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In this article we will discuss about:- 1. Meaning of Fish 2. Structure and Composition of Fish 3. The Microbiology of Primary Processing of Fish 4. Crustaceans and Molluscs 5. Spoilage of Fresh Fish.
Contents:
- Meaning of Fish
- Structure and Composition of Fish
- The Microbiology of Primary Processing of Fish
- Crustaceans and Molluscs
- Spoilage of Fresh Fish
1. Meaning of Fish:
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Here we are mainly concerned with what most people think of as fish; principally the free swimming teleosts and elasmobranchs.
The same term can also encompass all sea-foods including crustaceans with a chitinous exoskeleton such as lobsters, crabs and shrimp and molluscs such as mussels, cockles, clams and oysters. Microbiologically these have many common features with free swimming fish but some specific aspects.
Historically the extreme perishability of fish has restricted its consumption in a reasonably fresh state to the immediate vicinity of where the catch was landed.
This has detracted only slightly from it playing a significant role in human nutrition as, throughout the world, traditional curing techniques based on combinations of salting, drying and smoking were developed which allowed more widespread fish consumption. Poor keeping quality is a special feature of fish which sets it apart even from meat and milk.
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2. Structure and Composition of Fish:
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Although broadly similar in composition and structure to meat, fish has a number of distinctive features. Unlike meat, there are no visually obvious deposits of fat. Although the lipid content offish can be up to 25%, it is largely interspersed between the muscle fibres.
A further feature which contributes to the good eating quality offish is the very low content of connective tissue, approximately 3% of total weight compared with around 15% in meat. This, and the lower proportion of body mass contributed by the skeleton, reflect the greater buoyancy in water compared with air.
Muscle structure also differs. In land animals it is composed of very long fibres while in fish they form relatively short segments known as myotomes separated by sheets of connective tissue known as myocommata. This gives fish flesh its characteristically flaky texture.
Fish flesh generally contains about 15-20% protein and less than 1% carbohydrate. In non-fatty fish such as the teleosts cod, haddock and whiting, fat levels are only about 0.5%, while in fatty fish such as mackerel and herring, levels can vary between 3 and 25% depending on factors such as the season and maturity.
3. The Microbiology of Primary Processing of Fish:
As with meat, the muscle and internal organs of healthy, freshly caught fish are usually sterile but the skin, gills and alimentary tract all carry substantial numbers of bacteria. Reported numbers on the skin have ranged from 102– 107 cfu cm-2, and from 103-109 cfu g-1 in the gills and the gut.
These are mainly Gram-negatives of the genera Pseudomonas , Shewanella, Psychrobacter, Vibrio, Flavobacterium and Cytophaga and some Gram-positives such as coryneforms and micrococci. Since fish are cold blooded, the temperature characteristics of the associated flora will reflect the water temperatures in which the fish live.
The microflora offish from northern temperate waters where the temperatures usually range between — 2 and +12 °C is predominantly psychrotrophic or psychrophilic. Most are psychrotrophs with an optimum growth temperature around 18 °C. Far fewer psychrotrophs are associated with fish from warmer tropical waters and this is why most tropical fish keep far longer in ice than temperate fish.
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Bacteria associated with marine fish should be tolerant of the salt levels found in sea water. Though many do grow best at salt levels of 2-3%, the most important organisms are those that are not strictly halophilic but euryhaline, i.e. they can grow over a range of salt concentrations.
It is these that will survive and continue to grow as the salt levels associated with the fish decline, for example when the surface is washed by melting ice.
After capture at sea, fish are invariably stored in ice or refrigerated sea water until landfall is made. It is important that fresh, clean cooling agent is used as re-use will lead to a rapid build up of psychrotrophic contaminants and accelerated spoilage of the stored fish.
Gutting the fish prior to chilling at sea is not a universal practice, particularly with small fish and where the time between harvest and landing is short. It does however remove a major reservoir of microbial contamination at the price of exposing freshly cut surfaces which will be liable to rapid spoilage.
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Similarly any damage to the fish from nets, hooks, etc. that breaches the fish’s protective skin will provide a focus for spoilage. Subsequent processing operations such as filleting and mincing which increase the surface area to volume of the product also increase the rate of spoilage.
Fish can be further contaminated by handling on board, at the dock and at markets after landing, particularly where they are exposed for sale and are subject to contamination with human pathogens by birds and flies. Generally though, fish have a far better safety record than mammalian meat. A number of types of foodborne illness are associated with fish (Table 5.6).
4. Crustaceans and Molluscs:
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The propensity of crustaceans to spoil rapidly can be controlled in the case of crabs and lobsters by keeping them alive until immediately before cooking or freezing. This is not possible with shrimp or prawns, which are of far greater overall economic importance but die soon after capture.
In addition to their endogenous microflora, shrimp are often contaminated with bacteria from the mud trawled up with them and are therefore subject to rapid microbiological deterioration following capture. Consequently they must be processed either by cooking or by freezing immediately on landing.
Some aspects of the production and processing of frozen cooked peeled prawns can pose public health risks. Increasingly prawns are grown commercially in farms where contamination of the ponds, and thence the product, with pathogenic bacteria can occur via bird droppings and fish feed.
