Starch
Starch (
CAS# 9005-25-8) is a complex
carbohydrate which is
insoluble in
water; it is used by
plants as a way to store excess
glucose. Starch (in particular
cornstarch) is used in cooking for thickening
sauces. In industry, it is used in the manufacture of
adhesives,
paper, and
textiles. It is a white
powder, and is
tasteless and
odorless.
Biochemically,
starch is a combination of two
polymeric
carbohydrates (
polysaccharides) called
amylose and
amylopectin. Amylose is constituted by glucose monomer units joined to one another head-to-tail forming alpha-1,4 linkages. Amylopectin differs from amylose in that branching occurs, with an alpha-1,6 linkage every 24-30 glucose monomer units. The overall structure of amylopectin is not that of a linear polysaccharide chain since two glucose units frequently form a branch point, so the result is the coiled molecule most suitable for storage in starch grains. Both amylopectin and amylose are polymers of glucose, and a typical starch polymer chain consists of around 2500 glucose molecules in their varied forms of polymerisation. In general, starches have the formula (C
6H
10O
5)
n, where "n" denotes the total number of glucose monomer units.
Structurally, the starch forms clusters of linked linear polymers, where the
alpha-1,4 linked chains form columns of glucose units which branch regularly at the
alpha-1,6 links. The relative content of amylose and amylopectin varies between species, and between different cultivars of the same species. For example, high-amylose corn (
maize) has starch consisting of about 85% amylose, which is the linear constituent of starch, while
waxy corn starch is more than 99% amylopectin, or branched starch. The primary function of starch in plants, is to act as an energy storage molecule for the organism. In plants simple sugars are linked into starch molecules by specialized cellular organs called
amyloplasts.
Starches are insoluble in water. They can be digested by
hydrolysis, catalyzed by
enzymes called
amylases, which can break the glycosidic bonds between the 'alpha-glucose' components of the starch
polysaccharide. Humans and other animals have amylases, so they can digest starch.
Digestion of starches consists of the process of the cleavage of the starch molecules back into their constituent simple sugar units by the action of the amylases. The resulting sugars are then processed by further enzymes (such as
maltase) in the body, in the same manner as other sugars in the diet.
Starch is often found in the
fruit,
seeds,
rhizomes or
tubers of
plants. The four major resources for starch production and consumption in the
USA are
corn,
potatoes,
rice, and
wheat.
Pasta is an important dietary source of starch which is commonly prepared from wheat, rice or beans.
Bread is another important source of starch and is commonly prepared from wheat.
As an
additive for
food processing,
arrowroot,
guar gum,
locust bean, and
tapioca are commonly used as well. Commonly used starches around the world are:
arracacha,
buckwheat,
banana,
barley,
cassava,
konjac,
kudzu,
oca,
sago,
sorghum,
sweet potato,
taro and
yams. Edible
beans, such as
favas,
lentils and
peas, are also rich in starch.
When a starch is pre-cooked it can then be used to thicken
chilled foods. This is referred to on packaging as
modified food starch.
Agar,
carrageenan,
gelatins and
pectins are used in very much the same way.
Clothing starch or
laundry starch is a liquid that is prepared by mixing a
vegetable starch in water (earlier preparations also had to be boiled), and is used in the
laundering of
clothes. Starch was widely used in
Europe in the 16th and 17th centuries to stiffen the wide collars and ruffs of fine linen which surrounded the necks of the well-to-do. During the 19th century and early 20th century, it was stylish to stiffen the collars and
sleeves of men's
shirts and the ruffles of girls'
petticoats by applying starch to them as the clean clothes were being
ironed.
Aside from the smooth, crisp edges it gave to clothing, it served a practical purpose as well.
Dirt and
sweat from a person's neck and wrists would stick to the starch rather than fibers of the clothing, and would easily wash away along with the starch. Then, after each laundering, the starch would be reapplied.
Starch solution is used to test for elemental
iodine. A blue/black color indicates the presence of
iodine in starch solution. The details of this reaction are not yet fully known, but it is thought that the
iodine (I
3- and I
5- ions) fits inside the coils of
amylose, the charge transfers between the iodine and the starch, and the
energy level spacings in the resulting
complex correspond to the
absorption spectrum in the visible light region. A 0.4% w/w solution is the standard concentration for a dilute starch indicator solution. It is made by adding 4 grams of soluble starch to 1 litre of heated water; the solution is cooled before use (starch-iodine complex becomes unstable at temperatures above 35°C). This complex is often used in
redox titrations: in presence of an
oxidizing agent the solution turns blue, in presence of
reducing agent blue color disappears because I
5- ions break upinto iodine and iodide.
Under the
microscope, starch grains show a distinctive
Maltese Cross effect (also known as 'extinction cross' and birefringence) under
polarized light.
Animal starch is the common name of
glycogen. It is not the same as ordinary starch.
Starch can be hydrolyzed into simpler carbohydrates by
acids, various
enzymes, or a combination of the two. The extent of conversion is typically quantified by
dextrose equivalent (DE), which is roughly the fraction of the glycoside bonds in starch that have been broken. Food products made in this way include
*
Maltodextrin, a lightly hydrolyzed (DE 10–20) starch product used as a bland-tasting filler and thickener.
* Various
corn syrups (DE 30–70), viscous solutions used as sweeteners and thickeners in many kinds of processed foods.
* Dextrose (DE 100), commercial
glucose, prepared by the complete hydrolysis of starch.
*
High fructose syrup, made by treating dextrose solutions to the enzyme
glucose isomerase, until a substantial fraction of the glucose has been converted to fructose. In the
United States, high fructose corn syrup is the principal sweetener used in sweetened beverages.
* Jones, Orlando, "
US2000 Improvement in the manufacture of starch". (Class: 127/68; 48/119; 127/69). Middlesex, England,
USPTO.
* Detailed description and pictures of starch molecular structure: http://www.lsbu.ac.uk/water/hysta.html
