Mold and mycotoxins in cannabis

Microbiological safety in cannabis comprises two separate risk levels: visible mold infestation (mycelium, spores) and invisible mycotoxins — secondary metabolites formed by molds that remain in the product even after the mold has been removed.

Both risks are closely linked to water activity (aw), a measure of the proportion of water in the product that is actually available for microbial growth — independent of pure moisture content.

Botrytis cinerea (Botrytis/bud rot) preferentially infests dense, poorly ventilated bud structures from the inside out — visible surface infestation is usually a late stage, not the start of the infection.

Aspergillus and Penicillium species are the most relevant mycotoxin producers in cannabis; their growth risk increases significantly once water activity exceeds about 0.70–0.75, regardless of how dry the material appears on the outside.

Water activity (aw) measures the vapor pressure of water in the product relative to pure water (scale 0–1) — it differs from moisture content in percent because it only captures the water actually usable by microorganisms.

Below aw 0.65, the growth of most mold species relevant to cannabis is strongly inhibited — this value is considered the practical target upper limit for safe storage.

With improper drying, cannabis can already feel crispy dry on the outside while the core of the bud still has a significantly higher, microbiologically critical aw.

Visible early signs of Botrytis: a single wilted, discolored leaflet deep inside a dense bud, while the outside still appears unremarkable — if in doubt, carefully open the bud and check the center.

Mycotoxin risk CANNOT be reliably assessed visually: smell and appearance provide clues, but do not replace lab measurement, since toxins can remain in the tissue even after the visible fungal mycelium has died off.

A reliable risk assessment therefore always combines visual inspection, aw measurement and — in case of anomalies or in professional contexts — lab data.

Controlled drying at 55–65 % relative humidity and 18–21 °C over 7–14 days lowers the aw evenly, without excessive terpene loss from drying too quickly.

Curing (controlled post-ripening in sealed containers with periodic 'burping') stabilizes the aw throughout the entire bud volume, not just at the surface.

Storage well below aw 0.65 in tightly sealed, light-protected containers keeps the microbial risk low over weeks to months.

Relying solely on smell or appearance and considering aw measurement unnecessary.

Drying too fast at too low humidity — this quickly lowers the aw, but often seals the outer layer before the core has caught up, masking an internal moisture problem.

Removing conspicuous individual buds but classifying the rest of the batch as 'safe' without aw control.

In professional contexts, interlaboratory comparisons and ring tests supplement your own aw/visual inspection to uncover systematic measurement errors of individual labs.

Batch release criteria that combine aw threshold values with microbiological lab values are more reliable than any single criterion alone.