Recent Processing Methods for Preparing Starch-based Bioproducts - Food and Industrial Bioproducts and Bioprocessing - page 41

volume was observed between 300 and 420MPa. Gelatinization was 100% at 600MPa.
Pressure-induced changes were less for potato starch; there was no change in gelatinization
enthalpy at pressures up to 600 MPa. Below 300 MPa, the compressibility of 16% starch-
water suspensions was close to that of pure water; whereas over 300MPa the volume
reduction due to compression was higher for the starch-containing suspensions at the same
pressure. It was concluded from these experiments that the water molecules that enter the
starch granules during pressure-induced gelatinization occupy a reduced volume, compared
to pure liquid water. It was also shown that this volume reduction was maintained after the
wheat starch gels were returned to atmospheric pressure (Douzals et al ., 1996 b) .
The swelling index and the release of amylose from the starch granules depended on
whether starch was gelatinized by heat or by high pressure (Douzals et al ., 1998 ). When
iodine titrations were carried out on the soluble fractions obtained from gelatinizations at
5% starch solids, more iodine binding (and therefore more amylose release) was observed
with thermal gelatinization than with high-pressure gelatinization. This agrees with
microscopic observations that showed that the granule structure was better preserved during
high-pressure gelatinization. Young's modulus measurements carried out on 30% solids
starch gels showed that the gels produced by high-pressure gelatinization were weaker than
those produced by thermal gelatinization, since less amylose (that can retrograde at low
temperature) was released from the granules.
Effects of pressure-temperature combinations on the gelatinization of wheat starch
granules were determined, and a pressure-temperature gelatinization diagram was
constructed (Douzals et al ., 2001). Similarities between starch gelatinization and protein
unfolding under pressure were observed. Baks co-workers (2008) also measured the degree
of gelatinization as a function of pressure, temperature and treatment time for wheat starch-
water mixtures and obtained phase diagrams relating the degree of gelatinization to both
pressure and temperature. Stute co-workers (1996) reported the effects of high-pressure
treatment on the gelatinization and properties of a number of different starch varieties.
Above a certain pressure, an additional DSC peak was observed at about 50 °C; this was
identified as a retrogradation peak, in agreement with the fact that no water-soluble amylose
was observed in pressure-treated starches. When wheat starch was treated at 600 MPa for
15 min at 20 °C, a minimum of about 50% moisture was necessary for pressure-induced
gelatinization to occur. Higher gelatinization temperatures were needed in the presence of
sugar or ethanol. Starches that exhibited restricted swelling during pressure-induced
gelatinization also had lower viscosities when they were pasted by heating in water; and due
to their restricted swelling, smooth-textured pastes were obtained at high starch concentrations
(15-30%). The high-pressure treatments of pea starch (Fernandez-Martin et al ., 2008 ),
buckwheat starch (Vallons and Arendt, 2009a), sorghum starch (Vallons and Arendt, 2009b),
and Japonica rice starch (Tan et al ., 2009 ) were also studied.
Katopo co-workers (2002) pressure-treated normal, waxy, and high amylose maize
starches, as well as tapioca, potato and rice starches at 690MPa (100000psi) at room
temperature under the following treatment conditions: (1) starch in powder form with origi-
nal moisture contents of 10-16%; (2) starch suspended in ethanol (1:1 ethanol:starch); and
(3) starch suspended in water (1:1 and 2:1 water:starch). Crystalline structures, thermal and
pasting properties, molecular weight distributions, and morphologies of the treated starch
granules were investigated. X-ray diffraction showed that high-pressure treatment converted
the starches that originally displayed the A-type pattern into starches having X-ray patterns
that resembled the B-type. With the exception of high amylose maize starch, the degree of
gelatinization was higher with 2:1 water:starch than with 1:1. No gelatinization peaks were