After cooking, which should be sufficient to eliminate vegetative bacterial contaminants derived from the ponds, the edible tail meat is separated from the chitinous exoskeleton.
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Peeling machines are used in some operations but large quantities are still peeled by hand, particularly in countries where labour is cheap. The handling involved gives an opportunity for the product to be contaminated with human pathogens after the bactericidal cooking step and prior to freezing.
The flesh of molluscs such as cockles, mussels, oysters and clams differs from that of crustaceans and free swimming fish by containing appreciable (« 3%) carbohydrate in the form of glycogen. Though many of the same organisms are involved, spoilage is therefore glycolytic rather than proteolytic, leading to a pH decrease from around 6.5 to below 5.8.
Molluscs are usually transported live to the point of sale or processing where the flesh can often be removed by hand.
Although contamination may occur at this stage, the significant public health problems associated with shellfish arise more from their ability to concentrate viruses and bacteria from surrounding waters, the frequent pollution of these waters with sewage and the practice of consuming many shellfish raw or after relatively mild cooking.
5. Spoilage of Fresh Fish:
A number of factors contribute to the unique perishability of fish flesh. In the case of fatty fish, spoilage can be non-microbiological; fish lipids contain a high proportion of polyunsaturated fatty acids which are more reactive chemically than the largely saturated fats that occur in mammalian meat. This makes fish far more susceptible to the development of oxidative rancidity.
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In most cases though, spoilage is microbiological in origin. Fish flesh naturally contains very low levels of carbohydrate and these are further depleted during the death struggle of the fish. This has two important consequences for spoilage. Firstly it limits the degree of post mortem acidification of the tissues so that the ultimate pH of the muscle is 6.2-6.5 compared with around 5.5 in mammalian muscle.
Fish which have a lower pH such as halibut (approx. 5.6) tend to have better keeping qualities. Secondly, the absence of carbohydrate means that bacteria present on the fish will immediately resort to using the soluble pool of readily assimilated nitrogenous materials, producing off-odours and flavours far sooner.
The composition of the non-protein nitrogen fraction differs significantly from that in meat (Table 5.7). Trim-ethylamine oxide (TMAO) occurs in appreciable quantities in marine fish as part of the osmoregulatory system. TMAO is used as a terminal electron acceptor by non-fermentative bacteria such as Shewanella putrefaciens and this allows them to grow under microaerophilic and anaerobic conditions.
The product of this reduction is trim-ethylamine which is an important component in the characteristic odour offish (Figure 5.6.) TMAO also contributes to a relatively high redox potential in the flesh since the Eh of the TMAO/TMA couple is +19 mV.
Elasmobranches such as dogfish and shark contain high levels of urea. Bacterial urease activity in the flesh can produce ammonia very rapidly giving the product a pungent odour. Not only does this render the flesh itself uneatable but it can also taint the flesh of other fish stored nearby. It is for this reason that in many areas fishermen will discard all but the fins of shark when they catch them.
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Shellfish such as lobster have a particularly large pool of nitrogenous extractives and are even more prone to rapid spoilage; a factor which accounts for the common practice of keeping them alive until immediately prior to consumption.
Fish proteins are less stable than mammalian protein. As with meat, extensive proteolysis does not become apparent until the product is already well spoiled, but limited protein degradation may improve bacterial access to the nutrient pool of extractives.
The speed with which a product spoils is also related to the initial microbial load on the product: the higher the count the sooner spoilage occurs. Since fish from cold waters will have a larger proportion of psychrotrophs among their natural microflora, this can shorten the chill shelf-life appreciably.
Spoilage of chilled fish is due principally to the activity of psychrotrophic Gram-negative rods also encountered in meat spoilage, particularly Shewanella putrefaciens and Pseudomonas spp. The uniquely objectionable smell of decomposing fish is the result of a cocktail of chemicals, many of which also occur in spoiling meat.
Sulfurous notes are provided by hydrogen sulfide, methyl mercaptan and dimethyl sulfide and esters contribute the ‘fruity’ component of the odour. A number of other amines in addition to TMA are produced by bacterial catabolism of amino acids.
Skatole, a particularly unpleasant example produced by the degradation of tryptophan, also contributes to the smell of human faeces. The level of volatile bases in fish flesh has provided an index of spoilage, although this and other chemical indices used are often poor substitutes for the trained nose and eyes.
Figure 5.7 illustrates some of the different products made from fish, most of which are discussed elsewhere in terms of the general processing technologies used. One interesting aspect that relates to some of the discussion above will be discussed here.
The combination of a near neutral pH and availability of TMAO as an alternative electron acceptor means that vacuum and modified-atmosphere packing of fish does not produce the same dramatic extension of keeping quality seen with meat.
Typically the shelf-life extension of vacuum and modified-atmosphere-packed cod will vary from less than 3 days to about 2 weeks. Shewanella putrefaciens can grow under these conditions producing TMA and hydrogen sulfide to spoil the product.
Work in Denmark has also demonstrated that CO2 tolerant marine vibrios like Photo-bacterium phosphoreum may be responsible for a non-sulfurous spoilage of these products in some instances.
