Proteolytic changes of myofibrillar proteins in Podolian meat during aging: focusing on tenderness

The aim of this study was to understand the relationship between changes in postmortem degradation of myofibrillar proteins and tenderness development in 3 different muscles from 10 Podolian young bulls aged 1, 7, 14, and 21 d. Psoas major (PM), longissimus dorsi (LD), and semitendinosus muscle (STD) were removed from each half carcass 24 h postmortem. Meat chemical composition, Warner-Bratzler shear force (WBSF), myofibril fragmentation index (MFI), total collagen content, and changes in myofibrillar proteins were estimated in triplicate at each aging time. A significant muscle effect was found on meat chemical composition. Semitendinosus muscle showed the lowest intramuscular fat percentage (P < 0.01) and the highest total collagen content (P < 0.01) with respect to LD and PM. Warner-Bratzler shear force decreased during aging in all muscles (P < 0.001), and semitendinosus was the toughest muscle whereas PM was the most tender (P < 0.001). Myofibril fragmentation index significantly increased (P < 0.001) in LD and PM meat throughout aging time whereas in semitendinosus it increased from 14 d of aging. Proteolysis was investigated by SDS-PAGE, western blotting, and 2-dimensional electrophoresis. Throughout postmortem aging, some structural proteins changed in intensity in all muscles analyzed. The blotting profile highlighted that desmin and troponin-T (TnT) bands were affected by both muscle and aging effects. Desmin degradation was more intense and faster in LD muscle than in semitendinosus and PM muscles. A progressive increase of the degraded isoforms of TnT (33 and 30 kDa polypeptides) was found during aging in LD, while, in PM these bands appeared earlier showing a greater intensity from 1 d. During aging, 2-dimensional gel electrophoresis (2DE) image analyses results showed a significant increase of the total number of spots reaching the highest value in the LD muscle at 21 d of aging (P < 0.01). Proteins separation also revealed differences in the spot number and expression patterns of myosin light chain (MLC) isoforms among muscles. A principal components analysis applied to meat chemical composition, tenderness, and myofibrillar proteins accounted for approximately 96.09% of total variance. Data highlight that aging affects the meat tenderness and proteolysis with different intensities in each muscle. These results provide knowledge about the tenderness mechanism in different muscles from a rustic breed and could be useful for the development of muscle specific strategies for improving the quality and value in different commercial cuts.