Soy protein-polysaccharides interactions at the air-water interface

This work studies the interfacial behavior of mixed soy protein (SP) + polysaccharide (PS) systems to gain knowledge on the interactions between these biopolymers at the air-water interface under dynamic conditions at neutral pH where a limited incompatibility between macromolecules can occur. The PSs used were: hydroxypropylmethylcellulose (HPMC) as surface-active PS; lambda carrageenan (lambda C) and locust bean (LB) gum as non-surface-active PSs. Protein and PS concentration in the mixed systems were 2% and 0.25%, respectively. The dynamic surface pressure and rheological properties of films were evaluated with a drop tensiometer at 20 degrees C, pH 7 and ionic strength 0.05 M. The presence of HPMC and lambda C greatly increased the surface pressure, surface dilatational elasticity and relative viscoelasticity on the basis of different mechanisms. HPMC competed for the interface with SP, but due to its unusual strong surface activity it could dominate the surface pressure and improve film viscoelasticity. The modification of surface pressure and rheological properties of adsorbed SP films in the presence of lambda C necessarily suggests the participation of lambda C+ contaminants at the interface. Pure lambda C could influence the interface by a complexation mechanism, or indirectly by a depletion mechanism in the vicinity of the interface. In addition surface-active contaminant of lambda C if strongly bound to the PS could bring some PS molecules at the interface. LB little affected the surface pressure and rheological properties of SP films even if surface-active contaminants were present in the commercial preparation. Differences in the interaction of lambda C and LB gum with the protein should be mainly ascribed to different degrees of incompatibility and to the fact that LB is not charged. (C) 2006 Elsevier Ltd. All rights reserved.