Mechanistic study on in vitro disintegration and proteolysis of whey protein isolate gels: Effect of the strength of sodium ions

In this study, the influence of the strength of sodium ions on the mechanical properties, microstructure and proteolysis of whey protein isolate (WPI) gels during in vitro gastrointestinal digestion was investigated. The results showed that heat treatment of 15% (w/v) WPI solutions in the presence of 0-0.1 M NaCl resulted in fine-stranded gels with a smaller coarseness length scale, while the presence of higher salt concentrations (0.15-0.5 M) induced coarse-stranded gels with larger particle sizes of the protein aggregates. The elastic shear modulus was found to increase with increasing NaCl concentration, followed by a reduction under higher ionic strength. During in vitro gastrointestinal digestion, the kinetics of disintegration of all the gels were dominated by a surface erosion mechanism. The half disintegration time (t1/2) significantly increased from 136.2 min to 416.0 min with the increase of NaCl concentration from 0 to 0.5 M, respectively. The coarse-stranded gels were more resistant to digestion with less mass loss and a lower rate and extent of proteolysis throughout the in vitro digestion than the fine-stranded gels. This could be attributed to the limited swelling and more severe protein aggregation found in the coarser gels with more compact structures, which would retard the accessibility of acids and enzymes during digestion. The present work has provided mechanistic insights into the microstructural changes induced by sodium salts on disintegration and proteolysis of WPI gels during in vitro digestion. This is meaningful for future designing protein-based structured food products with designated digestion rates for targeted dietary demands.

Automated summary

This study investigated the influence of sodium ion strength on the properties and microstructure of WPI gels during in vitro digestion. Results showed that lower salt concentrations formed fine-stranded gels, while higher salt concentrations formed coarse-stranded gels. Salt concentration also affected the elastic modulus and digestion rate of the gels.