ruminants. Somatotrophins can be produced in relatively large quantities by bacteria using recombinant DNA techniques to insert the appropriate genes. However, because they also occur naturally, they may be more acceptable to consumers than #-adrenergic agonists even though not currently permitted in the EU.
beef classification is shown in Table 2.13. The EU beef classification scheme also requires recording of cold carcass weight and sex. Other characteristics that may be included in classification and grading schemes, especially for beef, are breed/type, age, muscle colour and graininess, the amount of intramuscular fat (marbling), and colour and texture of the subcutaneous fat. In the USA, marbling in beef is considered to be very important because larger amounts tend to be associated with more tender, juicy meat. Beef with large amounts of evenly distributed marbling therefore receives a higher quality grade in the United States Department of Agriculture (USDA) system.
The ideal classification system is one that is absolutely precise and where the classes can be exactly related to commercial value. Because of the importance of the relative amounts of fat and lean in determining this value, the EU pig scheme predicts percentage lean in the carcass from measurements of fat and/or muscle depth. For example, the equation used to predict percentage of lean meat in a pig carcass from the backfat thickness (P2) alone is:
lean meat = 65.5 ![1.15 !"P2(mm)] + [0.077 "cold carcass weight (kg)].
The P2 value is the backfat thickness measured in millimetres at the level of the head of the last rib and 6.5 cm from the mid-line of the carcass. The EU class is based on this calculated percentage lean (Table 2.14). If the precise amounts of lean and fat in a carcass can be predicted, ‘component’ pricing can be used. In this, the value of the whole carcass is determined exactly by the sum of the values of lean and fat components separately.
Much of the grading of beef and lamb carcasses is done subjectively by trained graders because of the difficulty of finding appropriate instrumental methods. Grading of pig carcasses is inherently easier due
Table 2.13. Percentage distribution of carcasses from steers, heifers and young bulls in the UK in 1994 (based on data in the MLC Beef Yearbook1995).
Fat class (increasing fatness)
1 2 3 4L 4H 5L 5H
E 0.1 0.1 0.1
U+ 0.2 0.8 1.1 0.4
U! 0.5 2.9 6.0 2.5 0.2
R 0.1 1.2 8.3 21.3 9.1 0.7 0.1
O+ 0.1 1.1 7.1 14.8 5.8 0.7 0.1
O! 0.1 0.9 3.7 5.3 1.7 0.3 0.1
P+ 0.3 0.6 0.6 0.2
P! 0.1 0.1 0.1
Conformation class (increasing muscularity)
to the much greater proportion of fat in the subcutaneous depot. The thickness of this can be measured directly. However, there is much interest in looking for instrumental methods for the ruminant species, for example, using such techniques as video-image analysis (Allen, 1984).
Methods of predicting body and carcass composition
Because of the importance of carcass composition, particularly the proportion of lean to fat, many methods have been devised to estimate it from simple measurements or using more sophisticated procedures.
Subcutaneous fat depth in pig carcasses can be measured using a ruler or an optical probe. Optical probes are pushed through the skin and fat.
The boundary between fat and lean is seen in an illuminated window and the fat depth is read from a scale. In the 1980s electronic grading probes, such as the Fat-o-Meater from Denmark and the Hennessy Grading Probe from New Zealand, were developed. These detected the boundary between fat and lean by the change in light absorbance as the probe tip passed through. A major advantage over the optical probe was that by pushing the probe through the underlying muscle tissue the depth of this could be measured as well. The combination of fat and lean measurements potentially produces much more accurate estimates of lean content (Fortin et al., 1984; Kempster et al., 1985).
Traditionally, carcass grading in pigs has been based on single measurements of fat thickness made over the m. longissimus dorsi (see Chapter 3) at the level of the last rib (for example, in the UK the P2 value is made 6.5 cm from the dorsal mid-line). However, this tends to underestimate the leanness of more muscular carcasses. This can be important in assessing carcasses from pigs from a diverse genetic back- ground with relatively large differences in conformation and fatness.
