Evaluation of Crude Drugs
Evaluation of Crude Drugs
Pharmacognosy | B.Pharm | GPAT | UPSC Drug Inspector
1. Introduction
Definition: Evaluation of crude drugs is the systematic process of determining the identity, purity, quality, strength and possible adulteration of crude drugs by using appropriate scientific methods.
Objectives
- To establish the identity of crude drugs.
- To determine purity and quality.
- To detect adulteration and substitution.
- To estimate active chemical constituents.
- To determine moisture, ash and extractive values.
- To ensure batch-to-batch consistency.
- To verify compliance with pharmacopoeial standards.
2. Classification of evaluation methods
Evaluation of crude drugs
1. Organoleptic
Colour, odour, taste
2. Morphological
External characters
3. Microscopic
Cells and tissues
4. Chemical
Constituents and assays
5. Biological
Activity and potency
6. Physical / physicochemical
Quality parameters
7. Molecular and instrumental methods
DNA barcoding, TLC, HPTLC, HPLC, GC and spectroscopy
The exact classification may vary between textbooks. Organoleptic evaluation is sometimes included under macroscopic evaluation.
3. Organoleptic evaluation
Definition: Examination of crude drugs using the sense organs to assess their characteristic sensory properties.
Characters examined
- Colour — Clove: dark brown.
- Odour — Fennel: aromatic.
- Taste — Gentian: bitter.
- Texture — Rough, smooth, hard or soft.
- Fracture — Fibrous, short or granular.
Advantages: Simple, rapid and inexpensive; useful for preliminary identification.
Limitations: Subjective, less reliable for powders, and insufficient to establish identity or purity by itself. Never taste unknown or potentially toxic samples.
4. Morphological evaluation
Definition: Identification and assessment of crude drugs by examining their external macroscopic characters.
Characters examined
- Shape and size
- Surface and colour
- Fracture
- Venation and margins
- Scars, hairs and other special features
Examples: Clove — nail-shaped flower bud; Ginger — branched rhizome; Cinnamon — dried bark.
Exam distinction: Morphology examines external features; microscopy examines internal structures.
5. Microscopic evaluation
Definition: Examination of cellular structures, tissues and diagnostic microscopic characters of crude drugs using a microscope.
Important microscopic characters
- Stomata
- Trichomes or plant hairs
- Calcium oxalate crystals
- Starch grains
- Fibres and vessels
- Stone cells or sclereids
- Epidermal cells
- Secretory structures
Types of microscopic evaluation
- Qualitative microscopy: Identifies diagnostic structures such as stomata, crystals, fibres and starch grains.
- Quantitative microscopy: Measures numerical microscopic parameters, especially in leaf drugs.
| Parameter | Meaning |
|---|---|
| Stomatal number | Number of stomata per unit area |
| Stomatal index | Percentage of stomata relative to stomata plus ordinary epidermal cells |
| Palisade ratio | Average number of palisade cells beneath each epidermal cell, according to the prescribed method |
| Vein-islet number | Number of vein-islets per square millimetre |
| Veinlet termination number | Number of veinlet terminations per square millimetre |
Stomatal index
Stomatal index = SE + S × 100
Where S = number of stomata and E = number of ordinary epidermal cells.
Example: If S = 20 and E = 80, stomatal index = 20
%.
- Powder microscopy: Examines powdered drugs for diagnostic fragments of tissues, fibres, vessels, crystals and starch grains.
Advantages: Useful for fragmented or powdered drugs and helps detect certain adulterants.
Limitation: Microscopy alone may not establish complete identity.
6. Physical and physicochemical evaluation
Definition: Determination of measurable physical properties and physicochemical quality parameters of crude drugs.
6.1 Foreign organic matter
Foreign matter includes unwanted plant parts, other organisms, soil, stones and extraneous materials.
% Foreign matter = Weight of foreign matterWeight of sample × 100
Example: 2 g foreign matter in a 100 g sample = 2%.
6.2 Moisture content and loss on drying
Excessive moisture may promote microbial growth, hydrolysis and deterioration during storage.
% LOD = Initial mass − Final massInitial mass × 100
Example: Initial mass = 10 g; final mass = 9 g. LOD = 10%.
