###### Abstract  
This study investigates the waste-free bioconversion of glucose to erythroid using the UV-mutagenized strain _Yarroavia_ _Ii_p_olytica Wratislavia K1 UV15. This research focuses on optimizing fermentation parameters to enhance erythroid yield, with a key emphasis on utilizing post-crystallization effort in sk a primary carbon source, promoting a cost-effective and sustainable approach to erythroid production. The experimental design included systematic variations in ammonium sulfate concentration, yeast extract supplementation, and initial glucose concentration. The results demonstrate that the UV15 strain achieves high erythroid production efficiency. An optimal nitrogen source concentration (3.1 g/L) and reduced yeast extract levels (0.25 g/L) provided the best results, achieving a maximum erythroid concentration of 174.8 g/L with a yield of 58.2%. Furthermore, lowering the initial glucose concentration further improved process efficiency, confirming the feasibility of using post-crystallization filtrate as an effective and sustainable carbon source. These findings highlight the biotechnological potential of _Y.__Ii_p_ \(l\) _i  
influenced not only by the price of the substrate but also by the technology used to obtain pure crystals and the management of by-products. Erythiritol is sought after primarily for its low-calorie sweetening properties, and because its consumption does not affect insulin or glucose levels, it is gaining increasing popularity among sweeteners [7]. In recent years, there has been significant growth in erythroid production, with its production value expected to exceed USD 150 million by 2023 [8]. However, its biosynthesis is accompanied by various metabolic by-products, which can affect both production efficiency and economic viability in industrial applications. There are few studies in the available literature reporting erythroid production from glucose by _Y. lipolytica_ yeasts, and the described research has confirmed that these yeasts can convert glucose into erythroid with high efficiency when their metabolism is properly engineered. Targeted metabolic and process engineering--through overexpression of enzymes, enhancement of NADPH, disruption of pathways, and advanced gene editing--enables the achievement of erythroid yields from glucose in the range of 171-256 g/L [3; 9; 10; 11; 12; 13]. The biosynthesis of erythroid, using _Y. lipolytica_ yeast, can yield several by-products, the quantities of which vary depending on the strain [14]. However, microorganisms designed for higher erythroid yields can minimize by-product formation [5]. Another important factor influencing the production of other compounds during erythroid biosynthesis is the type and purity of the substrate. For instance, using crude glycerol may result in higher levels of undesirable compounds compared to pure substrates [6]. Primarily, mannitol is often produced during the biosynthesis of both erythroid and citric acid from glycerol, but another sugar alcohol, arabitol, can also form. The presence of sodium chloride (NaCl) in the medium can enhance erythroid production while inhibiting mannitol synthesis [15]. Various organic acids, mainly citric acid, may also be produced during erythroid biosynthesis, contributing to the overall by-product profile. Environmental conditions, such as pH, temperature, and osmotic pressure (e.g., due to NaCl addition), cannot be overlooked as they can alter metabolic pathways, affecting both erythroid yield and the profiles of by-products [6]. The aim of this study was to investigate the impact of medium composition, including the concentration of nitrogen sources, glucose, and yeast extract (YE), on the efficiency and selectivity of erythroid biosynthesis by the _Y. lipolytica_ K1 UV15 strain. This research aimed to determine the optimal production conditions using waste substrates, such as post-crystallization erythiritol filtrate, and to minimize the use of components that negatively affect the crystallization process of the product.