Description
Evaluation of a drug ensures the identity of a drug and
determines the quality and purity of drugs. The main
reasons behind the need for evaluation of crude drugs are
biochemical variation in the drug, effect of treatment and
storage of drugs, and the adulterations and substitutions.
Improvements in analytical methods have definitely
led to improvements in harvesting schedules, cultivation
techniques, storage, activity, stability of active compounds,
and product purity. All of these gains have resulted in tre-
mendous improvements in the quality of herbal prepara-
tions now available.
Methods currently employed in evaluating herbs are
organoleptic, microscopic, physical, chemical, and biologi-
cal parameters.
11.2. ORGANOLEPTIC EVALUATION
Organoleptic evaluation means the study of drugs using
organs of senses. It refers to the methods of analysis like
colour, odour, taste, size, shape, and special features, such
as: touch, texture, etc. Obviously, the initial sight of the
plant or extract is so specific that it tends to identify itself. If
this is not enough, perhaps the plant or extract has a char-
acteristic odour or taste. Organoleptic analysis represents
the simplest, yet the most human form of analysis.
Talka gum, which is used as a substitute for acacia gum
could be identified by its colour and form. Talka gum is
usually broken and also some tears are brown in colour
and other colourless, whereas acacia is white to yellow in
colour. Mangosteen fruits are a substitute for bael fruits
and can be identified by darker rind and the wedge-shaped
radiate stigmas. Cuprea Bark (Remijia pedupiculata) differs
in its morphological character with cinchona. Blood Root
used as an adulterant for hydrastis is dark reddish-brown
in colour, whereas hydrastis is yellow in colour. Rheum
rhaponticum are much smaller than those of the Chinese
rhubarb and are easily distinguished.
Ginger and capsicum have pungent taste, whereas gentian
and chirata have bitter taste. Morphological differentiation
of leaves and pods of Indian senna and Alexandrian senna,
sweet taste of liquorice, odours of umbelliferous fruits,
disc-shaped structure of nux vomica, conical shape of
aconite, quills of cinnamon, etc. are few examples of this
organoleptic evaluation.
11.3. MICROSCOPICAL EVALUATION
Microscopic evaluation is indispensable in the initial iden-
tification of herbs, as well as in identifying small fragments
of crude or powdered herbs, and in the detection of adulter-
ants (e.g. insects, animal faeces, mold, fungi, etc.) as well as
identifying the plant by characteristic tissue features. Every
plant possesses a characteristic tissue structure, which can
be demonstrated through study of tissue arrangement, cell
walls, and configuration when properly mounted in stains,
reagents, and media. Lignin stains red or pink with a drop
of phloroglucinol and concentrated hydrochloric acid.
Mucilage is stained pink with rhuthenium red, and N/50
iodine solution stains starch and hemicellulose blue.
The characteristic features of cell walls, cell contents,
starch grains, calcium oxalate crystals, trichomes, fibres,
vessels, etc. have been studied in details. Surinam quassia
is recognized by the absence of calcium oxalate and pres-
ence of uniseriate medullary rays, crystal fibres, and wavy
medullary rays of cascara bark, lignified trichomes, and
plasmodesma in nux vomica. Stone cells are absent in the
frangula bark, whereas they are present in cascara. Presence
of pith in rhizomes and absence in roots, warty trichomes
of senna, and presence or absence of crystals of aloin
indicates different varieties of aloes, glandular trichomes
of mint, etc. The powder of clove stalks contains sclereids
and calcium oxalate crystals, but cloves do not contain these
two. Rauwolfia micrantho, R. densiflora, and R. perokensis are
found to serve as an adulterant for R. serpentine. The roots
of these species can be differentiated from R. serpentine by
the presence of sclerenchyma in the above species which
is absent in R. serpentine.
The techniques like microscopic linear measurements,
determination of leaf constants and quantitative microscopy
are also used in this evaluation.
Linear measurements include size of starch grains, length
and width of fibres, trichomes, etc. The diameter of starch
grains present in ipecacuanha assists in distinguishing
its varieties. The diameter of starch grains in cassia bark
distinguishes from cinnamon and detects senna stalk in
powdered senna leaf. The size of the stomata in leaves
of Barosma betulina distinguishes it from other species of
Barosma. The diameter of phloem fibres aids the detection
of cassia in cinnamon, and the width of the vessel helps
to detect clove stalks in powdered cloves. Measurements
of diameter for the identification of commercial starches
and for the detection in them of foreign starch are few
examples of linear measurements.
Determination of leaf constants include: stomatal number,
stomatal index, vein islet, vein termination number, and
palisade ratios. Stomatal number is average number of
stomata per sq. mm of epidermis of the leaf.
Stomatal index: It is the percentage which the numbers
of stomata form to the total number of epidermal cells,
each stoma being counted as one cell. Stomatal index can
be calculated by using the following formula:
Stomatal Index (S.I.) =
S
× 100
E + S
where,
S = number of stomata per unit area and
E = number of epidermal cells in the same unit area.
Timmerman (1927) and Rowson (1943) were amongst
the first few to investigate leaf drugs for stomatal number
and stomatal index.
Vein-islet number: It is defined as the number of vein
islets per sq. mm of the leaf surface midway between the
midrib and the margin. It is a constant for a given species of
the plant and is used as a characteristic for the identification
of the allied species. Levin in 1929 determined vein-islet
numbers of several dicot leaves.
