## 3 Results
### Biosynthesis of Volatile Organic Compounds
For _K. barnettii_ D1, with an increase in the amount of inoculum, a decreasing tendency in the concentration of compounds was visible. For _W. anomalous_ D11, an increasing tendency in production was observed with an increase in the level of inoculation. The composition of higher alcohols did not differ in both analysed media. An almost twofold increase in the concentrations of all alcohols was observed. The total concentration of these compounds was much higher in the analysed samples compared to the samples of untreated apple pomace. With increasing inoculation levels, increased productivity of these compounds fermented by _H. uvarum_ D9 and _W. anomalous_ D11 strains was demonstrated. Such fermented component should not contain high concentrations of fused alcohols for further use in food. Among the quantified VOCs, their precursors were, among others, valine, leucine, and glucose precursors. These findings are closely related to both the composition of the apple pomace and the enzymatic abilities of the analysed yeast strains. This indicates that the formation of specific volatile organic compounds is influenced by the available precursors in the substrate as well as by the metabolic pathways active in the yeast. For instance, leucine arylamidase and valine arylamidase hydrolyse peptide chains release amino acids that serve as precursors to various aroma compounds, including 2-methylpropanal, 3-methylpropanol, isobutyl acetate, 3-methylbutanal, 3-methylbutanol, and isoamyl acetate [78]. In this study, experiments were conducted at 15 \({}^{\circ}\)C. Nonetheless, significant amounts of VOCs were produced. Future studies could further inquire into the connection between the production of VOCs and the fermentation temperature, as well as into whether it is possible to modulate the composition of volatile compounds in fermented products depending on the temperature. This could also be connected to the specific product that manufacturers want to obtain since the desired combination of VOCs may vary from product to product, as each compound gives a different aroma impression. Overall, the amounts of VOCs produced during the experiment were high, which suggests that the selected yeast strains maintained good metabolic activity at the temperature of 15 \({}^{\circ}\)C. _H. uvarum_ D9 presented the best productivity of aroma compounds during the fermentation of untreated apple pomace regardless of the inoculum level. However, the samples collected after the fermentation of the hydrolysed medium showed higher summary concentrations of higher alcohols, which are not desirable compounds in this experiment. This means that this yeast produces a better aroma bouquet in the medium without enzymatic treatment. A similar observation can be made for the sample fermented by _K. barnettii_ D1. The other yeast, _W. anomalous_ D11, produced carbonyl compounds and fused alcohols at a similar level but produced significantly lower amounts of esters. Further research is needed to determine how they would contribute to the sensory profiles of selected fermented products. However, there is evidence to suggest that fermentation performed at a low temperature by selected non-_Saccharomyces_ yeast strains could positively contribute to the aroma of fermented products. Apple pomace was used as a fermentation medium. The composition of the medium could have influenced the fermentation process, especially in terms of the carbohydrates used as a carbon source for the yeasts [79]. However, the obtained results show the potential of low-temperature microbial fermentation of apple pomace in the production of valuable bioactive compounds. Furthermore, the process was designed in such a way that it eliminates the need for additional methods, such as extraction, to obtain the desired components. The utilisation of the ferment in this form has the potential to reduce the production cost of the final product and prevent the generation of unwanted by-products, such as solvents and waste microbial biomass.