Combined Hot Air And Microwave Vacuum Drying Of Seabuckthorn(hippophae Rhamnoides L.)Berries:Effects Of Pretreatment On Drying Characteristics And Quality Attributes

Seabuckthorn berries（Hippophae rhamnoides L.）are rich in a variety of nutrients and have good biological activity.Freshly picked seabuckthorn berries possess high moisture contents of up to 70%～85%,which makes them very easy to soften and deteriorate after harvesting at room temperature.Furthermore,the thin skin of seabuckthorn berries is easy to break,which is not conducive to long-term preservation and long-distance transportation.Drying is an effective method that can not only reduce the loss of active ingredients during long-term cryopreservation but also reduce storage and transportation cost.At present,the common drying methods of seabuckthorn berries are natural drying and hot air drying.This technology has the problems of low mass transfer efficiency and poor drying quality,and the drying cycle is easy to be prolonged due to"crust"and"shrinkage"in the later period.The combination drying method can combine the advantages of a single drying technology to solve this problem.Furthermore,the outermost surface of seabuckthorn berries is covered with a dense wax layer,which severely hinders moisture transfer from inside to outside during the drying process,leading to difficulties in drying.However,pretreatment technology can solve such problems effectively.The purpose of this work was to investigate the effects of pretreatments and combined drying on the drying characteristics and quality of seabuckthorn berries,and to optimise the combined hot air-microwave vacuum drying（HAD-MVD）process.The main research results are as follows:（1）The results of the study on the drying characteristics and quality of seabuckthorn berries during hot air drying under different pretreatment methods and parameters show that:the samples treated with the combination of ultrasonic-assisted lye（time,15min）and pricking holes（number,6）in the skin（UA15+PH6）had the shortest drying time（15.8h）,the highest drying rate(1.40 g·g^-1·h^-1),the best values ofΔE（29.61）and the highest rehydration ratio（1.15g/g）.The microstructures showed that the wax layer was dissolved and the micropores were produced by the chemical action of alkali,mechanical and cavitation effects of ultrasonic.Finally,the coefficient of variation method was used to calculate the comprehensive scores of samples that from different groups,and the highest score（1.013）was observed in the group of UA15+PH6.That is,UA15+PH6 treatment was the most appropriate pretreatment method for seabuckthorn berries.（2）The drying characteristics and kinetics of HAD-MVD under pretreatment were analyzed and it was concluded that the higher the moisture content at the conversion point,the shorter the drying time of HAD-MVD.The moisture content at the conversion point was 65%had the shortest drying time（5.55 h）.Under different microwave power conditions,increasing the microwave power can improve the effective moisture diffusivity(D_eff)of moisture in the microwave vacuum drying（MVD）stage,shortening the drying time.Under different microwave interval ratios,the higher the interval ratio,the lower the effective moisture diffusivity,the slower the drying rate and the longer the drying time in the MVD stage.The effective moisture diffusivity was calculated using Fick’s second law,the D_effobtained in the stage of HAD and MVD are ranged from 8.28×10^-11～8.98×10^-11m²/s and 2.33×10^-9～5.92×10^-9m²/s,respectively.The average activation energy（E_a）of MVD stage is 17.07 W/g according to Arrhenius equation.（3）On the basis of single factor experiment,the HAD-MVD process for seabuckthorn berries under pretreatment was optimized using the response surface method,and the optimum parameters were obtained:the moisture content at the conversion point was 65%,the microwave power was 395.583 W,and the microwave intermittent ratio was 2.By analyzing the drying kinetics and quality of HAD,HAD（UA15+PH6）and HAD-MVD（UA15+PH6）,it was found that HAD-MVD（UA15+PH6）had the shortest drying time and the best dry product quality.Finally,the color change of seabuckthorn berries during the drying process of HAD-MVD（UA15+PH6）was studied.The results show that in the HAD stage,the change law of a^*andΔE with time follows the fractional conversion model.In the MVD stage,the change law of L^*and b^*with time follows the zero-order model.