Effect of ultrasound pretreatment assisted drying methods on structural and thermal properties of soy protein isolate

Soy protein isolate, a essential component in various food products, can be greatly impacted by the drying process, influencing its quality and functionality. This study evaluates the effect of ultrasound pretreatments (5, 10, and 15 min at 350 W) and different drying techniques, namely, freeze drying (FD), spray drying (SD) and oven drying (OD) on structural and functional properties of soy protein isolate (SPI) powder. Among all drying conditions, maximum peak temperature T_d (175.31 °C) was achieved with FD after 15 min of ultrasound pretreatment. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis revealed that ultrasound pretreatment significantly affected protein fragmentation in soy protein isolate (SPI), altering its molecular weight and exhibiting distinct bands compared to control (with distinct bands around 48 kDa and 75 kDa range); demonstrating the most prominent effect of ultrasound pretreatment, which further enhanced functional properties. Notably, surface hydrophobicity (H₀) of SPI was significantly higher after ultrasound pretreatments compared to their control samples, suggesting that this application time effectively enhanced the exposure of hydrophobic groups. The relative crystallinity of FD-control, SD-control and OD-control samples were decreased by 6.83 % for FD, 8.14 % for SD and 5.30 % for OD after 15 min of ultrasound pretreated drying, respectively. Ultrasound pretreatment significantly reduced contents of off-flavor compounds, such as 3,5-Octadien-2-one, Decanal, (E)-2-octenal and 2-Pentylfuran in SPI. Moreover, relative content of 3,5-Octadien-2-one in the major volatile off-flavor compound of FD dried SPI reduced from 2.27 % to 1.00 %, 1.05 %, and 0.96 % at 5-, 10-, and 15-minute ultrasound (350 W) application times, respectively. These findings provided valuable insights into selecting appropriate ultrasound pretreated drying techniques to optimize the quality and performance of SPI in various applications.