Biomedical Engineering Reference
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and the ratio of cell wall thickness to cell diameter were found to be good predictors of
compressive and flexural mechanical properties. Compressive modulus data suggested that
the anisotropy of cell shape, due to cell elongation in the longitudinal direction, affected the
mechanical properties of the extruded products.
Since starch microcellular foams are readily dispersible in water, they cannot be used for
end-use applications in aqueous environments. To reduce the water sensitivity of these
foams, Ayoub and Rizvi (2008) studied the cross-linking of foams with epichlorohydrin
during their preparation by supercritical CO 2 extrusion. A mixture of native and pregelatinized
wheat starch was used; the in-barrel moisture content of the starch melt was maintained at
about 45%. Three levels of epichlorohydrin were used and sodium hydroxide was added to
the formulation to provide the alkaline pH needed for the cross-linking reaction to occur.
The relationship between cross-link density and cell size was studied and a decrease in
average cell size was observed with increasing concentrations of epichlorohydrin. The
cross-linked starch foams prepared by this process were insoluble in water at room
temperature. Patel and co-workers (2009) prepared cross-linked and uncross-linked starch
foams by first using supercritical CO 2 extrusion to prepare the foams (with and without
epichlorohydrin) and then immersing the extrudates in ethanol/water solutions of varying
concentration to solvent exchange the water remaining in the foams with ethanol. For
comparison, molded aqueous starch gels (aquagels) were also solvent exchanged in a similar
manner. The aquagel-based products showed a finer micropore structure than the products
obtained by extrusion, they also had lower density and higher brightness than the extruded
The effect of supercritical fluid extrusion of ground sorghum grain on the enzymatic
production of ethanol was investigated by Zhan and co-workers (2006). Analysis of
supercritical CO 2 -extruded sorghum showed increases in measureable starch content and
free sugar content; high levels of gelatinized starch were also observed. The supercritical-
extruded and non-extruded sorghum were further liquefied, saccharified and fermented to
ethanol. The yield of ethanol from supercritical-extruded sorghum was greater than that
obtained from non-extruded material, indicating that supercritical extrusion was an effective
pretreatment method that could improve the bioconversion of sorghum starch into ethanol.
Improvements in the bioconversion process were attributed to the enhanced release of
fermentable starch from the protein matrix, a decrease in fiber content, and the fine, porous
structure produced by the extrusion process.
Supercritical fluids have been used as reaction media for the preparation of starch
derivatives. Yalpani (1993) prepared a phosphate ester derivative of amylose (phosphorus
content, 3.7%) by heating an aqueous mixture of amylose and sodium tripolyphosphate in
supercritical CO 2 for 1.5 h at 60 °C. Oxidized starches were obtained when corn starch and
maltodextrins were treated with 19:1 mixtures of carbon dioxide and oxygen for two hours.
Carbonyl absorption bands in infrared spectra provided evidence for oxidation. A European
patent application (Harris et al ., 1999) describes a process for chemically modifying starch
to produce starch derivatives, such as esters and ethers, in supercritical CO 2 . The process
yields starch derivatives with a high degree of substitution without the use of organic
solvents and by-products of the reaction are easily separated. For example, high amylose
starch was esterified with acetic anhydride in supercritical CO 2 using sodium acetate catalyst
at 115 °C and 4000 psi for 2.5 h to yield starch acetate with a DS of 2.47. The acetylation of
starch in supercritical CO 2 was also studied by Muljana and co-workers (2008). When
supercritical CO 2 was compared with water as a solvent for the acetylation of potato starch,
better results were obtained with supercritical CO 2 with respect to degree of substitution and
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