For the Urhobo cuisine dish known as starch see usi
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined
by glycosidic bonds. This polysaccharide is produced by most green plants as an energy
store. It is the most common carbohydrate in human diets and is contained in large amounts
in such staple foods as potatoes, wheat, maize, rice, and cassava.
Pure starch is a white, tasteless and odorless powder that is insoluble in cold water or
alcohol. It consists of two types of molecules: the linear and helical amylose and the branched
amylopectin. Depending on the plant, starch generally contains 20 to 25% amylose and 75
to 80% amylopectin by weight. Glycogen, the glucose store of animals, is a more branched
version of amylopectin. Starch is processed to produce many of the
sugars in processed foods. Dissolving starch in warm water gives wheatpaste, which can
be used as a thickening, stiffening or gluing agent. The biggest industrial non-food use
of starch is as adhesive in the papermaking process. Starch can be applied to parts of
some garments before ironing, to stiffen them.
Etymology The word "starch" is from sterchen, meaning
to stiffen. "amylum" is for starch, from the Greek αμυλον, "amylon" which means "not
ground at a mill". The root amyl is used in biochemistry for several compounds related
to starch. History
Starch grains from the rhizomes of Typha as flour have been identified from grinding stones
in Europe dating back to 30,000 years ago. Starch grains from sorghum were found on grind
stones in caves in Ngalue, Mozambique dating up to 100,000 years ago.
Pure extracted wheat starch paste was used in Ancient Egypt possibly to glue papyrus.
The extraction of starch is first described in the Natural History of Pliny the Elder
around AD 77-79. Romans used it also in cosmetic creams, to powder the hair and to thicken
sauces. Persians and Indians used it to make dishes similar to gothumai wheat halva. Rice
starch as surface treatment of paper has been used in paper production in China, from 700
AD onwards. In addition to starchy plants consumed directly,
66 million tonnes of starch were being produced per year world-wide by 2008. In the EU this
was around 8.5 million tonnes, with around 40% being used for industrial applications
and 60% for food uses, most of the latter as glucose syrups.
Energy store of plants Most green plants use starch as their energy
store. An exception is the family Asteraceae, where starch is replaced by the fructan inulin.
In photosynthesis, plants use light energy to produce glucose from carbon dioxide. The
glucose is stored mainly in the form of starch granules, in plastids such as chloroplasts
and especially amyloplasts. Toward the end of the growing season, starch accumulates
in twigs of trees near the buds. Fruit, seeds, rhizomes, and tubers store starch to prepare
for the next growing season. Glucose is soluble in water, hydrophilic,
binds with water and then takes up much space and is osmotically active; glucose in the
form of starch, on the other hand, is not soluble, therefore osmotically inactive and
can be stored much more compactly. Glucose molecules are bound in starch by the
easily hydrolyzed alpha bonds. The same type of bond is found in the animal reserve polysaccharide
glycogen. This is in contrast to many structural polysaccharides such as chitin, cellulose
and peptidoglycan, which are bound by beta bonds and are much more resistant to hydrolysis.
Biosynthesis Plants produce starch by first converting
glucose 1-phosphate to ADP-glucose using the enzyme glucose-1-phosphate adenylyltransferase.
This step requires energy in the form of ATP. The enzyme starch synthase then adds the ADP-glucose
via a 1,4-alpha glycosidic bond to a growing chain of glucose residues, liberating ADP
and creating amylose. Starch branching enzyme introduces 1,6-alpha glycosidic bonds between
these chains, creating the branched amylopectin. The starch debranching enzyme isoamylase removes
some of these branches. Several isoforms of these enzymes exist, leading to a highly complex
synthesis process. Glycogen and amylopectin have the same structure,
but the former has about one branch point per ten 1,4-alpha bonds, compared to about
one branch point per thirty 1,4-alpha bonds in amylopectin. Amylopectin is synthesized
from ADP-glucose while mammals and fungi synthesize glycogen from UDP-glucose; for most cases,
bacteria synthesize glycogen from ADP-glucose. In addition to starch synthesis in plants,
starch can be synthesized from non-food starch mediated by an enzyme cocktail. In this cell-free
biosystem, beta-1,4-glycosidic bond-linked cellulose is partially hydrolyzed to cellobioase.
