Hydrodynamic pressure processing got a bad rap for meat tenderizing in the 1990s, but USDA scientists believe there’s still hope for this and other technologies to assist the process.

Experiments involving hydrodynamic pressure processing at the Agricultural Research Center lab in Maryland are conducted in this 54-liter steel vessel. Source: USDA.

Tenderization is one of the oldest value-added processes for beef, but knowing which animals’ cuts will yield the best payback remains largely a guessing game. Many different techniques and processes exist, including aging and mechanical tenderization with electricity, but inconsistent end results or lengthy aging processes often work against them. One possible solution is hydrodynamic pressure processing (HDP), which relies on the percussive power of an explosion in a water medium to generate a shock wave to break down tissue and protein molecules in beef, pork and poultry.

Beef has been the particular focus of research that began in the early 1990s at USDA’s Agricultural Research Service (ARS) labs in Beltsville, MD. The effort began in conjunction with Hydrodyne Inc., an engineering firm that introduced its system at the 1999 Worldwide Food Expo (see “Bigger bang for the buck,” Food Engineering, January 2000). With assistance from Halliburton’s Kellogg Brown & Root division, Hydrodyne installed a single commercial unit. A water-filled 6-ton steel tank lined with dynamite charges was used to tenderize 600-lb. batches of beef without damage or discoloration. Unfortunately, the scale-up from the lab system was based on some inaccurate assumptions, and the unit failed to perform as expected. The result was a poor reputation for HDP and disinterest in additional equipment fabrications.

Research on HDP continues, however. Building on work in the ‘90s that identified disruption of the myofibril structure and the structural integrity of the muscle and connective tissue as the key to effectiveness, scientists at the Beltsville lab have focused on better understanding of the molecular changes that occur and addressing the process’ drawbacks, notably the use of explosives. Efforts to develop HDP as a food safety intervention were dropped, but a group of three scientists, including protein chemist Brian Bowker, continue working on the quality-enhancement potential. Bowker joined the ARS Food Quality Lab in 2004. A native of Indiana, he attended Purdue University, studying animal science and muscle protein while earning undergraduate and doctoral degrees. Bowker received a fellowship from the Federation of Animal Science Societies in 2003, working for a year on Capitol Hill to help advance key issues of animal agriculture.

Brian Bowker, protein chemist, Food Quality Laboratory, USDA, Beltsville, MD.

FE: Who first proposed using explosive concussion to tenderize meat?

Bowker: The idea of using shock waves from underwater detonation of explosives was the focus of a 1970 patent, and refinements to address deficiencies were proposed by John Long, an engineer with a company called Hydrodyne. A cooperative R&D agreement was signed between the US Department of Agriculture’s Food Technology and Safety Lab and Hydrodyne in 1992 to develop the process, called hydrodynamic pressure processing (HDP).

FE: How does HDP tenderize?

Bowker: The shock wave created in underwater detonation moves at a speed exceeding the speed of sound and passes through the meat in a few milliseconds. Because boneless meat is three-quarters water, it is an acoustical match, and the wave passes through it, compressing the meat without raising temperatures more than 2° or 3°C. The optimal pressure of the wave is 70-100 megapascals, or about 10,000-14,500psi. The bottom of the vessel is an acoustical mismatch, and the wave is reflected off the bottom, almost doubling in pressure and changing to a tensile wave. Managing the relationship between wave velocity and the wave period is critical. Extensive research has been done on optimizing the shape of the reflective surface and the shape of the explosion.

FE: What are the drawbacks to HDP?

Bowker: The use of dynamite in an industrial environment is, of course, a big concern. Even in a government lab, it presents many issues. Two members of the staff are licensed to handle the explosives, and one must be present for every experiment. A lot of early work was done in cooperation with underwater demolition experts from the Navy. After September 11, 2001, security was heightened. Dr. Morse Solomon, the lab’s director and one of the licensed explosives handlers, has some interesting stories involving the FBI visiting him at his home.

Use of a batch process also hampers commercialization, particularly with the throughput needs of a modern meat plant. The meat also has to be vacuum packed and the air removed to prevent rupturing or tearing during the HDP process. One technology allows the process to be done in air, eliminating the need for a package.

FE: Are you exploring alternatives to dynamite?

Bowker: Yes, often in partnership with companies that have developed complementary technology. ARS is encouraged to work with private companies, and oftentimes they are small companies working in other industries. One is a firm that uses a plasma-pulse sparking device to generate electrical waves to clean zebra mussels out of pipes with an acoustic field. It’s the same principle used to treat kidney stones, using a noninvasive, electrically induced shock wave lithotripter. It’s nonexplosive and would be conducive for use in a continuous process using a conveyor belt. It’s also fairly inexpensive technology and could lend itself to small-scale tenderizing in a restaurant or at retail.

Some people think sticks of dynamite are used, but that was never the case. A binary explosive weighing a total of 100g is used. The shape of the sachet impacts the shape of the explosion and the differential pressure across the wave.

FE: Why didn’t the first commercial unit deliver the anticipated tenderization effect?

Bowker: The developer had the engineering expertise to push the project through, but because of the unfortunate falling out with ARS, the process itself wasn’t understood quite well enough, which resulted in scale-up problems. For example, shock absorbers were placed around the tank, and that reduced the reflective power of the wave. There was a lot of trial and error in the early years, as researchers tried to optimize the system while still evaluating factors such as the shape of the wave, the distance from the meat, how different cuts of meat responded, whether they should be parallel or perpendicular to the wave and so on.

The focus for the last five years has been on the molecular level, understanding what proteins are affected and how various meat and poultry react to additional processes, such as brine pickup by poultry. I’m trying to better understand the how and why of tenderization. Even with a month of aging, control groups never reach the same level of tenderization as HDP-treated meats. With maybe 10 percent of a sample, there can be a small initial response but a big impact later. My hypothesis is that proteins are disrupted enough that the aging process is accelerated.

FE: Besides HDP, what other protein research are you conducting?

Bowker: Predicting the tenderization potential of a given cut has been studied for 100 years. I’m looking at early post-mortem techniques to segregate primals by using predictor proteins. There’s a lot of interest in a noninvasive, rapid test that could use protein markers to predict tenderness. As part of the HDP work, we’ve looked at the proteins that are breaking down as tenderization occurs. Those proteins can be found in the moisture that forms when a cut is in a package. The next step would be to create an antibody to those proteins, put it on a sensor and get real-time readings of the tenderization potential of the meat. It would create a new grading system, based on new quality parameters.

FE: Are you continuing to research HDP?

Bowker: There still is quite a bit about the how and why of tenderization that we don’t understand. I’m still focused on the molecular level of tenderization and which proteins are affected.

It seems likely HDP would be best used commercially with other technologies. One recent study we conducted involved HDP in conjunction with brine injection and marination of pork loins. When applied before injection and vacuum tumbling, HDP was particularly effective in improving brine retention and processing yield, though it also improved texture and moisture when applied after those steps.