Natural colours: analysis

Lloyd, A.G., Extraction and chemistry of cochineal. Food Chem., 5, 91,1980. Yamada, S. et al.. Analysis of natural colouring matters in food (IV). Methylation of cochineal colour with diazomethane for analysis of food products, J. Agric. Food Chem., 41, 1071, 1993. 

Numerous CE separations have been published for synthetic colours, sweeteners and preservatives (Frazier et al., 2000a Sadecka and Polonsky, 2000 Frazier et al., 2000b). A rapid CZE separation with diode array detection for six common synthetic food dyes in beverages, jellies and symps was described by Perez-Urquiza and Beltran (2000). Kuo et al. (1998) separated eight colours within 10 minutes using a pH 9.5 borax-NaOH buffer containing 5 mM /3-cyclodextrin. This latter method was suitable for separation of synthetic food colours in ice-cream bars and fmit soda drinks with very limited sample preparation. However the procedure was not validated for quantitative analysis. A review of natural colours and pigments analysis was made by Watanabe and Terabe (2000). Da Costa et al. (2000) reviewed the analysis of anthocyanin colours by CE and HPLC but concluded that the latter technique is more robust and applicable to complex sample types. Caramel type IV in soft drinks was identified and quantified by CE (Royle et al., 1998). 

See A Cappelli Behaviour to tintonal analysis of naturally coloured animal and vegetable fibres. Ann. Labor. Chtm. centrale GabeUe, Vol. VII, p. 213. 

When receiving natural colour products from the supplier it is normal to carry out a quality control. The extent of this control will typically depend on the type of colour product, the iirformation available on the certificate of analysis, the expected processing conditions, the finished product and the standard quality control procedures of the food or beverage manufacturer. In some cases the colour strength or colour hue and intensity may be the most relevant parameter. In others the microbiological standard and absence of pathogens is also important. For powder formulations to be used in dry blends, the particle size is... 

Scotter, M.J., 2011. Methods for the determination of European Union-permitted added natural colours in foods a review. Food Additives and Contaminants Part A Chemistry Analysis Control Exposure Risk Assessment. 28 527-596. 

Kamikura, M. and Nakazato, K., Natural yellow colours from gardenia fruit and colours found in commercial gardenia extract analysis of natural yellow colours by high performance liquid chromatography, J. Food Hygiene Soc. Japan, 26, 150,1984. 

Symmetrical cyanine dyes, because of the resonance shown in Figure 6.4 (in which the two contributing structures are exactly equivalent), are completely symmetrical molecules. X-ray crystal structure determinations and NMR spectroscopic analysis have demonstrated that the dyes are essentially planar and that the carbon-carbon bond lengths in the polymethine chain are uniform. The colour of cyanine dyes depends mainly on the nature of the terminal groups and on the length of the polymethine chain. The bathochromicity of the dyes is found to increase... 

According to the predominant component, the binders are usually divided into protein, oil, polysaccharide, and resin binders. In this section we shall focus on protein binders but it is worth mentioning that in the majority of natural non-protein binders a minority protein component is usually present as well. Thus many of the analytical techniques described here can be (with certain limitations) applied to them as well. Although in colour layers of artworks and particularly in paintings protein binders are relatively abundant (up to 10%), their identification is often limited by a small amount of sample that is usually available for analysis (tens or hundreds of micrograms at most [6]). 

Identification and quantification of natural dyes need high performance analytical techniques, appropriate for the analysis of materials of complicated matrices containing a small amount of coloured substances. This requirement perfectly fits coupling of modern separation modules (usually high performance liquid chromatography in reversed phase mode, RPLC, but also capillary electrophoresis, CE) with selective detection units (mainly mass spectrometer). 

Elaboration of the method for the identification of colour compounds by RPLC MS should comprise four steps (1) spectral characterization of reference materials (standards) and subsequent optimization of detection parameters, as well as those of their chromatographic separation (2) analysis of natural dyestuffs used as colouring materials in historical objects (3) analysis of model samples (dyed fibres, paintings) prepared according to old recipes (4) application of the acquired knowledge to identification of colourants present in historical objects. 

Extraction or rather leaching of colourants is the first and a very important step in the analytical procedure for the analysis of natural dyes. The chemical composition of extracts from historical materials depends on many factors, such as the source of natural dyes, the technological procedure of their production, storage conditions over the centuries, ageing processes and extraction conditions. The choice of the extraction method depends on the properties of the components and the matrix from which they are isolated, as well as on the mechanism of dyeing with the particular dyestuff. In this regard, they are usually divided into three groups direct, vat and mordant dyestuffs. 

Analysis of dyed fibres allows identification of real colouring components of natural dyestuffs taking part in the dyeing process. Wool threads dyed with madder (Rubia tinc-torum) as well as Our Lady s bedstraw (Galium verum), were studied by HPLC DAD ESI MS" (SIM mode).[8] Chromatograms of the extracts from wool dyed with madder... 

In the contemporary investigation of artworks and especially in the identification of natural organic dyestuffs the applicability of HPLC MS cannot be questioned. This technique allows recognition of almost all common colourants in one run , which decreases the probability of losing specific information (Table 13.3). In comparison with GC-MS, HPLC MS has wider application, as it is not limited by the presence of polar and nonvolatile compounds, and therefore it usually does not require the derivatization step. The number of published papers, which has doubled in the last 3 years in comparison with the period 2000 2004, proves that HPLC-MS performs a pivotal role in the analysis of the colourants discussed. 

The last example of ToF-SIMS analysis of natural fibres is of a structural characterization of wood species for an eventual dendrochronological study [Saito et al. 2008], The aim of this research was to develop a new method to differentiate heartwood and sapwood. In dendrochronology, when bark is not present on the samples, the presence of sapwood is the key to determining felling date. Usually, heartwood and sapwood can easily been differentiated by their colour, with heartwood being much darker. Nevertheless, in the case of... 

In his Elements of the Art of Assaying Metals, Johann Andreas Cramer pointed out the close association of bismuth with arsenic and cobalt. Every ore of Bismuth, said he, as is shewn by the chemical analysis, is reduced to the State of Ore by Arsenide For this goes out of it by Sublimation. You find m the same Ore that Kind of Earth that gives an azure Colour to Glasses, of which we have already spoken in the Article of Cobalt. Whence it is evident that the Ore of Bismuth may widiout Impropriety be called Cobalt of Bismuth The more, because you will find in any ore of Bismuth the same Principles as in Cobalt, only in a different Propoition (55). This close association of bismuth and cobalt m nature made it difficult for eaily chemists to distinguish between them (56). 

The discussion above has addressed the assessment of a product s colour or perceived colour in basic terms. In the next two sections, methods to determine which coloured compounds are present in a product will be addressed. For the purpose of this chapter, the section on synthetic dyes will cover the analysis of the water-soluble dyes, or so-called coal tar dyes, and the section on natural pigments will cover the anthocyanin pigments, such as grape skin extracts, and the carotenoid-based materials, even if they are of synthetic origin. 

The physical and chemical characters of margarine, after melting and filtering, are naturally related to the fatty substances used in its preparation. For its analysis, use is made of the methods indicated for the various oils and fats (see Fatty Substances, Vol. I), and its distinction from butter is effected by the methods described for butter. Further, the methods there given are used for testing for various extraneous matters (flour, mineral substances), preservatives (salicylic acid, boric acid, etc.) and colouring matters. 

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