## 3 Results and Discussion  
### GC-MS Analysis  
The analysis of chromatograms generated for the individual samples allowed determination and assignment of over 99% of compounds to the main chemical groups (Table 1) constituting the odor  
Figure 1: Scheme of a typical sensorgram for metal oxide semiconductor (MOS) sensors with a ratio of reaction times of \(T_{R}\) and \(T_{CL}\) marked on the graph [14]. profile of the _Arabica_ coffees ([PERSON], 2011). Figure 2 presents a composite graph of the percentage of the chemical compounds present in the coffees. The Wiley 138 library with the highest quality of match was used to identify the compounds ([PERSON], 1976). The compounds were combined into the main chemical groups of VOCs and presented in percentages in the graphs. Alcohols, acids, aldehydes,azines, hydrazides, and ketones were the most abundant (Figure 2). The Brazilian Santos coffee contained almost 30% of aromatic compounds, mainly represented byazines. The simplest compound in this group was pyridine ([PERSON], 1976) (28.7% content). Another group of compounds that contributed to the odor profile of the Santos coffee contained aldehydes, which accounted for about 17.5%. Alcohols, acids, and hydrazides represented approximately 12-13%, and ketones constituted approximately 7% of volatile compounds. The largest contribution to the smell profile of the Ethiopian Sidamo coffee was ascribed to aldehydes, alcohols, hydrazides, andazines, which in total accounted for approximately 80% in the odor profile; in turn, ketones constituted ca. 10%. Two _Arabica_ coffees, i.e., Comal from Guatem and La Pastor from Costa Rica, were characterized by a similar percentage of hydrazides (22-25%), alcohols (18%),azines and aldehydes (14-17%), and ketones (8-10%), respectively. The El Palto Organic _Arabica_ coffee from Peru had similar contents of five groups of compounds: from 15 to 22% of alcohols, acids, aldehydes,azines, and hydrazides and 7% of ketones. The data shown in Figure 2 and Table 1 suggest that coffee aroma is generated by a combination of many chemical compounds containing various functional groups ([PERSON], 1976; [PERSON], 1976). The complex mechanisms of formation of the compounds are associated with many chemical reactions (e.g., degradation of sulfur amino acids, phenolic acids, fats, sugars, and pigments, Strecker degradation, etc.) (Brazil, 2011). However, aroma and taste are primarily generated via the Maillard reaction between sugars and amino acids (Brazil, 2011). The coffees from Brazil and Peru contained the highest content of pyridine, i.e., 28.7% and 21.9%, respectively. In the other three coffee brands, pyridines accounted for 13-15%. This compound is perceived as a bitter vegetable odor. In turn, aldehydes, ketones, esters, alcohols, and acids, i.e., compounds from the group of furans, were abundant in all coffee species. They are characterized by  
Figure 2: Percentage share of groups of volatile substances for all coffees beans after roasting. the ethereal smell of grass and hay. The coffee aroma also contained compounds with a pyrazine ring, which have a bittersweet aroma (Table 1). The differences in the contents of volatile compounds were related to the different conditions of coffee growth and harvest, interspecies differences, and pre-roasting [7] storage conditions [22]. They did not depend on the degree of roasting [21], as the final parameters of the roasting process were identical for all coffee types. Figure 2(a) shows the projection of the variables on the PC1 and PC2 planes. Chemical compounds located within the two circles have a strong effect on the possibility to determine the country of coffee origin [23]. Compounds in the negative quadrant of principal component PC1 differentiate the Brazilian and Peruvian coffee from the others. Noteworthy, pyridine and 2-oxopropanol (compounds located in the PC1 and PC2 negative quadrant) are most abundant in the Brazilian and Peruvian coffees.