Occurrence In Nature

As indicated in Table 1 the carotenoids are widely distributed in foods and are the most widespread group of pigments in nature. They are present without exception in photosynthetic tissue; occur with no definite pattern in nonphotosynthetic tissues such as roots, flower petals, seeds, fruits, vegetables; and are found sporadically in the Kingdom Protista, including the (fungi-yeast mushrooms and bacteria). The red, yellow, and orange pigments in the skin, flesh, shell, or exoskeleton of some animal species are due to these pigments. These would include the lobster, shrimp, salmon, goldfish, and flamingo. People who consume large amounts of tomato juice may turn orange or red because of an intake of the pigments. The carotenoids, which cannot be made by animals, are the precursors to vitamin A, but unlike vitamin A, are not toxic in large doses (26).

Generally the concentration of the carotenoids in various tissues is low but can vary widely. The cornea of the eye narcissus contains 18% /J-carotene (dry), whereas the concentration in foods is much lower. It has been estimated that the annual production of carotenoids is in on the order of 108 tons. The major pigment is fucoxanthin, the pigment in marine brown algae. Other major pigments include those found in green leaves, mainly lutein, violaxanthin, and neoxanthin. By contrast, yS-carotene occurs widely but in smaller amounts. It is of interest that (note: zea is major pigment in some algae) 3,3'-dihydroxy-a-carotene (lutein) is a major pigment, whereas 3,3'-dihydroxy-/?-carotene (zeaxanthin) is a minor pigment in nature. In contrast, the major hydroxarbon carotenoid found in nature is /?-carotene, whereas a-carotene is a minor pigment. A typical percentage profile of the xanthophylls of a green plant could be lutein, 40%; violaxanthin, 34%; neoxanthin, 19%; cryptoxanthin, 4%; and zeaxanthin, 2%. /i-carotene, lycopene, and capsanthin can be high in some tissues such as the sweet potato, tomato, and red pepper. By contrast, the carrot is a major course of a-carotene where the concentration can be 30 to 40%.

Figure 3. Common carotenoids.

The level of carotenoids is often directly related to the quality of food. The lack of pigments as in white butter or salmon, for example, is a quality defect (27). However, flour is bleached to destroy the carotenoids, and /S-carotene is often removed from red palm oil to make it colorless.

The acyclic pigments phytoene, phytofluene, zeta-carotene, neurosporene, and lycopene are often found in carotenoid-producing systems such as higher and lower plants. They are not often found in animal tissue. Of these, lycopene is the most common and may occur in large amounts in the tomato, watermelon, orange, pink grapefruit, and some apricots. Further reactions in products of lycopene lead to the closing of one or both rings. However in some oranges (Valencia) 3,4-didehydrolycopene is found. There have been a number of acyclic xanthophylls isolated mainly from the purple photosynthetic bacteria. The 1,2-

epoxide has been isolated from tomatoes as a minor pigment. Because phytofluene is fluorescent, it must be accounted for in the fluorometric analysis of vitamin A (25).

The alicyclic carotenoids (eg, ^-carotene) are very common in higher plants, bacteria, fungi, algae, and animals. It has been estimated, on the basis of structure, that some 50 to 60 alicyclic compounds have potential vitamin A activity. Apricots are an excellent source of p-carotene (60%), whereas the level in tomatoes (10%) and peaches (10%) is much lower. Papaya, if it is yellow/orange, is a good source, whereas the red flesh papaya contains mainly lycopene. Where the flesh is green, some plastid pigments would be present and thus some /9-carotene would be present. Red grapes, pears, figs, red apples, and beet root, although brightly colored, are not good sources of /^-carotene.

¿-Carotene has been isolated from certain tomato varieties where apo-polycis-isomers of p, a, and e-carotene, and lycopene have been found.

Vegetables may be excellent sources of the provitamin A carotenoids. Spinach may have up to 4.0 mg/100 g [i-carotene and carrots may have as high as 6.0 mg/100 g of /(-carotene. Yellow corn (maize) contains small amounts of j?-carotene and /(-zeacarotene. Sweet potatoes may be purple, white, or bright orange. The two former potatoes are low in carotenoids and the latter may be greater than 9 mg/100 g. Squash, broccoli, peas, and pumpkin contain relatively large amounts of carotenes. Most vegetable oils are slightly yellow indicating a small amount of carotenoids. The red palm oil produced in Brazil, Southeast Asia, and parts of Africa may contain large amounts of/?- and a-carotene (50 mg/100 g total carotenoids). Egg yolks are a rich source of carotenoids (lutein and zeaxanthin) but relatively poor sources of /i-carotene. Green peppers are a better source of carotene than are the red peppers. Wheat flour is low in carotene and this amount is further lowered in the bleaching process.

Hydroxylated carotenoids are very common in plants, where lutein and zeaxanthin are to a lesser extent, fi-cryptoxanthin are major pigments. Lutein is a common pigment in freshwater fish, whereas tunaxanthin (3,3'-dihydroxy e-carotene) is common in marine fish. Peaches are a good source of /?-cryptoxanthin. Many fruits contain xanthophyll esters. In the persimmon the carotenoid alcohol is /(-cryptoxanthin. Corn varieties are selected for their high content of zeaxanthin and lutein. These pigments are fed in order to pigment the skin and egg yolks.

The xanthophylls with hydroxyl or keto substitution in the four position are common in various animal tissues. Echinenone (4-keto-/?-carotene) is widely found in marine invertebrates and algae and canthaxanthin (4,4'-keto-/?-carotene). The usual epoxides are in the five, six position and these are easily converted to the 5,8-epoxide with acid. Where a keto group is in the four position, in-chain epoxides are formed (eg, 9,10-epoxides of canthaxanthin). The 1,2-epoxides of acylic pigments have been reported in tomato leaves. The allenic seaweed pigment fucoxanthin is thin. The cyclopentyl ketones are found in red peppers and are probably formed by a pinacolic rearrangement of the epoxides of zeaxanthin.

The acetylenic carotenoids are found in diatoms, Eu-glena, giant scallops, mussels, and starfish. A number of apo-carotenoids are formed in some fruit, bacteria, orange peel, and fungi. The most well known apo-compound is bixin, the pigment in annatto seeds. Crocin is an apo-compound occurring in saffron. In a sense, retinol can be considered a degraded ^-carotene. The carotenoid acid, to-rularhodin, is a common pigment in Rhodotorula yeast. Canthaxanthin is found in edible mushrooms, blue-green algae, trout, brine shrimp, and flamingos.

Astaxanthin is the pigment of the salmon and the invertebrates, annelida and crustacea. Recently it has been isolated from red yeast and Adonis flowers (28). Shrimp raised in intensive culture are often blue because of a protein complex with astaxanthin. This complex extends the chromophore resulting in a blue color. This has been mistakenly described as a disease, the condition, however, re sults in a lower price for the shrimp, particularly on the Japanese market. When cooked, the complex is broken and the red color is seen. The lobster can be green, black, red, or blue because of various protein-astaxanthin complexes all of which are broken with heat, yielding a red pigment. When treated with alkali, astaxanthin is converted to as-tacene.

Aromatic carotenoids have recently been isolated from photosynthetic bacteria and some sea sponges. Carotenoid epoxides are fairly common in nature. The so-called vio-laxanthin cycle is an example of the natural occurrence of epoxidation.

zeaxanthin ^ antheraxanthin ^ violaxanthin

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