What influences herbal quality, how is it controlled and why is it important? Barry Moore explores analytical methods and offers guidance to purchasing high quality herbs.
What influences herbal quality and how is it defined?
The starting point for herbal quality begins in the field, as the genetic lineage of the plant and the environment it is grown in, all play a key role in determining the final quality of the herb. Conditions such as light, temperature, precipitation, CO2 concentration, soil composition, pH, nutrient content and salinity, water stress, biodiversity and farming practices can all affect the formation of the active constituents in the plant and its susceptibly to various forms of contamination from the environment.
Selecting the right place to grow the herb is an important first step in ensuring the production of good quality herbs. Managing the growing conditions and the subsequent harvesting, drying, storage and processing is important, so that the risks of microbiological, chemical and physical contamination are minimised.
Quality practices are also defined by the regulatory environment; for instance, how the herb is used, as a cosmetic, food or medicine, or the markets the herb is supplied to. These play a part in determining the quality standards that are used and the type of analysis that is selected to control the quality of the herbs.
How the herbs get to us is also very important, herbal value chains are prone to challenges in supply. These include over-harvesting, adulteration and contamination at different points in the chain and a lack of sufficient quality control of the herb during different stages of production (1). This complexity creates significant challenges in quality control.
What is herbal quality control and why is it important?
Quality control is a process that is fundamentally about ensuring that the herbs we use are safe and efficacious. Poor quality herbs produce poor quality medicine and even pose toxicity risks, owing primarily to contamination or adulteration. On a larger scale, the consequences have wider implications on the reputation of herbal medicine and the regulations imposed on the use of herbs in practice. The recent ban on ashwagandha in Denmark following incidences of hepatotoxicity in hand with root product contamination with leaf, exemplifies this. So, sourcing high quality herbs is paramount to protect both patient and practice.
Herbal quality control covers all stages from growing through to production, storage and the final consumption of the herbs. Quality control ideally assures the correct botanical identity of the herb, its purity, the control of contaminants and the required level of active constituents. Standards are set for these criteria which are often codified in the form of a pharmacopeia that defines the tests required and their limits. Meeting these quality criteria defines the herb as pharmacopeial grade.
The standards that are set need to account for the considerable variation found in herbal materials and the type of herbal product (2). Meeting these standards offers consumers assurance that the correct herb is used, that the herb is safe, and that it contains a defined concentration of active constituents, which offers an indication of its potency and efficacy.
The starting point for herbal quality control analysis is to ensure the correct botanical identity of the herb, which includes the species, plant part and the plant maturity when harvested (3). Typical methods used in quality control analysis comprise morphological identification using macro/microscopic techniques, phytochemical analysis using chromatography/spectroscopy/spectrometry and analysis through the use of molecular markers, for example DNA or proteins (4). These approaches are also used for checking for purity, contamination and defined concentrations of active constituents in the herb (such as essential oils).
Analytical methods for herbal quality control
Morphological analysis
Macroscopic examination has long been used for the authentication and quality assessment of herbs by examining their gross morphological features and organoleptic properties (colour, odour and taste). This method is easy to undertake and can identify obvious quality issues quickly such as foreign matter and the presence of other plant species. It does, however, have some drawbacks as it relies heavily on personal experiences and it is not an effective way to distinguish closely related herbs that have very similar morphological features. Factors such as the growing environment or how the herb is processed can greatly influence the macroscopic characteristics (4).
Although on an industrial level, organoleptic assessment is not the most reliable means of testing, on an individual level as a practitioner, developing the skills involved are valuable to draw associations between flavours and constituents, and the herbs with their supplier, origins and efficacy as medicine as observed in practice.
Microscopy has an advantage over macroscopic methods as it can assess at the cellular level. Certain parameters such as the shape and size of trichomes, crystals formed by calcium oxalate, glandular hairs, pollens granules, the stomata guard cells or the cell walls constitution, are often indicative to a given organ and species or genus (3,4,5). Microscopy can also be used to assess herbal powders such as liquorice root and for the identification of microbial contaminants such as fungi.
Often these techniques will use dyes to make identifying the diagnostic features in the herb much easier. There are some limitations in the technique, as discerning between similar species can be challenging, diagnostic features may not be present in heavily processed samples and the analysis requires specialist training to carryout (3).
Phytochemical analysis
Phytochemical methods are widely used in herbal quality control for identification, checking for adulteration and quantifying the active constituents. Modern techniques use fingerprints which provide highly characteristic profiles for the herbs typical approaches include chromatography, spectroscopy and spectrometry.
Chromatography is a separation technique that is commonly used in which different compounds in the herb can be identified through a chemical process. Common techniques include, high performance liquid chromatography (HPLC) used for quantification of the chemical constituents such as curcuminoids in turmeric; gas chromatography (GC) which is mainly used in the analysis of essential oils; high performance thin layer chromatography (HPTLC) is used for identification and detecting contamination, such as adulteration with different plants or plant parts (3).
Some challenges with these chromatographic methods occur when an adulterant shares a similar chemical profile to the herb which is being analysed — this can make identification more complicated. Chemical constituents can vary depending on the stage the plant is in its growth cycle. The level of processing can also make a difference; it can change or degrade the chemical constituents making the analysis more complex (6).
Spectroscopic techniques such as near infra-red (NIR) and Raman are becoming increasing popular in herbal quality control. The technique works by measuring how the molecules in the sample you are analysing absorb vibrations at specific wavelengths and from this you can determine particular chemical constituents in the herb. These techniques are popular as they are very rapid, require less specialist training and are portable allowing greater flexibility in where you analyse the sample.