Improved prediction comes from the use of a larger number of sampling sites as has been pioneered in Denmark with the use of auto- matic multiple-probe grading systems. Better predictive measurements are always being sought, more modern approaches using combinations Table 2.14. The percentage lean in pig carcasses given different EU grades.
EU grade Lean meat percentage
S 60+
E 55–59
U 50–54
R 45–49
O 40–44
P <40
of measurements or sophisticated electronic scanning devices, for example using total body electrical conductivity (Hicks et al., 1998).
For research purposes a wide range of often very complex techniques has been used to measure body composition in both the live animal and the carcass. In live animals the methods have included dilution techniques, combinations of linear measurements, magnetic resonance imaging, computer-assisted tomography and dual energy X-ray absorptiometry (Lister, 1984; Mitchell et al., 1996). The com- position of whole carcasses has been predicted either by dissection into fat, lean and bone, or by chemical analysis of lipid, protein, ash and water, of sample or ‘indicator’ joints. Often part of the loin joint is chosen (Planella and Cook, 1991; Nour and Thonney, 1994; Swensen et al., 1998). A very thorough review of carcass evaluation techniques is that of Kempster et al.(1982b).
A particular technique that has been used to assess the composi- tion of both live animals and their carcasses is measuring the velocity of ultrasound through the body tissues. Ultrasound has a frequency above 20 kHz, the upper limit of human hearing. Measurements are usually made at frequencies of 1–5 MHz. When a pulse of ultrasound is passed through an animal’s body a small proportion (2%) is reflected back as ‘echoes’ from the boundaries between different tissues. The characteristics of these echoes can be used to generate a picture of the relative positions of these tissues. This is the principle used in machines such as the Scanogram (Ithaco Incorporated, USA) and the Vetscan (BSC, UK) which produce two-dimensional displays of the tissues below the scanning head. From these the thickness of fat layers or the cross-sectional areas of muscles can be deduced.
As well as recording these echoes to estimate body composition, the speed at which the ultrasound passes through the tissues can be used. This is because the speed is different in muscle and fat. It is about 1.6 km s!1in muscle and 1.4 km s!1 in fat (Miles and Fursey, 1974). By measuring the speed through a part of the animal’s body consisting of soft tissues the proportions of muscle and fat can be calculated. It is important that the pathway of the sound is not deflected by bone and, if used in the live animal for predicting lean and fat in the carcass, that it does not pass through organs or body cavities.
Additionally, the speed is influenced by temperature and, importantly, the effect is different for lean and fat. This is a slight limitation to the value of the technique for assessing the composition of carcasses. It can therefore be used for large carcasses, for example from adult cattle, when measured reasonably soon after slaughter but is less useful for rapidly cooling lamb carcasses.
A comparison of the use of various ultrasonic techniques for measuring beef carcass composition in the live animal was made by Porter et al. (1990) and a good review of the use of the speed of
ultrasound to measure composition is that of Fisher (1997). There can be significant between-operator variation in the interpretation of images from pulse echo techniques (McLaren et al., 1991). Measuring the speed of ultrasound through muscles has also been used to estimate the amount of intramuscular fat present in beef (Park et al., 1994).
Considering its complexity, an animal’s body consists of relatively few kinds of chemical substances. About 55–60% is water. This, and the 3–4% or so of minerals, make up the inorganic component. The remaining 35–40% consists of organic substances. These are complex compounds of carbon (C), hydrogen (H) and oxygen (O), sometimes together with nitrogen (N), sulphur (S) or other elements, which are in general found only in living organisms. Three major categories of organic compound are of importance to us: proteins, fats and carbo- hydrates. The approximate composition of an animal in terms of these components is given in Table 3.1. These figures are given only as an illustration. As we have seen in Chapter 2, the proportions of some components, especially fat, can vary greatly. Muscle tissue consists of about 75% water and 20% protein. A large part of the remaining 5% is fat with very small amounts of carbohydrate (principally glycogen), free amino acids, dipeptides and nucleotides.