VISCOELASTIC PROPERTIES OF MAIZE STARCH HYDROCOLLOID PASTES AND GELS

Maize starch aqueous suspensions (concentration: 4%) have been pasted in the presence of another polysaccharide (guar gum, locust bean gum or xanthan gum) at a concentration ranging from 0.1 to 0.5%. The viscoelastic behaviour of these mixed systems has been investigated by performing harmonic measurements in three steps: (i) a mechanical spectrum at 60-degrees-C, (ii) a gel-cure experiment at 25-degrees-C and (iii) a mechanical spectrum of the gel at 25-degrees-C. The starch dispersion being described as a composite, its viscoelastic properties in the pasted and in the gelled state, as well, are primarily governed by the volume fraction occupied by the swollen particles which accounts for two-thirds of the total volume. The continuous phase makes an additional contribution due to its own viscoelastic properties. The added polysaccharide is concentrated within the continuous phase which makes it play an increasing role in the viscoelastic behaviour of the composite as the hydrocolloid content is increasing. More spectacular effects are experienced with xanthan gum than with the galactomannans in relation to the 'weak' gel properties of xanthan systems. Furthermore, the mechanical spectra of the mixed systems indicate that they are less elastic than with starch alone. Complex modifications in the starch dispersion are thus brought about by the hydrocolloid; these can be ascribed to phase separation processes in relation to incompatibility phenomena between unlike polysaccharides.