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The Tradition of Mold Count Testing for Herbal Products

EVIO Labs fields a lot of questions from producers and retailers about mold and mold counts. Total mold count testing is a fundamental part of basic quality assurance for most herbal medicine commodities. Whether we’re looking at Russian Rhodiola, North American Echinacia or Kava paste from Vanuatu, you can bet that the commodity buyer will expect a mold count result (among other things) from and independent laboratory. At one time, mold count testing was required by rule, in Oregon, for all Cannabis being transferred to dispensaries. Despite that up to 15% of batches of Cannabis ‘flower’ failed mold count testing, mold count testing disappeared from the requirements sometime early in 2016. Despite this, some Oregon Cannabis wholesalers and retailers still required a mold count test for their own Quality Assurance purposes.

Basic mold count testing in bulk herbs, Cannabis included, is relatively simple and typically cost less than $50. The principle of mold count testing with herbs and herbal preparations is to shake a quantity of the sample with a buffered water solution and then ‘plate’ some of that liquid on a petri dish containing a general-purpose agar media that will germinate molds and mold spores. You can plate the liquid on either a petri dish or, more commonly, on a disposable petri-like device called 3M PetriFilm®. PetriFilm is like a dehydrated petri dish formed on a card. When a watery extract of the herb or herbal preparation is plated onto the PetriFilm card, the water rehydrates the agar which germinates the spores. After 48 hours incubation, any germinated fungi or fungal spores appear as a colored colony spot which can be counted. Data is reported in Colony Forming Units per gram of sample (CFU/g). Basic mold count assays are typically non-specific—meaning that it is a metric of all active molds and spores, both harmful and harmless. In truth, almost no herbal commodities will have NO mold count. In general, acceptable limits for mold counts in raw medicinal plant material intended for processing and raw herbal material intended for internal use are 10,000 CFU/g and 1000 CFU/g, respectively.

The World Health Organization states the following, with respect to the need to test herbal medicines for microbiological contamination:

“Herbs and herbal materials normally carry a large number of bacteria and molds, often originating in soil or derived from manure. While a large range of bacteria and fungi form the naturally occurring microflora of medicinal plants, aerobic spore-forming bacteria frequently predominate. Current practices of harvesting, production, transportation and storage may cause additional contamination and microbial growth. Proliferation of microorganisms may result from failure to control the moisture levels of herbal medicines during transportation and storage, as well as from failure to control the temperatures of liquid forms and finished herbal products. The presence of Escherichia coli, Salmonella spp. and molds may indicate poor quality of production and harvesting practices”(1)

Mold counts are one basic measure of herbal Cannabis and Cannabis products. While many molds typically found in cured cannabis are not regarded as being pathogenic, too much natural mold content can spoil the organoleptic quality of the herb (appearance and aroma). More seriously, high mold count can be a sign of too much time at a high moisture content, e.g., improper curing. In some cases, toxic molds can also be present. Toxic mold species such as Aspergillus can cause potentially fatal respiratory infections in immunocompromised patients and it can also produce significant quantities of aflatoxin, ochratoxin and other mycotoxins that are persistent and seriously toxic--even after processing or even sterilization.

Despite once being a required quality assurance test in Oregon, mold count testing is no longer required. In other legal Cannabis states, mold counts and other microbiological contaminant checks are required testing (California, Colorado, Massachusetts, Nevada, etc.). If you think basic mold testing is important, you should ask your Cannabis producer, wholesaler or retailer about it and let them know you value it as a metric of quality and safety.


WHO Guidelines for Assessing Quality of Herbal Medicines with Reference to Contaminants and Residues. ISBN 978 92 4 159444 8. World Health Organization 2007