Effects of different drying methods on extractable phenolic compounds and antioxidant properties from lemon myrtle dried leaves

Lemon myrtle (Backhousia citriodora) is one of the most commercially grown native herbs in Australia. This study aimed to evaluate the effects of different drying methods on phenolic compounds and antioxidant properties of lemon myrtle leaves to identify the most suitable drying conditions. The drying methods include hot air drying, vacuum drying, microwave drying, sun drying, shade drying and freeze drying. The results showed that drying conditions significantly (p < 0.05) affected the retention of total phenolic content (TPC), total flavonoids (TFC), proanthocyanidins, gallic acid, hesperetin, and antioxidant properties of lemon myrtle leaves. The optimal conditions for hot air drying and vacuum drying are 90 °C for 75 min and 90 °C for 120 min, respectively; whereas optimal drying conditions for microwave drying are 960 W for 7 min, and the time required for sun drying and shade drying are 2 days and 12 days, respectively. The freeze dried leaves contained the highest level of TPC, TFC, proanthocyanidins, gallic acid and hesperetin (74.11 ± 2.87 mg GAE/g dw, 87.15 ± 2.70 mg CE/g dw, 123.49 ± 6.12 mg CE/g dw, 53.77 ± 0.22 mg/g dw and 38.99 ± 0.26 mg/g dw, respectively). The freeze dried leaves also contained higher antioxidant capacity as compared to other samples. No significant difference in phenolic compounds and antioxidant capacity was observed between tested other drying methods. Therefore, any of these methods can be selected for dehydration of lemon myrtle leaves for industrial purposes. However, microwave drying can be selected for drying of lemon myrtle leaves for an industrial scale as it was the most time and/or energy efficient technique.