Effect of in-barrel CO2 saturation level on the morphology and structure development of milk protein extrudates

Abstract The combined effect of the operating pressure and supercritical carbon dioxide (SC–CO2) injection level on structure development during puffing of milk protein concentrate by supercritical fluid extrusion (SCFX) was studied. The extrudates were analysed using scanning electron microscopy, texture analyser, and electrophoresis. Extrudates obtained with SC–CO2 injection rate corresponding to the equilibrium solubility demonstrated a highly porous structure (mean pore size < 500 μm and mean pore wall thickness < 40 μm). In contrast, both undersaturated and oversaturated conditions formed extrudates having fewer large irregular pores (mean pore size > 700 μm and mean pore wall thickness > 80 μm). Reduction in interfacial tension with increasing CO2 concentration favoured homogeneous nucleation up to saturation CO2 levels, resulting in uniformly porous extrudates. SC–CO2 input rate above the saturation levels resulted in undissolved gas vacuoles in the melt, allowing dissolved CO2 to diffuse into them at die exit, favouring coalescence. These findings could have significant utility in designing novel milk protein-based puffed products of predefined structural and physicochemical properties.