Recent Processing Methods for Preparing Starch-based Bioproducts - Food and Industrial Bioproducts and Bioprocessing - page 46

typical of tuber starches to those resembling cereal starches. There was also a reduction in
starch granule solubility and a change in the X-ray diffraction pattern of potato starch was
observed. Although SEM showed some distortion of starch granules after microwave
treatment, the granule structure was maintained. Similar studies were carried out with wheat
starch, corn starch and waxy corn starch, in which the moisture contents were adjusted to
30% prior to microwave irradiation (Lewandowicz et al ., 2000 ). Investigations of pasting
properties showed that microwave heating of wheat starch and corn starch for 60 min caused
a reduction in viscosity during pasting whereas there was little effect on the viscosity of
waxy corn starch. The degree of crystallinity of wheat starch and normal corn starch was
decreased by microwave heating whereas the crystallinity of waxy corn starch remained
almost unchanged. Dyrek co-workers (2007) used electron spin resonance (ESR) to
investigate the free radicals formed in corn starch, potato starch and oxidized potato starch
when the starches were heated by microwave irradiation and by conventional thermal
heating. Both methods of heating generated free radicals with ESR signals.
A number of publications describe the use of microwave heating to form ester and ether
derivatives of starch. Koroskenyi and McCarthy (2002) reacted potato starch with acetic
anhydride and sodium acetate (an acetylation catalyst) for five minutes to yield starch acetate
with a DS of 0.37. Evaporation of acetic anhydride during microwave heating prevented the
use of longer radiation times. Succinate esters of starch were also prepared and a succinate
ester with a DS 0.59 was obtained in the presence of glacial acetic acid solvent. Ether
derivatives of starch that contained tertiary amine and quaternary amine substituents were
prepared by microwave-heating starch with aqueous solutions of 2-(diethylamino)ethyl
chloride and (3-chloro-2-hydroxypropyl)trimethylammonium chloride under alkaline
conditions for 1-10min. DS values varied from 0.21 to 1.54, depending on experimental
conditions and were comparable to the DS values obtained by conventional heating for two
hours at 70 °C. Cationic amine derivatives of starch are used extensively as retention aids in
papermaking. Singh and Tiwari (2008) also used microwave heating to prepare methyl ethers
of starch; complete methylation of soluble starch was observed in less than five minutes by
heating starch with iodomethane and potassium hydroxide.
Shogren and Biswas (2006) used microwave heating to prepare starch acetates and
determined the effects of reaction conditions and starch type (waxy, normal, or high amylose)
on product properties such as water solubility, swelling, and viscosity. Reactions were
carried out with mixtures of dry starch, acetic acid and acetic anhydride in a sealed Teflon
vessel. Maximum temperatures varied from 120 to 175 °C, and DS values for the resulting
starch acetates varied from 0.19 to 0.77. Molecular weights of the starch acetates were lower
than those of the native starches. Heating was also carried out in a conventional block heater
that was adjusted to give a temperature profile similar to that used with microwave heating.
With a heating time of six minutes, the starch acetate prepared by microwave heating had a
DS of 0.72 whereas a DS of 0.56 was obtained with conventional block heating. The
iodine-catalyzed acetylation of starch with acetic anhydride in the absence of acetic acid
was also studied (Biswas et al ., 2008). Since iodine is a good absorber of microwave
radiation, it was assumed that addition of iodine would accelerate the microwave-heated
reaction. Acetic anhydride was used with three different concentrations of starch, and
varying amounts of iodine were added. Mixtures were then microwave-heated to 100 °C and
held at this temperature for two minutes. In the absence of iodine, the DS was only 0.03.
However, the DS increased with increased amounts of iodine and values approaching
three could be obtained if sufficient iodine was added. Acetylation under these conditions
reduced the molecular weight of starch, probably because of the action of iodine as an