Quality Research And Simulation By Microwave Assisted Foam-Mat Drying Of Berrys

Berries are rich in nutrients, with high medical and health value. Fresh berries are soft and perishable, with initial moisture content of 90%. Drying is a feasiable way to process fresh berries. Microwave assisted foam-mat drying(MFD) has the advantage of high drying efficiency, high energy efficiency, maintaining a greater degree of berry flavor and nutrients of berries, is suitable for the dehydration of materials with high sugar content, high viscosity and thermal sensitivity compounds. Due to uniform distribution of the temperature and moisture within berry puree, anthocyanins and other active ingredients were obviously partial degraded. In this research, the effect of microwave assisted foam-mat drying processing conditions on the temperature, moisture content, anthocyanin content and the structure of anthocyanin were studied. The influence of process parameters on the quality of berry puree under microwave foam drying was studied by using the design of the quadratic regression orthogonal rotary. The process parameters were optimized, mass and heat transfer under Microwave foam drying were analyzed and simulated to provide a theoretical basis for non-uniformity of Microwave foam drying. Results were obtained as follows:(1) The change of moisture and the distribution of temperature of berry puree were studied under various microwave intensity and time. The results show that temperature changes of the foamed berry puree have three obvious stages including ascending, steady, and sharp rising stages. In ascending stage, the temperature rapidly increased to about 70 ℃, general change temperature tended to increase at descending trends from the rims to center. At steady stage, the average temperature of berry puree was in constant level, and the duration of this stage was shorted with the increase of microwave intensity. In rising stage, the temperature rapidly rised up to high level.Declines of moisture content tend to three stages including stable, gentle and rapid stage. In stable stage, moisture content of foamed berry puree were in invariant level. In gentle decline stage, due to the conversion of microwave energy into thermal energy, foam zone formed and moisture content started to evaporate at low drying rate. In rapid decline stage, the moisture content obviously decline to safe moisture. No significant difference of moisture content in different location was found within berry puree in this stage. Critical point between ascending stage and steady stage gives the guidance for the processed fruit powder wih high-quality.(2) The influence of processing parameters on the quality of berry puree under microwave foam drying was studied by using the design method of the quadratic regression orthogonal rotary. The regression models between factors and objective function were established, and the factor contribution rate of the objective function was obtained by the regression analysis of variance. In order to obtain high quality and high content of anthocyanins and vitamin C, the processing parameters were optimized as the microwave intensity of 7.0-7.5 W/g, microwave drying time of 7-8 min, material thickness of 2.0 cm.(3) Anthocyanin varieties in raspberry puree was detected by High Performance Liquid Chromatography as cyaniding, pelargonidinm, malvidin. During the microwave foam drying, delphinidin formed and the content of the four kinds of anthocyanins decreased, malvidin was completely degraded after drying. The content of cyanidin were in highest level. The absolute stereochemistry of anthocyanin at C4 changed between 4β and 4α, and the degree of polymerization changed.(4) According to the drying properties of berry puree and conservation laws of mass and energy, sectional models of heat and mass transfer may be developed to characterize the change and distribution of temperature and moisture within berry puree in microwave foam drying. In wet zone, moisture content was stable and temperature was variable, microwave energy was converted into heat. In foaming zone, temperature was stable and the moisture content was variable, the absorption of microwave energy was assumed to be consumed by water evaporation. In drying zone, temperature and moisture content were variable. Models of heat and mass transfer provide theoretical guidance for the scale-up application, and improve the controllability of the microwave process.