Decarboxylation of Tetrahydrocannabinolic acid (THCA) to active THC

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Motivation and Summary

The background of this study is as follows: On average hemp food stuffs contain up to 90% of the nonpsychoactive THCA. Nonetheless, regulation for THC limits in food often uses total THC = THCA + THC. This leads to an overestimation of the content of psychoactive THC in hemp food. The argument is that THCA can be transformed in THC after heating (decarboxylation). This
study analysed how relevant this conversion is in realistic scenarios.

Figure 2 shows the results for the complete decarboxylation depending on temperature and time. One realistic example scenario demonstrates, that the total THC measurement method can lead to an overestimation of the content of psychoactive THC of ca. 60%.

To evaluate to what extent the THCA content of hemp food stuffs can influence their THC-levels, we performed a literature survey about the THCA carboxylation to THC (Fig. 1). In a second step we plotted these on a timetemperature graph using two different methods. On the one hand, curve fitting with various mathematical functions was applied, where logarithmic trendlines had
the best R-squared values of 0.99. This was in line with the behaviour of a first order reaction and research about reaction kinetics of the decarboxylation reaction proves this (Perrotin-Brunel et al., 2011).

After thorough research it became apparent that the most publications concerned with THCA decarboxylation to THC after heating, were papers describing HPLC or GC methodologies and mostly only had one timepoint and temperature where they measured THCA and THC. Figure
2 shows the results of the literature survey of studies which had at least two timepoints and temperatures. What the graphic means Taking the combined trendline into account, it would
need 3 hours at 100 °C to convert THCA fully into THC and 4 hours at 98 °C. At high temperatures above 160 °C only about 10 minutes and at 200 °C only seconds are
needed to convert THCA fully into THC.

It has to be pointed out that various techniques were used  in the THC measurements e.g. Veress and colleagues (1990) used THCA in hexane on a glass plate whereas Taschwer et al. (2015) used confiscated cannabis and heated it in a closed cabinet. These differences could explain the discrepancy in some temperatures and times.
Moreover, the different cited papers use various units of THC measurement e.g. mg/g vs. percentages. The graphic shows the assumed complete THCA decarboxylation
to THC, where we used the peaks shown in the graph of Veress and colleagues (1990), for example, to indicate complete decarboxylation.

Side reactions affecting THC levels It is important to note, that starting from ca. 157 °C THC
evaporates. Consequently, the peak THC levels given here, are only present in the sample for a short amount of time.

For instance, the highest THC content is reached at 145 °C after 7 min, but after 40 min the total THC amount is already halved (Veress et al., 1990). Taschwer and colleagues (2015) show in their experiments peak THC levels after 3 min heating at 150 °C, but a return to near zero THC
percentages after 7 min. Figure 1: Decarboxylation reaction of THCA to THC (adapted from Perrotin-Brunel et al., 2011).


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