# Characterization of Thermal Properties of Ruby Chocolate Using DSC, PDSC and TGA Methods
## 3 Results
### Oxidative Stability Measuring by PDSC  
Although several methods have been used to analyze and monitor lipid oxidation [34], oxidation reactions cannot be measured by a single method due to their complexity. The history of food preparation--thermal history in the case of fats--may be crucial in understanding thermal stability. RC is produced from cocoa butter pressed from cocoa beans that have not previously been fermented nor roasted. Therefore, the melanoidins produced during the Maillard reaction, assumed by some researchers to have antioxidant properties [35], are rather of minor occurrence and may influence the induction time of the oxidation of the fat present in RC. Table 2 presents the onset and maximum induction time (\(\tau_{\text{on}}\), \(\tau_{\text{max}}\)) values obtained during the oxidation of the fat extracted from the RC in non-isothermal mode. Figure 3: Melting profile of fat extracted from ruby chocolate. The higher the rate of heating of the samples, the bigger the onset temperature of oxidation (Figure 4). At slow heating rates, products with a low molecular mass and intermediate oxidation are formed as a result of the oxidation reaction between the primary oxidation products (hydroperoxides) with an excess of oxygen. These compounds can accelerate the degradation process. At fast heating rates, as a result of evaporation, these intermediate products are not available in the reaction with the lipids. As a consequence, the threshold DSC signal is shifted to a high value [36]. In the non-isothermal oxidation of lipids, performed with a constant flow rate (>25 mL/min), oxygen is generated to a large excess. This allows the creation of peroxides independently of the oxygen concentration. Such a process of autooxidation can be considered as a first-order reaction [37]. This allows calculation of the kinetic parameters (E\({}_{\text{a}}\), Z) using Equations (1)-(4). A commonly used iso-conversion method is the Ozawa-Flynn-Wall method. \[\log\beta=a\ \frac{1}{T}+b \tag{1}\]  
\[\log\tau=a\ \frac{1}{T}+b \tag{2}\]  
where a and b are adjustable coefficients [38]. Linear correlations were found between the logarithm of the heating rate (\(\beta\)) and the inverse temperature (1/T) (Equation (1)) and between the logarithm of the induction time and the inverse temperature (Equation (2)). \begin{table}
\begin{tabular}{c c c} \hline
**Temperature (\({}^{\circ}\)C)** & **Induction Onset Time \(\tau_{\text{on}}\) (min)** & **Induction Maximum Time \(\tau_{\text{max}}\) (min)** \\ \hline
110 & 19.84 \(\pm\) 0.69 & 113.08 \(\pm\) 3.73 \\
115 & 14.88 \(\pm\) 0.14 & 87.19 \(\pm\) 4.88 \\
120 & 10.28 \(\pm\) 0.48 & 61.65 \(\pm\) 1.12 \\
125 & 6.10 \(\pm\) 0.40 & 40.48 \(\pm\) 0.95 \\
130 & 4.32 \(\pm\) 0.68 & 32.69 \(\pm\) 1.39 \\ \hline \end{tabular} Values represent means \(\pm\) standard deviations.  
\end{table} 
Table 2: DSC onset and maximum induction time at different temperatures (\(\tau_{\text{on}}\) and \(\tau_{\text{max}}\)) for the fat extracted from ruby chocolate. Figure 4: DSC curves of thermal-oxidative decomposition of fat isolated from ruby chocolate; heating rates: 2.5, 7.5 and 15 \({}^{\circ}\)C/min. The related regression parameters are listed in Tables 3 and 4, respectively. \[E_{a}=-2.19~{}R\frac{d\log\beta}{d\frac{1}{T}} \tag{3}\]  
\[Z=\frac{\beta E_{a}e^{\frac{E_{a}}{kT}}}{RT^{2}} \tag{4}\]  
\[k=Z~{}exp\left(\frac{-E_{a}}{RT}\right) \tag{5}\]  
where R is a gas constant and Z is a pre-exponential factor. The results given in Table 3 show that the fat extracted from the RC is characterized by high values of activation energy of the oxidation process, 82.54 and 80.45 kJ/mol, and high pre-exponential factor Z, both calculated regarding onset and maximum temperatures, respectively. For pure milk fat, [PERSON] et al. [39] obtained similar but lower values of E\({}_{\text{a}}\) (80.58 and 73.89 kJ/mol) and Z (9.52 \(\times\) 10\({}^{8}\) and 1.33 \(\times\) 10\({}^{7}\)) characterizing the oxidation process, calculated for the onset and maximum temperatures, respectively.