## 3 Results and Discussion
### Formation of Free Aging Aldehydes during Beer Aging  
The elucidated precursor compounds react further during beer aging and form aging aldehydes, causing sensory deterioration. This study focused on the Strecker degradation reaction because this is a typical reaction of the observed precursors amino acids, dicarbonyl compounds, and Amadori products. Figure 4 shows the concentration of 2 MP, 2 MB, 3 MB, meth, and PA during natural and forced aging in two barley varieties (B1 and B2) and three different malt modification levels (P1 < P2 < P3). Data of all sampling points are provided in the Supplementary Excel file (Table S1). The barley variety showed no effect. The formation of the aging aldehydes was comparable for both barley varieties. During aging, aldehydes were formed. As already shown in our preceding study, the rise was caused by the release from bound-state forms [1]. However, due to the observed decreasing levels of 3-DG and 3-DGal as Strecker aldehyde precursors during aging, _de novo_ formation was suggested to be occurring simultaneously. In what follows, the aging behavior of the observed Strecker aldehydes is discussed separately. The 2 MP concentration showed a strong increase during natural and forced aging. The concentration after forced aging represented levels comparable to the natural aging of 0-3 months. During natural aging, the 2 MP concentration increased up to 6 months and was then reduced up to 9 months. A shift to masked forms can be excluded because, after 9 months of natural aging, no bound-state form was observed in the preceding study [1]. It can be assumed that aldehydes are degraded by alternative reactions such as aldol condensation or further oxidation of the carbonyl group. Furthermore, 2 MP surpassed its threshold (86 \(\upmu\)g/L) [34] after 3 months of natural aging at the highest malt modification level (P3) and after 5 months at low (P1) and medium (P2) levels. Sensory attributes showed significant changes after natural aging ranging from 3 to 5 months [1]. The conformity indicates that 2 MP played a key role in the sensory changes during beer aging in the study. It is worth noting that 2 MB and 3 MB were also formed during natural and forced aging but less strongly than 2 MP. The concentration of both compounds after forced aging was comparable to the concentration after 3-6 months of natural aging. As already discussed for the 2 MP concentration, the degradation of both aldehydes occurred from 6 to 9 monthsof natural aging. This effect could also be caused by alternative degradation pathways such as aldol condensation or oxidation reactions. In this study, 2 MB and 3 MB did not surpass their flavor thresholds (2 MB: 45 \(\upmu\)g/L; 3 MB: 56 \(\upmu\)g/L) [34] but could still contribute to sensory flavor changes by synergistic effects. The limit of detection of meth is 0.8 \(\upmu\)g/L. Therefore, the Strecker aldehyde was majorly quantifiable after forced aging in B1 and after 3 months of natural aging in both barley varieties. For meth, forced aging was comparable to the natural aging of 3-6 months. After 3 months of natural aging, a constant increase in meth concentration was observed. The aldehyde surpassed its threshold of 4.2 \(\upmu\)g/L [34] after 9 months of natural aging. This result indicates that meth critically contributes to sensory deterioration in an advanced stage of beer aging (>6 months of natural aging). PA concentration increased constantly after 3 months of natural beer aging. The forced method yielded PA concentrations comparable to 6-9 months of natural beer aging, with an exception of B2P1 and B2P2 (3-6 months of natural beer aging). The aldehyde did not surpass its threshold at 105 \(\upmu\)g/L [34] but could contribute to sensory deterioration by synergistic effects such as 2 MB and 3 MB. Generally, an increasing malt modification level caused an enhanced formation of all Strecker aldehydes during natural and forced aging.