Starch Nanocomposite and Nanoparticles: Biomedical Applications - Modern Trends in Chemistry and Chemical Engineering - page 53

Amylopectin double-helical chains can either form the more open hydrated type
B hexagonal crystallites or the denser type A crystallites, with staggered monoclinic
packing, dependent on the plant source of the granules.
Modification of Starch
Current starch research has focused on the search for non-conventional starch sources
with diverse physicochemical, structural, and functional characteristics that provide
them with a broad range of potential industrial uses. Physicochemical and function-
al properties must be identified before determining the potential uses of starches in
food systems and other industrial applications. A fundamental characteristic of native
starches from different vegetable sources is that their granule size distribution and
molecular structures. These properties then influence a starch's usefulness in different
applications.
Native and modifi ed starches are used widely in food processing operations in or-
der to impart viscosity and texture. In their native form, gels or pastes of starches tend
to breakdown either from prolonged heating, high shear, or acidic conditions and also
they have the tendency to retrograde and undergo syneresis. Starch derivatisations like
etherifi cation, esterifi cation, acetylation, and cross-linking have been used to improve
the gelatinization and cooking characteristics and to prevent retrogradation. Each an-
hydroglucose unit of starch contains two secondary hydroxyls and a primary hydroxyl
group. These hydroxyls potentially are able to react with any chemical capable of
reacting with alcoholic hydroxyls. This would include a wide range of compounds
such as acid anhydrides, organic chloro compounds, aldehydes, epoxy, ethylenic com-
pounds, and so forth where the specifi c chemical contains two or more moieties capa-
ble of reacting with hydroxyl groups. There is a possibility of reacting at two different
hydroxyls resulting in cross linking between hydroxyls on the same molecule or on
different molecules. The most common modifi cation to starches to impart structural
integrity is chemical cross-linking. These chemically cross-linked starches are usu-
ally resistant to shear, pH, temperature during food processing conditions. The most
widely used cross-linking reagents for modifying food starches are mixtures of adipic/
acetic anhydride, and phosphorus oxychloride, or sodium trimetaphosphate, which
yield distarch adipates, distarch phosphates, respectively. Cross linking reinforces
the hydrogen bonds in the granule with chemical bonds which act as bridge between
the molecules. Several research groups have studied the rheological characteristics
of starches mixed with other hydrocolloids to improve their rheological properties.
Commonly used hydrocolloids are gum arabic, guar, carboxymethylcellulose (CMC),
carrageenan, xanthan, xyloglucan, and so forth.
The literature regarding the rheology of starch-hydrocolloid systems has attracted
more attention than other systems. Blends of native starches and other polysaccha-
ride hydrocolloids have been used in the modern food industry to modify and control
the texture, improve moisture retension, control water mobility, and eating quality of
food products. Polysaccharide blending affected the pasting, and rheological proper-
ties of cationic, native, and anionic starches differently. Components compatibility,
coupled with their individual properties like gelation, ageing, and so forth leads to
a large variety of structures and properties of prepared materials. The interactions