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. © 2017 Society of Chemical Industry