# Potential Applications of Yeast Biomass Derived from Small-Scale Breweries
## 4 Brewery Yeast Biomass  
Brewer's yeast represents an initially highly manageable by-product within the context of the beer production process, concurrently serving as a desired substrate across diverse industrial sectors. Not all batches of yeast obtained in the beer production process are considered waste due to the possibility of reuse in the fermentation of fresh wort. Following each inoculation event, the yeast population undergoes substantial augmentation, increasing by a factor of three to five. The frequency of reuse cycles is contingent upon several factors that involve the specific microbial species employed, the nature of the beer being produced, the composition of the brewing wort extract, and the meticulous adherence to hygiene practices. Conventionally, yeast is subjected to reuse cycles ranging from 3 to 10 iterations, or until such time that its application does not introduce noticeable alterations to the sensory attributes of the beer [24]. Consequently, with depleting recycling efficacy, the removal of yeast from the production process becomes imperative, thereby generating a waste stream that poses formidable challenges for disposal. Brewing yeast constitutes the second-largest by-product (after spent grain--20 kg/hL of beer [25]) in terms of mass within the framework of the beer brewing process. According to the previously provided data, the annual global beer production is approximately 1.9 billion hectoliters, with the mass of produced yeast exceeding 4.321 million tons (assuming an average biomass production of 2.25 kg per hectoliter of beer), resulting in about 864 thousand tons of dry biomass globally. The quantifiable production of biomass is contingent upon many factors, including the parameters governing fermentation (such as temperature, pH, and aeration), the taxonomy of microorganisms employed, the concentration and specifications of the inoculum, and the compositional intricacies of the nutrient milieu [26]. After fermentation is finished, yeast cells are set on the bottom of the fermentation tank and may be collected. Yeast sludge along the cells contains the remains of fermented wort (beer) and potentially some solid debris (such as hops). Beer usually contains the following components: ethanol (3-5%), carbohydrates (1-6%), hop-derived compounds (20-60 mg/L), and organic acids (50-250 mg/L). It also includes amino acids (mainly proline) and peptides (total nitrogen: 300-1000 mg/L) [27]. Moreover, yeast cell walls absorb bitter chemical compounds derived from hops (humulones and isohumulones) [28]. Depending on the further use of biomass, yeast cells may require additional washing to remove impurities. The exemplary composition of biomass is presented in Table 2. The main groups of organic compounds constituting yeast cells are proteins (up to 49% DW) and carbohydrates (up to 54% DW). Free amino acids constitute only a small part of the entire cell biomass (below 0.5% of DW). The amino acid composition of yeast biomass (including amino acids derived from protein) is presented in Table 3. The most abundant amino acids are glutamate/glutamic acid, aspartate/aspartic acid, alanine, leucine, and lysine. Notably, the proteins inherent in yeast slurry exhibit a noteworthy biological value, as evidenced by the quantity of essential amino acids within their structural composition, encompassing a range from 70 to 85% of the value observed in casein [9]. Carbohydrates are mainly localized in the yeast cell wall and can be divided into the following groups: glucans (1-3 beta-glucans, 1-4 beta-glucans, and 1-6 beta-glucans), chitin, and mannans (bound to proteins localized in the cell wall). The share of individual polymers in the cell wall structure of yeast is presented in Table 4. The composition and structure of a brewery's yeast cell wall change depending on growth conditions and culture states [34]. Thicker cell walls seem to develop in rich culture media (like YPD) or under stress conditions such as low pH (4) or high temperature (37 \({}^{\circ}\)C) [35].