Starch Nanocomposite and Nanoparticles: Biomedical Applications - Modern Trends in Chemistry and Chemical Engineering - page 50

attracted growing interest with respect to their potential application in drug delivery
systems. Ayame et al recently developed a self-assembly method for preparing physi-
cally cross-linked nanogels (<50 nm) through the controlled association of hydro-
phobically modifi ed polymers in water. Microscale hydrogels of controlled sizes and
shapes are useful for cell-based screening, in vitro diagnostics, tissue engineering, and
drug delivery in a sustained manner [33-50].
STARCH
Starch is an important naturally occurring polymer of glucose, with diverse applica-
tions in food and polymer science, found in roots, rhizomes, seeds, stems, tubers, and
corms of plants, as microscopic granules having characteristic shapes and sizes. Each
starch typically contains several million amylopectin molecules accompanied by a
much larger number of smaller amylose molecules. The largest source of starch is
corn (maize) with other commonly used sources being cereals (e.g., corn, wheat, rice,
oat, barley) contain 60-80%, legumes (e.g., chickpea, bean, pea) 25-50%, tubers (e.g.,
potato, cassava, cocoyam, arrowroot) 60-90%, and some green or immature fruit (e.g.,
banana, mango) contain 70% starch in dry base . Genetic modification of starch crops
has recently led to the development of starches with improved and targeted functional-
ity. Annual worldwide starch production is 66.5 million tons and the growing demand
for starches has created interest in identifying new sources and modifications or de-
rivatives of this polysaccharide.
Amylose
Starch is tightly and radially packed into dehydrated granules with origin-specific
shape and size. Granules contain both crystalline and amorphous areas, consists of
two types of molecules, amylose (normally 20-30%) and amylopectin (normally 70-
80%). The granules are insoluble in cold water, but grinding or swelling them in warm
water causes them to burst. Both amylose and amylopectin consist of polymers of α-D-
glucose units in the conformation. The relative proportions of amylose to amylopectin
and α-(1, 6) branch-points both depend on the source of the starch. The starch granule
absorbs water; they swell, lose crystallinity and leach out amylose. The higher the
amylose content, the lower is the swelling power and the smaller is the gel strength for
the same starch concentration. Of the two components of starch, amylose has the most
useful functions as a hydrocolloid. Its extended conformation causes the high viscos-
ity of water soluble starch and varies relatively little with temperature. The extended
loosely helical chains possess a relatively hydrophobic inner surface that is not able to
hold water well and more hydrophobic molecules such as lipids and aroma compounds
can easily replace this. Amylose forms useful gels and films. Its association and crys-
tallization (retrogradation) on cooling decreases its storage stability, causing shrinkage
and the release of water (syneresis).
According to X-ray studies starch can be classifi ed to A, B, and C forms. In the
native granular forms, the A pattern is associated mainly with cereal starches, while
the B form is usually obtained from tuber starches.