Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
Studies on polymorphic forms of short amylose crystallization have shown that
chain length of short amylose molecules 42,74 concentration of amylose solution, 74
temperature, 74 and presence of other chemicals such as salts 92 and alcohol 73 affect
the type of polymorphism of retrograded amylose. Amylose of DP 10 to 12 crystallizes
to develop A-type polymorphism, whereas that of DP 13 and longer develops
B-type polymorphism. 42,74 High concentration of amylose (~50%), high crystallization
temperature (30°C), 74 presence of salts of higher lyotropic number, 92 and presence
of 30% ethanol (w/w) 73 favor A-type polymorphism. Most salts and those that
have structure-breaking effects, such as NaCl, LiCl, KCl, RbCl, KI, KCN, KBr,
favor A-type polymorphism. Sulfate salts, such as K 2SO 4, (NH 4) 2SO 4, and Na 2SO 4,
however, favor the formation of B-type polymorphism. The trend of amylose chain
length effect on the crystalline structure is in agreement with the polymorphism of
starch that consists of amylopectin with different branch-chain lengths. 39 Starches
having more long branch chains, such as potato and high-amylose maize, display
B-type polymorphism, whereas starches with more short branch chains, such as rice,
corn, taro, and wheat, display A-type polymorphism.
Potato amylose in an aqueous solution (3.5 mg/ml) displays the highest rate of
retrogradation at 5°C, and retrogradation rate decreases with increase in temperature
up to 45°C. 93 When incubation temperature reaches 45°C, only small amylose
molecules (DP 110) crystallize, whereas amylose of larger molecular weight remains
in the supernatant. Size of the crystallite in retrograded amylose increases with
incubation temperature. After the amorphous region of retrograded amylose has been
hydrolyzed and removed by α-amylase or by acid, chain length of the resistant
crystalline region increases from DP 39 to DP 52 as incubation temperature increases
from 5 to 45°C. Retrograded amylose displays exclusively B-type polymorphism,
which does not change with incubation temperature. Repeated freeze–thaw treatments
of amylose solutions enhance amylose retrogradation and result in a spongelike
structure.
Crystallization of short-chain amylose in an aqueous solution produces spherical
particles. 73 Scanning electron micrographs of the spherical particles show diameters
of 10 to 15 µm, and the particles are mostly clustered. Polarized light micrographs
of the particles show Maltese cross, indicating a spherulitic morphology. Potato
amylose retrogrades and develops to nodule-shaped particles when the solution is
incubated at 5°C, but develops to a well-defined network when incubated at a higher
temperature (e.g., 15 or 25°C).
7.4.4 GLASS-TRANSITION TEMPERATURE OF STARCH
Glass transition is the kinetic transition of amorphous polymers from a glassy state
to a rubbery state. The glass-transition temperature (T g) is specific to each material
and depends on molecular weight of the polymer and presence of plasticizers. T g of
a homologous polymer increases with the increasing number-average molecular
weight up to a plateau limit 94 and decreases with an increase of plasticizer concentration.
Plasticizers are small chemicals compatible with the polymer and increase
flexibility and extensibility. Water is known to be the most powerful plasticizer for
starch and many hydrophilic biopolymers. 95,96 Glass-transition temperature of native