Starch and Modified Starch
Starch is a polysaccharide
, a chain of many glucose molecules.
It is the main carbohydrate store in roots and seeds.
There are two types of glucose chains in starch. One is a simple
chain called amylose
, and the other is a complex branched
form called amylopectin
In the starch grains in a plant, amylopectin makes up the bulk of
the material, between 50 to 80 percent by weight, made up of several
million amylopectin molecules per starch grain. The rest is a
much larger number of the smaller amylose chains, made up of 500
to 20,000 glucose units in each chain. Amylopectin molecules are
made of several million glucose units.
Amylopectin forms branched structures with about 30 glucose units
in a chain between branches. This makes the molecule somewhat
striped in appearance, with the knotted branch points all in
a row, and the smooth chains separating them. These molecules
are so large that this striped appearance shows up under a light
microscope, forming what appear to be "growth rings" in the
Starch is a major source of calories in grains and tubers, and
foods made from them.
When starch is added to products as an ingredient, however, it
is the functional properties of the starch that are usually important,
not the calories.
Starch is the main thickener in gravies, sauces, and puddings. It
absorbs water, and becomes a gel when cooked. As the starch swells
up with water, the amylose leaches out, and the amylopectin forms
the gel. Some starches have higher amylopectin content, and make
better gels than those containing lots of amylose.
As a thickener (as opposed to a gel), it is the amylose that has
the main function. The long water-soluble chains increase the
viscosity, and that viscosity doesn't change much with temperature.
Amylose chains tend to curl up into helices
the hydrophobic parts inside. This allows them to trap oils and
fats inside the helix, as well as aroma molecules.
Because starches are so good at absorbing water and bulking up,
they are important in the "mouth feel" of many food products, and
are used as fat substitutes.
Not all of the starch in a food ends up being digested. The
starch that is not absorbed by the body is called "resistant
starch", and is consider dietary fiber. It is also a source
of nutrition for intestinal flora, which make important vitamins
(and intestinal gas).
Starches are added to processed meats (lunch meats, hot dogs,
sausages, etc.) as a filler, binder, moisture retainer,
and fat substitute. They are added to soups, sauces and gravies
as thickeners. They are used in extruded cereals and snacks
to hold the shape of the material.
Starches can be modified in several ways to change their function
as additives in products. They can be cross-linked, where the chains
get stuck together into a mesh. They can be heated to break the
long chains down into simpler molecules like dextrin, polydextrin,
and malto-dextrin. These are simply short starches.
Starches can have a hydrogen replaced
by something else, such as a carboxymethyl group,
making carboxymethyl starch
Adding the carboxymethyl group makes the starch less prone to
damage by heat and bacteria. Carboxymethyl starch is used as an
additive in oil drilling mud, and is used in the goo that makes
ultrasound examinations messy. Carboxymethyl starch is also called
a starch ether
, or misnamed starch glycolate
due to a
Carboxymethyl groups make the starch more hydrophilic (water loving),
and aid in cross-linking. This makes carboxymethyl starch useful in
aspirin and other tablets to make them disintegrate quickly.
Longer carbon chains can also be added, such as carboxyethyl groups,
or carboxypropyl groups.
Adding bulky functional groups like carboxymethyl and carboxyethyl groups
reduces the tendency of the starch to recrystallize. When the starch
stays as a gel, a product is softer, and we say it is "fresh". When
the starch regains its crystalline form, the product becomes firmer,
and we say it is "stale". The technical term for this is starch
Starches can be esterified
by modifications with an acid.
An ester is the result of reacting an alcohol with an acid.
The starch loses a hydroxyl group, and the acid loses a hydrogen.
These combine to form water as the other product of the reaction.
Using acetic acid, starch acetates are formed, which are used as
film-forming polymers for pharmaceutical products, and as the
polymer in biodegradable packing foam "peanuts". Starch acetates
have a lower tendency to create gels than unmodified starch.
Acids can also break the long chains into shorter molecules, much
like heat does, to form polydextrins, malto-dextrin, or dextrin.
Enzymes are also used to do the same thing.
Cross-linking occurs when a hydroxyl group (OH) on one chain bonds
with a hydroxyl group on an adjacent chain. This toughens the
starch, and helps it resist heat and acids.
Cross-linking can be done by heating, or by reacting with compounds
Starches are also sometimes "pregelatinized" to make them easier
to dissolve during product manufacture.
Starches, especially modified starches, are also used as glues
in cardboard manufacturing.
Starches such as Gum Arabic and Gum Tragacanth are used as the
glue for stamps and postal envelopes.
Oxidized starch, usually oxidized with
is whiter than unmodified starch, has increased clarity, and a lower