Biomedical Engineering Reference
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The extent of granule erosion thus depended on the nature of the atmospheric gas under
which the irradiations were carried out, and it was observed that the sizes of the pits were
inversely proportional to the solubility of the atmospheric gas in water. The extent of granule
damage increased with increasing irradiation time but decreased with increased starch
concentration in the aqueous starch suspension. Granule damage was more extensive at the
lower frequency of 280 kHz.
Luo and co-workers (2008) examined the effect of ultrasonic treatment on the properties
of maize starch with different amylose contents. Waxy maize starch, normal maize starch
and amylomaize starch containing about 50% amylose were used. Starches were adjusted to
70% moisture content and ultrasonic treatment was carried out for 30 min at 60 °C. Although
treatment with ultrasound did not alter the size or shape of the starch granules, pores were
observed on the surfaces of normal and waxy maize starch granules, and fissures could be
seen on the granule surfaces of amylomaize starch. Treatment with ultrasound increased the
swelling power and solubility of all three starches. X-ray diffraction patterns of the ultra-
sound-treated starches were similar to those of the corresponding native starches, indicating
that the crystalline structures were essentially unchanged by the ultrasonic treatment.
Syneresis of water from frozen and thawed starch gels decreased after ultrasonic treatment
of normal and waxy maize starches, but increased after ultrasonic treatment of amylomaize
starch. The paste-viscosity curve for sonicated waxy maize starch was nearly the same as
that observed for native starch; however, normal maize starch showed a small lowering of
viscosity due to ultrasonic treatment, and a larger decrease in viscosity was observed for
amylomaize starch. It was concluded that ultrasonic treatment preferentially degrades the
amorphous regions of the starch granule and that amylose is more easily degraded than
amylopectin. The changes resulting from ultrasonic treatment thus depend not only on the
crystal structure of starch, but also on its amylose content. Jambrak and co-workers (2010)
examined the effects of ultrasound on the physical properties of corn starch. Aqueous starch
suspensions were treated at 24 kHz using either an ultrasonic probe with intensities ranging
from 34 to 73 W cm -2 or an ultrasonic bath with an intensity of 2 W cm -2 . With the ultrasonic
probe, temperatures after sonication varied from about 18 to 56 °C, depending on the
intensity. Rheological parameters, turbidities, and swelling powers of the treated samples
were compared with those observed for untreated starch.
The ultrasonic degradation of waxy rice starch was studied by Isono and co-workers
(1994). An ultrasonic homogenizer was used at 29 kHz and samples were continuously
pumped through the cell and returned at a rate of 0.7 l/min. The pasting properties of samples
sonicated at 40, 50, and 60 °C indicated that ultrasonic degradation was accelerated at the
temperature where gelatinization begins. A sonication temperature of 60 °C was, therefore,
used for the remainder of the study. Molecular weight decreased with increasing sonication
time and the rate of ultrasonic degradation was independent of polymer concentration at
concentrations below 7%. With long sonication times, the molecular weight approached a
constant value; the molecular weight distribution tended to be fairly narrow.
A number of articles have been published on the effect ultrasound on aqueous solutions
of starch and on aqueous dispersions of gelatinized starch granules. Azhar and Hamdy
(1979) heated potato starch in sodium acetate buffer at concentrations of 1.00, 0.75, and
0.50%, and the dispersions were sonicated at 1-2 °C at 20 ± 0.4 kHz. A rapid decrease in
viscosity was observed during the first five minutes of sonication; the viscosity then leveled
off at longer sonication times. Even though the sonicated swollen granules were disintegrated
by the ultrasonic treatment, and the starch molecular weight was reduced, no reducing
sugars were detected; the rate of enzymatic hydrolysis by
-amylase was not affected.
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