CHEMICAL MODIFICATION OF SAGO AND TAPIOCA STARCHES AND A STUDY OF THE PRODUCT PROPERTIES

Abstract

Natural starches have deficiencies which limit their use for manufacturing processes. Low concentrations of native starch suspensions on heating and subsequent cooling are characterized by association of amylase chains which precipitate out of solution; higher concentrations form gels. Introduction of phosphate groups into amylose and amylopectin chains leads to irregularities which prevent chain association, thereby stabilizing cooked pastes. Clarity of a paste varies with the botanical source of starch; it can be manipulated by chemical modification of the granules. Sago starch has limited commercial value as its properties vary, depending on its source and purification conditions. The functional and chemical properties of three different batches of sago starch were analyzed to determine the extent of variation in quality. Starches were modified to produce starch phosphate esters. Modification appeared to lessen the interbatch variation in functional properties, thus modification with phosphate salts may be a means whereby the variability in certain functional properties of different batches of sago starch could be reduced. This project utilizes a Noncentral Composite Design together with Response Surface Methodology (RSM) to produce urea-starch phosphates from sago and tapioca starches, as well as to investigate the effect of processing conditions on functional properties of the products. Also, an attempt was made to "copy" the functional properties of a commercial urea-potato starch phosphate by modifying sago and tapioca starches. RSM simultaneously considers all independent variables affecting a desired response and their interrelationships. The commercial modified starch is used to bind clay pigments which are applied to high-quality water-resistant printing paper in the coating process. Such coatings strengthen the paper during formation in the wet state and improve its surface properties in the dry state. The paste viscosity of starch must remain constant during coating, otherwise the paper will be undersized or oversized. Native starch is too viscous to be used as a binder in paper coatings. The use of modified starch allows a higher concentration (commonly 20 - 30%) for coating while maintaining a low and stable viscosity. 10g sago or tapioca starch, baked in the presence of 0.5g urea and 3M phosphate at pH 3.5 and 120°C, gave functional properties similar to the commercial sample. The R2a values for most of the regression models were high (greater than 0.700), but only 0.424 for the tapioca starch viscosity model. Contour plots were useful in predicting functional properties of products made under specified conditions; only tapioca starch viscosity could not be easily predicted, which perhaps indicated the significance of factors other than those considered in the model. The importance of residual analysis in RSM is highlighted Winsorization of outliers led to improved R2a values, but the predictive value of the models decreased. Structural analysis of urea-starch phosphates revealed that phosphates might have crosslinked amylose and amylopectin chains during modification. Processing conditions seemed to play a role in determining the relative proportions of ammonium-N, and nitrate-N formed during modification.