Join Dr. Nadia Sabeh and Kaitlin Urso of the Colorado Department of Health and Environment as they discuss odor control in the cannabis industry! This webinar was originally hosted on April 28, 2020.
Recently, I had an interview with Mary Kate McGowan of the ASHRAE Journal. The American Society of Heating, Refrigerating and Air-Conditioning Engineers is an enduring organization, and we were honored to have our story featured on their April cover! To access the annotated article, click below, or contact me for more information.
Why it's Important:
1. Facilitates gas exchange
2. Mitigates mold growth
3. Increases weight of leafy greens
Air flow and air movement is important for facilitating gas exchange between your plants and enviornment. Air movement helps to deliver carbon dioxide to the leaves, while transporting away water vapor produced by evapotranspiration. In this capacity, air movement enhances photosynthesis and nutrient delivery within your plant. Because air movement transports moisture away from the plant, it also helps to prevent growth of certain pathogens, such as molds and bacteria.
Air movement also helps to build up the cellular structure within the vegetative structures of your plant, including the leaves and stems. Therefore, it is important to move air across the plants at the right speed: If you are primarily selling the weight of leafy greens, higher air velocity will bulk them up. But if you are primarily selling fruit weight of tomatoes or cucumbers, then you only want enough vegetative structure to support your plant, without comprising the reproductive yield or quality of your crop.
Therefore, the desired air speed (or velocity) across the plants will depend on what type of plant you are growing. In general, the desired range of air speed is 0.3-0.5 m/s (60-100 fpm).
How It's Achieved:
2. Wind and Buoyancy
3. Air ducts
In a greenhouse, air movement is achieved both through the ventilation process (with mechanical fans or natural ventilation), as well as the use of horizontal air flow (HAF) fans. HAF fans are typically hung from the greenhouse structure (eg. trusses) and configured to blow air in a circular pattern around the greenhouse and above the top of the plants. These fans provide air circulation to create a more uniform environment, and help to break up the stagnant layer of air that would otherwise build up at the top of the plants. Duct socks may also be installed beneath grow benches or above the crop to deliver air from a central heating and/or cooling system.
In a vertical farm, air movement is more challenging because the racks themselves obstruct airflow and also due to the chimney effect, causing the top grow racks to be warmer than the bottom racks. Many growers opt to attach small computer fans (or similar) to the racks to blow air directly over the racks. Air ducts can also be designed to deliver air uniformly and consistently over the racks without the need for many, small pieces of equipment.
Odor control is one of my favorite topics to discuss in the cannabis industry. Not because we have it figured out or because it’s easy to implement. Quite the contrary; it is one of the most challenging aspects of designing a cannabis facility, precisely because it hasn’t been figured out.
Up until a few years ago, there were about 30 compounds that had been identified as generating the distinctive aromas of the cannabis flower, including limonene, α-pinene, and β-myrcene. Recently, research out of Iowa State1 identified over 200 new volatile compounds emitted from cannabis, whose odors were rated from practically imperceptible to incredibly intense. Not surprisingly to most growers and connoisseurs, the concentration of these compounds changed over the course of drying, such that different smells were more perceivable when fresh, while others became more prominent after desiccation (curing and storage).
One of the more interesting findings of this study demonstrated that some of the most “offensive” odors to the human nose are found in such small concentrations that they were barely detectable by mass spectrometry. In fact, the five most “characteristic” aromas detected by the human nose, “were not the most chemically abundant.” That’s right: The human nose is so sensitive and attuned to the cannabis plant’s aromatic compounds, we are better at detecting them than the advanced technology created to measure them.
What’s more, of the compounds detected, nearly 70% of them have very little information documented in the chemical databases, including their odor detection threshold. This lack of data is most likely because they are found in such small quantities that they did not warrant much thought or research. It turns out these molecules pack a punch, claiming responsibility for much of what we consider “odor.”
How do we control something we cannot measure or detect with modern instrumentation?
The current industry standard is to throw the kitchen sink at the problem, in hopes that available technology will adsorb, unravel, or otherwise eliminate the aromatic compounds emanating from the plant. The problem is that many of these devices are either ineffective (air filters), deficient in humid environments (carbon filters), energy intensive (UV irradiation and constantly running fans), or downright dangerous to humans (ozone). Additionally, these devices do not specifically target the odor-producing compounds we want to remove. They remove everything, good or bad, large or small, pungent or pretty.
So what can we do?
The first step is to continue the research and create new pages in the catalog of chemical compounds that includes Cannabis. Second, measuring tools need to be developed that have greater resolution and sensitivity to these compounds and the concentrations in which they can be found. Only then – after we know what and how much – will we be able to develop technologies that can effectively control odors through targeted elimination.
