It’s Not Too Late: Post-Harvest Solutions to Microbial Contamination Issues: Page 3 of 5

December 16, 2019
Abstract / Synopsis: 

Passing state regulations for microbial contamination can be challenging. Excessive levels of mold, yeast, and bacteria can cause health problems for consumers. What happens when your flower doesn’t measure up to your state’s standards? You do have post-harvest treatment options that can save your buds. Microbial contamination can be remediated without damaging your final product.

Killing Microbes in Post-Harvest Cannabis

There are many methods to reduce total mold and yeast count (TMYC) in post-harvest cannabis, but the two written about below seem to have no impact on the quality of the flower (terpenes, cannabinoids, and appearance). They are effective treatments and are not cost-prohibitive. The decontamination machines may be leased or purchased.

X-Ray Chamber Decontamination
X-ray chamber decontamination is highly effective and has a history of use in medicine and even airport security. X-rays are a form of high-energy electromagnetic radiation, that is, short wavelength light. X-ray wavelengths are shorter than those of ultraviolet (UV) radiation. This means they pack more punch with an intrinsically higher level of energy and therefore are much more effective in killing microbes than UV. The instrumentation for X-ray decontamination is well-tested and uses technology that has been proven for decades. The cannabis is placed in a lead-lined chamber that ensures the safety of the operator during the decontamination process. X-rays are produced from an internal vacuum tube once the lead lined chamber is sealed. The X-rays penetrate the cannabis and kill the microbes. The system is foolproof and does not allow the operator to be exposed to X-rays. It is highly effective in the destruction of the full complement of microorganisms to be tested at the state level. After the processing by X-ray chamber decontamination, your cannabis will be ready to pass the state tests.

Ozone Chamber Decontamination
Ozone chamber decontamination has a history of use in agriculture to help keep the food supply safe. Ozone is highly effective in reducing the levels of TYMC to meet state standards. Ozone gas (O3) is a highly reactive oxidizing molecule, meaning it is harmful to cells. It destroys the microbes’ cell wall, which enables the ozone to destroy all of the cell’s components: enzymes, proteins, DNA, and RNA. An ozone chamber generates its ozone gas as needed in a foolproof sealed chamber that cannot be operated until the door is sealed. Once placed in the chamber and sealed in, the cannabis will be exposed to ozone, and the microbes on the surface will be killed. Any ozone not consumed by the disinfection process is deactivated by the system before the door can be opened. This method greatly reduces the number of microbes but does not reduce the number to zero. Ozone chamber decontamination has been demonstrated to be effective against the full range of microorganisms expected to be tested by the states (4).

Controlling Microbial Growth Inside Packaged Cannabis Products

In the rare instances when regulators find contaminated products, such as flower and pre-rolls, on the shelf from lots that have previously met state standards, it is likely because of post-harvest environmental control of the products. A measurable value, called water activity, must be considered in assessing the probability of on-going microbial growth even while packaged. With an out-of-spec water activity value, plant material that passed the 10,000-count-limit test with a count of 7000 need only double over a period of weeks to render the material noncompliant with a measure of 14,000. The microbial growth that can take place after packaging is the reason that 11 of 29 states now require the testing of water activity, a measure that indicates whether continued microbial growth of the packaged cannabis is likely. We anticipate that additional states will include water activity in their slate of mandated testing.

  1. K. McKernan, Y. Helbert, H. Ebling, A. Cox, L.T. Kane, and L. Zhang, “Microbiological examination of nonsterile Cannabis products” (2018).
  2. T. Mitchell, Westworld, October 30, 2019
  3. G.D. Molim, M. de Souza Braga, et. al., Curr. Pharm. Des. 22(27), 4264–4287(24) (2016).
  4. A.J. Brodowska and K. Smigielski, “Ozonation-an alternative decontamination method for raw plant materials” Food Sciences and Biotechnology (2013).
  5. B.P. Carter, Cannabis Science and Technology 2(4), 30–35 (2019).
  6. C. Hellerman, PBS News Hour,
  8. Y. Gargani, P. Bishop, and D.W. Denning, Mediterr. J. Hematol. Infect. Dis. 3(1), e2011005, (2011).
  9. R. Kern, Cannabis Science and Technology 2(4), 16–29 (2019).

About the Coauthor

Dr. Jacklyn R. Green is a project manager, systems engineer, and scientist focused on today’s commercial cannabis enterprise. She is the cofounder and Chief Executive Officer of Agate Biosciences LLC, an agricultural consultancy. She earned her PhD at The University of Texas in astronomy, and subsequently applied her big-picture view and systems approach to problem solving at NASA’s Jet Propulsion Laboratory for more than 25 years. In her tenure there, she conducted detailed scientific research, managed complex projects and programs, and led teams of engineers and scientists to solve near impossible problems in a collaborative team setting. Now, she brings the best practices of management and systems engineering to cannabis businesses enhancing their financial and technical success in today’s stringent government-regulated environment.

About the Columnist

Roger KernDr. Roger Kern is a scientist and technologist who cares deeply about the cultivation and health of plants in the cannabis industry. With his PhD in microbiology from the University of California, Davis, Plant Growth Laboratory, he solves the most challenging problems in hydroponics, from studying the root microbiome to developing nutrients and lighting systems to ensure plant health and a disease-free lifecycle. He spent 22 years at NASA’s Jet Propulsion Laboratory as a scientist, technologist, and research leader before becoming the President of Agate Biosciences, a consulting firm for project management, systems engineering, and science in CEA for the past eight years. He leads developments to optimize sustainability, consistency, quality, and yield without compromising plant health. Direct correspondence to: [email protected]

How to Cite this Article

R. Kern and J.R. Green, Cannabis Science and Technology 2(6), 15-19 (2019).