Right after slaughter, meat is tender. But for the next 12 hours or
so, rigor mortis takes place, stiffening the muscles and making the
meat tough. Toward the end of the toughening phase, while the carcass
is hanging in a cooler, the tenderization phase begins, which makes
most meatbut not allsuitably tender.
The researchers found that tenderization was caused by the enzyme µ-calpain
degrading some muscle proteins. They believe industry shouldn't sell
meat before it has aged for 14 days, because the postmortem research
found that before that time, steaks are more likely to be tough. But
those extra days add considerable cost to production because of the
requirements for space and controlled temperature.
Koohmaraie explains that not all meat reaches the same level of tenderization.
His group wanted to find out why some meats become more tender while
others stay too tough. First, through protein chemistry and electron
microscopy, they found that µ-calpain breaks down muscle proteins,
making meat tender. Because the calpain enzyme requires calcium for
this activity, Koohmaraie and Wheeler developed a way of injecting calcium
into meat to help the tenderization process along. This marination technique
could be used to produce meat that has both improved tenderness and
Doing more research, Koohmaraie and food technologist Steven D. Shackelford
found that although µ-calpain causes the protein degradation that
improves tenderness, it's actually the activity of a protein called
calpastatin that determines how much calpain is activeand thus
how tender the steak will be. "The most significant part of this
project was learning that it's calpastatinnot calpainwhich
controls tenderness," Koohmaraie says.
Since that discovery, other ARS and non-ARS laboratories have confirmed
that the calpastatin system indeed greatly influences how tender steaks
are. But attempts to develop a tenderness classification system based
on calpastatin activity have not been successful. This is because calpastatin
explains just 45 percent of the variation in tenderness, which is not
high enough for accurate classification.
Koohmaraie's collaborators at MARC, chemist Timothy P.L. Smith and
geneticist Eduardo Casas, have taken a complementary genetic approach
to studying the µ-calpain system. They used a large population
of crossbred cattle to identify genes influencing tenderness. They found
that variation in the gene that produces µ-calpain appears to
"We're using genetics to identify DNA markers that can track variation
in the calpain gene," Smith says, "and we're looking for DNA
tests that can predict the likelihood that a given animal will produce
tender meat." This will permit breeding cattle that ranchers will
know are more likely to produce consistently tender offspring.
The researchers have made some progress. By sequencing the gene that
produces µ-calpain in both tender and tough cattle, Smith and
Casas identified differences that can be used as DNA markers. They have
released a DNA test that was effective in identifying tough animals
among a large herd of beef cattle representing the most popular beef
breeds in the United States. The researchers continue to develop additional
markers for use in other breeds.
And since research indicates that there may be many genes other than
µ-calpain and calpastatin that influence tendernesseach
having a relatively small effectSmith and Casas warn that more
genetic markers will be required to explain enough of the variation
to accurately guide breeding choices.
Koohmaraie and Smith stress that tenderness is only about half due
to genetics of the animal; the rest is the result of nongenetic, environmental
factors such as stress or diet. "There is only so much we can do
with genetics," Smith says. The rest is determined by how the animal
and the meat are handled throughout the various steps of beef production.
About 10 years ago, Koohmaraie, Shackelford, and Wheeler developed
a tenderness classification system that determines the quality from
individual animals. It's been used by some beef processors and retailers
to market a guaranteed-tender product line. An updated system is expected
soon.By David Elstein, Agricultural
Research Service Information Staff.
This research is part of Food Animal Production, an ARS National
Program (#101) described on the World Wide Web at www.nps.ars.usda.gov.
Timothy P.L. Smith, Steven
D. Shackelford, Tommy
L. Wheeler, and Eduardo
Casas are with the USDA-ARS Roman
L. Hruska U.S. Meat Animal Research Center, Spur 18D, Clay Center,
NE 68933; phone (402) 762-4100, fax (402) 762-4149.
"Working Towards a Consistently Tender Steak" was
published in the February
2005 issue of Agricultural Research magazine.