Walnut Drying Characteristics And Quality Changes When Subjected To Combined Radio Frequency And Hot Air Heating

Walnut(Juglans regia L.),a member of the Juglandaceae family,is mostly cultivated in China.Walnut kernels are nutrient-rich food due to their high polyunsaturated fatty acids and antioxidants.In industrial process of walnuts,drying is a very important process for walnut storage in the future.Radio frequency(RF)heating is a rapid dehydration technique that can be applied for drying walnuts.In this paper,the following experiments of RF drying walnuts were conducted:(1)A pilot-scale,27.12 MHz,6 kW RF unit was used to reduce the whole walnut moisture content from 20% to 8.0%(d.b.),and determine drying curve and protocol parameter.(2)Analyze and compare the effects of hot air,vacuum and hot air assisted RF drying on the drying characteristics and product quality of in-shell walnuts with regard to acid value,peroxide value,saponification value,iodine value,fatty acid compositions,total phenolics concentration and total antioxidant capacities.(3)Study desorption adsorption characteristics of walnuts,desorption adsorption curve and lag phenomenon.The main conclusions are as follows:(1)An electrode gap of 18.0 cm combined with hot air temperature of 50 °C provided an acceptable heating rate and stable sample temperatures during RF drying.Total drying times to reduce the whole walnut moisture content from 20 to 8.0% on dry basis required 240 and 100 min using hot air drying and hot air-assisted radio frequency drying,respectively.The drying curve for hot air-assisted RF drying was fitted to five different kinetic models,and the best one was the Page model.Quality analysis showed that there were no significant differences in peroxide value,free fatty acid,and colour between RF treatments and untreated controls during drying and storage periods(P > 0.05).(2)Compared with vacuum drying and hot air drying,hot air assisted RF drying had the shortest drying time(138 min).RF drying single layer walnuts needed more time than multiple layers walnuts(100 min),but the temperature of sample was lower.Both the reduced power assay and free radical scavenging capacity tests showed that air-assisted RF drying and vacuum drying had little effect on the total antioxidant capacity and total phenol content of walnuts during the drying process and storage.Hot air assisted RF treated walnut quality was better than that of the other two drying methods.(3)At the same temperature and water activity conditions,walnut shell moisture content should be significantly higher than walnut kernel.Walnut kernel and shell desorption adsorption isotherm type was different,because of the composition and construction differences in walnut kernel and shell.There was a significant lag phenomenon in the desorption adsorption isotherm both of walnut kernel and shell,but walnut shell lag phenomenon was greater.The water activity of 0.6 as a safety storage limit,during storage,the water content of walnut kernel and shells must be reduced to below 4.35% and 9.31%(d.b.),respectively.