Pressure Tactics: Winning the fight for healthier foods and new medicines

By Angela Correa, College of Agriculture and Life Sciences, Virginia Sea Grant

Dianne Bourne can’t tell the difference. “The taste is the same, the looks are the same but from a microbiological standpoint, the oysters over here” – she points to a tray full of unshucked Virginia oysters – “are cleaner by far than the waters they came from. That’s not what I typically see.”

A food science microbiologist, Bourne has seen similar effects in a variety of foods being processed in a new way, from cold cuts to fresh juices to salsa and guacamole.

Foods like these are sold based on freshness – often with a brief product life. Shucked oysters are typically unusable within 15 days of processing, even if they have been maintained at proper temperatures. The same goes for fresh, ready-to-eat foods, such as raw salsa. In some foods, such as prepared entrees and ready-to-eat deli foods, preservatives are used to lengthen the shelf life, but these additives can change the flavor of foods and cause allergic reactions in some consumers.

A Virginia Tech laboratory in the College of Agriculture and Life Sciences is employing an unusual technique – high hydrostatic pressure processing, or HPP – to extend the shelf life of fresh foods without additives. High pressure processing essentially operates on the principle that when large amounts of hydrostatic pressure are applied to an object, anything living and replicating within it is going to have a hard time carrying on normal functions. HPP works without additives and isn’t likely to make people wary the way that other techniques, like irradiation, have. HPP also works well with many raw foods: in fresh juices it zaps E. coli without pasteurization, in fresh guacamole it extends the shelf life without additives, and in sushi it wipes out most parasites without altering the delicate flavors of the fish.

The technique is based on a discovery made in 1899 at the Agricultural Experiment Station labs at West Virginia University. Chemist Bert Hite used a manually operated steel cylinder to pressurize milk, and was able to show that shelf life and quality could be extended by several days. This early research was promising, but the equipment was prone to catastrophic failure due to the extreme pressures Hite and his colleagues were trying to achieve. Hite’s 1899 paper catalogs the failure of one pressure rig after another. In one experiment, milk was inoculated with live Salmonella typhi (the bacteria that causes typhoid fever), but in the first attempt to pressurize the sample to 90 tons, “the steel cylinder, the lead tube, the tin tube, and one of the pistons went to pieces, scattering the germs all over the room.” After one of the staff members contracted typhoid, that portion of the experiment was discontinued. The initial research showed that the idea had potential, but the technology would have to log more than a century of refinement.

Meanwhile, the public’s expectation that food be safe has continued to increase. It seems that the further removed the average consumer is from the farm, the higher the expectation of purity in farm products becomes. Consumers in developed nations want to have the assurance that the foods they buy will not cause disease. As a result, processors have to be more meticulous than ever about the food safety hazards that may be present in their products. Even in cases where the potential pathogen is common throughout the environment, as with Listeria monocytogenes, processors of ready-to-eat foods are required to have a perfect record when it comes to keeping it out of the finished product. “This isn’t easy,” says George Flick, University Distinguished Professor of food science and technology. “Companies are always looking for a cost-effective way to maintain safety and eliminate pathogens. High hydrostatic pressure processing is one way some of them can make their foods safer without making the products look, smell, or taste different.

How does HPP work?

No one knows for sure how high pressure processing works, but the prevailing theory is that it alters the folding and assemblage, or tertiary and quaternary structure, of membrane-bound proteins, thus affecting membrane integrity and function. In a steak, for example, this effect – a subtle but highly significant shift in the shape of the membrane-bound proteins – is observed in both the muscle cell membranes and the flora that may be present on the steak’s surface. HPP leaves both types of cells essentially unchanged, with the difference being that the bacterial flora are now just as inert as the muscle tissue they reside upon.

This same effect is also seen in functional proteins, such as enzymes. HPP makes it possible to inactivate enzymes, and may retard spoilage in this way. Without treatment, enzymes produced when the food was alive and whole can hasten spoilage in the finished food product.

These findings open up a wealth of possibilities for research on many types of foods, vaccines, biologics, and seeds. The HPP laboratory at Virginia Tech has several projects underway in these areas.

Optimizing parameters for different foods

Not every food can be treated with high pressure – bread, for example, is flattened by the process, and marshmallows don’t survive well either. HPP is suitable for foods with high water content and few or no gas pockets within their structure. So while it is great for strawberries, it’s better if they are sliced first, so that the little hollow in the middle of the berry doesn’t become a liability. The requirements for processing foods under high pressure also differ in terms of the holding period, level of pressure, and temperature at which pressurization is conducted. These parameters are developed by trial and replication, and are different for each food.

The Virginia Tech laboratory has been used to optimize processing schedules for a wide variety of foods, from fresh salads to cold cuts to pureed fruits. Foods are analyzed thoroughly before and after processing for changes in flavor, texture, color, microbial counts, and chemical attributes. As a result, the lab has helped several companies to develop and refine high pressure processing protocols for such products as ready-to-eat sandwich meats, shrimp, clams, and potato salad.

Preventing foodborne disease

Certain foods, especially those that are often eaten raw, carry a higher risk for harboring pathogens. Norovirus infection, known as the “cruise ship virus,” sickens 22 million people annually in the United States, and is the most commonly occurring foodborne illness in the world. At Virginia Tech, USDA-funded work is underway to determine the HPP parameters needed to eliminate noroviruses from selected food products.

Phase one of this project has been completed. “We determined the HPP parameters needed to inactivate norovirus using a mouse model, both in culture and in oysters,” says Daniel Holliman, MD, who directs research at the laboratory. “It takes somewhat more pressure to kill this virus than is now being used in the oyster industry, but the pressure levels are achievable with the technology currently in use.”

