Biomedical Engineering Reference
In-Depth Information
Cell size and uniformity as well as cell density were determined. Addition of proteins
reduced the shrinkage of extrudates before oven drying and reduced the bulk density of the
extruded products. The cell size distribution was more uniform than that observed with
steam-expanded products. Supercritical fluid extrudates also had a relatively non-porous
skin surrounding the internal cells, which reduced water penetration (a desirable property
for a breakfast cereal). Compared to whey protein concentrate, it was observed by Gogoi
and co-workers (2000) that addition of egg white gave a softer skin and a fragile but well-
formed cellular structure. Also, when drying was carried out between 70 and 100 °C, the
structure was more homogeneous when egg white was added.
To better understand the supercritical extrusion process, Alavi and co-workers (2003a)
developed a mathematical model for bubble growth, as well as models for the flow of
starch melt through the extruder die, the bulk diffusion of CO 2 and water to the atmosphere,
and the heat transfer in the extruded product. The same authors also published simulation
results and comparisons with experimental data (Alavi et al ., 2003 b). Data obtained with
extrudates of pregelatinized corn starch and potato starch with 4-7% added whey protein
concentrate were used for validation. Since the high diffusivity of CO 2 favors escape of the
gas to the environment (and thus reduces the amount of CO 2 available for diffusion into the
bubbles) Alavi and Rizvi (2005) examined two approaches for minimizing this problem.
The first approach was to increase the rate of nucleation of CO 2 bubbles, which was
achieved by reducing the diameter of the extruder nozzle to obtain a higher rate of pressure
drop when the starch-CO 2 melt was forced through the nozzle. This approach increased the
expansion ratio by as much as 160%. The second approach was to reduce the temperature
of the melt in order to decrease the diffusion coefficient of CO 2 ; this was achieved by intro-
ducing a cooling zone in the extruder barrel prior to entry of the melt into the nozzle. A
34% increase in expansion ratio was observed when the melt temperature was decreased
from 60 to 40 °C.
Chen and Rizvi (2006b) used a slit-die rheometer fitted to a twin extruder to measure
the viscosity and expansion characteristics of starch-based melts plasticized with water
and supercritical CO 2 . Supercritical CO 2 was shown to be an effective plasticizer for
starch-water mixtures, and the increase in free volume due to addition of supercritical
CO 2 was shown to be an effective method for viscosity reduction. Mariam and co-workers
(2008) prepared extruded foams having thermal properties (such as heat capacity, thermal
conductivity and thermal diffusivity) comparable to commercial products. Pregelatinized
corn starch was extruded with different concentrations of whey protein and supercritical
CO 2 , and the expansion and density of the extruded foams were varied by changing the
amount of whey protein added and the injection rate of supercritical CO 2 . Cho and Rizvi
(2009a) studied foams prepared from pregelatinized corn starch containing five different
concentrations of whey protein isolate and extruded at four different levels of supercritical
CO 2 . The melt rheology of the extrudates was determined with an on-line slit die
rheometer; X-ray microtomography was used to determine the microstructural features of
the foams, such as average cell diameter, cell wall thickness, and void fraction. Rheological
properties were then correlated with the expansion characteristics and microstructures of
the foams.
In a second article by Cho and Rizvi (2009b), microstructural data were correlated with
mechanical properties, as determined using compression testing and three-point bending
tests. Cell sizes, observed in cross-sections of the extrudates, decreased with radial distance
from the center; while in the longitudinal direction, the cell shapes were more elliptical than
spherical, and were aligned along the extrusion direction. Both the density of the extrudate
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