Effect of β-lactoglobulin phenotype on whey protein nitrogen index and sulphydryl content of skim milk powder

The heat labile nature of whey proteins suggests that compositional differences in these protein fractions may affect physicochemical characteristics and functional properties during the thermal processing of milk products. For this reason, a study was set out to determine the effect of β-lactoglobulin (β-lg) polymorphism on the protein content of skim milk, its denaturation during skim milk powder (SMP) manufacture and the consequential impact on the heat classification of the resultant powders. Furthermore, the effect of β-lg variant on sulphydryl (SH) group content of SMP and its potential to augment the whey protein nitrogen index (WPNI) method as a means of heat classification was explored. The study revealed that the β-lg AA phenotype was associated with higher levels of whey protein and its respective β-lg fraction in raw milk, as well as a higher total SH content in SMP irrespective of preheat temperature used during manufacture. This was reflected in the lower WPNI values for the AA phenotype measured at each processing step (preheated skim: P < 0.004, concentrate: P < 0.023 and powder: P < 0.001). A significantly lower α-lactalbumin (α-la) content was associated with the AA phenotype compared to the AB and BB phenotypes for preheated skim milk (P < 0.042), concentrate (P < 0.001) and powder (P < 0.046). Moreover, it was found that the β-lg AA phenotype SMP contained a higher percentage of slow-reacting SH groups (as a percentage of total), which suggests greater resistance to heat denaturation. A potentially interesting relationship between slow-reacting SH groups and WPNI emerged in the course of data analysis, which may warrant further study. Manufacture of milk powders to meet low-heat classification is favoured by selective segregation of β-lg AA phenotype milks.