The cannabis plant produces an amazingly diverse array of medicinal compounds, with over 100 known cannabinoids and 200 terpenes contained in its resin. However, the plant bio-synthesizes its cannabinoids in their acidic forms, such as THCA, CBDA or CBGA, which are characterized by the presence of a carboxyl group attached to the phenolic ring. These acidic cannabinoids can be rapidly converted into their ‘neutral’ and more medically potent analogues (THC, CBD, CBG etc) under the influence of heat or extended storage which causes loss of the relatively unstable carboxyl group in the form of carbon dioxide. This process is called “decarboxylation”, and is generally used when preparing extracts which will be ingested orally or used topically at low temperatures, and not vaporized or smoked.
A side effect of decarboxylation is the loss of terpenes, which are aromatic compounds in the plant thought to work synergistically with cannabinoids to alleviate symptoms. Terpenes are volatile oils which can evaporate at relatively low temperatures. Thus, when making extracts meant for ingestion at low heat, achieving effective decarboxylation while minimizing terpene loss presents itself as the most medically beneficial approach. For extracts meant to be vaporized or otherwise ingested at high heat, pre-decarboxylation is unnecessary, since the process occurs during ingestion; this results in more preserved terpenes reaching the patient.
According to a 2013 study  by Luigi L Romano and Arno Hazekamp, of University of Siena and Leiden University, respectively, a comparison of decarboxylation methods revealed that a mild water bath treatment, with cannabis flower in an open glass vial at a low boil (temp. 98-100°C) for 5 min, did not lead to significant changes in the acidic-to-neutral cannabinoid ratio. In contrast, an oven treatment with material heated at 145°C for 30 minutes resulted in a complete decarboxylation of the major cannabinoids detected. THCA, cannabigerolic acid (CBGA) and cannabichromenic acid (CBCA) had all fully converted into THC, cannabigerol (CBG) and cannabichromene (CBC), respectively. Further conversion of THC into its’ main degradation product cannabinol (CBN), which produces a sleepy effect, only took place to a small degree during the oven treatment.
Compared to the untreated control, monoterpenes (the most volatile class of terpenes) were reduced to about half of their original levels even after exposing the plant material to the boiling water bath for just 5 min. After the more intense oven treatment, only small traces of the monoterpenes terpineol, myrcene and terpinolene could still be detected. The less volatile class of sesquiterpenes was found to be more resistant to the mild water bath treatment. However, most of them were lost in the oven treatment, and only traces of gamma-cadinene and eudesma-3,7(11)-diene remained.
With this data, one can see that effective pre-decarboxylation of material while making extract will unfortunately result in an almost complete loss of monoterpenes. Heating the material at a temperature lower than 145°C for a period longer than 30 minutes, may be the best approach to preserve sesquiterpenes while also creating a medicinal extract with the right acid-to-neutral cannabinoid ratio for low-heat ingestion.
Because of the beneficial “entourage effect” that the terpenes create when interacting with cannabinoids, more studies on terpene preservation techniques using different solvents and heating processes should be a focus of the medical community, including experiments carried out with cutting edge cold-extraction technology which only expose the extract to 100°C for 30 seconds as part of the solvent evaporation process. This minimal exposure to heat offers the best terpene preservation method currently, although the extract produced must be decarboxylated through high heat ingestion or other means.
by Sama’a Djomehri
 Luigi L Romano, Department of Pharmacy, University of Siena, Italy,
Arno Hazekamp, Plant Metabolomics group, Institute of Biology, Leiden University, The Netherlands, “Cannabis Oil: chemical evaluation of an upcoming cannabis-based medicine”, Cannabinoids, Vol 7, Issue 1, May 5, 2013