# Biological Properties of Sandalwood Oil and Microbial Synthesis of Its Major Sesquietrepenoids
## 2 The Source of Sandalwood Essential Oil, Its Main Components, and Its Applications  
Sandalwood oil is a highly prized plant essential oil primarily obtained from Santalum species such as _S. album_, _S. austrocaldonicum_, _S. yasi_, and _S. spicatum_. It is mainly extracted from the heartwood of a well-matured sandalwood tree through the steam distillation process [15; 16]. Other methods have also been used to extract sandalwood oil from the heartwood, including solvent extraction, hydro-distillation, liquid CO\({}_{2}\) extraction, and supercritical fluid extraction (SC-CO\({}_{2}\)) [17]. The amount, composition, and physiochemical characteristics of sandalwood essential oil depend on the species of origin, the tree's age, the region where it is grown, the season of harvest, and the extraction method used [9]. The main compositions of sandalwood essential oil are sesquiterpenoids ((Z)-\(\alpha\)-santalol, (Z)-\(\beta\)-santalol, (Z)-\(\alpha\)-_exo_-bergamotol, (Z)-_epi_-\(\beta\)-santalol) and sesquiterpenes (Figure 1) (\(\alpha\)-santalene, \(\beta\)-santalene, _epi_-\(\beta\)-santalene, and \(\alpha\)-_exo_-bergamotene) [18; 19; 20].  
Santalol makes up about 90% of the sesquiterpenoid content in the oil (Table 1); it is composed of about 40-60% \(\alpha\)-santalol and from 20 to 25% \(\beta\)-santalol [9]. In general, sandalwood essential oil shall have (Z)-\(\alpha\)-santalol content ranging from 41-55% and (Z)-\(\beta\)-santalol in the range of 16-24% [18]. However, at present, _S. spicatum_ extracts do not meet that requirement for two main reasons: a low combined santalol content and high levels of E-farnesol isomer, a suspected allergen [21; 22]. Meanwhile, the Indian native sandalwood tree species _S. album_ is historically said to provide the bulk of sandalwood products, which contain a higher oil content rich in santalol (approximately 90%) [23]. Extracts of _S. spicatum_ are typically less valuable than those of _S. album_ due to the lower total santalol content and diverse sesquiterpene composition [24].  
The (Z)-\(\alpha\)-santalol and (Z)-\(\beta\)-santalol are the most vital ingredients contributing to sandalwood oil's bioactivities and unique woody odor characteristics. The double-bond configuration of the \(\alpha\)-santalol molecule plays a crucial role in determining its odor [25]. The presence of a high content of santalol and santalene makes sandalwood oil possess several pharmacological/medicinal activities, such as anticancer activity [26; 27; 28], antimicrobial activity [29; 30], antiviral activity [31; 32], and neuroleptic/antipsychotic activity [33]. Preclinical and clinical research has indicated that sandalwood extracts have shown anticancer and antimicrobial activities and suggests that they possess antioxidant, anti-inflammatory, antihyperglycemic, and antihyperlipidemic activity [34].  
\begin{table}
\begin{tabular}{l c c c} \hline \hline
**Native/Trade Name** & **Scientific Name** & \(\alpha\)**-Santalol (\%)** & \(\beta\)**-Santalol (\%)** \\ \hline Indian sandalwood & _Santalum album_ & 41–55 & 16–24 \\ \hline New Caledonian & _Santalum_ & 48–49 & 20–22 \\ sandalwood & _austracaldonicum_ & & \\ \hline Fiji sandalwood & _Santalum yasi_ & 37–39 & 26–28 \\ \hline Australian & _Santalum spicatum_ & 15–25 & 5–20 \\ sandalwood & \multicolumn{1}{c}{} & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Percentage composition of \(\alpha\)-santalol and \(\beta\)-santalol in various Santalum species.