Introduction

According to the Spanish Food Code, fish are defined as “edible vertebrate animals, either marine or freshwater (fish, mammals, cetaceans and amphibians), fresh or preserved by various authorised methods.”

In Spain, practically 90% of the fish consumed comes from the sea.

Fish muscle is composed of muscle fibres, just like meat. However, the structure of these fibres is entirely different from that of birds and mammals, as it is adapted to the need to flex the body for propulsion in water.

Fish muscle is organised into myotomes, which are W-shaped segments connected by thin layers of oily connective tissue called myocommata.

Characteristics of the tissue

The muscle tissue of fish breaks down more easily because actin and myosin proteins denature more readily. Moreover, the collagen in the connective tissue does not become tougher with the age of the animal, as no cross-links are formed.

In fish and shellfish, post-mortem rigor sets in much earlier, and the tenderisation phase lasts for a shorter time than in meat.

The pH drop is less pronounced than in meat, making it easier for microorganisms to proliferate, hence cooling must be applied more quickly.

It is believed that red muscle is more suitable for sustained activity, whereas white muscle is probably more adapted for vigorous activity over short periods. Additionally, white muscle in fish is compositionally very uniform regardless of its location, while dark muscle varies depending on its location.

The chemical composition of fish is as follows:

• 70–80% water
• 0.1–25% invisible fat
• 17–20% protein
• 0.05–0.2% glycogen
• 1% vitamins and minerals

PROTEINS

Fish and shellfish proteins have a high biological value due to their amino acid composition, even surpassing that of meat, and are approximately 85% bioavailable. They are rich in the least abundant amino acids and are suitable for complementing cereal proteins.

Several types of proteins are found in fish:

• Stromal proteins (5–10%): collagen, present in smaller amounts than in meat.
• Myofibrillar proteins (75%): actin, myosin, troponin, tropomyosin.
• Sarcoplasmic proteins (20%): less myoglobin than in meat, resulting in a paler colour.

FATS

The fat content varies greatly among different types of fish.

Lean fish such as hake, sole, or cod contain between 0.1–1% lipids. In these fish, triglycerides accumulate mainly in the liver (up to 75%), meaning the muscle tissue is largely free of fat. This makes them very suitable for preservation by salting, without developing rancid flavours.

Fatty fish can contain up to 25% lipids, with most of the fat found in the muscle tissue and hardly any in the liver. Fish oils are highly unsaturated, with iodine values much higher than those of other animal fats. This is why they are liquid at sea temperatures. However, due to the high degree of unsaturation, these fats are easily oxidised.

Marine fish contain more unsaturated fatty acids than freshwater fish, and also possess fatty acids with four or six double bonds, such as cuplanodonic acid and others.

Cholesterol content is high in some fish and even higher in shellfish.

Non-protein nitrogenous compounds

These compounds are dissolved in the plasma and intercellular fluids, significantly contributing to the flavour of fish and acting as substrates for microbial spoilage. Most of them are free amino acids, such as histidine in fatty fish like herring and salmon, taurine in molluscs, or glycine in prawns.

Classification of fish

According to the Spanish Food Code, fish may be classified as:

Fresh fish: fish that have not undergone any preservation treatment since capture, except the addition of pure crushed ice or a mix of ice and salt.

Frozen fish: fish, whole or cut, that have been subjected to cold until the core temperature drops from 0ºC to –5ºC within no more than two hours, then stored at –23ºC.

Salted fish: fresh fish, whole or cut, eviscerated and unspoiled, that have been subjected to prolonged exposure to common salt in solid form or brine.

Smoked fish: whole or cut fish previously brined and dried, then exposed to wood smoke, as specified in the smoking regulations.

Dried fish: fish dried by exposure to dry air or any authorised method to reduce water content over time (depending on environmental conditions and fish size) to ensure longer preservation.

Consumers generally base their preferences on organoleptic properties. Thus, they tend to favour fish with fewer bones and those that are fattier and tastier over leaner varieties.

Another classification is based on fat content:

• Fatty or oily fish: northern bonito, anchovy, mackerel, horse mackerel, pomfret, scad, salmon, sardine.
• White or lean fish: wedge sole, blue whiting, red bream, megrim, sole, hake, monkfish, turbot.
• Semi-fat fish: gilthead bream, sea bass, swordfish, red mullet, trout.

Quality attributes

The quality of fish or shellfish is largely determined by its freshness. And how can we tell if a fish is fresh?

Many components of fish contain substances that are toxic to the human body.

Natural toxic substances

Some marine species produce toxic substances such as neurotoxins (e.g., river barbel roe, eel blood).

Bacterial decomposition leads to the formation of substances such as biogenic amines (histamine, putrescine, etc.), which are toxic to humans.

External toxic substances

Pesticides: the most dangerous and targeted today are organochlorines, as they degrade slowly and accumulate in the liver and fatty tissues of fish and shellfish.

Heavy metals: primarily mercury, lead, and cadmium, all carcinogenic. Cadmium settles in sediment and is ingested by bottom-dwelling, non-predatory fish, especially near river mouths.

Substances resulting from food processing

Benzopyrene and other polycyclic aromatic hydrocarbons. These are highly soluble in oils and are therefore found in high concentrations in preserved fish oils. Consequently, consuming these oils is not recommended.

Components added to fish and shellfish

Colourants: used only in salmon substitutes, anchovy paste, prawn preserves, and black fish roe.

Preservatives: prevent biological decomposition, although their effectiveness is time-limited. Some may form harmful compounds over time. The most used preservative is wine vinegar, which can be applied in large quantities. Other authorised preservatives include sorbic acid (up to 2.5 g/kg), benzoic acid (4 g/kg), and formic acid (1 g/kg). Formaldehyde is also used for preserving prawns.

Nutritional importance of fish and shellfish

They are rich in protein and in antioxidant vitamins such as A, D, and E. In some cases, they also contain high levels of important minerals like iodine.

They are easier to digest because they contain little connective tissue and very little fat.

White fish contain a lot of water and protein but little fat, whereas oily fish have a high fat content.

Crustaceans and molluscs contain 1.2–4% carbohydrates.

Oysters have a high mineral content, including zinc, copper, and calcium.

Octopus, squid, and cuttlefish contain only structural proteins and water.