Consumers generally do not consider beef as a cheaper source of protein in the diet. They eat beef because they like it. One of the most important components of beef quality and acceptability is tenderness. This is why the tenderest cuts (beef tenderloin for example) are the most expensive. However, there is considerable inconsistency in tenderness from one carcass to another. There are several factors that contribute to the tenderness of beef at consumption, and they are additive to the tenderness of the final product.
The U.S. Department of Agriculture (USDA) quality grading system generally does a poor job of identifying tenderness in the carcass. The USDA quality grade (Prime, Choice, Select, Standard) is based on marbling in the rib eye muscle at the 12th rib and on estimates of animal maturity from bone ossification. In general, the relationship between marbling and tenderness is weak (Parrish, 1974). Actually, the relationship between age classification and tenderness may be higher. The results in Table 1 indicate that there is a window of acceptability for quality grade for beef by consumers, and this result would likely include general differences in tenderness. Further, quality grade at mid-Choice or higher has a significantly greater impact on acceptability, while Standard will have a significantly lower impact on acceptability, with tenderness being a part of these results.
It is well-documented that there is a genetic contribution to tenderness, but control of tenderness through selection will not change tenderness as effectively as management and processing methods (Koohmaraie et al., 1995). Gene markers are now available for tenderness, and many are specific for the presence of a calpastatin gene. Calpain enzymes are responsible for changes that result in meat tenderization (Koohmaraie et al., 1995), and calpastatin is a specific inhibitor to calpain. Selection for traits by only using gene markers is not as effective as using a combination of phenotypic and genotypic selection (Spangler et al., 2007).
There are no consistent results documented for differences in tenderness among breeds when compared in unbiased trials. The exception is when Bos taurus (English and Continental breeds) are compared with Bos indicus (Brahman and Brahman-derivative breeds). Wheeler et al. (1994) have documented that meat from Brahmans and Brahman crosses is less tender than meat from English and Continental cattle handled in the same way, regardless of the marbling score.
A summary produced by Tatum et al. (2007) indicates that heifers may produce tougher meat than steers. In eight out of the 10 studies reviewed, heifers produced loin steaks with a significantly higher Warner-Bratzler Shear Force (WBSF), which measures tenderness by the force required to cut through a core sample of cooked steak. The authors concluded from an analysis across the studies that the average difference in WBSF for meat from heifers was +0.25 kilogram compared to meat from steers.
As cattle age, there is formation of more connective tissue and maturity of the muscle fiber. These features contribute to a loss of tenderness. In an extensive study by Shorthose and Harris (2006) measuring meat tenderness in cattle from 1 to 60 months of age, the results clearly indicated that tenderness is reduced with advancing age. In this study, the increase in toughness was linear up to about 40 months of age.
After the animal is killed, tenderness will be reduced, as rigor mortis results in a shortened muscle fiber. Beginning soon after death, tenderness factors are simultaneously engaged, and these factors continue after rigor mortis ends, through the carcass aging phase. In a process called postmortem proteolysis, proteins that are responsible for the structural integrity of the muscle are degraded, and the meat becomes tenderer. The rate and extent of proteolysis is the source of variation in meat tenderness from one carcass to another. It's likely there are both genetic and environmental factors involved.
Aging is the process where meat is stored for a period of time either as a whole side (dry aging) or fabricated to wholesale or smaller cuts and stored in a vacuum-packed bag (wet aging) to capture proteolysis of the muscle fibers. Flavor changes also occur due to microbial activity during aging. Dry aging will usually result in more flavor changes, and is more costly due to occupying more cooler space and any trim from the carcass that dries and is discolored. Some consumer research has shown that consumers prefer beef that has been wet-aged for the same length of time as a dry-aged comparison (Sitz et al., 2006).
Most beef in retail food chains is wet-aged to some degree, although the aging time will vary due to transportation, further processing needed and marketing. In general, Gruber et al. (2006) have shown that for Select and Choice carcasses, tenderness changes will be rapid for the first 14 days of aging, and then will become less pronounced. By 15 to 21 days of aging, very little additional tenderization is occurring.
Cold shortening is the term used to describe what happens when carcasses are chilled rapidly after slaughter. This does not allow the glycogen in the meat to be converted to lactic acid, and the presence of the glycogen creates an energy source that allows irreversible muscle fiber contraction (Davey et al., 1976). The production of lactic acid reduces the pH of the meat, and carcasses that are at 10 degrees Celsius or less before a pH of 6.2 or less is reached will experience cold shortening of the muscle fibers and a permanent loss of tenderness.
One method to encourage the reduction of pH and prevent cold shortening is with electrical stimulation of the carcass. Bendall et al. (2006) have shown that 700 volts at 25 hertz for two minutes for a total of 3,000 pulses to a carcass shortly after slaughter is optimum. This study indicated a pH change to 5.7 occurred about 70 percent faster after stimulation.
While tenderness has great value for both producers and consumers of beef, it is a difficult trait to manage. There is a matrix of related factors on farms and in processing plants that must be addressed to provide a tender product, and failure of any of these factors can result in an undesirable eating experience.