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ENC scientist develops test for safely consuming raw oysters


You may be thinking about oysters as a part of your romantic Valentine’s dinner.  They’re packed with trace minerals and vitamins A and D.  Something you hope they're not tainted by is bacteria.  The good news is now is one of the best times to consume this shellfish delicacy.

“The beginning of February is usually one of the coldest times and it’s usually when we see the lowest levels of bacteria.”

That’s Research Assistant Professor at the Institute of Marine Sciences in Morehead City Brett Froelich talking about vibrio bacteria, a naturally occurring bacterium common in coastal waters.   

“Raw oysters as with any raw food product, being uncooked, any bacteria that are present in that are going to be going right in your body.  This is particularly important for oysters because they are filter feeders.  As they filter the water they are growing in, they are concentrating those bacteria in the water into their meat.”

Even though the chance of a vibrio infection is relatively low, it does happen. Symptoms include nausea, vomiting, fever and in very rare cases death.  People with diabetes, alcoholism and especially liver disease are at increased risk of serious illness.

Credit E. Woodward/UNC Institute of Marine Sciences
Fringing reef in Middle Marsh

In addition to temperature, salinity is another factor that determines the levels of vibrio bacteria in an oyster.  The saltier the water, the less bacteria.

“So it’s only when waters are really warm and have a lower salinity usually from rain or just because of where it’s located that the bacteria levels tend to be higher.”

In North Carolina, oyster season runs from October to the end of March.  So there is some truth to the old saying “oysters are best in the months with the letter “R.”  However, Froelich says during September and April – which both have an “r”- vibrio bacteria levels can still be high.

“We’ve done a three year study and we’ve found that October is a particularly high season for these bacteria, coming off the summer season, and while even though this is an in-season time and it’s generally accepted okay to eat the oysters, there are lingering populations of these harmful bacteria.”

The simple solution is to just cook them and the risk of a vibrio infection goes away.  But for those who enjoy raw oysters, Froelich has developed a quick, simple test that will determine whether they are safe to eat. 

“Essentially, it’s a color coded chart and just by looking at the temperature of the water and the salinity, which say you’re an at-home oyster grower, you can measure the salinity using a $20 device that you can buy on Amazon.  And in just a few seconds, you can follow along on this chart and see where the color is red, yellow or green which kind of gives you an indication of how many bacteria might be in these particular oysters.”

The new testing method would be beneficial to people who grow their own oysters in their backyard, part of the Under Dock Oyster Culture program.   Commercial fishermen could also take the measurements before harvesting from a particular area to ensure the oysters are safe to eat raw.  Because salinity is a component, Froelich says fishermen would notice site to site differences. 

“These bacteria both replicate very fast, and they are also flushed by oysters pretty quickly.  So this could be day to day changes.  What may be poor one day could be okay the next.”

Using a thermometer, a salinity reader and the new color coded chart takes less than a minute to get results.  That’s compared to nearly 24 hours that it takes Froelick to collect oysters, pulverize the meat, and analyze the bacteria levels in the laboratory.

Credit ML Parker/ UNC Research

“Around the Institute, everybody refers to me as the man who makes the oyster milkshakes.”

The project was conducted in collaboration with the Division of Marine Fisheries and University of North Carolina Charlotte.  Right now, Froelich says the information on the chart is “eastern North Carolina specific,” and that there are plans to broaden the range in the future.

“And what we’re looking at next is the difference between farmed oysters and wild oysters.  They experience different conditions at different times and what’s true for the wild oysters may not be true for the farmed oysters. And we’re beginning that project this year.”

Jared Brumbaugh is the Assistant General Manager for Public Radio East. An Eastern North Carolina native, Jared began his professional public radio career at Public Radio East while he was a student at Craven Community College earning his degree in Electronics Engineering Technology. During his 15+ years at Public Radio East, he has served as an award-winning journalist, producer, and on-air host. When not at the station, Jared enjoys hiking, traveling, and honing his culinary skills.