Calculation of relaxation modes of the dehydrated surface layer of the whiting during the drying process

Abstract

During cold drying of fish, the surface layers, which have lost a significant part of the water, become compacted and prevent the diffusion of water from the inner layers of the fish to the surface. In order to restore the diffusion properties of the surface layer of fish, it is necessary to create conditions for the redistribution of moisture along the thickness of the object, for example, temporarily limit external mass transfer. To do this, reduce the temperature, the speed of air movement in the drying chamber and increase its relative humidity. Due to diffusion, water from the central layers of the fish will flow to the dehydrated surface layer and moisten it. As a result, the mass-conducting properties of the surface layer will increase. The purpose of this work is to study the effect of relaxation processes on changes in the thickness of the dehydrated surface layer of fish for the subsequent development of scientifically sound rational drying regimes. The calculation of moisture redistribution in this work was carried out under the condition (assumption) that there is no external mass transfer of moisture from the surface of the processing object. The kinetics curve and water distribution curves in the processing facility were constructed during the production of dried fish products in 2; 2.5; 13.5; 55 hours respectively. The average humidity of the whiting after dehydration lasting 2 hours was 333% in terms of dry matter. The thickness of the dehydrated layer S = 1.6 mm. After 1.5 hours of relaxation, the moisture on the surface of the fish increased from 272 to 330% and approached the average humidity of the sample. During dehydration of fish lasting 13.5 hours, the average moisture content of the sample per dry mass was 217%, the thickness of the dehydrated surface layer S = 0.9 mm. After 2.5 hours of relaxation, the moisture content on the surface of the layer was 174%. It was found that at the beginning of the drying process, the dry dehydrated layer has 
 a positive tendency to restore the lost moisture-conducting (diffusion) properties, but at the end of the drying process it takes more time to restore them. The paper proposes a method for calculating relaxation curves, which can be used to study the processes of moisture redistribution in the processing object in order to create rational drying modes.