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98 Textbook of Pharmacognosy
Solanum tuberosum
Potato: Flower
Geographical Source
Starch is commercially produced in tropical and subtropical countries. USA,
Canada, Australia, China, India, Russia, Thailand, Vietnam, Pakistan are
the major producers of starch in the world. Rice is produced in India, China,
Japan and other countries. Wheat is cultivated in many temperate countries
of the world. Maize is produced in USA, Argentina, India and other tropical
and subtropical countries. Potato is cultivated in many countries.
Collection and Preparation
In general, the cereal grains (corn, rice and wheat) mostly comprise of starch
bundles, oil, soluble protein, and the insoluble protein termed ‘gluten’ while
potato contains starch, mineral salts (inorganic), soluble proteins and
vegetable tissues. The various specific methods are normally employed to
separate the starch either from the grains of cereals or from potato.
The method of preparation of starch depends upon the nature of the
raw material used and the purity of starch desired. Cereal starches have to
be freed from cell debris, oil, soluble protein matter and the abundant
insoluble proteins (glutelins and prolamins) known as ‘gluten’. Potato
starch, on the other hand, is associated with vegetable tissue, mineral salts
and soluble proteins.
Collection and Preparation of Maize (Corn) Starch
The grains of corn (maize) are first washed with running water to remove
the dust particles and organic matter. Later, the grains are softened by
soaking them in warm water at 40–60°C for 48–72 hours along with a
solution of sulphur dioxide (0.2–0.3%) to check the fermentation. The grains
that are swollen are passed through attrition mill to split and crush them
partly to separate the endosperm. The embryos (germs) are isolated by the
addition of water as a result of which they float and are segregated by
Systematic Pharmacognostic Study of Carbohydrates and Derived Products 99
skimming off. The corn oil (germ oil, a source of vitamin E) is recovered
from the embryos by expression method. The water that is used to soften
the grains dissolves most of the minerals, soluble proteins and
carbohydrates from the grains. This can be used as a culture medium for
the production of antibiotics such as penicillin. Following the germ removal,
the liquid mass is freed from the cell debris and gluten (insoluble protein)
by passing through fine sieves and washing. The slurry (milky) obtained
is a mixture of starch and particles of gluten are subjected to centrifugation
by starch purification centrifuge. The starch being heavier settles at the
bottom and the particles of gluten being lighter floats on the surface (this
is removed by a jet of water). Finally, the starch is washed thoroughly
with fresh water, centrifuged many times and/or filtered, pressed and
finally dried either on a moving belt dryer or flash dryer, and finally packed.
Collection and Preparation of Rice Starch
The broken pieces of rice grains are adequately soaked in an aqueous solution
of sodium hydroxide (0.4–0.5% w/v) which softens the grains and the
dissolution of the insoluble protein, gluten. The grains are then wet-milled
and taken up with water. The resulting suspension is purified by passing
through the sieves and the starch is recovered by a process of centrifugation.
The starch is washed finally, dried, powdered and stored in bags.
Collection and Preparation of Wheat Starch
Wheat being the common staple food, its utility for making the starch is
restricted. The wheat flour is first made into a stiff ball of dough which is
kept aside for a short duration. During this process, gluten swells up and
the balls are shifted to grooved-rollers which move forward and backward
slowly. The water is constantly sprinkled to carry off the starch along with
it while gluten remains as a soft elastic mass. The slurry is purified by
centrifugation, washed, dried, powdered and packed in bags.
Collection and Preparation of Potato Starch
The potato tubers are thoroughly washed to remove the soil, and are
chopped (reduced) into small pieces and made into a pulp by crushing in
a rasping machine (or in a disintegrator of the hammer-mill type). The
resulting slurry is filtered through metallic sieves to remove the cellular
matter (vegetable tissues). The milky slurry is purified by centrifugation,
and the residue is washed with water, dried and stored in bags.
General Description of Starch
Starch generally occurs in nature as irregular, angular, white masses that
may easily be reduced to powder. Starch of rice and maize appears white;
wheat starch appears creamy white; while the starch of potato appears
100 Textbook of Pharmacognosy
pale yellow in colour. The starch is odourless and the taste is bland and
mucilaginous. All the four types of starch possess a definite shape and
characteristic features.
Maize (Cor
n) Starch
Synonyms
Corn flour (Commonwealth countries except in Canada), Maize starch
(Europe), Corn starch or Corn flour (UK and Australia), Maisstarke
(German), Fecule de Maize (French).
