As produced by the cannabis plant, the cannabinoids are produced in carboxylic acid form, which means that there is a carboxyl group (COOH) attached. Note the THCa molecule below with the COOH carboxyl group in the upper right hand corner.
THCa Molecule above, THC molecule below:
Notice the carbon with a single bond to the THC molecule, a double bond with one of the oxygens, and a single bind with the remaining oxygen, which has its other bond taken up with a hydrogen atom.
That Carboxyl link must be removed for applications requiring THC to have access to the CB-1 receptors in our brains, because the THCa molecule can't pass the blood brain barrier.
Removing that Carboxyl link is called decarboxylation and is typically accomplished using heat. The following is a graph from the Journal of Chromatography, courtesy of Jump117, which shows five different decarboxylation time and temperature curves.
Journal of Chromatography decarboxylation graph
Note that the total THC for each curve peaks and then starts down again. That occurs at approximately 70% decarboxylation, where it was previously ASSumed that after 70% the rate that remaining THCa converting to THC was slower than the rate that existing THC was converted to CBN. Lower THC and higher CBN makes the medication more sedative.
Enter the Swiss Institute of Legal Medicine study, as well as the European Industrial Hemp Association paper on Decarboxylation of Tetrahydrocannabinolic acid (THCA) to active THC, and that perception was changed. What it shows is that about 70% is the maximum yield, because THC is also being converted to CBN at the same time.
The reason for the plunge in THC level after 70%, is that around 30% THC has be degraded reaching that peak, and further processing continues to degrade existing THC to byproducts eventually leading to CBN.
Institute of Legal Medicine Study
European Industrial Hemp Association Paper Graph
The charts of course are just an estimate, because material decarboxylates naturally with time and drying, so every case is different. It is the best estimate we have for decarboxylating plant material in an oven.
To decarboxylate plant material in an oven, preheat the oven to 250F and spread the plant material in a thin layer in a cookie tray. Place in the center of the preheated oven and bake for the prescribed period of time, stirring once in the middle.
Decarboxylating concentrates after extraction is more precise in that you can determine your decarboxylation progress by watching the CO2 bubbles produced as a byproduct of decarboxylation. The COOH link doesn’t just fall off, it breaks down and is given off in the form of CO2 and H2O.
These bubbles are small equally sized fizzy bubbles as opposed to solvent bubbles, which are random sized an some quite large. If you are watching their production, and keeping them stirred and dissipated, you will note a sudden significant reduction in bubble production when the solution hits the top of the curve and starts down the other side.
That is about 70% and time to stop if you are seeking maximum THC levels, but if you want a more sedative concentrate, continue until no more bubbles are produced.
Here is a picture showing the random sizes of solvent bubbles, followed by one showing CO2 bubbles from decarboxylation. This concentrate is being decarboxylated after extraction in a stainless bain marie pot, sitting in a 250F hot vegetable oil bath.
Larger random sized solvent bubbles
Below is a good example of the small fizzy CO2 bubbles from decarboxylation. The larger bubbles present are just an accumulation of small bubbles and if you keep the pool stirred so as to keep them dissipated, the larger bubbles don’t form and you can better tell exactly when the bubble action drops off.
Fizzy CO2 bubbles from decarboxylation
As you approach 70% decarboxylation, the CO2 bubble production will suddenly start to drop off, at which point it is time to remove the concentrate from the hot oil.
Lastly, below is what fully decarboxylated quiescent oil looks like.
Fully decarboxylated and quiescent