Colouring principles

AUevi, P et al.. Synthesis of carminic acid, the colourant principle of cochineal, J. Chem. Soc. Perkin Trans., 1, 575, 1998. 

The important dyeware known as cochineal consists of the dried female of an insect, the Coccus cacti coccinellifera, which lives on certain species of cactus. The colouring principle of cochineal is glucoside carminic acid, of the formula CnHigOjo [77]. 

There are few examples nowadays of the classical chromatographic detection method based on visible colour, since the sensitivity is low and other approaches provide the necessary quantitative accuracy. The visible-colour principle, however, remains common in thin-layer chromatographic analysis (TLC Section 4.19). Elution of amino acids and peptides from liquid-chromatographic columns can be monitored using a short-wavelength light source (A = 214 nm commonly reliable lamps for A = 200 nm are becoming more widely used) and UV detectors. 

Identification and Assignment of Structure The identity of a synthetic product is often established by comparison of its absorption curve with that of the natural product or another standard sample. The absorption curves of synthetic and natural vitamin A2 are shown in Figure 7.1. The identification pf coloured substances is easier since colour can almost always be correlated with the presence of certain types of chromo-phoric groups. Thus, visible absorption spectra are employed for the detection and identification of the natural pigments (e.g. carotenoids, anthocyanins md porphyrins), dyes and other colouring matters. Hie absorption spectra of these colouring principles will be discussed separately in Chapter 12. 

Haemoglobin, which is the colouring principle of human venous blood, shows absorption bands at about 430 and 560 mu. When haemoglobin... 

Carminic acid, the active colourant principle of cochineal, is employed in the form of its derivatives, notably, the aluminium chelate which is obtained by semi-synthesis. 

The structures are shown of cis-bixin (18 R = Me), norbixin (18 R = H) and crocetin (19), the parent acid of the colourant principle of saffron, sometimes described as a natural substance having a secondary colouring effect, which exists as the bis-gentibiose ester, crocin, although crocetin and crocin do not have E numbers and are... 

The colourant principle of cochineal is carminic acid (30) which is used as carmine in the form of its aluminium and other metallic chelates (calcium-aluminium) in much the same way that alizarin was formerly employed as a mordant dye by chelation of the 1,2-dihydroxy system. 

Saffron, from the dried stigma of Crocus sativa, the colourant principle of which is crocetin is produced in several sub-tropical countries, for example. North Africa, Spain, France. It is an expensive yet quite stable material although not a permitted colourant with an E number. 

Cochineal extract (Colour Index 75470, E 120)is the final alcohol-free material obtained after aqueous ethanolic extraction of the dried bodies of the female scale insect Dactylopius coccus Costa which lives on cacti such as Opuntia or Nopalea coccinellifera a species indigenous to Peru and Mexico, although also found in the Canary Islands. In practice a simpler procedure consists of aqueous alkaline extraction. The colourant principle is carminic acid which is more well-known in the form of carmine, an aluminium chelate of carminic acid, a material insoluble in water and stable on the acidic side. A uv spectrophotometric study has been made (ref. 21). Thin layer chromatography of cochineal has been examined on acetylated cellulose with the solvent system, ethyl acetate-tetrahydrofuran-water (6 35 47) in which the Rf was 0.94 (refs.22,23). The history of the chemistry of cochineal has been discussed (ref.24). 

Curcumin is the colourant principle of turmeric (Colour Index 75300, E 100) and tumeric oleoresin. The dried and ground powder from the rhizome or root of Curcuma longa of the Zingiberaceae family... 

In a discussion of archil that is, orchil q.v.) a lichen dye - Salter (1869) states that Soluble in water and alcohol, this colouring principle yields by precipitation with chloride of calcium a compound known as Sohd French Purple . Bersch (1901) adds to this, remarking that French Purple is a compound of the archil colouring matter with hme. .. a garnet red precipitate is produced . 

Natural dyestuffs have been structured according to source, with divisions according to plant, lichen, scale insect and shellfish. To derive a list of dye compounds, species identified in the hterature were collated and the colouring principles determined from scientific sources (primarily Schweppe, 1992 and Harbome and Baxter, 1999). These were used for the development of the primary compounds listing (that is, not all known dyestuffs have been included, only those where there is evidence for the particular source having been used in a pigment context). 

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