Post-mortem evolution of non-protein nitrogen and its peptide composition in growing lamb muscles

Twenty-two 4-month-old lambs were used in two experiments to assess the effects of growth rate, induced by different feeding levels, and ageing (21 days, 4°C) on non-protein nitrogen (NPN) contents and their peptide composition. Muscle (Longissimus dorsi, Semimembranosus) and sex (male, female) effects were also studied. During ageing, NPN contents increased (15–20%). This variation represented 1.6–2.8% of total nitrogen. Peptides extracted from fresh muscles were in the range of molecular weight (MW) up to 12 kDa, the majority being less than 2.4 kDa (45–48%). During ageing, peptide average MW decreased (11.5–27.6%). High growth rate increased NPN contents at slaughter (+6–14%) and during ageing. Peptide average MW and post-mortem proteolysis rate as with to NPN variations were also higher. Differences between sex were observed at the end of the ageing period. Muscle effects were observed on the peptide class from 6.8 to 3.4 kDa.