Recent Processing Methods for Preparing Starch-based Bioproducts - Food and Industrial Bioproducts and Bioprocessing - page 44

were heated to 180-230°C (a temperature range commonly used for food preparation)
(Blaszczak et al ., 2008). Waxy maize starch exhibited a greater ability to generate free
radicals than high amylose starch; however, the presence of water and the high-pressure
pretreatment of the starches reduced the amount of free radicals observed.
Aqueous 10% dispersions of normal rice, waxy rice, normal corn, waxy corn, tapioca and
potato starches were pressure-treated at 400 and 600MPa for 30min at 20°C and the
dispersions were characterized by their pasting properties, degree of swelling, and changes
in birefringence (Oh et al ., 2008a). As observed by earlier workers, potato starch was least
affected by pressure treatment and retained its birefringence after treatment for 30 min at
600 MPa. Under the same conditions, tapioca starch and the waxy starches showed complete
gelatinization; normal corn starch and rice starch were partially gelatinized. In a subsequent
publication, the effects of treatment pressure (100-700 MPa), treatment time (up to 30 min)
and treatment temperature (10-60 °C) on the gelatinization of normal and waxy rice starches
were described (Oh et al ., 2008b). Pasting behavior, viscosity, degree of swelling, changes
in birefringence and leaching of amylose and amylopectin were studied. Degree of
gelatinization during pressure treatment depended upon the type of starch and the pressure,
temperature, and duration of the treatment. Although normal and waxy rice starches both
lost all birefringence after treatment at 500 MPa, they exhibited different gelatinization char-
acteristics. Normal rice starch maintained more of its granular structure and the particulate
solid obtained consisted of swollen starch granules as opposed to granule ghosts (Obanni
and BeMiller, 1995). Also, the leaching of soluble starch was minimal. In contrast, waxy
rice starch appeared to contain granule ghosts and no additional changes related to gelatini-
zation were observed on further heating.
The effects of dissolved salts and sugars on the pressure-induced gelatinization of wheat,
tapioca and corn starches were generally comparable to those observed when the starches
were gelatinized thermally (Rumpold and Knorr, 2005). For example, addition of sucrose
led to a reduction in the degree of gelatinization of all three starches; at a sucrose concentration
of about 20%, gelatinization was almost completely suppressed. The effects of different
solutes on gelatinization were attributed to their effect on the availability of free water.
When normal corn starch was suspended in 2 M sodium chloride or in 50% w/w sucrose, as
opposed to water, these two solvents showed a protective effect against starch gelatinization
during high-pressure treatment at 600 MPa (Kweon et al ., 2008a ), and the protective effects
of sucrose were more dramatic than those of sodium chloride. Kweon co-workers (2008b)
also studied the pressure-induced gelatinization of wheat starch in sodium chloride solutions
of varying concentrations. Although high-pressure treatment at all sodium chloride
concentrations from 0.1 to 5 M resulted in reduced gelatinization compared to water alone,
maximum stabilization with respect to gelatinization was observed with sodium chloride
concentrations near 2M. When aqueous sodium dodecyl sulfate solution was added to
potato starch, and the sample was pressure-treated at 1.5GPa for one hour at 40°C, an
amylose-sodium dodecyl sulfate helical inclusion complex was formed, as indicated by
DSC and the appearance of a V-type X-ray diffraction pattern (Yamamoto et al ., 2002 ). Use
of this technique to prepare amylose complexes of commercial interest, such as flavorings
and emulsifiers, was suggested.
Selmi co-workers (2000) compared the amyloglucosidase-hydrolyses of corn and wheat
starches that were pressure-treated for 15 min at 600 MPa and 25 °C, or 400 MPa and −20 °C,
with starches that were thermally gelatinized by heating in a water bath at 80 °C for 15 min.
Although a higher conversion to glucose after five minutes was observed for the
thermally-gelatinized corn starch, a conversion of about 90% was observed after 30 min for