By Prof. John Abiodun Daramola
Fish are processed in many different ways in different parts of the world. Heavy salting, freezing, drying, hot smoking, canning and pasteurisation are all recognised methods of fish preservation. All affect the microorganisms on the fish in different ways and will result in a different type of microflora and different risks from spoilage organisms and pathogens.
However, traditionally processed fish products (TFPs) are reported to carry high potential risk for human health for halophilic pathogenic bacteria, histamine and parasites (Köse, 2010).
The key to any preservation system is for the producer to understand how the process works and what needs to be controlled to get a safe stable product. For example, a correctly processed canned fish will be commercially sterile and can be stored at ambient temperatures for long periods without spoilage or risk to consumer safety. Once the can is opened, the contents become open to contamination and as there is usually no preservative within the product and the contaminating microorganisms will be able to grow.
Therefore, canned fish products should be consumed shortly after opening. In dried or frozen fish, the microflora is prevented from growing by the storage conditions used and the product may have a long shelf life in the preserved state.
Bacterial contaminants of fish and seafood
The most common cases of food poisoning which involve fish and seafood usually occur as a result of eating contaminated prawns or other shellfish. Oysters is a good example of seafood which some people enjoy consuming whilst raw, but these often contain bacteria which are likely to cause food poisoning and other similar conditions. These shellfish filter seaweed and algae from the surrounding water but bacteria which live in this water get to enter the oyster during the filtration process. Consequently, these bacteria then take root inside the fish. Usually, it is easy to think of food poisoning as an issue associated with contaminated chicken, beef or pork but unfortunately, fish is included. For example, Escherichia coli is a bacterium that is present in nearly all types of shellfish as a result of contact with water contaminated by raw sewage.
Five bacterial species namely: Staphylococcus aureus, klebsiella sp. Salmonella sp., Escherichia coli and Pseudomonas sp. are the major bacterial pathogens associated with post-harvest fish spoilage. According to the findings by Gram and Huss (2001), who reported that these microorganisms were the major causes of microbial spoilage of fresh fish after capture and the microbial count on different media suggests contamination. The total bacterial count on fish rarely indicate the quality of the fish but it gives an indication of the risk of spoilage induced since each of these microorganisms had different ways of affecting health conditions of consumers of such contaminated fish (Gram et al., 2000).
Conversely, Miller et al., (1973) observed that not all bacteria present on fresh fish are spoilers but there are certain active spoilers which are the major pathogens associated with fish spoilage.
The presence of Klebsiella and Salmonella spp. in the fresh fish samples is an indication that the water used for processing was faecally contaminated. The presence of Staphylococcus aureus, a normal flora of skin and mucous membrane of humans can be attributed to human contact during handling and processing (Dalgaard et al., 2006).
Staphylococcus aureus produces a variety of extra cellular enzymes and toxins that have been found to be responsible for food poisoning and can rapidly develop resistance to many antimicrobial agents and pose health risk with therapeutic problems (Thrower, 2000 and Abolagba et al, 2011). However, Clostridium botulinum, the bacterium causes botulism, is considered as the most harmful of these bacteria (Long, 2009).
Fungal contaminants of fish
Dried fish has a storage life of several years and thus gives fungi a greater opportunity to contaminate it. Fungi are omnipresent in the environment, being found wherever water, suitable organic nutrients and an appropriate temperature occur. They secrete enzymes outside their body structure and absorb the digested foods (Prescott et al., 1999). The growth of filamentous fungi in foods and food products results in waste and is costly as well as sometimes hazardous.
Mycotoxins are secondary metabolites produced by moulds that are capable of causing disease and death in humans and animals (Bennett and Klich, 2003). Drying to moisture content below 15% prevents the growth of many spoilage microorganisms while mould growth is only suppressed at 10% moisture content (Buere, 2005).
Fafioye et al. (2002) studied the fungal infestation of five traditionally smoked dried freshwater fish in Ago-Iwoye, Nigeria and isolated and identified eleven different fungal species of which Aspergillus flavus was the most frequently encountered fungi on the fish species. Also, in a study of mycoflora of smoke-dried fishes sold in Uyo, Eastern Nigeria by Adebayo-Tayo et al. (2008), twelve different fungi specie were found to be associated with the smoke-dried fish samples. The associated fungi were Aspergillus flavus, Aspergillus tereus, Aspergillus fumigatus, Absidia sp, Rhizopus sp, Aspergillus niger, Mucor sp, Cladosporium Sp, Penicillin italiculum, P. viridatus, Candida tropicalis and Fusarium moniliformis. Similarly, Junaid et al (2010) in a study aimed at isolating and identifying the fungi associated with stockfish, showed that seven different fungi species were found.
However, moulds may be present without producing any toxin (Bennett and Klich, 2003), but the presence of toxigenic fungi increases the risk for mycotoxin production (Jacobsen et al., 2008). The reason being that even though the fungus is no longer alive, while it was growing, it can produce mycotoxin which can poison the food (Wong, 2007). Mycotoxins greatly resist decomposition or being broken down in digestion so they remain in the food chain and even temperature treatments such as cooking and freezing, therefore, do not destroy the mycotoxins.
Viral contaminants of fish
Numerous viruses of human or animal origin are found in the environment and infect people via water and food: bivalve molluscs, vegetables and prepared foods are classified by the World Health Organization as priority hazards. Hepatitis A virus (HAV), genogroup GI, GII, and GIV norovirus (NoV), enterovirus (EV), rotavirus (RoV), hepatitis E virus (HEV), adenovirus (AdV), and bocavirus (BoV) have been detected in food (especially shellfish), water samples and surface swabs by nested (RT) PCR, real-time PC.
Prof. Daramola is the HOD Department of Agriculture and Agricultural Technology, Bells University of Technology, Ota,Ogun state