Interaction of soy protein isolate fibrils with betalain from red beetroots: Morphology, spectroscopic characteristics and thermal stability

Betalain has been reported as water-soluble, nitrogenous pigments with biological activities. However, their sensitivity to thermal degradation limits their application during food processing. In this report, the interaction of betalain and soy protein isolate fibrils (SPI-F) was investigated through multiple microscopic and spectroscopic techniques and the thermal stability of betalain was assessed after heating treatment. AFM and TEM images showed that the complex of betalain and SPI-F was small, amorphous aggregate. Fluorescence spectroscopy revealed that SPI-F bound with betalain via hydrophobic interactions. Circular dichroism results showed the secondary structures of SPI-F was changed by betalain binding, with a decrease in α-helix (7.5–6.9%), β-turn (13.1–12.9%) and random coil (41.1–40.7%) and an increase in β-Sheet (38.3–39.5%). The addition of SPI-F decreased color loss and increased the thermal retention of betalain from 55.3% to 75.9%. These findings suggest that SPI-F has a protective effect on the thermal stability of betalain and facilitate the widespread application of betalain as natural colorants.