Biomedical Engineering Reference
In-Depth Information
degermed corn flour. The preferred method of isolation was neutral protease digestion for
2h followed by sonication for 30min at 50°C. It was suggested that this method could
replace sulfur dioxide and shorten the steeping time in the wet-milling process used for the
isolation of corn starch.
In summary, irradiation of a liquid with ultrasound results in acoustic cavitation. This
high-shear process results in intense local heating and high pressure followed by very high
rates of cooling. The effect of ultrasound on granular starch, aqueous starch solutions, and
aqueous dispersions of gelatinized starch has been studied. Since high-shear cavitation
cleaves polymeric materials at the center of gravity, ultrasound has been used to prepare
starch degradation products with narrow molecular weight distributions. Proposed
applications for these degraded starch products include their use as blood plasma substitutes
and as components in the preparation of pharmaceuticals. Treatment with ultrasound has
also been used to prepare low-viscosity, latex-like dispersions from starch graft copolymers.
When applied to soils, these aqueous dispersions functioned as stabilizers to inhibit water
erosion. Ultrasound has been used in the preparation of starch derivatives, to enhance the
reactivity of starch with enzymes used for ethanol production, and to facilitate the separation
and isolation of starch granules from cereal grains during wet milling.
2.6 PROCESSING USING SUPERCRITICAL FLUIDS
When a fluid substance such as carbon dioxide or water is heated under pressure, the density
of the liquid phase will decrease as the temperature is increased, whereas the density of the
gas phase will increase with increased pressure. If a temperature-pressure phase diagram is
constructed, and if we move upward along the curve separating the liquid phase from the gas
phase, a temperature and pressure will be reached where the densities of the liquid and gas
phases become identical and there will be no distinction between the gas and liquid phases.
This point on this phase diagram is known as the critical point, and a supercritical fluid is
any fluid that is at a temperature and pressure greater than those at the critical point. The
critical temperature and pressure will vary according to the chemical structure of the fluid.
For example, carbon dioxide (CO 2 ) has a critical temperature of 304 K and a critical pressure
of 74bar, whereas the critical temperature and pressure of water are 647K and 221bar,
respectively (Williams et al ., 2002). Above the critical temperature, a gas cannot be liquefied
by pressure.
Supercritical fluids have properties intermediate between those of gases and liquids.
Densities and viscosities of supercritical fluids are less than those of liquids, and they are
able to diffuse readily through solid substances. Their solvation power is dependent upon
temperature and pressure and can be changed by altering these two variables. Although a
number of substances have been used as supercritical fluids, carbon dioxide has been the
most widely used because of its convenient critical temperature and pressure, as well as its
low price, stability, non-flammability and low toxicity. Its functionality as a solvent lies
between non-polar hydrocarbons and weakly-polar solvents. To improve the ability of
carbon dioxide to dissolve polar molecules, a co-solvent such as methanol or ethanol may
be added. One of the first practical uses for supercritical fluids was for the extraction of
caffeine from coffee, and extraction of non-polar materials is a major application for these
fluids. Supercritical fluids have been used as the mobile phase in chromatography and as
solvents in chemical reactions and processing. They have also been used for the preparation
of micometer-size particles and for the drying of biological specimens for microscopy, since
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