Experimental Studies On The Microwave-Assisted Foam-mat Drying Of Blackcurrant Pulp

Blackcurrant is a kind of special berry fruit, and the furits are rich in various nutritious components and health care function. The juice of blackcurrant is transparent and is presented purplish red color.The fresh furit is too acid to eat, but it is very fit for processing. In this paper, microwave-assisted foam-mat drying methods was applied to dry blackcurrant pulp, and microwave drying characteristics of foamed blackcurrant pulp and the effect of drying parameters on the main qualities of blackcurrant powder were studied. Kinetics model of microwave-assisted foam-mat drying for blackcurrant pulp was established and different drying methods of blackcurrant pulp were compared. Consequently, the optimum technological conditions of drying blackcurrant pulp with microwave-assisted foam-mat drying methord were deterimined, which provided a novely method for drying blackcurrant pulp and important theoretical support for subsequent industrialisation production.In the foaming experiments, the blackcurrant pulp was foamed by the addition of glyceryl monostearate (GMS) and soy protein isolate (SPI) at different concentrations, and the foam was stabilized with the addition of methyl cellulose at 0.5% concentration. The study proved that the best foaming properties were obtained by treating the blackcurrant pulp with GMS (6%), SPI (3%) and methyl cellulose (0.5%), as well as whipping 6 min. The foam density is 0.6g/cm3.Based on the results of microwave drying characteristics of foamed blackcurrant pulp, the effect of the moisture content and temperatures on the drying rate and quality was found in the process of drying. The results showed that initial microwave intensity and material thickness had significant influence on the drying rate of foamed blackcurrant pulp. The drying rate could be improved by increasing the microwave intensity and decreasing the thickness of pulp. It was found that the process of microwave drying could be basically divided into three stages, including preheating, drying at constant rate and drying at decelerating rate when initial microwave intensity was lower than 14.0W/g. In this case the temperature of foamed pulp was in a relatively stable statement. When using higher microwave intensity (higher than 14.0W/g), constant rate stage was not found.Acorrding to the drying kinetics characteristics of foamed blackcrrant pulp, the exponential model, the monomial proliferation model and Page equation were used to describe the process of microwave drying. It was found that the correlation coefficient of Page model was the highest which was 0.9920. The Page equation model was followed as: exp[exp(3.87350.1830.0032)]MR = ??+P?P2 t1.5935+0.0187P?0.00027P2The verified results showed that the Page equation had significant effect and highly fitting precision, and it could be used to predict and control the changes of moisture content in the process of drying foamed blackcurrant pulp with microwave drying method.Comparative experiment between hot-air and microwave drying of foamed blackcurrant pulp has been completed. It is showed that the drying rate of microwave drying was more than that of hot-air drying method. Quality indexes in terms of color value, Vitamin C content, anthocyanin content and sensory quality were measured to evaluate the quality of dried blackcurrant powder. The results showed that the quality of the dried blackcurrant powder with the microwave foam-mat drying was better than that with the convential hot-air foam-mat drying method.Response surface analysis at four factors and five levels was applied to optimize the drying parameters by integrated indicator of color, Vitamin C content, anthocyanin content and moisture content (d.b.), and the corresponding regression model was established. The results showed that the sample load had significant influences on the color and moisture content, the equivalent thickness had extremely significant influences on Vitamin C and anthocyanin content, and microwave power and drying time had significant influences on moisture content. The reasonable technologic conditions of drying method were obtained as following: the microwave intensity was 8.62W/g, the drying time was 8 min, and the equivalent thickness of foamed pulp was 4.46mm. The parameters were resonable according to verification experiment.