Rheological Properties and Microstructure of Dephosphorylated Whole Casein Rennet Gels

¹ Eastern Regional Research Center, USDA, ARS, North Atlantic Area, 600 East Mermaid Lane, Philadelphia, PA 19118

The formation of rennet gels made with native or dephosphorylated (40 and 93%) whole CN was monitored using a dynamic oscillatory shear spectrometer with a Couette geometry sample holder. Casein solutions were prepared with different Ca²⁺:CN ratios and treated with rennet prior to loading into the sample holder. Gels formed when all the individual CN had either all or none of their phosphate groups, but only weak localized gels formed when the CN were at multiple levels of dephosphorylation. The rheological properties of gels with the highest storage (elastic) and loss (viscous) moduli 1 h after gel formation were compared [Ca²⁺:CN ratios of .6 (native) or .3 (dephosphorylated)]. Rennet gels of maximally (93%) dephosphorylated CN had similar coagulation times and gel strengths, had lower tan (the viscous modulus divided by elastic modulus), and formed at lower Ca²⁺:CN ratios over a narrower range than did native CN. Partially (40%) dephosphorylated CN rennet gels were similar to maximally dephosphorylated casein gels in tan and Ca²⁺:CN ratios of formation but, compared with both native and maximally dephosphorylated CN gels, were much weaker and had much longer coagulation times. Electron microscopy showed that native CN micelles had fused together to form smooth columns but that the dephosphorylated CN had formed smaller discrete aggregates that clustered together to form a more compact matrix.

Key Words: casein • gel • dephosphorylated • microstructure

Submitted on July 2, 1993
Accepted on November 29, 1993