Holliman is the principal investigator in a number of projects taking aim at decreasing the infectivity of high-risk foods. He says, “The better we can understand the cellular, biochemical, and enzymatic consequences of HPP treatment on bacteria, viruses, and parasites, the more reliably we can apply this technology to make the food supply safer.”

Holliman’s current research also includes developing HPP protocols to inactivate hepatitis A virus in oysters and to eradicate a variety of parasites in meats and fruits.

Simplifying vaccine development

The unique mechanism by which HPP inactivates bacteria, viruses, and parasites can yield an organism that is non-infective, but still immunologically active. This curious possibility opens the way for simplified development of new vaccines. Preliminary studies are underway to generate a vaccine against Burkholderia cepacia, an organism that can cause fatal lung infections in patients with cystic fibrosis. In phase one, Virginia Tech’s HPP researchers determined the high pressure inactivation parameters for the organism in vitro. Many other illnesses, including new and emerging infections, may be candidates for HPP-generated vaccines. Holliman’s team has plans to examine a variety of organisms, with the goal of developing vaccines for them using HPP. Preliminary studies for HPP-inactivation/vaccine generation for Mycobacterium marinum, a fish and human pathogen, began in August. M. marinum is similar in many ways to Mycobacterium tuberculosis, the organism that causes tuberculosis in humans. “We hope that the data generated in this study will lead to new vaccine approaches for human tuberculosis, in addition to the veterinary applications,” says Holliman. “There is no limit to the potential candidates for new vaccines that could be generated using our techniques.”

In the future this technology could also be used to enhance or purify existing vaccines and biopharmaceuticals, and to develop novel compounds using HPP-induced alterations of conformational and chemical structures.

Helping industry

HPP can provide food manufacturers an edge in the marketplace. By extending shelf life and halting enzymatic processes that lower food quality, HPP can boost the availability and consumer appeal of many products, leading to improved profit potential. The Virginia Tech laboratory works with many outside companies to assist them in considering the application of high pressure in their food products.

Lab manager Laura Douglas says, “Competition is fierce among food manufacturers, and they’re always looking for ways to provide a better product or a novel presentation for their consumers. We provide a complete package of testing services for private label firms. We can develop a process schedule that will work for their needs; test our results thoroughly from a sensory, microbiology, and chemical standpoint; and deliver sample batches of product along with a complete report to decision makers within the company. Confidentiality is always assured.”

In these ways, the lab serves as an unbiased provider of information and can support the adoption of the new technology for use in products when it is a good fit.

Flick says, “The shellfish industry in particular finds that HPP opens up some exciting possibilities, because while it is extending shelf life and eliminating risks from Vibrios and other pathogens, it also releases the shell of bivalves, making them very easy and quick to shuck. They open right up.”

Oysters processed with HPP have been on the market for a few years, but only a few processing firms have made the transition to using high pressure processing equipment. Crab and lobster products are also great candidates for HPP, as are many high-cost foods with a short shelf life. “Shellfish processors need to carefully consider the benefits and costs of HPP for their own operation,” says Flick.

Farmers and harvesters can follow every safety guideline out there and still be unable to completely prevent microbial contamination of their products – that’s the way it is. But processors now have the option to mitigate food safety hazards later in processing without altering their product with heat, additives, or freezing.

The technology is a boon for those who want to increase their intake of raw or fresh foods, and for those who try not to eat anything they can’t pronounce. It’s also a great safeguard for infants, the immunocompromised, and the elderly. In sectors of the food industry that produce fresh salads, juices, and salsas; deli meats; and high value seafood, HPP is well known and has become the gold standard for food safety and quality. For other sectors, more research is required before the technology can be adopted as a safe and palatable alternative for processing. At Virginia Tech, the pressure is on to develop safer foods and new medicines.

Oysters ready for processing by Virginia Tech researchers George Flick and Daniel Holliman. The oysters are banded prior to processing to ensure that the shells stay closed after the procedure. View the complete photo at a larger size.

Research Director Daniel Holliman loading the high-pressure vessel with a basket of product. You can view the complete photo at a larger size.

“Companies are always looking for a cost-effective way to maintain safety and eliminate pathogens. High hydrostatic pressure processing is one way some of them can make their foods safer without making the products look, smell, or taste different.”
– George Flick

George Flick at work in the high-hydrostatic pressure processing lab. You can view the complete photo at a larger size.

Daniel Holliman uses the hoist to pull a heavily loaded basket from the high-pressure vessel.

Before and after: standard-sized styrofoam coffee cups become suitable for a shot of espresso with just a brief pressure treatment. You can view the complete photo at a larger size.

Daniel Holliman peers into the processing vessel.

The Virginia Tech HPP laboratory has the only full-scale commercial hydrostatic press controlled by a university on the North or South American continents. The 10-ton, 35-liter Quintus press enables researchers to achieve results that can be easily correlated to a full-production scenario.

Processed vs. unprocessed: High-value meats and produce fare well with high pressure, emerging virtually unchanged with regard to flavor, texture, and appearance, but with dramatically improved shelf life and safety. The top photo is of a plate of processed food; the bottom photo shows the unprocessed food. You can view the complete photo of the plates side-by-side, at a larger size.

Farmers and harvesters can follow every safety guideline out there and still be unable to pcompletely prevent microbial comtamination of their products – that’s the way it is. But processors now have the option to mitigate food safety hazards later in processing without altering their product with heat, additives, or freezing.

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