Pigments natural

The natural iron oxide pigments are termed the ochres which are yellow and contain goethite (10-50%) as the Fe oxide constituent, the reds, with a high content of hematite, the medium to dark yellow siennas, the umbers and the blacks, which consist of magnetite (Benbow, 1989 Buxbaum Printzen, 1993). 

Considerable effort is needed to transform natural ochres into commercially acceptable pigments (Bee, 1986). In Southern France, for example, the ochre is mined using mechanical shovels and then washed, decanted, dried and ground. This extracted material is separated from the associated clay and sand with an electric cen- 

2) Ochra (latin) from ochros (greek) = pale yellow, although there are also red ochres. In the 3 millenium BC, the dead, in some societies, 

A further important application of these pigments is in paints. The natural red iron oxides are also used in primers for steel structures and cars, for marine coatings and for anti-fouling paints. In the USA, the metallic browns are used for these purposes. The level of soluble salts in the latter pigments is low and this reduces corrosion problems. The metallic browns are also used in heat resistant enamels. 

MIO is nontoxic, it is more environmentally acceptable than the lead and chromate based paints which are used to provide chemical protection in other anticorrosion paints and primers (Etzrodt, 1933). The highest quality (with respect to the flatness of the particles, aspect ratio, etc.) MIO comes from the mine at Waldenstein in Austria (Producer Karntner Montanindustrie), but there are also mines in Spain, South Africa and Western Australia. The supply of highest quality MIO, which used to be mined in Devon in England, was exhausted during the 1970s. This was one of the factors which prompted the U.K. workers to seek a method of making synthetic MIO (Carter, 1988 see Chapter 20). 

Kirchner (1978) has discussed the utility of thin-layer chromatography for the following natural pigments chlorophylls, carotenoids, xanthophylls, flavonoids, anthocyanins, porphyrins, and bile. From the standpoint of practical TLC, the most important of these pigments are the chlorophylls, carotenoids, xanthophylls, and anthocyanins, and it is these pigments that are considered further in this chapter. For practical TLC of the flavonoids, consult Harbome (1984, 1992). For information on the TLC of porphyrins, see Doss (1972), Dolphin (1983), and Jacob (1992). Jain (1996) has provided useful information on the examination of porphyrins (in studies on clinical porphyrias) by TLC in clinical chemistry. 

Practical TLC studies on bile pigments have been discussed by Scott (1969) and Dolphin (1983). Heirwegh et al. (1989) reviewed analytical procedures, including TLC, for the separation and identification of bile pigments including the biliverdins and bilirubins. 

Isaksen (1991) provided an extensive review on the TLC of natural pigments. He stated that TLC is the most common method used to analyze natural pigments. Pigments considered by Isaksen include the flavonoids, anthocyanins, carotenoids, chlorophylls, chlorophyll derivatives, and porphyrins. Optimal chromatographic systems (i.e., sorbents and mobile phases for each pigment group) are considered in detail in the review. The Isaksen (1991) review was revised and updated by Andersen and Francis (1996). 

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Davies and Kost (1988) noted that TLC of carotenoids is one of the most modem methods of earotenoid separation. TLC of carotenoids has been done on a variety of layers using many different solvent systems. Davies and Kost (1988) provided an extensive list of tables on the TLC of carotenoids. Cikalo et al. (1992) used automated multiple development (AMD) techniques for the separation of complex plant carotenoid mixtures they also evaluated the use of scanning densitometry for spectral identification and quantification of these carotenoids. 

This section examines the analysis of two major classes of compounds  

A description is given of a discovery in 1833 by Bartolomeo Bizio, a Venetian chemist, of the origin and chemical properties of Tyrian Purple. This was subsequently shown 

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Porphyrins and chlorophylls are the most widespread natural pigments. They are associated with the energy-converting processes of respiration and photosynthesis in living organisms, and the synthesis of specific porphyrin derivatives is often motivated by the desire to perform similar processes in the test tube. The structurally and biosynthetically related corrins (e.g. vitamin B,j) catalyze alkylations and rearrangements of carbon skeletons via organocobalt intermediates. The biosyntheses of these chromophores are also of topical interest. 

Carotenoids are natural pigments characterized by a tail to tail linkage between two C20 units and an extended conjugated system of double bonds They are the most widely dis tributed of the substances that give color to our world and occur m flowers fruits plants insects and animals It has been estimated that biosynthesis from acetate produces approximately a hundred million tons of carotenoids per year The most familiar carotenoids are lycopene and (3 carotene pigments found m numerous plants and easily isolable from npe tomatoes and carrots respectively... 

During storage, natural pigments often deteriorate with time because of exposure to light, heat, air, and moisture or because of interaction of the components of the food with each other or with the packaging material. The color of maraschino cherries, for example, fares so poorly with storage that they are routinely bleached then artificially colored. 

Phthalocyanines are analogues of the natural pigments chlorophyll and heme. However, unlike these natural pigments, which have extremely poor stabihty, phthalocyanines are probably the most stable of all the colorants ia use today. Substituents can extend the absorption to longer wavelengths, iato the near iafrared, but not to shorter wavelengths, and so their hues are restricted to blue and green. 

The carotenoids are the most widely distributed group of pigments, occur naturally in large quantities, and are known for their structural diversity and various functions. The carotenoids constitnte a widespread class of natural pigments that occur in all three domains of life in the eubacteria, the archea, and the eucarya. Carotenoids are ubiquitous organic molecules, but they are not produced by the human body. They have been fonnd to be essential to human health based on the nutritional understanding of vitamin A (retinol) and (i-carotene. ... 

Delgado-Vargas, R, Jimenez, A.R., and Paredes-Lopez O., Natural pigments carotenoids, anthocyanins, and betalains — characteristics, biosynthesis, processing, and stability, Crit. Rev. Food Sci. Nutr., 40, 173, 2000. 

Britton, G., The Biochemistry of Natural Pigments, Cambridge University Press, Cambridge, 1983. 

Delgado-Vargas, F. and Paredes-Lopez, O., Natural pigments global perspective, in Natural Colorants for Pood and Nutraceutical Uses, CRC Press, Boca Raton, FL, 2003, chap. 6. 

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