Biomedical Engineering Reference
In-Depth Information
agents for water treatment and purification. Phosphate and acetate esters of starch have food
and non-food applications. The solubility of the triacetate ester of starch in some organic
solvents extends its range of commercial applications.
Although many processes have been used for the conversion of starch into commercially-
viable products, newer processing methods are being developed that could supplement the
well-established methods now used. Examples of these methods are (1) annealing and heat-
moisture treatment of starch granules, (2) pressure treatment of starch granules, (3) use of
microwave heating and ultrasound, (4) use of supercritical fluids, (5) reactive extrusion, and
(6) steam jet cooking at high temperatures and pressures. This review will focus on the
current literature in these areas.
2.2 ANNEALING AND HEAT-MOISTURE TREATMENT
Annealing and heat-moisture treatments are physical, hydrothermal methods that have been
used to modify the properties of starch granules without destroying the granule structure
(Jacobs and Delcour, 1998 ; Stute, 1992 ; Zavareze and Dias, 2011 ). Both treatments involve
heating starch granules for various time periods at specific temperatures and moisture levels,
and both treatments are carried out at temperatures above the glass transition temperature of
the amorphous regions of the starch granule, but below the gelatinization temperature at the
moisture levels used.
The process of annealing has been reviewed by Tester and Debon (2000). Annealing is a
process that occurs when starch granules are heated in excess water (>60% w/w) or at an
intermediate water content of about 40% w/w. Temperatures used for annealing are typically
in the 50-60 °C range (Jacobs and Delcour, 1998). Annealing does not alter the morphology
of the starch granules and no soluble carbohydrates are leached from the granules during the
annealing process. Also, annealing does not result in significant changes in crystal type and
degree of crystallinity, as measured by X-ray diffraction. The effect of annealing on the
properties of starch granules is an increase in the gelatinization temperature and a decrease in
the gelatinization temperature range. In maize starches, the largest effects observed by DSC
were with starches having the highest amylose contents. Annealing takes place most rapidly
at temperatures just below the gelatinization temperature, which depends upon the water
content of the starch during the annealing process. The effects of annealing on the pasting
properties of starch granules and their susceptibility to acid and enzymatic hydrolysis depend
upon the botanical source of the starch. For example, with wheat, potato, oat and lentil
starches, annealing decreases the granule swelling power and also decreases the amount of
amylose leached from the granules when the treated samples are heated in excess water.
Heat-moisture treatment is a process that occurs when starch granules are heated at
moisture contents lower than those used for annealing (i.e., below 35%). As in the annealing
process, temperatures are above the glass transition temperature but below the gelatinization
temperature of the starch at the moisture level used. Temperatures ranging from 84 to 120 °C
have been used (Jacobs and Delcour, 1998). As observed for the annealing process, starch
granule morphologies are not significantly changed. A reported effect of the heat-moisture
treatment of potato starch is a change in the X-ray diffraction pattern from the B-type to
A- or C-type. The A-type pattern for cereal starches was not changed by heat-moisture
treatment, although new V-type peaks were observed in the X-ray diffraction patterns,
presumably due to the formation of amylose inclusion complexes with the native lipids
normally present in small amounts in these granules. Gelatinization temperatures were
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