## 2 Results and Discussion
### An Overall View of the Volatiles Associated with Baking in Broas
The volatile compounds that strongly contributed to differentiating the samples along PC1 (Figure 5) are placed in group B of the correlation heatmap (Figure 6 and Tables S4 and S5) and consisted, among others, of several pyrazines. The major route for the formation of pyrazines is thought to be from \(\alpha\)-aminoketones, which are products of the condensation of a dicarbonyl with an amino compound via Strecker degradation during the Maillard reaction [20; 24]. Very strong and positive correlations were found between Pz7 (2-methyl-5-vinylpyrazine) and Pz3 (2,6-dimethylpyrazine), both products from the reaction between leucine and fructose [59]. The _proa_ prepared from the available commercial hybrid variety maize flour (B12) showed a higher content in pyrazines (_p_\(<\) 0.05) than all the traditional varieties (Table S2 and Figure 7), especially in 2-methylpyrazine (Pz2) and 3-dimethylpyrazine (Pz5) (Table S3). The higher amounts of total ferulic acid in the traditional maize varieties [82] might have inhibited the formation of pyrazines [80] in the corresponding _proas_. Lastly, the volatiles which negatively contributed to PC1 and PC2 are located in group C (Figure 6). These volatiles show strong and negative correlations with several of the volatiles which belong to groups A and B. Furfural (F12) was strongly and negatively correlated to both benzofuran (F21) and furfurylfuran (F28). Thus, _proas_ with higher amounts of furfural also showed lower amounts of benzofuran and furfurylfuran, and vice versa. Furfural is mainly generated in the initial stages of the Maillard reaction and participates in further reactions [14; 24; 83]; thus, differences in the precursors which reacted with furfural and generated benzofuran and furfurylfuran, or the presence of interferents of this reaction [27; 80], may have influenced its extent. Similarly, negative correlations were found among several sulfur compounds and both furans and furanones, which have been described as important precursors of thiazoles and thiophenes [24], and between oxathane (S6) and sulfur-containing compounds, such as dimethyl disulfide (S3), dimethyl trisulfide (S8), 2-acetylthiophene (Tp5) and benzothiazole (Tz3), which might have been precursors  
Figure 7: Representation of the percentage of chromatogram area for the families of chemical compounds studied in _proas_ prepared from traditional maize varieties (_n_ = 11) and commercial maize flour. of the formation of oxathiane. A recent study in wines has proposed that thiols act as precursors of the formation of oxathiane (76). Taken together, these results suggest that the main differences among the volatile compounds of the studied _proas_ are likely due to the presence of specific precursors or interferets which affect the extent of some reaction pathways of the Maillard reaction, as the formation of pyrazines and oxathiane, rather than the overall Maillard reaction. These results can partially explain the contradictory reports on the effect of phenolic compounds on the Maillard reaction (27; 74; 80). Differences in precursors or interferets of certain pathways of this reaction in maize varieties can cause differences in _proas_ aroma. As previously stated, the analyzed commercial hybrid variety sample (B12) showed lower scores in a sensory analysis (77). The results from the present study have shown that this sample showed a higher content in pyrazines (Figure 7), particularly in 2-methylpyrazine (\(p<0.05\)). These results were somewhat surprising since pyrazines are strong odorants usually associated with positive sensorial characteristics, giving roasted and nutty aromas to bread (5; 51). However, pyrazines may contribute to aroma characteristics in _proas_ that are not typical of this type of bread, giving rise to some of the negative comments associated with B12, such as a 'weak typical flavor', 'wheat bread flavor', 'with a weak maize flavor' and 'no history' (27). Similarly, it has been reported that most pyrazines contributed negatively to the aroma of popcorn (33).