Fresh fish and its conservation

Fish degradation

From the time of capture to its food use, fresh fish must necessarily be refrigerated. The conservation of the catch in ice at 0 ° C (constant temperature) does not alter its composition for about 4 - 7 days.

After 7 - 10 days, or sooner if the temperature is higher than 0 ° C or undergoes important fluctuations as happens when it is exposed in the market stalls, the first important alterations begin. Initially, we witness the transformation of triethylamine oxide into trimethylamine and, subsequently, into dimethylamine by bacterial and endogenous enzymes. Over time the reaction continues leading to the formation of monoethylamine and formaldehyde (responsible for the typical smell of battered fish). There is also formation of sulfidric acid (which contributes to giving the fish a nauseating aroma) and biogenic amines (histamine, tryptamine, cadaverine, putrescine, tyramine). Histamine, in particular, is present in discrete quantities already in fresh fish and the increase in its concentration can give rise to pseudo-allergic reactions in sensitive subjects (appearance of red dots on the skin, nausea, abdominal pain, etc.). The same reactions occur following the ingestion of strawberries when there is a basic predisposition.

In addition to chemical transformations, fish can also be affected by biological contamination. In particular, the fish is attacked mainly by bacteria belonging to the genera Pseudomonas, Moraxella and Flavobacterium-Cytophage .

Evaluation of freshness: how to know if a fish is fresh?

Sensory methods

	FRESH FISH	DROPPED FISH
Smell	Soft and pleasant	Acre, acid, ammoniacal
General appearance	Brilliant, metallic, iridescent	Dull, opaque
Body	stiff	Springs (if the meat is pressed with a finger the impression remains)
Consistency	soda
Scales	Strongly adherent	They are easily removed
Skin	Tesa, colored	Flaccid, faded
Eye	Clear, lively	Dull, vitreous
Gills	Rose-red, united	Gray, uplifted
Anus	Closed	protruding
Viscera	Smooth, clean, brilliant	softened
Plug	Adherent to meat	relieved
Meats	Solid, white / red	crumbly

Physical methods

Determination of the electrical conductivity of the tissues: as the fresh product deteriorates, the electrical conductivity increases.

Chemical methods

Determination of trimethylamine, volatile basic nitrogen, formaldehyde, histamine, peroxides and thiobarbituric acid.

Biochemical methods

Search for specific enzymes that escape from cells during thawing; if they are present it means that the fish has been frozen and refrozen.

Microbiological methods

They are based on the development of cultures of microorganisms, but are too long to be applied to the evaluation of the freshness of the fish.

When buying frozen fish, remember that the cold chain should be kept as intact as possible; for this reason:

put the frozen fish in the cart only at the end of the shopping (the seriousness of a supermarket is also evaluated based on the position of the frozen food counter, which must be close to the crates and on the opposite side from the entrance).

In addition to evaluating the temperature of the thermostat in the refrigerated counter, preference should be given to the packages on the bottom, as those closest to the opening door are more prone to temperature changes.

Fish contaminants

They derive above all from the environment in which they live. They can be divided into:

chemical contaminants:

• heavy metals: lead, cadmium, mercury

• pesticides

• organic chlorine and bromine compounds

and biological contaminants:

• toxins produced by algae and some fish species, such as puffer fish, can cause: poisoning with PSP paralysis; neurotoxic poisoning NSP; amnesic poisoning ASP; Diarrheal DSP poisoning.

• parasites and microorganisms (they are more common in farmed fish).

storage

The best way to preserve fresh fish is to refrigerate at 0 ° C, which can occur in cells or in flake ice. In this case the commercial life of the fish is 7-8 days; 9-10 if it is vacuum packed.

Deep-freezing (-18 -30 ° C), on the other hand, must take place in a maximum time of 2 hours and for this reason it is often made directly in fishing vessels. Other preservation processes, even if now used only to impart particular organoleptic characteristics, are:

• salting, in which the fish is alternated with layers of salt or immersed in concentrated brines (10-30% NaCl);
• smoking;
• drying (the final humidity must be less than 15% and its effectiveness is acceptable only for the conservation of lean fish, such as cod that is used to make stockfish);
• marinating (using vinegar and salt as preservatives);

The canning, widespread in the industrial field, is mainly used for tuna, small eyes and anchovies). By law, the addition of antioxidant, antifungal and antibacterial additives is allowed in canned fish:

• Sorbic acid and its salts

• Benzoic acid

• Ethyl and propyl paroxybenzoate

• Sulfur dioxide (salt cod and shrimp and shellfish preserves)

• Ascorbic acid

The fish that remains unsold or the fished species without commercial value are used for the production of fish meal and oil destined above all for animal husbandry.