Acid-induced gels from soy and whey protein thermally-induced mixed aggregates: Rheology and microstructure

In this study, we explored how substituting whey protein isolate (WPI) with soy protein isolate (SPI) affects the linear and non-linear rheological behavior of acid-induced gels, and their microstructures. Commercial SPI and WPI dispersions (pH 7.0, 3.0 mS/cm) were preheated (95 °C, 30 min) at different protein concentrations (2%, 4%, 6%, and 8% w/w) and SPI: WPI ratios (0: 4, 1: 3, 2: 2, 3: 1 and 4: 0). The resultant thermally-induced aggregates were characterized before gelation was induced by glucono-δ-lactone (GDL). Small and large amplitude oscillatory shear (SAOS and LAOS) tests showed that replacing WPI with SPI decreased the strength (lower G′) and stretchability (lower γc) of acid-induced gels in the linear viscoelastic (LVE) regime. Gels containing SPI behaved more similar to pure SPI gels in the non-linear viscoelastic (NLVE) regime: displaying a relatively elastic response at large strain and a gradual transition to plastic behavior. The changes in rheological properties were explained by the differences in the gel microstructures, via fractal scaling theory, multiphoton laser scanning microscopy (MLSM) and scanning electron microscopy (SEM). WPI gels formed denser and homogenous gel networks with very strong inter-floc links, while hybrid gels and pure SPI gels formed coarser and more porous networks with intermediate inter-floc links. The constituent flocs in the latter were larger, with rougher, more elongated and branched structures. The present results provide useful information for future attempts to replace WPI with SPI in food products based on acid-induced gelation.