Effect Of Counter-Rotating Twin Screw Extruder On The Physicochemical And Functional Properties Of Sago (Metroxylon Sagu) Starch
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Date
2007-03
Authors
Aisah Bujang
Journal Title
Journal ISSN
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Publisher
Universiti Sains Malaysia
Abstract
This research project was conducted to study the extent of destruction of sago starch
(Metroxylon sagu) by using counter-rotating twin-screw extruder. The effect of primary
extrusion parameters, screw speed (100, 125, 150, 175 and 200 rpm) and moisture
content (3, 40, 50 and 60% w/w dry basis) was evaluated with barrel temperature
kept constant. The changes in both physicochemical and functional properties
induced by extrusion processing were studied. High shear forces of the counterrotating
twin screw have been suggested in this study to caused not only granular
destruction but also destructions at the molecular level. Initial granular structural
destruction was observed on the physical structures of the extrudates obtained.
Maximum expansion ratio of 7.83 was obtained at 125 rpm screw speed and 30% w/w
db moisture content, with lowest bulk density of 0.128 g/cm3 and minimum breaking
strength of 0.548 Nm2• Cross section of the extrudate by SEM micrographs also
showed highly porous structures with polyhedral gas cells formed. Swelling capability,
shape of the gas cells and smoothness of the inner gas cells surface points out that
starch granules breakdown have occurred. The destruction of starch granular
structure was evident by the increase of water solubility capability up to 72.26% and
decrease in water absorption capacity (4.37 gIg). The destruction was also observed
in the changes of pasting behaviour. Partial loss of crystallinity observed from x-ray
diffractograms showed shifting of native sago starch x-ray pattern form C-type to Btype
for extruded sago starch. Formation of amylose-lipid complex was induced by
availability of amylose released due to melting of granular crystallinity which is
facilitates by extrusion at high shearing rate. Reduction in the intensity stretching of glycosidic C-C starch bond linkages, increase intensity of OH groups stretching and
formations of a and 13 anomers as showed by FTIR spectra have qualitatively suggested that higher level of destructions have occurred. Maximum degree
hydrolysis of 86.77% obtained showed improvement on the in-vitro digestibility of
extruded sago starch compared to the native sago starch (63.25%) after 12 hours of
incubation at gODe with thermostable a-amylase. Reduction in intrinsic viscosity from
127.62% for native sago starch to 40.35% for sago extrudate (100rpm screw speed.
30% w/w db moisture content) suggested that reduction of starch molecular weight
with more linear starch chains was released.
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Technology