APPLICATION OF POLYMER BLENDING LAWS TO STARCH-GELATIN COMPOSITES

The small-deformation rheology of a range of starch-gelatin composites has been studied, to test the applicability of the isostress and isos train blending laws to biphasic biopolymer networks. Waxy maize starch (crosslinked and uncrosslinked) was used to minimise release of solub le material into the gelatin phase, and was gelatinised at 80 degrees C. Phase volumes were determined by measurement of the increase in con centration of the gelatin phase due to swelling of the starch granules . The swelling volumes obtained for the crosslinked and uncrosslinked samples were 9.0ml/g and 9.65ml/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 q uenching to 5 degrees C and measured under a controlled time-temperatu re regime (150 min at 5 degrees C; heated to 20 degrees C at 0.25 degr ees/min). The value of the storage modulus (G') of the gelatinised sta rch granules, which is the only unknown parameter, was varied by a sta ndard minimisation procedure to obtain the best least-squares fit betw een observed values of log G' for the composite gels and those calcula ted by the polymer blending laws. Good agreement between observed and calculated values was obtained using the isostress model when the tria l value of the starch modulus was higher than that of the gelatin phas e, and the isostrain model for the converse situation. The fitted modu li for the swollen granules (730 Pa and 55 Pa for the crosslinked and uncrosslinked samples, respectively) are close to those observed exper imentally for the same starches gelatinised in water to a packing frac tion of similar to 75%. Copyright (C) 1996 Elsevier Science Ltd