Enhancement of phytochemical content and drying efficiency of onions (Allium cepa L.) through blanching

BACKGROUND: This study investigated the effect of blanching (60, 70 and 80 °C for 1, 3, 5 and 10 min) combined with oven drying at 60 °C on the phenolic compounds, antioxidant activity, colour and drying characteristics (drying time, drying rate constant, effective moisture diffusivity and activation energy) of onion slices. RESULTS: Blanching of onion slices at 60 °C for 3 min and at 70 °C for 1 min prior to drying increased their bioactive compounds and antioxidant activity compared to the control samples and other treatments. Eighteen drying models were evaluated. The Modified Page and two-term exponential models best represented the drying data. The effective diffusivity ranged from 3.32 × 10⁻¹¹ m² s⁻¹ (control) to 5.27 × 10⁻¹¹ m² s⁻¹, 5.01 × 10⁻¹¹ m² s⁻¹, and 4.74 × 10⁻¹¹ m² s⁻¹ for onions blanched at 60 °C, 70 °C and 80 °C, respectively. The higher activation energy was observed for the control (unblanched) sample and slightly lower values were found for 1 min- and 3 min-blanched samples, confirming the higher drying efficiency as a result of the blanching pre-treatment. CONCLUSION: The use of blanching as a pre-treatment before drying of onions resulted in enhanced phytochemical content and drying efficiency.