RHEOLOGICAL PROPERTIES AND MICROSTRUCTURE OF DEPHOSPHORYLATED WHOLE CASEIN RENNET GELS

The formation of rennet gels made with native or dephosphorylated (40 and 93%) whole CN was monitored using a dynamic oscillatory shear spec trometer with a Couette geometry sample holder. Casein solutions were prepared with different Ca2+:CN ratios and treated with rennet prior t o loading into the sample holder. Gels formed when all the individual CN had either all or none of their phosphate groups, but only weak loc alized gels formed when the CN were at multiple levels of dephosphoryl ation. The rheological properties of gels with the highest storage (el astic) 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 delta (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 g els were similar to maximally dephosphorylated casein gels in tan delt a and Ca2+:CN ratios of formation but, compared with both native and m aximally dephosphorylated CN gels, were much weaker and had much longe r coagulation times. Electron microscopy showed that native CN micelle s had fused together to form smooth columns but that the dephosphoryla ted CN had formed smaller discrete aggregates that clustered together to form a more compact matrix.