Recent Processing Methods for Preparing Starch-based Bioproducts - Food and Industrial Bioproducts and Bioprocessing - page 45

all starch samples. With wheat starch the two high-pressure treatments gave higher
equilibrium yields of glucose than the thermally-gelatinized sample.
Since batch-wise, high-pressure treatment of starch for finite time periods has limited
practicality with respect to commercial processing, Wang co-workers (2008) studied the
pressure treatment of maize starch using a commercial, high-pressure homogenizer with a
pressure range up to 150 MPa. Aqueous starch suspensions were prepared at 1.0% solids
and were subjected to single-pass, high-pressure treatment at 60, 100 and 140 MPa. DSC
showed a decrease in gelatinization temperature and gelatinization enthalpy with increasing
homogenization pressure, and X-ray diffraction indicated a loss of crystallinity after
treatment at 140 MPa.
In summary, high-pressure treatment reduces the temperature at which the gelatinization
of starch takes place, and the effects of high pressure vary with the type of starch used.
Cereal starches such as corn, wheat and rice, that exhibit A-type X-ray diffraction patterns,
are affected the most by high-pressure treatment, whereas starches with B-type diffraction
patterns, such as potato, are more resistant to the effects of high pressure. Also, waxy
starches are more sensitive to high-pressure treatment than amylose-containing starches.
A major application of this processing technique will be the tailoring of starch properties for
food applications, for example, food products processed with minimal heating. Compared
to thermal gelatinization, the starch granule structure is better preserved when high-pressure
gelatinization is used, and high-pressure treatment therefore has the potential for creating
new food products with unique textures. The shelf-life of foods can also be improved by
high-pressure treatment without the detrimental side effects of heat on food texture, flavors,
and heat-sensitive nutrients. Since covalent bonds are not affected by high-pressure
treatment, high pressures have only a minimal effect on low molecular weight materials
such as flavors and nutrients, whereas protein-based sources of food spoilage, such as
enzymes and microorganisms, can be denatured and inactivated by high-pressure treatment
without the application of heat.
2.4 MICROWAVE PROCESSING
The use of microwave heating to carry out chemical reactions between organic compounds
has been studied since the mid-1980s and several reviews of this technology have been
published (Caddick, 1995 ; Corsaro et al ., 2004 ; Gabriel et al ., 1998 ; Galema, 1997 ; Lidström
et al ., 2002). The modification of starch by microwave radiation was reviewed by Tomasik
and Zaranyika ( 1995 ).
Lewandowicz co-workers (1997) studied the microwave heating of potato and tapioca
starches in the absence of chemical reactants. Time-temperature profiles of starch-water
systems during microwave heating were established, and the effects of microwave radiation
on the properties and structure of the heated starch granules were determined. The increase
in temperature with irradiation time depended upon the moisture content of the starch.
At low moisture contents (about 1-5%), a rapid rise in temperature was observed whereas a
slower temperature rise was observed with moisture contents of about 7-15%. When the
moisture content exceeded about 20%, the temperature leveled off at about 80-90 °C and
remained fairly constant, even with prolonged heating times. A correlation was found
between the moisture content during microwave heating and the pasting properties of the
heated starch granules. With moisture contents over 20%, there was a rise in the gelatinization
temperature of the microwave-heated starches; the pasting properties changed from those