## 3 Results and Discussion
### Investigation of the Apple Wines
#### 3.2.2 Volatile Compounds from Apple Wines
Strong dependence on the fermentation strategy was observed for esters, alcohols and free acids, whereas carbonyl compounds and terpenoids showed only small concentration differences in the five different fruit wines. Figure 3 gives a comparison of the relative proportions of esters, alcohols and acids in the apple wines AW1 to AW5. From Figure 3 and from data given in Table 3, it can be seen very clearly, that the fermentation strategy significantly impacts the volatilome, whereas the addition of the firing agent Gerbinol(r) to AW4 and AW5 does not significantly influence the composition of the volatilome. The volatile compounds in fermented products are, on the one hand, determined by the volatilome of the starting material; on the other hand, the secondary metabolism of the used strain is an important source for the volatile compounds in the final product. Furthermore, the composition of the volatile fraction can be influenced by the selection of the yeast genera and by the nutrients that are available for the microorganisms. Two recent articles on _Saccharomyces cerevisiae_ summarize the role of the respective enzymes on the biosynthesis of secondary metabolites in the course of the fermentation process [33; 34]. Higher alcohols (fused alcohols) as reaction products from the corresponding amino acids formed via the Ehrlich pathway belong to the most abundant volatile compounds in fermented beverages. In our study, 2-methyl butanol, 3-methyl butanol, 1-pentanol and phenylethyl alcohol were present in partly very high relative concentrations, with a highest average relative concentration of approx. 3760 ug L\({}^{-1}\) for 3-methyl butanol and the lowest average relative concentration of 7.7 ug L\({}^{-1}\) for 1-pentanol. Even though the concentration differences were statistically significant, with the exception of 3-methyl butanol, the effect of the fermentation strategy on compound concentrations was only minimal. The same behaviour was observed for the corresponding acids, 2-methyl- and 3-methyl butanoic acids. By contrast, for the corresponding acetates (2-methyl- and 3-methyl butyl acetate, phenethyl acetate), and also the straight-chain acetates ethyl acetate, propyl acetate and hexyl acetate, significantly lower concentrations were observed in the mash fermented apple wines AW3 and AW5 than in the other fruit wines. Alcohol acetyl transferases are required to catalyse the biosynthesis of acetate esters from the corresponding alcohols. The observed behaviour indicates a down-regulation of alcohol acetyl transferase activities in AW3 and AW5 as no nutrients were provided during mash fermentation. Highest concentrations of the mentioned acetates were found in AW1; the quick start of the fermentation without maceration nor mash fermentation and, thus, sufficient energy supply for the yeast to catalyse the acetate formation might be the reason for this behaviour. All other esters that were identified in the apple wines belong to the group of fatty acid esters (mainly ethyl or methyl esters of straight chain fatty esters C\({}_{3}\) to C\({}_{12}\) or of methylated acids). These esters are of particular importance for the flavour of apple wines as their sensory properties are described as fruity, apple- or pear-like, pineapple or strawberry like, depending on the chain length. The biosynthesis of esters mainly occurs intracellularly by the fermenting yeasts and is followed by a diffusion through the yeast's plasma membrane [35]. As a consequence, their formation is strictly controlled by the  
Figure 3: Relative amounts of the compound classes with highest relative concentrations in the apple wines AW1 to AW5; \(\bullet\)\(\bullet\) esters, \(\bullet\)\(\blacktriangle\) acids, \(\bullet\)\(\blacktriangle\) alcohols. activities of the related yeast alcohol acetyltransferases. Straight-chain fatty acids are released from the cytoplasmic fatty acid synthase complex; the subsequent formation of fatty acid esters is a result of the reaction of the fatty acid with the acyl-CoA component of ethanol [33; 34].