FOOD SCIENCE AND TECHNOLOGY NOTES

TUNA

By James Daniels and Chieko E. Hebard

Illustrations by Linda Thompson

Occasionally, tuna are caught in the South Atlantic and landed in Virginia. Should you find yourself with one someday, this paper may help answer some of your questions and give you some ideas about what to do with it.

There are several species of fish canned and/or sold under the name tuna in this country. They are all related, belonging to the family, Scombridae, which also includes bonitos and mackerels. They travel in large schools and are among the fastest swimmers of all fish. The bodies are streamlined and smooth, offering the least resistance to water.

Background on Tuna

Canned tuna comprises the bulk of the fish consumed in this country, and the price and the grade of canned tuna is dependent upon the kind of tuna used. The most prized species is the albacore tuna, Thunnus alalunga, which has firm and very light-colored meat. Albacores are sometimes known as longfins because of their long, sabre-sharp pectoral fins. They normally weigh 10 to 60 pounds, and range from Southern California to mid-Mexican waters. Albacore is the only tuna that can be identified as "white" in the can.

The second most popular species is the yellowfin tuna, Thunnus albacares. Found from the Gulf of California to the waters off northern Chile, yellowfins are characterized by their yellowish dorsal and anal fins as well as the yellowish coloring on the sides. Slightly darker in color than albacore, the yellowfin tuna nevertheless is one of the most commercially valuable fish with its firm texture and fine taste. It is generally identified as "light" tuna.

As supplies of such choice species as albacore and yellowfin declined in recent years, commercial fishermen and canners have turned to skipjack tuna. The smallest of the four tunas, skipjack, Katsuwonus pelamis weigh from 4 to 24 pounds. Found in tropical and temperate waters all around the world, they are a popular sport fish on both the Pacific and Atlantic Coasts. Skipjack is characterized by dark stripes on the lower sides of the body, and has darker meat with stronger flavor than the above two species.

Bluefin tuna, Thunnus thynnus, is similar to skipjack in color and flavor. It can weigh up to 1,000 pounds, but those commercially harvested weigh from 15 to 80 pounds. Unlike other fishes, the bluefin tuna maintains a body temperature warmer than the ambient water temperature due to its high metabolic rate. Bluefins are usually found from Alaska to lower California. They also are present on the Atlantic Coast in smaller numbers. The chances are the tuna you catch in Virginia are bluefins, since they are by far the most common in this part of the world.

White meat is the designation given only to albacore meat and light tuna includes the rest of the tuna species. Except for the color characteristics, they can be used interchangeably.

Canned tuna is available in four different pack styles: solid-pack, chuck-style, flake, and grated, solid-pack being the most expensive and grated the least expensive. Sizes of the can vary from 3.25 - 3.5, 6.5 - 7, 9.25 to 12.5 - 13 ounces. Tuna is packed in oil, water, vegetable broth, and oil with vegetable broth. Lemon juice may be added for flavor.

Fresh Tuna

Fresh tuna is very tasty and not at all like tuna from a can. Fresh tuna can be preserved by freezing or canning.

Whether to be eaten right away or held for preservation, fresh tuna should be iced immediately to insure quality and freshness. The viscera should be removed at once, especially from fish caught while feeding, because the digestive juices in their iscera cause the flesh to become soft and flabby, thus hastening spoilage. The quality of tuna is at its peak only for the first 4 to 5 days, then declines until the fish is finally considered spoiled at about two to three weeks.

As with other types of fish, there are several indicators you can use to gauge freshness. The gills should be bright and blood red. The eyes should be clear and protruding slightly. The skin should be bright and shiny. The odor should be fresh and sweet rather than the ammoniacal smell of old fish. The flesh should be firm and elastic.

Freezing Tuna

Most tuna, particularly the bluefin tuna from the Atlantic, are too large to frozen whole. In most cases, the fillets are too large for a home freezer and too much for a single meal. Therefore, the fish should be cut into steaks or chunks prior to freezing. Convenient-sized pieces should then be rinses in cold water, drained, and dried with paper towel.

For the prolonged storage of fatty fish like tuna, it is important to keep the fish from contacting air, which causes darkening and rancidity. The fish therefore should be wrapped in moisture-vapor proof packaging such as plastic freezer wrap. To keep air out and to keep moisture from escaping, the fish may also be dipped in ascorbic acid dip (2 tablespoons ascorbic acid to 1 quart of water) for 20 seconds or glazed.

Freeze the fish as quickly as possible. This can be accomplished by turning the freezer to the coldest setting several hours ahead of time, then placing the wrapped fish in direct contact with the bottom or side of the freezer. Do not overload your freezer. If large quantities of fish are placed in the freezer at one time, the freezing time could be substantially extended.

Once the fish is thoroughly frozen (allow up to 24 hours for whole fish or large pieces), the thermostat can be returned to the normal setting and the fish moved to a more convenient location in the freezer. For long storage, the fish should be stored at 0F or lower. The lower the temperature, the longer the storage life of the frozen fish.

