# Bioconversion of Apple Pomace to _Meyerozyma guilliermondii_ and _Scheffersomyces stipitis_ Biomass
## 1 Introduction  
According to recent statistical data, apple production in European countries has been estimated at around 18 million tonnes. Around 4 million tonnes are Polish apples. One of the main byproducts of apple processing is pomace (AP), which can account for 20-35% of the harvested apples [1, 2]. Apple pomace, a source of valuable bioactive components, can serve as a food ingredient either directly or indirectly (after bioconversion with fodder yeast), making it particularly valuable given the increasing demand for feed and dietary protein [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]. The war in Ukraine, given the country's significant share of cereal and sunflower production, is causing a substantial economic impact worldwide. Consequently, a pressing global issue is determining what should be used to feed animals. Soya has been a popular animal feed ingredient due to its high content of proteins and valuable amino and fatty acids. There are two challenges associated with the feed use of soya. The first is the competing use of soya for feed with its direct use as human food. The second is the relatively high price of soya [15]. The chemical composition of pomace can vary greatly depending on the variety of apples, the ripeness, the climate, and the soil properties of the region from which the apples originate. Compared to the spent biomass of other fruits, apple pomace is particularly notable for its high pectin content, a polysaccharide that is part of the plant's cell wall. In addition, apple pomace is characterised by a high content of cellulose, hemicellulose, and lignin, and proper processing of pomace results in the hydrolysis of cellulose intomonosaccharides that are more bioavailable to microorganisms. The pomace is also rich in organic acids, sugars, fatty substances, and vitamins. Compounds are also characterised by strong antioxidant activity--polyphenols [4; 7; 16; 17]. The pomace produced directly as waste from the fruit and vegetable industry typically contains a high water content of around 30% of the raw material. As a result, fresh AP is highly susceptible to undesirable microorganisms. Its storage life can be significantly extended by ensiling or drying processes. Dried AP, which usually consists of 90-95% dry matter, can be directly incorporated into animal feed, thereby enriching it with essential nutrients. Another alternative use for AP is in the production of pectins. Pectins also possess important properties such as the ability to gel, stabilise, and thicken [17; 18; 19]. For this reason, the pectins in apple pomace, when adequately modified enzymatically, chemically, or thermally, are used as hydrocolloids in industry [4; 17; 20]. AP can also serve as a microbial substrate in propionate-acetic fermentation, employing the bacteria _Propi-onibacterium freudenreichii_ to obtain acetic acid and propionic acid [21]. AP is reportedly used in various technologies and end-products, such as baking, extruded foods, meat products, confectionery and dairy products, alcoholic beverages, edible mushroom cultivation, anthocyanins, and flavonoids [17]. Cellulose, hemicellulose, and lignins form a complex that is an integral part of plant biomass. To use it for the production of biofuels or microbial feedstocks, it is necessary to break down this complex to release the sugars it contains. The cell wall of plant cells consists of microfibrils, with hemicellulose occupying the outer part interspersed with lignin, while cellulose is centrally located. Cellulose is the most abundant component in this complex, typically comprising 40-60% of plant biomass, followed by hemicellulose at 20-50% and lignin at 15-35%. Cellulose is a polysaccharide composed of D-glucose residues linked by \(\beta\)-1,4-glycoside bonds with cellobiose (containing two glucose subunits) being the repairing unit in this chain. Consequently, efficiently breaking down lignocellulosic raw materials to glucose makes them an excellent source of sugar for yeast in culture media [22].