6.5 The effect of fish species, fishing ground and season

Contents - Previous - Next

Influence of handling, sue, pH, skin properties

The spoilage rate and shelf life of fish is affected by many parameters and, as stated in section 5, fish spoil at different rates. In general it can be stated that larger fish spoil more slowly than small fish, flat fish keep better than round fish, lean fish keep longer than fatty fish under aerobic storage and bony fish are edible longer than cartilaginous fish (Table 6.6). Several factors probably contribute to these differences and whereas some are clear, many are still on the level of hypotheses.

Table 6.6 Intrinsic factors affecting spoilage rate of fish species stored in ice

Rough handling will, as outlined in section 5.2, result in a faster spoilage rate. This is due to the physical damage to the fish, resulting in easy access for enzymes and spoilage bacteria. The surface/volume ratio of larger fish is lower than that of smaller fish, and, as bacteria are found on the outside, this is probably the reason for the longer shelf life of the former. This is true within a species but may not be universally so.

Post mortem pH varies between species but is, as described in section 5.2, higher than in warm-blooded animals. The long rigor period and the corresponding low pH (5.4-5.6) of the very large flatfish, halibut (Hippoglossus hipoglossus), has been offered as an explanation for its relatively long iced storage life (Table 6.7). However, mackerel will often also experience a low pH and this seems to have little effect on shelf life. As can be seen from Table 6.7, fatty fish are in general rejected sensorically long before lean fish. This is mainly due to the appearance of oxidative rancidity.

The skin of the fatty pelagic fish is often very thin, and this may contribute to the faster spoilage rate. This allows enzymes and bacteria to penetrate more quickly. On the contrary, the thick skin of flatfish and the antibacterial compounds found in the slime of these fish may also contribute to the keepability of flatfish. As described earlier, the slime of flat fish contains bacteriolytic enzymes, antibodies and various other antibacterial substances (Hjelmland et al., 1983; Murray and Fletcher, 1976). Although large differences exist in the content of TMAO, this does not seem to affect the shelf life of aerobically-stored fish but rather the chemical spoilage profile of the species.

Table 6.7 Shelf life of various fish species from temperate and tropical waters. Prepared from data published by Lima dos Santos (1981); Poulter et al. (1981); and Gram (1989)

1) fat content and shelf life subject to seasonal variation

In general, the slower spoilage of some fish species has been attributed to a slower bacterial growth, and Liston (1980) stated that "different spoilage rates seem to be related at least partly to the rate of increase of bacteria on them".

Influence of water temperature on iced shelf life

Of all the factors affecting shelf life, most interest has focused on the possible difference in iced shelf life between fish caught in warm, tropical waters and fish caught in cold, temperate waters. In the mid- and late sixties it was reported that some tropical fish kept 20-30 days when stored in ice (Disney et al., 1969). This is far longer than for most temperate species and several studies have been conducted assessing the shelf life of tropical species. Comparison of the data is, as pointed out by Lima dos Santos (1981), difficult as no clear definition has been given on a "tropical" fish species and as experiments have been carried out using different sensory and bacteriological analyses.

Several authors have concluded that fish taken from warm waters keep better than fish from temperate waters (Curran and Disney, 1979; Shewan, 1977) whereas Lima dos Santos (1981) concluded that also some temperate water fish species keep extremely well and that the longer shelf lives in general are found in fresh water fish species compared to marine species. However, he also noted that shelf life of more than 3 weeks, which is often observed for fish caught in tropical waters (Table 6.7), never occurs when fish from temperate waters are stored in ice. The iced shelf life of marine fish from temperate waters varies from 2 to 21 days which does not differ significantly from the shelf life of temperate freshwater fish ranging from 9 to 20 days. Contrary to this, fish caught in tropical marine waters keep for 12-35 days when stored in ice and tropical freshwater fish from 6 to 40 days. Although very wide variations occur, tropical fish species often have prolonged shelf lives when stored in ice as shown in Table 6.6. When comparisons are made, data on fatty fish like herring and mackerel should probably be omitted as spoilage is mainly due to oxidation.

Several hypotheses have been launched trying to explain the often prolonged iced spoilage of tropical fish. Some authors have noted an absence in development of TMA and TVN during storage and suggested that the spoilage of tropical fish is not caused by bacteria (Nair et al., 1971). The lack of development of TMA and TVN may be explained by a spoilage dominated by Pseudomonas spp.; however, qualitative bacteriological analyses must be carried out to confirm or reject this suggestion. Low bacterial counts have been claimed in some studies, but often inappropriate media have been used for the examination and too high incubation temperatures ( > 30°C) have not allowed the psychrotrophic spoilage bacteria to grow on the agar plates.

Reviewing the existing literature on storage trials of tropical fish species leads to the conclusion that the overall sensory, chemical and bacteriological changes occurring during spoilage of tropical fish species are similar to those described for temperate species.

Psychrotrophic bacteria belonging to Pseudomonas spp. and Shewanella putrefaciens dominate the spoilage flora of iced stored fish. Differences exist, as described in section 5.3, in the spoilage profile depending on the dominating bacterial species. Shewanella spoilage is characterized by TMA and sulphides (H₂S) whereas the Pseudomonas spoilage is characterized by absence of these compounds and occurrence of sweet, rotten sulphydryl odours. As this is not typical of temperate, marine fish species which have been widely studied, this may explain the hypothesis that bacteria are not involved in the spoilage process of tropical fish.

