Eating raw oysters is getting safer, thanks to a new practice called post-harvest processing, or PHP, that virtually eliminates harmful Vibrio vulnificus bacteria from the shellfish, say University of Florida researchers.
Recently enacted federal guidelines require oyster processors in Florida and several other states to use PHP, said Victor Garrido, a research coordinator with UF's Institute of Food and Agricultural Sciences.
The technology involved isn't cheap, so UF experts are helping processors in Apalachicola, heart of Florida's oyster industry, evaluate various methods and learn to use them successfully, he said.
"Florida's oyster processors have to change with the times to survive," Garrido said. "The industry is making a big effort, putting money and equipment into post-harvest processing. There's also a risk in putting out a new product."
For decades, freshly harvested oysters were simply stored under refrigeration to discourage bacterial growth, he said. That practice is still in use, but PHP takes things a step further, by reducing the number of Vibrio vulnificus bacteria in the shellfish before they leave the packing house for wholesalers, retailers and restaurants.
Three cold-based methods appear most promising for the Florida industry, Garrido said. One flash-freezes oysters with liquid nitrogen, another does the job with powerful blast freezers, a third uses immersion in hot water, then ice slush, and finishes with a trip to the freezer. All three result in raw, half-shell oysters that are frozen solid and may be stored for several months without loss of quality.
UF researchers have developed sensory profiles to better direct fine-tuning of PHP methods to assure that the texture, color and flavor of treated oysters are similar to the original characteristics of fresh oysters. UF's oyster sensory team, led by biological scientist Laura Garrido, has developed the nation's first sensory standards for raw oyster products.
Concurrent UF projects are assessing the PHP methods -- which were developed by private companies -- to determine how well they kill bacteria, he said. The methods using liquid nitrogen and blast freezers reduced Vibrio vulnificus bacteria levels to less than 30 organisms per gram of meat, sufficient to win approval by the Florida Department of Agriculture and Consumer Services and the U.S. Food and Drug Administration; the third method is expected to be validated later this year.
To check their bacterial content, the oysters are analyzed at a UF laboratory in Apalachicola, using state-of-the-art procedures. Charlene Burke, a UF biological scientist, is operations director of the facility, which is the first in the United States to offer immediate measures confirming that oyster processing operations are turning out safe products.
The UF oyster research is funded by the U.S. Department of Agriculture and Florida Sea Grant, a program of coastal research and education affiliated with UF.
The federal mandate for PHP arose from concerns about Vibrio vulnificus, which naturally occurs in seawater and can accumulate in the tissues of raw shellfish, Victor Garrido said. It's more abundant during warm weather, a phenomenon that has long caused seafood aficionados to shun raw oysters from May through August.
Among healthy people, moderate exposure to Vibrio vulnificus does not result in any health problems, he said. But those with certain health conditions could experience life-threatening blood infections. This includes people with liver diseases, diabetes, stomach disorders, history of alcoholism and other conditions that weaken the immune system.
Physicians often recommend that people at higher risk of infection avoid raw shellfish altogether, he said. So PHP could put raw oysters back on the menu for people who avoid them out of necessity, or just caution. Restaurants serving raw oysters may find these products convenient, because they can be stored for long periods and thawed when needed.
Currently, Florida's oyster industry must have enough PHP capacity to handle 25 percent of the oysters harvested between May and September that will be sold raw for the half-shell market, said Bill Mahan, Franklin County extension director and Florida Sea Grant agent.
The requirement went into effect in January 2005, and its effectiveness is still being assessed, said Mahan, also a member of the Interstate Shellfish Sanitation Conference, a body that dictates guidelines for oyster fishing, processing and commerce. Vibrio vulnificus infections are rare -- Florida averaged about nine cases per year in the late 1990s, according to the state agriculture department. That translates to .6 cases for every one million residents, a figure now used as a baseline for gauging improvement.
Next year, experts will review Vibrio vulnificus infections reported in Florida for 2005 and 2006, looking for a 40 percent reduction from baseline levels. Mahan expects that goal to be met -- if so, current requirements for PHP capacity will remain as is. If not, Florida's oyster industry will be required to double its PHP capacity or comply with new limits on harvesting, processing and marketing oysters.
Currently, three Florida oyster processing firms are using or preparing to use PHP. In 2004, Leavins Seafood became the first producer in Florida to offer PHP-treated oysters commercially, using a liquid nitrogen method that owner Grady Leavins developed.
"The demand has been wonderful," said Leavins, who has a patent pending on his method. "We sold twice as many of them the second year as the first, and I believe we'll sell three times as many the third year."
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