Enzymatic hydrolysis of native granular and modified starch below gelatinization temperature

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Date
2010
Authors
Yussof, Norshariffa
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The effects of mild heat treatment, cross-linking and annealing towards the susceptibility of granular starch to enzymatic hydrolysis at sub-gelatinization temperature (35°C) were investigated. Mild heat-treatment was carried out at 60°C for 30 minutes. Cross-linking was performed by re-acting the starches with 1% epichlorohydrin (ECH), while for annealing the starches were annealed at 50°C for 72 hours. Tapioca and sweet potato starches were subjected to enzymatic hydrolysis with a mixture of fungal a-amylase and glucoamylase at 35°C for 24 hours. The dextrose equivalent (DE) value of heat-treated starch increased significantly compared to native starch i.e. 36% to 50% and 27% to 34% for tapioca and sweet potato starch, respectively. The DE of cross-linked tapioca starch decreased significantly from 36% to 28%. Both annealed tapioca and sweet potato starches were hydrolyzed to a greater degree compared to native starch. Scanning Electron Microscopy (SEM) examination on hydrolyzed starches showed that enzymatic erosion occurred mainly at the surface of starch granules. Hydrolyzed heat-treated starch exhibited rougher surface and porous granules compared to native starch. SEM micrograph revealed that hydrolyzed cross-linked starches had less porous granules as compared to their native counterparts. Both hydrolyzed annealed starches had more porous granules and rougher surface as compared to native starch due to enzymatic erosion. The swelling power and solubility of hydrolyzed heat-treated, hydrolyzed cross-linked and hydrolyzed annealed starches decreased significantly as compared to their native counterparts for both tapioca and sweet potato starch. Crosslinking and annealing had affected the pasting properties of the starches significantly and increased the gelatinization temperature. The amylose content increased significantly in hydrolyzed heat-treated and annealed tapioca and sweet potato starches. X-ray analysis suggested that enzymatic erosion preferentially occurred in amorphous areas of the granules for all starches as no significant changes were observed in the x-ray diffraction patterns of hydrolyzed native and pre-treated starches. The hydrolyzed pre-treated tapioca and sweet potato starches basically showed smaller value of particle size distribution in contrast to their native counterparts. Evidently, heating treatment below gelatinization temperature was effective in enhancing the degree of hydrolysis of granular starch. Cross-linking was found to reduce the extent of hydrolysis of tapioca starch by reducing the swelling power and solubility of the starch granules. This study also showed that arlnealing treatment had allowed the enzymes to hydrolyze the starch more ext~nsively by the presence of pores on the starch granules created during the treatment.
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