Visual appearance, especially colour, is the most important characteristic of foods in determining their selection prior to actual consumption.  "Carotenoids" are of particular importance because they serve as one of nature's most important pigments. In biology, any substance that can impart colour to the tissues or cells of animals or plants can be called a pigment.

Carotenoid is the generic name for one of the most common groups of naturally occurring pigments found in the animal and plant kingdoms.  The most familiar is beta-carotene, the orange-red pigment in carrots, that was discovered almost 200 years ago.  It is from the word carrot, that the group name, "carotenoid" is derived.  Carotenoids are synthesized around the globe by plants, fungi and some bacteria at the rate of some 3 tons every second.  Carotenoids show great diversity in structure, function and colour.  To date, over 600 carotenoids have been identified in nature, varying in colour from yellow to red.  Carotenoids are present without exception in photsynthetic tissue and occurring with no definite pattern in nonphotosynthetic tissues such as roots, flower petals, seeds, and fruits.  They are also found in fungi, yeast, molds, mushrooms, and bacteria, and in many cases they are the major pigment in some tissues of aquatic and avian species, that is, lobster, prawn, salmon, and in the feathers of many birds. 

Most carotenoids are polyunsaturated hydrocarbons, containing 40 carbon atoms, and comprising two terminal ring systems (see below).  Carotenoids that are composed entirely of carbon and hydrogen are known as carotenes, while those that contain oxygen are termed xanthophylls. 

Figure 1.  A typical carotenoid - beta-carotene

What Carotenoids do

Carotenoids have many other roles besides coloration.  In many cases coloration is not the first role for carotenoids.  Research has shown that carotenoids can function as a provitamin A source and as potent antioxidants.  Carotenoids have been shown to enhancing the immune response, the may be important in preventing cardiovascular diseases, age-related macular degeneration and cataracts, protection from certain cancers.  They have been shown to need for normal reproduction in cattle.

Animals, including humans, cannot make carotenoids from simpler molecules, they are totally dependent on the amounts supplied in their food.  In this respect, carotenoids are like vitamins; if intakes are inadequate, health and survival may be compromised.

Coloration in Fish

Coloration plays a role in social structure as well as defence of fish in the wild.  Fish depend a great deal on vision as a source of sensory information.  All but a few species (mainly cave dwellers) have well-developed eyes.  As is the case in many other groups of animals, the body colours of fish are predominantly dependent on the presence of special cells in the skin, called chromatophores.  These cells contain pigments or light-scattering or light-reflecting organelles.

Carotenoids also have excellent antioxidative characteristics.  Cold-water fishes, like salmon, have a high level of polyunsaturated fat in their membranes, and protection of lipid tissue from peroxidation seems to be a metabolic function for astaxanthin.  Astaxanthin has been shown to be one hundred times more effective than vitamin E as an antioxidant.

Information provided by Roche S.A.