Drying and curing CBD flowers: what changes on the analysis

Most of the cannabinoids in a freshly cut flower are in acidic form. CBDA is the main one, and heat turns it into CBD, as our page on CBDA describes. In the Cornell study covered below, fresh flowers held 74.9 to 80.6% water, and there was over 100 times more of the acidic forms than of the neutral ones.

Then come two steps. Drying takes out the water, in eight hours in a drying oven or in a week or two in air. For curing, the dried flowers rest in a closed container, in the dark, until the core and the surface reach the same moisture. Heat, time and air all act during those weeks.

A Cornell University team published a comparison of three drying methods in the journal Plants in 2025, each followed by three weeks of curing, on two hemp varieties grown in a greenhouse. How much CBDA turns into CBD depends almost entirely on heat.

The authors' main finding is that neither drying nor curing significantly changed the total cannabinoid content. Two lots from the same plot, dried in different ways, will show different CBD and CBDA lines with the same total. If you compare two flowers on the "CBD" line alone, you are partly comparing how they were dried.

Total CBD corrects this bias: CBD + 0.877 × CBDA. The coefficient accounts for the mass CBDA loses as it becomes CBD. A flower with 1.0% CBD and 12.0% CBDA therefore holds about 11.5% total CBD. Our guide to reading a certificate of analysis shows where both lines appear, and the reaction itself is described in the article on decarboxylation.

After low-temperature drying, CBD stays almost entirely in acidic form. In nine commercial hemp flower varieties analysed in a study published in 2022, CBDA came to 20.7 to 67.8 mg per gram, against 0.16 to 1.10 mg for CBD.

The Cornell team cured the dried flowers for three weeks in the dark, at about 25 °C and 45% relative humidity, in glass jars or metallised bags. Their water content rose again, up to 13.6% depending on the lot. Whether the container was a jar or a bag made no significant difference to it.

The conversion carried on slowly during curing. In the lots dried in ambient air, the share of CBDA converted went from under 0.1% to about 1.5 to 1.8%. As long as temperature and humidity were held steady, curing did not increase the yeast and mould load.

Hot-air drying had left the lowest microbial load, up to 100 times below the other two methods, and the darkest, least green flowers. The authors recommend hot air, then curing in a glass jar. They did not measure terpenes, and they name the terpene profile during curing as one of the questions still open.

In 2024, a team at Israel's Volcani Center published a comparison of open-air drying and controlled-chamber drying on medical chemovars rich in THCA, which goes through the same kind of reaction as CBDA. The traditional method took 14 days at 15 °C and 50 to 55% relative humidity. The chambers ran at the same 15 °C with under 10% relative humidity, and drying lasted six days.

The differences measured were large:

• THCA dropped by about 20% in open air while THC increased 3.5-fold; in the chambers, THCA held steady;
• one chemovar carried 31 million colony-forming units (CFU) of Alternaria alternata per gram after 12 days in open air, compared with 2 million after six days in a chamber;
• of that chemovar's inflorescences dried in open air, about 80% were heavily infested, while the chamber lots showed no visible mould;
• drying and curing together took at least 60% less time.

A lot can carry two million CFU per gram with nothing showing, so a visual check is not enough. Our guide to spotting a mouldy flower separates what you can see from what needs a lab test.

Water activity measures only the water available to microorganisms, so two flowers with the same moisture content can have different water activities. For dried cannabis flower, the ASTM D8197 standard sets a window of 0.55 to 0.65, to be maintained from the end of drying until sale.

Both limits have a purpose. Under 0.55, the flower crumbles during handling and storage. Above 0.65, the standard aims to prevent the growth of microorganisms such as mould. Curing brings the flower into that window and storage keeps it there, as explained in our article on storing flowers.

Terpenes evaporate, the lightest ones first. In the 2024 Israeli study, drying removed 3 to 20% of the beta-myrcene depending on the chemovar, and up to 58% of the d-limonene depending on the conditions. The heavier sesquiterpenes rose by 15 to 50% in relative share. A dried flower's aroma profile therefore differs from that of the fresh plant. These families are described in our article on terpenes.

The same team tracked inflorescences stored for 130 days at 4 °C. Kept under air, the control lot lost 23% of its THCA. The best group, treated with hexanoic acid before harvest and then stored under vacuum, retained 96%. In every group, alpha-pinene, beta-pinene and linalool held steady, and beta-myrcene dropped by 19 to 51%.

The general rule is about THC: 0.30% Δ⁹-THC at most. French law sets no yeast, mould or mycotoxin limit specific to hemp flower sold outside the medical channel.

Medical cannabis does have such requirements. The order of 16 October 2020 sets the specifications of the medicines used in the French pilot programme. It refers to the European Pharmacopoeia monographs and to the European Medicines Agency guideline on the microbiological aspects of herbal medicinal products. A CBD flower producer is free to follow them and has no obligation to.

When you buy, you can still make four checks:

• read the CBD and CBDA lines side by side, or the total CBD;
• check for a microbiological analysis (total yeast and mould, CFU per gram) if the seller publishes one;
• choose a recent packing date, because terpenes decline even when kept cold;
• never treat a flower with no visible mould as proven clean.

Our laboratory page lists the catalogue products. Our selection criteria are set out in the guide to smokable flowers, and you will find the catalogue flowers in the CBD flowers category.