Recent Processing Methods for Preparing Starch-based Bioproducts - Food and Industrial Bioproducts and Bioprocessing - page 40

increased by heat-moisture treatment, and the endotherms observed by DSC were broadened.
The pasting properties of starches were also affected by heat-moisture treatment. Viscosity
increases during pasting were observed at higher temperatures and final viscosities of the
pasted starches were lower, compared with untreated starch samples. As with annealing,
decreased granule swelling was observed. The effect of heat-moisture treatment on the
hydrolysis of starch with acids and enzymes varied with the botanical source of the starch.
In summary, explanations suggested for the effects of annealing and heat-moisture
treatment on the properties of starch granules include changes in crystallinity and crystallite
growth, increased order and interactions between amylose and amylopectin in the amorphous
fraction, and increased interactions between the amorphous and crystalline areas of the
granule. Since annealing and heat-moisture treatments are non-chemical in nature and
significantly affect the gelatinization and pasting properties of starches, most applications
for these processing methods will probably be in the food area. For example, annealing and
heat-moisture treatments have been examined as methods for increasing the percentages of
slowly-digestible and resistant starches in food products (Chung et al ., 2009 ).
2.3 HIGH-PRESSURE TREATMENT
The literature in this area has been reviewed by Fernandez-Martin and co-workers (2005)
and Pei-Ling co-workers (2010). Pressures used in the high-pressure treatment of starch
have typically ranged from about 100 to 1000 MPa (987-9869 atm). If sufficient water is
present and the applied pressure is sufficiently high, some starches can be gelatinized at
room temperature or below, as evidenced by the loss of birefringence or the reduction or
absence of gelatinization endotherms in DSC thermograms. Compared to heat-induced
gelatinization, pressure-gelatinized starch granules are less swollen and retain more of their
original granular appearance.
Numerous articles have been published that describe the effects of high pressure on
starch properties. Gelatinization of wheat starch and potato starch as a function of pressure
and water content was studied (Muhr and Blanshard, 1982); it was observed that high
pressure did not affect the microscopic appearance of the granules or their staining behavior.
Reduction in gelatinization temperature was greatest at higher pressures and water contents,
and wheat starch was more easily gelatinized than potato starch. Later research also showed
that starches with A-type X-ray diffraction patterns (such as corn, wheat and rice) were
affected the most by high-pressure treatment (Ezaki and Hayashi, 1992). Starches with
B-type X-ray diffraction patterns (such as potato) were more resistant to pressure, and
C-type starches (such as sweet potato and tapioca) showed intermediate properties. The
X-ray diffraction patterns of normal corn starch, defatted normal corn starch, waxy corn
starch and rice starch showed few changes up to 200MPa; however, after treatment at
500 MPa, considerable destruction of the A-type pattern for corn starch was observed (Hibi
et al ., 1993). The B-type pattern for potato starch was not significantly changed. To demon-
strate the role of water in the changes observed in X-ray diffraction patterns, corn starch and
potato starch were dried to moisture contents of 5.6-7.6% and pressure treatments were
carried out in hexane at 500 MPa for 60 min. Both the A-type and B-type patterns remained
the same, showing that in the absence of water the crystalline structure was not changed.
Volume increases in wheat starch and potato starch granules during the compression and
release phases were measured microscopically (Douzals et al ., 1996a ). Gelatinization of
wheat starch began at pressures above 300MPa and the largest increase in granule