## 3 Results and Discussion
### Influence of the Proteolytic Malt Modification Level on De Novo Formation  
The previous sections focused on several individual precursor compounds and aging aldehydes. However, the contribution to _de novo_ formation should be further regarded for each substance class in a holistic perspective. Hereby, the influence of malt modification level on the aging potential is focused in the following lines. Figure 6 shows the molar distribution of the sum of free aldehydes, bound aldehydes (4-VP releasable aldehydes) [1], and precursor compounds (amino acids, dicarbonyls, and Amadori products) during aging due to different malt modification levels (P1 < P2 < P3) and barley varieties (B1 and B2). For amino acids, the sum of Strecker-relevant amino acids (leucine, isoleucine, valine, methionine, and phenylalanine) was calculated. Dicarbonly comprised the sum of 3-DG, 3-DGal, and 3-DM concentrations. Amadori products were calculated as the sum of FL and ML content. All three substance classes are precursor compounds contributing to Strecker degradation. The 4-VP releasable aldehydes from the preceding article were included in the evaluation to further elucidate comprehensively the influence of _de novo_ formation and release for the formation of flavor-active volatiles. All observed compounds contribute to the actual aging potential at several aging stages [19]. Figure 6: Malar distribution of free and bound aldehydes and precursor compounds (amino acids, dicarbonyls, Amadori products) during aging in beer at three different malt modification levels (P1 < P2 < P3) and two barley varieties (B1 and B2); **(A)** B1P1; **(B)** B2P1; **(C)** B1P2; **(D)** B2P2; **(E)** B1P3; **(F)** B2P3. In what follows, all substance classes are considered, starting with precursors. Strecker-active amino acids represented the highest percentage of the overall sum of the calculated compounds. Notably, 73-93% of the sum belonged to this substance class. Furthermore, the concentration of amino acids was 5-16 times higher than that of dicarbonyls, accounting for only 5-20% of the aging potential (sum of all compounds). The ratio of amino acids and \(\alpha\)-dicarbonyls indicated that the dicarbonyl concentration was the critical factor for _de novo_ Strecker degradation from both classes. This fact is further substantiated by the higher reactivity of dicarbonyl compounds [5]. Dicarbonyls were the only precursor substance class in which degradation was observed for almost all variations. This fact could be shown by the changes in the sum from fresh beer to 9 months naturally aged beer (B1P1: 95 to 75 \(\upmu\)M; B1P2: 114 to 103 \(\upmu\)M; B1P3: 143 to 120 \(\upmu\)M; B2P1: 90 to 94 \(\upmu\)M; B2P2: 120 to 113 \(\upmu\)M; B2P3: 156 to 134 \(\upmu\)M). The degradation indicated Strecker degradation and confirmed previous references that also observed dicarbonyl degradation during beer aging [15; 36]. The Amadori products accounted for only a minor percentage of 1-8% of the aging potential at all aging stages. Here, it should be considered that only two Amadori products as direct precursors of dicarbonyls were observed. Amadori products formed at the \(\alpha\)-amino group of amino acids as direct Strecker aldehyde precursor compounds [8] might be more important. For example, [PERSON] and [PERSON] (1989) determined fructose-derived Amadori products from valine, leucine, and isoleucine in the sum of 105 \(\upmu\)M in fresh beer [37]. Regarding free and bound-state aldehydes, it was revealed that both forms in sum accounted for only 0.7% of the aging potential. The precursor substance classes were highly elevated, compared with the free aldehydes in this study, whereby the ratio increased toward free aldehydes during aging. In fresh beer, the precursor concentration (sum of amino acids, dicarbonyls, and Amadori products) was 3050-13,790 times higher than free aldehyde concentration. After natural aging of 9 months, the ratio decreased to only 334-787 times.