Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
Root and tuber starches, such as potato and tapioca, consist of little lipids and
thus display lower pasting temperatures and greater peak viscosities. 48 Potato starch
has an exceptionally low pasting temperature and high peak viscosity, which can be
attributed to its large content (~0.08%) of phosphate monoester derivatives and also
its large granule size (diameter up to 75 µm). Phosphate monoester derivatives carry
negative charges that generate repulsion and facilitate swelling and pasting of starch
during cooking, resulting in a low pasting temperature and a high paste viscosity.
In presence of salts, negative charges of phosphate groups are masked and viscosity
of the starch paste is substantially reduced.
Pastes of normal cereal starch, such as normal wheat and normal maize, are
more opaque than those of waxy and tuber starches. 85 This is because of light
reflected by the dense, limited swollen granules of normal cereal starches. Normal
cereal starch is known for consisting of amylose and lipids/phospholipids and has
restricted swelling. 85 Pastes of waxy starch and tapioca starch display greater clarity
because the starches disperse to greater extents, and the highly dispersed starches
do not reflect light. Potato starch and chemically modified starches that carry charge
groups display the greatest clarity, resulting from charge repelling.
7.4.3 STARCH RETROGRADATION
Dispersed amorphous starch in pastes, gel, or solutions gradually develops doublehelical
crystalline structures and loses its water-binding capacity. This process is
known as starch retrogradation. Retrograded starch that consists of large crystalline
size, such as retrograded amylose having 31 AGU in the crystalline region, is highly
resistant to enzyme hydrolysis. 34 Resistant starch is produced commercially from
high-amylose maize starch 86–89 for bulking agent and is used in low-caloric diet food
products.
Amylose molecules having linear structures develop double-helical crystallites
faster. Amylopectin molecules with branched structures, in general, crystallize
more slowly. The rate of retrogradation or crystallization of amylopectin depends
on branch-chain length. Amylopectin molecules that have long branch chains, such
as ae waxy maize starch, crystallize faster than those with short branch chains,
such as waxy rice starch and sugary-2 maize starch. 23,40 Starches that consist of
higher concentrations of lipids and phospholipids are also known to retrograde
faster, which might be attributed to limited dispersion of starch during cooking.
When starch chains are entangled and more clustered, they crystallize more
promptly.
Studies have shown that linear-chain amylose molecules of DP 80 to 100 display
maximal rate of retrogradation. 90,91 Linear amylose molecules with chain length
longer or shorter than DP 80 to 100 display slower retrogradation rates. Amylose
having chain length less than DP 110 precipitates from a solution (0.1%), and that
having chain length more than DP 1100 forms predominantly gels. Amylose of DP
250 to 660 develops mixtures of precipitates and gel. 91 Chains of monodispersed
molecular-weight amylose shorter than DP 10 do not retrograde. In a mixture with
longer-chain linear molecules, amylose chains as short as DP 6 can cocrystallize
with other longer chains. 91