Canning Tuna

Tuna is a low acid food and, when improperly processed, there is a danger of botulism poisoning. Botulism poisoning, one of the most severe food poisonings known, is caused by a toxin produced by the bacterium, Clostridium botulinum. This organism often is found in mud samples taken from seafood producing waters.

Two things should be done to avoid botulism poisoning. First, keep hands, equipment, and the working area clean to minimize the possibility of contamination. Second, always follow the processing instruction carefully and use the pressure canner instead of a boiling water bath or the oven. This is because it takes higher-than-boiling temperatures to completely destroy or inactivate Clostridium botulinum, and the pressure canner along can attain sufficient heat.

Before you start, there are a few things you should check to insure the safe use of your pressure canner. First, make sure the pressure gauge on the canner is accurate. It should be checked for accuracy once year. Have the proper amount of water in the pressure canner. Process at either 10 or 15 pounds pressure. Begin counting the processing time after the correct pressure is reached. If you are doubtful as to whether your fish was properly processed, reprocess for the entire period; or refrigerate the fish and eat within 3 to 5 days; or freeze in tightly sealed containers. Re-read the directions on how to use the pressure canner and on canning fish.

There are different methods of canning tuna using either raw or precooked tuna. There are two reasons canned tuna sold commercially are precooked prior to canning. First, precooking removes much of the body oil of tuna; that oil causes the meat to be somewhat bitter. Second, precooking improves the appearance, flavor, and texture of the product. After precooking, tuna meat is packed in oil (cottonseed, soy, or vegetable), water, or oil and water.

Tuna may be precooked by pressure cooking at 240F at 10 pounds pressure for 2 hours. It may be steamed for 2 to 4 hours; or baked in a slow-to-moderate oven (250 ­ 350F) for 1 to 4 hours, depending on the size of the fish, until cooked through. If cooking in the oven, the meat thermometer inserted into the center of the fish should register at 165 to 175F when cooking is complete. Cool fish to room temperature, then store overnight, or 12 to 24 hours, in a cool room or refrigerator to firm the meat.

Before or during the packing of the precooked meat into jars, inspect for two possible defects. One is called "greening," characterized by grey or grey-green coloring of meat. It usually is accompanied by an unpleasant, urine-like odor. "Green" meat is not harmful but is unappealing and is best discarded. The other defect is called "honey-combing," which develops after the precooking step. It is characterized by pitted, spongy-looking meat, generally localized near the head of the fish. Since the defect is a result of fish decomposition, any tuna exhibiting honeycombing should be discarded. Off-odors usually accompany the honeycombing.

Before canning, the fish should be skinned and cut into chunks small enough to fit jars. Half-pint and pint jars with wide mouths are recommended. Pack fish pieces into jars, leaving 0.5 to 1-inch space at top. Add 0.5 to 1 teaspoon salt to half-pint jars. Add 4 to 6 tablespoons of hot oil, boiling water, or equal amounts of oil and water to each pint; 2 to 4 tablespoons to each 0.5 pint.

Seal jars and process at 15 pounds pressure for 80 minutes. If using 10 pounds of pressure, process half-pint containers for 90 minutes and pint jars for 100 minutes. Allow jars to cool and store in a cool, dark place.

Tuna may be canned raw without precooking. The dry pack method calls for cut pieces of raw tuna to be packed into half-pint jars to within 0.25 inch of the rim. One-half teaspoon of salt is added to the jars which are then processed at 10 pounds pressure for 1 hour and 50 minutes (80 minutes at 15 pounds pressure).

The brine pack method, similar to the dry pack method, calls for soaking cut pieces of tuna for 1 hour in a brine made of 0.75 cup salt mixed with one gallon of water. The fish should be completely submerged in the brine. After soaking, the fish is drained for several minutes and packed into jars to within 0.25 inch of the rim. No water or salt is added. Half-pint jars are then processed at 10 pounds pressure for 1 hour and 50 minutes (80 minutes at 15 pounds pressure). One gallon of brine is enough for about 25 pounds of cleaned fish. Only use the brine once.

Scombroid Poisoning

Whether the tuna is bought in a store or canned at home, one danger you should be aware of is scombroid poison. This substance is produced in scombroid fishes when naturally occurring spoilage bacteria in the meat convert the amino acid histidine in the fish protein into a biologically active amine poison, saurine. The saurine is produced when fish are left in the sun or at room temperature for several hours.

The incidence of scombroid poisoning is rare and generally not fatal, but the poisoning resembles a severe allergy with such symptoms as headaches, dizziness, nausea, and vomiting. Tuna containing the toxin in such high quantities as to bring on illness is said to taste "sharp and peppery." The toxin is relatively heat-stable and not completely destroyed or inactivated during the canning process. In order to avoid scombroid poisoning, fish should be processed immediately after capture, and should be discarded if left in the sun for more than two hours.

Mercury in Tuna

In the past, some large tuna had higher levels of mercury than that allowed by the government, and consequently were rejected for consumption. However, in recent years, it has been demonstrated that the limit set by the government was too low, and the government has since increased it. Most tuna, except, perhaps, very large ones exceeding 250 pounds, fall into the acceptable range and can be consumed without the danger of mercury contamination.

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