Effect of thermomechanical treatment on the rheological properties of crosslinked waxy corn starch

Abstract

Cross-linked waxy maize starch (CWMS) are currently used as thickening or gelling agents in the food industries under high heating temperature and high shear conditions such as occurs, for example, in homogenizers. The rheological properties of CWMS dispersed in water have been studied in relation to its swelling behaviour. The main parameters that have been dealt with are starch concentration (2–4%), pasting temperature (90–130 °C) and shear conditions (∼10 5 s −1). The swelling behaviour was assessed by means of swelling experiments and particle size analysis. The rheological study was performed by means of steady shear measurements (viscometry) and in oscillatory shear (viscoelasticity). The rheological properties of CWMS suspensions were strongly dependent on the concentration regime of the suspension. A yield volume fraction of ∼0.4 was found to be critical. Beyond this value, starch dispersions exhibited the behaviour of suspensions of swollen particles, that means shear-thinning behaviour with a yield stress and measurable viscoelastic properties, typical of a gel-like system. All these properties strongly depended on the pasting temperature with an optimum around 124 °C for which the starch suspension was considered as “well-cooked”. Sensitivity of starch granules to shear forces was also strongly dependent on the pasting temperature. When starch granules were undercooked, their swelling properties, and hence their rheological properties were reinforced by additional high shearing. In contrast, when the starch granules were well-cooked or overcooked the rheological properties were depressed by shearing as a result of their high fragility. These overall results allowed evaluation of the role of the combined effect of heating temperature (above 100 °C) and shear conditions on the swelling behaviour and rheological properties of suspensions of CWMS in the formulation of starchy products.