## 3 Results and Discussion
### Influence of Aldehyde Structure on Occurrence in a Bound State  
The chemical structure of aldehydes influences the degree of binding in various ways. Generally, steric effects might hinder the binding of Fur, Benz, and PA. Positive inductive effects (+I) lower electrophilicity and thus the binding affinity (e.g., for 2 MB compared with 3 MB). Positive mesomeric effects (+M) hinder the binding of Fur, Benz, and PA due to a higher electron density at the carbonyl group [33]. [PERSON] et al. investigated the equilibrium constants (K\({}_{\text{a}}\)) of a variety of aldehydes and HSO\({}_{3}\)\({}^{-}\) in model wines. Higher values indicated that the equilibrium is more on the side of the bound state aldehyde.  
Figure 4: ACA-releasable aldehydes (\(n=6\)): heatmap shows relative concentration of bound state aldehydes in comparison to the free form (B1: barley variety 1; B2: barley variety 2; P1: low proteolytic malt modification; P2: medium proteolytic malt modification; P3: high proteolytic malt modification).  
They found that along with ACA (K\({}_{\rm a}\) = 485 \(\times\) 10\({}^{3}\)), Meth (K\({}_{\rm a}\) = 50 \(\times\) 10\({}^{3}\)) and 3 MB (K\({}_{\rm a}\) = 29 \(\times\) 10\({}^{3}\)) showed higher affinity toward HSO\({}_{3}\)\({}^{-}\) compared to PA (K\({}_{\rm a}\) = 17 \(\times\) 10\({}^{3}\)), 2 MP (K\({}_{\rm a}\) = 2.8 \(\times\) 10\({}^{3}\)), 2 MB (K\({}_{\rm a}\) = 2.6 \(\times\) 10\({}^{3}\)), or Fur (K\({}_{\rm a}\) = 0.1 \(\times\) 10\({}^{3}\)). Higher values indicate an increased affinity toward nucleophiles [14; 15].  
In fresh beers, 2 MP (up to 1970%), Fur (up to 760%), and PA (up to 290%) showed the highest relative release rates in the case of 4 VP and, at the same time, together with 3 MB, the highest absolute concentrations (Table S3). Thus, not only Ka but also process technology needs to be considered. Although 2 MP is the most volatile of these compounds, it was also present in the free form in the highest quantities and might withstand processes such as evaporation during boiling at relatively high concentrations compared to other compounds. Fur and PA, in contrast, have relatively low volatilities and thus are not evaporated to the same extent but rather remain in the liquid phase. Therefore, these compounds might react more with nucleophiles in the matrix and thus occur more in a bound state. The results obtained in this study are in agreement with the literature. [PERSON] et al. found that T2N, Fur, and 2 MP could be released from fresh commercial beers by 4 VP, each at an increase of more than 100% [17].  
ACA revealed the presence of 2 MP (up to 490%), 2 MB (up to 220%), 3 MB (up to 120%), Hex (up to 102%), and Benz (up to 620%) in bound states in fresh beers and their depletion during aging. Fur (except for B1/P3) and Meth did so, too, but to lesser extents. Interestingly, PA and T2N showed the same behavior, with the exception that after M9, more aldehydes were again released upon the addition of ACA. The highest absolute concentrations in the bound state were obtained for 2 MP, 3 MB, and PA (Table S3). The same aldehydes were among the ones that could be released by 4 VP at high concentrations.