Abstract: Walnut meal protein powder was processed from walnut meal by four drying technologies: hot air drying (HA), heat pump drying (HP), vacuum freeze drying (VF) and spray drying (SD). The aim of this study was to investigate the effect of different drying technologies on the quality characteristics and microstructure of walnut meal protein powder. The results showed that the walnut meal protein powders processed by HA and HP had lower hygroscopicity and consequently better storability compared to those processed by the other drying methods, but their color and quality were poorer, and their microstructure exhibited irregular blocky shapes of varying?size with rough surfaces. The product dried by VF, with the highest yield of 29.18%, had the highest protein, essential amino acid and total amino acid contents of 87.70%, 24.61 g/100 g and 86.11 g/100 g, respectively, as well as the lowest moisture of 3.05%, but it had poor color, lamellar microstructure with smooth surfaces, and large particle size. The product dried by SD, with the lowest yield, had the highest sensory score, L* value, moisture content, specific surface?area and hygroscopicity of 75 points, 90.63, 3.91%, 271.47 m2/kg and 2.16%, respectively. In addition, it had good nutritional quality, the lowest wettability (settlement time of 139.33 s), and the smallest and uniform particle size, and its microstructure was maintained well. Taking into consideration the quality characteristics and microstructure of walnut meal protein powder, and the drying efficiency and cost, SD was the method of choice for preparing walnut meal protein powder.

Key words: walnut meal protein powder; drying technology; quality characteristics; microstructure