## 3 Recent Advances in Biotransformation of Terpenes: Study Case of Limonene and Alpha Pinene
### Limonene Biotransformation  
Limonene is the most abundant naturally occurring monoterpene found in orange peel oil, comprising up to 90% of its composition [21]. Due to its abundance and low cost, limonene has been extensively studied as a precursor to produce high-value derivatives, offering a potential strategy for enhancing the commercial value of agro-industrial residues like orange peel oil [22] (Figure 2). The applications of limonene as a flavour and fragrance additive in food products, cosmetics, household cleaning products, and textiles are well known [23]. Additionally, limonene finds utility as a solvent, a feedstock for fine chemicals, a precursor for polymeric biomaterials, and as an active ingredient in medicine [24]. Currently, most of the limonene is obtained from citrus finds, a significant byproduct generated during fruit processing in citrus juice industries [25]. However, many citrus juice industries, particularly those in developing countries, lack the necessary infrastructure and technology for on-site recycling of large quantities of citrus waste [26]. Consequently, a substantial number of citrus finds are disposed of in landfills rather than being utilised for limonene extraction. Limonene is extensively studied as a precursor in the biotechnological production of monoter, as it can be transformed into various value-added compounds such as carvone, carved, perillyl alcohol, terpineols, menthol, and pinenes (Table 2). It serves as a cyclic monoterene with multiple industrial applications as a biosolvent, in surfactant formulations, as an antibacterial agent, and as a pesticide in nutrition, as well as a precursor for fine chemicals in the food, perfume, medicine, agrochemical, and oral hygiene industries [27]. Given the structural similarity between limonene and oxygenated monoterneols with pleasant fragrances, such as perilyl alcohol, carvool, carvone, menthol, and \(\alpha\)-terpineol, limonene can be used as a precursor for the synthesis of these flavour compounds [24]. Several studies have reported the biotransformation of limonene into \(\alpha\)-terpineol, using microorganisms such as _Cladosporium_ sp., _Pseudomonas gladioli_, and an \(\alpha\)-terpineol dehydratase isolated from the same strain [28]. Orange peel oil analysis has revealed that D-limonene is the major component, accounting for 96.1% of the total content [29]. The biotransformation of D-limonene to a-terpineol was conducted using a strain of _Penicillium digitatum_ NRRL 1202. Experiments employed two distinct media: malt yeast broth (MYBB) and malt extract broth (MEB). Among them, the MYB medium resulted in the most efficient bioconversion of D-limonene to a-terpineol. Optimisation of the bioconversion process using synthetic media supplemented with orange peel oil demonstrated increased yields of \(\alpha\)-terpineol, reaching 79% at 3 h of fermentation and 95.5% after 7 h with the addition of MYB in the medium. This demonstrates that supplementation of synthetic medium (medium rich of reducing sugar, minerals and organic nitrogen) allows the microorganism to perform and increase the biotransformation of limonene. However, it should be noted that the use of expensive synthetic media and the concentration of orange peel oil were limitations of this study as the cost to scale-up the process was high because of the high cost of nutrients. Instead, the use of residual medium as wastewater coming from agroindustry could be a way to decrease the cost of the process and maintain high performance of biotransformation [29]. [PERSON] et al. [7] investigated the bioconversion of limonene using two isolated strains, resulting in the production of high-value derivatives carvone and \(\alpha\)-terpineol. The aspergillus LB-2038 strain achieved a higher concentration of carvone (47 mg/L), while the Penicillium LB-2025 strain stood out in \(\alpha\)-terpineol production (10 mg/L).