Veinlet termination number: It is defined as the
number of veinlet termination per sq. mm of the leaf surface
midway between midrib and margin. A vein termination is
the ultimate free termination of veinlet. Hall and Melville
in 1951 determined veinlet termination number of distin-
guishing between Indian and Alexandrian Senna.
Palisade ratio: It is defined as the average number of
palisade cells beneath each epidermal cell. Unlike vein-
islet number for the determination of which an unbroken
portion of the leaf is required, palisade ratio can be deter-
mined with the powdered drug. The technique of palisade
ratio determination was introduced by Zorning and Weiss
(1925) in their studies on Composita phytoconstituents are given under their respective chapters
in the text, where it could be referred. Examples of iden-
tification of constituents are: copper acetate used in the
detection of colophony present as an adulterant for resins,
balsams, and waxes; Holphen’s test for cottonseed oil and
Baudouin’s test for sesame oil in olive oil; the test with
acetic and nitric acids for Gurjun balsam in copaiba; Van
Urk’s reagent for ergot; Vitali’s morins reaction for tropane
alkaloids; iodine for starch; murexide test for purine bases,
etc. are examples of this evaluation.
Quantitative chemical tests such as acid value (resins,
balsams), saponification value (balsams), ester value
(balsams, volatile oils), acetyl value (volatile oils), etc. are
also useful in evaluation of a drug by means of chemical
treatment.
Chemical assays include assays for alkaloid, resin, volatile
oil, glycoside, vitamins, or other constituent. Few examples
are the assay of total alkaloid in belladonna herb, the total
alkaloid and nonphenolic alkaloid in ipecacuanha, the alka-
loid strychnine in nux vomica, the resin in jalap, and the
vitamins in cod-liver oil. The results obtained can conclude
the presence of inferior or exhausted drug and, by proving
absence of the assayed constituent, it will suggest complete
substitution of a worthless article.
Instrumental analyses are used to analyse the chemi-
cal groups of phytoconstituents using chromatographic
and spectroscopic methods. Chromatographic methods
include paper chromatography, thin-layer chromatography,
gas chromatography, high-performance liquid chroma-
tography, and high-performance thin-layer chromatogra-
phy. Spectroscopic methods include ultraviolet and visible
spectroscopy, infrared spectroscopy, mass spectroscopy, and
nuclear magnetic spectroscopy.
11.5. PHYSICAL EVALUATION
In crude plant evaluation, physical methods are often used
to determine the solubility, specific gravity, optical rotation,
viscosity, refractive index, melting point, water content,
degree of fibre elasticity, and other physical characteristics
of the herb material.
Solubility
Drugs specific behaviours towards solvents are taken into
consideration. This is useful for the examination of many
oils, oleoresins, etc. Few examples are the solubility of
colophony in light petroleum, the solubility of balsam of
Peru in solution of chloral hydrate, the solubility of castor
oil in half its volume of light petroleum and the turbidity
produced with two volumes of the solvent; the solubility
of balsam of Peru in an equal volume of alcohol, 90%, and
the production of a turbidity with a larger volume; castor
oil is soluble only in three volumes of 90% alcohol, while
the adulterated form it shows good solubility in alcohol.
Alkaloidal bases are soluble in organic solvents and alkaloidal
salts are soluble in polar solvents.
Optical Rotation
Anisotropic crystalline solids and samples containing an
excess of one enantiomer of a chiral molecule can rotate
the orientation of plane-polarized light. Such substances
are said to be optically active, and this property is known
as optical rotation. The enantiomer that rotates light to
the right, or clockwise when viewing in the direction of
light propagation, is called the dextrorotatory (d) or (+)
enantiomer, and the enantiomer that rotates light to the
left, or counterclockwise, is called the levorotatory (l) or
(-
) enantiomer. Few examples of drugs with this property
are eucalyptus oil (0° to +10°), honey (+3° to –
15°), Che-
nopodium oil (-
30° to –
80°), etc.
Refractive Index
Refractive index is defined as the property of a material that
changes the speed of light, computed as the ratio of the
speed of light in a vacuum to the speed of light through
the material. When light travels at an angle between two
different materials, their refractive indices determine the
angle of transmission refraction of the light beam. In
general, the refractive index varies based on the frequency
of the light as well; thus, different colours of light travel
at different speeds. High intensities can also change the
refractive index. This could be used as a parameter in
evaluating the herbal drugs; for example castor oil 1.4758
to 1.527, clove oil 1.527 to 1.535, etc.
Specific Gravity
It is also known as relative density. The ratio of the mass of
a solid or liquid to the mass of an equal volume of distilled
water at 4°C (39°F) or of a gas to an equal volume of air
or hydrogen under prescribed conditions of temperature
and pressure. Some examples of specific gravity of drugs
are cottonseed oil 0.88–0.93, coconut oil 0.925, castor oil
0.95, etc.
Viscosity
Viscosity is the resistance of a fluid to flow. This resistance
acts against the motion of any solid object through the fluid
and also against motion of the fluid itself past stationary
obstacles. Viscosity of a liquid is constant at a given tempera-
ture and is an index of its composition. Viscosity also acts
internally on the fluid between slower- and faster-moving
adjacent layers. Since it is constant at a given temperature,
it is used as an evaluation parameter; for example, pyroxylin
kinematic viscosity, 1100–2450 centistokes.
Subject:- Pharmacognosy 1
Semester:- Sem 3
Course:- Bachelor of pharmacy
