Effects Of Degree Of Cross-Linking On Physical Properties, Pasting And Freeze-Thaw Stability Of Cassava Starch Modified By Reactive Extrusion Process (Rex)
This study aimed to investigate a modification of cassava starch through phosphorylation and cross-linking reaction using a reactive extrusion process (REX). It combined chemical and physical alteration of starch in a single process. Cross-linking reaction of cassava starch was achieved using a pilot scale twin screw extruder under controlled temperature (40°C) and screw speed (200 rpm) conditions. A range of sodium trimetaphosphate (STMP) (0.5 – 2.0% dry weight starch), NaOH concentration (0.05 – 0.15 M) and moisture content (50 – 85% dry weight starch) was preliminarily studied to produce cross-linked starch with three different degree of cross-linking; 1) low degree of cross-linking (50-69%); 2) medium degree of cross-linking (70-89%); and 3) high degree of cross-linking (>90%). Physical properties (water solubility, WSI and swelling power), pasting properties, gel properties and freeze-thaw stability of cross-linked cassava starch samples were studied in comparison with those of extruded starch control (non-cross-linked) and native cassava starch samples. The results indicated that the clarity and peak viscosity of starch decreased with an increase in degree of cross-linking. Cross-linking reduced the water solubility index and swelling power of starch. Introduction of phosphate cross-links into the starch restricted the mobility of the molecular structure, leading to a reduction in WSI and viscosities of starch. Extruded starch controls and all cross-linked cassava starch samples showed higher swelling power at 30 °C than the native cassava starch. At 80 °C, the swelling power of all cross-linked cassava starch samples was similar to the native starch but higher than the starch control equivalents. The maximum peak and final viscosities were shown in lowest degree of cross-linking. Gels made from cross-linked starch samples showed greater strength and stability against freeze-thaw process compared to the native starch. This study suggested that the low and medium degree of cross-linked starch could be use as thickener and stabilizer in a wide variety of food products.
Keywords- Cassava starch, Cross-linking, Reactive Extrusion, Stabilities, Physical Properties.