Application of polymer blending laws to starch-gelatin composites

Abstract

The small-deformation rheology of a range of starch-gelatin composites has been studied, to test the applicability of the isostress and isostrain blending laws to biphasic biopolymer networks. Waxy maize starch (crosslinked and uncrosslinked) was used to minimise release of soluble material into the gelatin phase, and was gelatinised at 80 °C. Phase volumes were determined by measurement of the increase in concentration of the gelatin phase due to swelling of the starch granules. The swelling volumes obtained for the crosslinked and uncrosslinked samples were 9.0 ml/g and 9.65 ml/g, respectively, and were independent of gelatin and starch concentration over the range used (0.88–1.50 wt% gelatin; 1–5 wt% starch). The gelatin-gelatinised starch composites and gelatin-phase samples isolated by centrifugation were gelled by quenching to 5 °C and measured under a controlled time-temperature regime (150 min at 5 °C; heated to 20 °C at 0.25 °/min). The value of the storage modulus ( G′) of the gelatinised starch granules, which is the only unknown parameter, was varied by a standard minimisation procedure to obtain the best least-squares fit between observed values of log G′ for the composite gels and those calculated by the polymer blending laws. Good agreement between observed and calculated values was obtained using the isostress model when the trial value of the starch modulus was higher than that of the gelatin phase, and the isostrain model for the converse situation. The fitted moduli for the swollen granules (730 Pa and 55 Pa for the crosslinked and uncrosslinked samples, respectively) are close to those observed experimentally for the same starches gelatinised in water to a packing fraction of ~75%.