Biomedical Engineering Reference
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oxidizing agent. Although esterification with acetic anhydride occurred readily, reaction
with propionic anhydride was more difficult, even with iodine catalysis. A similar method
of acetylation was used by Sánchez-Rivera and co-workers (2010) to prepare acetate esters
of banana starch.
Microwave heating was also used by Xing and co-workers (2006) to prepare maleate
esters from dry corn starch. Starch samples with different moisture contents were mixed
with maleic anhydride and heated, either in a microwave oven for different periods or in a
conventional oven at 100 °C for 1-6 h. Shorter reaction times and higher DS values were
obtained with microwave heating. Some moisture in the starch sample was necessary to
obtain good reaction efficiency; maximum efficiency was observed with a water content of
12.1%. SEM showed that esterification did not greatly change the morphology of the starch
granules, since not enough water was present to cause gelatinization. Biswas and co-workers
(2006a) observed a rapid reaction of corn starch with maleic anhydride when the reaction
mixtures were microwave-heated in DMSO in the presence of pyridine. The effects of
reaction time, microwave power, and the amounts of maleic anhydride and pyridine used
were determined; addition of pyridine yielded products with DS values up to 0.30. DMSO
is a good solvent for microwave reactions, since it readily absorbs microwave radiation and
heats up rapidly. When the reactivities of maleic anhydride, succinic anhydride, and octenyl
succinic anhydride were compared in DMSO without addition of pyridine, succinic
anhydride and octenyl succinic anhydride gave starch esters with a DS of 0.3, compared to
a DS of only 0.1 with maleic anhydride. Compared to conventional block heating, microwave
heating produced starch esters with higher DS values, and in slightly higher yields. Jyothi
and co-workers (2005) prepared succinate esters from cassava starch; the highest DS of
0.051 was obtained when the starch sample had a moisture content of 20% and the heating
time was seven minutes at 120°C. SEM showed that esterification did not change the
outward appearance of the starch granules. Citric acid esters of cassava starch were also
prepared (Jyothi et al ., 2007). DS values for the resulting esters ranged from 0.005 to 0.063,
with the highest obtained by heating starch with 0.45 mole of citric acid per mole of
anhydroglucose unit (AGU) for seven minutes at 160°C. Examination of the pasting
properties showed that paste viscosities increased with increasing DS and the highest peak
viscosity was observed for the sample with DS 0.063.
Microwave heating was also used to prepare starch derivatives by reaction with urea,
thiourea, biuret and thiosemicarbazide. Siemion and co-workers (2004) blended air-dried
potato starch, containing residual adsorbed water, with either urea or biuret. The mixtures
were then microwave-heated to obtain starch derivatives in which hydrogen atoms of the
hydroxyl groups were substituted with -CONH 2 or -CO-NH-CO-NH 2 , respectively.
Reaction products of starch and urea have been suggested as additives in the diets of ruminant
animals as a source of metabolized nitrogen. Reactions of thiourea with potato starch were
also carried out with both microwave and conventional heating (Siemion et al ., 2005a ). The
reaction with thiourea was similar to that observed with urea; however, the reaction products
underwent further decomposition and dextrins contaminated with isothiocyanates were
obtained. Potato starch was also microwave-heated with thiosemicarbazide; TGA, DSC and
FTIR were used to characterize the resulting products (Siemion et al ., 2006 ). Cationic
starches were similarly prepared by reacting starch with semicarbazide hydrochloride
(Siemion et al ., 2005b ).
Microwave heating was also used to prepare inorganic derivatives of starch. Potato starch
was sulfated with pyridine. SO 3 complex in a microwave-assisted solid state process, and a
maximum DS of 1.05 was obtained (Staroszczyk et al ., 2007a ). Although conventional oven
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