Fish Safety Considerations

After the fish has died, the enzymes which are present in the gut, digest the walls of the intestinal tract and start spoiling the flesh. Most of the bacteria is present in the slime that covers the body of the fish, the gills and in its digestive tract. The skin produces this slime to protect the fish from the outside bacteria and to decrease the resistance of water when swimming. When the fish dies, its skin releases more slime which becomes an ideal substrate for bacteria to multiply. They start penetrating the skin and the spoiling process begins.

Fish can be the source of the following diseases: Salmonella, Shigella, Staphylococcus aureus (known as “stuff” infection), Clostridium perfringens and Clostridium botulinum. The last one is hardest to eliminate, and is commonly known as “food poisoning.” Clostridium botulinum is not associated with fish only, but with all low acid foods such as meats, fish, poultry and vegetables. Canning industry safety procedures are based on elimination of Clostridium botulinum spores using reasoning that “if the procedure can kill Clostridium botulinum spores, it will kill all other microorganisms as well.”

Fish like any other meat is susceptible to food poisoning given the right conditions for the development of C. Botulinum spores into toxins. Those conditions (lack of oxygen, humidity, temperatures 40-140° F (4-60° C) always exist when smoking meats. Furthermore many times fish will be packed by the Reduced Oxygen Packaging Method that can create favorable conditions for C.botulinum to become toxin even after fish was hot smoked and cooked. To eliminate the possibility of such a danger Cure # 1 is added the same way it is used when smoking meats or sausages. The Food and Drug Administration currently allows nitrites to be used in salmon, sablefish, shad, chubs, and tuna.

Adapted from Torry Notes #22, Torry Research Station, UK:

Botulism in Fishery Products

What is botulism?

Botulism is an often fatal food poisoning disease caused by one of the most powerful toxins known to man. A toxin is a poison produced by certain bacteria when they grow on food; the kind of bacteria that forms the toxin causing botulism is called Clostridium botulinum. When food containing the toxin is eaten, the nervous system is affected and death can follow within hours.

Clostridium Botulinum in Fish

There are seven known types of Clostridium botulinum, referred to as types A-G; of these, A, B, E and F consistently produce botulism in humans, and B, E and F are frequently found in the sea. Clostridium botulinum grows only in the absence of oxygen, and type E, and some varieties of B and F, have two important properties. First, they are found in fish intestines and gills and in mud from the sea, whereas the other types are found mostly in soil. Secondly, they grow and form toxin at a much lower temperature than the other types; they can grow at 5°C in fish products. Fortunately the toxin is readily destroyed by cooking since it does not survive exposure to 158° F (70°C) for 2 minutes.

Over the years public taste has changed and with it the methods of curing. At one time the product was so heavily salted, smoked and dried that Clostridium botulinum, where present in fish, could not grow and produce toxin. Such a heavily cured product needed to be neither refrigerated nor vacuum packed during distribution. Nowadays hot-smoked fish are distributed with much less smoke and salt, and much more moisture, with the result that any Clostridium botulinum present can more readily form toxin when the fish are kept warm for some length of time.

Smoked Fsh and Botulism

Salmon flesh is either dry salted or brined before being cold smoked, the time of salting varying with the size of salmon being cured. The presence of salt in the product has a great effect on the growth of Clostridium botulinum, but the concentration of salt in smoked salmon is not usually high enough to prevent growth altogether; commercial smoked salmon usually contains 1 to 4 per cent salt. The concentration required to prevent growth at room temperature can vary from as low as 3 per cent to 5 per cent or more, so that the amount of salt present in smoked salmon is on its own no guarantee against the danger of botulism. Trout and mackerel are brined and then hot smoked, either as gutted whole fish or as fillets. The range of salt concentration is similar to that found in salmon.

How to Control Botulism

Before the toxin of Clostridium botulinum can develop in a fishery product a number of factors must coincide: the organism must be present in the fish, the time and temperature of storage must be favorable for toxin production, and the chemical composition of the product must be such that it supports the growth of the organism. Elimination of any one of these factors will make the product safe.

In practice it is not possible to rid a contaminated fish of botulinum organisms but removal of the guts and gills, followed by thorough washing of the belly cavity with tap water, can reduce contamination by as much as 90 per cent.

Summary

The bacteria can grow at a temperature as low as 38° F (3.3°C) but its toxin is destroyed by heating it at 158° F (70° C); any product that is properly cooked and eaten the same day is therefore safe; the main danger lies in products that either can be eaten raw, such as smoked salmon, or need not be cooked by the consumer, for example hot-smoked fish. Changes in traditional method of processing can introduce new hazards; for example, reduction in the amount of salt or smoke can allow the bacteria to flourish, or the use of vacuum packs can extend the shelf life of the product sufficiently to allow the product to become toxic during storage. The safety of any product is assured if the fish are stored at a chill temperature below 40° F (4°C) from the time they are caught until they are eaten.

Parasites

Wild fish are infected with a large variety of parasites which can infect a person that likes to eat raw or lightly preserved fish such as sashimi, sushi, ceviche, and gravlax (salted salmon). Parasites can attach themselves to fish body, penetrate its flesh or settle down in the gills or the liver. They are not a health concern in thoroughly cooked fish. Raw fish should be frozen to an internal temperature of -20°C (-4°F) for at least 7 days to kill parasites. It is important to be aware that most home freezers are not cold enough to kill parasites.

Cooking Fish

The fish is considered safe to consume when submitted to 145º F (63º C) inside temperature for 30 minutes. The U.S. Food and Drug Administration “Good Manufacturing Practice” for “hot smoked fish” recommends that commercially prepared smoked fish be subjected to one of the following:

  • Center temperature must reach at least 180º F (82º C) for 30 minutes if water phase salt (WPS) content is a minimum of 3.5% OR
  • Center temperature must reach at least 105º F (41º C) for 30 minutes if water phase salt content is a minimum of 5%. Without chemical analysis it is hard to tell what is the WPS content of the fish, however, most 1 to 2 inch thick pieces of fish will reach these conditions if salted 1-2 hours in 60º SAL brine (15.8% salt by weight) and smoked for 4-5 hours at 180-200º F (82-94º C), followed by 4-8 hours of smoking without heat.

Guidance for Processing Seafood in Retail Operations states that:

  • Cold Smoked Seafood – seafood that has been produced by subjecting the product to mild heat and smoke to achieve a partial coagulation of proteins.
  • Oven/smoker temperature not to exceed 90º F (32.2º C), for a drying and smoking period that does not exceed 20 hours; OR
  • Oven/smoker temperature not to exceed 50º F, (10º C), for a drying and smoking period not to exceed 24 hours.
  • Hot Smoked Seafood – smoked seafood that has been produced by subjecting the product to heat during the smoking process to coagulate the proteins throughout the seafood.
  • Product internal temperature must be maintained at a continuous temperature of at least 145º F (62.8º C), for a minimum of 30 minutes.

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