RESEARCH OF THE COMBINED DRYING PROCESS OF APPLE RAW MATERIAL OF HIGH HUMIDITY

Abstract

The food market of Ukraine needs a significant amount of vegetable and fruit products in a dried state, most often in the form of semi-finished products for long-term storage or the finished product for consumption (vegetable and fruit chips, snacks, etc.).

The developers of technologies and equipment, first of all, are faced with the task of ensuring the proper quality of products. An alternative to thermal moisture removal is electrical technology in which clean energy is fed in a concentrated form directly to the objects of drying.

In Sumy National Agrarian University proposed a combined method of drying high-moisture sliced ​​fruit and vegetable raw materials [10], which uses surface convective heating in combination with the supply of energy throughout the volume with direct transmission of electric current. At the first stage of drying, apples have a large amount of moisture in a free state, therefore, to intensify the process, it is advisable to use the transmission of electric current.

The developed experimental facility for studying the processes of combined convective and electric drying of apple samples, in the first stage, represents a drying chamber in which the desired air temperature from 25-55 °C was maintained. The dried electric samples were supplied through flexible contacts with an alternating electric current of industrial frequency 50 Hz with a voltage of 10-20 V. At certain time intervals, the current magnitude, the mass of the samples and their internal temperature were recorded. Calculated were a certain electrical resistance and drying rate of objects.

Measurements of the electrical resistance of dried apple samples showed their electrical conductivity up to 1/3 of the original weight. It was found that due to the intense juice production from the apple pulp cells and a corresponding decrease in the resistance of the plant material under conditions of limited removal of water vapor, a sharp increase in the electric current and sample temperature to peak values ​​is observed. At the same time, the drying rate increased to the highest value.

Limit parameters of intensity up to 2-3 V/mm and volumetric energy saturation up to 1 W/cm³ in the dried material were established at which the temperature of apple samples does not exceed the allowable peak values ​​of 50-55 С.

With allowable ranges of initial energy supply conditions, a 2-3-fold reduction in the duration of the first stage of drying was established compared to convective heating at the same temperature of the heat transfer media.

Analysis of the drying curves and changes in the process rate revealed the absence of the classical first drying period with a constant rate of moisture release, which requires further study of the kinetics of drying with a combined energy supply.

The results of research can be the basis for the development of technological processes of periodic drying of sliced apples in the apparatus of average productivity.