Co-gels of whey protein isolate with crosslinked waxy maize starch: Analysis of solvent partition and phase structure by polymer blending laws

Abstract

Co-gels of whey protein isolate (WPI) and phosphate-crosslinked waxy maize starch (PCS) were formed by heating from 50 to 80 °C at 1 °C/min, holding for 30 min at 80 °C, cooling to 20 °C at 1 °C/min and holding for 30 min at 20 °C, with measurement of storage modulus ( G′) and loss modulus ( G″) at 1 rad s −1 and 0.5% strain throughout. G′ and G″ remained essentially constant on holding at 20 °C, and average values of log G′ during the holding period were used in analysis of co-gel moduli by the Takayanagi isostrain and isostress blending laws. WPI in the absence of starch and PCS in the absence of WPI were subjected to the same time–temperature regime as the mixtures, to provide calibration curves for use in the analyses. As anticipated, WPI and PCS showed independent thermal transitions in DSC heating scans, with no indication of any associative interactions between the two materials. During cooling, the mixtures showed appreciable increases in G′, similar to those observed for WPI alone, whereas slight decreases were observed for PCS, indicating a continuous WPI phase and dispersed starch phase. The magnitude of the increase in G′ observed on gelation of WPI also indicated development of a continuous protein network. Blending law analyses in which the starch phase was assumed to consist of fully swollen PCS granules dispersed individually through a WPI gel matrix were unsuccessful in matching the observed values of log G′ for the composites, irrespective of whether the swelling volume of PCS was held fixed at the value of Q≈11.9 mL/g determined experimentally by the polymer-exclusion method using blue dextran, or treated as a variable parameter, along with the modulus of individual swollen granules, which was varied in both analyses. However, an analysis in which PCS was regarded as a second gelling polymer gave a high standard of agreement ( r 2=0.994) between observed and fitted values of log G′ for all mixtures studied (18 in total, with concentrations of PCS ranging from 1 to 5 wt% and concentrations of WPI ranging from 1 to 10 wt%). The only variable in this analysis was the “solvent avidity parameter”, p, defined as the ratio of water to polymer in the (continuous) WPI phase divided by the corresponding ratio for the (dispersed) PCS phase. The best fit was obtained with p≈3.3. This high relative affinity of WPI for water implies restricted swelling of PCS except at low concentrations of either or both components of the mixtures. The changes in G′ observed on heating indicated that at high concentrations of WPI separation into a dispersed starch phase in a continuous protein matrix occurred prior to gelatinisation of PCS. At lower concentrations of WPI, however, gelatinisation of PCS was accompanied by large increases in G′, indicating formation of a starch network which was then fragmented by gelation of WPI. Fragments of starch network (on the length scale of ∼10–100 μm) embedded in a WPI gel matrix were visualised directly by light microscopy.