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

Abstract

This study aimed to investigate if differences in β-casein polymorphic structure would affect the acid-induced (glucono-δ-lactone) gelation behaviour of corresponding skim milks. Gels obtained from skim milk containing A2/A2 β-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 acid-induced 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 β-casein contained greater amounts of α-helixes and aggregated β-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 κ-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 β-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. • Effects of acidification on milks with various β-caseins were studied. • Gels carrying A1 β-casein possessed greater water retention and lower permeability. • Increased β-sheets in gels carrying A1 β-casein related to higher storage modulus. • A2/A2 gel possessed a high amount of polyproline II structures. • Gels prepared of A1/A1 and A1/A2 milks possessed a denser microstructure.