## 3 Results and Discussion
### TBARS Values  
During cooking, a moderate lipid oxidation occurs, which is an important source for the formation of meat flavour and odour compounds [9; 50]. In this research, this oxidation was determined using the TBARS value, meaning an index of malondialdehyde concentration (MDA), which is a secondary product of lipid oxidation [51]. The results, expressed as mg MDA/kg of the _Biceps femoris_ muscle, are presented in Figure 1. As we can see, there were significant (\(P<0.01\)) differences regarding the diet provided in all thermal treatments. More concretely, the inclusion of chestnuts in the finishing diet seemed to increase the value of TBARS. These outcomes do not agree with those shown previously by [PERSON] et al. [52] and [PERSON] et al. [53], who observed that the inclusion of chestnuts reduced the TBARS index in pork meat and its derivates. At the same time, our research also do not agree with other studies that did not find any significant difference in lipid oxidation after using chestnuts in pig fattening [54; 55; 56; 57]. The increase in TBARS value that occurred in our inquiry could be due to the fact that CH samples present a higher percentage of intramuscular fat (Table 1); therefore, it would be normal to present a higher oxidation. However, in our research, it is difficult to explain this because we have seen that TBARS are not correlated with the percentage of fat, since very low Pearson correlationswere obtained for both diets (r = 0.047; \(P<0.05\); and r = \(-0.073\); \(P>0.05\), for CH and CF, respectively). Even so, a possible reason for the datum obtained could be that CH pigs tend to accumulate a higher concentration of unsaturated fatty acids [6; 8; 18], which are more susceptible to oxidation [58; 51; 59]. In relation to the different thermal treatments, we have obtained TBARS values ranging from 0.32 to 1.52 mg MDA/kg. These aftermaths are similar to those reporter in different cooked pig meat and products where values between 0.29 and 3.26 mg MDA/kg were come across [9; 27; 60; 61]. However, other authors found lower TBARS rates, which did not exceed 0.29 mg MDA/kg [62; 63; 64]. One reason why our study shows high values of TBARS may be the fact that the Celta pig breed tends to accumulate more unsaturated fatty acids than other breeds [2]. Additionally, [PERSON] et al. [65] indicated that the TBARS value from which the perception of rancidity dominates the flavour of the pig cooked meat was between 0.5-1 mg MDA/kg. According to the previous rank, the samples belonging to the pigs fed with chestnut would present a rancid flavour, since they obtained a TBARS index higher than 1 mg MDA/kg (except for frying method). However, other studies for distinct products showed less restrictive values. Thus, [PERSON] and [PERSON] [66] and [PERSON] et al. [67] showed a threshold value over 2 mg MDA/kg for beef meat. On the other hand, the different cooking methods significantly (\(P<0.01\)) affected the TBARS index for both diets. Fried samples presented significantly lower values (\(P<0.05\)) of TBARS than the other cooking methods that did not differ significantly among them. More concretely, microwaved and grilled treatments displayed the highest TBARS values (1.52 and 1.39 mg MDA/kg for CH samples and 0.51 and 0.57 mg MDA/kg in CF samples, respectively). These outcomes do not agree with those obtained by [PERSON] et al. [68], who observed that microwave cooking did not generate large TBARS values in bacon. But other authors obtained results that adapt to ours. Thus, [PERSON] et al. [60], [PERSON] et al. [69] and [PERSON] et al. [34] concluded that the microwaving produced a greater lipid oxidation than other thermal treatments. Keep in mind that this cooking method is characterized by the use of a low time and temperature, the data obtained in this work suggest that there may be an interaction between microwave and meat fat which causes the oxidation of unsaturated fatty acids [9].