Nutritive Quality of Walnutdried by Variable Temperature Drum Catalytic Infrared-Hot Air

In order to improve the quality of dried walnut, the changes of walnut oil (acid value, iodine value, peroxide value, oleic acid, linoleic acid, linolenic acid and palmitic acid contents), proteinstructure (FTIR, CD, SEM, fluorescence, surface hydrophobicity, amino acid composition) and polyphenols (content, IC_50(DPPH), IC_50(ABTS) and iron ion reduction ability) were investigated after the variable temperature drum catalytic infrared-hot air drying (VTDCIR-HAD), and compared with those of single hot air drying (HAD) and constant temperature drum catalytic infrared-hot air drying (CTDCIR-HAD). The results showed that compared with HAD, the VTDCIR-HAD and CTDCIR-HAD treatments reduced the oleic acid value by 17.31% and 28.85%, respectively, and decreased the peroxide value by 4% and 20%, respectively, while the iodine value, oleic acid, linoleic acid and linolenic acid contents had no significant changes. Among the three drying methods, the quality of walnut oil treated with VTDCIR-HAD was the best. Compared with HAD, the VTDCIR-HAD and CTDCIR-HAD treated walnut protein structures were more elongated, but there was no significant difference between them. The results showed that the infrared absorption intensity increased, the α-helical contents decreased by 50% and 18.75%, respectively, the fluorescence intensity increased, and the surface hydrophobicity increased by 28.27% and 12.51%, respectively. The proportion of free hydrophobic amino acids decreased by 19.24% and 9.49%, respectively, while the proportion of hydrophilic amino acids increased by 9.84% and 4.85%, respectively, and the apparent structure of protein had no significant change. Compared with HAD, the VTDCIR-HAD and CTDCIR-HAD treatments increased polyphenols contents by 5.23% and 10.92%, respectively, while IC_50(DPPH) and IC_50(ABTS) decreased by 24.36%, 11.58% and 14.72%, 10.60%, respectively, and iron reduction ability was significantly improved. Among the three drying methods, VTDCIR-HAD treatment had the highest content of polyphenols in walnut. In conclusion, among the three drying methods, VTDCIR-HAD treatment was the best, which can slow down the rate of oil oxidation and rancidity, improve the protein structure, increase the content of polyphenols and antioxidant activity, and was more conducive to protect the quality of dried walnut.