STUDIES ON THE ENZYMATIC HYDROLYSIS OF RAW SAGO STARCH

Abstract

The development of starch hydrolytic procedures without the need for gelatinization would eliminate the need for energy input and the problems associated with handling viscous pastes. Sago starch has been used as a model for the action of enzymes on starch granules. Five batches of sago starch from different botanic and geographic sources were hydrolysed by glucoamylase, ?-amylases, pectinase and debranching enzymes. Results were compared with those of tapioca and corn starches. Native sago starch was found to be very resistant to the action of all enzymes tested. A strong synergistic effect between gluoamylase and bacterial ?-amylases on the hydrolysis of starch granules was observed. Debranching enzymes and pectinase had no significant effect on the hydrolysis of sago starch granules by ?-amylase, glucomylase and the mixture of these two types of enzyme. A pretreatment process of incubating starch slurry at 600C for 2 h prior to hydrolysis greatly increased the susceptibility of starch granules to enzyme action. The degree of hydrolysis by Termamyl, an ?-amylase, and AMG, a glucoamylase, was increased from 30% to 81% at 32 h. the synergistic effect between termamyl and AMG was even stronger than that on native or non-treated starch. High performance size and pretreated sago starch with single or enzyme mixture, did not change the apparent molecular weights and the ratio of the two major components, amylose and amylopectin, in hydrolysed granule residues. Scanning electron microscope observation revealed that pretreatment altered the sago starch degradation pattern. Degradation for native starch was observed mainly as surface erosion. In contrast, digestion of pretreated sago starch occurred in a single specific loci regardless of the enzymes used. The result of this hydrolysis was the development of a single deep round hole on the starch granules. These degradation patterns were further confirmed by coulter counter granule size distribution analysis, light microscope photographs of sections of granule residues. The “single round hole” degradation pattern was not found with pretreated corn and tapioca starches. Differential scanning calorimeter indicated that while the enthalpy of gelatinization for non-treated starch remained constant during the course of hydrolysis, enthalpy of gelatinization for treated starch decreased in proportion to the extent of hydrolysis. The narrowed gelatinization for treated starch decreased in proportion to the extent of hydrolysis. The narrowed gelatinization range and increased gelatinization temperature suggested that annealing had occurred in starch granules during pretreatment. Investigation of the effect of pretreament conditions on hydrolysis illustrated that there was a correlation between extent of annealing and sago starch granule hydrolysis.