## 1 Introduction  
Multi-product biorefineries play a crucial role in the development and implementation of a sustainable bioeconomy, particularly in the context of significant environmental challenges, including fossil fuel depletion, climate change, and waste management. These facilities target the complete fractionation and valorization of a wide range of biomass resources for producing both bulk and specialty co-products, making the bioprocessing cost-competitive [1]. Biomass residues such as agro-food waste (e.g., discarded vegetables) and lignocellulosic residues (e.g., woody residues, agricultural wastes, and energy crops) are primary raw materials for these multi-product biorefineries [2]. Therefore, the efficient use of these feedstocks is crucial to obtain a wide range of value-added products, including biofuels and other bio-based products with applications in the chemical, pharmaceutical, food/feed, or material sectors. However, the heterogeneity and diverse nature of waste resources may represent a significant limitation during bioconversion processes [2]. This fact is due to the need to use different methodologies to obtain the sugars contained in these feedstocks (e.g., agro-food residues mainly require crushing and liquid/solid separation steps, while lignocellulosic require pretreatment and enzymatic hydrolysis steps), as well as the need of converting a wide range of carbon sources and facing different stress factors (e.g., the presence of inhibitory compounds). One of the most attractive products that can be obtained from biowastes is microbial oil, also known as Single-Cell Oil (SCO) [3]. The production of microbial oil has grown considerably in recent years due to their potential to replace the vegetable oil used in sectors as diverse as food (nutritional supplements or food additives), energy (biofuels), the pharmaceutical industry (arachidonic acid, docosahexaenoic acid) and the material sector (biopolymers, bioplastics). Yeast, microalgae, or bacteria produce SCO. These microorganisms exhibit faster growth cycles compared to plant crops, can utilize a wide range of carbon sources for lipid accumulation, and can be cultivated under well-controlled conditions without the need to use fertile land, thus extending the possibility of dedicating these areas to other crops. In addition, oleaginous microorganisms can be subjected to genetic engineering to design and develop novel strains, targeting increased process efficiency, providing relevant insights into the conversion pathway, and/or reducing process costs through the simultaneous co-production of value-added compounds [4]. The red yeast _Rhodosporidium toruloides_ is an oleaginous microorganism with great biotechnological potential [5; 6; 7]. This is due to its capacity to grow on a wide range of carbon sources, its tolerance to different biomass-related inhibitory compounds, and its robustness to other stress factors associated with bio-based processes [8]. In addition to accumulating SCOs, this yeast naturally produces carotenoids, which confer its red colour and are important value-added products with antioxidant properties. Due to their antioxidant and colourant properties, carotenoids may have multiple applications in the food industry (dietary supplements), cosmetics (colourants and photoprotective additives), or the pharmaceutical sector (vitamin precursors, active compounds that modulate the immune system) [9]. Currently, carotenoids are obtained from plants, microbial-based biosynthesis, or chemical synthesis--the synthetic production of carotenoids. However, the growing demand for natural products by global consumers calls for the development of new research with emerging and alternative ways to obtain these products [10]. The Polaris Market Research report valued the global natural beta-carotene market in 2024 at $1.28 billion [11]. These figures are projected to exceed $2.06 billion by 2034. All these advantages make _R.