# Edible Films Based on Ovine Second Cheese Whey with Oregon Essential Oil
## 3 Results and Discussion
### Thermogravimetric Analysis (TGA)  
Thermogravimetric analysis (TGA) and the first derivative of TGA (DTG) plots of the thermal degradation pattern of protein films are shown in Figure 4. DTG curves of percent weight loss versus temperature are obtained from raw thermogravimetric data. Table 9 shows the weight loss and residual weight of the films. Four weight loss stages were observed, as has also been reported by other authors [45]. The first period was 32-120 \({}^{\circ}\)C and corresponded to the evaporation of free water and hydrogen-bonded water to the film matrix  
The second period occurred in the range of 120-235 \({}^{\circ}\)C and involved the thermal decomposition of glycerol, the degradation of low molecular weight compounds (including essential oils) and the vaporization of structure-bound water within the film [44]. The lowest weight losses were observed in WS films compared with WPI and SCW samples, and they were lower at the highest SCW proportion; interactions among WPI and SCW compounds (denatured proteins, lipids) may improve the thermal stability of WS films. The weight loss at the third stage (235-370 \({}^{\circ}\)C) was primarily caused by polymer degradation. The highest weight loss was found in WS 1:2 films. The fourth period was in the range of 370-600 \({}^{\circ}\)C; polymer carbonization was responsible for the weight loss above 370 \({}^{\circ}\)C.  
Total weight loss was in the range of 86.3-88.6%, and the residual weight between 11.3% and 13.7%. The films with the highest values of residual weight were the mixture of WPI-SCW in a 2:1 proportion (WS 2:1) and the presence of EO did not modify this parameter. EO addition did not influence the weight loss of films. Table 10 shows the temperature peaks observed at each stage. Three peaks were observed at around 63 \({}^{\circ}\)C, 193 \({}^{\circ}\)C and 313 \({}^{\circ}\)C in all of the films. Nevertheless, two peaks (at 257 \({}^{\circ}\)C and 313 \({}^{\circ}\)C) were only found at the third stage in the WS films, which involved the presence of two chain structures with different thermal stabilities. The peak at around 313 \({}^{\circ}\)C appeared in all samples and may correspond to the degradation of parts of the protein matrix in which molecules established better molecular interactions. The peak at 257 \({}^{\circ}\)C in the polymer degradation stage could indicate a lower thermal stability of films when SCW was added to WPI, which may be due to the degradation of the denatured proteins from the second cheese whey protein concentrate that interacted with whey proteins from WPI. This peak was found at lower temperatures, 246-252 \({}^{\circ}\)C, in WS films with EO. The interaction of EO with SCW and WPI proteins may cause this decrease. However, weight losses were similar in all samples, as mentioned above. Further studies are needed for the determination of the positive or negative influence of SCW and EO addition on the thermal stability of WPI in view of the industrial application of these films in food packaging. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Film** & **Stage I** & **Stage II** & **Stage IIIa** & **Stage IIIb** \\  & **32-120 \({}^{\circ}\)C** & **120–235 \({}^{\circ}\)C** & **235–370 \({}^{\circ}\)C** & **235–370 \({}^{\circ}\)C** \\ \hline WPI & 62.7 & 193.0 & nd & 308.1 \\ WS 2:1 & 62.3 & 192.8 & 257.3 & 307.8 \\ WS 1:1 & 63.5 & 193.2 & 257.4 & 311.3 \\ WS 1:2 & 63.8 & 190.6 & 257.3 & 318.3 \\ SCW & 61.1 & 194.0 & nd & 312.5 \\ WS 2:1O1 & 63.6 & 195.9 & 246.2 & 311.9 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Thermal degradation of the films described in the materials and methods section–temperature peaks (\({}^{\circ}\)C).