Effect of different drying methods and conditions on drying performance and volatile retention of Backhousia citriodora (lemon myrtle) leaves

Abstract

The effect of different drying methods i.e., oven (OD), vacuum (VD), and heat pump assisted (HPD) drying on lemon myrtle leaf (LML) as well as the retention of volatiles in the dried product was investigated. Following a constant rate period, each method entered the falling rate period, whereby drying was governed by internal diffusion. The moisture diffusivity (Deff), calculated using Fick’s second law, was found to fall in the range of 8.07 × 10-10 to 4.35 × 10-9 m2/s across the varying drying methods and conditions. The activation energy (Ea), calculated using the Arrhenius equation, was 13.42, 45.41, and 72.85 kJ/mol for HPD, VD, and OD, respectively. Essential oils (EO) were obtained from dried LML through the simultaneous distillation and extraction (SDE) method, and thereafter, the EO was analysed using gas chromatography-mass spectrometry (GC-MS). Citral, an important volatile in LML, was among the EO components identified. Commonly found in its cis- and trans- forms, citral retention differed between the drying methods. The highest retention of volatiles was observed in the HPD method, followed by VD and OD. As high citral content is indicative of successful post-harvest processing, and given its ability to retain such volatiles in dried LML, HPD is therefore the most suitable drying method for LML drying. HPD can also be applied for the preservation of various agricultural products especially heat-sensitive herbs. The waste heat recovery ability attracts agropreneurs’ attention on adapting HPD into their industry. The versatile feature of this controlled dryer not only ensures good quality herb products, but is also economical, efficient, and time-saving.