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Cannabis shelf life: How long does it last?

An experiment on the shelf stability of cannabis flowers

Like any agricultural commodity, marijuana has a shelf life.

There must be some period of time after which the product has lost its optimum flavor or effect or is no longer fit for human consumption. In other words, at some point the cannabis is no longer representative of the quality advertised by the producer.

Unlike other agricultural commodities, standards do not exist in the cannabis market that specify how long a product should remain on the shelf or if/when it should expire.

About the Research Methods

Following a proposed change to quality assurance standards in Washington, Confidence Analytics conducted a research experiment to learn more about the shelf life of cannabis.

The pool of samples examined in this experiment was selected out of convenience; Confidence Analytics, a state-certified cannabis testing laboratory, stores samples for several months before destroying them, which allowed its scientists to retest some samples to evaluate trends over time as the material decays. The company investigated changes in viable microbial bioburden and cannabinoid content over a period of 125 days.

Microbiological analysis was performed using routine methods of microbial enumeration via plate count as described by the Food and Drug Administration’s Bacterial Analytical Manual. Endpoints included colony-forming units per gram (CFU/g) of total aerobic bacterial (or just aerobic), total fungal (or just fungal) and total bile-tolerant gram-negative bacterial (or just enterobac). Three groups of samples representing storage durations of 45 days (sample size=22), 60 days (sample size =21) and 90 days (sample size =11) were each examined to compare initial versus final microbial bioburden using a paired t-test with two tails. Microbiological results were also assessed in comparison to Washington state’s quality assurance failure thresholds for microbial contamination (100,000 CFU/g for aerobic; 10,000 CFU/g for fungal; 1,000 CFU/g for enterobac).

Cannabinoid content analysis (sample size=81) was performed using high-pressure liquid chromatography separation and diode array detection as described by the American Herbal Pharmacopoeia (Cannabis Inflorescence and Leaf), with individual reference standards for each analyte. Endpoints described in this publication include quantifications of both the acidic (THCA) and decarboxylate (THCd) forms of the main psychoactive cannabinoid ∆-9-tetrahydrocannabinol over a continuous date range of one to 125 days from the initial test. Trends in the decarboxylation rate were examined using the equation below, which computes the percentage loss of THCA as it converts to THCd through the process of decarboxylation.

In all calculations, a conversion factor of 0.877 was used to account for the molecular weight difference between THCA and THCd; the measured value of THCA was multiplied by the conversion factor for all calculations in this publication. Relative decarboxylation was fit to a linear regression for statistical analysis. A p-value of less than 0.05 was interpreted as statistically significant for all statistical tests.

This proposed rule, if put into effect, would have some profound consequences for many stakeholders in the cannabis industry. One potential positive outcome may be that it prompts a discussion about shelf stability, so stakeholders can begin to ask: When should marijuana expire? Should it ever? Does it have a “best by” date? How should it be stored prior to packaging? How do different storage methods affect the rate of change to the marijuana’s quality? What is the best way to package it, and why? How do we know these things? These are complex questions with complex answers, and they warrant our attention.

This article will examine one method of storage and investigate the rate of change in analytical test values over time as the product experiences consistent storage conditions.

Producers and processors of useable marijuana continuously submit samples of flower for quality assurance testing, either as a state-mandated requirement prior to sale, or for their own research and development purposes. Samples of cannabis flower, upon receipt by Confidence Analytics, are completely dried in a desiccator and homogenized into a fine powder before subsamples are drawn for various microbiological and analytical chemistry assays. Most of the homogenized sample remains unused and is stored in an individual plastic bag, sealed with a zip-lock, and kept in a cool, dry, dark environment.

For this experiment, flower samples from 135 lots were selected semi-randomly from the pool of samples tested in the previous 125 days. Samples were intentionally selected to cover a wide range of storage duration and original microbiological contamination values.

These samples had all been initially tested for microbial bioburden and/or cannabinoid content immediately prior to storage.

The initial test values were compared to values obtained after post-storage reexamination, representing a change in value over time.

Microbiological contamination values for all three contaminants of interest (aerobic, fungal and enterobac) observed under these storage conditions demonstrated a decrease in viable colonies over time. The downward trend was statistically significant at 45 days for aerobic (p-value=0.012) and fungal (p-value=0.035), but not enterobac (p-value=0.444). The decreasing trend was consistent for all three time comparisons. None of the samples in this experiment changed from passing microbial contamination values to failing after storage, and one sample changed from failing to passing.

Decarboxylation of THCA resulted in a statistically significant linear decrease in the concentration of that molecule over time, with the average sample losing 0.0167% of its THCA content per day (95% confidence interval=0.0113-0.0221; p=2.91e-8). This amounts to a loss of 0.5% of THCA in 30 days. If we assume that degradation of THCA through decarboxylation follows this pattern of decay, we’d predict the half-life of THCA under these storage conditions to be somewhere in the range of eight to nine years.

