Enzymatic hydrolysis of native granular and modified starch below gelatinization temperature
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Date
2010
Authors
Yussof, Norshariffa
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Abstract
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.