How Long Does Cannabis Stay Fresh For? Scientists May Have the Answer
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A new kinetic study from researchers at the National Research Council Canada (NRCC) could better explain how cannabis material degrades when left on the dispensary shelf.
The study, published in the journal Analytical and Bioanalytical Chemistry, is one of the first to explore the effects of temperature on cannabinoid degradation using room temperature conditions. Its authors found the degradation to follow a simple first-order kinetic model.
The researchers hope that the study can provide a less ambiguous framework for talking about cannabinoid degradation and encourage others to build empirical models to explore the kinetics of cannabis aging.
The problem with cannabis age studies
Most cannabinoids naturally exist inside the cannabis plant in their acidic forms. It is only when the plant is smoked or vaporized that these acid forms convert into their more biologically active forms.
But even prior to lighting, cannabinoid acids will naturally begin to decarboxylate. And this natural decomposition is not just limited to the acidic cannabinoids – studies have shown that the levels of neutral THC can also decrease over time as it decomposes to form cannabinol (CBN).
Understanding the degradation of cannabinoids, whether that is THCA-to-THC or THC-to-CBN, is important for producers and consumers who might want to know more about the shelf life of their products and for analytical chemists wanting to perform accurate chemical characterizations on cannabis products of different ages.
“The United Nations formula for the age of cannabis just says ‘ratio of THC to CBN.’ Well, what does that mean?” asked Dr Juris Meija, a senior research officer at the NRCC, and lead author of the new study.
“Is it a ratio on a mass basis? Is this ratio on an amount basis? You can come up with ten different ratios within that vague phrase. So, we are trying to raise attention that the last thing we need is ambiguity,” he told Analytical Cannabis.
Despite the interest in understanding cannabis degradation, many of the studies currently available in the literature have not distinguished between the acidic and neutral cannabinoids in their results, or have used high-temperature conditions over shorter timeframes to simulate longer periods of degradation. Mindful of this, the NRCC team created an experiment that would provide precision measurements of the cannabinoids in a long-term stability study using realistic storage condition temperatures.
Cannabis degradation follows simple first order kinetics
The researchers first created a homogenous blend of two different cannabis strains, chosen to achieve a good spread of common cannabinoids present in easily detectable levels. The material was then placed in one of six different simulated storage conditions ranging from temperatures of -20°C to +40°C (including one held at room temperature) for up to one year. Samples were taken from these feedstocks regularly to assess the extent of degradation in seven different cannabinoids over time, with two additional samples stored at -80°C acting as a control.
On analyzing the data, the scientists determined that a network of first-order kinetic models provided a good fit to model all the observed changes across the seven cannabinoids of interest.
First-order reactions are reactions that progress at a rate that linearly depends on the concentration of only one reactant. This means that, within cannabis, it would be possible to examine the decarboxylation of THCA to THC with respect to time and temperature, using only parameters relating to THCA and THC. The behavior of the other cannabinoids would be irrelevant.
Hence, it is possible for analysts to accurately calculate the age of a product or the half-life of a cannabinoid (time taken for half of it to decarboxylate) without dealing in approximate cannabinoid ratios.
“We challenged the UN classical age determination formula, because that’s kind of pulled out of thin air, on top of it being ambiguous,” said Meija. “To me, the biggest feature of the paper is actually the suggestion that you can do half-life estimates from the cannabinoid ratios themselves – you can go within the same kind of cannabinoid, as opposed to going from the ratio of different cannabinoids – because you can get much better estimates.”
The importance of good mathematical models
As Meija says himself, “these equations are nothing new.” But where past researchers have used approximations to extrapolate cannabinoid degradation over time, the NRCC team has created a more data-driven, fleshed out kinetic model that fits well with what has been seen experimentally in the past.
Early cannabis stability studies estimated that the THC content in cannabis would decrease at a rate of 3-5 percent per month when kept at room temperature. More recently, estimates have confirmed a 12 percent degradation in THC over the first 100 days of storage, equivalent to a rate of 3-4 percent each month. The NRCC researchers found the average monthly THCA+THC degradation rate to be 2 percent at 20°C and have also created an interactive cannabis stability calculator to better illustrate these findings for each cannabinoid across the full experimental temperature spectrum.
“We certainly do talk about the shelf life in our paper, but we’re not focusing on the shelf life,” said Meija, in response to a question on how these results might go on to impact the wider industry. “Really, we’re just showing that these are the side effects of the mathematical model and that the mathematical model doesn't have to be backward-looking, it could be also forward-looking. But the point is, you need to have a mathematical model if you want to talk about [these things].”
“We just wanted to have a data-driven science-based addition to the existing materials on how we can understand the material and how it changes. Only once we have that can we start to say if we need [things like] use-by dates or not.”
While the results from this kinetic model track well with other observational studies done on raw cannabis, Meija says that the team is interested to see how the kinetics of cannabinoid degradation will change when it comes to modeling different strains, hemp, or even different product formulations such as oils or infused beverages.
“The biggest unknown for us is how well you can extend this, how robust are these kinetic parameters to different strains or different systems,” Meija explained. “Because we certainly don’t expect the same kinetic parameters for CBD oils or CBD infused wine or something like that, but there is no need to pretend that everything is going to be different.”