may not be. However, if meat is labelled as organically produced some consumers may be persuaded to believe that it will taste better, especially if, for ethical or other reasons, they normally buy meat from extensively or organically reared animals.
This effect is illustrated by the findings of a Dutch experiment. In the Netherlands some pigs are produced under a free-range, organic system (the animals are known as ‘Scharrelvarkens’). It is prescribed that the animals must be kept in groups, sows must have access to the open air and that their diet must contain 10% roughage but no anti- biotics or growth promoters, for example. This system is perceived to be beneficial to the welfare of the pigs and to produce better pork.
Packs of this pork were compared with equivalent packs of pork but derived from pigs reared in the normal, commercial, intensive system.
The pork was assessed by two groups of consumer. One comprised general members of the public, the other, people who normally preferred to buy meat from extensively reared animals. When the two sorts of pork were presented unlabelled to the consumers, practically no differences were detected by either consumer group. When the trial was repeated, but with the packs labelled as coming either from
‘normal’ or extensively reared pigs, the general public still did not detect material differences. However, the people who normally bought pork from extensively reared animals recorded very significant improved eating quality in this meat, but the results of the first trial with unlabelled packs had shown that there were really no inherent differences in quality between the two types of pork. The implication is that the consumers who normally bought meat from extensively reared animals had unwarranted preconceptions about its palatability and their assessments reflected these. Because they expectedthe meat to taste better they persuaded themselves that it actuallydid. This is an example of a general phenomenon. Delizaet al.(1996) have shown that consumers change their rating of the sweetness and bitterness of pure solutions of sucrose or quinine after being given (erroneous) informa- tion about them which led to expectations that the solutions were more or less sweet or bitter than they really were.
Beef quality
On-farm factors include breed, rearing system, feeding and nutrition, and age and maturity at slaughter. The effects of breed have perhaps been overrated in the past. There are genetic influences on meat quality factors but they usually relate to very specific characteristics.
The best beef is often thought to come from the traditional beef breeds of cattle like the Hereford and Aberdeen Angus rather than the dairy breeds like the Friesian, Holstein and Jersey. Certainly, as we have already seen (Table 2.6), the beef breeds produce carcasses with better conformation and bigger muscles. The Hereford and Angus are early maturing breeds so they start laying down subcutaneous and intra- muscular (marbling) fat earlier. At a fixed slaughter weight they there- fore tend to have more such fat than the later-maturing dairy breeds.
This effect is emphasized by the fact that at the same level of fatness dairy breeds also tend to store most of their fat internally as abdominal fat rather than associated with the muscles. Marbling fat contributes to juiciness and tenderness so on average, meat from the beef breeds should have better eating quality at the same carcass weight. However, if slaughtered at the same overall level of fatness, rather than at the same weight, the dairy breeds often have higher amounts of marbling in their meat. In practice, the eating quality of meat from beef and dairy breeds seems very similar and, in fact, the latter may even be better (Knapp et al., 1989; Thonney et al., 1991). The beef from humped cattle (Bos indicus) tends to be tougher than that from European breeds (Bos taurus). As we have seen (Chapter 5), part of the reason for this is that the post-mortem proteolytic breakdown of the myofibrillar system is less in meat from B. indicusbecause of differences in the activities of the calpains and calpastatin.
Pork quality
Major factors influencing pork quality are those that influence whether the muscle shows PSE or DFD characteristics, whether it has an abnormally low ultimate pH, and the level of intramuscular fat present.
Major genes associated with the factors influencing muscle pH are the halothane gene and the RN!gene. A review of the production factors known to affect meat quality in pigs is given in Woodet al.(1992). An excellent review of the genetic factors is that of Sellier and Monin (1994).
The halothane gene
The halothane (Haln) gene is so called because its presence in a pig as the double recessive genotype confers susceptibility to the common
anaesthetic gas halothane. Susceptible pigs, referred to as halothane- positive, exhibit a characteristic response when they breath the gas through a face mask. Their limbs become extended and rigid and they develop a raised body temperature or hyperthermia. The hyperthermia gets progressively worse and is referred to as ‘malignant’. If the response is not rapidly rectified the pigs die. The halothane test only identifies animals with the double recessive genotype (nn), not the heterozygote (Nn). These carrier animals, like the normal homozygote (NN), are not influenced adversely by halothane, and go into a relaxed state of stable anaesthesia. They are referred as halothane-negative. The inability of the halothane test to identify the heterozygote was a con- siderable disadvantage in attempts to control the presence of the gene in pig breeds by selective breeding. This situation was made worse by difficulty in interpretation of the test results. The length of exposure to halothane in the test was restricted to 3–5 min to reduce deaths in positive animals but this might not be sufficient time to detect some positive pigs. Also, the physiological state of the animal seems to affect the test. This is important since some stress is inevitably associated with carrying out the test itself. However, recently a DNA-based test has been developed which enables exact differentiation of all three genotypes (NN, Nn, nn). This test is based on detection of the ryanodine receptor gene (Ryr). The ryanodine receptor is the calcium release channel of the sarcoplasmic reticulum and a single mutation of the gene controlling it has been identified by Fujii et al. (1991) as likely to be the cause of halothane sensitivity in pigs.
