## 3 Discussion
### The Content of Volatile Compounds That Contribute to the Aroma of Apple Wines  
The sensory profile of wines is shaped by the volatile compounds present in them. These compounds have different sources of origin. Some of the volatile compounds are introduced with the raw material, which is the fruit must. The composition of the must is related to the species and variety of fruits, ripening conditions, sunlight, etc. First of all, these are terpenes, in which only 10% of the must are in volatile form. The majority of these compounds are linked to saccharides by glycosidic bonds [60; 61; 62]. The dominant amount of volatile compounds that shape the aroma of wines is formed during fermentation and maturation. This group includes higher alcohols, esters, volatile acids and aldehydes [63; 64; 62]. An important group of compounds are those formed as a result of wine maturation, during which wine components undergo chemical reactions, such as oxidation or esterification, creating unique aromatic substances. Furthermore, the ageing process in contact with wood also affects the volatile compounds of wines. The structure of wood (porosity, permeability) and its chemical composition (polyphenols, tannins and volatile compounds) determine the biochemical processes occurring during wine ageing, which adds complexity to the aroma [65]. Tables 4 and 5 list the volatile compounds that shape the odour sensations perceived during olfactometric analysis. The majority of alcohols have a fruity or floral scent. Isobutyl alcohol, in addition to the fruity scent, also produces a chemical aroma. Six-carbon alcohols and 2-nonanol have an herbal scent. The aromas of all esters determined in the olfactometric analysis were described as fruity, with the exception of diethyl butyrate, which had an herbal scent. The identified terpenes had a floral and herbal scent, and furfural was responsible for the roasted aroma. Floral and fruity aromas are most desirable in white and fruit wines [65; 66]. They impart freshness to wines and are clearly associated with the raw material from which they originate. In the case of red wines, particularly those that have been aged for an extended period, the importance of floral and fruity notes is diminished [64; 67]. Previous studies on the impact of wood chips on the sensory profile have primarily focused on red wines [20; 68; 50]. The experience in ageing white wines with wood chips has been limited to the assessment of polyphenic compounds and possibly sensory analysis [69; 51]. [PERSON] et al. [50] demonstrated in their research on red wine that the profile of volatile compounds aged with wood depends on the type of fruit and the type of wood. In certain instances, wines aged with cherry wood chips exhibited a higher concentration of ethyl acetate than those aged with apple wood, while oak wood had the greatest effect on the production of phenylethanol. Other studies [70] have demonstrated that the maturation of red wines in the presence of cherry wood chips results in higher concentrations of most aroma-forming compounds compared to apple wood chips. However, no such changes were observed in this study. Although some volatile compounds were more abundant in wines aged with cherry chips, in some cases, e.g., 3-methyl-1-butanol, ethyl butanoate and \(\alpha\)-pinene, the use of apple wood contributed to their higher concentrations in the wines than cherry wood (Table 4). In wines aged with the addition of apple chips, the presence of ethyl butanoate, which is characterised by a pineapple aroma, had a particularly significant impact on the aroma activity (Table 5). The study by [PERSON] et al. [18] demonstrated that oak wood extracts exhibited a greater diversity of volatile compounds than cherry wood extracts. However, some compounds were present in higher concentrations in cherry wood extracts, including palmitic acid, cyclopropyl carbonl and pyranone. These findings were based on wood extracts, not wine. The aforementioned compounds were not identified in apple wines in this study (Tables 4 and 5).