🎯 Exam Tip: Loss on drying can include volatile substances other than water. Use a water-specific method, such as Karl Fischer titration, when required.
6.3 Ash value
Ash value measures inorganic residue remaining after treatment under prescribed incineration conditions.
| Type | Significance |
|---|---|
| Total ash | Total inorganic residue after incineration |
| Acid-insoluble ash | Estimates acid-insoluble residue, often associated with silica and siliceous matter |
| Water-soluble ash | Estimates the water-soluble portion of total ash |
| Sulphated ash | Residue obtained after treatment with sulphuric acid and ignition |
% Total ash = Weight of ashWeight of drug × 100
Example: 0.8 g ash from 10 g of drug = 8%.
Importance: Abnormally high values may indicate contamination or excessive inorganic matter. Compare results with the relevant monograph.
6.4 Extractive values
Extractive values estimate the amount of material extracted by a specified solvent under prescribed conditions.
Types include water-soluble and alcohol-soluble extractive values.
% Extractive value = Mass of dried extractMass of crude drug × 100
Example: 1.5 g dried extract from 5 g of drug = 30%.
Extractive values help assess extractable constituents and may reveal exhausted or substituted materials.
6.5 Volatile oil content
Commonly determined by distillation using a Clevenger-type apparatus when specified.
Examples: Clove, Fennel, Peppermint and Coriander.
Principle: Volatile constituents are distilled with water or steam, condensed and separated from the aqueous phase. The oil is measured under prescribed conditions.
6.6 Other physicochemical parameters
| Parameter | Application |
|---|---|
| Refractive index | Characterization of oils and suitable liquids |
| Optical rotation | Characterization of optically active substances |
| Specific gravity / relative density | Characterization of liquids and oils |
| Viscosity | Flow properties of liquids |
| Swelling index | Swelling capacity of mucilage-containing materials |
| Foaming index | Foaming capacity, often associated with saponins |
| Bitterness value | Relative bitterness under prescribed conditions |
| Haemolytic activity | Assessment where relevant, particularly for saponin-containing materials |
Not every test is required for every crude drug. The relevant monograph determines the required parameters.
7. Chemical evaluation
Definition: Identification and quantitative determination of chemical constituents in crude drugs using chemical reactions and appropriate analytical methods.
7.1 Qualitative chemical tests
Qualitative tests indicate the possible presence of particular classes of constituents.
| Constituent | Test | Typical positive indication |
|---|---|---|
| Alkaloids | Dragendorff's test | Orange to reddish-brown precipitate |
| Alkaloids | Mayer's test | Cream-coloured precipitate |
| Tannins / phenolics | Ferric chloride | Blue, blue-black or greenish colour |
| Flavonoids | Shinoda test | Pink, red or magenta colour may develop |
| Saponins | Foam test | Persistent foam |
| Anthraquinone glycosides | Bornträger's test | Pink to red alkaline layer for responsive compounds |
| Cardiac glycosides | Keller–Killiani test | Characteristic interfacial colour reaction |
| Reducing sugars | Fehling's test | Brick-red precipitate under suitable conditions |
These are preliminary screening tests. A positive reaction does not prove the exact identity of a crude drug or constituent; confirmatory methods may be needed.
7.2 Quantitative chemical analysis
Methods include
- Gravimetric analysis: Measurement based on mass.
- Volumetric analysis: Measurement using a standard solution of known concentration.
- Spectrophotometry: Measurement based on light absorption.
- Chromatography: Separation and analysis of constituents.
- Validated assays: Quantification using a prescribed or validated analytical procedure.
7.3 Examples of chemical evaluation
| Crude drug | Important constituent | Common analytical method |
|---|---|---|
| Clove | Eugenol | GC or GC-MS |
| Senna | Sennosides | HPLC or monograph-specified assay |
| Cinchona | Quinine and related alkaloids | Suitable alkaloid assay or chromatography |
| Liquorice | Glycyrrhizin | HPLC or validated assay |
| Tea | Caffeine | HPLC or spectrophotometry |
| Fennel | Volatile oil constituents, including anethole | GC or GC-MS |
| Digitalis | Cardiac glycosides | Validated chromatographic or monograph-specified assay |
7.4 Chromatographic evaluation
TLC / HPTLC: Separate compounds on a planar stationary phase. HPTLC can also support fingerprinting and quantification.