The halothane gene was early on discovered to be closely associated with the occurrence of stress-susceptibility in some pig breeds or strains. Susceptible breeds showed difficulty in coping with stressful episodes, relatively high mortality during transport and produced carcasses that exhibited a high prevalence of PSE meat. These charac- teristics are sometimes referred to as the porcine stress syndrome (PSS). However, halothane sensitivity was also associated with the positive attributes of muscularity and good carcass conformation, undoubtedly the reason for its inadvertent selection for in some breeds, notably the Pietrain and Belgian Landrace in Europe and the Poland China in North America. Because of the recessive nature of its inheritance it was thought at one time that the benefit of the halothane gene – better carcass quality – could be exploited while eliminating the problems of poorer meat quality – a high level of PSE meat – by producing a carefully controlled heterozygous slaughter generation.
This would be done by retaining a halothane-negative, homozygous population of the female parent (gilts and sows) and a halothane- negative, but heterozygous, population of boars. When crossed together these would produce a halothane-negative slaughter genera- tion of half normal homozygotes and half heterozygotes:
Boar Sow/gilt
Parents: Nn " NN
Slaughter population: half NN, half Nn↓
Unfortunately, the meat quality in the heterozygote is in fact generally not as good as in the normal, halothane-negative pigs. In other words, the gene is not fully recessive in its effects on these traits. Therefore, the strategy adopted by most pig-breeders appears to be to eliminate completely the recessive mutation from their herds, aided by the DNA- based test. An example of the differences that have been found between pigs exhibiting the three halothane genotypes is given in Table 6.5.
The RN!gene
The RN!gene has been identified in certain strains of Hampshire pigs, particularly in France and Sweden (Monin and Sellier, 1985; Lundström and Enfält, 1997). The name of the gene reflects a major effect in its dominant form, that is, to reduce the yield of cooked, cured ham, and the names of the originators of the technique used to measure this yield.
Hence, RN!refers to ‘Rendement Napole’, rendement from the French word for ‘yield’, and Napole from the initial letters from the surnames of N. Naveau, P. Pommeret and P. Lechaux. The yield is measured by curing and cooking a sample of 100 g of the m. semimembranosus under defined conditions. The three genotypes are RN!RN!, RN!rn+ and rn+rn+(normal). The yield of muscles from pigs carrying the RN!gene (either homozygotes or heterozygotes) is reduced by up to 8%.
Table 6.5. Growth and carcass and meat quality characteristics of British Landrace pigs selected as halothane-positive, halothane-negative, or their crosses, and fed ad lib.(data from Simpson and Webb, 1989).
Halothane-positive Crosses Halothane-negative
(nn) (Nn) (NN)
Daily live weight gain (kg) 0.91 0.94 0.89
FCR (food eaten/weight gain) 2.40 2.51 2.58
Average backfat (mm) thickness 22.4 23.8 23.2
LD cross sectional area (cm2) 34.0 33.4 32.6
Killing-out percentage 76.6 76.7 75.9
Conformation scorea 7.4 7.1 7.1
Reflectance of LD (EEL units) 51 43 42
Percentage of carcasses visually
scored as pale, wet, or PSE 51 16 6
aTen-point visual scale, higher is better.
FCR, food conversion ratio; LD, m. longissimus dorsi; EEL, Evans Electroselenium Reflectance meter; PSE, pale, soft, exudative.
The RN!gene is thought to increase the glycogen content of the muscles, particularly those with a high complement of white, glycolytic fibres, redder muscles being little affected. The gene also produces lower ultimate pH values, resulting in higher drip losses and paler meat, and lower muscle protein concentrations. There may also be effects on eating quality and carcass composition, although this is not clear. The lower ultimate pH values have led to the use of the term
‘acid meat’, and its association with the Hampshire breed, the
‘Hampshire effect’.
Intramuscular fat
The potential role of intramuscular (marbling) fat in promoting juici- ness and tenderness has already been mentioned. Several studies suggest that low levels of intramuscular fat reduce eating quality. In contrast higher levels are beneficial. Wood (1995) has suggested several reasons for this. Improved tenderness may be because the soft fat ‘dilutes’ the effects of tougher myofibrillar elements so reducing shear force, or the fat may reduce the ‘rigidity’ of the muscle structure or it may allow fibre bundles to separate from one another more easily.
