Alkamide stability in Echinacea purpurea extracts with and without phenolic acids in dry films and in solution

Degradation of the major alkamides in E. purpurea extracts was monitored under four different accelerated storage conditions, phenolic-depleted and phenolic-rich dry E. purpurea extracts and phenolic-depleted and phenolic-rich DMSO E. purpurea extracts at 70, 80, and 90 degrees C. Degradation of alkamides followed apparent first-order reaction rate kinetics. Alkamides degraded faster in dry films than in DMSO solution. The phenolic acids acted as antioxidants by limiting the loss of the alkamides in dry E. purpurea extracts. In contrast, E. purpurea alkamides in DMSO degraded faster when the phenolic fraction was absent. The overall order of degradation rate constants was alkamides 1 approximately 2 approximately 6 > 9 approximately 8 > 3 approximately 5 approximately 7. The energy of activation (Ea) predicted for alkamide degradation averaged 101 +/- 12 kJ/mol in dry films +/- phenolic acids, suggesting the oxidation mechanism was the same under both conditions. In DMSO solutions, Ea values were about one-half of those in dry films (61 +/- 14 kJ/mol), suggesting a different mechanism for alkamide oxidation in solution compared to dry. Predicted half-lives for alkamides in extracts suggested very good stability.