CHILLED AND FROZEN RAW FISH

Associate Prof. Covadonga Arias
Department of Fisheries and Allied Aquacultures
Auburn University
Auburn
Alabama 36849
United States of America

1.1 Definitions

Fish are classified as any of the cold-blooded aquatic vertebrates of the super class Pisces typically showing gills, fins and a streamline body. In addition, 'fish' also refers to the flesh of such animals used as food. This super class of vertebrates includes all the bony and cartilaginous finfish, and excludes molluscs and Crustacea. However, some regulatory agencies such as the US Food and Drug Administration (FDA) will include molluscan shellfish, crustaceans, and other forms of aquatic animal as part of their 'fish' definition. In this chapter, "fish" will be used for fresh and seawater finfish.

Fish are an important part of a healthy diet since they contain high quality protein, but typically present a low fat percent when compared to other meats. In addition, most fish contain omega 3-fatty acids and other essential nutrients.

Although fish is broadly similar in composition and structure to meat there are a number of distinctive features. Protein content in fish fillet varies typically from 16 - 21%. The lipid content, which can be up to 67%, typically fluctuates between 0.2 - 20%, and is mostly interspersed between the muscle fibres. Fish fillets are a poor source of carbohydrates, offering less than 0.5% (1). Fish fillet composition can vary significantly within the same species due to feed intake, migratory patterns, and spawning season. The lipid fraction is the component showing the greatest variation; it shows a typical season pattern especially in migratory species such as herring or mackerel. Fish can be divided into fatty and lean fish; lean fish are those fish that store most of their fat in the liver, while fatty fish have fat cells distributed along their bodies. Muscle composition and structure of fish also differ from those found in other meat. Fish flesh is dominated by the abundance of white muscle in relatively short segments, giving it its characteristically flaky structure. The connective tissue content of fish is also lower than that found in meat, typically 3 and 15% of total weight, respectively (1).

Chilled fish is fish that has been cooled to, and maintained at or below 7 °C, but not below 3 °C during storage, transportation and sale.

Controlled-atmosphere packaging (CAP) refers to packaging in an atmosphere where the composition of the gases is continuously controlled during storage. This technique is primarily used for bulk storage.

DMA is dimethyl amine.

Evisceration is the removal of the viscera from a fish.

Fresh fish is raw fish that has not been processed, frozen or preserved.

Frozen fish is fish that has been cooled to, and maintained at or below 2 °C (normally below -12 °C) during storage, transportation and sale.

Modified-atmosphere packaging (MAP) refers to packaging systems in which the natural gaseous environment around the product is intentionally replaced by other gases, usually carbon dioxide (CO2), nitrogen (N2) and oxygen (O2). The proportion of each component is fixed when the mixture is introduced, but no further control is exercised during storage.

Organoleptic refers to qualities such as appearance, colour, odour and texture.

Quality refers to palatability and organoleptic characteristics such as tenderness, juiciness, and flavour based on the maturity, marbling, colour, firmness, and texture of the fish.

Raw fish refers to fish that has not been cooked but excludes fish treated with curing salts and/or subjected to fermentation.

Shelf life is defined as the time of storage before microbial spoilage of a fish is evident.

Spoilage describes changes that render fish objectionable to consumers; hence, spoilage microflora describes an association of microorganisms that, through their development on fish, renders that fish objectionable to consumers.

Spoilage potential is a measure of the propensity of microorganisms to render fish objectionable to consumers through the production of offensive metabolic by-products.

Superchilled fish is fish that has been cooled to, and maintained at temperatures just below the freezing point, at -2 to -4 °C, during storage, transportation and sale.

Vacuum packaging (VP) refers to packaging systems in which the air is evacuated and the package sealed.

1.2 Initial Microflora

The subsurface flesh of live, healthy fish is considered sterile and should not present any bacteria or other microorganisms. On the contrary, as with other vertebrates, microorganisms colonise the skin, gills and the gastrointestinal tract of fish. The number and diversity of microbes associated with fish depend on the geographical location, the season and the method of harvest. In general, the natural fish microflora tends to reflect the microbial communities of the surrounding waters. It is difficult to estimate how many microorganisms are typically associated with fish, since they heavily depend on the type of sample analysed and the protocol used for isolation. In fact, standard culture-dependent methods can only recover between 1 to 10% of total bacteria present in any given sample. More accurate, molecular-based methods have not yet been used to address this issue. Gastrointestinal tracts and gills typically yield high bacteria numbers, although these are influenced by water quality and feed. Fish harvested from clean and cold waters will present lower bacterial numbers than fish from eutrophic and/or warm waters. However, potential human pathogens may be present in both scenarios.

The autochthonous bacterial flora of fish is dominated by Gram-negative genera including: Acinetobacter, Flavobacterium, Moraxella, Shewanella and Pseudomonas. Members of the families Vibrionaceae (Vibrio and Photobacterium) and the Aeromonadaceae (Aeromonas spp.) are also common aquatic bacteria, and typical of the fish flora. Gram-positive organisms such as Bacillus,Micrococcus, Clostridium, Lactobacillus and coryneforms can also be found in varying proportions (1). It is crucial to mimic the environmental physico-chemical parameters when isolating bacteria from fish. For example, some species (most Vibrios) require sodium chloride for growth; whenever possible, several culture media containing sodium chloride and more than one incubation temperature should be used. It must be noted that mesophilic bacteria can rapidly overgrow psychrotropic organisms.

Human pathogenic bacteria can be part of the initial microflora of fish, posing a concern for...