placed in a refrigerator or directly in a sublimation drying equipment for freezing.In the process of processing the product in short-term UHF electromagnetic waves, the temperature can rise to 110-1200 ºC.In the selected onion samples, the temperature of the onion samples was lowered to -25 ºC in 8 hours by pre-treatment of the product in the UHF range, then the temperature was gradually increased to 35 ºC, and the process was continued until 10% residual moisture remained.Samples dried by dielectric freeze drying are exposed to a microwave field.Both vacuum and atmospheric microwave drying can be performed in a dielectric freeze-drying chamber.The vacuum environment during drying is maintained with the help of a special vacuum pump, the temperature of the condensation collection (trap) of the generated steam is sufficient.Three magnetrons are located at different angles to avoid uneven distribution of microwave energy.Microwave power can be continuously adjusted.The temperature of the dried samples is determined by an optical sensor that works well in a microwave oven.

Results and discussion
The quality indicators of the researched products include: organoleptic indicators, moisture, recoverability, amount of dry matter, acidity, amount of dyes, amount of vitamin C, as well as other indicators.Organoleptic assessment was performed based on appearance, consistency, taste, smell and color.Moisture content was determined classically by drying the sample in a vacuum-drying cabinet at 105 °C to constant mass and using a Mettler Toledo HG-63 moisture analyzer.
Sublimation drying technology includes the following processes: first, it is treated with the UHF range, then the samples are placed in a refrigerator, and the frozen samples are transferred to a vacuum-sublimation chamber.The door of the chamber is hermetically closed and a vacuum-assisted air extraction system is activated.The pressure in the chamber is set to about 10-15 Pa.After that, the sublimation drying process takes place in the chamber.After a certain period of time, the pressure in the chamber begins to decrease depending on the mass of the product placed for drying.When the pressure reaches 10-15 Pa, the vacuum system is turned off and the chamber is filled with drying agent.When atmospheric pressure is reached in the chamber, the door is opened and the products in the tray are taken out for examination.
In sublimation drying technology, the main process is carried out in a cylindrical drying chamber.The chamber has a desublimator and a vacuum pump is connected.At the bottom, the chamber is connected to the unloading auger through a vacuum valve.Product drying process mode is monitored and controlled.In the process of drying, the samples are carried out in a deep vacuum, due to the evaporation of moisture.If we assume that the freezing process is constant for both methods A=const, then the optimal method of drying the product can be determined depending on the obtained result or the quality indicators of the product.In this case, we need vitamin C values obtained during product drying (Table 2-3).Using one of the simplest methods of optimization, using the comparison method, based on the amount of vitamin C stored during the drying period, the drying period and the amount of electricity consumed, the microwave sublimation drying method can be considered more effective or optimal compared to the traditional method.
It was observed that plum samples dried 6 hours earlier than samples dried by conventional sublimation method, as the moisture content in the samples decreased to 28% during sublimation drying after pretreatment of the product in the UHF range.At the initial stage of the drying process, the temperature in the drying chamber is 0-50 °C.The pressure in the chamber during the drying process is 10-15 Pa.The kinetics of the drying process was analyzed by changing the values of the product weight every 3 hours.The samples were dried to a residual moisture content of 10%.It can be seen that during the sublimation drying of product samples, preliminary product treatment in the UHF range reduces the drying time by 20-25%.Working principle of microwave freeze dryer: 1. Freeze in a freezer at -30 to -35°C.2. Drying the product in a sterile chamber using vacuum and microwaves.

Sealing dried products (hermetic packaging).
Often, the initial processing of products is carried out by heat treatment (washing) and subsequent grinding (cleaning, cutting).The advantage of the freeze-drying technology is that, after such treatment, the products do not differ much in terms of organoleptic and physico-chemical properties.Digestion and assimilation are also maintained as needed.
During sublimation drying of fruits and vegetables, the chemical composition of the research object almost does not change.The main reason for this is that the drying process is carried out at extremely low temperature and pressure.Carbohydrates make up the main part of the dry matter of the fruits, and they are mainly lost due to the formation of melonoidin and caramelization reactions at high temperature.Considering that the temperature inside the sublimation chamber does not exceed 40 -45 °C even in the microwave sublimation drying method, it is sufficient to analyze the vitamins and organoleptic indicators of the products.

Conclusion
It was observed that the sublimation drying process with product pretreatment in the UHF range dries 5 hours faster than the dried samples without pretreatment.When sublimation drying after pretreatment of the product in the UHF range, the moisture content of the samples decreased to 28%, so it was observed that they dried 6 hours earlier than the samples dried by the traditional sublimation method.At the initial stage of the drying process, the temperature in the drying chamber is 0-50 °C.The pressure in the chamber during the drying process is 10-15 Pa.The kinetics of the drying process was analyzed by changing the values of the product weight every 3 hours.The samples were dried to a residual moisture content of 10%.It can be seen that during the sublimation drying of plum and cherry samples, preliminary treatment of the product in the UHF range reduces the drying period by 20-25%.

Table 1 .
The degree of variability of control factors.

Table 2 .
Results of an experiment on drying plums.

Table 3 .
Energy consumption and performance indicators in plum freeze-drying technology.