Outbreaks & Recalls

Could Freezing Oysters Reduce the Risk of Vibrio Infections?

Could freezing oysters reduce the risk of Vibrio infections? Scientists say, yes!

The Centers for Disease Control and Prevention (CDC) estimates that around 80,000 people become sick with vibriosis – the illness associated with Vibrio bacterial infections – in the United States each year. Around 100 of those infections will be fatal. In fact, Vibrio vulnificus is responsible for over 95% of seafood-related deaths in the United States, making it the highest fatality rate of any foodborne pathogen.

Oysters are the most associated food when it comes to foodborne Vibrio infections. So, it is no surprise that scientists are looking at ways to make this popular seafood a bit safer. Several studies have been focused on freezing oysters to kill potential Vibrio bacteria lurking within.

A recent study out of New Zealand examined optimal temperatures to kill this deadly bacterium.

Why Are They So Dangerous, and How Can Freezing Oysters Be the Answer?

You may have heard that eating oysters at certain times of the year is dangerous. However, eating raw or undercooked oysters is never a good idea – regardless of time of year!

Why does time of year influences oyster safety? It all comes down to water temperature. Certain bacteria flourish at warmer temperatures, particularly in salt or brackish water. Vibrio vulnificus and other Vibrio species are known for this phenomenon.

Oysters are filter feeders. This means they take in water and sort out their “foodstuffs,” then release the water back into the environment. On repeat.

One adult oyster can filter up to 190 liters of water a day.

That is just over 50 gallons!

Oysters are filter feeders.

One adult oyster can filter over 50 gallons a day!

Considering the size of these small animals, that is impressive! Taking in that much of the surrounding environment allows for pathogenic bacteria (including Vibrio vulnificus and Vibrio parahaemolyticus) to concentrate in the oyster 100-fold.

This natural bioaccumulation of Vibrio bacteria presents a food safety risk.

Should we just make oysters off limits?

Fortunately, oyster connoisseurs can release their collectively held breaths.

Most of the time seafood is frozen to preserve freshness, but scientists have discovered another benefit. Freezing oysters can inactivate and kill Vibrio bacteria.

Here’s how.

Freezing Oysters Kills Vibrio Bacteria

Freezing oysters also freezes the bacteria present inside them. While toxins can survive this treatment, bacteria can be more vulnerable.

As the oyster meat freezes, the bacteria freeze as well. Scientists describe this as a five-step process.

Five Step Freezing Process

Step 1: Extracellular Ice Formation

During extracellular ice formation, the freezing temperature begins to freeze areas just outside cells.

Step 2: Intracellular Ice Formation

During the second step of the freezing process, ice begins to form inside the cells.

Step 3: Concentration of Extracellular Solutes

As more extracellular ice forms on the organism, more water present outside of the cell becomes frozen, increasing the concentration of other chemicals – such as salt – in the area.

Step 4: Concentration of Intracellular Solutes

This same concentration process occurs inside the cells.

Step 5: Low Temperatures

Once both outside and inside areas of the cell become frozen, temperatures in the entire organism reduce.

This activity shears the cells, resulting in bacterial death.

Gram negative bacteria, such as Vibrio vulnificus, respond well to this treatment. The cell walls are much thinner than the gram-positive counterparts, such as Vibrio parahaemolyticus.

The Study

New Zealand scientists wanted to hone in on optimal freezing temperatures to determine safe processes to recommend to fisherman and processing facilities in an effort to reduce Vibrio illnesses.

Researchers exposed Pacific oysters to six different strains of Vibrio vulnificus, allowing the organism to bioaccumulate those harmful pathogens. Then they air-blast frozen them at -55 °C (-67 °F) and then put them in frozen storage at -8 °C, -13 °C, -18 °C, and -28°C. Samples were taken at different times from 0 to 360 days.

Blast freezing followed by frozen storage showed promising results.

Air-blast freezing, and subsequent frozen storage, showed significant results in decreasing Vibrio vulnificus in Pacific oyster samples. Rapid bacterial inactivation was observed in the first 3 days after air-blast freezing. The optimum temperature for achieving the strongest results was -18 °C for 123 days.

For gram-positive bacteria, such as Vibrio parahaemolyticus, a flash freeze of -95.5 °C for 12 minutes followed by -21 °C for five months showed optimal results.

What Other Methods Are Used Instead of Flash Freezing Oysters to Reduce Bacterial Load?

A variety of methods to reduce the bacterial load are available to the oyster industry. Some of these methods require a significant capital investment. Others are time or labor intensive.

Here are a few of the current options:

  • Irradiation
  • Mild heat processing
  • High-pressure processing (HPP)
  • Individual quick freezing (IQF) with extended frozen storage

Oyster harvesters and/or processors have several methods to choose from. However, flash freezing oysters followed by frozen storage seems to be the most optimal method.

Vibriosis Symptoms

Symptoms of Vibrio infection can vary from mild, to serious, and even deadly. Most people will experience diarrhea and vomiting, however, as many as 1 in 5 people with a Vibrio vulnificus infection die.

1 in 5 people with Vibrio vulnificus infection die

Vibrio vulnificus can lead to bloodstream infections, severe skin blistering, and limb amputations. Contact with raw oysters or other shellfish and spread through open wounds or breaks in the skin such as eczema or a rash.

People at Higher Risk

While anyone can become sick, people with certain health factors are more likely to become sick or experience severe illness.

You are at increased risk if you:

  • have liver disease, cancer, diabetes, HIV, or the blood disorder thalassemia.
  • receive immune-suppressing therapy for the treatment of disease.
  • take medicine to lower stomach acid levels.
  • have had recent stomach surgery.
  • are 65 years or older.

These individuals should avoid oysters and shellfish to prevent exposure to this deadly bacterium.

Reduce Your Chances of Vibrio Infection

Avoidance is the key to help reduce your chances of Vibrio infection. However, this is not always the preferred option.

If you do not have any of the high-risk factors, consider these prevention strategies to avoid Vibrio infection.

Don’t Eat Raw or Undercooked Oysters or Shellfish

Never eat raw or undercooked oysters or other shellfish, even if they have been previously frozen. Contrary to popular belief, hot sauce, lemon juice, and alcohol do not kill Vibrio bacteria.

Keep It Separate

Keep cooked seafood separate from raw seafood and its juices. This is a rule of thumb when it comes to all foods. Avoid cross-contamination by keeping them separate.

Wash Your Hands

Always wash your hands. Wash your hands with soap and water after handling raw seafood.

Does Freezing Oysters Impact Your Culinary Choices?

Does freezing oysters impact your dinner choices? If a package of raw oysters included the label indicating this new flash freeze and frozen storage process, would that impact your shopping decisions?

Stay in Touch with Make Food Safe!

If you’d like to know more about food safety topics in the news, like Could Freezing Oys check out the Make Food Safe Blog. We regularly update trending topics, foodborne infections in the news, recalls, and more! Stay tuned for quality information to help keep your family safe, while The Lange Law Firm, PLLC strives to Make Food Safe!

By: Heather Van Tassell (contributing writer, non-lawyer)

Heather Van Tassell

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