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

In this study, the effect of steaming and roasting treatment on the physicochemical and functional properties of walnut kernel at 95°C for different time (15, 20, and 30 min) was investigated. Steaming and roasting treatments significantly increased the enthalpy change for protein denaturation, in-vitro digestibility, viscosity, storage modulus (G′) and loss modulus (G″) (p < 0.05), the order from high to low was steaming (7.11–8.69 J g⁻¹; the gastric and intestinal digestion: 1.21%–17.83% and 1.51%–27.31%, respectively; 134.04–450.49 Pa s; 214.28–1047.14 and 61.72–196.09 Pa) > roasting (6.24–7.07 J g⁻¹; the gastric and intestinal digestion: 1.26–15.42% and 1.21–22.37% a, respectively; 16.92–86.07 Pa s; 6.58–209.85 and 3.08–67.14) > untreated (4.53 J g⁻¹; the gastric and intestinal digestion: 0.24%–4.18% and 1.00–7.58%, respectively; 17.06 Pa s; 5.78 and 1.79 Pa). All samples contained the essential amino acids, the amino acid score of samples by steaming was the highest. In addition, the protein of walnut kernel after heating treatment 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.