Understanding the differences in gelling properties between lupin protein isolate and soy protein isolate

The gelling properties of lupin protein isolate (LPI) were compared with those of soy protein isolate (SPI). It was found that LPI behaves fundamentally different than SPI, evidenced by the formation of weaker and deformable gels. Further investigation shows that both protein isolates can be considered particle gels and that LPI particles do not swell as much as SPI particles inside the network. Besides, heating hardly affects LPI particles while SPI particles show additional swelling. To explain the differences, the sulfhydryl reactivity of LPI was tested. The amount of free sulfhydryl groups on LPI was higher than the amount of free sulfhydryl groups on SPI. Upon heating the amount of free sulfhydryl groups on LPI increases. We hypothesize that the compact, heat stable structure of the protein particles suppresses the intermolecular bonding through disulphide bridge formation and favours intramolecular crosslinking. The small sulphur-rich proteins that are not incorporated within the particles but are present in the surrounding solution cannot strengthen the particle network, due to their low concentration. LPI did not form gels of similar consistency as SPI and may therefore be less useful for solid food products. The thermal stability of LPI could offer opportunities for high-protein foods that require low viscosity after heating.