Coffee is made up of modified seed of the fruit of various tropical to subtropical trees or coffee shrubs. Coffee has a large number of varieties, only a few of them have economic significance. Almost all of the world's coffee production is provided by two varieties, called ‘Arabica’ and ‘Robusta’ (Butt and Tauseef Sultan, 2011). Arabica (Coffea arabica), is the most important botanical species, especially for the high quality of its fruits. It comes from about 70% of the world's green coffee production. Robusta (Coffea canephora), is the second most important variety of coffee, and its share of world production is steadily growing mainly due to its greater adaptability to habitats and disease resistance. Other reasons to increase demand of Robusta coffee in the market are the growing demand for instant coffee, which is preferentially made from Robusta coffee and last but not least, lower price of Robusta coffee compared to Arabica coffee (Kemsley et al., 1995). World coffee production in 2017 was about 9.5 million tonnes, which makes coffee the second most important commodity of world trade (FAOSTAT, 2018).

After processing the coffee beans by wet or dry drying technologies (Arya and Jagan Mohan Rao, 2007; Guimar, Berbert and Silva, 1998), coffee beans are roasted. During coffee roasting, coffee beans get brown colour and their characteristic flavour and aroma (Yeretzian et al., 2002). In different countries, different roasting styles have been created according to population preferences. These roasting styles differ from each other at the roasting temperature used and the total coffee roasting time (Moon, Yoo and Shibamoto, 2009; Dórea and DaCosta, 2005).

Due to the popularity of coffee in the world many researchers were involved in coffee research in the last quarter of a century. Most studies on coffee are healthrelated studies (Ciaramelli, Palmioli and Airoldi, 2019; Poole et al., 2017; Ludwig et al., 2014; Butt and Tauseef Sultan, 2011; Dórea et al., 2005). Other studies are focused on the role of roasting conditions of the coffee in the level of selected compounds. Information on what is happening in roasted coffee beans is quite sufficient in the available literature, but there are only few studies that deal with the complex monitoring of changes in the chemical composition of coffee beans during roasting (Wei et al., 2012).

In the present study the fate of sucrose, chlorogenic acid, acetic acid, formic acid, lactic acid, caffeic acid, total phenolic compounds and 5-hydroxymethylfurfural was studied in coffee roasted in two ways (Medium roast and Full city roast).

Coffee beans contain, in addition to water and minerals, a large number of organic substances. The main component of coffee beans are carbohydrates. The coffee beans contain various hemicelluloses, starch, oligosaccharides and mainly sucrose. The amount of monosaccharides is relatively small. Other substances contained in coffee are proteins, non-protein nitrogenous substances, phenolic substances, non-volatile organic acids, volatile substances and oils (Arya and Jagan Mohan Rao, 2007; Farah and Marino Donagelo, 2006; Redgwell and Fisher, 2006).

Concentration of sucrose in green coffee beans and in roasted coffee beans is presented in Table 1 and Table 2. The results show that sucrose degradation occurs during the roasting process. Sucrose degradation is explained by sucrose hydrolysis to glucose and fructose, which may be further fragmented to form aliphatic acids, or which may participate in Maillard reactions with proteins or amino acids (Ginz et al. 2000). The sucrose concentration decreased in the middle of the roasting process by 47% in the case of Medium roast and by 59% in the case of Full city roast. At the end of the roasting process, almost all sucrose has already been degraded (96 – 98%).

Another substance that has been observed to reduce the concentration during the roasting process was chlorogenic acid. Concentration of chlorogenic acid in green coffee beans and in roasted coffee beans is presented in Table 3 and Table 4. Chlorogenic acid concentration decreased in the middle of the roasting process by 45% in the case of Medium roast and by 42% in the case of Full city roast. At the end of the roasting process, 67% of chlorogenic acid contained in green coffee was degraded during Medium roast and 85% during Full city roast. Chlorogenic acid is involved in colour, flavour and aroma formation of coffee Farah and Marino Donagelo, 2006; Farah et al., 2005). Major degradation products of chlorogenic acid are melanoids and low molecular weight compounds.

Strong significant correlation was found between sucrose concentration and concentration of organic acid during coffee roasting (r >0.9).

Concentration of organic acid in coffee during the roasting process is shown in Table 3 and Table 4. While the sucrose concentration in coffee decreases during roasting, the concentration of organic acids significantly increases. The most significant change was found in the lactic and formic acid content. The content of these acids in coffee beans rose almost 100 times after coffee roasting. Trend of changes in the concentration of organic acids was similar for Medium roasting and French roasting, however in the case of French roasting decrease in organic acid content was observed in the later stage of roasting. Ginz et al. (2000) lists the content of organic acids in Robusta coffee after roasting to be approximately 2 mg.g^-1 in the case of formic acid and acetic acid and 0.2 mg.g^-1 in the case of lactic acid. Formation of organic acids in coffee is described by Lobry-deBruyn-vanEckenstein rearrangement reaction in which fructose or glucose produced by sucrose hydrolysis is involved, and by formation of 1,2-endiole or 2,3-endiole as acid precursors (Ginz et al., 2000). Formation of organic acids in coffee during roasting process did not significantly affect the pH of the coffee (Table 1 and Table 2.). This finding can be caused due to highly complex buffering effects and the wide distributions of salts and acids present in coffee. Jeszka- Jeszka-Skowron et al. (2016) measured pH value of water treated Brazil Arabica green coffee to be 4.92.

