Influences Of Pre-Treatments On Quality Of Kiwi Slices Under Hot Air And Radio Frequency-Vacuum Drying

Dehydration is an important way to reduce postharvest loss of kiwifruit.The development of high-quality dried kiwifruits may greatly promote the development of the kiwifruit industry.At present,the practical methods used for drying kiwifruit in the industry are osmotic drying and hot air drying.However,osmotic drying leads to problems such as high sugar content and inadequate drying in dried product,and hot air drying causes problems such as long treated time,low ascorbic acid content and browning.Therefore,proper treatments of kiwifruit before dehydration are necessary.In this study,hot water blanching treatment,sucrose osmotic treatment and ascorbic acid soaking treatment were used for studying the influences of different pretreatments on quality of kiwifruit slices under hot air drying and RF-vacuum drying.Firstly,effects of three drying pretreatment methods on color and drying characteristics of dehydrated kiwifruit were compared,in which the best parameters were determined for each pretreatment method.Then drying kiwifruit with different storage time was performed to determine the suitable storage time of kiwifruit for RF vacuum-hot air drying.Finally,effects of the three pretreatment methods on RF vacuum-hot air drying characteristics of kiwifruit and quality of dehydrated kiwifruit were evaluated,and compared to quality of kiwifruit dried by hot air to provide a reference for further applications of the combined drying methods of kiwifruit.The major research results are as follows:(1)To compare the effects of different pretreatments on the color of kiwifruit under hot air,based on the total color difference(ΔE)of dehydrated kiwifruit.Compared to unpretreatment,the soaking and the osmotic treatments significantly(P<0.05)reduced the ΔE and no significant difference(P>0.05)existed between each,while the blanching treatment did not significantly reduce ΔE.The best parameters for hot water blanching treatment were hot water temperature of 70 ?C with blanching time of 1 min.The best parameters of sucrose osmotic treatment were permeation concentration of 60% with osmotic time of 180 min.The best parameters of ascorbic acid solution soaking treatment were ascorbic acid concentration of 1% with soaking time of 15 min.(2)In the study of the effect of storage time before drying on the drying characteristics and quality of dehydrated kiwifruit,the drying rate increased and the drying time decreased with the extension of storage time.The storage time before drying had an important effect on quality of dehydrated kiwifruit.For example,when stored for 7 days before drying,dried kiwifruit had the lowest ascorbic acid content.The hardness of dried kiwifruit decreased with the storage time before drying.Dried kiwifruit that stored 14 or 21 days before drying had higher ascorbic acid content and browning value.If browning value and ascorbic acid content are used as evaluation indexes for quality,the dried kiwifruit without storage before drying has the best,but had the longest drying time and the largest hardness.(3)In the study on the effects of different pretreatments on the drying characteristics and quality of dehydrated kiwifruit,all pretreatments shorten the RF vacuum-hot air drying time.For the samples pretreated,comparing with hot air drying alone,samples dried by RF vacuumhot air drying had higher(P<0.05)ascorbic acid content.Among the dried samples with pretreatment,samples pretreated by osmosis had lower BI,and higher ascorbic acid content for soaking.So osmotic and soaking treatments can be used as pretreatments of RF vacuumhot air drying for kiwifruit to improve the product quality.