PHYSICAL MODIFICATION OF SAGO STARCH FOR INDUSTRIAL USES

Abstract

Native starch is of limited value for most applications or processes. Modifications of native starches are carried out to provide starch products with the properties required for specific uses. Physical modifications like extrusion, drum-drying and dry heat treatment have been widely used in producing modified starches for food and non-food applications. Sago starch is one of the major starch source in the Asia Pacific region. In spite of the availability, quality and price, this starch is not fully utilized due in part to limited understanding of its behaviour in conversion processes. Experiments were undertaken to establish a range of extrusion working conditions for sago starch. The feasible extrusion conditions comprise of two combinations of processing variables, namely low moisture/ high temperature and high moisture/ low temperature. Response Surface Methodology was employed to study the effects of processing variables, namely feed moisture content, barrel temperature and extruder screw speed, on product characteristics. The responses investigated were degree of polymerization (DP), intrinsic viscosity ([?]), degree of gelatinization (DG), Water Solubility and Water Absorption Index (WSI & WAI), enzyme susceptibility (ES) and degree of degradation (DGR). The variation of each of these responses could be accurately described by a fitted regression model. A model for the extrusion of sago starch was proposed. Extrusion of sago starch seemed to follow either one of the two routes depending upon extrusion conditions. Low-moisture extrusion resulted in early mechanical fragmentation of starch granules. Extrudates of very low intrinsic viscosity were formed. Low WAI and ES were also observed. Amylopectin was extensively degraded to smaller molecular weight materials. In contrast, gelatinization was found to be the predominant mechanism in high-moisture extrusion, with little starch degradation. High-moisture extrusion appeared to be a simpler system than low-moisture extrusion, with negligible unknown interfering factors. Dry heat treatment of sago starch could be well described in terms of moisture content, roasting temperature and time. However, moisture content did not appear to affect the product properties. The predominant reaction below l60°C seemed to be depolymerization of starch molecules whilst transglucosidation became more significant at a roasting temperature above l 60°C. The extent of depolymerization of starch generally increased with increasing roasting temperature and time. High roasting temperature seemed lo promote branching of dextrins. Dry heat treatment was found to be an efficient process for dextrin production from sago starch. 100% soluble dextrins could be formed under comparatively mild conditions ( 140°C for 10 hrs or 160°C for 5 hrs).The relative ease in obtaining completely soluble dextrins from sago starch should enable them to be more widely applicable in the food and cosmetics industries. The effect of HCl as catalyst was also examined. A greater variety of dextrins could be produced in the presence of catalyst. The data obtained in these investigations establish the conditions for physical modification of sago starch, and should prove useful in its commercial applications.