The storage conditions examined in this experiment demonstrate a decrease in both viable microbiological contamination and THCA content over time. While the downward trends for both are statistically significant, only the change in microbial contamination is particularly meaningful at this time scale. With a predicted half-life of nearly a decade, the rate of THCA loss over 125 days is insubstantial, even if it is statistically significant, and an accurate decay curve would be better demonstrated by a study of longer duration. It was observed that microbial contaminants in cannabis flower lose viability over time when stored at desiccated conditions, and the implications of this finding to the marijuana industry could be profound.

A very important point to consider as a caveat to any conclusions drawn from these data is that the trends observed here are true only for the storage conditions described. Certainly, most producers and processors of useable marijuana do not store their flowers completely dried, as was done in this experiment. From anecdotal experience, this author can confirm that even when modest amounts of moisture are present in the container, marijuana flowers can grow mold in the packaging.

A very wide variety of storage methods exist in the cannabis domain, some of which (such as packing with inert gas) might outperform the results of this study in terms of product stability over time. Other forms of storage, particularly when high levels of heat or moisture are present, will certainly perform less well.

Perhaps the most important conclusion to be drawn from these results is the realization that there is much more to learn about the shelf stability of cannabis. Studies such as this can be highly informative to manufacturing processes and legislative decisions, especially in such a young industry. With similar methods to those described here, it should be possible to validate relatively long periods of shelf stability. To that end, we encourage manufacturers of marijuana products to partner with their analytical labs to further the science of this commodity and enhance the professionalism of their craft.

Cannabis shelf life: How long does it last? An experiment on the shelf stability of cannabis flowers Like any agricultural commodity, marijuana has a shelf life. There must be some period

How to Tell If Cannabis Is Past Its Prime

Weed doesn’t go bad the way a jar of mayo or some other food product might, but it can definitely be “off” or even moldy.

Old weed likely won’t lead to any serious health issues if you don’t have any underlying conditions.

It can, however, have a noticeable drop in potency, which can be a big deal if you’re using it for medical purposes. Older weed can also undergo changes in taste and texture.

When stored properly (more on this later), dried cannabis keeps for 6 months to 1 year. Over time, it begins to lose its aroma and potency.

According to some older research, weed loses roughly 16 percent of its THC after 1 year, and it just keeps dropping from there:

  • 26 percent THC lost after 2 years
  • 34 percent THC lost after 3 years
  • 41 percent THC lost after 4 years

It’s mostly in the smell. Weed that’s past its prime will smell different or lose its aroma entirely. Some weed might even smell and taste harsh when it’s been sitting too long.

Its appearance can also give you a clue as to whether or not it’s old. Fresh weed shouldn’t crumble or feel spongy when you break it off. If it does, it’s old and either too dry or too moist.

Consuming it shouldn’t harm you, but be prepared for changes in texture and potency. The exception is weed that’s grown mold, which could potentially make you sick.

Mold is often hard to see unless you look very closely. It typically looks like white powdery or fuzzy spots, some of which can be pretty small.

Moldy weed usually smells musty, kind of like hay. It also tends to have a bit of an “off” taste.

Even if your weed isn’t super old, it’s best to do a mold inspection. A study by researchers from University of California, Davis found bacteria and mold on 20 cannabis samples bought from dispensaries and pot growers in Northern California.

Mold on weed isn’t likely to cause major health problems, but it can lead to nausea, vomiting, and coughing.

In people with weakened immune systems, inhaling smoke or vapors from weed containing bacteria or fungi could cause serious illness or even death.

If it looks or smells off, then you’re better off tossing it, even if you just bought it.

Light, humidity, temperature, and oxygen can all mess with cannabis and affect its aroma, taste, and potency potential.

Here’s what to consider when storing weed to help keep it fresh and maintain its quality for as long as possible.

Choose the right container

Ditch plastic baggies and containers. Plastic holds static that can affect delicate trichomes — the tiny, crystal-like hairs on flowers that produce cannabinoids and terpenes — and mess with potency.

And forget those funny little tins, too, because they let in too much oxygen.

Glass jars with an airtight seal, like mason jars, are the way to go. They don’t have any static charge and limit oxygen exposure. Plus, they’re inexpensive and easy to find.

Most dispensaries also sell containers designed to keep weed fresh for as long as possible.

If you have kids or pets in your household, invest in a child- and pet-proof container.

Watch the humidity

Weed is best kept at a relative humidity of 59 to 63 percent. Any higher and you run the risk of trapping moisture, which can lead to the growth of mold. Anything lower can cause your weed to dry out.

To help you preserve your stash, you can add humidity packs to your containers if you really want to get fancy. You can also go the extra mile and store your weed in a humidor made specifically for cannabis.

Keep it cool, dark, and dry

Keeping weed in a cool and dry spot away from sunlight is as important as the container you use, if not more so.

Direct sunlight can cause cannabis to break down, and too much heat can hold moisture and lead to mold.

Keeping it somewhere too chilly, on the other hand, could dry it out and lose those precious trichomes, which is why the fridge and freezer aren’t recommended.

Aim to store cannabis in a dark place, like a closet or cabinet, with a temperature below 77°F (25°C).

Weed doesn't go bad in the way perishable food does, but it can definitely degrade over time. Here's what to look for.