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Figure 1. Basic structure of the phycobilins. (a) is a bilin in the conventional linear form, (b) is the same structure in a cyclic form to show its relationship with the porphyrins, (c) is a blue phyco-cyanobilin pigment and (d) is a red phycoerythrobilin pigment.

attachment of the bilin chromophore to its protein is very stable due to covalent bonds between the ethylidene group on position 3 (see Colorants: chlorophylls, Fig. 2) of the bilin with a cysteine group in the protein. A second covalent bond may be on position 18. The major function of the bilin pigments is to act as light absorbers in the energy transport system.

Phycobilin preparations can be obtained by simply freeze-drying an algal cell suspension, producing a highly colored powder. The chlorophyll and carotenoid components can be removed by centrifuging after breaking the algal cells. The centrif├║gate is a brightly colored solution of water-soluble protein of which 40% may be phycobilin. Further purification by precipitation with ammonium sulphate and ion-exchange yields almost pure phycobilin. The free chromophore can be obtained by extended refluxing in methanol. An alternative to extensive purification would be to grow an organism such as Cyanidium caldarium, which, on addition of 5-aminolevulinic acid to the growth medium, yields pure phycocyanobilin (1). Another approach is to use the existing technology for growth of Spi-rulina platensis in open ponds. This organism has been used for eons in Africa and Mexico, probably because of its high protein content and digestibility. It is important in health food stores as a dietary supplement. The cells are simply harvested and dried. Another unicellular organism, the red algae Porphyridium omentum can also be grown in open ponds or in tubular reactors for production of phycobilin and phycoerythrin. A number of patents (2) exist for the extraction, stabilization, purification, and use of phycocyanins from Spirulina and Aphanotheca nidulans, and three firms are marketing the product.

The presence of a protein in the pigment suggests that the colorant would be used for products requiring a minimum of heat treatment. But the fact that it takes 16 hours in boiling methanol to separate the protein from the chromophore suggests that applications could include products with mild heat treatment. Suggested applications for phy-cocyanin colorants include chewing gums, frozen confections, soft drinks, dairy products, sweets, and ice cream. No patents have been filed for the use of phycoerythrin, but it seems like a good candidate for a red colorant.

There are several industrial applications for phycobilins such as fluorescent tracers in biochemical research, fluorescence-activated cell sorting, fluorescence microscopy, and so on (1). Existence of other markets will certainly help to obtain the critical volume necessary for production of food colorants. Colorants containing phycobilins are currently not permitted in the United States.

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