Effects of Spray Drying and Freeze Drying on the Properties of Protein Isolate from Rice Dreg Protein

This work was done to investigate the effects of different drying methods in the preparation of rice protein isolates (RPIs), as the understanding could provide useful information regarding applications of plant proteins in the food industry. RPI from rice dreg protein, a cheap by-product from the production of rice syrups, was extracted using an alkali solution along with isoelectric precipitation, and subsequently dried by freeze drying (FD-RPI) or spray drying (SD-RPI). The differences in biochemical, physical, and structural characteristics were observed for the dried proteins. SD-RPI had higher protein solubility and emulsifying activity at the pH values between 5 and 11, with higher foaming capacities (127.08 +/- 2.25 % compared to 118.83 +/- 2.71 %) than FD-RPI. However, FD-RPI had larger mean diameter (2,114.2 +/- 79.6 nm compared to 490.4 +/- 44.8 nm of SD-RPI), higher water/oil holding capacity (p < 0.05), and thermal stability. In addition, FD-RPI contained more beta-turn structures (43.04 % compared to 25.81 %), and less beta-sheet and random coils than SD-RPI, indicating that the more compact and ordered conformations of FD-RPI might be related to their physicochemical and functional properties. The choice of drying method could significantly influence physicochemical and conformational properties of RPIs, consequently determining their specific functional properties. The understanding of drying effects on their properties could assist in selecting the appropriate drying method to optimize the utilization of RPIs in the food industry.