Optimization of solvent extraction and matrix solid-phase dispersion methods for agarwood oleoresin and essential oil production

Abstract

Agarwood essential oil is the most expensive resinous wood fragrance, indicating that it is extremely valuable. Generally, Malaysian producers use traditional method of hydro-distillation for extracting agarwood essential oil. However, this method has several limitations such as low product yield, essential oil quality inconsistency, time and energy consuming which resulting in a high operating cost of agarwood essential oil production. A low-grade agarwood (grade D) from the species of Aquilaria malaccensis was utilized in this study to improve the yield and quality of oleoresin and its essential oil using two consecutive solvent extractions, that were ethanol and n-hexane, respectively. The oleoresin was firstly extracted using reflux extraction, and then the essential oil was extracted using matrix solid-phase dispersion (MSPD). Based on their polarity, the combination of these solvents is the most ideal for producing high quality agarwood essential oil. Central composite design was used to investigate the effect of process parameters on extraction yield and the presence of chemical compound in the extract using regression analysis. Oleoresin yield of 7.08 ± 0.38 % (w/w) with total resin content of 5.75 ± 0.81 % (w/w) were obtained under the following optimal conditions: particle size of 0.5 - 1.0 mm, raw material to ethanol ratio of 1:29.9 g/ml and extraction time of 4.97 hours. Agarwood essential oil of 0.46 ± 0.10 % was achieved through optimization of essential oil extraction using the MSPD method at optimal condition: oleoresin to sorbent ratio of 1:1.46, oleoresin to n-hexane ratio of 1:39.19, extraction duration of two hours and applied silica gel as the solid support material. There were seven sesquiterpenes compounds detected through gas chromatography – mass spectrometry (GCMS) profiles for qualitative assessment, with the highest relative peak areas of 14.11 % and 16.80 % in oleoresin and essential oil extracts, respectively. This study revealed that both optimal extraction conditions produced the desired extracts in lesser extraction time and lower agarwood to solvent ratio. The best quality of agarwood essential oil obtained in improved extractions was proven by comparing with GCMS chromatogram pattern. There was a 3.67-fold increase in peak area % of the discovered seven compounds when compared to Soxhlet extraction, and the amount of 4-phenyl-2-butanone compound was found to be the highest (0.53 mg/ml) when compared to other analyzed extraction methods. Overall, the MSPD extraction approach was able to produce the highest yield of agarwood essential oil, with a 21.05 % increase of essential oil yield compared to the Soxhlet extraction. The findings of this study provide a new insight about mass transfer theory by observing the surface morphology and particle size distribution on oleoresin-silica gel blended powders prior to extraction. On top of that, the disrupted mixture sample could achieve a smaller particle size and increase solute diffusion in the dispersion process, resulting in a high essential oil yield and retain most of the agarwood components. In conclusion, the findings strongly suggested that extraction using MSPD is a potential strategy for improving the overall extract yield and quality in the production of agarwood essential oil.