Fish Facts, Fresh or Frozen
For: Fish: What’s the Catch? July 2003
Eating fish once or twice a week is a heart-healthy idea (see Wellness Letter, March 1999). America’s favorite fish is canned tuna, and there’s nothing wrong with getting your fish allotment from canned fish. But a wide variety of whole fish, fillets, and shellfish is now available in fish markets and even in super-markets that used to stock little beyond frozen, breaded shrimp. Fresh fish adds variety and interest to a heart-healthy diet. Of course, it should really be fresh: fish and shellfish are among the most perishable of foods and require careful handling—from fisherman to merchant to you.
Shop in markets that smell meticulously clean, look clean, and are clean; that display fresh fish and shellfish atop a bed of shaved ice, in refrigerated cases, or both. Fish should not be stacked, but displayed in a single layer atop the ice. A merchant handling fresh fish should wear plastic gloves. Frozen fish should be stacked below the freeze-line in the freezer display.
Know a good fish when you see one
• Fresh fish should be shiny and should have a fresh sea smell—not a fishy odor.
• Another good test, particularly for fresh fish that’s already packaged, is to press it lightly with your finger. Fresh fish will be firm and spring back, leaving no indentation. The problem is that the seller won’t want customers poking the fish, but you can ask about freshness and ask him to do the finger test.
• Still another clue, if the fish is whole: its eyes should bulge a little and look clear. (A few fish, such as walleye pike, have white-ish eyes.) Cloudy, sunken eyes are a sign that it is not fresh.
• Prewrapped fresh fish in the supermarket may not be fresh. If it isn’t dated, ask when it was packaged.
• Much fish is now flash-frozen at very low temperatures soon after it is caught. When kept at 0° Fahrenheit and properly thawed (for several hours in the refrigerator or—if you’re really in a hurry—in the microwave), frozen fish is comparable to never-frozen. Thawing at room temperature is a bad idea for two reasons: flavor deteriorates, and spoilage is more likely.
• When buying frozen, select fish in tightly sealed packages with no ice crystals or any sign of refreezing. Fish should be rock hard; it may not be as shiny as fresh, but that’s okay. Sometimes frozen or thawing fish will be displayed along with fresh fish.
• Raw fish should not be displayed with cooked fish in such a way that the raw can contaminate the cooked.
• Plan to cook fish the day you buy it, or the next day at the latest. Make fish your last purchase, take it straight home, and refrigerate it right away. One option: have it packed in crushed ice.
Shellfish is different
• Cook shellfish the day you buy it. Don’t eat raw shellfish.
• Most shrimp have been frozen when caught and then thawed. This is perfectly okay, so long as they smell fresh.
• Mollusks (clams, oysters, mussels) should be bought live. If the shells are open, tap them sharply. Any that don’t close should be discarded—they are dead and may have started to decompose. After cooking, discard any whose shells fail to open.
• Scallops are the exception to the rule above. They are never sold in the shell because they deteriorate too quickly. Buy shiny, mild-smelling ones. Cook until the interior becomes opaque.
• If you buy cooked shellfish, remember that some dealers cook it only when it’s about to spoil. Ask when it was cooked. And be sure it was cooked enough.
• Live shellfish should be stored in the refrigerator, well aerated so that they stay alive. They’ll suffocate in a tightly sealed plastic bag. Immersing them in fresh water kills them. Wash them just before cooking.
Cooking tips
• Fish is one of the easiest of all foods to cook. Fat-free or low-fat methods include grilling, broiling, baking, poaching, and microwaving (a form of poaching). Frying or sautéing in a small amount of oil is okay, too. Avoid breading and deep-frying, which can turn fish into a high-fat dish. Marinades or herb rubs add wonderful flavors—just marinate in the fridge. If you want to use the marinade for a sauce, heat it to boiling.
• The secret to making fish taste good is to use high heat
but not overcook. Cook just until the flesh flakes when tested with a sharp knife or fork; the flesh should be opaque, but the center can be slightly translucent. Shellfish like shrimp and mussels will cook in five minutes. But steam mussels and clams past the point where the shell opens. Boiling for five minutes kills any bacteria.• When microwaving boneless fish, cook on high for three minutes per pound (depending on the oven); let stand three minutes to finish cooking. Some microwaves cook unevenly, so check for doneness in more than one spot.
• A delicious stock can be made with fish heads, bones, and other inedible parts. Discard all innards. Clams, mussels, lobsters, and shrimp yield a good broth when boiled or steamed. Stock can be used as a basis for fish soups or boiled down for a sauce or used as a poaching medium. Fish stock freezes well.
• Leftover cooked fish can be safely refrigerated and kept for two or three days. But refrigerate leftovers as soon as possible.
• If you have other questions as you deal with fish, call the FDA Hotline (888-SAFEFOOD) or visit their website.
Fish dangers: you can control them
• Raw fish and shellfish (such as oysters, clams, and mussels) may be contaminated by parasites, viruses, and bacteria. We advise against eating them raw. Cook them adequately, as described above. Pregnant or nursing women, children, the elderly, and those with compromised immune systems should never take a chance on raw fish or shellfish.
• Fish and shellfish, particularly those from local waters, may also contain pollutants such as PCBs, pesticides, and lead. Cooking won’t eliminate these substances. You should check with the health department in your area before you eat fish caught locally in streams and lakes. (These are unlikely to turn up in your fish market.) Pregnant or nursing women and children should avoid freshwater fish.
• Ocean fish are generally less polluted than freshwater fish, and all commercially marketed fish undergo inspection. But eat fish in reasonable amounts—no more than three times a week. Vary your choices. Don’t eat fish every day, and don’t eat the same kind of fish more than once a week. Pregnant or nursing women and children should avoid large game fish such as swordfish, which may contain heavy metals, and should limit fish consumption to once or twice a week.
UC Berkeley Wellness Letter, September 1999