Developing Radio Frequency-Vacuum And Combined Dehydration With Hot Air Drying For Kiwifruit Slices

China is the major kiwifruit producer,accounting for approximately 50% of the total world production.Microbial activity is a major cause of spoilage for fresh kiwifruits during storage,transportation and retailing,which would deteriorate their quality and shelf life at ambient temperature.Drying is one of the oldest preservation methods,which rely on lowering the water activity of food products for improving their storage stability.However,some common problems associated with traditional drying for kiwifruits are long drying times,high energy consumption and poor product quality.Radio frequency(RF)heating has been long proposed as an effective thermal treatment of agricultural commodities.In this study,the dielectric properties of kiwifruits were firstly measured using an open-ended coaxial probe system and the effect of pre-treatments on the dielectric and physicochemical properties of kiwifruits were also studied,and then the effect of RF process variables on the RF-vacuum drying(RFVD)characteristics of kiwifruit slices and the drying kinetics were determined using 3 kW,27.12 MHz RF-vacuum drying system,and finally the combination of RF-vacuum and hot air drying(HAD)was developed and compared with single RFVD and HAD in terms of drying uniformity,energy efficiency and quality of final products.The main results are as follows:(1)Both the dielectric constant and loss factor decreased with decreasing moisture content and increasing frequency.At high moisture contents(> 65% w.b.),the dielectric constant decreased slightly with increasing temperature,whereas at lower moisture contents(< 50% w.b.),the dielectric constant increased sharply with increasing temperature.In addition,the osmotic dehydration(OD)resulted in less kiwifruit quality deterioration than HAD in terms of titratable acidity,ascorbic acid,soluble solids and color.Dielectric permittivities of OD kiwifruits were slightly lower than those of HAD samples.(2)The electrode gap,vacuum pressure and sample thickness had major effects on the drying characteristics of RFVD.The RFVD was associated with internal heating and rapid drying resulting in 65% reduction of HAD(60oC)time.Moreover,RFVD samples were associated with better color stability,higher vitamin C retention and higher rehydration capacity(P<0.05)as compared with HAD samples.Based on acceptable drying rate,stable temperature and avoiding arcing,a RFVD protocol with the electrode gap of 60 mm,vacuum pressure of 0.02 MPa and sample thickness of 8 mm was identified.However,the non-uniformity is still a main issue in developing an effective and practical RFVD protocol for kiwifruits.(3)The total drying time was the shortest(480 min)when using RFVD,followed by RFVD+HAD(600 min;20% longer)and HAD(900 min,almost double).However,non-uniform drying patterns were observed in both RFVD and HAD methods,but the RFVD+HAD drying process gave better moisture distribution not only within the individual kiwifruit slices but also among the other slices dried in the same batch.Additionally,the average energy efficiency of hot air system was remarkably improved from 9.92% to 22.93% by applying the RFVD+HAD method.Finally,the RFVD+HAD samples had better quality attributes like color,shrinkage ratio and rehydration capacity.Therefore,the combined RFVD+HAD method is recommended as an effective,practical and uniform drying method for kiwifruits.