Cruciferin versus napin – Air-water interface and foam stabilizing properties of rapeseed storage proteins

Rapeseed protein extract has high nutritional value and good techno-functionalities, e.g. in foam systems. Yet, its exact interface and foam stabilizing mechanisms are not well understood. Rapeseed proteins comprise mainly of cruciferin and napin. Our aim was to systematically investigate the interface stabilization behaviors of cruciferin, napin and combinations of both at air-water interfaces. We used surface (dilatational and shear) rheology and microstructure imaging (AFM) and linked their behavior to their foaming properties. We observed that napin adsorbed faster at the air-water interface than cruciferin due to its smaller size, leading to 90% higher foam overrun than cruciferin (320%). The interfaces showed distinct differences in structure and mechanical properties, as cruciferin formed stiff solid-like interfaces with E_d’ = 72.5 mN/m and G_i’ = 9.0·10⁻³ Pa m, leading to high foam stability (half-life time 220 min). Napin formed weaker less stretchable interfaces (E_d’ = 61.8 mN/m; G_i’ = 6.7·10⁻³ Pa m), leading to substantially lower foam stability (half-life time 23 min). Cruciferin and napin were also mixed at 3:1, 1:1 and 1:3 (w/w) ratios. Napin increased the foamability of all mixtures with foam overrun between 400 and 420%. The mixture at 3:1 cruciferin-to-napin ratio had comparable foam stability with pure cruciferin since cruciferin dominated the mechanical properties of the air-water interface. Higher napin contents largely decreased foam stability with half-life time decreasing to 80 min. These findings provide a comprehensive understanding of the behaviors of rapeseed proteins at air-water interfaces and their link to foaming properties, which can be used to tailor the properties of aerated products stabilized by rapeseed proteins.