Cellobiose phosphorylase cleaves to glucose 1-phosphate and glucose; the other enzyme—potato
alpha-glucan phosphorylase can add glucose unit from glucose 1-phosphorylase to the non-ruducing
ends of starch. In it, phosphate is internally recycled. The other product—glucose—can
be assimilated by a yeast. This cell-free bioprocessing does not need any costly chemical
and energy input, can be conducted in aqueous solution, and does not have sugar losses.
As a result, cellulosic starch could be used to feed the world because cellulose resource
is about 50 times of starch resource. Degradation
Starch is synthesized in plant leaves during the day, in order to serve as an energy source
at night. Starch is stored as granulates. These insoluble highly branched chains have
to be phosphorylated in order to be accessible for degrading enzymes. The enzyme glucan,
water dikinase phosphorylates at the C-6 position of a glucose molecule, close to the chains
1,6-alpha branching bonds. A second enzyme, phosphoglucan, water dikinase phosphorylates
the glucose molecule at the C-3 position. A loss of these enzymes, for example a loss
of the GWD, leads to a starch excess phenotype. Because starch cannot be phosphorylated, it
accumulates in the plastid. After the phosphorylation, the first degrading
enzyme, beta-amylase is able to attack the glucose chain at its non-reducing end. Maltose
is released as the main product of starch degradation. If the glucose chain consists
of three or less molecules, BAM cannot release maltose. A second enzyme, disproportionating
enzyme-1, combines two maltotriose molecules. From this chain, a glucose molecule is released.
Now, BAM can release another maltose molecule from the remaining chain. This cycle repeats
until starch is degraded completely. If BAM comes close to the phosphorylated branching
point of the glucose chain, it can no longer release maltose. In order for the phosphorylated
chain to be degraded, the enzyme isoamylase is required.
The products of starch degradation are to the major part maltose and to a less extensive
part glucose. These molecules are now exported from the plastid to the cytosol. Maltose is
exported via the maltose transporter. If this transporter is mutated, maltose accumulates
in the plastid. Glucose is exported via the plastidic glucose translocator. Now, these
two sugars act as a precursor for sucrose synthesis. Sucrose can the be used in the
oxidative pentose phosphate pathway in the mitochondria, in order to generate ATP at
night. Properties
Structure
While amylose was traditionally thought to be completely unbranched, it is now known
that some of its molecules contain a few branch points. Although in absolute mass only about
one quarter of the starch granules in plants consist of amylose, there are about 150 times
more amylose molecules than amylopectin molecules. Amylose is a much smaller molecule than amylopectin.
Starch molecules arrange themselves in the plant in semi-crystalline granules. Each plant
species has a unique starch granular size: rice starch is relatively small while potato
starches have larger granules. Starch becomes soluble in water when heated.
The granules swell and burst, the semi-crystalline structure is lost and the smaller amylose
molecules start leaching out of the granule, forming a network that holds water and increasing
the mixture's viscosity. This process is called starch gelatinization. During cooking, the
starch becomes a paste and increases further in viscosity. During cooling or prolonged
storage of the paste, the semi-crystalline structure partially recovers and the starch
paste thickens, expelling water. This is mainly caused by retrogradation of the amylose. This
process is responsible for the hardening of bread or staling, and for the water layer
on top of a starch gel. Some cultivated plant varieties have pure
amylopectin starch without amylose, known as waxy starches. The most used is waxy maize,
others are glutinous rice and waxy potato starch. Waxy starches have less retrogradation,
resulting in a more stable paste. High amylose starch, amylomaize, is cultivated for the
use of its gel strength and for use as a resistant starch in food products.
