Sucrose is added to foams to alter taste and improve stability. With protein-stabilized polyhedral foams, sucrose could be improving stability by reducing the rate of fluid draining from the lamellae, or have a direct effect on interfacial properties. In this investigation, we examined the mechanisms by which sucrose (0 to 63.6% w/v) altered the stability of foams made with different proteins (egg white protein, EWP; and whey protein isolate, WPI). Foam drainage ½ life and dynamic viscosity of pre-foam solutions were measured. Confocal laser scanning microscopy was used to identify the structural changes of foams over time. Air/water interfacial dilational elastic modulus (E′) and interfacial tension (γ) were determined by a pending drop method.

Sucrose addition increased drainage ½ life of protein foams and dynamic viscosity of pre-foam solutions. The drainage ½ life values of EWP foams were exponentially increased while those of WPI foams were linearly increased with sucrose concentration. The dynamic viscosities of EWP and WPI solutions were increased in a parallel pattern. Therefore, the exponential increase in stability of EWP foams was not due to viscosity alone. Microscopic images revealed that sucrose slowed the average bubbles size growth in both EWP and WPI foams; however, WPI bubbles developed to a bi-model distribution after 20 min, suggesting disproportionation. Interfacial rheology measurements showed that sucrose increased the E′ of EWP but decreased E′ of WPI. Relationship between bubble size growth (changes of average bubble area over time) and E′/γ suggested that interfaces with E′/γ>2 can effectively retard bubble size growth in EWP foams, confirming the theoretical predictions (Walstra. 2003. Physical Chemistry of Foods. New York: Marcel Dekker, Inc. 537 p.); however, WPI foams exhibited a different behavior. These results showed that sucrose caused a general increase in stability due to increased viscosity of the continuous phase, and a protein-specific effect on stability factors associated with interfaces.