## 4 Conclusions  
The objective of the study was to investigate the _de novo_ formation of free aldehydes during beer aging due to different malt modification levels. Thus, the investigation focused on the oxidation of higher alcohols, Strecker degradation, and the Maillard reaction. No formation of Strecker aldehydes from their corresponding alcohols could be confirmed. A higher malt modification level increased the concentration of amino acids, dicarbonyls, and free Strecker aldehydes. This increased aging potential explains the decreased sensory evaluation in the preceding article by a higher malt modification level [1]. The dicarbonyl compounds 3-DG and 3-DGal were degraded during aging, indicating Strecker degradation. Amadori products showed no consistent trend during aging. The effect might be caused by a balance between aldehyde formation and degradation, which could further lead to observed increased 3-DM concentrations during aging. AGEs remained constant during beer aging. Therefore, dicarbonyls reacted more strongly to Strecker aldehydes or alternative pathways such as HMF or melanoidin formation. For Strecker degradation, dicarbonyl concentrations were found to be more critical than the amino acid concentration. Finally, it was concluded that _de novo_ formation and release of bound-state aldehydes occurred simultaneously. Up to four months of natural aging, the release of bound-state aldehydes was the predominant way for aging aldehyde formation. Significant sensory changes occurred at a range of 3-5 months of natural aging. Therefore, it can be assumed that the release of aldehydes basically provided the potential for sensory changes and needs to be regarded as a separate aging mechanism. In conclusion, _de novo_ formation mainly reinforced and accelerated sensory beer aging after 3-5 months in this study.  
**Supplementary Materials:** The following are available online at [[https://www.mdpi.com/article/10.3390/foods10112668/s1](https://www.mdpi.com/article/10.3390/foods10112668/s1)]([https://www.mdpi.com/article/10.3390/foods10112668/s1](https://www.mdpi.com/article/10.3390/foods10112668/s1)), Figure S1: Concentrations of (A) 2-methylpropanol, (B) 2-methylbutanol, and (C) 3-methylbutanol in fresh (FR), forcibly aged (FO), and naturally aged (M3, M6, and M9) beer at different malt modification levels (P1: white bar; P2: grey bar; P3: black bar) at two barley varieties (B1 and B2); n = 3, Table S1: All data from amino acids, aldehydes, Amadori products, dicarbonyls, and AGEs.  
**Author Contributions:** Conceptualization, A.N., M.K. and M.G.; investigation, A.N., M.K. and F.L.; writing--original draft preparation, A.N., M.K. and F.L.; writing--review and editing, M.H., M.G., T.H. and T.B.; visualization, A.N. and M.K.; supervision, M.H., M.G., T.H. and T.B.; funding acquisition, M.G. All authors have read and agreed to the published version of the manuscript.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Variation** & **Release Dominant Period of Natural Aging** \\ \hline B1P1 & 0–4 months \\ B1P2 & 0–4 months \\ B1P3 & 0–4 months \\ B2P1 & 0–3 months \\ B2P2 & 0–2 months \\ B2P3 & 0–1 months \\ \hline \hline \end{tabular}
\end{table}
Table 2: Release dominated aging period of aldehydes of pale lager beer from two barley varieties (B1 and B2) and three different malt modification levels (P1 < P2 < P3).  
**Funding:** These IGF Projects (AiF 20624 N and AiF 20151 N) were supported via AiF within the program for promoting the Industrial Collective Research (IGF) of the German Ministry of Economic Affairs and Energy (BMWi), based on a resolution of the German Parliament.  
**Data Availability Statement:** The data presented in this study are available within the article.  
**Acknowledgments:** Special thanks to O.K. for technical support.  
**Conflicts of Interest:** The authors declare no conflict of interest.