79 Postmortem proteolysis in pork does not depend on fibre type distribution M. Christensen 1 , P. Henckel 2 and P.P. Purslow 1 1 The Royal Veterinary and Agricultural University, Department of Dairy and Food Science, Rolighedsvej 30, DK-1958 Frederiksberg C, Denmark 2 Danish Institute of Agricultural Sciences, Department of Product Quality, Postbox 50, DK-8830 Tjele, Denmark Introduction Proteolytic degradation is known to be faster in white muscles than in red muscles (Whipple & Koohmaraie, 1992). Variation in eating quality between muscles has often been correlated to their metabolic properties, as determined by the fibre type distribution. Correlations between fibre type distribution and postmortem proteolysis could result from two possible effects: (1) Due to their inherent differences in metabolic potential, composition and content of proteolytic enzymes, fibres of some types may degrade more than others. (2) The balance of fibre types controls postmortem (p.m.) metabolic characteristics of the muscle as a whole, with all fibre types within it being equally affected. An experiment was conducted to compare the rate of postmortem proteolysis in five porcine muscles differing in fibre type distribution and to compare the rate of proteolysis in type II fibres isolated from these muscles. Materials and methods Three pigs (all the same crossbreed of Duroc, Landrace and Yorkshire) were used in this study. At 1 hour p.m. Semitendinosus (ST), Semimembranosus (SM), Longissimus dorsi (LD), Soleus (S) and Vastus intermedius (VI) were removed from the left side of the carcasses. Muscle samples ( 10 g) were cut and frozen in liquid nitrogen. At 24 hours p.m. the five muscles from the right side of the carcass were removed and muscle samples were either frozen immediately or stored for 2 and 7 days, respectively, at 2 o C. Fibre types were identified by staining for myofibrillar ATPase activity (Brooke & Kaiser, 1970). Muscle homogenates and type II fibres were dissolved in urea buffer (Fritz et al., 1989). Desmin and its degradation products were resolved by SDS-PAGE on 10% separating gels (Novex, USA). Proteins were electrophoretically transferred to polyvinylidene fluoride membranes and incubated with primary mouse anti-desmin (DE-R-11, 1:5.000, Dako, UK) antibody. Antibody binding was visualised by exposure to BCIP/NBT. Densitometric scans of membranes were performed using a CREAM software program (Kem- En-Tek, Denmark). Results The rate of desmin degradation in the five muscles is shown in figure 1. Desmin degraded faster in LD and SM than in ST, S and VI. The rate of desmin degradation was expected to be similar for ST, LD and SM because the fibre type distribution of these muscles is similar (data not shown). However, ST exhibited the same rate of degradation as VI and S, even though VI and S have more type I and IIa fibres than ST. The inter-muscle differences can therefore not be explained solely by the fibre type distribution but may also be influenced by other muscle-specific traits (e.g., proteolytic potential). Desmin degradation in type II fibres is illustrated in figure 2. The relative change in the band intensity of native desmin from day 1 to 8 p.m. was calculated and used as an estimate of the rate of degradation because degradation products could not be detected. The highest relative change of desmin occurred in type II fibres isolated from LD, while desmin did not change from day 1 to 8 p.m. in ST. These results indicate that type II fibres show different patterns of proteolysis depending on the muscle in which they are located. Conclusions Differences between muscles in the rate of degradation do not seem to be a direct result of the fibre type distribution. The rate of degradation probably depends more on the local environment (i.e., pH and proteolytic potential) within the muscle rather than on variations between individual fibres of given types. References Brooke, M.H. and K.K. Kaiser. 1970. Arch Neurol. 23: 369. Fritz, J.D., D.R. Swartz and M.L. Greaser. 1989. Anal. Biochem. 180: 205. Whipple, G. and M. Koohmaraie. 1992. J. Anim. Sci. 70: 798. Figure 1 . Western blot of muscle homogenates at 0 (lane a, e, i, m, q), 1 (lane b, f, j, n, r), 3 (lane c, g, k, o, s) and 8 (lane d, h, l, p, t) days postmortem. Purified desmin appears in the single track on the right. Each muscle was analyzed from three different animals. 54 kDa 39 kDa LD SM ST S VI std a b c d e f g h i j k lm n o p q r s t Figure 2. Western blot of type II fibres at 0 (lane a, e, i, m, q), 1 (lane b, f, j, n, r), 3 (lane c, g, k, o, s) and 8 (lane d, h, l, p, t) days postmortem. Purified desmin appears in the single track on the right. Each muscle was analyzed from one animal. LD SM ST S VI std 54 kDa a b c d e f g h i j k l m n o p q r s t https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1752756200004610 Downloaded from https://www.cambridge.org/core. Gothenburg University Library, on 01 Feb 2020 at 22:06:11, subject to the Cambridge Core terms of use, available at