Ginz et al. 2000 monitored pH changes in Robusta coffee during the roasting process and recorded pH change from 6.1 to 5.7 similar to this study (Table 1 and Table 2).

Another strong correlation was found between chlorogenic acid concentration and caffeic acid concentration in the case of Medium roast (r = 0.8025). However, in the case of Full city roast, this correlation was not significant and week (r = 0.2403). Chlorogenic acid is an ester of quinic acid and phenolic acid, mostly caffeic, ferulic or 3-hydroxycinnamic acid (Farah and Marino Donagelo, 2006). During shorter roasting at lower temperatures chlorogenic acid can be hydrolysed and concentration of caffeic acid in coffee beans may temporarily increase. On the other side, with longer roasting times and higher temperatures, caffeic acid released from chlorogenic acid can be further degraded. From the results summarized in Table 4 it can be seen a significant increase of caffeic acid concentration in roasted coffee beans and subsequent reduction of caffeic acid concentration over longer periods of roasting. The formation of phenolic substances during roasting of coffee is also evident from the total concentration of phenolic compounds presented in Table 1 and Table 2. Measured concentrations of chlorogenic acid, caffeic acid or total phenolic compounds in this study are comparable with data published in literature. Chlorogenic acid concentration in green coffee beans is reported within the range of 34 – 57 mg.g^-1 while in roasted coffee beans in the range of 2 – 19 mg.g^-1 (Ludwig et al., 2014; Narita and Inouye, 2015; Farah and Marino Donagelo, 2006; Moon, Yoo and Shibamoto, 2009).

Total phenolic compounds content in coffee is reported to be 14 – 30 mg.g^-1 (gallic acid equivalent) while caffeic acid content in coffee is reported to be 1.4 – 3 mg.g^-1 (Bauer et al., 2018; Hall, Yuen and Grant, 2018).

Almost all foods that are heat-treated are monitored for content of 5-hydroxymethylfurfural. This compound is generated in food by the Maillard reaction (Antal et al., 1990). Increased interest in 5-hydroxymethylfurfural stems from a partially verified suspicion that this compound is a health hazard compound that may be mutagenic, carcinogenic and cytotoxic (Abraham et al., 2011). The content of 5-hydroxymethylfurfural in roasted coffee is reported to be 0.3 – 1.9 mg.g^-1 (Murkovic and Pichler, 2006). In this study, maximum concentration of 5-hydroxymethylfurfural was measured to be 0.549 mg.g^-1 in coffee during Full city roast. During Medium roast maximum content of 5-hydroxymethylfurfural was found to be 0.346 mg.g^-1. Relatively interesting is the course of 5-hydroxymethylfurfural concentration during roasting. During both Medium and Full city roast two sharp maxima in 5-hydroxymethylfurfural concentration were recorded, which could correspond to the first and second crack in coffee beans. After reaching the second maximum, concentration of 5-hydroxymethylfurfural in coffee decreases, because of its degradation to organic acids (Murkovic and Bornik, 2007).

To investigate the overall composition changes during the roasting process, PCA was performed on the data for all coffee bean extracts at different time of roast. The roasting process was divided into three categories according to the total roasting time (short, medium and long time). The results are shown in Figure 1 and Figure 2. The PCA plots confirmed that sucrose and chlorogenic acid degraded during the roasting process and also that with longer roasting times the content of organic acids in coffee increases. Both Figures 1 and Figures 2 also show that content of 5-hydroxymethylfurfural is the highest in medium roasting times.

This study proved that coffee roasting is a complex chemical process. The basic processes detectable during coffee roasting are the decomposition of sucrose and chlorogenic acid. Almost all sucrose is degraded during roasting independently of the roasting method. Degradation of chlorogenic acid is higher with longer roasting at higher temperatures. During the Full city roast (225 °C, 19 min) up to 85% of chlorogenic acid was degraded. Degradation products of sucrose and chlorogenic acid are low molecular organic acids and phenolic acids. Formic or lactic acid concentrations in coffee beans increased up to 100-fold during roasting. The increase in the concentration of phenolic compounds was not so steep, but it was observable. From the measured results, it cannot be clearly stated that with higher temperature and with higher roasting time concentration of 5-hydroxymethylfurfural is increasing. Highest concentrations of 5-hydroxymethylfurfural were measured at medium roasting times at both Medium and French city roasting temperatures. Conversely, at shorter roasting time and lower temperature higher concentrations of 5-hydroxymethylfurfural were found at the end of roasting.