Recent Processing Methods for Preparing Starch-based Bioproducts - Food and Industrial Bioproducts and Bioprocessing - page 56

solvent penetration is improved and the absence of surface tension in supercritical fluids
allows samples to be dried without distortion. Supercritical fluids and their applications in
biotechnology and the processing of food products have been reviewed by King (2000),
Williams and co-workers (2002), and Darani and Mozafari (2009).
The gelatinization of cassava, wheat and potato starches in supercritical CO 2 was studied
by da Cruz Francisco and Sivik (2002). Aqueous starch dispersions were prepared at 28%
solids and the dispersions were treated with supercritical CO 2 for 20 min at pressures of 8 and
30 MPa. The percentage of gelatinized starch granules was determined using light microscopy
to observe the loss in birefringence. Compared to gelatinization at atmospheric pressure, the
percentage of gelatinized cassava and potato starch granules was greater when supercritical
CO 2 was used at a pressure of 8 MPa, whereas less gelatinization was observed at 30 MPa.
With wheat starch, higher percentages of gelatinized granules were observed at both CO 2
pressures (compared with the control gelatinization at atmospheric pressure), and a higher
percentage of gelatinized granules was observed at the lower CO 2 pressure of 8MPa.
Pressurizing the starch at 8 MPa with nitrogen rather than CO 2 reduced the percentages of
gelatinized granules for all three starches. The gelatinization behavior in supercritical CO 2
was attributed to the plasticizing effect of CO 2 on starch at low to moderate pressures.
The influence of supercritical fluid extraction on the starches obtained from ginger and
turmeric was studied in order to evaluate the potential use of these extracted starches by the
food industry after the ginger and turmeric tubers were extracted to obtain oleoresin and
essential oil (Braga et al ., 2006). Extraction of ginger was carried out with supercritical CO 2
plus isopropyl alcohol as a co-solvent at 250bar and 35°C. Extraction of turmeric was
carried out with CO 2 plus a co-solvent comprised of a 1:1 mixture of ethanol and isopropyl
alcohol at 300 bar and 30 °C. SEM showed that supercritical fluid extraction did not modify
the surface and overall morphology of the starch granules, and did not cause the granules to
swell. Small effects of the extraction processes on the X-ray diffraction patterns, viscosities,
and gelatinization temperatures of the starches were observed. Muljana and co-workers
(2009) studied the gelatinization of potato starch (containing 16.2% water) in supercritical
CO 2 at pressures up to 25 MPa and temperatures ranging from 50 to 90 °C. The intensities of
infrared absorption bands associated with amorphous starch increased with both temperature
and CO 2 pressure, indicating a higher degree of gelatinization. These observations were
confirmed by DSC measurements. A maximum degree of gelatinization of 14% was
observed at 90 °C and a CO 2 pressure of 25 MPa.
When experiments were carried out using nitrogen instead of supercritical CO 2 , a maximum
degree of gelatinization of only 3.9% was observed under the most extreme conditions,
indicating that the gelatinization observed in supercritical CO 2 was not solely due the effect of
hydrostatic pressure. The plasticizing effect of supercritical CO 2 was attributed to the dissolving
of supercritical CO 2 in the starch granule matrix, which would induce swelling and plasticize
the amorphous regions, thus accelerating the diffusion of water into the granule. X-ray
diffraction was also used to evaluate changes in the degree of gelatinization of the treated
samples. Although the diffraction pattern for samples treated with supercritical CO 2 at 90 °C
was similar to that of the native starch, the area of the amorphous region increased due to
partial gelatinization, and higher CO 2 pressures produced lower orders of crystallinity. SEM
showed no changes in the appearance and morphology of the starch granules due to the
supercritical CO 2 treatment.
The solubility and the diffusion coefficients of CO 2 in starch-water mixtures were
determined by Chen and Rizvi (2006a) using native and pregelatinized corn starches for
their experiments. Starch mixtures containing 50:50, 75:25, and 100:0 ratios of pregelatinized