Eating satisfaction from beef results from the interaction of tenderness, juiciness and flavor. Of these, beef tenderness is the most sought-after and least consistent attribute available to consumers. 
 
Beef tenderness can be based on species (Bos Taurus European cattle are thought to produce more tender meat than Bos indicus, Zebu or “humped” cattle); pre-mortem nutrition, handling and slaughter technique; and animal genetics. 
 
The genetically controlled variation in beef tenderness can be expressed pre-mortem or post-mortem and is currently thought to be due to variations in three genes. These are myostatin, calpain and calpastatin. The genetic effects of myostatin are expressed pre-mortem. 

The genetic variant that produces a non-functional myostatin gene has the largest pre-mortem effect on beef tenderness of any single genetic feature investigated to date. Polymorphisms (genetic variants) in the gene myostatin are responsible for “double muscling” which has been noted as associated with increased tenderness, particularly in Piedmontese and Belgian Blue cattle. This increase in tenderness is thought to be related to a decrease in connective tissue in the muscle. 

The other two genes shown to be related to beef tenderness are active in the post-mortem events associated with aging of beef. These post-mortem events are thought to be responsible for 90% of the changes responsible for the development of tender beef. 

The mechanism of increasing tenderness associated with beef storage at refrigerated temperatures has been shown to be related to two proteins--the m-calpain protease and to its inhibitor calpastatin. The m-calpain protease is an enzyme that catalyzes the degradation of key myofibrillar and associated proteins. These are structural proteins in the beef muscle. More post-mortem activity of this enzyme leads to increased breakdown of muscle structural protein and therefore increase beef tenderness. Calpastatin is an inhibitor of the m-calpain protease and regulates 60% of the tenderness effect of aging. Increased activity of this inhibitor protein blocks the action of the m-calpain protease, thus less muscle breakdown occurs during the aging process and the beef is less tender (Figure 1).

Genotyping for two genetic variants in the gene coding for the m-calpain protease and one in the gene coding for calpastatin has become commercially available. The presence of these genetic variants has been correlated with Warner-Bratzler shear force (WBSF) measurements which are currently the best objective measurements of beef tenderness. The more the WBSF is reduced, the more tender the beef. 

The variants in the gene coding for the m-calpain protease correlating with increased tenderness result in a more active enzyme. The variants in the gene coding for calpastatin that correlate with increased tenderness result in a less active inhibitor.  This relationship is shown in the table below. 

This table gives the genotype and estimated change in W-B shear force measurement from the NBCI trials of the Merial Igenity product. A similar product with slightly different genotypes is available from Bovigen. An Igenity tenderness score has been designated to facilitate an understanding of the relationship between genotype and shear force. 

Studies of the Igenity tenderness genotypes from 171 Belted Galloway cattle showed that these cattle scored better for tenderness than other cattle breeds. These data show that 71.1% of Belted Galloway cattle scored 6 or above on the Igenity tenderness scale (10 is the most tender), versus 41.1% of the 1600 reference cattle tested (Figure 2).
 

Figure 2

This difference is primarily due to the fact that 99% of the Belted Galloway cattle tested had a genetic variant of the gene coding for calpastatin that resulted in markedly decreased activity of this inhibitor of the m-calpain protease. This is an extremely unusual finding among breeds of cattle and bodes well for the marketability of this breed.