CO-GELATION OF AGAROSE AND WAXY MAIZE STARCH

The rheology of composite gels formed by gelatinization of (uncrosslin ked) waxy maize starch (WMS; swelling volume approximate to 12 ml g(-1 )) in agarose solution (0.25 wt%) at 80 degrees C, and gelation of the agarose component by rapid quenching to 5 degrees C, has been examine d by small-deformation oscillatory measurements. At low concentrations of starch (up to similar to 2 wt%) the composite moduli (G'(C)) follo w those of the agarose phase (isolated by centrifugation in the pre-ge l solution state), indicating that the swollen granules are present as a dispersed phase within a continuous biopolymer matrix. At higher co ncentration of starch (3-5 wt%) there is a sharp reduction in G'(C) an d the experimental moduli show close agreement with values calculated for a bicontinuous network. Bicontinuity was confirmed by the presence of a residual agarose network after elimination of the starch compone nt by enzymic hydrolysis. On further increase in WMS concentration to 6 wt% (starch phase-volume approximate to 72%) there is a second sharp reduction in G'(C), towards the modulus of WMS alone, indicating that starch has now become the dominant component. The reduction continues until the starch concentration reaches the reciprocal of the swelling volume (at similar to 8.3 wt%); at higher concentrations, where swell ing is restricted by availability of water, the modulus rises, consist ent with a simple increase in the local concentration of the dominant starch network. In the bicontinuous region (3-5 wt% WMS), but not at h igher or lower concentrations of starch, the composites show extreme s train-sensitivity, with structural breakdown occurring at similar to 0 .3% strain. From comparison with previous studies of the behaviour of WMS and phosphate-crosslinked waxy maize starch (PCS) in composites wi th other biopolymers (gelatin and xanthan), it is suggested that the d istribution of the two constituent phases is determined by (i) the eas e of dissociation of the weak starch network formed on gelatinization, to reduce topological restrictions on formation of a stronger (i.e. m ore enthalpically favourable) biopolymer matrix, and (ii) the extent t o which the biopolymer promotes association of starch granules by depl etion flocculation. (C) 1998 Elsevier Science Ltd. All rights reserved .