Oysters

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Key Factspending-external-review

  • In addition to being popular culinary delicacies, oysters help protect coastlines, clean water by filtering, and provide a habitat for many other species such as fish, shrimp, and crabs.
  • Several coastal regions of the U.S. are known for oyster production. Oysters are among Florida’s top commercial seafood products in terms of dockside value.
  • There are many harvesting and processing techniques. To ensure the prevention of foodborne disease outbreaks, Hazard Analysis and Critical Control Points (HACCP) plans must be in place for processing, packaging, and storage of oysters.
  • Oysters are filter feeders and may harbor harmful pathogens, such as Vibrio, norovirus, and Hepatitis A in their tissue. These pathogens are associated with foodborne outbreaks and can lead to recalls and harvest site closures.

IntroductionOyster_OutbreakMuseum

Oyster cultivation began over 2,000 years ago when the Romans harvested them from the Adriatic Sea and eventually, from all over the Mediterranean. Today, they are a popular commodity consumed around the world.

The oyster is a bivalve mollusk, meaning it has two shells held together by a strong muscle. The thick, larger lower shell permanently attaches itself to a hard substrate while the top shell generally has a smaller and flatter appearance. Oysters play a valuable role in the coastline ecosystem. They are filter feeders, filtering the water around them at a rate of 25 gallons per day to feed on single-cell plants. Oysters are considered a keystone species*, detoxifying their surrounding environment while providing a habitat for fish, shrimp, crabs, and other sea animals.

*A keystone species is a plant or animal that plays a unique and crucial role in the way an ecosystem functions. Without keystone species, the ecosystem would be dramatically different or cease to exist altogether.

Geography

There are two types of oyster species that are largely harvested in the United States, the Eastern oyster (Crassostrea virginica) and the Pacific oyster (Crassostrea gigas). The Eastern oyster is native to the Eastern coastline from Newfoundland, Canada down to Panama and parts of northern Colombia, including the Gulf of Mexico. The Pacific oyster originated from Japan but can now be found in parts of East Asia and Australia, and from Southeast Alaska down the west coast to the tip of the Baja peninsula of Mexico. Both species can also be found in the British Isles and the Mediterranean.

Gulf of Mexico showing were Apalachicola Bay
Gulf of Mexico showing Apalachicola Bay

Eastern oysters naturally spawn in Gulf waters. In Florida, the majority of harvest locations are in Apalachicola Bay off the panhandle region. Freshwater from the Apalachicola River flows into the bay and mixes with saltwater, creating an estuarine environment. This moderate-salinity environment is preferable to oysters, and helps to limit parasites, pathogens, and predators that often prefer saltier waters. The Chesapeake Bay also is a similar estuary environment preferable to the Eastern oyster. Pacific oysters have formally flourished in Willapa Bay, Oakland Bay, and Puget Sound, but today they are only commercially harvested in the southern bays of the Puget Sound.

Foodborne Outbreaks

When an outbreak has been identified and epidemiologically linked to oysters, regulatory agencies take appropriate action to protect public health. Information from the tags on the oyster bags are used to pinpoint the exact location of harvest. In some cases, foodborne illness outbreaks can lead to recalls and harvest location closures.

Below are examples of outbreaks and recalls associated with oysters reflecting the diversity of vehicles, pathogens, and other circumstances:

A 2006 outbreak of Norovirus occurred in frozen oysters that were thawed and then consumed raw in Marion County, Oregon.

In late 2014, the Washington State Department of Health identified an outbreak of norovirus-like illnesses associated with the ingestion of raw oysters. Upon a brief shoreline inspection, a failing septic system was discovered as the potential source of illness. A recall was issued involving three Washington companies, nine additional states, and two additional countries.

In August 2015, a food recall warning was issued for oysters harvested from British Columbia coastal waters on or before August 18, 2015 due to unacceptable levels of V. parahaemolyticus. The recall applied to oysters sold for raw consumption and was targeted at consumers, dealers, retailers, and food service establishments. The recall was triggered by an investigation into a foodborne illness outbreak by the Canadian Food Inspection Agency.

On September 2, 2015, the Massachusetts Department of Health announced a 7-day closure of a harvesting site due to the epidemiological linkage of over three cases of V. parahaemolyticus. The harvest site was declared open on September 10, 2015.

Between December 16, 2018 and April 17, 2019 sixteen cases of gastrointestinal illness were identified and associated with the ingestion of raw oysters in five different states. The Centers for Disease Control and Prevention (CDC) confirmed that the outbreak included cases of Shigella flexneri, V. parahaemolyticus, Shiga toxin-producing Escherichia coli non-O157 coinfection, Campylobacter lacri, V.albensis, norovirus, and Vibrio of unknown species, traced back to raw oysters imported from Baja California Sur, Mexico. On May 6, 2019 Estero El Cardon issued a voluntary recall and on May 7, 2019 Mexico’s public health authorities closed Estero El Cardon until June 13, 2019.

