Biomedical Engineering Reference
In-Depth Information
starch to native corn starch were prepared, and sufficient water at 50 °C was then added to
obtain a water content of 40%. The starch-water mixtures were then placed in a diffusion
cell and the solubility and diffusion coefficients of CO 2 were determined at pressures up
to16 MPa at 50°C. The experimentally-measured solubility accounted for the estimated
swollen volume; the solubility increased with pressure. The solubility of CO 2 showed no
dependence on the degree of gelatinization of the starch. The diffusion coefficient of CO 2
increased with the concentration of CO 2 (which was pressure dependent) and decreased with
increasing degrees of starch gelatinization.
Supercritical CO 2 has been used to extract various materials from the starch matrix. Artz
and Myers (1995) prepared a model corn starch system to evaluate extraction with supercritical
CO 2 as a method for analyzing emulsifiers in food products. Twin-screw extrusion was used
to mix acetylated monoglycerides with corn starch at a concentration of 3%, and extractions
of the mixtures were then carried out. Supercritical fluid extractions with CO 2 and with CO 2
containing 5% methanol as a co-solvent resulted in higher extraction percentages than those
observed when extractions were carried out with chloroform/methanol. A European patent
application describes a process for removing off-flavors, off-odors and off-colors from
starches by extraction of the starches with supercritical fluids (Koxholt et al ., 2003 ). Due to
environmental concerns and the safety hazards associated with conventional methods of
extraction using organic solvents, Hubbard and co-workers (2004) investigated alternative
methods for the extraction of lipids from wheat flour using supercritical CO 2 . Different
pressures, temperatures and concentrations of co-solvent were used for the extractions. The
best results were obtained with 12% ethanol as a co-solvent, a pressure of 51.7 MPa, and a
temperature of 80 °C. Under these conditions, the amount of extracted lipid was similar to
that obtained by conventional extraction using American Association of Cereal Chemists
(AACC) and American Oil Chemists' Society (AOCS) methods. Ashraf-Khorassani and
co-workers (2005) examined supercricital fluid extraction with CO 2 for its ability to remove
impurities from several common pharmaceutical excipient powders, including corn starch.
Solutions of formic acid, formaldehyde and hydrogen peroxide were added to the excipients
(or sand control), and the samples were extracted at 450atm and 40°C. Surface-bound
impurities were removed more effectively than imbedded impurities, and extractions from
starch were less effective than extractions from some of the other excipients tested. Extraction
of hydrogen peroxide was not effective, since supercritical CO 2 is a poor solvent for
this material.
Peterson and co-workers (2007) studied the critical fluid extraction of native lipids from
corn starch using 80:20 CO 2 :ethanol and 100% ethanol as solvents. The effects of lipid
extraction on the pasting properties of the extracted starch were determined, and the
extraction results and pasting properties were compared with those obtained with corn
starch that was defatted using refluxing 75:25 (v/v) n -propanol:water. Since the temperatures
and pressures used in this study were below the critical point for ethanol, this procedure was
designated as a critical fluid-extraction rather than an extraction with a supercritical fluid.
No extractable lipid was obtained when supercritical fluid extractions were carried out with
pure CO 2 . More native lipid was extracted with pure ethanol than with 80:20 CO 2 :ethanol,
and improved extractions of native lipid were observed when the moisture content of the
starch was increased from 10 to 19%. Compared to refluxing 75:25 n -propanol:water,
critical fluid extraction resulted in incomplete removal of native lipid, and was not selective
with respect to the chemical composition of the lipid removed. However, despite the
incomplete removal of native lipids, critical fluid extractions of corn starch reduced the
swelling and deformability of the starch granules when they were heated in water, and also
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