An efficient approach to obtain essential oil, oleanoic acid, 3-O-acetyloleanolic acid and betulinic acid from fresh leaves of Melaleuca bracteata F. Muell. 'Revolution Gold' using microwave and gravity-assisted solvent-free distillation followed by extraction

A green and efficient microwave and gravity-assisted solvent-free distillation followed by extraction (MGSFDE) approach was presented to extract essential oil, oleanoic acid, 3-O-acetyloleanolic acid and betulinic acid from Melaleuca bracteata F. Muell. 'Revolution Gold' leaves. In this process, essential oils were obtained by solvent-free distillation with the synergistic action of microwave and gravity using in situ water in fresh plant cells. Meanwhile, most of the water contained in fresh plants is removed, which simplifies the operation for the subsequent extraction of strongly alcohol-soluble triterpenic acids such as oleanoic acid, 3-O-acetyloleanolic acid and betulinic acid (avoiding the drying process of plant materials), increasing the yield of triterpenic acid. For the extraction process of essential oil, the second-order kinetic model showed a better fit than the first-order kinetic model. Through single-factor, face-centered central composite design and BoxBehnken design, the operational parameters affecting MGSFDE were systematically optimized. Under optimized parameters, the yield of M. bracteata leaf essential oil was 6.05 ± 0.17 mL/kg DW, and the yields of oleanoic acid, 3-O-acetyloleanolic acid and betulinic acid were 28.99 ± 1.45, 9.21 ± 0.46 and 0.91 ± 0.05 mg/g DW, respectively. The types of compounds in the essential oil extracted at 540 W were the ones that were most similar to those in the traditional hydrodistillation (HD), and the relative content was higher, according to the results of GCMS, principal component analysis, and heatmap analysis. Because of its greater yields, lower power consumption, lower CO2 emissions, and lower water wastage, MGSFDE is assumed to be an effective and ecologically friendly extraction process with the potential to replace traditional methods and provides a great reference for extraction studies of other aromatic plants containing volatile and nonvolatile active ingredients.