## 1 Introduction  
Essential oils can be derived from various plant parts, including flowers, fruits, leaves, buds, seeds, twigs, bark, herbs, wood, and roots. These oils are predominantly composed of monoterepne hydrocarbons [1]. These terpene hydrocarbons, such as limonene, pinenes, and terpen, serve as ideal starting materials for microbial transformations to obtained high value molecules such as carveol, terpineol, verbenol and verbenol. Microorganisms and enzymes can act as catalysts in two different processes: de novo synthesis and biotransformation of natural precursors. De novo synthesis involves utilising the entire metabolic pathway of microorganisms to produce a combination of flavour compounds, whereas biotransformation focuses on specific reactions that lead to the production of a major compound. In de novo synthesis, microorganisms metabolise carbohydrates, fats, and proteins, converting the breakdown products into flavour components [2, 3]. However, this process yields only trace amounts of flavours and is not economically viable for industrial production due to low concentrations of the desired compounds [4]. In biotransformation, a precursor is introduced into the process, inducing the microorganism to follow a specific pathway and produce the final product through one or two chemical reactions [5]. Additionally, agro-industrial byproducts can be utilised as nutrient and substrate sources in fermentation processes, reducing fermentation costs and enabling industrial viability [6]. Terpenes are noteworthy substrates for biotransformation, as they are secondary metabolites of plants and can be found in byproducts generated during fruit and plant processing. Monoterpenes (such as limonene, \(\alpha\)-pinene, and \(\beta\)-pinene) and sesquiterpenes (such as valencene and farnesene) are common volatile compounds found in various essential oils. Structurally, they are closely related to aroma compounds of significant interest in the industry, requiring only a few chemical reactions to obtain high-value products [7] (Table 1). From an economic perspective, the advantages of bio-transformation become evident when comparing the prices of substrates and products. For example, in 2015, R-limonene has a reference price of US$34/L, while its oxygenated form, carved, is priced around US$530/L [7, 8]. Selecting the appropriate biocatalyst is another crucial step in natural aroma production. Microbial cells offer advantages over enzymes, as they possess higher stability and do not require additional cofactors for the reaction to occur. Microorganisms also exhibit rapid growth rates, and fermentation parameters can be easily manipulated, allowing for genetic engineering to enhance strain performance in large-scale processes [9]. The objective of this review is to provide approaches and strategies to improve the biotransformation of two of the most promising biomolecules found in agroforestry residual biomass. The review will discuss general knowledge of terpenes, recent advances of biotransformation of limonene and alpha-pinene, strategies to improve the biotransformation of these two molecules using solid-state fermentation and finally various extraction technology to recover the produce molecules.