Although many of the outbreaks associated with oysters caused fairly small numbers of illness, in 1998 over 400 illnesses caused by V. parahaemolyticus serotype 03:K6 were attributed to oysters harvested from Galveston Bay.

Production

Harvesting

When wild oysters grow and reproduce without human interference, the larvae, known as ‘spat,’ attach to oyster reefs or other structures in the water. The cultivation of oysters involves growing immature oysters in captivity and transporting them to man-made oyster beds where they reach maturity. Oysters harvested in the northwest and northeast are primarily cultivated whereas oysters harvested from the Gulf of Mexico are primarily harvested from natural beds. Oyster beds in Florida waters are managed by the Florida Fish and Wildlife Conservation Commission (FWC) and processing facilities are regulated by the Florida Department of Agriculture and Consumer Services (DACS).

Harvester tonging for oysters. Source: Tampa Bay Times
Harvester tonging for oysters. Source: Tampa Bay Times

Florida’s oyster industry is concentrated in the Apalachicola Bay, where ninety percent of Florida’s oysters are harvested. Along Florida’s Gulf Coast, oysters have been harvested the same way for over a century. Oystermen on small boats use long-handled, rake-like tongs to scoop up oysters from their beds. This method is sustainable, unlike dredging which can severely damage the seafloor by crushing, burying and exposing marine organisms. Tonging can also be detrimental to the environment if not properly regulated. The oysters are brought on the boat where they wait to be culled. The oystermen use a culling iron to break off barnacles and spat on the outside of the oyster shell. The culling iron also serves as a measurement tool to ensure the oyster meets the legal Florida harvesting size of three inches in length. Legal harvesting sizes vary from state to state. Adult oysters smaller than the legal harvesting size are tossed back into the water to reach maturity. The excess shells that are brought up with the oysters in the tongs are returned to the water for bed preservation.

Harvesters delivering directly to dealers/processors. Source: Florida Department of Agriculture and Consumer Services
Harvesters delivering directly to dealers/processors. Source: Florida Department of Agriculture and Consumer Services

Oysters that are harvested from Apalachicola Bay are brought back to shore and handled according to strict procedures outlined by DACS. Harvesters are required to deliver oysters directly to the dealer (also referred to as a processor) by statute, so most dealers are located in this region. Harvesting starts at dawn from open/approved harvest area waters and return by specific times. Oysters are bagged, tagged, and delivered to dealers by boat or vehicle; dealers are located on either dockside or on land. Oysters that are traditionally harvested on a boat without on-board cooling capability must be cooled to 55°F within 8 hours of mechanical refrigeration.

Harvesting methods, equipment, boat size, cooling methods, harvest times, and seasonal management strategies vary according to state regulations. For example, Florida and Alabama only allow small boats for individual harvesters while Louisiana and Texas allow large boats with multiple crew members and full size coolers. These boats are capable of staying on the water for days at a time. Other states allow boats that harvest and process oysters.

Aquaculture

Oyster aquaculture is of growing interest in Florida. On submerged land leases, aquaculturists either bottom plant oyster stock, attached spat, or submerge mesh bags filled with oyster seed (immature oysters). This process is similar to growing fruit on land. Planting usually occurs during the fall and winter months to avoid damage during summertime harvest. A successful oyster reef will have enough clutch material for spat so that seed planting in the future is not necessary. In Texas and Louisiana, lease-holders are allowed to harvest oysters at designated times from natural oyster reefs and transplant them to their lease site for harvest upon maturity.

Processing Certifications

Once received by the dealer, oysters are weighed and each bag is retagged. In Florida, there are several levels of certification for which a dealer may qualify. A “shellstock shipper” may resell the shellstock in the original bag in which it was received, or may wash, sort, re-bag, or box the product. Shellstock refers to whole, in-shell oysters. A “shuck packer” may perform the duties of a shellstock shipper, but is also approved to shuck oysters and distribute the meat in regulated containers. Facilities certificated for “post-harvest processing” (PHP) are approved to perform individual quick freezing (IQF), high hydrostatic pressure, irradiation, and quick freeze on the half shell. PHP methods have the ability to reduce Vibrio spp. sometimes found in oyster meat to non-detectable levels, although validation of PHP must occur.

Post-Harvest Processing: Individual Quick Freezing

Each processing facility adheres to the same Interstate Shellfish Sanitation Conference (ISSC) regulations, but each facility usually has a unique method of processing. At a local PHP-certified facility in Florida, oysters are received on a refrigerated truck that is kept below 41°F. The truck backs up to the facility loading platform where the oysters are unloaded; the storage compartments of the trucks are temperature controlled to maintain safe internal temperatures. The oysters are delivered in bags where one bag is equal to one 10-gallon bucket, or 60 pounds of culled oysters in the shell. They are then transferred to a large, refrigerated storage room that is maintained between 38-41°F and organized by geographical region. The National Shellfish Sanitation Program (NSSP) requires shellfish coolers to be maintained at 45°F. Anything below this temperature is the dealer’s choice.

