Food Source Information Canned Tuna

Canned tuna is an affordable and popular source of protein around the world. There are five species of tuna that make up most of the commercial market: skipjack (Katsuwonus pelamis), albacore (Thunnus alalonga), yellowfin (Thunnus albacares), bigeye (Thunnus obesus), and bluefin (Thunnus thynuss). All tuna belong to the Thunnini tribe, a classification within the Scombridae, or mackerel, family. Of these five species, skipjack and albacore make up the majority of canned tuna, while bigeye, bluefin, and yellowfin are primarily used in sushi and sashimi. Species vary in size and lifespan from the 40-pound skipjack that lives up to ten years, to the 1500-pound bluefin with an average lifespan of 40 years. Tuna are both predatory and migratory and are found in most of the large oceans around the globe. The biggest threat posed to tuna species is overfishing.

Canned tuna is usually classified into two categories: light meat tuna, which is mostly skipjack and sometimes yellowfin, and white meat tuna, which is usually albacore. The three major brands of canned tuna sold in the United States are Starkist, Chicken of the Sea, and Bumble Bee. These brands make up over 80 percent of canned tuna consumption in the U.S. (though several smaller labels are available commercially). Most tuna fishing occurs in the Pacific Ocean. Other fishing sources, in order of fishing yield, are the Indian Ocean, the Atlantic Ocean, and the Mediterranean Sea. The countries that produce the most in tuna fishing yield are Indonesia, Japan, and the Philippines. Thailand is now the site of the majority of the world’s tuna processing plants, processing approximately 25% of the world’s canned tuna. The majority of Thailand’s canned tuna is exported to the United States and the European Union.

Between 2000 and 2020, at least 1 canned tuna-associated outbreak was reported to CDC’s National Outbreak Reporting System (NORS), causing 3 illnesses, and no hospitalizations or deaths. Two of the few recorded foodborne illness outbreak associated with canned tuna in the United States involved two cases of botulism in California in April 2009. Both cases were hospitalized after consuming home-canned tuna, but neither died. Botulism is a potentially fatal illness caused by a neurotoxin produced by the bacteria Clostridium botulinum. The toxin is found naturally in soil and the intestines of fish, animals, insects, and birds. Humans are usually exposed to the toxin via ingestion of foods that have been stored in anaerobic environments that allow the Clostridium botulinum bacteria to grow, typically in canned foods that have undergone improper sterilization during the canning process. Most fresh produce contains the bacteria on the exterior, but Clostridium botulinum bacteria only grow in anaerobic conditions, so these foods are safe to consume.

In early 2019, fifteen cases of infection due to the strain Salmonella Newport were reported across eight states. Traceback evidence linked these cases to frozen raw ground tuna supplied by Jensen Tuna Inc. that were processed in Houma, Louisiana and imported from JK Fish in Vietnam. Of the fifteen people with reported illness, ages ranged from 24 to 85 years wherein two individuals were hospitalized, and no deaths were reported. A survey of food intake history found that 75% of the cases reported eating sushi from a restaurant before in the week before becoming ill. Through these interviews, frozen ground tuna from Jensen Tuna was identified as the likely source of the outbreak, which subsequently led to a recall by Jensen Tuna on all frozen ground tuna.

Yellowfin tuna was at the center of a voluntary recall in November 2019 due to elevated histamine levels. Northern Fisheries of Compton, Rhode Island initiated a voluntary recall of their frozen, wild-caught yellowfin Tuna Medallions that were produced in June 2019. Histamine is produced in the skin, gills, and gut of fish when bacteria breakdown the amino acid histidine; incorrect storage of fish at inappropriate temperatures is directly related to excessive production of histamine after a fish is caught. Elevated histamine levels may lead to histamine fish poisoning, also known as scombroid fish poisoning, which resembles symptoms of an allergic reaction including facial swelling, tingling sensations in the mouth, rash, hives, and nausea.

