Comparative studies of meat. VIII. The percentage of fat in the fatty and muscular tissues of steers and the iodine number of the extracted fat, as affected by breed and level of nutrition

Abstract

1. An investigation has been carried out concerning the percentage of fat in the subcutaneous and intermuscular fatty tissues and in the muscular tissues, and the iodine number of the extracted fat from eight anatomical joints (foreshin, neck, shoulder, thorax, loin, pelvis, leg and hindshin) and from the perinephric fatty tissue and psoas muscles of the carcasses from twenty-four animals. The animals were from three breeds (Hereford, Dairy Shorthorn and Friesian) and were on four levels of nutrition—high-high and medium-high, and high-medium and medium-medium. The first two groups—finished on concentrates—were younger than the second two groups—finished on grass.2. Using an analysis of variance for the resultant data for percentage of fat and iodine number, it was found that breed had affected the percentage of fat in the tissues of the various joints significantly—on the average the order was Shorthorn (highest), Hereford and Friesian (lowest). This result could be predicted from the fact that this was the order of fatness of the carcasses (as measured by the percentage of fatty tissue in the carcass—see Callow, 1961).3. The data for iodine number showed no significant effect for breed in either the subcutaneous or intermuscular tissues. The significant effect of breed in the case of muscle could be attributed to significant differences in the overall level of fatness (see Callow, 1961) of the carcasses of the three breeds.4. A good correlation existed between the average percentage of fat in a joint and the average iodine number of fat extracted when the data for various joints was used to calculate straight lines of best fit. (In the case of muscular tissue, it was necessary to use the inverse of the percentage of fat to calculate the average—because the relation between the percentage of fat in muscular tissue and its iodine number is a rectangular hyperbola.)5. Using such lines of best fit, it was possible to show that certain joints had tissues which gave abnormal values for iodine number. Thus, deepseated tissues, like psoas muscle and kidney fat, had unexpectedly low values, whilst tissues from the hindshin had unexpectedly high values. This was attributed to the effect of local temperature—a high local temperature in the body giving a lower iodine number and a low temperature a higher iodine number than would otherwise be expected. The special case of subcutaneous fat from the thorax—with a higher iodine number than expected—is explained by the presence of brisket fat (which has a high iodine number).6. The data were used graphically to show the effect of growth gradients, thus the average percentage of fat in all the tissues is lowest in fore- and hindshins and highest in thorax (muscular and intermuscular tissues) and in the pelvis (subcutaneous tissue). Similar effects, but in the reverse direction, were shown by average values for iodine numbers.7. There were systematic differences in the percentage of fat in the subcutaneous and intermuscular tissues of joints and in the iodine number of the extracted fat. These too showed marked growth gradients. Although the percentage of fat in the intermuscular tissue was greater than that in the subcutaneous tissue in the fore- and hindshins, it was less in the thorax, loin, pelvis and leg. In spite of this, the iodine number of the subcutaneous fat was always higher than that of the intermuscular fat. This is attributed to the former being a colder tissue than the latter.8. It was shown that the relation between iodine number and percentage of fat in the various joints was different for the animals finished on grass (highmoderate and moderate-moderate levels of nutrition) as compared with those finished on concentrates (high-high and moderate-high levels). This was attributed to the rate of fattening being greater in the second case and to this giving rise to lower iodine numbers.9. Beyond this effect of rate of fattening, no reason could be found for the fact that the general level of iodine numbers showed variation from animal to animal.10. The extreme variation in percentage of fat in the tissues was (a) muscular tissue from 1·3 to 14·5%, (b) intermuscular tissue from 29·5 to 82·7%, and (c) subcutaneous tissue from 25·2 to 89·8%. For iodine number the variation was (a) 50·0 to 73·9, (b) 42·9 to 67·9 and (c) 46·3 to 67·9 It is thus clear that beef can be a very variable foodstuff.