CHAPTER 3
Quality changes and spoilage of fish

3.1 Introduction

What is quality? The term of quality has different meanings. Most often 'quality' refers to the aesthetic appearance and freshness or degree of spoilage that the fish has undergone. It may also involve safety aspects such as being free from harmful bacteria, parasites or chemicals. It is important to remember that 'quality' implies different things to different people, and is a term which must be defined in association with an individual product type. In the fishing industry the term 'quality fish' often related to expensive species or to the size of fish. Fish considered by a processor to be of inferior quality may be too small or to poor a condition for a certain process, resulting in low yields and profit. For example, it is often thought that the best quality is found in fish which are consumed within the first few hours post mortem. However, very fresh fish which are in rigor-mortis are difficult to fillet and skin, and are often unsuitable for smoking. Thus, for the processor, slightly older fish which have passed through the rigor process are more desirable (Huss 1995).

Quality usually refers to the same meaning as freshness. Freshness is the major contribution to the quality of seafood products. For all kinds of seafood products, freshness is essential for the quality of final product (Alasalvar et al. 2010). Postharvest quality of seafood alters rapidly. Quality attributes of fish flesh - including food safety, organoleptic features, nutritional quality and aptitude to industrial transformation - influence consumption and acceptability of fish as food. Fish sensorial changes and texture properties are closely linked to freshness (Delbarre-Ladrat et al. 2006).

3.2 Factors affecting quality of fish

The type and number of factors influencing quality of fishery products are numerous. Postharvest biochemical and microbial changes in fish tissue depend very significantly upon the factors which effect the concentration of substrates and metabolites in the tissues of the live fish, the activity of the endogenous enzymes, the microbial contamination and the conditions after catching (Sikorski et al. 1990). Along with ante-mortem muscle biochemistry, post-mortem biochemical processes are directly linked to final quality attributes. The understanding of post-mortem mechanisms is a prerequisite for an accurate control of the quality of commercialized fish by the identification of objective markers or indicators (Delbarre-Ladrat et al. 2006).

The physical, chemical and bacteriological characteristics of fish tend to differ with species, feeding habits, season, spawning cycles, methods of catching, fishing grounds, size, age, microbiological load and geographical location (Shewan 1977; Huss 1995). Nevertheless, temperature and rigor-mortis are the main underlying factors in fish spoilage.

3.2.1 Species

The rate of spoilage is species dependent. It is well-known fact, that when chilled or frozen, fatty species such as sardine and mackerel will spoil more rapidly than a lean species like cod. The fat content of pelagic fish species can vary considerably throughout the year. Differences in composition within a species may be the cause of secondary influences on quality. When placed in refrigerated storage, lean fish in poor condition spoil much more rapidly than specimens of the same species in good condition. This may be explained by glycogen content of the flesh. Low flesh pH also has undesirable effects on the quality of fish. The final pH of cod flesh has some influence over the rate at which deterioration occurs during frozen storage. Those species of fish caught in warm waters keep longer on ice than those caught in cooler waters. The reason for this is bacterial flora growing on the surface of fish. Another species effect is related to migratory routes. Those species migrating for long distances prior to capture will, in all probability, not be in as good physical condition as those species or specimens of the same species that follow shorter routes (Wheaton &Lawson 1985).

3.2.2 Size

Size heterogeneity develops in fish populations as the animal matures because differences exist in feeding behaviour and growth rate. Heterogeneous growth may impact composition and other muscle attributes at harvest because of developmental differences in tissue compartments at harvest (Jittinandana et al. 2003). Large fish keep better than small fish. One of the main mechanisms of spoilage is penetration of microorganisms from the surface to the interior of the fish. Larger fish have a smaller surface area to volume ratio so that in the same time period, less of the interior of larger fish is affected. Also, large fish such as cod, tuna and salmon are generally eviscerated aboard the fishing vessel. Flesh pH is another size effect. Small fish tend to have higher post-rigor pH than larger fish of same species (Wheaton & Lawson 1985). In general, large fish of a given species fetch the highest prices. Consumers are prepared to pay more for large samples of commodities like shrimps, scampi, crab and lobster because they are visually and gastronomically more satisfying. That is to say, large fish are not necessarily more finely flavoured and textured than small ones. Processors place a high value on large fish because the percentage yield of edible material is high, handling costs per unit weight are lower, they often keep better, and often more uniform products can be made from them. On the other hand, for some purposes the optimum size is less than the largest. Large sizes of trout, clams and oysters are not favoured for table use, because portion size is then too large or expensive. For canning, specific sizes of sprat, herring, sardine and similar species are required to ensure correct can fill (Connell 1975).

3.2.3 Distance to port

How quickly the fish are eviscerated and placed into cold storage may be related to the distance the vessel must travel from its home port to the fishing grounds. The problem of distance from fishing grounds to port is more pronounced in tropical and subtropical regions than it is in the colder climates. The heat from the sun quickly overheats the fish and accelerates post-mortem changes (Wheaton & Lawson 1985).

3.2.4 Diet of fish

Heavily feeding fish tend to be more susceptible to autolytic tissue degradation than the petite feeders. The type of feed/food on which fish is feeding on may similarly have an effect on their spoilage rate during storage. Non-feeding fish have been found to have low levels of bacteria in the intestines as compared to the heavily feeding fish (Huss 1988).

The chemical composition of fish varies greatly depending on feed, environment and season. It is reported that the lipid content of farmed common carp can vary widely depending on the feed used. The quantity of n-3 fatty acids varies largely with the origin of diets and its composition (Ljubojevic et al. 2012).

Fattiness in cultured fish generally is undesirable because consumers consider eating fish as part of a low-fat diet. Fat in fish may not only reduce consumer acceptance, but also may reduce storage time and decreased processed yield of the fish. There are several factors that affect the amount of fat in fish such as feeding (Lovell 1998). Diet of fish has significant effect on flavour of its flesh. Some freshwater fish suffer from a muddy odour and flavour, which may reduce consumer acceptance (Wheaton & Lawson 1985).

3.2.5 Fishing grounds and methods

The fishing grounds and geographical locations exclusively tend to determine fish spoilage in so far as temperature, food types, the level of pollution; microenvironments are concerned (Shewan 1977) The location of the fishing grounds plays an indirect role in determining the quality of fishery product. Within a species, flavour can vary from one ground to the next, depending upon the nature of food and physiological condition of species of interest. Winds, tides water conditions and migratory patterns also have some influence on the condition and quality of the before harvest. The time of year when spawning and subsequent poor condition develops in fish varies with the fishing grounds (Wheaton & Lawson 1985). Geographical location, and or the type of waters, has a remarkable influence on the type of micro fauna that will grow on the fish. The microfauna found in fish from warm waters (tropical waters) differs from those from cold waters. The predominant bacteria on the surface of fish in warm waters consist of the Gram-positive bacteria; Bacillus, Micrococcus and Carnobacterium, which constitute 50-60% of the total microflora. Those found in cold waters comprise Gram-negative bacteria; Psychrobacter, Moraxella, Pseudomonas ssp, Actinobacter, Shewanella, Flavobacterium, Cytophaga and Vibrio (Gram et al. 1989). The common spoilage organisms associated with seafood stored in ice and air, whether of temperate, sub-tropical or tropical origin, are the Gram-negative psychrotrophic bacteria such as Pseudomonas and Shewanella putrefaciens. Other pertinent spoilage bacteria include: P. phosphoreum, commonly isolated in packed fish and other meats; Vibrionaceae, associated with fish at elevated temperatures anaerobic spoilers such as Lactobacillus spp. and Leuconostoc spp. (Huss...