HYDROLYSIS OF STARCH BELOW GELATINIZATION TEMPERATURE USING AN AMYLOLYTIC ENZYME
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Date
2010-04
Authors
UTRA, UTHUMPORN
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Abstract
The action of amylolytic enzymes (a-amylase and glucoamylase) for their
ability to hydrolyze starch in native granular state (below gelatinization temperature)
and the effect of molecular organization and fine structure of starch polymers on the
extent of hydrolysis had been studied. Hydrolysis of raw starches below
gelatinization temperature was studied in order to understand the action of the
enzyme during hydrolysis. Starches from different botanical origin (com, mung bean,
sago and potato) were chosen and hydrolysis was carried out at 35°C for 24 hours.
Then, the susceptibility of cereal starches with different amylose: amylopectin ratios
(normal com, waxy com, high amylose com, rice and waxy rice) towards hydrolysis
were evaluated in order to establish a correlation between physicochemical and
molecular structure of starch. Wheat starch (large and small granules) was
hydrolyzed as to evaluate the effect of granule size on the rate and extent of
hydrolysis. Starches were also pre-treated with various pre-treatment before
subjected to hydrolysis in order to give maximum conversion of starch to
fermentable sugars. The effect of heat treatment (50°C for 30 minutes) on the
susceptibility of com, mung bean, sago and potato starches towards enzymatic
hydrolysis was investigated. Then, the effects of removing lipid and protein
(surface/indigenous) of cereal starches on the extent of hydrolysis were studied.
Starches were defatted (75% aqueous n-propanol for 7 hours) and protein in starches
was removed by treating the starches with NaOH (0.1 %) and protease treatment
(0.3% w/v). Hydrolyzed starches were analyzed for the degree of hydrolysis,
physicochemical and functional properties. Com starch yielded the highest value of
XXV
dextrose equivalent (DE) (53%), followed by mung bean starch (36%), sago starch
(15%) and potato starch (12%). SEM micrograph showed the presence of natural
pores on com starch and hydrolyzed com starch exhibited more porous granules
compared to other starches. For different ratio of amylose: amylopectin, waxy com
and normal com starch was the most susceptible being converted into 54% and 53%
DE, followed by high amylose com (15%), waxy rice (14%) and rice (7%). The
small granule of wheat also showed higher DE compared to large wheat with DE at
37% and 27%, respectively. All the pre-treated starches (protease treatment, NaOH
treatment, heat-treatment and defatting) showed significant increased in the DE
compared to native starches after hydrolysis. SEM micrographs revealed that
hydrolyzed treated starches had more porous granules and more susceptible to
hydrolysis compared to native starches. The particle size distribution and amylose
content of treated starches decreased significantly. However, no significant changes
were observed in X-ray diffraction pattern of hydrolyzed starches, suggesting that
hydrolysis preferentially occurred in amorphous region. The action of enzymes
caused significant changes in swelling power and solubility, pasting and
gelatinization properties. Evidently, the enzymes were able to hydrolyze granular
starches and pre-treatment before hydrolysis significantly increase the rate of
hydrolysis
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Keywords
HYDROLYSIS OF STARCH BELOW GELATINIZATION TEMPERATURE