HPLC: Separates compounds using a liquid mobile phase under high pressure.
GC / GC-MS: Used for suitable volatile compounds and for additional chemical characterization by mass spectrometry.
Chemical fingerprinting: A chromatographic or spectroscopic profile used to compare the overall chemical pattern of a sample with an appropriate reference.
7.5 Marker compounds
A marker compound is a constituent selected to help identify, characterize or control the quality of a herbal material or preparation.
- Active marker: Has known or relevant biological activity.
- Analytical marker: Is selected for identification or quality control, even if its therapeutic contribution is not established.
A marker does not necessarily represent the full pharmacological activity of a complex herbal preparation.
8. Biological evaluation
Definition: Assessment of biological activity or potency by measuring the effect of a crude drug on an appropriate living organism, isolated tissue, cell system, enzyme system or microorganism.
8.1 Objectives
- Determine biological potency.
- Compare a test preparation with a reference standard.
- Assess a defined biological response.
- Detect changes in activity during processing or storage.
- Evaluate activity when chemical content alone is insufficient.
8.2 Methods
- In vivo assay
Performed in a whole living organism. Examples include studies of analgesic, antipyretic and other defined pharmacological effects.
- In vitro assay
Performed outside the whole organism using isolated tissues, cells, enzymes or other biological systems.
- Microbiological assay
Measures the response of suitable microorganisms to a substance under specified conditions.
8.3 Types of biological assays
| Type | Principle |
|---|---|
| Graded response assay | Measures response magnitude at different doses or concentrations |
| Quantal response assay | Records whether a defined response occurs |
| Comparative assay | Compares the test preparation with a reference standard |
| Microbiological assay | Measures a biological response involving microorganisms |
8.4 Examples
| Drug / substance | Biological attribute |
|---|---|
| Papain | Proteolytic enzyme activity |
| Senna preparations | Laxative-related activity, where specified |
| Ergot preparations | Relevant pharmacological activity of ergot alkaloids |
| Digitalis preparations | Cardiac glycoside-related activity |
| Plant extracts | Antimicrobial or other defined biological activity |
Advantages: Measures a relevant biological response and may detect changes in potency not reflected by a single chemical measurement.
Limitations: Biological variability, cost, time, assay dependence and ethical requirements for animal studies.
🎯 Exam Tip: In vitro activity does not by itself establish clinical effectiveness. Biological screening is also not automatically equivalent to a validated potency assay.
9. Molecular and DNA-based evaluation
Definition: Molecular evaluation uses DNA-based or other molecular techniques to help authenticate the biological source of crude drugs.
Techniques
- DNA barcoding: Uses selected DNA regions to assist species identification.
- PCR: Amplifies a selected DNA sequence.
- DNA sequencing: Determines the nucleotide sequence of a DNA region.
- DNA fingerprinting: Examines genetic markers or patterns to distinguish biological sources.
Applications: Botanical authentication, detection of certain substitutions, and identification when morphology is unavailable.
Limitations: Processing can degrade DNA; closely related species may be difficult to distinguish; DNA analysis does not directly establish chemical composition, potency or contaminant levels.
10. Contaminant and adulterant evaluation
A crude drug may appear genuine but fail quality requirements because of contamination, deterioration or substitution.
| Parameter | Purpose | Common methods |
|---|---|---|
| Heavy metals / toxic elements | Detect harmful elemental contaminants | AAS, ICP-MS |
| Pesticide residues | Detect agricultural chemical residues | GC-MS/MS, LC-MS/MS |
| Microbial contamination | Assess microbial quality and specified pathogens | Enumeration and prescribed culture methods |
| Aflatoxins | Detect toxic fungal metabolites | HPLC, LC-MS/MS |
| Foreign matter | Identify extraneous material | Visual examination and separation |
| Adulterants / substitutes | Assess authenticity | Macroscopy, microscopy, chromatography and DNA methods, as appropriate |
No single method detects every contaminant or adulterant. The tests must be selected according to the drug, likely hazards and applicable specifications.
