Introduction
While fish is recommended by nutritionists as a source of low-fat, high-quality protein, it is, however, susceptible to disease-causing micro-organisms (pathogens). In this incisive article, Dr Daramola John explains the best preservative steps such as dehydration, refrigeration, irradiation, chilling and canning to take to keep them safe and valuable to the consumers.
With that, the consumers would benefit from fish as a veritable source of omega-3 fatty acids and vitamins such as D and B2 (riboflavin). Others include minerals such as calcium, phosphorus, iron, zinc, iodine, magnesium and potassium.
The perishable nature of fish
Fish is an extremely perishable food that begins to spoil as soon as it is caught perhaps, even before it is taken out of the water. Physical damage, dehydration and contamination are part of the causes of quality loss in fish. In tropical countries, spoilage of fish is rapid due to the high temperature and lack of required facilities to preserve like refrigeration, irradiation, chilling and canning amongst other factors. Meanwhile, the quality of fresh seafood depends on the condition of the fish after it has been caught, as well as factors such as hygiene and handling during pre and post-processing.
A contaminant is a substance, artificially or naturally produced, that is found in a place where it should not be or in an amount greater than it should be. On the other hand, microbial food spoilage is an area of global concern as it has been estimated that as much as 25% of all food produced is lost through microbial activities. Microbial spoilage could predispose consumers to health hazards resulting from food poisoning. Therefore, an improved science-based understanding of the growth and activity of spoilage micro-organisms in seafood and other foods is crucial for the development of preservation techniques and subsequent reduction of losses due to spoilage. It is also important to look into the area of pre-cooling and choice of packaging with respect to temperature control during transport, processing and storage.
Gram-positive organisms such as Bacillus and Micrococcus tend to be found on fish caught in tropical waters. Beside these, the flora will tend to be the same as that found on temperate water fish. There are several human pathogens in tropical waters. For example, Vibrioparahaemolyticus, Vibrio cholera, and Vibrio vulnificus, the potential presence of these microorganisms should be noted when handling fish from these waters. The microflora of farmed fish from tropical water does not differ significantly from that of wild fish. However, as farms tend to be near areas of human habitation, the water may be contaminated with higher-than-normal levels of human and animal wastes.
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This brings concerns of contamination with enteric organisms such as Salmonella and Escherichia coli. However, traditionally processed fish products (TFPs) are reported to carry a high potential risk for human health for halophilic pathogenic bacteria, histamine and parasites.
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. 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. Drying to moisture content below 15% prevents the growth of many spoilage microorganisms while mould growth is only suppressed at 10% moisture content.
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 PCR and sequence analysis. Acute viral gastroenteritis is an important and often unappreciated cause of morbidity and mortality worldwide.
Fish and food safety
The same rules about food safety and preparation apply to fish as with any other food product. Raw and cooked fish must not come into contact with each other due to the risk of cross-contamination. All of the pre-treatments ought to be carried out under conditions of good manufacturing practice, which means that the rudimentary steps of process hygiene should be implemented. Satisfactory control of contamination from operating surfaces and raw materials is achievable with regular cleaning and limiting the duration of exposure at temperatures suitable for the growth of spoilage microorganisms.
Parasites, contaminated water and careless handling of fish increases microbial contamination. Very low temperature or high-temperature techniques i.e canning and other methods of fish preservation, such as, drying which reduces the moisture content will reduce microbial growth or eliminate them. Fresh and processed fish in the markets have been reported to be either contaminated with bacteria or fungal toxins. Contamination occurs by an invasion of toxigenic strains ubiquitously found before and during harvesting or processing or by improper storage, thus the prevention of contamination is not an easy task.
Therefore, there is a high need for good hygiene, buying from a reputable seller, keep seafood cold and live shellfish alive, refrigerate shellfish properly, do not cross-contaminate coupled with proper cooking and at the correct temperature before consumption.
