Meat Quality

The principles that link meat quality with animal welfare have been described already (Gregory, 1998). This chapter gives examples that support those principles and brings together new information on genetic and farm management factors that influence meat quality.

Stress and Meat Quality

Stressing an animal before it is killed can make its meat more tender. Several historical accounts describe ante-mortem treatments used in the past to improve tenderness (e.g. Anon., 1910). Bulls were baited by dogs, pigs were whipped in a similar way to that still used on dogs in parts of Asia, and cattle were chased through the streets by mounted stockmen equipped with goads. Similarly, meat from wild animals that had been severely stressed, such as hares caught in chases, was renowned for its tenderness.

The duration of the stress determines whether the meat will in fact be more tender. Physical activity that lasts for hours before an animal is slaughtered will deplete muscle glycogen. If muscle glycogen is reduced below a critical level, the pH ult of the meat will be raised. In the case of pigs, pH ult is only affected if muscle glycogen concentration at the time the pig is stunned is less than 53 mmol/kg (Henckel et al ., 2002). Meat with a high pH ult has higher calpain activity, which imparts tenderness as the meat ages, and it is prone to being dark in colour. It also retains moisture more effectively when stored and cooked. In pigs this type of meat is known as dark, firm, dry (DFD) pork, and in cattle it is called dark-cutting beef (DCB). High pH ult meat from stressed animals does not keep as well as meat with a normal pH, and it has a poor shelf life when sold fresh. In these ways, stress benefits some meat quality features and has a detrimental effect on others.

Sometimes, relatively mild stresses can upset the suitability of meat for a particular market. At an export abattoir that specialized in supplying kobi beef to Japan, feedlot cattle produced meat that lacked sufficient brightness in colour because of a high prevalence of intermediate pH ult cuts. This developed because the animals did not sit down on the hard concrete floor in the holding yards on the night before slaughter, and so their postural muscles were not sufficiently rested when the animals were slaughtered.

Stress before slaughter can affect meat quality in other ways. If an animal is stressed briefly just before it is slaughtered, it may die whilst its muscle is acidotic. In this situation, there is a risk that the meat will be pale and have poor waterholding capacity (WHC). Pig breeds and strains that are double recessive for the halothane gene ( nn ) are prone to accelerated post-mortem muscle acidification and producing pale pork which has a poor WHC (PSE meat).

PSE and DFD are now recognized features in poultry meat but they are not thought to be causing consumer dissatisfaction. They can be recognized from the criteria shown in Table 12.1, and, in the case of PSE, the prevalence is claimed to be high. For example, in turkey meat in the USA, the prevalence of L ≥ 53 is 40% (Owens et al ., 2000a). Poultry breast meat is naturally pale in colour, and so high L values are not thought to be abnormal. If complaints do arise in the future about PSE poultry meat, they are more likely to be connected with its texture. PSE chicken meat can be abnormally soft and almost mushy in texture (Zhang and Barbut, 2005b). This situation is different from PSE pork, which is often chewier in texture. DFD broiler meat can be more prone to developing thiobarbituric acid reactive substances (TBARS) when stored, and this could give rise to complaints about off flavours (Young et al ., 2003). However, DFD broiler meat can be ideal for producing processed products because of its ability to retain water during cooking (Zhang and Barbut, 2005a; Table 12.2).

Table 12.1. Classifying criteria for PSE and DFD broiler meat.

Table 12.2. Cooking loss and softness in chicken meat.

The DFD condition has been responsible for carcass condemnations by meat hygiene services. In 2004, about 9% of turkey carcasses in Canada were condemned for ‘cyanosis’. This abnormality was misdiagnosed, and it was due to an excessive dark colour caused by the DFD condition. Cold stress during transport was probably a contributory factor (Mallia et al ., 2000a, b). At the other extreme, high temperatures can produce PSE in turkeys but this would not normally be recognized during routine meat inspection (Babji et al ., 1982).