Characters
Corn starch is a fine white powder with irregular white masses. There is
no distinct odour. It tastes starchy first and later sweet. The aqueous mixture
is neutral. The grains are simple, polygonal to round, 5 to 35 microns in
diameter with hilum either circular or 2 to 5 -rayed. The lamellae are
indistinct and with iodine it becomes deep blue.
Maize grains
Refined corn oil Quality corn
Starch grains: maize
Systematic Pharmacognostic Study of Carbohydrates and Derived Products 101
Corn starch
Corn flour
Rice Starch
Characters
The granules are simple or compound. The simple granules are polygonal
and 2 to 12 microns in diameter, while the compound granules are oval
Paddy – Source of starch
Starch grains of Rice
Rice starch
Rice flour
102 Textbook of Pharmacognosy
and 12 to 30 microns × 7 to 12 microns in size. The hilum is usually indistinct
with indistinct lamellae.
Wheat Star
ch
Characters
Dough prepared from wheat flour is kept for some time to allow the
insoluble proteins, gluten to swell. The mass is shaken constantly with
water on rollers. The suspension coming out is dried. The starch grains are
simple, circular and large about 28 to 45 microns in diameter. The hilum is
central which appears as a dot. The lamellae are indistinct. Starch occurs
in irregular, angular masss or as a white powder. It is insoluble in cold
water but forms a colloidal solution on boiling with about 15 times its
weight of water, the solution forming a transluscent jelly on cooling. Wheat
starch is highly refined and closely resembles flour. The starch grains vary
in shape and size depending upon the source.
Wheat-starch grains
Wheat grains
Wheat-sugar
Wheat product—bread
Systematic Pharmacognostic Study of Carbohydrates and Derived Products 103
Starch grains of wheat
Potato Starch
Synonyms
Potato starch flour, Perunajauho (Finnish), Potatismjol (Swedish), Potetmel
(Norwegian), Kartoffelmel (Danish), Kartoflusterkja or Kartoflumjol
(Icelandic).
Characters
The granules are larger in their size, simple, oval to subspherical, somewhat
flat, the size ranging from 30 to 100 microns. The hilum is present near the
narrower end with clear concentric striations. The starch is very refined
and contains minimal amount of protein or fat. The powder is clear white
in its colour and exhibits neutral taste after cooking, good clarity, high
binding strength, long texture and a minimal tendency of foaming or
yellowing. It is more viscous, slightly anionic, with a low gelatinisation
temperature and high swelling power.
Tubers—source of starch
Potato—starch grains
104 Textbook of Pharmacognosy
Potato starch
Potato flour
Tests for Starch
Starch taste is mucilaginous. The maize starch is neutral, wheat and potato
starch shows acidic reaction and the rice starch is alkaline. Normally, iodine
solution (by dissolving iodine in water in the presence of potassium iodide)
is used to test for starch and a dark blue colour indicates the prersence of
starch. The details of the reaction are not yet fully known. Amylose in the
starch is responsible for the formation of a deep blue colour in the presence
of iodine. The intensity of the colour depends on the amount of amylose
present. If amylose is not present, the colour will be orange or yellow.
Waxy starches with little or no amylose will appear red in colour.
Amylose
Systematic Pharmacognostic Study of Carbohydrates and Derived Products 105
Amylopectin
Chemical Constituents
Pure starch is a soft, white, granular, tasteless and odourless powder that
is insoluble in cold water or alcohol or other solvents and can be stored
more compactly. The basic chemical formula is (C
6H10O5)n
. Starch is a
polysaccharide which comprises of glucose monomers joined in alpha-1,4
linkages. It consists of a linear and helical amylose (20 to 25%) and the
branched amylopectin (75 to 80%). In animals, the glucose store is in the
form of glycogen which is a more branched amylopectin. The molecules of
starch arrange themselves as semi-crystalline granules in plants and each
plant species has a unique granular size. The amylose molecules are about
150 times more than amylopectin molecules and smaller in size. When
heated, the starch gets dissolved in water and the granules swell and burst
and the semi-crystalline structure is lost and the mixture is more viscous.
This process is referred to as gelatinisation of starch. It becomes a paste
and becomes more viscous during cooking. Upon cooling or prolonged
storage, this paste partially recovers and gets thickened by losing water
within. This process is called retrogradation. If subjected to dry heat, starch
breaks down to form pyrodextrins (brown) and the process is called
dextrinisation. Some of the cultivated plant varieties contain pure
amylopectin without amylase. These are called waxy starches (maize, rice
and potato). Starch is usually adulterated with tapioca starch obtained from
Manihot esculenta.
