Towards Higher Oil Yield and Quality of Essential Oil Extracted from Aquilaria malaccensis Wood via the Subcritical Technique

Abstract

A method that delivers a high yield and excellent quality of essential oil, which retains most of its value-added compounds, and undergoes least change after the extraction process, is greatly sought after. Although chemical free methods are acceptable, they call for an extensive processing time, while the yield and quality from these methods are often disappointing. This work utilizes subcritical water technology to address these issues. In this undertaking, essential oil was extracted from Aquilaria malaccensis wood by way of subcritical conditions, and characterized through gas chromatography/mass spectroscopy (GC/MS). Optimization through response surface methodology revealed temperature to be the most critical factor for the extraction process, while the optimum conditions for temperature, sample-to-solvent ratio, and time for subcritical water extraction was revealed as 225 °C, 0.2 gr/mL, and 17 min, respectively. The subcritical water extraction technique involves two simultaneous processes, which are based on good fitting to the two-site kinetic and second order model. In comparison to the hydrodistillation method, GC/MS results indicated that the quality of A. malaccensis’ wood oils, derived through the subcritical water technique, are of significantly better quality, while containing many constructive value-added compounds, such as furfural and guaiacol, which are useful for the production of pesticides and medicines. Pore size, functional groups, and morphology analysis revealed the occurrence of substantial damage to the samples, which facilitated an improved extraction of bio-products. In comparison to conventional methods, the use of the subcritical method not only involves a shorter processing time, but also delivers a higher oil yield and quality.

Subcritical Water Extraction of Natural Products

“…Grains, seeds, corn, and fruits have also been investigated using SBWE [ 21 ]. Among all papers reviewed, only one article reported that the wood of Aquilaria malaccensis has useful medicinal applications, and is used in traditional medicine to treat pain, fever, rheumatism, and asthma [ 22 ].…”

“…Essential oils have been used in folk medicine in ancient China, Egypt, Arabia, and Greece throughout centuries. Therefore, some researches have attempted to utilize SBWE as a green solvent to extract essential oils from plants, and recent researches are illustrated in Table 6 [ 12 , 22 , 28 , 174 , 175 , 176 , 177 , 178 , 179 , 180 , 181 , 182 , 183 , 184 , 185 , 186 , 187 , 188 , 189 , 190 , 191 , 192 , 193 , 194 , 195 , 196 , 197 , 198 , 199 , 200 , 201 , 202 , 203 , 204 , 205 , 206 ]. The most common essential oils are extracted by distillation [ 174 , 176 , 180 , 181 ], n -hexane [ 174 ] or supercritical carbon dioxide [ 181 ], or dichloromethane [ 181 ].…”

Effects of various artificial agarwood-induction techniques on the metabolome of Aquilaria sinensis

“…Essential oils typically arise from secondary metabolites with several hundreds of constituents. However, oil quantity and quality in agarwood are reported to be markedly dependent on agarwood induction techniques [ 8 , 9 , 21 – 23 ]. There have been prior studies on the metabolite profiles of agarwood [ 8 ].…”

“…A large number of studies have indicated that both chromones and sesquiterpenes are essential for the medicinal and incense applications of agarwood [ 4 , 8 , 13 ]. Recently, it has been revealed that the quantity of chromones in agarwood fluctuates throughout agarwood production [ 25 ], but 2-(2-phenylethyl) chromones have been shown to accumulate significantly in high-quality agarwood [ 8 , 13 , 23 , 25 ].…”

“…Several traditional methods may approximate the formation process of wild agarwood. However, traditional methods are laborious and time-consuming [ 4 , 11 , 23 ]. New non-traditional treatment methods have been reported to yield higher but slower accumulation of secondary metabolites and oil in agarwood [ 8 , 15 ].…”