11. Comparison of evaluation methods
| Method | Main basis | Major application |
|---|---|---|
| Organoleptic | Sensory properties | Preliminary identification |
| Morphological | External characters | Whole-drug identification |
| Microscopic | Cells and tissues | Powder and fragment identification |
| Physical / physicochemical | Measurable physical properties | Quality and purity assessment |
| Chemical | Chemical reactions and analysis | Identification and estimation of constituents |
| Biological | Biological response or potency | Activity assessment |
| Molecular | DNA or molecular markers | Botanical authentication |
| Contaminant analysis | Specific contaminant measurement | Safety and purity |
12. Worked numerical examples
Example 1: Total ash
A 5 g sample produces 0.35 g ash.
% Total ash = 0.355 × 100
Ans. 7%
Example 2: Loss on drying
A 10 g sample weighs 9.2 g after drying.
% LOD = 10 − 9.210 × 100
Ans. 8%
Example 3: Extractive value
A 4 g sample produces 0.6 g dried extract.
% Extractive value = 0.64 × 100
Ans. 15%
Example 4: Foreign matter
A 50 g sample contains 0.5 g foreign matter.
% Foreign matter = 0.550 × 100
Ans. 1%
13. General workflow for evaluation
1
Sampling
Obtain a representative sample.
2
Preliminary examination
Check appearance and visible defects.
3
Authentication
Perform macroscopic and microscopic examinations.
4
Physicochemical testing
Determine relevant moisture, ash and extractive values.
5
Chemical analysis
Identify or quantify appropriate constituents.
6
Additional tests
Assess activity, contaminants or DNA when indicated.
7
Interpretation
Compare results with the applicable official specification.
This is a general workflow rather than a mandatory sequence for every crude drug.
14. High-yield revision table
| Parameter | Key point |
|---|---|
| Stomatal index | S/(E + S) × 100 |
| Total ash | Total inorganic residue after incineration |
| Acid-insoluble ash | Estimates acid-insoluble residue |
| Extractive value | Material extracted by a specified solvent |
| Loss on drying | Mass lost under specified drying conditions |
| Volatile oil content | Commonly determined by a prescribed distillation method |
| HPLC | Separation and analysis of suitable compounds |
| GC | Analysis of suitable volatile compounds |
| DNA barcoding | Supports species authentication |
| Biological assay | Measures a defined biological response or potency |
| Chemical fingerprint | Comparative pattern of multiple constituents |
15. Practice MCQs
Self-assessment test
0/8 answered
Select one answer per question. Answers are revealed after submission.
Q1. 1. Which method is particularly useful for identifying powdered crude drugs through diagnostic tissues?
- Morphological
- Microscopic
- Organoleptic
- DNA barcoding only
2. Which ash parameter helps assess siliceous contamination?
- Total ash
- Water-soluble ash
- Acid-insoluble ash
- Sulphated ash
3. A 10 g sample produces 0.5 g ash. Total ash percentage is:
- 2%
- 5%
- 10%
- 20%
4. Which method is commonly used to analyse suitable volatile oil constituents?
A. GC
- Stomatal index
- Ash determination
- DNA barcoding
5. Which technique can support botanical authentication?
- Total ash only
- Loss on drying only
- DNA barcoding
- Extractive value only
6. Does a positive preliminary phytochemical test prove exact drug identity?
- Always
- No, confirmation may be required
- Only for alkaloids
- Only for flavonoids
7. Which method assesses biological potency through a defined response?
- Biological assay
- Ash value
- Foreign matter test
- Organoleptic evaluation
8. Which is the correct formula for stomatal index?
A. E/(E+S) × 100
B. S/E × 100
C. S/(E+S) × 100
D. (E+S)/S × 100
Submit answers
16. Recommended references
- World Health Organization (WHO), Quality Control Methods for Herbal Materials (2011). Official WHO publication.
- WHO, Quality Control Methods for Medicinal Plant Materials (1998). Official WHO publication.
- WHO, guidance on selecting marker substances of herbal origin for quality control. Technical guidance on marker substances.
For publication and examination use, consult the applicable current Indian Pharmacopoeia or other official monograph for drug-specific procedures and acceptance limits.
References
- Official WHO publication — who.int
- Official WHO publication — who.int
- Technical guidance on marker substances — cdn.who.int