Enzymatic Conversion of Sweetpotato Starch Residue into New Food Materials

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The manufacture of starch from tropical rootcrops like sweetpotato (SP) and cassava (CA) produce considerable amounts of starch residue as by-product. This residue is rich in cell wall material (CWM) which makes it a potential source of dietary fiber. This study characterized the CWMs obtained from the starch residue of three rootcrops, SP, CA, and potato (PO) in terms of their sugar composition and attempted to convert them into soluble dietary fiber enzymatically.To characterize the CWMs from the rootcrops, their sugar compositions were analyzed first. For relatively insoluble samples, pretreatment with 12 M H_2SO_4 prior to hydrolysis with 1 M H_2SO_4 was required for complete hydrolysis. For monosaccharide analysis, HPAEC-PAD method with a CarboPac PA10 column, instead of a PA1 column resulted in reliable sugar composition of these samples. Using this method of analysis, the sugar composition of the CWMs and their polysaccharide fractions was determined. Among the polysaccharide fr … More actions, it was only in the hemicellulose fraction where significant differences in the sugar composition, especially in the galactose content were observed among the three rootcrops. This has been the first report on the sugar composition of CA CWM.To convert the CWMs into soluble dietary fiber, a bacteria producing a CWM-degrading enzyme was isolated from soil, Bacillus sp. M4, and the crude enzyme solution from its culture filtrate was used to solubilize the residues. Bacillus sp. M4 has been found to secrete polygalacturonic acid lyase and glycan depolymerase activities especially arabinanase, but cellulase activity was nearly absent. Sugar analysis of the solubilized product from the CWM of the three rootcrops revealed that it is mainly composed of galacturonic acid, and the neutral sugars found commonly in the pectin fraction, suggesting the presence of a protopectinase activity in M4 enzyme. The mode of action of the crude enzyme was determined by terminal sugar analysis using HPAEC-PAD after hydrolysis of the reduced products. Results revealed that M4 enzyme attacked the galactan as well as rhamnogalacturonan moieties of the protopectin, resulting in the release of a soluble dietary fiber of pectin fraction. Less