## 1 Introduction  
After bottling, beer flavor is affected by various chemical reactions, leading to flavor instability: One main reason for a change in flavor in beers is the rise of aging indicators such as sensory-active aldehydes. In our preceding study (part one), the sensory and chemical effects of beer aging in lager beer were introduced [1]. The concentration of aging-relevant aldehydes increases during storage, and these aldehydes can be either formed by their release from bound-state aldehydes forms or by _de novo_ formation pathways [2]. The preceding study focused on the release of bound-state aldehydes (part one). Important reactions for _de novo_ formation of aging aldehydes are the Strecker degradation, the Maillard reaction, and oxidation reactions such as lipid oxidation or oxidation of the corresponding alcohols [3, 4]. Despite moderate storage temperatures of approximately 20 \({}^{\circ}\)C, _de novo_ formation of aging aldehydes can occur with low reaction rates, starting in fresh beer. Here, the concentrations of precursor substances are a critical factor for reaction kinetics [4]. Figure 1 summarizes the reaction pathways for the _de novo_ formation of aldehydes observed in this study. Regarding Strecker degradation and the Maillard reaction, amino acids, Amadori products, and dicarbonyls are important precursor substances [5; 6; 7]. During Strecker degradation, dicarbonyls and amino acids react to Strecker aldehydes. Here, they form an unstable hemiaminal adduct by water elimination, which further undergoes irreversible decarboxylation. After water addition, the adduct is decomposed into Strecker aldehydes and an \(\alpha\)-ketoamine compound [4]. Important aging-relevant Strecker aldehydes are 2-methylpropanal (2 MP) from valine, 2-methylbutanal (2 MB) from leucine, 3-methylbutanal (3 MB) from isoleucine, phenylacetaldehyde (PA) from phenylalanine, and methional (meth) from methionine. Alternatively, Strecker aldehydes could be directly formed from Amadori products [8] or by oxidation of their corresponding alcohols, e.g., 2 MP from the oxidation of 2-methylbutanol [9]. Regarding the direct oxidation of higher alcohols, the pathway could be deemed insignificant due to minimal O\({}_{2}\) levels after bottling (<0.1 mg/L) [10]. Further, a study on wine recently found that oxidation of higher alcohols to their respective aldehydes is significantly less relevant than the degradation of amino acids during the Strecker reaction [11]. A competing reaction pathway for dicarbonyls and amino acids is the final stage of the Maillard reaction [12]. Here, the amino groups of arginine or lysine can react with dicarbonyls forming \"advanced glycation end products\" (AGEs). For example, Pyrraline and N\({}^{\varepsilon}\)-carboxyethyllysine (CEL) can be formed via the reaction of the \(\varepsilon\)-amino group of lysine with the dicarbonyl 3-deoxyglucose (3-DG) [13] or with methylglyoxal [14], respectively. [PERSON] et al. demonstrated an 8% conversion from artificially spiked 3-DG to AGEs during aging [15]. Therefore, AGE formation can influence dicarbonyl reactivity during aging. Thus far, several studies have shown a negative sensory effect of artificially increased precursors. [PERSON] et al. pointed out that an elevated content of amino acids increased protease activity during the mashing procedure had a negative effect on beer sensory quality during aging [16]. [PERSON] et al. reported negative sensory effects by spiking phenylalanine and methionine to fresh beer [17]. [PERSON] et al. showed enhanced Strecker aldehyde formation after prolonged storage by adding 3-DG to fresh beer [15]. However, there is a lack of knowledge about the effect of technologically varied initial precursor content on _de novo_ aging aldehyde formation during beer aging (forced and natural). As introduced in our preceding study (part one), accelerated proteolytic malt modification, in particular, is likely to be a good adjusting tool because it increases the levels of amino acid and dicarbonyls in malt [18] and wort [19].