1. Rice S, Koziel JA (2015) Characterizing the smell of marijuana by odor impact of volatile compounds: An application of simultaneous chemical and sensory analysis. PLoS ONE 10(12):e0144160 https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1994&context=abe_eng_pubs
More often than not, E. coli outbreaks in lettuce, spinach, and other leafy greens are sourced back to a field-grown operation in Arizona or California; two regions that, combined, grow 98% (!) of the lettuces consumed in the U.S. Inherently, these fields are susceptible to the risks of contamination from animals pooping in a field (probably while munching on some salad), polluted irrigation water (also probably caused by animals – dead or defecating), and poorly enforced sanitation practices.
Even if just one lettuce plant is contaminated, the chance that other lettuces get contaminated is very high. Not only are the lettuces comingled into a single box or container in the field, but they then follow a long and diluted chain of custody route from the farm to the packing shed, off to regional distribution centers, and then trucked to grocery stores and markets, all before it ever lands on your dinner plate (and that was a condensed version). And in the case of packaged lettuce, like your favorite Caesars mix, those lettuces may have originated from 5 or more farms. That’s a hard bread crumb (or crouton) trail for the FDA to follow.
Enter Controlled Environment Agriculture, or CEA. This method of farming shelters crops from the outdoors by growing plants in greenhouses or buildings. The simplest term used – and almost lost to history – is “protected agriculture,” a term that is most appropriate in the discussion of food safety. By farming inside buildings – regardless of whether it’s made of glass, plastic, or concrete – the risk of animal contamination is all but eliminated. Risk of water pollution also practically evaporates to nothing; as typical hydroponic irrigation systems are built with filters and purification systems to remove any potential contaminants before water is delivered to the plants. Water is also delivered directly to the roots, rather than sprayed or flooded over a field; so even if the water was contaminated, it wouldn’t get on the parts we eat.
As for the chain of custody concerns, they practically disappear with CEA-grown crops. Most, if not all, of produce grown indoors is marketed under a single, specific brand, with little to no comingling of produce sourced from different locations. In fact, that would be a pretty challenging feat, considering the hyper-local market many of these farms serve. Not only do indoor and greenhouse growers brand their product, they also label where it’s grown to appeal to their local market. Indeed, during this most recent E. coli outbreak, indoor farms and greenhouses were the first lettuce producers crossed off the FDA’s list of possible offenders.
The current conversation around indoor food production is often overshadowed by the debate over energy use and whether or not it justifies the reduction in carbon footprint by eliminating long food miles. But the recent E. coli outbreak in Romaine lettuce has highlighted an often overlooked potential benefit of growing indoors: improved food safety. As consumers continue to eat more fresh produce, food safety is going to become a greater factor in their purchasing choices. And until deer or raccoons can figure out how to open doors or cut through plastic, greenhouses and vertical farms will stand as a reliable source of safe and healthy fresh produce.
Humidity control is one of the most important factors in designing and operating an indoor farm. The amount of moisture in the air (aka “humidity”) affects the transpiration rate of plants, which is responsible for moving water and nutrients from the root zone to other parts of the plant. When the humidity levels are too high or too low, transpiration will slow, inhibiting plant health, growth, and development.
There are many terms used to describe the moisture content of the air, including relative humidity (RH), absolute humidity (AH), and dewpoint temperature. In horticulture, vapor pressure deficit (VPD) is most commonly used, as it describes the relationship between water in the air and water at the surface of the leaf. VPD is the difference in water vapor pressure at a given air temperature and RH and the vapor pressure at the same air temperature when it is saturated with water (100% RH). This difference in vapor pressure is literally what “forces” water through the plant.
The majority of horticultural crops are C3 plants, which have evolved to thrive in temperate climates with a good source of water (as opposed to CAM plants, such as cactus, which evolved to thrive in hot and dry climates). Much research has been performed demonstrating that, in general, horticultural crops enjoy a VPD within the range of 0.65 to 1.25 kPa when well-watered. For vegetative plants, such as leafy greens, they tend to do better in the lower half of the range. Whereas fruiting and flowering plants, such as tomatoes and cannabis, tend to like the VPD in the higher range, especially when they are mature crops. When the VPD is lower than 0.65 kPa, stomata begin to close, restricting transpiration and CO2 assimilation. When the VPD is higher than 1.25 kPa, the plant will begin to stress and, in an effort to conserve water, will also close their stomata.
Many growers are afraid of “high” humidity levels, for fear of harboring molds and bacteria, and they will try to drive the RH as low as possible to avoid these pests. Unfortunately, this action can be counterproductive. Just like over-worked humans who have suppressed immune systems, plants that are water-stressed will be more susceptible to microscopic invaders and diseases. And just like your mom telling you to “drink lots of water” and “eat chicken soup,” the same prescription is good for plants. By properly managing the moist air environment and providing an adequate supply of water at the root zone, plants can freely absorb water and nutrients, build up their defense systems, and put more resources into carbon fixation, photosynthesis, and reproduction.
When it comes to managing humidity, plants are like Goldilocks: they like it “just right”; not too high and not too low.