The Drying Characteristics And Drying Quality Of Wolfberry Undergoing Pulsed Vacuum Drying

Drying is a critical step of wolfberry processing.As to the most prominent problems of long time consuming,instability products quality,the constant drying stage could not meet the need of changeful material,electronic panel contact heating assisted pulsed vacuum drying(EPC-PVD)and far-infrared radiation heating assisted pulsed vacuum drying(FIR-PVD)was applied in this paper to dry wolfberry.The effect of vacuum pressure duration(10,15,and 20 min),atmosphere pressure duration(2,4,and 6 min)and drying temperatures(60,65,and 70 ?)on drying kinetics of wolfberry was investigated in this study.Additionally,the drying quality of the dried wolfberry in terms of color value,volatile component,rehydration characteristics,microstructure,total flavonoid content,total antioxidant capacity were detected.The multi-stage analysis were applied to optimize the drying process base on the optimal constant drying temperature and constant pulsed cycle parameters(65 ?,15:2).Meanwhile,the drying quality was analyzed to provide the theoretical foundation for multi-stage analysis.The pulsed vacuum drying kinetics of wolfberry indicated that vacuum pressure duration,atmosphere pressure duration and drying temperature had significant influence on drying time(p<0.05).The better drying parameter was the atmospheric pressure duration for 2 min and vacuum pressure duration for 15 min.To compare with the time cost by EPC-PVD,the time cost by FIR-PVD would decrease by 17%-19%with the same drying parameters.Weibull distribution model was used to calculate the moisture effective diffusivity of FIR-PVD and EPC-PVD for wolfberry,which ranged from 3.72×10-10 to 7.31×10-10 m2/s,and 3.34×10-10 m2/s to 6.88×10-10 m2/s,respectively.In addition,the drying activation of FIR-PVD and EPC-PVD were 54.30,and 68.59 kJ/mol by Arrhenius equation respectively,when vacuum duration was 15 min and atmosphere duration was 2 min.During the FIR-PVD and EPC-PVD,high and low temperature of wolfberry internal temperature occurred alternately with the pressure change of the drying chamber.According to the internal temperature of wolfberry,the process of heat and mass transfer during the drying was studied.The FIR heating mode is more suitable to apply into PVD,which was attribute to the high efficient in heat energy utilization.Eight popular agricultural products thin drying models were used to describe the drying experiments data of wolfberry.Chi-square test value(?2),residual sum of squares(RSS)and coefficient of determination(R2)were used to evaluate and analyse the fitting.Weibull model and Wang and Singh model fitted well with the drying data.The regression analysis between the model parameters and drying parameters(vacuum pressure duration,atmosphere pressure duration,and drying temperature)was carried out,the new model were set up to describe the pulsed vacuum drying of wolfberry.The result indicated that the water activity of samples dried with FIR-PVD and EPC-PVD ranged from 0.312 to 0.351 at room temperature(24.7?25.6 ?).Colour values obtained for FIR-PVD were close to those of the fresh samples according to the less ?E value and bigger L*,a*,b*,C,and h° value.Generally,the proportions of aldehydes,esters,phenols,and the heterocyclic compound in total volatile components of FIR-PVD sample are higher than those of EPC-PVD samples.And the proportions of the alcohols,ketones and acids in FIR-PVD sample are lower than those of EPC-PVD dried samples.The samples dried with FIR-PVD had more micropore structure compared with samples dried with EPC-PVD,which was benefited to the moisture migration.The samples dried with FIR-PVD had more total flavonoid content,which resulted in higher total antioxidant capacity.To compare with the optimal constant drying temperature and constant pulsed cycle parameters(65 ?,15:2),three kinds of multi-stage regulation(variable drying temperature and constant pulsed cycle parameters,constant drying temperature and variable pulsed cycle parameters,and variable drying temperature and variable pulsed cycle parameters)could decreased the drying time by 8.3%to 15.9%and improve the efficiency in energy utilization.There is no obvious difference in colour value between the sample dried with constant drying temperature and constant pulsed cycle parameters and multi-stage regulation.The rehydration characteristic and total flavonoid content were improved after multi-stage regulation.