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  • Writer's pictureTravis Cesarone

How does CBD turn purple, more than polyphenol?

Chemists found special molecules known as cannabinoids in cannabis before other plants. But one member in this class of natural compounds holds a colorful mystery. An old test utilizes a specific chemical that turns the cannabinoid, CBD, into purple byproducts. The reaction can identify certain cannabinoids in a sample. An abundant and essential chemical, however, turns poorly packaged CBD into likewise purple compounds, according to recent experiments.

Cannabinoids break down into various end products given enough time and oxygen. And the beam test contains a basic reagent, which is a chemical with a PH above water. The base oxidizes a compound in cannabis into new a intermediate that appears yellow under neutral conditions. But, CBD extracts slowly turn purple in a clear bottle exposed to enough light and oxygen. Previous experiments on a specific chemical reaction found the yellow substance, HU-331, but simply missed an elusive violet molecule.

HU-331 would decrease in concentration, and then we would see this purple color form.

The beam test

Brodie Thomson, a Ph.D. candidate in organic chemistry, found unknown routes to purple end products. He completed the work with Delic Labs under the supervision of Dr. Markus Roggen in collaboration with colleagues from the University of British Columbia and the University of Calgary. Uprooted Concepts discussed CBD degradation routes with Thomson and Roggen, expanding on four-decade-old research.

In the late 1960s, Professor Raphael Mechoulam and Dr. Zvi Ben-Zvi from the Hebrew University School of Pharmacy worked alongside Weizmann Institute colleague, Yehiel Goani, and quantified one colorful chemical in the beam test. According to their work, CBD oxidizes into a quinone known as HU-331. Speaking with Uprooted Concepts, Dr. Markus Roggen elaborated on their efforts.

"The thought was that you take what you expect to be cannabis, like, for a police check. You dissolve the cannabis in methanol or ethanol and then you add a base — potassium hydroxide — and if the sample turns purple then that is your quick check for cannabis.
Mechoulam went in and figured out why it turns purple. And his paper shows that it is not THC, but CBD that turns purple when mixed with a base and one reaction, or product he showed is this hydroxy-quinone — HU-331." —Dr. Markus Roggen, Founder and President, Delic Labs.
CBD oxidizes into HU-331, a yellow-orange solid, which then turns into a purple, negatively charged derivative under light. Photo courtesy of Thomson et al. 2023 | Nature.

Seeing beyond the beam test

Their experiments suggest that CBD first turns into HU-331 —a middle-person that light helps degrade into a more mysterious purple compound. Expanding on the newly discovered possibility, Thomson found the truly purple culprit — a negatively charged molecule that easily escapes normal lab tests. They based their journey on vague assumptions, though.

"All we knew was that it was potentially an HU-331-driven process. But there was no concrete evidence of that in the first place." — Brodie Thomson, Department of Chemistry, UBC.

"The problem is that HU-331, as a neutral compound, is yellow. It’s only the anion that’s purple. But if you have a [CBD] vape cartridge, no one added any base to it, right? It just sits there. So why does CBD oil turn purple color if no one added a base?" — Dr. Roggen

"What we saw when we measured HU-331 in samples of CBD aliquots was — one — the HU-331 concentration increased slowly over time, which was showing us that [CBD] was potentially oxidizing by itself and probably in the presence of oxygen. But then also, when we started to [expose] samples that contained a known quantity of HU-331 to light, the HU-331 would decrease in concentration. And then we would see this purple color form, which suggested that HU-331 was potentially causing this, but it wasn’t the actual color." — Thomson

Color and chemistry

Purples are especially intricate on the color wheel, deriving from combinations of violet’s short-wave and orange’s longer wavelengths of light. But many cannabinoids, even cannabinol, do not produce color. Molecules that do produce color must contain several tightly packed connections, or in technical terms, the compound must contain enough conjugated parts. THC and CBD have carbon and hydrogen-based ring systems that are primarily connected by single electron bonds. They contain double-bonds too loose to absorb light and produce color.

But extra oxygen molecules form new groups and double-bond connections with CBD, which occurs during the production of HU-331 and related compounds. In other words, oxygen turns CBD into HU-331 before it forms new conjugated anions (negatively charged molecular groups) that emit shorter wavelengths of light. Contrasting natural conditions, though, protons within CBD behave differently in a strongly basic solution. This means that oxygen uniquely reacts with CBD depending on the surrounding environment, especially basicity — a scale of hydrogen atom acceptance.

