Effect of limited hydrolysis of soy protein on the interactions with polysaccharides at the air–water interface

The objective of the work was to study the effect of limited hydrolysis of soy protein on the interactions with polysaccharides with and without surface activity at the air–water interface at neutral pH where a limited incompatibility between macromolecules can occur. The surface pressure and phase angle as a function of time were evaluated with a drop tensiometer at 20 °C, pH 7 and ionic strength 0.05 M. Hydrolysates of 2% (H1) and 5.4% (H2) degree of hydrolysis (DH) with neutral protease from Aspergillus oryzae were obtained from a commercial soy protein isolate. The polysaccharides used were: hydroxypropylmethylcellulose (HPMC) as surface active polysaccharide; lambda carrageenan (λC) and locust bean gum (LB) as non-surface active polysaccharides. It was found that increasing DH decreased the surface pressure and increased film viscoelasticity (determined as the phase angle, θ) of soy protein hydrolysates and the nature of protein–polysaccharide interactions was strongly affected by DH. The presence of polysaccharides led to an increase of surface pressure of H1 but when added to H2, HPMC and λC decreased the surface pressure. The less hydrolyzed protein H1 gave rise to a higher surface pressure and film viscoelasticity in combination with the polysaccharides. This result points out that a limited protein hydrolysis was sufficient to improve the surface properties of soy proteins if used in combination with polysaccharides. Polysaccharides used in admixture with hydrolyzed soy proteins could control and improve the stability of foams and emulsions not only by increasing bulk viscosity but also by improving film viscoelasticity.