The aggregation of soy protein isolate on the surface of Bifidobacterium

In this study the interaction between soy protein isolate (SPI) and Bifidobacterium was investigated to reveal the protecting mechanism of SPI on Bifidobacterium. The results indicated that the average particle size of the aggregate of SPI and Bifidobacterium longum (B. longum, the better surface hydrophobicity of four Bifidobacterium) exhibited more considerable expansion than SPI alone, from 295.9 nm to 404.8 nm (the aggregate). It was confirmed that B. longum was localized in the microparticle core, while SPI acted as a wall-coating material, as determined by zeta potential and laser scanning confocal microscopy (LSCM). Surface hydrophobicity, which was described by fluorescence intensity, of the aggregate of SPI and B. longum was decreased to 72% of SPI. The main binding force of the interaction between SPI and B. longum originated from a hydrophobic interaction was verified by isothermal titration calorimetry (ITC). The enthalpy (ΔH) was determined by ITC to be − 3.24 kJ mol− 1 for the adsorption of SPI on B. longum at 25 °C and pH 7.0. Furthermore, the aggregation was testified to be an endothermic process, and the process was spontaneous and irreversible. The hydrophobic forces between SPI and Bifidobacterium interpreted that SPI has the potential to be a useful food ingredient in protecting probiotics.