# Bioconversion of Apple Pomace to _Meyerozyma guilliermondii_ and _Scheffersomyces stipitis_ Biomass
## 1 Introduction
One of the most efficient methods for converting lignocellulosic raw materials to glucose, alongside non-organic acid thermohydrolysis, is bioconversion through enzymatic hydrolysis using cellulases. The rate and extent of cellulose degradation depend on various factors, including the enzyme dose, hydrolysis temperature, and cellulose structure. Cellulose from different sources may exhibit varying degrees of resistance to hydrolysis [23; 24; 25; 26]. Single-cell protein (SCP) is commonly produced from agri-food industry waste containing simple sugars or polysaccharides. During the First and Second World Wars, yeast SCP was used on a large scale in Germany to prevent hunger and malnutrition problems [25]. In light of this information, developing research on cultivating valuable yeast biomass using apple pomace as the primary nutrient medium seems advisable. Based on scientific predictions, the protein requirement for the estimated global population of 9.7 billion, assuming a minimum requirement of 1 g/kg of body weight per day, results in an annual minimum requirement (assuming 60 kg as the average body mass) of 212,430,000 tonnes of protein for human nutrition alone [27; 28]. Additionally, animals consume at least a comparable amount of protein annually. Regarding the anticipated number of livestock in the next 20 years and their protein needs, there are no precise statistics, but current trends indicate continued growth. However, given the current global situation, it is difficult to realistically forecast the protein demand for this sector [28; 29]. Agriculture, facing challenges from a changing climate [30; 31; 32; 33], may struggle to fully meet these needs. Consequently, there is growing interest in alternative protein sources, such as microbial protein. Yeast has been widely used by humans for thousands of years for alcoholic fermentation and dough rise but it also has the capability to produce valuable dyes, proteins, hormones, surfactants, and vitamins [34; 35; 36; 37; 38; 39; 40]. Yeast's biomass, rich in exogenous proteins and vitamins, makes it an appealing intermediate product for producing protein hydrolysates and for use in feed. Maintaining yeast cultures on an industrial scale is more manageable compared to multicellular organisms, and its acquisition does not incur high costs [41; 42; 43; 44; 45; 46]. Due to its high nucleic acid content, yeast protein intake should not exceed 16% of the recommended daily protein dose [47]. _Scheffersomyces stipitis_ and _Meyerozyma guilliermondii_ are well-tested strains that reproduce efficiently by budding at 25-37 \({}^{\circ}\)C. These yeasts are capable of fermenting various carbohydrates such as glucose, galactose, maltose, and xylose [34; 48; 49; 50; 51; 52; 53; 54; 55; 56]. _Meyerozyma guilliermondii_ ATCC 6260, used in our experiments, is known as one of the yeast strains accepted for obtaining fodder SCP [57], while the other strain--_Scheffersomyces stipitis_ LOCK P0047--is unique in its properties to efficiently grow in the hydrolys of agricultural wastes, what was previously presented [50; 58; 59]. _M.guilliermondii_ or S. _stipitis_ have been used for years to convert lignocellulosic substrates to ethanol, xylitol, and biomass [50; 51; 59; 51; 52; 53; 54; 55; 56]. Therefore, we have set out to investigate the conversion of apple pomace to biomass by these two strains. Since industrial processes for biomass synthesis for feedstuffs include a thermal treatment step aimed at thermolysing the cells to improve the digestibility of the yeast biomass, the microbiological safety of this product containing thermally inactivated yeast cells is also ensured [34]. Considering all the factors mentioned above, it can be assumed that cultivating _S. stipitis_ and _M. guilliermondii_ cells using apple-derived wastes may also be a favourable method of their biovalorisation, particularly in light of the current geopolitical situation.