Biomedical Engineering Reference
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altered the paste viscosities observed at different concentrations. In a subsequent
investigation, critical fluid extractions with 75:25 ethanol:water were shown to be more
effective in removing native lipids from corn starch than previous extractions that did not
include water as a component in the solvent system (Peterson et al . 2008 ). The efficiency of
lipid extraction increased with the solvent/starch ratio, and over 99% of the native lipid
could be removed from normal dent corn starch using a solvent/starch ratio of 11.9 ml/g.
Comparison of critical fluid-extracted starches with starch that was conventionally defatted
by extraction with refluxing 75:25 n -propanol:water showed that neither of these extraction
procedures caused any changes in the X-ray diffraction pattern, the birefringence of the
starch granules, and the morphology of the granules, as observed by SEM. Pasting curves of
the extracted starches exhibited plateau-like peak viscosities, as opposed to the sharper peak
viscosities observed with native corn starch. Maximum viscosities and shapes of the pasting
curves depended on the concentration of starch, the amount of residual native lipid in the
starch sample, and the method used for lipid extraction (i.e., critical fluid vs. refluxing 75:25
n -propanol:water).
A process was developed for preparing extruded foams or puffed products from starch-
containing materials using supercritical fluids in the extrusion process (Rizvi and Mulvaney,
1992; Mulvaney and Rizvi, 1993). In this process, aqueous starch dispersions are first
cooked by extrusion at high temperature and pressure. The cooked starch is then cooled in
the extruder barrel to less than 100 °C (to prevent foaming and expansion of the extrudate by
steam). In the final step, supercritical CO 2 is injected into the barrel of the extruder just
before the cooked starch is forced through the extruder die (or nozzle). Under these
conditions, the added CO 2 dissolves in the cooked, aqueous starch dispersion and serves as
a blowing agent for the foaming process. Nucleation for the formation of bubbles is caused
by the sudden drop in pressure at the die, and bubble growth then occurs by diffusion of CO 2
into the nucleated bubbles. Further expansion of the extrudate then takes place, until the
foamed structure is set by post-extrusion oven drying. Extruded foams with regularly-
shaped pores can thus be produced at temperatures lower than 100 °C, which would not be
possible if conventional processing methods using steam as the blowing agent were used.
Since lower temperatures are used in the supercritical fluid process, starch degradation is
reduced, and heat-sensitive materials such as flavorings and colorants can be dissolved in
the supercritical CO 2 and easily mixed into the cooked starch dispersion before it passes
through the die. Extruder wear is also reduced at the lower temperatures used. The conditions
used in the supercritical fluid process produce denser, harder foamed products with a more
uniform cell distribution than those obtained by conventional steam puffing.
To aid in the design and control of the supercritical CO 2 extrusion process, Singh and
co-workers (1996) studied the diffusivity of carbon dioxide in extruded gelatinized starch at
343 K and pressures up to 117 bar (1700 psi). The diffusivity decreased with pressure but
was not affected by moisture content in the range 34.5-39%. Lee and co-workers (1999)
investigated the effects of various processing parameters on the physical properties of
extruded corn starch extrudates that were expanded using supercritical CO 2 injection. The
effects of barrel temperature, screw speed and water addition on the specific mechanical
energy input for the process were examined, as well as the expansion ratio, water absorption,
water solubility, breaking stress and elastic modulus of the extruded products. Alavi and
co-workers (1999) determined the effect of adding different levels of egg white protein and
whey protein concentrate on the expansion and cellular structure of starch-based supercritical
fluid extrudates. Since post-extrusion drying is used in the process, the effect of drying
temperature on the expansion of extrudates and their cellular structure were also studied.
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