The harvest tag on each bag is removed and replaced with a dealer tag containing a Florida certification number prior to sale and the original harvest tag is retained by the dealer. Tags are important during oyster-related foodborne disease outbreaks because they allow epidemiologists to identify harvest and processing locations (trace back investigations), where potentially contaminated oysters have been shipped (trace forward investigations), and aid in the identification of food safety violations. Once oysters are ready for processing, they are loaded into a mechanical sorting machine and undergo a quality check.

1) Oysters are weighed. 2) Cold storage in 50 lb bags. 3) Cold storage in 50 lb bags and 50 gallon containers. Source: David Dekevich
1) Oysters are weighed. 2) Cold storage in 50 lb bags. 3) Cold storage in 50 lb bags and 50 gallon containers. Source: David Dekevich

The local Florida PHP-certified facility distributes whole, in-tact oysters in the bag (shellstock), oysters on the half shell designated for raw consumption, and oyster meat designated for cooking, frying, and freezing. De-shelled oyster meat is referred to as shucked product. All shellfish dealers must have a specific Hazard Analysis and Critical Control Points (HACCP) plan and follow specific guidelines for the Critical Control Points, including receiving, processing (thawing, shucking, portioning), packaging, and storage. In addition to a dealer HACCP plan, PHP-certified dealers must have specific HACCP plans to validate the PHP.

The PHP at a local Florida dealer is one process that is composed of multiple steps: heat shocking, flash freezing, and cold storage. Heat shock involves briefly exposing shellstock to hot water on a conveyer belt prior to shucking. This process relaxes the abdominal muscles that are connected to the shell, reducing the difficulty of shucking the oyster. The oysters are easier to open and the risk of contamination by small shell pieces is decreased. For oysters that will be shipped frozen on the half shell, the heat shock water temperature is maintained between 145-150°F. For oysters that will be fully removed from their shell, the water temperature is maintained between 147-152°F.

Nitrogen freeze preparation. 1. Fresh water rinse. 2. Moving along the conveyor belt. 3. Quality check 4. Nitrogen flash freeze machine. Souce: David Dekevich
Nitrogen freeze preparation. 1. Fresh water rinse. 2. Moving along the conveyor belt. 3. Quality check 4. Nitrogen flash freeze machine. Souce: David Dekevich

After the oysters have been heat shocked, they move along the conveyer belt to another room where they are shucked. Depending on product demand, oysters are either left on a half shell or fully removed from their shell. Individuals who shuck the oysters stand on both sides of the conveyer belt at personal stations. Half shell oysters are placed back on the conveyer belt and fully shucked oyster meat is transferred to a container at the shucker’s station. Excess shells are dropped down a shoot onto a separate conveyer belt that empties them out onto a pile of shells outside of the facility. These shells are purchased by State or Federal programs to rebuild the oyster reefs in Apalachicola Bay. Oyster shells encourage regeneration of the beds because spat attach themselves to the shells where they mature into adult oysters.

Oysters on the half shell continue along the conveyer belt where they are rinsed with water that is about 50°F. Some oysters intended for half shell distribution that are improperly shucked fall out of the shell into a tub below the conveyer belt; these are later combined with the shucked product that is sold in pint and gallon sized containers. The half shells are then quality checked by hand and turned so that the meat side is facing up. Visually unappealing oysters are discarded. All oysters on the half shell deemed for shipping are flash frozen via liquid nitrogen. Once the oysters are properly positioned, they move slowly on the conveyer belt through the freezer and are exposed to -70°F for 7.5 minutes. When they exit the flash freezer, the temperature of the meat is about -20°F. The oysters are “glazed” one more time with 50°F water to prevent frost bite and to seal in natural juices during cold storage and shipping. After PHP, oysters are packed into a high quality box in layers of 12, separated by food grade Styrofoam and stored at -20°F for 7 days. After cold storage, they are ready for shipment. Once received, the boxes are to be kept on ice until ready to serve. Flash frozen oysters are intended to be served on the shell at food establishments for raw consumption.

In addition to PHP, some dealers ship whole oysters deemed for raw consumption in 50 lb. bags to other dealers, wholesalers, and restaurants. These oysters are not processed like the oysters on the half shell since they are shucked right before they are served.

