## 3 Results and Discussion
### Study of Coffee Silverskin Oil
#### 3.2.2 PDSC Study of Coffee Silverskin Oil  
Induction time is essential in the terms of fat stability and can be used as a primary parameter for the assessment of the resistance of the tested fat to its thermal oxidative decomposition. The PDSC analysis of fats was performed at isothermal conditions. The DSC curves presenting the oxidation induction times are shown in Figure 3. The time of oxidation reaction reached the values of 22, 48 and 108 min for experiments conducted at a temperature of 100, 110 and 120 \({}^{\circ}\)C, respectively. Comparing the obtained results with other oils, it can be stated that oil extracted from silverskin is of low stability. The oxidation induction time for rapeseed oil, hazelnut oil and extra virgin oil at 100 \({}^{\circ}\)C reached 200, 141 and 155 min, respectively [41; 42], while in the case of silverskin oil--about 105 min. A similar tendency was observed in the case of applying higher temperatures (110 and 120 \({}^{\circ}\)C).  
The DSC technique was also used in the study of thermokinetic parameters of the oxidation process. The results obtained show the effect of the sample heating rate on the T\({}_{\rm on}\) and T\({}_{\rm max}\) temperature values. With the increase in the heating rate, the temperature for initiating the oxidation process and  
Figure 3: DSC curves of oxidation induction time of silverskin fat samples.  
The dependencies shown in Figures 4 and 5 can be described by a regression equation:  
\[\text{Log}\ \beta=a(1/T_{\text{on}}\ \text{or}\ \text{T}_{\text{max}})+b \tag{1}\]  
where \(\beta\) is a heating rate and T is the temperature.  
Activation energy (\(\text{E}_{\text{a}}\)) values and pre-exponential factor (Z) were calculated using Equations (2) and (3), respectively.  
The activation energy values were calculated according to the Ozawa-Flynn-Wall method based on the equation:  
\[\text{E}_{\text{a}}\ =\ -2.19\ R.\frac{d\log\beta}{d(1/T)} \tag{2}\]  
where R is the gas constant.  
The pre-exponential factor was calculated from the dependence:  
\[Z\ =\ \frac{\beta E_{a}e^{\frac{E_{a}}{RT}}}{RT^{2}} \tag{3}\]  
Quantified regression coefficient values \(a\), regression constant \(b\) and the determination coefficients \(R^{2}\), as well as values of activation energy and pre-exponential factor, are summarized in Table 2.  
In the present study, \(\text{E}_{\text{a}}\) reached the value of 81.52 kJ/mol (calculated for \(\text{T}_{\text{on}}\)) and 84.12 kJ/mol (calculated for \(\text{T}_{\text{max}}\)). In terms of oils, the \(E_{\text{a}}\) value is affected mainly by the degree of unsaturation. It was previously reported that a high polyunsaturated fatty acid (PUFA) content would decreased, while high monounsaturated fatty acid (MUFA) and saturated fatty acid (SFA) content would increase, the \(E_{a}\) value for the lipid oxidation process [43]. When compared with other oils--hazelnut oil (\(E_{\text{a}}\) = 89.1 kJ/mol) and rapeseed oil (\(E_{\text{a}}\) = 92.69 kJ/mol)--it can be stated that silverskin oil is of low resistance to oxidation. Taking the above into consideration, the research was completed by defining fatty acids' composition, which was carried out by gas chromatographic analysis of fatty acid methyl esters.