Review on the Usefulness of Botanical Microscopy to Detect Herbal Ingredient Adulteration

Reviewed: Ichim MC, Häser A, Nick P. Microscopic authentication of commercial herbal products in the globalized market: Potential and Limitations. Front Pharmacol. 2020;11:876.

Keywords: Adulteration, authentication, bamboo, microscopy, Phyllostachys spp., Pseudosasa spp.

The data presented in this publication represent the results of a systematic review on authenticating commercial herbal ingredients using the microanatomical features of plants. Out of the 2,326 records obtained using the keywords (“medicinal plant” OR herbal OR botanical OR nutraceutical

OR TCM) AND (microscop* OR histolog* OR morpholog*) AND (identification OR authentication OR adulteration), only 28 papers were included because the large majority did not directly assess authenticity by using botanical microscopy, or the papers were excluded as they did not pertain to medicinal herbs, did not specify the country in which the commercial products were purchased, or the authors did not conclude if the samples were authentic or adulterated.

Of the 28 papers (number of product samples analyzed in parentheses), 10 were from Asia (n = 162), seven from Europe (n = 49), six from South America (n = 167), four from North America (n = 128), and one from Africa (n = 2). Fourteen of these publications presented data from less than 10 samples.

Some of the more interesting information in this review stems from the discussion of specific papers. As an example, the authors discussed results from the investigation into the authenticity of bamboo (Phyllostachys spp., or Pseudosasa spp., Poaceae) leaf tea, which reportedly has become popular in Europe.¹ Four of the eight samples analyzed contained leaf fragments that were identified as belonging to Chinese pink or Chinese carnation (Dianthus chinensis, Caryophyllaceae). The leaves of Chinese pink are used in traditional Chinese medicine (TCM) to enhance diuresis and are known in Chinese as shi zhu (stone bamboo). Therefore, this example of adulteration can be explained by the similarity of the vernacular names in Chinese.

Despite the relatively small number of papers in which botanical microscopy is used to detect adulteration, the authors conclude that “microscopic authentication is robust enough to be used along local, national, and international value chains for herbal products.” Advantages of a microscopic assessment are its low equipment and consumables cost and the ability to detect contamination by non-plant materials, e.g., insects, excessive levels of soil, stones, or the inadvertent presence of wire, nylon, or other types of fabric that cannot be detected using genetic or chemical methods. Limitations are the difficulties in distinguishing among closely related species and the inability to assess ingredients where morphological characteristics are lacking, such as extracts and essential oils.

Comment: Despite representing a valuable orthogonal approach to the assessment of crude whole, cut, or powdered herbal substances, botanical microscopy has mostly been replaced by chemical analysis methods in many academic and industry laboratories specializing in plant identification and authentication. This is in part due to the natural evolution of early analytical methodologies that progressed from micromorphology and organoleptic assessment to magnification that began with the advent of the microscope, to the reductionist Western scientific approach of looking for pharmacological actions and standardization to individual constituents, to the genetics used today. However, microscopic identification remains a standard entry and requirement in virtually every pharmacopoeia today and the work of organizations such as the American Herbal Pharmacopoeia (AHP) and training programs conducted by the Food and Drug Administration (FDA) and the American Herbal Products Association (AHPA) have helped to increase awareness about the value of this often-neglected tool. Lack of the inclusion of microscopy in quality control programs may also be due in part by the difficulties in finding experts in the field of botanical microscopy compared to the number of people who can run high-performance thin-layer chromatography (HPTLC) or high-performance liquid chromatography (HPLC) equipment, but more likely, the trend towards using processed materials, i.e., herbal extracts where microanatomical features are absent, by herbal dietary supplement manufacturers, may be the main driver towards chemical methods of analysis.

This paper provides some examples that show how useful botanical microscopy can be. It is worthwhile to read a few of the cited references. As an example, one study described how to easily distinguish between Lycium barbarum and L. chinense fruit by measuring the size of the seeds. In this publication, the authors evaluated 17 commercial goji fruit samples, and determined that all of them were made with L. barbarum using the microscopic features.² With so much high-tech equipment available to analyze plants, it is crucial to remind people that plant taxonomy is based on morphological features, and that experts in plant anatomy have an important role to play in determining the identity of herbal ingredients. There are some encouraging signs of increased interest in plant morphology by researchers, as explained in Heidi Ledford’s paper “The lost art of looking at plants.”³

References

1. Horn T, Häser, A. Bamboo tea: Reduction of taxonomic complexity and application of DNA diagnostics based on rbcL and matK sequence data. PeerJ. 2016;4:e2781.
2. Wetters S, Horn T, Nick P. Goji who? Morphological and DNA based authentication of a “superfood”. Front Plant Sci. 2018;9:1859.
   
3. Ledford H. The lost art of looking at plants. Nature. 2018;553:396-398.