I. Overview

Although tuna are found around the globe, most tuna fishing occurs in the Pacific Ocean, particularly in the west Pacific. The United States once had multiple tuna canning sites primarily located along the West coast, but most were shut down due to environmental impact concerns and to the lower cost of processing in other countries. In the eastern Pacific, dolphins frequently travel with tuna and can be unintentionally caught during fishing. This is one reason tuna fishing has shifted to the west Pacific, where dolphin safety is less problematic. The majority of processing plants for many large tuna companies in the United States are located internationally; the majority of canned tuna is processed in Thailand and across Southeast Asia. In 2017, approximately $653 million of tuna was imported for consumption, whereas the United States exported around $47 million of tuna.

II. The Catch

Most skipjack tuna are caught by purse seine fishing, which involves lowering a cylindrical net around a school of fish, closing the bottom, and lifting the net out of the water. This method of fishing reduces levels of bycatch while being efficient technique to catch large schools of fish.  Larger types of tuna are caught via other methods, like longlining, which uses a mainline equipped with many hooks and bait that can extend for kilometers in deep waters, or trolling, which entails trailing a short-baited line behind the boat. Fishing vessels store the fish on ice and transport the catch to shore. Fisheries and processing plants are usually near the docks for convenient unloading.

III. Fish Cleaning and Preparation

After being unloaded from fishing vessels and kept in cold storage, the tuna fish are thawed in tanks of water. The fish are then sorted by size on large racks to be pre-cooked, which strips away any unwanted oils. Sorting the fish by size before pre-cooking minimizes the weight loss that results from overcooking. This first cooking process can last from 45 minutes to three hours. After pre-cooking and cooling, the fish are de-boned, the skin is removed, and the meat is separated into light meat and red meat. Lighter meat is designated for the canning process while red meat is typically used in pet food. The parts of the fish for human consumption, called loins, are then cleaned before entering the canning phase.

IV. Canning

Following the cleaning and preparation phase, the tuna meat is packed in cans along with broth, oil, or water, and sometimes seasoning, such as salt. The cans receive lids and are vacuum sealed to prevent leakage and contamination. The cans are then washed before being cooked and sterilized at high temperatures that must reach over 240 degrees to ensure the absence of live bacteria inside the sealed containers. Of particular concern are spore-forming anaerobic bacteria, such as Clostridium botulinum. The cans are cooled, labeled, and checked for quality assurance. This process involves assessing damage to the cans, including dents, swelling, and malfunctioning seals, which could indicate potential contamination. Cans are then shipped to warehouses and sent out to stores around the United States where they can remain on shelves for 2–5 years.

I. Storage and Preparation

The most important step in the canning process is heat sterilization. This ensures that the contents of each can do not contain live bacteria and that each can is sealed to prevent bacteria from entering or growing inside the can at any point during storage. Because the low acidity of tuna does not successfully kill bacteria, it requires sterilization at much higher temperatures when canning. When home canning, only a pressure canner is sufficient in the sterilization step to kill the bacteria in food that produce botulinum toxin. Boiling-water canners are not capable of reaching temperatures of 240–250 degrees Fahrenheit necessary for successful sterilization. Food safety in home pressure canning can only be ensured if the correct guidelines are followed for elevation, proper venting, and sustained temperature range.

Unopened cans of tuna are virtually safe from foodborne illness forever. As such, “Sell-by” dates, “Best if used by” dates, “Use by” dates, and “Closed or coded” dates are in reference to food quality, not food safety. The recommended shelf life for canned tuna is 2–5 years, as long as the can is in good condition. Damaged or swollen cans of tuna could indicate that the contents are no longer safe to consume. Cans of tuna should also never be frozen or stored in environments exceeding 90 degrees Fahrenheit, as this could compromise the seals. Cans should also be washed before opening to prevent bacteria on the exterior of the can from contaminating the food. Canned tuna that has been opened should not be eaten if left out at room temperature for over two hours (or just one hour if the ambient temperature is over 90 degrees Fahrenheit). Opened tuna is only safe in refrigeration for 3–5 days.

