Rheology of caseinate fractions in relation to their water holding capacity

In this paper we relate rheological properties of whole caseinate and various caseinate fractions to the improved water holding capacity of caseinate gels on which we reported previously. In that separate paper it was found that as-casein (ASCN) enriched sodium caseinate (NaCN) fractions gave a substantially better water holding capacity. We hypothesized that this was due to the ` telechelic' character of ASCN. Here we systematically investigated the low shear rheology and viscoelastic properties as a function of concentration, salt content, pH and temperature. It was found that the ASCN enriched fraction had a considerably lower overlap concentration. The overlap concentration depends on the ` bulkiness' of a polymer but also on the interaction. By considering ASCN as a telechelic (two hydrophobic parts connected by a middle hydrophilic part) poly-electrolyte molecule it is understandable that ASCN has a considerably lower overlap concentration (0.058 w/w) than NaCN or b-casein, which both have an overlap concentration greater than 0.1 w/w. We tested the telechelic vis a vis the overlap concentration hypothesis by mildly cross linking NaCN and ASCN depleted NaCN using transglutaminase. Viscosity was increased by a factor 1.5 to 2 and found a lower overlap concentration by almost a factor of two. In a separate paper we also found that the water holding capacity of a caseinate gel depends directly on the cross linking of the gel, which may be physical e. g. by a telechelic ASCN, or chemical, though enzymatic cross linking. (C) 2015 Elsevier Ltd. All rights reserved.