Impact of steaming and roasting heat-treatment on physico-chemical, functional and digestibility of walnut kernel

Abstract

Abstract In order to develop the application of walnut kernel, the effect of steaming and roasting treatment on the amino acid composition, digestibility, conformation, physicochemical rheology and functional properties of walnut kernel at 95 ºC for different time (15, 20 and 30 min) was investigated, and compared to those of untreated sample. Scanning electron microscopy suggested that heating treatment had a notable effect on the microstructure of walnut kernel, especially the steam heating. Both treatments significantly increased the enthalpy, vitro protein digestibility, viscosity, G′ and G″ (p < 0.05), the order from high to low was steaming > roasting > untreated. All samples contained the amounts of essential amino acids, the amino acid score (AAS) of samples by steaming was the highest compared to that of the untreated and roasting, and the only limiting amino acid of walnut kernel before or after heat treatment was lysine. In addition, the protein of walnut kernel after heating treatment with the extension of time contained more α-helix and random coil structures compared to the untreated sample, while β-sheet and β-turns structures decreased. Moreover, the thermal treatment could cause the changes of the water/oil holding capacity, foaming and emulsifying properties of walnut kernel flour. When there were differences between the results of steaming and roasting samples, it was concluded that the water played an important role in steaming. These results suggested that the thermal treatment as an effective approach could improve the physico-chemical, structural and functional properties of walnut kernel and be potentially applied in the food processing.