Lipids

The term lipids, synonym of fat, is frequently used to refer oils and fats. Typically fats are solid at room temperature and oils are liquid. This group of water-insoluble organic compounds comprising a wide variety of substances is broadly divided into two classes: (i) neutral lipids, which include triacylglycerols, free fatty acids, cholesterol and other sterols and also isoprene and respective esters, (ii) polar lipids, particularly phospholipids, also termed structural lipids because of their fundamental role in the formation of cell membranes (this class comprises mainly phosphatidyl choline, phosphatidyl ethanolamine, sphingomyelin and cardiolipins).

Lipids perform several biological functions important for the living organisms, namely storage and transport of energy, formation of cell membranes and maintenance of their structural integrity, supply of essential fatty acids for the cell structures and prostaglandins synthesis and transport of fat-soluble vitamins as well.

From all seafood constituents, lipids are those whose content presents the largest changes, varying the values in the edible part, commonly, between 0.1 % and 22.0 %. The variation range of the lipid content is the result of multiple factors, particularly species, sexual maturity, water temperature, food abundance and stress. These oscillations are most notorious in fatty fish, which may exceptionally reach values higher than 25.0 % (for instance, 1 to 22 % for sardine, 0.8 to 25 % for herring and 0.7 to 23 % for chub mackerel). Reproduction, growth and prolonged fast can affect the fat depots in different body areas.

Regarding lipid content, seafood are classified into three categories: lean, when lipid content in the muscle fraction is below 5 %, such as sole, cod, hake and almost all crustaceans; medium-fat, if the content varies between 5 and 10 %, where brill and silver scabbard fish are examples; and fatty fish, in those species whose lipid content is higher than 10 %, at least during a part of the year, such as sardine, tuna, herring and salmon.

Besides the observed differences in the lipid content, there are also considerable differences in the distribution of fat. Hence, lean fish store fat in the liver, which is relatively large and rich in triacylglycerols while in the muscle portion phospholipids are the most important lipids. Furthermore, the so-called fatty and medium-fat fish store this constituent as triacylglycerols in the muscle, in a fat layer underneath the skin - specially, during times of intense feeding - and in the muscle and viscera.