Effect of Heating Time on Phenolic Compounds, Flavonoids, and Antioxidant Activity of Aqueous Leaf Extracts of Eucalyptus camaldulensis Used as Biopesticides in Market Gardening in Burkina Faso

Abstract

The use of aqueous extracts of Eucalyptus camaldulensis as biopesticides requires a preliminary concentration step involving prolonged heating of the extracts. The aim of this study was to evaluate the effect of heating duration on the total phenolic content and total flavonoid content of the aqueous leaf extract of Eucalyptus camaldulensis. Aqueous extracts obtained by hydrodistillation of Eucalyptus camaldulensis leaves were heated under reflux for 0, 1, 2, 3, and 4 hours, then lyophilized. Flavonoids were identified using HPTLC. Total phenolic compounds, condensed tannins, and total flavonoids were determined using the Folin–Ciocalteu reagent, sulfuric vanillin, and aluminum trichloride methods, respectively. Antioxidant activity was evaluated using the DPPH radical-scavenging assay. After 4 hours of heating, the contents of total phenolics, condensed tannins, and antioxidant activity decreased simultaneously, whereas the flavonoid content increased accordingly. Total phenolic content decreased from 185.34 ± 7 to 146.95 ± 7 mg GAE/g of dry extract; condensed tannins decreased from 23.3 ± 1.4 to 13.02 ± 1.2 mg CE/g of dry extract. In contrast, flavonoids increased from 55.99 ± 0.03 to 73.11 ± 0.08 mg QE/g of dry extract. IC₅₀ values increased from 20.67 ± 0.5 to 56.33 ± 9 µg/mL, indicating a reduction in antioxidant activity with longer heating duration. This study shows that the extract concentrated after 4 hours of heating retains about 80% of its phenolic compounds, suggesting that it can be used in a biopesticide formulation.