## 3 Results and Discussion
### Volatile Compounds of Cold Brew Samples
#### 3.3.1 Aromatic Alkyl Pyrazines  
Hundreds of compounds have been identified as coffee flavor components where pyrazines in coffee are determined in predominant amounts in similar studies [26]. Pyrazines are monocyclic aromatic rings with two nitrides (in the para position). The formation of pyrazines is related to the process of heating food--amino acids react with sugars in Maillard reactions. A longer roasting profile also resulted in their increase. Many of these compounds are intense and exhibit very low odor thresholds--for example, 2,5-dimethylpyrazine has an odor threshold of 1 ppm in water [27]. Pyrazine compounds give a coffee, nutty, roasted, earthy aroma [28]. Its derivatives may contribute to reducing free radicals in the body [29]. In the case of pyrazine, the lowest value was determined for sample A 12h 5 \({}^{\circ}\)C--0.3 \(\mathrm{\SIUnitSymbolMicro g}\), whereas an average was achieved at 4.0 \(\mathrm{\SIUnitSymbolMicro g}\)/g. Furthermore, 11 samples from 27 reached more than average. It has a pungent, sweet smell that becomes floral when diluted [30]. 2-methylpyrazine was the most abundant compound among the pyrazines group in the experiment. This is in agreement with the results of [PERSON] et al. [31], who reported a range of concentration of the aromatic compound in coffee from 34.8 ppm to 91.6 ppm. The mean value in our study was 47.9 ppm, the overall range was between 1.0 and 121.5 ppm (\(\mathrm{\SIUnitSymbolMicro g}\)/g). The knowledge gained from the effects for the example of time and temperature suggest that 2-methylpyrazine is made by asparagine and fructose rather than glucose [32]. It is defined as nutty, roasted, grassy and chocolity in dilution [30]. It is noteworthy that its thresholds of odor detection in water, oil and air are as follows: 30-100, 27 ppm and >2000 \(\mathrm{\SIUnitSymbolMicro g}\)/m\({}^{3}\)[33]. Ethyl pyrazine--a derivative of methylpyrazine--is characterized by completely different odor properties, which is largely due to an additional methyl group in the side chain. Its aroma is described as nutty, sweet, buttery, rum-like with an odor detection in water of 4-21 ppm and oil 17 ppm [34]. In a study by [PERSON] et al. [35], the authors reported the percentage range of ethyl pyrazine in macerated coffee, at a ratio of 1.0 g coffee ground per 5 mL water for 18 h with occasional stirring, to be in the range of 0.65-4.16%. After converting the concentration in our research from the mean value 25.6 \(\mathrm{\SIUnitSymbolMicro g}\)/g to %, we obtained a higher average ethylpyrazine content in samples. This was 5.94%, summarized by samples A, B and C, respectively: 1.02%, 2.70% and 2.22%. This suggests that the concentration of pyrazine compounds and their derivatives increases with the degree of coffee roasting (more than 50% comparing 210 \({}^{\circ}\)C with 220 \({}^{\circ}\)C and 230 \({}^{\circ}\)C). Figure 6: Main volatile formation reactions during the coffee roasting process. Precursors are formed in rectangles. Pyrazine, 2,6-dimethylJ Pyrazine, 2,5-dimethyl: The research carried out by [PERSON] shows that especially when heat treatment is too intense, DMP is formed [34]. The results of [PERSON] and [PERSON] [36] also showed that the reaction between L-ascorbic acid and L-threonine/L-serine leads mainly to the formation of pyrazines. Many of these are alkylpyridazines, such as 2-methylpyrazine, 2,5-dimethylpyrazine and 2-ethylpyrazine [36]. On the other side, [PERSON] et al. indicated a significant decrease in DMP with an increase in polyphenol concentration [37]. They are characterized by a nutty, sweet, toasted aroma [34]. The concentration values of dimethylpyrazine ranged from 3.70 to 145.4 \(\upmu\)g/g in the analyzed extracts and we confirmed that concentration increases with the higher final temperature roasting of green coffee beans. In a similar study for cold brew, the concentration was calculated to range from 4 to 23 ng/mL of the extract by SPME-GC/MS method [5].