Interfacial properties, thin film stability and foam stability of casein micelle dispersions

Foam stability of casein micelle dispersions (CMDs) strongly depends on aggregate size. To elucidate the underlying mechanism, the role of interfacial and thin film properties was investigated. CMDs were prepared at 4°C and 20°C, designated as CMD_4°C and CMD_20°C. At equal protein concentrations, foam stability of CMD_4°C (with casein micelle aggregates) was markedly higher than CMD_20°C (without aggregates). Although the elastic modulus of CMD_4°C was twice as that of CMD_20°C at 0.005Hz, the protein adsorbed amount was slightly higher for CMD_20°C than for CMD_4°C, which indicated a slight difference in interfacial composition of the air/water interface. Non-linear surface dilatational rheology showed minor differences between mechanical properties of air/water interfaces stabilized by two CMDs. These differences in interfacial properties could not explain the large difference in foam stability between two CMDs. Thin film analysis showed that films made with CMD_20°C drained to a more homogeneous film compared to films stabilized by CMD_4°C. Large casein micelle aggregates trapped in the thin film of CMD_4°C made the film more heterogeneous. The rupture time of thin films was significantly longer for CMD_4°C (>1h) than for CMD_20°C (<600s) at equal protein concentration. After homogenization, which broke down the aggregates, the thin films of CMD_4°C became much more homogeneous, and both the rupture time of thin films and foam stability decreased significantly. In conclusion, the increased stability of foam prepared with CMD_4°C appears to be the result of entrapment of casein micelle aggregates in the liquid films of the foam.