Conjugation of soy protein isolate (SPI) with pectin by ultrasound treatment

The Maillard reaction in the aqueous system with and without ultrasound treatment was used to prepare conjugates between soy protein isolate (SPI) and citrus pectin (CP)/apple pectin (AP). Ultrasound treatment at a power of 450 W and a temperature of 70 °C significantly accelerated the conjugation processes between SPI and pectin samples and led to much greater grafting extents compared to the traditional wet heating. A higher degree of graft of the SPI-CP conjugates was achieved at a shorter ultrasound duration compared to the SPI-AP conjugates, possibly attributed to the larger molecular weight and the more flexible structure of AP. SDS-PAGE analysis confirmed the formation of SPI-pectin conjugates. Analysis of the protein secondary and tertiary structures suggested that the attachment of CP or AP changed the spatial conformation of SPI and led to a looser protein structure. In addition to the grafting process, ultrasound was also observed to play a marked role in unfolding the SPI resulting in more favorable structures for the Maillard reaction. Furthermore, the application of ultrasound to the conjugation process significantly increased the surface hydrophobicity and emulsifying properties of both conjugates, indicating that ultrasound can be a desirable method for protein-polysaccharide conjugation.