# Thermal and Kinetic Properties of Brazilian Coffee Arabica Beans
## 3 Results and Discussion
### Oxidative Stability and Thermokinetic Parameters  
The parameter that is used to determine the oxidative stability is induction time. The DSC curves presenting the oxidation induction times are shown in Figure 2.  
The PDSC analysis of the fats obtained from the roasted coffee samples, performed at an isothermal temperature of 120 \({}^{\circ}\)C, indicates that the induction time increased in comparison to the fats from green coffee beans. In this case, the fats obtained from green coffee beans are less stable compared to the fats extracted from light and dark roasted beans. The oxidative stability of coffee fat is associated with the groups of compounds that have a major contribution to the overall activity of coffee beans, i.e., polyphenols, melanoidins and tocopherols. During the roasting process, oil-soluble Maillard reaction products (including melanoidins) are developed, which causes a great increase in antioxidant activity of the roasted coffee in comparison to green coffee beans [33]. At the same time, polyphenols and tocopherols are partially degraded and partially bound to the structures of the newly formed melanoidins [34]. The remaining phenolic compounds have a positive, non-linear relationship to the antioxidant activity of roasted coffee beans [35]. These phenomena could cause a slight decrease in antioxidant activity in the dark roasted coffee compared to light roasted beans, which influences the thermal stability of coffee fat. Figure 2: The oxidation induction time of Brazilian coffee samples. To evaluate the kinetic data of the samples, experimental onset (T\({}_{\text{on}}\)) and maximal (T\({}_{\text{max}}\)) oxidation temperatures at six heating rates were determined and presented in Tables 2 and 3. The determined T\({}_{\text{on}}\) values (\({}^{\circ}\)C) were recalculated to match an absolute scale (K). Then, the absolute onset temperatures were presented as a function of a heating rate logarithm (log\(\beta\)) and fitted to a linear regression model (Equation (1)), as follows:  
\[\log\beta=\text{a}(1/\text{T}_{\text{on}})+\text{b} \tag{1}\]  
where a and b are the regression coefficients and the regression constant, respectively. For the established initiation conditions, fat oxidation reactions occurring in excess of oxygen are first-order reactions [36; 37; 38]. Assuming that the reaction degree is constant and independent of the heating rate while a DSC curve reaches its peak, the activation energy (Ea) was calculated based on the following Ozawa-Flynn-Wall method (Equation (2)):  
\[\text{E}_{\text{a}}=2.19\;\text{R}\;\frac{\text{d}\log\beta}{\text{d}(1/\text{ T})} \tag{2}\]  
where R--gas constant; \(\beta\)--heating rate (\({}^{\circ}\)C min\({}^{-1}\)); T--temperature (K). Pre-exponential factor (Z) was also calculated using the Arrhenius Equation (3), as follows:  
\[Z=\frac{\beta\text{E}_{\text{a}}\frac{\text{E}_{\text{a}}}{\text{R}\text{T}^{ 2}}}{\text{R}\text{T}^{2}} \tag{3}\]  
The calculated values of kinetic parameters characterizing the thermal-oxidative decomposition of the coffee fat samples are summarized in Table 4. The activation energy of the samples ranged from 94 to 105 kJ mol\({}^{-1}\) and 29 to 33 kJ mol\({}^{-1}\) for T\({}_{\text{on}}\) and T\({}_{\text{max}}\), respectively. The estimated values of activation energy for the onset temperature were higher than the values for the maximal temperature. Similar regularity was also shown in the case of the pre-exponential factors. The difference between the activation energy values at the start of thermal-oxidation and maximum temperature may have resulted