Recent Processing Methods for Preparing Starch-based Bioproducts - Food and Industrial Bioproducts and Bioprocessing - page 48

heating gave about the same DS as microwave heating, longer reaction times were required.
Granular potato starch was also esterified by reaction with the sodium salts of selenous and
selenic acids (Na 2 SeO 3 and Na 2 SeO 4 ) (Staroszczyk et al ., 2007b ). Molar ratios of
AGU:selenium salt were 1:1 or 1:0.1, and mixtures were heated in a microwave oven at
either 450 W or 800 W for 20 min. A maximum DS of about 0.03 was observed and the
granular appearance of the starch was retained. Microwave heating in the solid state was
used by Gui-Jie and co-workers (2006) to cross-link corn starch with sodium trimetaphosphate
(STMP). Varying amounts of STMP and sodium carbonate, in water solution, were added to
corn starch and the samples were dried at 40 °C for 24 h to give a water content of about 8%.
About three minutes at 270 W was required to obtain a DS of 1.89 × 10 -2 , whereas
conventional oven heating required 24 h to obtain a DS of 0.70 × 10 -2 . Cross-linking by
microwave heating caused no significant changes in starch granule morphology. Since
STMP has low toxicity, the cross-linked starches can be used as thickeners in food products.
Zinc derivatives of starch were prepared by solid state reactions of potato starch with sodium
tetrahydroxozincate [Na 2 Zn(OH) 4 ], which was prepared by the reaction of zinc oxide with
sodium hydroxide (Staroszczyk and Janas, 2010).
Microwave heating was employed in procedures used to prepare starch graft copolymers.
For example, with microwave heating, free radical graft polymerization of acrylamide onto
potato starch could be achieved with low concentrations of potassium persulfate initiator
(Singh et al ., 2006). Maximum grafting was observed with 0.1 g of potato starch in 25 ml of
water, and with acrylamide and potassium persulfate concentrations of 0.10 M and 0.0025 M,
respectively. Heating for 60 s at 720 W produced a final temperature of 98 °C. Graft
polymerization was not observed with these reactant concentrations under conventional
polymerization conditions. Starch graft copolymers were also prepared by microwave
heating aqueous mixtures of potato starch and caprolactone for three minutes at 390 W
(Koroskenyi and McCarthy, 2002). Guan and co-workers (2006) used a different method of
grafting (based on Maillard-type reactions between amino groups of a protein and the
reducing carbonyl groups of a polysaccharide or sugar) to prepare co-polymers of soluble
starch and soy protein isolate. Reactions were carried out by mixing s oy protein isolate with
sugars, dextran or soluble starch and then rapidly heating the mixtures to about 90 °C in a
microwave oven. The resulting protein-polysaccharide co-polymers were shown to have
emulsifying and antimicrobial properties.
Finally, microwave heating was used to prepare silver nanoparticles, using starch as both
a stabilizer (or capping agent) and a reducing agent for the conversion of silver ions to
nanoparticulate metallic silver (Sreeram et al ., 2008). A solution of starch was first prepared
at a concentration of 1% and silver nitrate solution was then added. The following methods
of heating were then used for the reduction of silver ion: (1) uncontrolled heating to about
80 °C; (2) controlled heating at 2 °C/min; and (3) microwave heating for 30-120 s. Microwave
heating produced the smallest silver nanoparticles (average particle size: 12 nm) and the
particle size was more monodisperse, compared to the other two methods. Also, with
microwave heating, silver ion was reduced to metallic silver within 120 seconds, compared
to 60 min for the other methods. The fact that the nanoparticles of silver did not aggregate
showed that starch was acting as both a reducing agent and stabilizer. End-use applications
for silver nanoparticles include their use as catalysts and antimicrobials, and also their use
in medical and biological applications.
In summary, microwave heating allows reactions to be carried out with shorter reaction
times, more uniform heating, and less chance for overheating, since microwave radiation
heats only the reactants and solvent and not the reaction vessel itself. In contrast, heating