Effects of different drying methods on the quality of walnut green husks

The effects of different drying methods ( drying in the shade, sun drying, hot air drying, microwave drying and medium-short infrared wave drying) on the drying properties ( moisture content, color difference ( ΔE^*) , total phenols, total flavonoids, anthraquinones and drying time) of walnut green husks ( WGH) were studied.Results showed that it took the shortest time of 0.4 h to get constant weight by microwave drying, indicating the fastest drying rate, followed by medium-short infrared wave drying ( about 8 h) .In addition, contents of total phenols, total flavonoids and anthraquinones in the WGH dried by sun were 46.98, 13.79 and 5.37 mg/g, respectively, which were significantly higher than other drying methods ( p < 0.05) . Contents of total phenols, total flavonoids and anthraquinones in the WGH dried by medium-short infrared wave drying were 43.13, 12.23 and4.36 mg/g, respectively.The color of WGH powder by hot air drying at 50 ℃ was the lightest and the value of ΔE^*was 65.05, followed by medium-short infrared wave drying with ΔE^*of 67.98.By comprehensive comparison of the five drying methods, the quality of the WGH dried by sun was the best, but affected by the weather easily. In the rainy weather, there was no significantly difference between 50 ℃ hot air drying and medium-short infrared wave drying.Moreover, the time by mediumshort infrared wave drying was only 1/3 of the time by 50 ℃ hot air drying, therefore, taking drying efficiency into consideration, the medium-short infrared wave drying was more suitable for WGH drying.