Improved juiciness may be because fat promotes the flow of saliva in the mouth and better flavour may derive from reactions of the fat during cooking. A good illustration of this relation between fat level and eating quality is a study carried out in Denmark by Bejerholm and Barton-Gade (1986). They aged pork loins for 7 days then cooked slices to an internal temperature of 65°C before evaluating them using an experienced taste panel. They also made objective measurements of shear force using an instrumental method after cooking to 72°C (Table 6.6).
Table 6.6. The effect of fat content on the eating qualityaof pork (from Bejerholm and Barton-Gade, 1986).
Average Percentage of
percentage Overall samples with
intramuscular Tenderness Flavour acceptability Shear acceptable or
fat score score score force better quality
0.86 0.6 0.8 0 100 71
1.24 1.7 1.6 1.2 86 87
1.73 1.9 1.7 1.4 78 89
2.37 2.2 1.9 1.9 79 97
2.76 2.7 2.5 2.3 76 100
3.94 2.7 2.3 2.3 69 100
aMeasured using 11-point hedonic scales where +5 was ideal, 0 was neither good nor bad and !5 was poor.
With increasing fat levels, tenderness, flavour and overall accept- ability increased. Samples judged more tender by the panel also had lower shear values. As the fat level increased the percentage of samples judged as having acceptable or better eating quality increased.
However, overall, there was relatively little improvement in quality above an intramuscular fat concentration of about 2%. Higher intra- muscular fat was also found to improve the eating quality of PSE pork, although overall panel scores were lower than for normal pork.
Similarly, studies in North America by DeVol et al. (1988) found significant correlations between eating quality, and instrumentally determined shear force, and fat concentration in the muscle. When they grouped samples according to their instrumentally determined texture they found that higher fat levels were associated with more tender pork (Table 6.7).
The levels of intramuscular fat measured in both the Danish and American studies were high compared with concentrations measured in UK pigs. These seem to average about 1% or less, perhaps reflecting the reductions in the overall fatness of the carcasses. At these very low intramuscular fat levels, eating quality may well be less than optimal.
An important consideration is the method used to extract the fat. Fat solvents like diethyl ether do not extract phospholipids present in the cell membranes and therefore underestimate the true total fat content of the muscle (Chapter 11). At low fat concentrations this under- estimation may be a significant fraction of the whole.
The beneficial effects of fat on the eating quality of fresh pork are reflected in similar effects in minced (ground) meat products. In fact, since the range of fatness can be greater, because fat is added to levels of up to 20%, the effects are more pronounced.
Different breeds of pig have different inherent amounts of intra- muscular fat. The Duroc is generally credited with having the highest levels and has been used in breeding schemes in Denmark and the UK to improve pork eating quality because of this. The benefits of incorporating Duroc genes on increasing intramuscular fat content and eating quality are illustrated in Table 6.8. In fact, Duroc pigs may have
Table 6.7. The association between instrumentally determined texture and intra- muscular fat (from DeVol et al., 1988).
Average shear force (kg) Average percentage of fat in muscle
5.1 2.4
4.0 3.3
3.6 3.2
3.2 3.4
2.6 3.6
intramuscular fat levels which are too high (>3–4%) for both consumer acceptability of fresh pork (because of the preference for lean-looking pork) and for processing into cooked hams. The benefits of the breed are therefore normally used in a cross containing up to 50% of Duroc blood.
The relationship between intramuscular fat levels and eating quality is undoubtedly complicated by other factors, notably the influence of breed per se. This is illustrated by the results from a comparison of the eating quality of pork from a number of breeds carried out by Warrisset al.(1990c, 1996). The overall acceptability of meat from the different breeds compared with the concentration of intramuscular fat in the muscles is shown in Table 6.9. Although the Table 6.8. The effect of incorporation of different proportions of Duroc genes on the intramuscular fat content and eating quality of fresh pork from gilts and boars fed ad lib. (from MLC Stotfold Pig Development Unit, Second Trial Results, 1992).
Percentage Duroc genes
0 25 50 75
Intramuscular fat (%) 0.70 0.86 1.08 1.27
Tendernessa 5.0 5.0 5.3 5.4
Juicinessa 4.1 4.1 4.2 4.4
Pork flavoura 3.9 4.0 4.0 4.0
aTaste panel scores based on 1–8 scales, 1 being extremely tough, dry or weak in flavour, 8 being extremely tender, juicy or strong in flavour.
Table 6.9. The levels of intramuscular fat and eating quality (overall acceptability) of pork from 10 breeds of pigs killed at an average live weight of 62 kg (from Warriss et al., 1990c, 1996).
Breed Eating qualitya Percentage of fat in the muscleb
Pietrain 1.60 1.2
Gloucester Old Spots 1.67 1.3
Landrace 1.71 1.4
Berkshire 1.90 2.1
Large White 1.91 0.9
Large Black 2.01 1.6
Saddleback 2.10 1.4
Hampshire 2.51 1.3
Duroc 2.56 2.2
Tamworth 2.81 1.5
aOverall acceptability taste panel scores based on a scale of !7 to +7 with higher scores indicating better quality.
bDiethyl ether-extractable.