Despite the different odour profiles, the level at which the offensive off-odours are detected sensorially is more or less the same. In model systems (sterile fish juice) 108 109 cfu/ml of both types of bacteria is the level at which spoilage is evident.

As outlined in section 5.3, the relatively high post mortem pH is one of the reasons for the relatively short shelf life of fresh fish as compared to, for instance, chill stored beef. It has been suggested that tropical fish species, such as the halibut from temperate waters, reach a very low pH, and that this explains the longer shelf life. However, pH values of 6-7 have been found in the studies of tropical fish species where pH has been measured (Gram, 1989). As the differences in skin properties are believed to contribute to the longer shelf life of flatfish, it has been suggested that this factor explained the extended shelf lives. It is indeed true that fish from warm waters often have very thick skin, but no systematic investigation has been carried out on the skin properties.

As spoilage of fish is caused by bacterial action, most hypotheses dealing with the long iced shelf life of tropical fish species have centred around differences in bacterial flora. Shewan (1977) attributed the long iced shelf lives to the lower number of psychrotrophs on tropical fish. However, in 1977 only a very limited number of studies of the bacterial flora on tropical fish were published. During the last 10-15 years several investigations have concluded that Gram- negative rod-shaped bacteria (e.g., Pseudomonas, Moraxella and Acinetobacter) dominate on many fish caught in tropical waters (Gram, 1989; Surendram et al., 1989; Acuff et al., 1984). Similarly, Sieburth (1967) concluded that the composition of the bacterial flora in Narragansett Bay did not change during a 2-year survey even though the water temperature fluctuated with 23°C on a year-round basis. Gram (1989) showed that 40-90% of the bacteria found on Nile perch were able to grow at 7°C. The number of psychrotrophic bacteria is within one log unit of the total count, and the level of psychrotrophic organisms is not per se low enough to account for the extended iced storage lives of tropical fish; Jorgensen et al. (1989) showed that a two log difference in number of spoilage bacteria only resulted in a difference of 3 days in the shelf life of iced cod.

As described in section 5, the bacterial flora on temperate water fish species resume growth immediately after the fish have been caught and rarely is a lag phase seen. Contrary to this, Gram (1989) concluded that a bacterial lag phase of 1-2 weeks is seen when tropical fish are stored in ice. Also, the subsequent growth of psychrotrophic bacteria is often slower on iced tropical than on iced temperate water fish. This is in agreement with Liston (1980) who attributed differences in shelf life to differences in bacterial growth rates. Although a large part of the bacteria on tropical fish are capable of growth at chill temperatures, they will (as this has never been necessary) require a period of adaptation (i.e., the lag phase and slow growth phase). Gram (1989) illustrated this by investigating the growth rate at 0°C of fish spoilage bacteria that had either been pre-cultured at 20°C or at 5°C. For some strains, the same bacterial strain would grow more quickly at 0°C if pre-cultured at 5°C than if pre-cultured at 20°C (Table 6.8). Preculturing was done with several sub-culture steps at each temperature. Similarly, Sieburth (1967) showed that although the taxonomic composition of the bacterial flora in Narrangansett Bay did not change with fluctuating temperature, the growth profile of the bacteria fluctuated following the water temperature. However, the adaptation hypothesis does not explain why some tropical fish spoil at rates comparable to temperate water fish.

Table 6.8 Generation times at 0°C for fish spoilage bacteria pre-cultured at high (20°C) or low (5°C) temperatures

It can be concluded that many factors affect shelf life of fish and that differences in the physiology of the bacterial flora are likely to be of major importance.

Off flavours related to fishing ground

Occasionally fish with off-flavours are caught, and in certain localities this is a fairly common phenomenon. Several of these off-flavours can be attributed to their feeding on different compounds or organisms. The planktonic mollusc, Spiratella helicina, gives rise to an off- flavour described as "mineral oil" or "petrol". It is caused by dimthyl-ß-propiothetin which is converted to dimethylsulphide in the fish (Cornell, 1975). The larvae of Mytilus spp. cause a bitter taste in herring. A very well known off-flavour is the muddy-earthy taint in many freshwater fish. The flavour is mainly caused by two compounds: geosmin (1a ,10ß-dimethyl-9a -decalol) and 2-methylisohorneol, which also are part of the chemical profile of wine with cork flavour. Geosmin, the odour of which is detectable in concentrations of 0.01-0.1 mg/l, is produced by several bacterial taxa, notably the actinomycetes Streptomyces and Actinomyces.

An iodine-like flavour is found in some fish and shrimp species in the marine environment. This is caused by volatile bromophenolic compounds; and it has been suggested that the compounds are formed by marine algae, sponges and Bryozoa and become distributed through the food chain (Anthoni et al., 1990).

Oil taint may he found in the fish flesh in areas of the world where off-shore exploitation of oil is intensive or in areas where large oil spills occur. The fraction of the crude oil that is soluble in water is responsible for the off-flavours. This is caused by the accumulation of various hydrocarbon compounds, where particularly the aromatic compounds are strong flavourants (Martinsen et al., 1992).

Contents - Previous - Next