Flow and viscoelastic properties of cereal starch/hydrocolloid pastes and gels

The rheological behavior of cereal starch + hydrocolloids systems is discussed in relation to the interactions taking place in the mixed system. Examples are given with non-gelling (galactomannans) and gelling (K-carrageenan) polysaccharides. The rheological characterization is performed by means of measurements in steady shear (viscometry) and in oscillatory shear (viscoelasticity). Viscometry allowed one to describe the properties of the dispersions before gelation takes place. Viscoelasticity measurements provided a way to assess the properties of the hot pastes, the kinetics of gel formation as well as the final properties of the gel. These measurements are combined with a description of the swelling-solubility behavior of starch. It is showed that when dealing with starch keeping its granular structure after pasting (wheat or 'normal' maize, crosslinked waxy maize starch or CWMS), the polysaccharide remains located in the continuous phase upon swelling of starch granules. Large differences can be noted between amylose-free (CWMS) and amylose-containing starches. In the case of CWMS, the main effect of the added polysaccharide is to enhance the viscosity of the continuous phase. Let the polysaccharide be thickening or gelling, the system exhibits the properties of either a suspension of swollen particles dispersed in a macromolecular solution or the properties of a composite gel composed of highly deformable particles embedded in a continuous gel matrix. When dealing with amylose-containing starch (wheat, 'normal' maize), an additional mechanism is experienced due to the presence of amylose in the continuous phase together with the polysaccharide. As a result of incompatibility of amylose with the added polysaccharide, phase separation takes place which interferes with the gelling process of amylose. The kinetics of gel formation of starch + hydrocolloid mixed systems as well as the final properties of the gel are strongly influenced by the phase separation process.