Synthetic amylose made from cellulose has a well-controlled degree of polymerization.
Therefore, it can be used as a potential drug deliver carrier.
Hydrolysis The enzymes that break down or hydrolyze starch
into the constituent sugars are known as amylases. Alpha-amylases are found in plants and in
animals. Human saliva is rich in amylase, and the pancreas also secretes the enzyme.
Individuals from populations with a high-starch diet tend to have more amylase genes than
those with low-starch diets; chimpanzees have very few amylase genes. It is possible that
turning to a high-starch diet was a significant event in human evolution.
Beta-amylase cuts starch into maltose units. This process is important in the digestion
of starch and is also used in brewing, where amylase from the skin of seed grains is responsible
for converting starch to maltose. Dextrinization
If starch is subjected to dry heat, it breaks down to form dextrins, also called "pyrodextrins"
in this context. This break down process is known as dextrinization.dextrins are mainly
yellow to brown in color and dextrinization is partially responsible for the browning
of toasted bread. Chemical tests
Iodine solution is used to test for starch; a dark blue color indicates the presence of
starch. The details of this reaction are not yet fully known, but it is thought that the
iodine fit 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. The strength of the resulting blue color depends
on the amount of amylose present. Waxy starches with little or no amylose present will color
red. Starch indicator solution consisting of water,
starch and iodine is often used in redox titrations: in the presence of an oxidizing agent the
solution turns blue, in the presence of reducing agent the blue color disappears because triiodide
ions break up into three iodide ions, disassembling the starch-iodine complex. A 0.3% w/w solution
is the standard concentration for a starch indicator. It is made by adding 3 grams of
soluble starch to 1 liter of heated water; the solution is cooled before use.
Each species of plant has a unique type of starch granules in granular size, shape and
crystallization pattern. Under the microscope, starch grains stained with iodine illuminated
from behind with polarized light show a distinctive Maltese cross effect.
Food Starch is the most common carbohydrate in
the human diet and is contained in many staple foods. The major sources of starch intake
worldwide are the cereals and the root vegetables. Many other starchy foods are grown, some only
in specific climates, including acorns, arrowroot, arracacha, bananas, barley, breadfruit, buckwheat,
canna, colacasia, katakuri, kudzu, malanga, millet, oats, oca, polynesian arrowroot, sago,
sorghum, sweet potatoes, rye, taro, chestnuts, water chestnuts and yams, and many kinds of
beans, such as favas, lentils, mung beans, peas, and chickpeas.
Widely used prepared foods containing starch are bread, pancakes, cereals, noodles, pasta,
porridge and tortilla. Digestive enzymes have problems digesting
crystalline structures. Raw starch will digest poorly in the duodenum and small intestine,
while bacterial degradation will take place mainly in the colon. When starch is cooked,
the digestibility is increased. Hence, before humans started using fire, eating grains was
not a very useful way to get energy. Starch gelatinization during cake baking can
be impaired by sugar competing for water, preventing gelatinization and improving texture.
Starch industry The starch industry extracts and refines starches
from seeds, roots and tubers, by wet grinding, washing, sieving and drying. Today, the main
commercial refined starches are cornstarch, tapioca, wheat, rice and potato starch. To
a lesser extent, sources include rice, sweet potato, sago and mung bean. Historically,
Florida arrowroot was also commercialized. To this day, starch is extracted from more
than 50 types of plants. Untreated starch requires heat to thicken
or gelatinize. When a starch is pre-cooked, it can then be used to thicken instantly in
cold water. This is referred to as a pregelatinized starch.
Starch sugars Starch can be hydrolyzed into simpler carbohydrates
by acids, various enzymes, or a combination of the two. The resulting fragments are known
as dextrins. The extent of conversion is typically quantified by dextrose equivalent, which is
roughly the fraction of the glycosidic bonds in starch that have been broken.
