“Yeast and mold levels are influenced by genotype, environment, and handling practices”

“Total yeast and mold (TYM) levels in inflorescences of high THC-containing cannabis are regulated to ensure that users are not exposed to potentially harmful levels. In North America, the limits imposed range from 1,000–10,000 cfu/g of dried product to 50,000–100,000 cfu/g, depending on the jurisdiction.” In a recent study, over 2,000 fresh and dried samples were assayed for TYM over a 3-year period to identify specific factors which can contribute to TYM levels. “Our findings indicate that TYM in cannabis inflorescences are the result of a dynamic interaction between genotype, environment, and post-harvest handling methods. Some of these factors may be altered by cannabis producers to reduce the potential build-up of these microbes,” the researchers say.

What impacts TYM?
For the study, a range of different genotypes (strains) were grown under greenhouse conditions during the fall–winter months (October–April) and spring–summer months (May–September) over a 3-year period (2019–2022), which generally included four cropping cycles per year. All inflorescence samples were obtained from plants in the final 2 weeks of flower (usually week 7 or 8 in the flowering period) representing different genotypes being grown (fresh samples) or following drying using the rack or hang dry procedure (dried samples). “The variables that significantly (p < 0.05) increased these TYM levels in inflorescences were: the genotype (strain) grown, presence of leaf litter in the greenhouse, harvesting activity by workers, genotypes with a higher abundance of stigmatic tissues and inflorescence leaves, higher temperature and relative humidity within the inflorescence microclimate, time of year (May–October), method of drying buds after harvest, and inadequate drying of buds.”

According to the results, the total yeast and mold (TYM) levels in cannabis inflorescences are significantly influenced by the genotype (strain) that is cultivated. “The density of inflorescence leaves on different cannabis genotypes, which collectively created a warmer and more humid microclimate within the inflorescences, was shown to enhance TYM levels,” the researchers say. “Another feature of cannabis plants that can promote a build-up of TYM within inflorescences is the density of pistils (consisting of stigma, style and ovary tissues) and larger sizes of inflorescences observed in certain genotypes, such as 'Watermelon Kush' and 'Powdered Donuts,' that harbored consistently high TYM levels. Scanning electron microscopic observations showed that mycelium ramified over the pistils (in particular, the stigmatic tissues) as well as the trichomes. Pistils can provide sites on which bacterial and fungal populations can establish within flowers."

Leaf litter
"Leaf litter left on the greenhouse floor after pruning activities harboured a range of fungi and yeasts, as shown by plating assays, the most prevalent of which were species of Aspergillus, Cladosporium and Penicillium. Leaves harbor a significant population of microbes at various stages of decomposition. Harvesting activity, wherein workers remove foliage and prepare inflorescence stems for manual harvest, and step on the leaf litter, caused a significant increase in air-borne propagules of TYM," according to the results.

Drying process
A comparison of wet trim and hang-dry processes showed that TYM in samples processed using the hang-dry method were significantly lower. The study explains that the absence of injury sites for colonization by residents or introduced microbes in trimmed dried samples compared to trimmed fresh samples would account for the difference between the two handling methods.

“During the hang dry process, the foliage present on the stems caused a spike in TYM during the first 72 h. The air in the drying rooms also showed a significant increase in the population of air-borne propagules, likely to have originated from the leaf tissues,” the researchers continue. “Upon further drying to 96 h, the TYM levels were significantly reduced as the tissue moisture content was reduced to 12–14%. During moisture removal, microbial populations rapidly decline since drier conditions are unfavorable for the growth of most yeasts and molds, which occurs optimally at water activities (aw) near 0.9. Our data showed that even after 6 days of drying to a moisture content of 12–14% (translating to aw of ca. 0.62–0.7), there were still remnants of microbial activity that could be detected on the plating media. Inadequate drying of cannabis samples that leave pockets of moisture can result in a higher survival rate of TYM. In subsequent handling processes, such as curing, storage, and shipping, a proliferation of these residual microbes could take place.”

Read the complete study at www.frontiersin.org

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