Recent Processing Methods for Preparing Starch-based Bioproducts - Food and Industrial Bioproducts and Bioprocessing - page 42

detected in A-type starches pressure-treated at the 2:1 ratio, indicating total loss of crystalline
structure and molecular order. DSC thermograms after high-pressure treatment showed a
newly-developed peak that resembled that of retrograded B-type starch. Starches having the
B-type pattern were not changed by high-pressure treatment, and the high-pressure treatment
did not change the molecular weight distribution of the starch. Treatment of starches in
powder form and as suspensions in ethanol resulted in a reduction in the peak intensity of
the X-ray diffraction pattern; however no conversion from the A-type to the B-type pattern
was observed. Changes in pasting properties due to pressure treatment varied with starch
type; and when pressure-treated in water, increased pasting temperatures and decreased
paste viscosities were observed. The effect of high-pressure treatment on normal corn, waxy
corn, wheat, and potato starches at their ambient moisture contents of about 14-20% was
investigated by Liu co-workers (2008) at pressures up to 1500MPa. DSC showed that
high-pressure treatment reduced the gelatinization temperature as well as the corresponding
gelatinization enthalpy. These changes in properties were irreversible and were maintained
during storage for six months at 25 °C. The birefringence observed in the untreated- and
pressure-treated starches was not visually different, and the X-ray diffraction patterns were
not significantly changed by high-pressure treatment. However, scanning electron
microscopy (SEM) showed that pressure treatments caused some changes in the morphology
and surface appearance of the granules.
A high-pressure cell was developed that enabled optical examination of starch granules
in situ at pressures up to 300 MPa (Bauer et al ., 2004). Potato (B-type), wheat (A-type) and
tapioca (C-type) starches were dispersed in water at concentrations of 0.8%. Although the
maximum pressure of 300 MPa was not high enough to cause swelling of potato starch and
tapioca starch, most wheat starch granules swelled during pressure treatment. Pressure
treatment of iodine-stained wheat and tapioca starches resulted in discoloration of the
granules, whereas potato starch discolored only slightly. Discoloration during pressure
treatment was attributed to the loss of amylose-iodine complex from the starch granules and
the accumulation of complexed amylose in the aqueous phase surrounding the granules,
which resulted in an observed darkening of this phase. Resistant starch yields of about 12%
were obtained by high-pressure treatment processes and were in the same range as yields
obtained by conventional heat gelatinization (Bauer et al ., 2005 ).
When aqueous 10% dispersions of potato starch were subjected to high-pressure
treatment at 20°C and 600MPa, NMR spectra of the treated samples showed signal
resonances corresponding to amorphous sites within the starch structure (Błaszczak et al .,
2005a). FTIR spectra also showed that high pressure treatment affected the intensity of
bands corresponding to the amorphous and more ordered areas of the starch granules. DSC
thermograms showed a decrease in gelatinization temperature after high-pressure treatment
and a decrease in total enthalpy with increased treatment time. SEM images suggested that
the starch granule surface was more resistant to high-pressure treatment than the interior of
the granule. When 30% aqueous dispersions of waxy corn starch and high amylose corn
starch were pressure-treated at 20°C and 650MPa, changes in the starch structure were
observed in NMR spectra, and X-ray diffraction patterns of pressure-treated high amylose
starch showed a decrease in crystallinity (Blaszczak et al ., 2005b ). An amorphous structure
was observed for waxy corn starch after only three minutes at high pressure. These results
confirmed the NMR data and indicated that changes in structure were mainly related to the
crystalline areas of the starch granule. After pressure treatment for nine minutes at 650 MPa,
the birefringence remaining in the high amylose starch granules tended to be located in the
outer portions of the granules. While most of the high amylose starch granules retained their