Chemistry Reference
In-Depth Information
Amylopectin, on the other hand, is branched through carbon 6:
CH
2
OH
O
CH
2
OH
HO
O
OH
O
HO
m
OH
O
O
HO
OH
O
n
The ratios of amylopectin to amylose in many natural starches are about 3:1. The main commercial
source of starch in this country is corn. Lesser amounts of industrial starch are obtained from potatoes,
wheat, and tapioca (not necessarily in that order). The extraction of starch from plant material is done
by grinding the plant tissues in water. The slurry is then filtered to obtain a suspension of starch
granules. These granules are then collected with the aid of a centrifuge and dried.
When a water suspension of starch granules is heated to 60-80
C, the granules swell and rupture.
This results in formation a viscous colloidal dispersion containing some dissolved starch molecules.
Cooling this dispersion results in formation of a gel, due to aggregation of the amylose molecules.
It is essentially a crystallization phenomenon, known as
retrogration.
By comparison, amylopectin
molecules cannot associate so readily due to branching and will not gel under these conditions.
Starches are modified chemically in various ways. Some acetate and phosphate esters are produced
commercially, as well as hydroxyalkyl and tertiary aminoalkyl ethers. Both unmodified and modified
starches are used principally in papermaking, paper coating, paper adhesives, textile sizes, and as
food thickeners. There are many reports in the literature on graft copolymers of starch. The work
is often done in search of biodegradable materials for packaging and agricultural mulches.
Most chemical modifications of starch parallel those of cellulose.
8.2.3 Cellulose
This polysaccharide is found widely in nature. It is a major constituent of plant tissues (50-70%,
depending upon the wood), fibers, and leaf stalks. Chemically cellulose is 1,4-
-poly-anhydroglucose
b
[
12
](
trans
):
CH
2
OH
O
O
HO
n
OH
