Colloidal calcium phosphate in bovine casein micelles, and its impact on rheological properties of rennet-induced gels and cheese texture

A simple model for colloidal calcium phosphate (CCP) nanocluster was theoretically developed. Two important features in this model were (1) theoretical molecular weight (Mw) was 4,900&sim9,800 g/mol, and (2) 4 casein molecules were involved in a single CCP nanocluster. To experimentally support the derived Mw of CCP nanocluster, size-exclusion chromatography was used to separate CCP nanoclusters fraction from trypsin- or papain/proteinase-treated milk samples and multiangle laser light scattering was used to measure the Mw of this fraction. The Mw was 7,282&sim13,772 g/mol, which was close to the theoretically-derived Mw. To elucidate the possible role of the interactions between casein molecules and CCP nanoclusters in the textural properties of gels/cheeses, we measured the rheological properties of rennet-induced gels/cheeses made from milk that was acidified to lower pH values or that had added EDTA. The gelation profiles showed a maximum storage modulus (GM), and a decrease in storage modulus (G') values. In low pH gels or gels with added EDTA, low GM and high loss tangent (LT) values were observed due to the loss of insoluble (INS) Ca content. The loss of INS Ca reduced casein cross-linking and resulted in a more flexible network in these milk gels. The presence of GM was due to macro/microsyneresis reaction occurring in rennet-induced gels during very long reaction times. The loss of INS Ca stimulated inter/intra-molecular rearrangements of casein particles as indicated by higher LT values, which were responsible for the observed decrease in G' values after GM. A modified Carlson model was fitted to experimental G' values as a function of time and this model predicted this decrease, and also reasonably predicted the observed rheological changes in rennet-induced gels. The rate constant for macro/microsyneresis derived from this modified Carlson model could be useful for studying macro/microsyneresis and how this reaction is affected by variables. When milk with depleted levels of INS Ca was processed into cheeses, a decrease in G' values and an increase in LT values were observed at high measuring temperature. This indicated that the loss of INS Ca crosslinking can cause the higher melt and flow of these cheeses. Therefore, the concentration of CCP nanoclusters can be used as texture modifier in dairy gels/cheeses.