Quality and energy efficiency evaluation of Rosmarinus officinalis L. by intermittent and continuous microwave drying: Polyphenol composition, bioactive compounds quantification, antioxidant properties, physical characteristics, and energy consumption

Abstract

Improper drying is one of the major causes of beneficial bioactive compounds' degradation in bio-crops together with high energy consumption. Preservation of the compounds is crucial in retaining the bioactivities of the herbs with diverse industrial applications. Continuous microwave drying (CMD), despite being effective and efficient, has not merged with the industry as process intensification due to the challenges of overheating. This study investigates the application of intermittent microwave drying (IMD) on rosemary and its compounds, to address the issues of overheating and the subsequent loss of bioactive compounds resulting from CMD. This technology is implemented through the application of microwaves in a periodic on/off manner. Although CMD had the shortest drying duration (>20 min) and the highest drying rate (0.431 kg H2O/kg DB min), IMD had the highest effective diffusivities (1.292 × 10−10 m2/s) indicating better moisture kinetics and evaporation. CMD exhibited favorable physical characteristics of shrinkage (83.40%), bulk density (669.23 kg/m3), and rehydration capacity (34%). However, both methods displayed high antioxidant activity (90.36%–93.78% inhibition). Higher compound concentrations of Carnosic acid (4.85 mg/g), Carnosol (6.86 mg/g), and Rosmarinic acid (14.36 mg/g) were achieved using IMD. Lastly, IMD had a higher energy efficiency (1.70%) than CMD due to drying by residual heat generation and redistribution. Practical application: Drying is a crucial first post-harvest process to preserve aromatic and medicinal herbs and plants for downstream processing. However, the sensitive and diverse nature of bioactive compounds makes it practically impossible to implement one drying technology to process all. Microwave technology is highly favored by researchers as well as industries to replace traditional convective hot air drying due to its effectiveness and higher efficiency. Nevertheless, the overheating issue from the continuous microwave is still a challenge while processing delicate herbs and their bioactive compounds. Especially, herbs like rosemary which possesses thermolabile compounds. The current study implements microwave technology periodically to benefit from sustainable drying via microwave without overheating and subsequent loss of bioactive compounds. The outcome of this study demonstrated the effectiveness of the intermittent microwave technology which proved to be more energy efficient than continuous microwave while retaining higher bioactive compound concentration in rosemary leaves.