Several ways to safely thaw your fish
*This article cannot be substituted for safety advice and we encourage you to refer to USDA recommended guidelines.
Can you really get sick from thawing your vacuum-packaged fish in a vacuum-sealed package?
Yes, you absolutely can — if you do not thaw your fish properly. For that reason, most government agencies, industry organizations, and food safety websites urge you to puncture the package to release the vacuum. Here’s what to know.
Fish is to Clostridium Botulinum (C. bot) as beef is to E. coli and chicken is to Salmonella. Packaged fish is commonly known to contain C. bot either because its habitat generates the bacteria, or because of exposure somewhere between where the animal dies and your dinner plate. The good news is that C. bot only causes illness if it can produce the botulism toxin, and we’re here to tell you how to keep that from happening.
The short answer: botulism develops only at specific temperatures and if there is little to no oxygen present. To eliminate risk, avoid thawing your fish in an intact vacuum package. Botulism toxin, while scary, is easily destroyed by heating your food to at least 185℉ for five minutes.
Why would anyone vacuum-package fish if there is such a great risk?
Fish is one of the most delicate meats to store and prepare. The moment a fish takes its last breath, the quality will degrade if not protected. Vacuum-sealing the fish helps to maintain quality in a couple ways. For one thing, there’s a lot of water in the flesh of fish. Even in a freezer, that water will evaporate if unsealed and cause freezer burn. Vacuum packages also prevent the healthy fats and oils in fish from oxidizing, which can leave fish tasting rancid. And of course, vacuum seals keep your freezer (and home) from smelling like fish.
Vacuum seals ensure a high caliber of fish for consumers, and with the proper precautions, you don't have to sacrifice quality for safety.
Why would someone recommend opening the pouch to thaw if you keep the fish cold throughout the process of thawing?
There are various types of C. bot, and the most resilient types can grow and produce toxins at temperatures as low as 37.9℉. However, the product would have to be between 37.9℉ and 41℉ for seven days for the bacteria to accomplish this. Even between 42℉ and 50℉ it would take two days to grow and produce toxin.
What is the concern if it takes so long for this to happen at common refrigerated temperatures?
Fridge temperatures are sensitive and can fluctuate widely, meaning it’s not always easy to predict the conditions we store our food in — or assume that they aren’t fertile ground for bacteria. Out of curiosity, I checked the temperature of my 15-year-old refrigerator with my calibrated, accurate digital thermometer. The top shelf held at about 42.6 F, but jumped to 50 F within ten seconds of being opened. At the same time, the back of the lower shelves held at 34.4 F. That’s quite a range for a normal size refrigerator.
Common practices, like leaving the fridge door open or removing frozen foods, can increase the internal temperature of the fridge. And if your household is anything like mine, as soon as your food hits the back of the refrigerator it falls into a black hole until you clean it out or investigate the unpleasant odor. Leaving fish in the fridge for too long without accounting for temperature shifts can produce a time and temperature combination that might be deadly if you aren’t careful.
The Foolproof Method
To completely eliminate risk, we recommend you puncture the package to introduce oxygen and allow the fish to thaw in your refrigerator overnight. There are some things you should prepare for, however. For one thing, your fridge may smell like fish — even if you transfer it to a sealed container. Second, fish starts dehydrating in the refrigerator once the package is open, so we advise waiting to thaw your fish until you plan to eat it. And finally, if you aren’t going to cook or sear the fish and instead plan to eat it raw, you risk exposing it to bacteria normally found in a household refrigerator, like Listeria.
Overnight Method (when you know the temperature of your refrigerator)
Thawing fish in an intact vacuum seal is possible IF you are able to measure and control the temperature of your fridge AND plan to use the fish soon after thawing. Remember that C bot is a question of temperature versus time, so accounting for both is important.
Because temperatures fluctuate between and within refrigerators, we recommend installing a thermometer to monitor your fridge environment. You must place the fish in the coldest part of the refrigerator, one that you know is at or below 38℉. This is likely the place where you’ve found frozen leftovers or produce before. Leave it in the refrigerator to thaw overnight and no more than 24 hours before you’re going to prepare it. NEVER thaw the vacuum-packaged fish for any period of time at room temperature.
Less Than an Hour to Thaw
One quick option is to rapidly thaw the fish in cold water in the intact vacuum package. Use running water or fill up your sink. Note: Never use warm or hot water. It seems counterintuitive, but some types of C. bot can produce toxins in just six hours if at temperatures of 70℉ or above.
Moreover, thawing with warm or hot water isn’t even that effective. The surface of the fish may thaw (or even start to cook) more rapidly, but its center really does not thaw any faster.
The Bottom Line
There are several ways to safely thaw your fish, and the right one for you depends on several factors. C. bot is a question of temperature versus time, so consider both when deciding which method to use. Botulism poisoning is extremely preventable. If taking simple precautions, the risk of illness should not deter seafood lovers from enjoying high quality fish.
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Know Your Author
Justin Blaney is a Food Scientist with over 20 years working in food safety including working for the state of Wisconsin’s Division of Food Safety