## Results and Discussion
### Volatilome of Apples after Storage
[PERSON] et al., 2016 [61] reported a lower dependence of BCAT activity in Golden Delicious apples on ethylene than for enzymes involved in the fatty acid degradation. The observed alcohol concentrations in Crimson Crisp apples indicate a similar reaction on the reduction of ethylene formation. Esters are compounds that are of great importance for the flavor of apples. The fruity notes are mainly derived from this compound class [62]. The concentration patterns of esters follow the general trend of the straight chain alcohols. Whereas in primary aroma compounds, we observed a decrease in ester concentrations following fresh apples > RA > CA > MCP, the relative concentration of alcohols as part of the secondary aroma compounds was highest in RA after storage followed by fresh apples > CA > MCP. Esters are results of ester formation from the corresponding alcohols upon activity of the enzyme acyltransferase (AAT). Such as LOX and ADH, AAT activities are ethylene dependent [63; 64], thus, their formation is reduced along with the reduction of the respiration rate and the ethylene formation. As a consequence, ester concentrations follow similar patterns as alcohols which may also explain the observed high amounts of esters in juice from RA stored Crimson Crisp apples. Figure 2 shows the biplot as a result of the multivariate data analysis (principal component analysis PCA) calculated from the relative concentrations of the secondary aroma compounds and demonstrates the correlations of the Crimson Crisp apples stored under different conditions and the corresponding volatiles. PCA analysis calculated by means of the primary aroma compounds delivers a similar picture (results not shown). The results show a close relation of fresh apples after the harvest and apples stored under regular air on the one hand (quadrant I) and a correlation of CA and MCP stored Crimson Crisp apples on the other hand (quadrant IV). Even though we observed strong differences in the overall amounts of straight-chain alcohols, esters and aldehydes in dependence of the storage conditions, this is not reflected in the results obtained from PCA analysis. The bigger part of volatiles is located in a dense cluster in quadrant II and III close to the centroid of the PCA plot. Five distinct volatiles may be regarded as discriminative for the storage behavior of Crimson Crisp apples stored under different conditions. While the straight-chain alcohols 1-butanol and 1-hexanol together with the corresponding acetates butyl acetate and hexyl acetate are closely correlated with fresh apples and RA stored apples, the methyl-branched-chain alcohol 2-methyl butanol and its corresponding ester 2-methyl butyl acetate exhibit high correlations with CA and MCP stored fruits. These findings again indicate a strong down-regulation of enzymes by regulation of ethylene emission, while enzymes that are necessary for amino acids degradation are less affected by measures reducing or inhibiting ethylene formation by the fruits. However, CA-stored apples are more related--and probably more similar in their aroma properties--to fresh apples than MCP-stored apples. The prediction of the sensory properties for these products based on this dataset is scarcely possible. Reduced aroma--in both intensity and complexity--is expected from CA and even more pronounced from MCP apples, while RA samples would most probably show a more intense aroma. However, the results of the analysis of volatiles suggest that CA storage might best preserve the qualities of Crimson Crisp apples that are harvested at high ripeness.