Microwave-assisted solvent extraction of banaba crude oil and its antibacterial activity

Banaba (Lagerstroemia speciosa) is a deciduous tropical flowering tree found in India, the Philippines, and Vietnam. It is known worldwide especially for its antidiabetic and antiobesity abilities. The active ingredient responsible for these properties in humans is corosolic acid, a triterpenoid compound that stimulates glucose uptake by enhancing insulin receptor phosphorylation. Although conventional methods have been used to extract banaba, they lead to low extraction efficiency, long extraction time, and even loss of some volatile compounds. Recently, microwave - assisted extraction with solvent (MASE) has been found to be widely used for herbal extraction because of its increased efficiency, reduced extraction time, and environment-friendliness. Thus, MASE was investigated for banaba leaves and fruits in this study. Ethanol and ethyl acetate were used as solvents for extracting from banaba fresh leaves. The effect on yield of banaba extract was studied for the following parameters: 300W, 500W, and 800W microwave power; 2 minute and 6 minute extraction time; 6ml/g, 8ml/g, and 10ml/g solvent-to-material ratio; and, 50oC, 60oC, and 70oC temperature. The active compound in banaba extract, corosolic acid, was identified by high-performance liquid chromatography (HPLC) analysis. Furthermore, the antibacterial activity of banaba extract was tested by Kirby–Bauer method with two gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa) and two gram-positive bacteria (Staphylococcus aureus and Bacillus subtilis). Extraction of banaba by MASE obtained, on the one hand, the highest yield of 63.95% for ethanol, which made a good solvent for this process, compared to the highest yield of 7.9625% for ethyl acetate. The best conditions that gave the highest yields, on the other hand, were at 500W of power, 10:1(ml of ethanol/g of material) solvent-to-sample ratio, 6 minute, and 70oC yielding 63.95% for fresh banaba leaves; and, at 500W power, 8:1(ml of ethanol/g of material) solvent-to-sample ratio, 6 minute, and 60oC yielding 79.65% for fresh banaba fruits. Corosolic acid was identified by HPLC analysis with the peak at around 8.5 minute, the yield of corosolic acid was 0.248% at the best conditions for MAE extraction with banaba fresh leaves extract. However, no presence of corosolic acid was observed in banaba fruits extract. Measurement of five physico-chemical properties of the resulting banaba crude oil—i.e., pH, specific gravity, refractive index (RI), acid values, and ester values—gave the following characterization ranges: pH of 4–5, specific gravity of 0.927–1.012, and RI of 1.34–1.35. Banaba crude oil obtained a low range of acid values at 2.14–3.6 but obtained a high range of ester values at 12–15 for leaves, specifically giving 96.58 for fresh fruits and 117.67 for dried fruits. Antibacterial testing of the extracts of banaba showed positive inhibition for antibacterial activity, which demonstrated the applicability of banaba extracts of leaves and fruits in the pharmaceutical and medical industries. The level of antibacterial ability varied with the extract product and with the bacteria used. All four microorganisms, however, exhibited activity at 100mg/ml with E. coli as the most sensitive and with the extract showing the lowest zone of inhibition diameter with B. subtilis. The best inhibition was exhibited by the extract of fresh banaba leaves while the worst inhabitation was displayed by the extract MASE with ethyl acetate as solvent