Enzymatic Cross-Linking of Casein Facilitates Gel Structure Weakening Induced by Overacidification

Casein (α_S1, α_S2, β, κ) is the major protein fraction in milk and, together with heat denatured whey proteins, responsible for gel network formation induced by acidification. Rheological measurements during gelation typically reveal a maximum storage modulus (G') at a pH close to the isoelectric point (pI) of casein (~4.6). With further decreasing pH gel stiffness decreases because of increased electrostatic repulsion, which is referred to as overacidification. In this study we investigated the effect of casein cross-linking with microbial transglutaminase on gel structure weakening induced by acidification to pH below the pI. Although enzymatic cross-linking increased the maximum stiffness (G' _MAX ) of casein gels the reduction of G' during overacidification, expressed as ratio of the plateau value (G' _FINAL ) to G' _MAX , was more pronounced. Almost no soluble protein was detected in the serum of gels from cross-linked casein, whereas considerable amounts of α_S- and κ-casein were released from reference gels below the pI. This suggests that covalent cross-linking of casein retains charged molecules within the gel network and therefore causes a higher reduction of protein-protein interactions because of higher electrostatic repulsion. Furthermore, higher amounts of uncross-linked β-casein, which was the only casein type not found in the serum, resulted in higher G' _FINAL to G' _MAX ratios, underlining the important contribution of β-casein to acid gel formation and prevention of gel structure weakening.