Putting on the pressure for safer foods

Think of store-bought orange juice that tastes just like fresh-squeezed, lasts longer than fresh and is as safe as, or safer than, pasteurized OJ. This possibility is closer to reality than to science fiction, thanks to a partnership between the U.S. Department of Agriculture (USDA) and the University of Delaware.

Since 2000, USDA molecular biologist David H. Kingsley and Dallas G. Hoover, UD food microbiologist and professor in the animal and food sciences department of the College of Agriculture and Natural Resources, have been working together in the lab. They are evaluating the effectiveness of high-pressure treatment for inactivating food-borne viruses in shellfish and other foods, while preserving the uncooked character and flavor. If their research is successful, Kingsley says, the USDA may support the use of high pressure in commercial applications.

“A number of foods you buy in the grocery store have been prepared using high pressure, including some brands of guacamole, yogurt smoothies and ready-to-eat lunch meats,” he says. “High-pressure treatment effectively eliminates food-borne bacteria such as Listeria and Vibrio—and, as we are showing, food-borne viruses—yet keeps the food raw, thus maintaining its natural qualities that are altered by industry standard processes such as heat.”

Hoover and Kingsley are studying ways to use high pressure that will eliminate hepatitis A and Norovirus in shellfish and other foods. Norovirus, the No. 1 virus of food-borne illness in the United States, is common in shellfish. The food-borne virus causes viral gastroenteritis, infecting 20 to 25 million people each year.

“A lot of people really like to eat raw shellfish; in fact, they insist on it,” Kingsley says. “Shellfish live in estuaries in which boats are prevalent and into which septic tanks may leak. Shellfish are bio-concentrators, which means they take up everything they see, thus housing various bacteria and viruses. Since you don’t cook raw fish, science must find an alternative method to inactivate those bacteria and viruses. We believe that alternative is high-pressure treatment.”

In the lab, Kingsley puts viruses into flexible packages and seals them. Then, the package goes into a small pressure chamber the size of a soup can. Pistons start pumping water, and high pressure—6,000 times that of atmospheric pressure—is applied to the package.

“That much pressure is the equivalent of three elephants standing on a dime,” Kingsley says. Once the package housing the virus has been treated, Kingsley attempts to infect a cell with the virus. Whether the virus is able to infect the cell determines the effectiveness of the pressure.

Using high pressure to prepare foods is not a new idea. According to Kingsley, the method has been around since the turn of the last century, when a West Virginia researcher sought to pasteurize milk with pressure. However, because the technology at the time wasn’t practical, the early attempts failed to advance. That changed when Hoover arrived at the University back in the early ’80s.

Hoover says that the head of the food science department at the time, Dan Farkas, predicted that high pressure could inactivate bacteria and pathogens in foods. Hoover, who was working under Farkas as an assistant professor, had doubts about the hypothesis and was afraid the research might hurt his career, he says. On the contrary, Hoover today is known worldwide for his research into high-pressure treatment of foods.  

“At first, I thought it was a crazy idea,” he says, “but he was the head of the department, so I showed up every day and worked as a technician in these pressure treatments. We peddled the idea to various industries, eventually achieving credibility. Since then, I have worked on pressure research exclusively.”

Kingsley, who has worked for the USDA’s Microbial Food Safety Research Unit for five years, is an expert in aquaculture and food-borne viruses. While attending an aquaculture meeting, he says, he heard about the use of high pressure to kill Vibrio in foods and was intrigued. At a later meeting, the possibility of using high pressure to kill viruses was discussed, and Kingsley contacted Hoover.

A short time later, the two formed what Hoover calls a “loose collaboration,” a non-funded specific cooperative agreement between the USDA and the University. Hoover shares his mastery of high pressure and the high-pressure unit he acquired in 2003 from Avure Inc., while Kingsley shares his expertise in viruses and aquaculture.

“At this point, we have proven that high pressure is effective in inactivating hepatitis A,” Kingsley says. “The tricky part is dealing with Norovirus. Because it cannot be replicated in the lab, we can only test genetically related strands of the virus. We have tested the feline version of the virus and have successfully inactivated it. The next step is to test the true Norovirus in humans.”

Kingsley says they have not yet acquired funds to begin a human volunteer research study. “One human volunteer costs $11,000. You need about 30 people,” he says. “That’s a lot of money.”

In the meantime, Hoover and Kingsley say they will continue their research, defining the parameters of what affects inactivation, including such factors as temperature of a food, salinity, acidity or fat content. “I think it’s just a matter of time before the use of high pressure really snowballs,” Kingsley says. “The technology today is such that the process is more cost-effective.”

The large commercial version of the high-pressure unit Hoover and Kingsley are using would cost a food manufacturer about $1 million. But, Kingsley says, in the long run, the machine would cost a food company very little.

“Some food manufacturers estimate the cost to pressure-treat their products at about 5 cents per pound of meat and 7 cents per liter of juice,” he says. “This could end up costing the consumer a few more cents. In Japan, where consumers tend to care more about quality than cost, they already have pressure-treated, tastes-like-fresh-squeezed orange juice on store shelves.”

Hoover, who has worked with high pressure for more than 20 years, is convinced it is the best way to process food.

“When we go into grocery stores, we expect all foods to be wholesome and safe,” he says. “But, safety doesn’t sell; taste does. High pressure is superior to the heating method of processing foods because it is both safe and preserves the quality of taste.”

—Jaime Cherundolo, AS ’03