What You Can Learn From Tobacco Curing

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Cannabis used to be so colorful: Acapulco Gold, brown Colombian, bright green Oaxaca big bud and, our personal favorite, redder-than-red Thai stick. The colors were the result of a variety of drying techniques that often involved drying the product under the sun, whose energy enacted chemical changes in the plant material, changing its color. This change is not something we see much of today because everyone cures in the dark to purposely avoid color change. Maybe it’s time to turn back the hands of time and see if we can learn anything.

Ask anyone what is happening in the drying room, and they will all answer “moisture removal.” Ask them what is happening in the cure room, and the answer, more often than not, is going to be “burping.” Despite a lack of specific knowledge about what is actually happening to the plant during the process, some growers say they are able to change and enhance the potency and effects during cure. If they can, there must be some chemical reactions going on. This would be really exciting because if we can identify and understand those reactions, we can leverage them. Let’s see if we can find any signs of those reactions.

A Cure-ious Comparison

The current wisdom on cannabis curing seems to be borrowed from tobacco curing practices. Cannabis producers use the same terminology used in tobacco processing. But under the hood of tobacco curing, terminology is where the similarities with cannabis curing end.

Being the most researched horticulture commodity in the world, there is plenty of tobacco curing research to dig through. The Wikipedia entry on tobacco curing has a good summary: “Curing and subsequent aging allow for the slow oxidation and degradation of carotenoids in the tobacco leaf.”

The oxidation of carotenoids produces color changes, which explains why tobacco curing is often called color curing. If we are looking for signs of chemical reactions, the color change is a clear sign of them. In contrast, cannabis that comes out of cure the same color it went in suggests that compared to tobacco, cannabis is not undergoing significant chemical changes in the typical dry and cure process. Compared to four to eight weeks of tobacco drying and curing, sometimes followed by a fermenting process, we can guess that the chemistry of cannabis material dried in one week does not change much before moisture levels drop below where biological processes and many chemical reactions can continue.

Tobacco curing does not just change the color of the leaves, it changes the flavor and aroma as well. As carotenoids degrade, they produce aromatic compounds that contribute to the tobacco’s distinctive aromas and flavors. Curing also changes the amount of sugars in the leaves, which also affects flavor. Add practices such as flue curing (heat treatment) and fermentation to the picture, and we can see that tobacco manufacturers employ a wide range of processes to produce leaf-chemistry changes. In comparison, the quick drying processes we use with cannabis have little chance of inducing chemical reactions.

In tobacco, the color-change period happens first and at humidity levels of 80 percent to 90 percent, and temperatures up to 105°F. The drying phase happens next at temperatures up to 140°F, and the final drying phase happens at temperatures above 140°F.

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Tobacco leaf is commonly 25-percent sugar by weight, according to “Guidelines for Temperature, Humidity and Airflow Control in Tobacco Curing” by researchers at the University of Georgia. Flue-cured tobacco produces higher sugar content, while air and sun curing produce lower sugar content in the leaves.

If sugar levels can affect smoking smoothness in tobacco, maybe controlling it in cannabis could provide similar effects. To find out, we would need to look at using slower dry-down rates and heat applications, and, possibly even try to ferment the product to see exactly what changes we can induce in the plant’s chemistry. We would also need to examine whether any of those treatments produces a clearly superior cannabis product.

One thing is abundantly clear, whatever we are doing in our cure spaces, it’s not the same thing happening in tobacco curing barns.

Calling All Experimenters

The drying step for cannabis removes the majority of moisture from plants, and the humidity and temperature of the drying environment controls how fast moisture migrates out of the plant material. As opposed to cannabis, tobacco curing happens under the influence of heat, so here is something for someone to experiment with: heat curing. For that matter, someone should also try cold curing.

During the drying phase, water lost from the plant material carries with it aromatic compounds including ketones, alcohols, esters, terpenes and more. These terpenes are not the ones we typically talk about in the plant’s resin. They are contained in the plant material, like tobacco. It is reasonable to expect that as the “plant tissue” aromatics diminish, the aroma of the dried material becomes dominated by the terpenes contained in the resin on the trichomes.

This causes us to wonder if the dreaded “grassy” or “hay field” smell is the signature aroma of drying plant tissues, while the lasting aromas we get even after a year in storage are due to the terpenes contained in the resin that exists outside the plant material. Evaporation is not a chemical reaction (it’s a state, or phase change), so we don’t see much that suggests chemical reactions happening in during drying. We can, however, recommend a test for growers to vary the dry/cure conditions and monitor how long it takes for unwanted odors to dissipate, and which variable is responsible for the difference.

Test #1

Chlorophyll is a great place to look for chemical reactions. Popular literature ascribes unpleasant tastes to chlorophyll in leaves, and many people talk about getting rid of chlorophyll as a key goal of dry and cure. The green product rolling out of the typical cure room tells us chlorophyll is clearly not being destroyed, and people don’t seem to be turning it down, so does chlorophyll really play a role in aromas?

To find out, take some material and dark-cure it. Also cure some of the same material in sunlight, or under lights, until you get to the desired moisture level. Odds are that the light-cured material will not be the same color as the dark-cured material. The loss of the green color is the indication that the chemistry of the material has changed as the sunlight broke down the chlorophyll molecules, removing the “green.”

Now for the taste test. If the light-cured material gets higher marks from testers than the dark-cured material, we have fairly conclusively shown that chlorophyll is an important variable in the aroma equation—and that maybe we should be adding lights into our drying spaces immediately. If the tastes and aroma are largely the same for both green and not-green product, we will know chlorophyll is not the place to look for taste and aroma enhancements.

Test #2

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Where does this leave us? In need of testing that enables cultivators to relate drying and curing techniques and schedules to aroma. First, dry some flower quickly, and compare that to flower dried over a six-day period. This will quickly establish how important the dry period is and provide feedback for fine-tuning.

We are very aware that the typical quick-dry times for cannabis are driven by a financial pressure that is never going to let up. That pressure is going to require anyone trying to lengthen their dry and cure time for better quality to have some pretty strong evidence that a longer dry/cure process is going to produce more revenue. A few simple tests seem reasonable to validate the current dry time and resulting quality, or indicate that minor changes in dry and cure have a potential worth that warrants further experimentation.

The Lab Test

What about the claims of better potency and altered effects? With labs now readily available to growers, claims can be validated or bounced easily. So when looking to dial-in a cure room, lab results will tell us if product chemistry is significantly different before and after cure. No more guessing.

Drying and curing processes are not insignificant to production costs, and that makes them a lever. Since you can move anything with a big enough lever, we need to find out if dry and cure offers us a little leverage on quality—or a lot.

July 2017
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