Duroc breed had nearly the highest rated eating quality and the greatest amount of intramuscular fat, the best quality pork came from the Tamworth, a traditional, relatively unselected breed with only moderate amounts of marbling fat. The Berkshire, another traditional breed, had high levels of fat but only moderate eating quality. The Pietrain had low amounts of intramuscular fat, although not as low as the Large White, but very poor eating quality possibly to some degree because of the generally poor WHC of the meat of this breed. This tends to produce dry-tasting meat lacking in juiciness.
Breed effects
This work comparing quality characteristics of different breeds also serves to illustrate other changes in meat quality that have occurred with selection in the modern commercially-important breeds such as the Pietrain, Large White, Landrace, Hampshire and Duroc in com- parison with the ‘traditional breeds’. Table 6.10 shows the charac- teristics of the meat from the breeds grouped under three headings.
Group I comprises the six traditional breeds from Table 6.8, Group II the Large White, Landrace, Hampshire and Duroc, and Group III the Pietrain which perhaps represents the extreme case of selection for carcass muscularity. Improvements in carcass quality are associated with some less desirable characteristics of the meat. That from the improved breeds tended to be paler in colour (higher reflectance values) and to lose more exudate during storage than that from ‘tradi- tional’ breeds. These differences were attributable to a reduced initial muscle pH (pH45). There were also progressive increases in the coarse- ness of the grain of the muscles.
Fat firmness
In the early days of the European settlement of North America the colonists allowed their pigs to run wild in the woodlands to feed off
Table 6.10. Meat quality characteristics in different types of pig breeds (from Warriss et al., 1996).
Group I Group II Group III
pH45in LD muscle 6.48 6.34 5.83
Muscle coloura 46.3 47.8 60.1
Water-holding capacityb 7.3 10.4 12.9
Graininess of musclec 2.1 2.3 2.9
Group I = Large Black, Berkshire, Tamworth, Saddleback, Gloucester Old Spots;
Group II = Hampshire, Duroc, Large White, Landrace; Group III = Pietrain
aMeasured as reflectance (EEL units); higher values indicate paler muscle.
bMeasured as the percentage loss of exudate from stored meat.
cSubjective score based on a scale of 1 = fine, 4 = coarse.
the natural foods. However, pigs fattened entirely in this way, although they grew well, did not produce good pork. It was soft, oily and hard to preserve even though its flavour was originally good if eaten directly.
To overcome this, the pigs were rounded up and housed, being fed corn for several weeks before going to market. This resulted in their meat becoming solid and their fat white and firm. Corn-fed pigs therefore fetched a higher price than those fattened only on naturally available food. The softness of the fat from the naturally fattened pigs was caused by the unsaturated nature of the fats in the nuts and acorns they consumed. The unsaturated fat would have been prone to oxida- tion and rancidity so the meat would not store well.
Another disadvantage of soft fat is that it does not contribute to the firmness of joints of meat so detracting from their appearance when displayed for sale. This is a problem that has been associated with modern, very lean pork. The reduced fat firmness leads to meat that butchers refer to as not ‘setting’ well. The problem is caused both because there is less fat to support the lean tissues and because the fat is softer. As well as this softness, the fat layers tend to separate from the underlying muscle. The fat is softer because it is less saturated and because the adipose tissue contains more water and therefore propor- tionally less lipid.
Use of entire males (boars) for pork production
As we have seen, raising entire male animals, rather than castrating them soon after birth, is one of the most effective ways of reducing carcass fatness. Because lean is cheaper to deposit than fat in energy terms, entire males eat less food for the same weight gain. They also tend to grow faster. Compared with castrates, boars have shown improved daily live weight gain and food conversion efficiency of the order of 10% in some studies, although figures rather less than these are perhaps more usual in modern lean breeds of pigs. To exploit these advantages fully boars must be fed at high levels – probablyad lib. The carcass yield from boars is very slightly reduced: on average killing-out percentages are about one percentage point lower. This is generally attributed to the weight of the testicles (Table 6.11).
As well as these production benefits of rearing boars the castration procedure is avoided. Very young pigs are usually castrated without anaesthetic or analgesia and many people who are concerned with animal welfare therefore find the procedure unacceptable.
All these benefits led to the percentage of male pigs left entire (uncastrated) in the UK rising from less than 5% in 1976 to more than 30% in 1986 and 49% in 1991. In other words, by 1991, almost all male pigs were left uncastrated. Ireland and Spain also rear boars for slaughter but worldwide this is unusual. The reason for this is that there are also potential disadvantages associated with the production