Purple CBD, the shelf not the base

Mechoulam’s work with the beam test gave answers under lab conditions. But cannabis consumers and CBD processors know that extracts change and degrade over time. While some consumer-driven cannabis brands achieve success with brand recognition, potency, and craft cultivation. Aesthetic drives the agricultural market. But CBD and THC do not produce color, and when they do, it likely means new compounds now exist. Surprisingly, though, no one dove into the chemistry to update Mechoulam’s research for processor and consumer purposes until an unfriendly face stepped onto the scene.

If you asked Google the question, pages of articles reveal that polyphenols, a type of plant nutrient found in carrier oils, oxidize into a purple substance. Otherwise, cannabis produces anthocyanins, a purple substance defined as a flavonoid. But cannabis processing and storage practices often fail to consider oxygen’s impact. While certain carrier oils used in cannabis products might turn purple, new experiments reveal that the problem with color further lies within CBD’s ambient chemistry.

"The crux of the paper sits on the fact that when we were shining lights on samples of HU-331, both in the presence and absence of CBD, if we removed the oxygen component, we saw none of this novel intermediate. And as soon as we exposed the intermediate to any oxygen, we saw the degradation of that compound into these colored anions that are purple. So not only was this potentially decomposing to HU-331, but there are other things in [the reaction]… We saw other products forming in this process — such as hydrogen peroxide." —Thomson

Unfortunately, various side reactions occur while CBD oxidizes and turns into a purple anion in a vape pen, creating too much noise to detect a solitary signal. In other words, light and oxygen produce CBD conversions too messy to easily quantify tiny amounts of a purple substance at parts per billion.

"It was actually a member of Markus’s team, [Dr. Eric Janusson], who took an advanced method where, instead of looking at the whole compound itself, you can look at fragmented targets of the compounds within a solution… But as long as we had a quinone core that could be deprotonated, we could see the color form." — Brodie Thomson

THC, unlike CBD, has a third closed-ring system. This eliminates the resorcinol moiety found in CBD or CBG that is required for oxygen to grab and form a hydroxy-quinone. In other words, THC cannot oxidize into colorful hudroxyquinone under ambient conditions found in a stored vape pen.

Going to BAT with funding

The newly elucidated anion hid in common cannabis products behind its parent, HU-331, until Roggen and Thomson’s team noted purple CBD oil even after HU-331, an unstable intermediate, completely broke down. But their team’s discovery only came after requests and funding from British American Tobacco (BAT) who were working on CBD vape pens. Put to task, Roggen and Thomson, with assistance, began a long journey to answer the daunting question, “Why does my CBD turn purple?”

"BAT was working on CBD vape cartridges, and their cartridges turned purple. But BAT, being a larger international company and not a cannabis insider, was concerned if a new compound is formed — is that a health concern?" — Dr. Roggen

While chemists cannot quantify pharmacological nuances, Roggen expressed disappointment over tobacco executives conducting more thorough quality checks on CBD vape pens than cannabis professionals.

Batches of CBD turn purple, everyone on Reddit knows that. But no one in the cannabis industry felt compelled to say, ‘Oh, maybe we should put some money into this and figure this out.’ It had to be the bad guys from the tobacco industry that said this might be a problem." —Dr. Markus Roggen

The study occurred over a 10-week storage period on propylene glycol-based e-liquids containing 5% CBD. Oxygen and light turn CBD into purple molecules that degrade from HU-331 quinone under ambient conditions. Results suggest that developing a ‘use after X number of days after opening” label can benefit consumers and patients. It also demands that processors consider both light and oxygen, possibly by storing CBD products in amber glass vials, as an example.

Let us know in the comments if you have found CBD to turn purple over time. Does light impact your storage practices?


  1. Thomson, B.J., Hanna, S., Schwarzenberg, A. et al. CBD hydroxyquinone photo-isomerises to a highly reactive intermediate. Sci Rep 13, 6967 (2023).

  2. Mechoulam, R., Ben-Zvi, Z., & Gaoni, Y. (1968). Hashish–13. On the nature of the Beam test. Tetrahedron24(16), 5615–5624.

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