Rheological and structural properties of acid-induced milk gels as a function of β-casein phenotype

This study aimed to investigate if differences in beta-casein polymorphic structure would affect the acid-induced (glucono-delta-lactone) gelation behaviour of corresponding skim milks. Gels obtained from skim milk containing A2/A2 beta-casein had significantly lower elastic modulus, water holding capacity, and gel permeability compared to A1/A1 and A1/A2 gels. Microscopy images also showed a denser microstructure and smaller pore size in acidinduced gels prepared with A1/A1 and A1/A2 milks compared to A2/A2 milk. A number of reasons may account for these differences in gelation, specifically, gels with A1 beta-casein contained greater amounts of a-helixes and aggregated beta-sheets in their secondary structure compared to A2/A2 gel that was comprised mainly of random coils or polyproline II helixes. In addition, compositional differences such as greater total and micellar calcium and higher levels of total x-casein existed in A1/A1 and A1/A2 milks compared to A2/A2 milk and may have contributed to the difference in gelation and gel structure. Although the role of beta-CN phenotype on skim milk gelation may need further investigation, findings from this study clearly indicate that A2/A2 milk was associated with poor acid gelation properties.