These starch sugars are by far the most common starch based food ingredient and are used
as sweetener in many drinks and foods. They include:
Maltodextrin, a lightly hydrolyzed starch product used as a bland-tasting filler and
thickener. Various glucose syrups, also called corn syrups
in the US, viscous solutions used as sweeteners and thickeners in many kinds of processed
foods. Dextrose, commercial glucose, prepared by
the complete hydrolysis of starch. High fructose syrup, made by treating dextrose
solutions with the enzyme glucose isomerase, until a substantial fraction of the glucose
has been converted to fructose. In the United States sugar prices are two to three times
higher than in the rest of the world, which makes high fructose corn syrup significantly
cheaper, so that it is the principal sweetener used in processed foods and beverages. Fructose
also has better microbiological stability. One kind of high fructose corn syrup, HFCS-55,
is sweeter than sucrose because it is made with more fructose, while the sweetness of
HFCS-42 is on par with sucrose. Sugar alcohols, such as maltitol, erythritol,
sorbitol, mannitol and hydrogenated starch hydrolysate, are sweeteners made by reducing
sugars. Modified starches
A modified starch is a starch that has been chemically modified to allow the starch to
function properly under conditions frequently encountered during processing or storage,
such as high heat, high shear, low pH, freeze/thaw and cooling.
The modified food starches are E coded according to the International Numbering System for
Food Additives: 1400 Dextrin
1401 Acid-treated starch 1402 Alkaline-treated starch
1403 Bleached starch 1404 Oxidized starch
1405 Starches, enzyme-treated 1410 Monostarch phosphate
1412 Distarch phosphate 1413 Phosphated distarch phosphate
1414 Acetylated distarch phosphate 1420 Starch acetate
1422 Acetylated distarch adipate 1440 Hydroxypropyl starch
1442 Hydroxypropyl distarch phosphate 1443 Hydroxypropyl distarch glycerol
1450 Starch sodium octenyl succinate 1451 Acetylated oxidized starch
INS 1400, 1401, 1402, 1403 and 1405 are in the EU food ingredients without an E-number.
Typical modified starches for technical applications are cationic starches, hydroxyethyl starch
and carboxymethylated starches. Use as food additive
As an additive for food processing, food starches are typically used as thickeners and stabilizers
in foods such as puddings, custards, soups, sauces, gravies, pie fillings, and salad dressings,
and to make noodles and pastas. Gummed sweets such as jelly beans and wine
gums are not manufactured using a mold in the conventional sense. A tray is filled with
native starch and leveled. A positive mold is then pressed into the starch leaving an
impression of 1,000 or so jelly beans. The jelly mix is then poured into the impressions
and put into a stove to set. This method greatly reduces the number of molds that must be manufactured.
Resistant starch is starch that escapes digestion in the small intestine of healthy individuals.
High amylose starch from corn has a higher gelatinization temperature than other types
of starch and retains its resistant starch content through baking, mild extrusion and
other food processing techniques. It is used as an insoluble dietary fiber in processed
foods such as bread, pasta, cookies, crackers, pretzels and other low moisture foods. It
is also utilized as a dietary supplement for its health benefits. Published studies have
shown that Type 2 resistant corn helps to improve insulin sensitivity, increases satiety
and improves markers of colonic function. It has been suggested that resistant starch
contributes to the health benefits of intact whole grains.
In the pharmaceutical industry, starch is also used as an excipient, as tablet disintegrant
or as binder. Industrial applications
Papermaking Papermaking is the largest non-food application
for starches globally, consuming millions of metric tons annually. In a typical sheet
of copy paper for instance, the starch content may be as high as 8%. Both chemically modified
and unmodified starches are used in papermaking. In the wet part of the papermaking process,
generally called the "wet-end", the starches used are cationic and have a positive charge
bound to the starch polymer. These starch derivatives associate with the anionic or
negatively charged paper fibers / cellulose and inorganic fillers. Cationic starches together
with other retention and internal sizing agents help to give the necessary strength properties
to the paper web formed in the papermaking process, and to provide strength to the final
paper sheet. In the dry end of the papermaking process,
the paper web is rewetted with a starch based solution. The process is called surface sizing.