There are some limitations in these techniques. With NIR spectroscopy the spectra can sometime be complex meaning the peaks can be difficult to distinguish. Raman sometimes has an issue with autofluorescence (the emission of light from the sample) due to chlorophyll content so analysing the leafy part of the herb can be particularly problematic (7).
Often phytochemical chromatography techniques are combined with spectroscopy or spectrometry methods, for example GC-MS, were gas chromatography (GC) is combined with mass spectrometry (MS). These are commonly referred to as ‘hyphenated techniques’ and combining methods in these ways enhances the accuracy for adulteration detection (6).
Molecular analysis
More recent developments in molecular methods have seen an increasing interest in utilising these techniques in quality control. DNA barcoding is the main technique used for herbal quality control and the method involves sequencing a short, standardised region of the genome. DNA barcode sequences are very short compared to the entire genome and therefore it is a rapid and inexpensive test which is ideally suited to quality control analysis (3,6).
Although DNA barcoding is highly effective at confirming species identity, it does have some limitations if the herb is extensively processed, substituted with different plant parts, or is harvested at the incorrect growing stage. More complex herbal mixtures can also be challenging to analyse, although recent developments in DNA meta barcoding have demonstrated that more complex herbal mixtures can be analysed effectively (6).
DNA barcoding whilst suitable for identification and to a certain extent purity, cannot measure the active constituents and therefore needs to be used in conjunction with suitable phytochemical techniques.
Summary
There are a plethora of quality control methods available, though there is no all-in-one analysis solution. Robust quality control relies on taking a multi-method approach as both the morphological, chemical and molecular methods all provide different quality insights and have different strengths and weaknesses.
Testing everything is never a feasible option from both an economic and technical standpoint. Although specifications and standards like the pharmacopoeia provide guidance on requirements, herbal suppliers in the UK do not routinely run quality testing to pharmacopeial standards on all herbs that they stock. The individuality of the quality issues that different herbs face is important to be aware of so that when purchasing specific herbs, you know what testing is needed to mitigate associated risks.
Integrated supply chains where there is direct contact between the supplier and the local producer will generally have less quality issues than a non-integrated supply chain where the steps in the chain are less well understood. Supply chains have different levels of risk and, therefore, require different levels of oversight.
Purchasing from producers with third-party certification is one way to get a degree of independent oversight of your supply chain. Third-party certification is a process where compliance against a defined set of standards for example good manufacturing practices (GMP) is assessed by an independent third party. Although not a quality standard per se, organic certification includes a strong emphasis on traceability which is a critical part of any quality standard. So, alongside your quality certification, other schemes can contribute to providing assurances for your supply chain. Whilst third-party certification provides a good level of assurance you should not become fully reliant on this approach. Carrying out your own due diligence, such as auditing your supply chain are important activities that help assure the quality of the herbs you purchase.
As a herbalist purchasing from suppliers with third-party certification will provide you with a degree of assurance that the herbs supplied are safe and of high quality. There are specific quality standards such as the HerbMark, a scheme developed by the British Herbal Medicines Association (BHMA) that are specifically tailored to the supply of herbal medicines to practitioners. So, looking out for this or more general quality standards, such as GMP, is a good starting point.
Alongside this, asking your suppliers for a specification so that you know what standards the herb meets and a certificate of analysis, to know what was tested are important due diligence activities. Where it is feasible, undertaking your own quality assessment of the herbs through macroscopy will provide you with insights into the quality of the herbal material.
Successful herbal quality control is built upon understanding what factors influence the quality of the herbs. This will inform what actions are needed to ensure the herbs are safe and of the reliable quality. Understanding your herbs and their supply chain better will enable you to target your herb testing and assurance, so that it is carried out in the most effective way possible.
You can also read our other herbal quality article “A review of the methods for assessing the quality and adulteration of herbal medicines“.
References
- Booker A., Johnston D., Heinrich M. Chapter 2 – New perspectives on value chains of herbal medicines—Ethnopharmacological and analytical challenges in a globalizing world, Editor(s): Mukherjee, P.K. Evidence-Based Validation of Herbal Medicine (Second Edition), Elsevier, 2022, Pages 43-58.
- Evans, W.C. Trease and Evans Pharmacognosy: Sixteenth Edition London: Elsevier; 2009.
- Heinrich, M., Barnes, J., Gibbons, S., & Williamson, E. Fundamentals of Pharmacognosy and Phytotherapy. London: Elsevier; 2004.
- N.W. Muyumba, S.C. Mutombo, H Sheridan, A. Nachtergael, P. Duez, Quality control of herbal drugs and preparations: The methods of analysis, their relevance and applications, Talanta Open, Volume 4, 2021.
- Zhao, Z., Liang, Z., Ping, G. Macroscopic identification of Chinese medicinal materials: Traditional experiences and modern understanding, Journal of Ethnopharmacology, Volume 134, Issue 3, 2011, Pages 556-564.
- Nazar, N., Saxena, A., Sebastian, A., Slater, A., Sundaresan, V. and Sgamma, T. Integrating DNA Barcoding Within an Orthogonal Approach for Herbal Product Authentication: A Narrative Review. Phytochemical Analysis, 2024, https://doi.org/10.1002/pca.3466
- Beć, K.B., Grabska, J., Huck, C.W. NIR spectroscopy of natural medicines supported by novel instrumentation and methods for data analysis and interpretation, Journal of Pharmaceutical and Biomedical Analysis, Volume 193, 2021.
