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Special Feature: Oysters

Though not mandated, processing grows in the Gulf

Motivatit Seafoods uses a high-pressure chamber to kill bacteria in oysters. - Photo by Melissa Wood
By Melissa Wood
June 01, 2013

The number of illnesses caused by the bacteria commonly linked to raw oyster consumption is rising. Vibrio vulnificus infections increased 43 percent in 2012, compared to averages from 2006 to 2008, according to the latest Centers for Disease Control report on foodborne illnesses. The jump is even greater in a review of long-term trends, increasing 116 percent in 15 years.

The CDC recommends thoroughly cooking all oysters to prevent illness. The U.S. Food and Drug Administration has pushed for mandatory post-harvest processing (PHP) to rid oysters of the bacteria; in 2011 it wanted to mandate PHP for all oysters from the Gulf of Mexico during warmer months, when Vibrio is most common.

But the numbers don’t tell the whole story. Though the FDA has not yet implemented its plan, PHP has been increasing throughout the region that accounts for about two-thirds of U.S. oyster production, says Patrick Fahey of Ameripure Processing Co., in Franklin, La.

“You actually have more PHP processors today than you did three years ago as well as additional harvesting restrictions for raw oysters,” he says. Harvesters of white-tag oysters, which indicates they are approved for raw consumption, must be under refrigeration within one hour of harvesting. 

Steve Otwell, a seafood safety specialist at the University of Florida, doesn’t believe PHP will be mandated, but will remain driven by market forces. 

“You have all these options that are available, but it’s not mandated. The capacity has expanded, so that’s a good thing,” says Otwell. He says a popular topic in the food-science industry is whether warming oceans will spread Vibrio bacteria, but actual illnesses from eating raw oysters have not increased.

Vibrio are not a contaminant. They are part of the environment. So why don’t we all get sick? It’s because particular people are more vulnerable to this bacteria,” and their numbers are increasing as the population ages, he says.

As PHP grows, so does the number of available methods. Some companies use patented processes, like Ameripure’s cool pasteurization, which heats the oyster enough to kill bacteria without cooking them. Motivatit Seafoods in Houma, La., puts oysters into a high-pressure chamber, a process that, while effective, can be expensive, says Otwell. 

“The simplest one we’ve developed at the University of Florida and is available to anyone who wants to use it, is to freeze the product,” he says. Common freezing takes about a month, but a faster technique is to freeze oysters with carbon dioxide and liquid nitrogen, which takes less than a week to kill bacteria.

Otwell says the use of gamma irradiation, which takes an entire prepackaged product and runs it through a food-irradiation facility, has become more popular. 

Oregon State University researchers announced an improved oyster-purification method this spring that kills 99.9 percent of bacteria after four to five days. Depuration is a proven method, whereby oysters filter clean seawater and excrete most bacteria into the water, which is then filtered and sterilized with ultraviolet light. But since 10 percent of Vibrio remain after two days, the researchers tweaked that method by putting oysters in 45- to 55-degree (Fahrenheit) water sterilized with ultraviolet light.

“Temperature-controlled depuration uses less electricity than other methods that rely on freezers, heat and pressurization,” says Yi-Cheng Su, an OSU professor of seafood microbiology and safety. 

In early May, Fahey’s concern was supply as productive reefs in southern Texas were shut down until the season reopens in November and supply from the public reef in Louisiana was scant.

“We were able to get our orders out yesterday and today, but I’m worried about tomorrow,” he says. 

He and others point to a slower-than-expected recovery from the Deepwater Horizon oil spill in 2010. In efforts to slow the spread of oil, officials released fresh water, harming oysters that depend on a mix of salt and fresh water. This time, however, the culprit may be too much salt, says Otwell. A historic drought has dried up fresh water sources so oysters are getting more saltwater. 

But he predicts improved management is good news for oyster supply. “All of a sudden, it’s getting ready to roar back. If the drought doesn’t continue it should return to normal.”

Email Assistant Editor Melissa Wood at mwood@divcom.com 

Find other SeaFood Business articles discussing Vibrio vulnificus here.

June 2013 - SeaFood Business  

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