Emulsifying properties of milk protein genetic variants

The emulsifying properties of lactic WPCs containing beta-lactoglobulin variants A and B, pure beta-lactoglobulin A, B and C variants, mineral acid caseins containing beta-casein A(1) and beta-Cn A(2), and the pure beta-Cn A(1) and beta-Cn A(2) were studied. Significant differences between the emulsifying ability of the three pure beta-lactoglobulins, and between the variant WPCs were observed. The A variant was found to have the highest emulsifying activity index tin emulsions containing both the WPCs and the pure variant proteins), with B next and C the lowest. For the pure beta-lactoglobulin variants the trend of increasing emulsion stability was C > B > A, with no significant differences observed for the WPC samples. No significant differences between the surface coverages of the variant WPC samples or between the pure beta -lactoglobulins were found.

For the beta-cn, the A(1) variant showed a higher emulsifying activity than the A(2) for both acid caseins and for the pure proteins, and a lower emulsion stability when the pure proteins were used to make the emulsions. No significant differences were observed in the surface coverage of emulsion droplets made with either the acid casein or the pure protein.

The emulsifying activity of a protein is related to how quickly the protein is able to unfold and spread at the oil-water interface. Proteins that unfold rapidly will be able to stabilize smaller droplets before significant coalescence occurs. The emulsifying activity of the beta-lactoglobulin variants can be related to the known structural stability. beta-Lg C is known to be the least flexible variant, beta-Lg A the most. Therefore beta-Lg A can unfold at the oil-water interface more rapidly than beta-Lg C, and can stabilize smaller oil droplets.

Similarly, the ability of beta-Cn A(1), to form finer emulsion droplets than beta-Cn A(2) must also be related to the ability of the protein to spread at the surface. The extra proline residue substituted into the secondary structure of the beta-cn A(2) must confer enough inflexibility in the protein to alter significantly the rate of protein spreading at the oil-water interface.