Starch granules essentially contain two complex polysaccharides,
amylopectin (α-amylose) and amylose (β-amylose); the former constitutes
over 80% of most starches.
Amylopectin is water-insoluble swells up in water to give rise to a thick
paste upon boiling with water. It produces a distinct violet or bluish-red
colouration with a solution of 0.1N iodine. It has highly branched structure
composed of several 100 short chains of about 20–25 D-glucose units. One
terminal of each of these chains is joined through C-1 to a C-6 with the next chain.
106 Textbook of Pharmacognosy
Amylopectin
Amylose is water soluble, and gives an instant bright blue colour with a
solution of 0.1N iodine. Upon hydrolysis, this yields a disaccharide (maltose
+/–) and the only monosaccharide (D-glucose +), and as such it has been
suggested amylase comprises of chains of a number of D-glucose units,
and each unit is linked by an alpha-glycoside linkage to C-4 of the next
unit. This constitutes up to 25% of the total starch content and the
proportion varies with species. This is found to be either absent or present
to a very small extent (≤6%) in some glutinous or waxy starches in the
plant kingdom.
Amylose
Glucose units
History of Starch
Egyptians used starch paste to stiffen the cloth and also during weaving
linen and to glue the papyrus. Romans employed it in the preparation of
the cosmetic creams, to powder the hair and to thicken the sauces. It was
used to make the dishes similar to gothumai wheat halva by the Persians
and Indians. The rice starch was used for the surface treatment of paper in
China. In 1811, Vauquelin discovered that when starch is heated at higher
temperature, it is converted into a substance which is completely soluble
in water and resembles gum Arabic in its physical properties. Biot and
Persoz (1833) gave the name ‘dextrin’ to this gum like substance, while
Payen and L’ersoz gave the name ‘diastase’ to the agent in the malted grain
which can transform starch.
Systematic Pharmacognostic Study of Carbohydrates and Derived Products 107
General Uses of Starch
Starch possesses both absorbent and demulcent properties. It is employed
in dusting powder because of its unique protective and absorbent property.
It is used in the formulation of tablets and pills as a vital disintegrating
agent and a binder. It is also used as a diagnostic aid for the proper
identification of the crude drugs. It is used as diluents (or filler) and a
lubricant in the preparation of capsules and tablets. It is used as an indicator
in iodimetric analyses. It is also used as an antidote of choice for iodine
poisoning. The dietetic grades of corn starch are generally marked as
‘Maizena’ and ‘Mondamin’. ‘Glycerine of Starch’ is used not only as an
emollient but also as a base for the suppositories. The starch is the starting
material for the large scale production of liquid glucose, glucose syrup,
dextrose and dextrin. Starch finds its extensive industrial application for
the sizing of paper textiles. Starch also possesses nutrient properties as a
food and in cereal-based weaning foods for babies (Farex-R and Cerelac-R
of Nestle). The starch can also be used topically and externally to allay
itching. It is also used profusely in laundry starching. Starch is used to
make foam for packaging. The glucose form starch can be fermented to
biofuel ethanol. It is used as industrial alcohol and can also be used as
nitro-starch (for explosives).
How Bad Are Starches?
Starches are bad because they are a more concentrated source of
carbohydrates and calories than fruits, non-starchy vegetables and dairy
products though many of them are excellent sources of fibres, vitamins,
minerals and phytonutrients. They are an important part of a healthy,
balanced diet when chosen correctly and consumed in reasonable portions.
The good starches are potatoes, bread, cereal products, rice and grains and
pasta in the diet.
Health Risk of Starches
Too much starch in the diet is associated with dental caries, obesity, and
diabetes mellitus. The starch (especially cooked and contained in the
processed foods) can cause spikes in blood glucose levels after a meal.
Hence, consumption of starch is advised to be in moderation. The
individuals with celiac disease and congenital sucrose-isomaltase deficiency
may need to avoid the foods rich in starch.
Medicinal Uses
Starch is used as a nutritive, demulcent, protective and as an absorbent. In
the manufacture of tablets, it is used as a disintegrant. A starch suspension
may be swallowed as an antidote for iodine poisoning. It can be used as an
excipient and a binder in the preparation of the tablets.