Film-forming properties of a modified starch/κ-carrageenan mixture in relation to its rheological behaviour

The mechanical and calorimetric properties of acid hydrolyzed hydroxypropylated pea starch (HPPS)/κ-carrageenan (κC) cast films were investigated in relation to the rheological behaviour of the gelled film-forming dispersions. The mixture was prepared by mixing HPPS (25%, w/w), an easily slurried non-gelling polysaccharide, and a K+κ-carrageenan (⩽1% w/w) used for its gelling properties. Whatever the κ-carrageenan content, the total ionic concentration (K+) was maintained constant to control the transition temperatures. Compared to individual components, the rheological properties of the hot mixtures at 60 °C showed a dramatic increase of the viscosity. Upon cooling, stronger gels were formed compared to those of the pure κ-carrageenan in similar ionic conditions. Meanwhile, these gels set and melted at temperatures around 10 and 20 °C higher than those of the κ-carrageenan alone in the same conditions. These phenomena were attributed to excluded volume effects between κ-carrageenan and starch resulting in the increased concentration of the carrageenan forming the network. Drying was performed under controlled conditions at 25 °C and 40% relative humidity. The glass transition temperature Tg of HPPS at different water contents was obtained by calorimetry. The properties of the cast films were evaluated on the basis of mechanical and calorimetric measurements. Overall, the final film properties of the blends were similar to those of the films with starch alone. This means that the influence of the κ-carrageenan on the solid-state behaviour of the blends was hidden, in spite of the strong influence of κ-carrageenan on the rheological behaviour in the solution and in the gel state.