Foam properties of algae soluble protein isolate: Effect of pH and ionic strength

In this study the foam properties of algae soluble protein isolate (ASPI), a mixture of mainly proteins and polysaccharides, were investigated as function of isolate concentration (0.1–1.0 mg/mL) and pH (3.0–7.0) at 10 mM and 200 mM NaCl. In addition, adsorption kinetics and dilatational elasticity at the air–water interface were studied. Whey protein isolate (WPI) and egg white albumin (EWA) were used as reference proteins. The consistent dilatational behaviour of ASPI at all pH values and ionic strengths tested indicated a similar interfacial composition at all these conditions. Adsorption kinetics, in contrast, were influenced by varying environmental conditions. At increased ionic strength and close to the theoretical isoelectric point calculated based purely on the amino acid composition of ASPI (pH 7) adsorption increased. Since similar adsorption behaviour was also observed for WPI and EWA, the interfacial properties of ASPI are most likely dominated by its protein fraction. This is further confirmed by the fact that ζ-potential measurements suggested an overall isoelectric point of ASPI below pH 3, while adsorption kinetics varied between pH 5 and pH 7 (the theoretical protein-based isoelectric point of ASPI). The overall foam stability of ASPI stabilized foams was superior to those of WPI and EWA in the pH range 5–7. In conclusion, the molecular and interfacial properties of ASPI, a mixture of proteins and polysaccharides, seem to favour the production of very stable foams in this pH range by the selective adsorption of its protein fraction to the air–water interface.