Influence of ultrasound-assisted osmotic dehydration pre-treatment on total phenolic content, antioxidant capacity and p-cymene content of eucalyptus deglupta

Abstract

Convective drying is widely used in the drying of medicinal plants, however, long duration and high temperature often compromise the quality through degradation or loss of bioactive antioxidants. This problem can be overcome by incorporating pre-treatment such as ultrasound-assisted osmotic dehydration (UAOD) to efficiently and effectively remove excess moisture. Therefore, the main objective of the current research is to optimise the incorporation of UAOD pre-treatment before convective drying of Eucalyptus deglupta leaves for enhanced retention of phenolic compounds, p-cymene yield and antioxidant activity. This was achieved by evaluating the effect of four independent variables: UAOD temperature (20-60 °C), UAOD duration (60-100 min), ultrasound intensity (198-330 W) and UAOD sucrose concentration (30-50% w v-1). Optimal UAOD conditions were achieved at a temperature of 58.18 °C, duration of 60 min, ultrasound intensity of 330 W and sucrose concentration of 36.02%. The optimum UAOD pre-treatment produced higher total phenolic content, antioxidant activity and p-cymene yield compared to convective drying on its own by two-fold. Lewis model predicted the drying kinetics with the highest accuracy based on R2 (0.9889) and RMSE (6.52 × 10-8). The results justify the inclusion of UAOD pre-treatment before convective drying for improved phenolic compound extraction with high p-cymene yield and antioxidant activity from Eucalyptus deglupta.