Starches used have been chemically, or enzymatically depolymerized at the paper mill or by the
starch industry. The size - starch solutions are applied to the paper web by means of various
mechanical presses. Together with surface sizing agents the surface starches impart
additional strength to the paper web and additionally provide water hold out or "size" for superior
printing properties. Starch is also used in paper coatings as one of the binders for the
coating formulations which include a mixture of pigments, binders and thickeners. Coated
paper has improved smoothness, hardness, whiteness and gloss and thus improves printing characteristics.
Corrugated board adhesives Corrugated board adhesives are the next largest
application of non-food starches globally. Starch glues are mostly based on unmodified
native starches, plus some additive such as borax and caustic soda. Part of the starch
is gelatinized to carry the slurry of uncooked starches and prevent sedimentation. This opaque
glue is called a SteinHall adhesive. The glue is applied on tips of the fluting. The fluted
paper is pressed to paper called liner. This is then dried under high heat, which causes
the rest of the uncooked starch in glue to swell/gelatinize. This gelatinizing makes
the glue a fast and strong adhesive for corrugated board production.
Clothing starch Clothing or laundry starch is a liquid that
is prepared by mixing a vegetable starch in water, 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 practical purposes
as well. Dirt and sweat from a person's neck and wrists would stick to the starch rather
than to the fibers of the clothing, and would easily wash away along with the starch. After
each laundering, the starch would be reapplied. Today, the product is sold in aerosol cans
for home use. Other
Another large non-food starch application is in the construction industry, where starch
is used in the gypsum wall board manufacturing process. Chemically modified or unmodified
starches are added to the stucco containing primarily gypsum. Top and bottom heavyweight
sheets of paper are applied to the formulation, and the process is allowed to heat and cure
to form the eventual rigid wall board. The starches act as a glue for the cured gypsum
rock with the paper covering, and also provide rigidity to the board.
Starch is used in the manufacture of various adhesives or glues for book-binding, wallpaper
adhesives, paper sack production, tube winding, gummed paper, envelope adhesives, school glues
and bottle labeling. Starch derivatives, such as yellow dextrins, can be modified by addition
of some chemicals to form a hard glue for paper work; some of those forms use borax
or soda ash, which are mixed with the starch solution at 50–70 °C to create a very
good adhesive. Sodium silicate can be added to reinforce these formulae.
Textile chemicals from starch: warp sizing agents are used to reduce breaking of yarns
during weaving. Starch is mainly used to size cotton based yarns. Modified starch is also
used as textile printing thickener. In oil exploration, starch is used to adjust
the viscosity of drilling fluid, which is used to lubricate the drill head and suspend
the grinding residue in petroleum extraction. Starch is also used to make some packing peanuts,
and some drop ceiling tiles. In the printing industry, food grade starch
is used in the manufacture of anti-set-off spray powder used to separate printed sheets
of paper to avoid wet ink being set off. For body powder, powdered corn starch is used
as a substitute for talcum powder, and similarly in other health and beauty products.
Starch is used to produce various bioplastics, synthetic polymers that are biodegradable.
An example is polylactic acid based on glucose form starch.
Glucose from starch can be further fermented to biofuel corn ethanol using the so-called
wet milling process. Today most bioethanol production plants use the dry milling process
to ferment corn or other feedstock directly to ethanol.
Hydrogen production could use glucose form starch as the raw material, using enzymes.
See also Acrylamide, which is present in fried potatoes
Distilled beverage, brewing from starch alcohol Non-Newtonian fluid
Yeast extract Starch production
Starch analysis References
External links Starch, by Martin Chaplin
Starch - Stärke, scientific journal on starch CDC - NIOSH Pocket Guide to Chemical Hazards,
information for workers