Drying and curing CBD flowers: what decides quality

A freshly cut flower holds most of its cannabinoids in acidic form, mainly CBDA, the precursor of CBD described in our article on the acidic forms of cannabis. It also holds a great deal of water, and a microbial load inherited from the field or the greenhouse.

Drying removes the water. Curing, which follows, lets the material settle in a closed atmosphere over several weeks. These two stages determine the content displayed, the aromatic profile and the stability of the product.

That is the clearest result of a study published in Plants in 2025 by a team at Cornell University, which compared three methods on industrial hemp: hot-air drying, ambient-air drying and freeze-drying.

Hot-air drying, at 75 °C for eight hours, converted 8.5 to 9.0 % of the CBDA into CBD. Ambient-air drying, at 25 °C and 45 % relative humidity for seven days, converted less than 0.1 %. Freeze-drying likewise converted less than 0.1 %.

Two batches from the same plot, dried differently, will show distinct CBD and CBDA lines on their analysis, without the cannabinoid total having changed. A buyer comparing only the "CBD" line is therefore partly comparing drying processes. Our guide to reading a certificate of analysis explains why the two lines have to be read together.

An Israeli study published in Plants in 2024 confirms the logic on medicinal chemovars: drying in a controlled atmosphere, at 15 °C and under 10 % relative humidity, preserved THCA, whereas open-air drying lost 20 % of it and multiplied THC formation by 3.5. That controlled drying also cuts the total duration of drying and curing by at least 60 %.

In the Cornell study, the dried flowers were then given a curing period of three weeks in glass jars or pouches, in the dark, at around 24 to 25 °C and 45 % relative humidity. Their moisture rose from 3.3 % to 13.6 %, which corresponds to the equilibrium sought.

During that time, the ratio between acidic and neutral forms continued to fall, with a further decarboxylation of 1.5 to 1.8 % for the batches dried in ambient air. Total cannabinoid content, for its part, did not move significantly. Curing therefore redistributes the forms without increasing the total.

The useful notion here is not moisture content but water activity, which measures the water available to micro-organisms rather than total water. The sector's reference specification, standard ASTM D8197, sets a window of 0.55 to 0.65 for dried cannabis flower, to be maintained across the whole chain, from drying through to the shelf.

Both bounds have their reason. Below 0.55, the flower turns brittle and degrades on handling. Above 0.65, the risk of fungal growth rises: the food science literature puts the lower limit for the growth of moulds and yeasts at around 0.61.

The gap between slow drying and controlled drying reads directly in the fungal load. The Israeli study measured, on one chemovar, 31 million colony-forming units per gram of Alternaria alternata after twelve days of open-air drying, against 2 million after drying in a controlled atmosphere. It also reports that about 80 % of the inflorescences dried in open air were heavily infested, with no visible mould on the controlled-atmosphere batches. Our reference points for spotting a mouldy flower remain useful when buying.

Terpenes evaporate, and the lightest go first. The Israeli work reports monoterpene losses of 3 to 20 % for beta-myrcene depending on the chemovar, and up to 58 % for d-limonene depending on the method used. Sesquiterpenes, being heavier, see their relative share rise by 15 to 40 %.

Over time, a third study by the same team followed inflorescences stored at 4 °C for a hundred and thirty days. The control batch, kept in air, lost 23 % of its THCA; the batch kept under vacuum with prior processing, 4 %. On the terpene side, alpha-pinene, beta-pinene, linalool and d-limonene stayed stable in the best group, while beta-myrcene lost 37 %.

For a buyer, a rich terpene profile on the analysis indicates careful drying and a recent product, and justifies the storage advice set out in our article on keeping flowers.

Few sellers say so: there is no microbiological limit, in France or in the European Union, set by regulation specifically for hemp flower sold as a non-medicinal consumer product.

Thresholds do exist, but elsewhere: the European Pharmacopoeia sets microbiological quality criteria for herbal drugs for medicinal use, and the order of 16 October 2020 refers to them for cannabis for medicinal use. A CBD flower producer may choose to follow them, but nothing compels it.

That shifts the burden of proof onto the seller. At Phytogrammes, every batch is analysed by an independent laboratory and the certificate is published before purchase, and our selection criteria are set out in how to recognise the best flowers.