The thermal denaturation of proteins was evaluated in the natural state of bovine muscle using differential scanning calorimetry (DSC). The Longissimus lumborum muscle was selected according to ultimate pH (pHu) values classified into two groups: low pHu with values between 5.4 and 5.8, and intermediate pHu with values between 5.81 and 6.19. The muscles were cut and aged at 2 °C up to 21 d post mortem. The three maximum temperatures of denaturation (Tmax1, Tmax2, Tmax3) found in muscle were evaluated, showing higher thermal stability in the intermediate pHu group, which could be an indicator of protection of proteins against aggregation or enzymatic activity. The thermal behavior of muscle proteins could be defined by biochemical factors that are affected by pHu of the muscle, however, further studies are necessary to explain this process, which could have a great impact on the understanding of the final tenderness achieved in meat.