ii. Mercury and Recommended Servings

Methylmercury is an organic compound that can build up in an organism over time through ingestion or absorption. Most bodies of water contain mercury due to atmospheric deposition from industrial pollution, such as burning of coal or mining, and as a result of natural processes such as volcanic eruptions and forest fires. Mercury reacts with organic matter in the water and creates the compound methylmercury. Bodies of water that contain the most mercury are typically found near wetlands, forests, or mining sites.

All fish contain mercury, but the levels in each fish vary with species, size, and age. Older and larger fish tend to contain more mercury. Older fish have had more years to internally accumulate a significant amount of mercury due to their environments; this is referred to as bioaccumulation. Because larger fish often eat smaller fish, mercury levels increase in fish that are higher up the food chain, a process known as biomagnification. Of the two species of tuna used in canning (albacore and skipjack), albacore contains more mercury because the fish are larger in size; therefore, white meat canned tuna has a higher mercury content than light meat canned tuna. However, both albacore and skipjack tuna are considered to be low mercury seafood compared with other predatory fish, such as shark and king mackerel. The species that are typically used in sushi and sashimi (bigeye, bluefin, and yellowfin) have higher levels of mercury due to their large size.

As with predatory fish, mercury can build up in humans who regularly consume fish with high levels of mercury. Methylmercury exposure in humans can impact a variety of functions including cognition, memory, fine motor skills, and visual skills. Pregnant women are cautioned to heed recommendations for canned tuna consumption, as the effects of mercury can more severely impact birth outcomes and young children; exposure to methylmercury during critical fetal development periods can result in poor neurological development and hearing problems. However, canned tuna is also a source of some of the most important nutrients during pregnancy, including protein and omega-3 fatty acids. To balance these concerns, the amount of white meat canned tuna consumed should not exceed 1 serving per week (about one average meal), and for light meat canned tuna the recommended amount is 2-3 servings per week.

Global canned tuna consumption is increasing. It is a reasonably priced source of protein that is convenient to store and requires no refrigeration. The world’s top consumers of canned tuna are the European Union, the United States, and Japan. Its popularity is spreading now to many Latin American and Middle Eastern countries, while consumption is plateauing or decreasing in the United States and Europe, in part due to increased demand for fresh tuna. Canned tuna is one of the most commonly consumed seafood products in the United States, with the average person eating around 14 pounds per year. The type of canned tuna most frequently eaten in the United States is chunky light meat in water, which makes up about 75–80% of U.S. canned tuna consumption per year. One in four Americans eat canned tuna at least once per week, and studies have indicated that large households consume more canned tuna than average-sized households. Canned tuna is more popular in the summer than at any other time of year. Over half of the tuna eaten in the U.S. is in sandwich form, about 22% in salads, and 15.5% in casseroles.

Canned tuna is a good source of protein, omega-3 fatty acids, vitamin D, and selenium. Protein is a necessary building block of all tissues in the body. Omega-3 fatty acids are “essential” because the body does not produce them, meaning they must be consumed through food. These polyunsaturated fatty acids are important for healthy brain development and function, reducing inflammation, and cardiovascular function. Vitamin D is both transported and stored in fat tissues. Vitamin D has various roles throughout the body, which includes helping maintain levels of phosphorus and calcium, which contributes to maintaining bone health and prevent osteomalacia, a weakening of skeletal and bone structures. Selenium is an element present in very small amounts that plays an important role in thyroid and hormonal function, as well as prevention of harmful oxidative processes in the body.

The fear of mercury poisoning can prevent many people from purchasing canned tuna, but research indicates that the health benefits of eating the recommended amounts of canned tuna per week greatly outweigh any risks associated with the low mercury levels in most canned tuna.

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