WHY AN OYSTER THERMOMETER ?
KNOWLEDGE AND SAFETY GO HAND-IN-HAND
Here is the knowledge about vibrio from Bob Rheault, Executive Director of the East Coast Shellfish Growers Association. (It is all a quote.)
1) Vibrios are ubiquitous - (everywhere) - usually in moderate numbers. Shellfish will concentrate bacteria while they feed, but they also will digest and expel some - reaching some sort of equilibrium as long as they are feeding. If they are disturbed and clam up, the counts start to double depending on temperature.
2) In warmer waters there tend to be more – but, unless your oysters are baking in the sun in a tide pool or exposed on a tidal flat, they are likely to have a small or moderate dose
3) It takes a large dose to cause illness (we believe for total Vp that dose is about 30,000 cells, but there are many strains of Vp - About 80% of Vp cells on the east coast are totally benign, the remaining 20% are the ones we worry about - the strains that produce certain hemolysin toxins that cause illness )
4) A small dose becomes a big dose when the oyster is removed from the water and allowed to warm up or stay warm for hours.
5) Growth rates are tied to temperature - at 90 degrees F the population will double every hour, at 80 degrees it takes about two hours, at 60 degrees it takes over a day for the bacteria population to double, and below 50 the growth of Vp stops altogether.
6) We do not have a way to eliminate all bacteria in live oysters. (Holding in chilled seawater for 48 hours seems to have promise, but this is something only professionals should attempt and requires special permits.) Bacteria and viruses can be eliminated by cooking through. Bacteria can be reduced via many PHP processes that leave the animal dead.
It is rare, (but possible) that a dozen oysters coming right out of the water have enough Vibrios inside to cause illness.It is far more common that safe oysters were turned into unsafe oysters because they were not chilled before a few doublings occurred. (If you ever bet on Backgammon and used the doubling cube you know how fast your bets can go from a dollar to 2,4,8,16 or 32 dollars... and an affordable game becomes quite painful.)
So if you can at least get your shellfish down to 60 as soon as possible, and down to 50 within 5 hours - your risk of illness should be negligible. If you leave your shellfish in the sun for a few hours, all bets are off. If you hold them in the water, chances are better that they remain cool, but it is likely that they remain clamped shut if they have been disturbed so counts will continue to go up unless they open up and start feeding again which should allow the numbers to come back to equilibrium in a few hours.
Vibrio bacteria eat chitin - so if you have lots of dead crabs or shrimp shells in the growing waters you may elevate your risk substantially. About half of Vp illnesses come from improperly handled shrimps and crabs - or cross contamination between raw and cooked or partially cooked shrimp or crab.
Immuno-compromised individuals should not eat ANY raw proteins - no raw fish or shellfish, no undercooked crab or shrimp, no undercooked chicken or beef, no raw egg... period. If you are not sure if you are imuno-compromised you should ask your doctor.
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