## 1 Introduction  
p-Cymene (p-C) [1-methyl-4-(1-methylethyl)-benzene] is a monoterpene that can be obtained from a variety of plants and has biologically active effects of antioxidant, anti-infective, anti-anxiety, anti-inflammatory, antimicrobial, and anticancer. Due to its beneficial effects, it is used in biomedical applications and is considered a novel agent in the treatment of infectious diseases [1]. In particular, p-C has been shown to be one of the best anti-neuroidegenerative potential monoterpenes for Alzheimer's disease (AD). It may also play a positive role in improving memory and learning, reducing anxiety, and regulating sleep [2]. p-C exerts neuroprotective effects by reducing H\({}_{2}\)O\({}_{2}\)-induced caspase-3 expression in neurons. This leads to its role as a neuroprotective agent against oxidative stress, which helps prevent AD [3]. Among monoterpenes, p-C has shown protective effects on the pathogenesis of atherosclerosis through anti-inflammatory, antioxidant, and lipid metabolism modulation, and has been discussed as a new anti-atherosclerotic drug [4]. Essential oils, which are extracted from plants, are natural aromatic volatile compounds and are attributed to the treatment of various kinds of diseases due to their antimicrobial and antioxidant activities, and are beneficially used as applied ingredients in the cosmetics and perfumery industries [5, 6]. The component responsible for the antioxidant activity of eucalyptus has also been identified as p-C [7]. It includes the genera Origanum and Thymus and is also present in food-based plants in carrots, orange juice, graprefruit, raspberries, tangerines, and many spices [8]. In addition, p-C has been shown to inhibit and interfere with the expression of the suppressor of cytokine signaling 3 (SOCS3) in the inflamed colon [9]. Also, rosmarinicacid has intestinal anti-inflammatory activity associated with modulating antioxidant and immunomodulatory systems to protect barrier cells, maintain the mucus layer, and preserve communication junctions [10]. It is an important compound found in the essential oil of thyme, with antimicrobial activity against _Staphylococcus aureus_, _Salmonella typhimurium_, and _Streptococcus pyogenes_ in particular [11; 12]. These p-Cs are naturally occurring compounds, and there are m-Cymene (meta-substituted alkyl groups) and o-Cymene (ortho-substituted alkyl groups) that do not occur in nature [13]. The odor of p-C is described as woody and slightly pungent [14; 15]. The production of p-C has traditionally been achieved via the catalytic alkylation of petroleum feedstocks [16]. However, the use of hazardous acids, such as aluminum chloride (AICI3) and hydrogen fluoride (HF), can lead to safety, waste, corrosion, and environmental pollution, so an alternative approach to produce p-C from the dehydrogenation of terpen has emerged [17; 18; 19]. It is a dehydrogenation reaction of the p-C of terpen in a liquid or gas phase [20]. Also, high yields are obtained when zeolites and alkali metals are used as catalysts for liquid phase dehydrogenation and metal oxides are used as catalysts for gas phase dehydrogenation [21]. The formation of p-C can be expected from a wide range of volatile organic compounds (VOCs), including aromatic hydrocarbons obtained from the combustion and evaporation of fossil fuels, especially from the oxidation of \(\alpha\)-pinene [22; 23]. The potential importance of switchable mechanisms of p-C has been discussed, making extensive research on p-C very important [24]. In this context, increasing yields through an efficient approach by anticipating synergistic strategies of biological and chemical catalysis refers to the conversion of renewable biomass into chemicals and fuels, and thus the production of p-C [25; 26]. Also, the development of the microbial platform enables the conversion of limonee, 1,8-cinee, to p-C through fed-batch fermentation, which is a promising research topic at the convergence of biology and chemistry [27].