Assessment of microwave drying for rapid and convenient analysis of medicinal plants for quality assurance

Abstract

AbstractIntroductionThe safety and quality of many medicinally important herbs are compromised since farmers and small organizations are involved in the cultivation, aggregation, and primary processing of these herbs. Such organizations often lack adequate quality control facilities. To improve the safety and quality of herbal products, simple, rapid, and affordable quality control systems are required.ObjectivesThe aim of this study was to assess the suitability of microwave oven‐drying for moisture content (MC) determination and sample preparation of herbs in small organizations.MethodsMicrowave oven‐drying (720 W) and convective oven‐drying at 105°C for MC determination were compared. The effects of three different drying methods (microwave oven‐drying, low‐temperature convective drying, and freeze‐drying) on in vitro antioxidant and polyphenol oxidase (PPO) activity were determined, similarity analysis was conducted using HPLC signature spectra, and validation was performed with LC‐MS focusing on one herb.ResultsMicrowave oven‐drying at 720 W significantly reduced the drying time (from hours to minutes), whereas the spatial variation of temperature in convective ovens set at 105°C can cause about 10% underestimation of MC. Microwave oven‐drying showed similar macro‐properties like freeze‐drying and higher extractability (10%–20%) and in vitro antioxidant capacity (33%–66%) and lower PPO activity compared to low‐temperature convective drying. HPLC signature spectra revealed strong similarity of soluble components between freeze‐dried and microwave oven‐dried herbs. LC‐MS analysis demonstrated more common compounds between freeze‐dried and microwave oven‐dried Centella asiatica extracts, whereas convective tray‐dried samples had fewer compounds common with samples obtained by freeze‐drying or microwave oven‐drying.ConclusionsMicrowave oven‐drying is rapid (tens of min) and shows small batch‐to‐batch variation compared to oven‐drying at 105°C. The in vitro antioxidant assays and signature spectra can be used for assessing the source and purity or quality of a specific herb variety.