1) Oysters exit the nitrogen freezer. 2) Oysters are packed for shipping. Source: David Dekevich
1) Oysters exit the nitrogen freezer. 2) Oysters are packed for shipping. Source: David Dekevich

Other Post-Harvest Processing Methods

High hydrostatic pressure is another method of PHP performed by dealers. It is a patented process that exposes oysters to pressures of 35,000-40,000 pounds per square inch for three to five minutes to kill spoilage bacteria and reduce Vibrio sppto non-detectable levels. Irradiation is also used by some dealers in Florida. The goal of irradiation is to reduce Vibrio spplevels without changing the palatability of the oyster. Irradiation is an inexpensive, highly-effective PHP for oysters. A truckload of about 37,000 pounds of shell stock can be irradiated in about two hours and costs about eight to ten cents per pound. Irradiation levels from 0.82 kiloGray (kGy) to <2 kGy are proven not to alter taste while still reducing Vibrio spp. to non-detectable levels. The U.S. Food and Drug Administration has identified 5.5 kGy as the maximum dose of irradiation for shellfish.

Food Safety

Oysters pose health risks to healthy and immunocompromised individuals when consumed raw or undercooked. Oysters are filter feeders and may harbor Vibrio vulnificusVibrio parahaemolyticus, or other pathogenic Vibrio spp. in their tissues. Vibrio spp. are naturally occurring bacteria that are present in warm sea waters, such as the Gulf of Mexico. The bacteria may be present in approved harvesting waters since testing for contamination involves identification of other pathogen species. Concentrations of Vibrio spp. increase May through August when water temperatures are warmer, and when 60 percent of vibriosis cases occur. PHP oysters that are designated for raw consumption have reduced colony counts of Vibro spp. in the product, which decreases the overall risk of Vibrio infection. Hepatitis A and norovirus are viruses that may be present in waters with fecal contamination and oysters may harbor these viruses in their tissue as well.

These pathogens do not alter the look, taste, or smell of the oyster and can be consumed unknowingly. Among seafood-borne infections in the United States, V. vulnificus causes the highest number of fatalities, although this type of illness is generally rare.

Common Oyster Pathogens and Symptoms of Illness

Ingestion of V. vulnificus among healthy people can cause vomiting, diarrhea, and abdominal pain within 12-72 hours after ingestion. Ingestion among persons with liver disease and immunocompromising conditions can cause life-threatening illness characterized by fever, septic shock, and blistering skin lesions. V. vulnificus bloodstream infections are fatal in over 50% of infections. V. parahaemolyticus can cause diarrhea, vomiting, and abdominal pain in healthy individuals within 24 hours of ingestion. Similar to V. vulnificusV. parahaemolyticus can also cause serious complications in persons with liver disease, immunosuppression, and decreased gastric acidity. Recovery for healthy individuals generally takes place without medication after a few days.

Hepatitis A and norovirus are also concerns for those who consume raw oysters harvested from waters with fecal contamination or from unrecognized contamination in regulated harvest sites. Hepatitis A is a liver disease caused by the Hepatitis A virus. Symptoms include diarrhea, dark urine, jaundice, fever, headache, nausea, abdominal pain, and loss of appetite; which usually become apparent 28-30 days after ingestion. Norovirus is a highly contagious virus that causes stomach pain, nausea, vomiting, and diarrhea 12-48 hours after ingestion. Individuals with Hepatitis A or norovirus tend to recover without seeking medical treatment.

Consumption

To reduce the risk of contracting a foodborne illness when eating oysters in restaurants, order them fully cooked. According to the 2013 Food and Drug Administration Food Code, an oyster is fully cooked when the internal temperature reaches 145°F for 15 or more seconds. When preparing oysters at home in the shell, discard any oysters with open shells before cooking. Boil them for 3-5 minutes after the shells open. Ensure the internal temperature reaches 145°F for at least 15 seconds. Discard any oysters that did not open after they have been boiled for the appropriate amount of time.

There are several ways to safely cook shucked oysters to reach an internal temperature of 145°F:

  • Boil or simmer for at least 3 minutes.
  • Fry at 375°F for at least 3 minutes.
  • Broil 3 inches from heat for 3 minutes.
  • Bake at 450°F for 10 minutes.

Nutrition

Oysters are highly nutritious. They are a low-calorie, low-cholesterol source of protein; an exceptional source of zinc, which strengthens the immune system; and a rich source of omega-3 fatty acids, which are linked to reduced risk of heart attack, breast cancer, prostate cancer, and stroke.

1) Raw oysters on the half shell. Source: Monica Oluwek 2) Baked stuffed oysters on the half shell. Source: Saucy Girl's Kitchen
1) Raw oysters on the half shell. Source: Monica Oluwek 2) Baked stuffed oysters on the half shell. Source: Saucy Girl’s Kitchen

References

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Author

David Dekevich

David Dekevich

Florida Integrated Food Safety Center of Excellence Liaison at the Florida Department of Health

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