Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
pasting. On cooling, viscosity increases as temperature decreases, and starch molecules
in the paste develop a network and gel. After an extended storage period or
repeated freeze and thaw cycles, starch molecules in the paste or gel crystallize, and
the process is known as retrogradation.
The versatile physical structures of starch facilitate a wide variety of applications;
for example, granular starch is used for facial powder; large A-granules of wheat
starch are used in carbonless copy paper, and small particle starch 65 or small granular
amaranth starch (diameter 0.5 to 2 µm, resembling that of a fat micelle) is used for
fat substitutes. 66 Starch pastes are used as sizing agents in paper and textile industries
and as thickening agents for soup and canned foods in the food industry. Starch gel
is used to make tapioca pudding, desserts, and fillings. Retrograded or crystalline
high-amylose maize starch is used as resistant starch that has limited digestibility
by humans and is desirable for low-caloric diet foods.
7.4.1 STARCH GELATINIZATION
Starches of different botanical sources and genetic backgrounds display different
gelatinization properties, such as gelatinization temperature, enthalpy change, and
melting of the amylose–lipid complex. Gelatinization temperature of starch can be
determined by a light microscope equipped with a hot stage. The temperature at
which starch granules lose the Maltese cross in the presence of excess water is the
gelatinization temperature. The most common and reliable method for analysis of
starch gelatinization temperature and enthalpy change is by a differential scanning
calorimeter (DSC). Starch gelatinization is an endothermic reaction and requires the
presence of water or other plasticizers such as glycerol. Without water or other
proper plasticizers, starch will not gelatinize until it decomposes. Thus, water is
critical for starch gelatinization. Two times or more, by weight, of water to starch
is required to assure a constant gelatinization temperature. With insufficient water
content, starch gelatinization temperature increases and the range of gelatinization
temperature also broadens. 67
The gelatinization properties of starches vary substantially. For example, onset
gelatinization temperatures vary from 47.8 (sugary-2 maize starch) to 71.5°C (ae
waxy maize starch), ranges of gelatinization temperatures vary from 6.6 (barley
starch) to 58.8°C (high-amylose maize VII starch), enthalpy changes of starch
gelatinization vary from 10 (barley starch) to 22 J/g (ae-waxy maize starch), and
percentage retrogradation of gelatinized starch after being stored at 5°C for 7 days
varies from 4.3 (sweet rice) to 80.8% (high-amylose maize V starch). 40 The gelatinization
temperature of starch is highly correlated to the branch-chain length of
amylopectin. 22,68–71 Starches that consist of amylopectin with more long branch
chains, such as high-amylose maize starches and ae-waxy maize starch, display
higher gelatinization temperatures and greater enthalpy changes, gelatinization temperature
ranges, and percentage retrogradation. Potato starch amylopectin also has
more long branch chains but displays a very low gelatinization temperature (T 0 =
58.2°C). This may be attributed to amylopectin of potato starch consisting of a large
concentration of phosphate monoester derivatives. The negative charges of the phosphate
derivatives repel one another and destabilize the granular structure. Thus,