Recent Processing Methods for Preparing Starch-based Bioproducts - Food and Industrial Bioproducts and Bioprocessing - page 52

at reduced temperatures. Examination of the effect of sonication frequency on the viscosity
of a 5% waxy corn starch paste, using a bath-type sonicator at frequencies of 183, 143, 99,
and 44 kHz, showed that the final viscosities decreased as the frequency was lowered. Gel
permeation chromatography and light scattering showed that the molecular weight of waxy
maize starch decreased sharply with sonication times up to 30min, and then decreased
slowly. NMR spectra showed that although viscosities decreased during sonication, the
chemical structure of the polymer chains was not altered. Cheng and co-workers (2010)
studied the effects of ultrasonic treatment at 20 kHz on the properties of aqueous dispersions
of gelatinized maize starch, as well as the properties of cast films prepared from the sonicated
dispersions. Treatment with ultrasound greatly reduced the viscosities of the gelatinized
starch dispersions, and fragments of granule ghosts present in the gelatinized dispersions
were disintegrated by sonication. Cast films with good properties were obtained from the
sonicated dispersions.
As an alternative to dispersing, gelatinizing or dissolving starch in water prior to treatment
with ultrasound, Lima and Andrade (2010) pretreated high amylose maize starch (70%
amylose) by first blending with 25, 30, 35, and 40% glycerol and then melt-processing the
mixtures at 100°C. After melt-processing, the samples were extracted with ethanol to
remove glycerol and dried at 50 °C. The samples were then dispersed in water at 5 g/l at
50 °C and sonicated for 30 min at 10 °C at a frequency of 20 kHz. The largest reduction in
intrinsic viscosity was observed for the sample melt-processed with 40% glycerol, and the
higher degradation was attributed to a greater solubility of this sample in water prior to
sonication. NMR spectra showed that melt processing and ultrasonic treatment did not
change the chemical structure of the treated starch. Jackson (1991) examined the sonication
of corn starch dispersions in 90:10 DMSO:water and observed complete solubility, as well
as depolymerization of the starch, when the dispersions were sonicated for 15 or 20s.
Suguchi and co-workers (1994) prepared a 1% dispersion of wheat starch in 90% DMSO;
after cooling to room temperature, the sample was sonicated at 40 °C for 300-2700 s. Gel
filtration and reducing power were used to calculate molecular weights before and after
sonication. The molecular weight was reduced by sonication, and the large amylopectin
peak with MW > 10 8 changed to a broad peak with lower average molecular weight after
sonication for 600 s. A large reduction in molecular weight was observed after 1800 sec, and
a single sharp peak was observed.
Treatment with ultrasound has not only been used to disperse and dissolve starch in both
aqueous and non-aqueous solvents, but has also been used to reduce the viscosities of
aqueous polymer dispersions prepared by graft polymerizing water-insoluble polymers onto
gelatinized starch. Gugliemelli and co-workers (1974) used ceric ammonium nitrate to
initiate the graft polymerization of acrylonitrile onto gelatinized cationic starch (prepared
by reacting wheat starch with 2-chlorodiethylaminoethyl hydrochloride); graft copolymers
containing about 50% grafted polyacrylonitrile (PAN) were obtained. Ultrasonic treatment
of the viscous, aqueous graft copolymer dispersions at 20 kHz for 1-3 min at 5-10 °C yielded
low-viscosity, latex-like dispersions. The decrease in viscosity was attributed to the
disruption of associations between individual particles of graft copolymer. Treatment with
ultrasound had little effect on the molecular weight of grafted PAN. Low viscosity latexes
were also obtained by ultrasonic treatment of graft copolymers prepared using chloroprene
as the monomer (Gugliemelli and co-workers, 1976b). The scope of this research was further
extended to include graft polymerizations of acrylonitrile onto a gelatinized cationic starch
with quaternary amine substituents, as well as graft polymerizations using a mixed monomer
system composed of acrylonitrile plus t -butylaminoethyl methacrylate (Gugliemelli et al .,