## 3 Results and Discussion
### Behavior of Free Aldehydes  
The clear sensory differences, especially for the attribute bready, in the produced samples during aging indicated that the amount of soluble N affects the number of aromactive compounds. Amino acids, as part of the soluble N in beer, are reactants in various aldehyde-yielding reactions and direct precursors of Strecker aldehydes. These aldehydes are used as aging indicators because of the fact that they increase during aging but show high aroma activity [4]. Next, we discuss the behavior of free aldehydes: 2 MP, 2 MB, 3 MB, Meth, PA, Benz, Fur, Hex, and T2N.  
Figure 2 shows the concentrations of chosen aldehydes that showed statistically significant differences. Each boxplot contains the data of all aging points (FR, FO, M1-M9). The behavior of all other investigated aldehydes can be found in Table S3.  
In the fresh condition, we observed none to only minor differences in the analyzed aldehydes between different malt modification levels (Table S3 and part 2). Generally, all aldehydes increased during aging to different extents, depending on the amount of soluble N in the samples. This indicated either N-dependent _de novo_ formation or a release from the bound state. Especially, the concentration of Strecker aldehydes (2 MP, 2 MB, 3 MB, and PA) in beer significantly increased at higher malt modification levels. Again, higher proteolytic modification (P3) showed a significant increase in the content of these aldehydes. Meth was mostly present only below its LOQ, and thus the results showed no dependency on the malt modification level despite Meth being a Strecker aldehyde. Other aldehydes such as Fur, Benz, Hex, and T2N were not significantly influenced by the  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Tested Pair** & **Correct Answers** & **Wrong Answers** & _p-Value_ \\ \hline B1/P1–B1/P2 & 8 & 8 & \textgreater{}0.05 \\ B1/P1–B1/P3 & 12 & 4 & \textless{}0.001 \\ B1/P2–B1/P3 & 13 & 3 & \textless{}0.001 \\ B2/P1–B2/P2 & 10 & 6 & \textless{}0.01 \\ B2/P1–B2/P3 & 14 & 2 & \textless{}0.001 \\ B2/P2–B2/P3 & 14 & 2 & \textless{}0.001 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Triangle tests to determine sensory differences in beer after 9 months of natural aging (\(n=16\)) (B1: barley variety 1; B2: barley variety 2; P1: low proteolytic malt modification; P2: medium proteolytic malt modification; P3: high proteolytic malt modification).  
amount of soluble N. Fur is mostly influenced by the heat load during beer production [8]. Benz, even though being considered a Strecker aldehyde, appears to be more dependent on oxygen [6]. The formation of Hex and T2N is influenced by the concentration of lipids (varying by environment and cultivar) and the enzymes involved in their degradation during malting and brewing. Thus, their concentration is independent of the N species [20].  
The observed increases in Strecker aldehydes explain the sensory impressions of aged beers, since these compounds are important contributors to aged aroma and especially to bready impressions [32]. A higher proteolytic malt modification lead to a higher aging potential in fresh beer and thus to increased formation of aldehydes during aging. An increased number of reactants during beer production results in a higher aging potential in the wort, which is likely to be transferred to the final beer [13].