Meat Tenderness and Muscle Development:

Meat Tenderness and Muscle Development
Several studies have shown that some events in muscle development and postnatal growth are related to meat quality traits, according to Niels Oksbjerg, M. Therkildsen and P.M. Nissenn from the University of Aarhus in Denmark, in a paper presented to the ICoMST 2009 conference in Copenhagen this year. By Senior Editor, Chris Harris.

The rate of muscle growth is deeply linked to performance traits, such as daily gain, feed conversion rate and meat content of the carcase and for this reason selection for increased performance traits may influence some meat quality traits.

However, the report says that the relationship between events determining muscle growth and meat quality may also give rise to developing new concepts for production of meat with high meat quality.

Muscle growth can be divided into prenatal and postnatal events, the report authors say.

Prenatal muscle development occurs in various steps including specification of mesodermal cells to myoblasts, myoblast proliferation, alignment of myoblast and their fusion to primitive myofibres.

"Muscle fibre formation (myogenesis) takes place during foetal development. Highly specialised muscle fibres develop from mesodermal cells, which are specified to become myoblasts. These myoblasts proliferate several times before they align and fuse to become premature myofibres. By innervation these myotubes differentiate to mature muscle fibres. Two populations of fibres develop at different times," the report shows.

At birth, the muscle fibre number (MFN) is constant but apart from the muscle fibre number and fibre type distribution, postnatal muscle growth is also related to protein accretion of the fibres as a function of the rates of protein synthesis and protein degradations – or protein turnover. Satellite cell proliferation supports protein turnover.

This means that selection for performance of meat animals may cause correlated changes in various steps in moygenesis and/or in postnatal muscle growth.

However, the report says that changes in some steps may deteriorate meat quality traits, while others have no effect or may even improve meat quality.

In this way, increasing the muscle fibre number may increase lightness and lower redness because an increased muscle fibre number results in lower cross-sectional area of muscle.

Also, several studies suggest that the rate of muscle protein degradation in vivo is related to tenderisation probably because µM-dependent Calpain is limiting the rate of protein degradation in vivo as well as during tenderisation.

"This finding suggests that, during selection, periodical measurements of the Calpain system are needed but this finding may also be used to develop new concepts for high quality meat," the study says.

It shows that if it is possible to increase the rate of protein degradation just before slaughter, this may result in meat with a greater tenderness. A feeding strategy involving compensatory growth could meet this requirement.

For decades, it has been recognised that compensatory growth (or catch-up growth) following re-alimentation may occur after a period of feed restriction in most farm animals, and the phenomenon has been reported to take place in pigs in both conventional production systems and in organic pig production.

According to other findings in cattle, both the rate of protein synthesis and degradation are elevated. Consequently, compensatory growth response may be a mean to increase tenderness of meat.

"The protein turnover, however, develops dynamically through compensatory growth, and rises from a low value initially after onset of ad libitum feeding to a high value," the report says.

"However, the rate of protein synthesis increases at a faster rate than the rate of degradation, but is at a level higher than control animals 42 to 45 days after the beginning of the realimentation period."

The report shows that from a series of studies both castrated male pigs and female pigs exhibit compensatory growth, while tenderness is only improved in female pigs.

This result may be related to the effect of compensatory growth on intramuscular fat, which was unaltered in meat from female pigs but reduced in meat from castrated male pigs, the study says.

Intra-muscular fat (IMF) is positively related to tenderness and the beneficial effect of compensatory growth on tenderness may then be abolished by a reduction in IMF in castrated male pigs.

It is generally accepted that intra-muscular fat (IMF) has a positive effect on the sensory quality of fresh meat and up to three per cent IMF may be beneficial for the tenderness.

The report adds that while higher intramuscular far is desirable, existing means to increase IMF are costly and no shortcut is available at the moment.

In conclusion, some steps in muscle development and postnatal growth are related to meat quality, traits which should be taking into consideration following selection for increased performance but may also be used to develop new concepts for meat products with high quality, such as the compensatory growth response.

The report adds that more research is needed to clarify whether compensatory growth can be combined with factors in the production process that cause deterioration in tenderness.