Studies on the muscles of meat animals III. Comparative composition of various muscles in pigs of three weight groups
Authors: Lawrie R. A. , Pomeroy R. W. , Cuthbertson A. 
Published: March 2009

1. The contents of moisture, total nitrogen and intramuscular fat (and its iodine number), and the ultimate pH, were determined on longissimus dorsi (lumbar and thoracic regions), psoas major, rectus femoris, triceps (lat. head), superficial and deep digital flexor, sartorius and extensor carpi radialis muscles from ten pigs, of accurately known history, in each of three weight groups—pork, bacon and manufacturing (150, 200 and 250 lb. live weight, respectively).

2. In addition, the myoglobin content of longissimus dorsi (lumbar), psoas major, rectus femoris, triceps (lat. head) and extensor carpi radialis was also determined. In all five muscles there was an increase in pigment content with increase of body weight, but there was some suggestion that the myoglobin content of psoas major and triceps showed a proportionally greater increment between pork and bacon weights and rectus femoris and extensor carpi radialis between bacon and manufacturing weights.

3. In pigs of all three weight groups there were highly significant differences in fat-free moisture content between muscles and between animals. The mean, fat-free moisture content of longissimus dorsi (lumbar) (76·33%) is significantly less than that of longissimus dorsi (thoracic) (76·94%) and the mean value in the latter is significantly less than that in all the muscles except psoas major (77·28%). The mean fat-free moisture content of deep digital flexor (80·00%) is significantly higher than that in all the other muscles studied. There is evidence for a direct, but not linear, relationship between the contents of fat-free moisture and connective tissue (as calculated from hydroxyproline concentration).

4. Differences in ultimate pH between animals and between muscles were highly significant; but not those between the three weight groups, although there was a tendency for higher values in the muscles from bacon and manufacturing groups.

5. Both for the data overall, and within a given muscle, there was a direct proportionality between ultimate pH and fat-free moisture content. Except in the case of deep digital flexor correlation coefficients between these two parameters were significant. These correlations varied in significance for different muscles between the three weight groups.

6. Overall, the mean total nitrogen content (fat-free) of longissimus dorsi (lumbar) (3·77%) was significantly greater than that of longissimus dorsi (thoracic) (3·69%) and the latter greater than that of psoas major (3·58%). Such differences were also apparent within each weight group. The values were significantly greater than those for the six other muscles studied. The overall mean fat-free nitrogen content of deep digital flexor (3·17%) was very significantly less than that for all the other muscles. For the muscles of pork, bacon and manufacturing weights, overall nitrogen contents (fat-free) were 3·38, 3·44 and 3·55%, respectively. There was a significant inverse correlation between nitrogen (fat-free) and moisture (fat-free) (r = –0·88).

7. The ranking of muscles with respect to their contents of intramuscular fat was significantly different between weight groups. In contrast to the bovine, there was no significant difference overall between the intramuscular fat contents of longissimus dorsi in lumbar and thoracic regions; and these were both significantly higher than levels in psoas major, although the converse is true in the bovine.

8. Variation in the intramuscular fat content of the lumbar region of longissimus dorsi was strongly reflected by those in longissimus dorsi (thoracic), psoas major, rectus femoris, triceps and superficial digital flexor; but little with those in extensor carpi radialis and deep digital flexor; and not at all with that in sartorius.

9. There was no significant difference overall in the iodine number of intramuscular fat between weight groups. Iodine numbers in both thoracic and lumbar regions of longissimus dorsi were significantly lower than those in all the other muscles studied. The overall correlations between intramuscular fat and its iodine numbers were r = –0·70, – 0·71 and – 0·66 in pigs of pork, bacon and manufacturing weights: each of these was significant.

10. In discussing the results, it is suggested that those circumstances causing a high ultimate pH in muscle (e.g. fatigue) may concomitantly effect redistribution of body water and hence explain the observed proportionality between water content and ultimate pH. Significant differences in moisture content between muscles may be related to their relative proportion of connective tissue or to its nature. On the basis of much of the chemical data obtained, the muscles studied appear to fall into three groups—longissimus dorsi (lumbar and thoracic), deep digital flexor, and the rest, although such a conclusion does not accord with that suggested by anatomical dissection. Aspects of the relative maturity of the muscles are considered.