Pea protein-fortified extruded snacks: Linking melt viscosity and glass transition temperature with expansion behaviour

The addition of proteins to extrusion feed formulations can strongly affect the expansion behaviour of the melt and often results in limited expansion of the final product. Therefore, the aim of this study was to investigate the impact of pea protein isolate (PPI) content (0-30%) and extruder screw speed (300-700 rpm) on the (i) initial and final expansion, (ii) melt viscosities and (iii) glass transition temperatures of rice starch-based, extruded snacks. Highest initial expansion was observed at 30% PPI content, but significant (p < 0.05) shrinkage occurred at screw speeds of 500 and 700 rpm. Melts containing 30% PPI exhibited higher viscosities than those without PPI. Furthermore, addition of PPI reduced the glass transition temperature and broadened its temperature transition range. Addition of 30% PPI possibly delayed melt solidification, which resulted in shrinkage and non-uniform final extrudate structures. Extrudates containing 20% PPI exhibited the highest final expansion and no significant shrinkage was observed.