Food colours, separation

Masar, M., Kaniansky, D., and Madajova, V., Separation of synthetic food colourants by capillary zone electrophoresis in a hydrodynamically closed separation compartment, J. Chromatogr. A, 724, 327, 1996. 

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). 

Another RP-HPLC technique has been applied for the determination of synthetic food dyes in soft drinks with a minimal clean-up. Separation of dyes was obtained in an ODS column (150 x 4 mm i.d. particle size 5 pm). Solvents A and B were methanol and 40 mM aqueous ammonium acetate (pH = 5), respectively. Gradient conditions were 0-3 min, 10 per cent A 3-5 min, to 25 per cent A 5-8 min, 25 per cent A 8-18 min, to 75 per cent A 18-20 min, 75 per cent A. The flow rate was 1 ml/min and dyes were detected at 414 nm. The separation of synthetic dyes achieved by the method is shown in Fig. 3.35. The concentrations of dyes found in commercial samples are compiled in Table 3.21. The quantification limit depended markedly on the type of dye, being the highest for E-104 (4.0 mg/1) and the lowest for E-102 and E-110 (1.0 mg/1). The detection limit ranged from 0.3 mg/1 (E-102 and E-110) to 1.0 mg/ml (E-104 and E-124). It was suggested that the method can be applied for the screening of food colourants in quality control laboratories [113]. 

The same technique has been employed for the purification of Food Colour Red No. 106 (Acid red). The chemical structure of the dye is shown in Fig. 3.118. The separation process was controlled by analytical RP-HPLC carried out in an ODS column (150 X 4.6 mm i.d. particle size 5 /.an). The mobile phase consisted of ACN-0.01 M TFA (27 73, v/v). The flow rate was 1 ml/min and analytes were detected at 254 nm. Equilibrated n-butanol and water were employed for CCC separation. OP was acidified with 40 mM of sulphuric acid, and 30 mM of aqueous ammonia was added to the LP. The coil was rotated at 800 rpm and LP was pumped at a flow rate of 1 ml/min. Fractions of 1 ml volume were taken from the effluent. The CCC profile of Food Colour Red No. 106 in CCC is shown... 

Fig. 3.119. Separation of the components of Food Colour Red No. 106 by pH-zone-refining CCC. For conditions see text. Reprinted with permission from H. Oka et al. [173],...

Fig. 3.144. Electropherograms of synthetic food colourants in two different ice cream bars (a,b), grape soda (c) and mango soda (d). Separation solution, 0.025 M borax-NaOH buffer containing 5 mM /4cyclodextrin, pH 9.5 detection wavelength, 200 nm. Reprinted with permission from K.-L. Kuo et al. [187].

In 1935, the Committee was renamed the Analytical Methods Committee (AMC) but the main analytical work was carried out by sub-committees composed of analysts with specialised knowledge of the particular application area. The earliest topics selected for study were milk products, essential oils, soap and the determination of metals in food colourants. Later applications included the determination of fluorine, crude fibre, total solids in tomato products, trade effluents and trace elements, and vitamins in animal feeding stuffs. These later topics led to the publication of standard methods in a separate booklet. All standard and recommended methods were collated and published in a volume entitled Bibliography of Standard, Tentative and Recommended or Recognised Methods of Analysis in 1951. This bibliography was expanded to include full details of the method under the title Official, Standardised and Recommended Methods of Analysis in 1976 with a second edition in 1983 and a third edition in 1994. 

Jaworska, M. Szulinska, Z. Wilk, M. Anuszewska, E. Separation of synthetic food colourants in the mixed micellar system. Appheation to pharmaceutical analysis. J. Chromatogr., A 2005,1081,42-47. 

At some point in the near future you should watch the video entitled Thin-layer chromatography in use an application from the food industry in the multimedia activity Practical techniques on the Experimental techniques CD-ROM that accompanies this book. There you will see an experiment on the separation of food colourings. At various times you will be asked to take notes or make measurements from the screen, so you should make sure that you have an experiment notebook and pen to hand. This activity should take about... 

Fig. 6.1 Separation of colours extracted from a dry dog food (poultry, cereals, green...

There is a recent trend towards simultaneous CE separations of several classes of food additives. This has so far been applied to soft drinks and preserved fruits, but could also be used for other food products. An MEKC method was published (Lin et al., 2000) for simultaneous separation of intense sweeteners (dulcin, aspartame, saccharin and acesulfame K) and some preservatives (sorbic and benzoic acids, sodium dehydroacetate, methyl-, ethyl-, propyl- and isopropyl- p-hydroxybenzoates) in preserved fruits. Ion pair extraction and SPE cleanup were used prior to CE analysis. The average recovery of these various additives was 90% with good within-laboratory reproducibility of results. Another procedure was described by Frazier et al. (2000b) for separation of intense sweeteners, preservatives and colours as well as caffeine and caramel in soft drinks. Using the MEKC mode, separation was obtained in 15 min. The aqueous phase was 20 mM carbonate buffer at pH 9.5 and the micellar phase was 62 mM sodium dodecyl sulphate. A diode array detector was used for quantification in the range 190-600 nm, and limits of quantification of 0.01 mg/1 per analyte were reported. The authors observed that their procedure requires further validation for quantitative analysis. 

Chlorophylls occur very frequently in the plant kingdom, they are responsible for the colour of vegetables and some fruits. They also occur in algae and several bacteria. Chlorophylls in plants are photoreceptors and in photosynthesis the presence of a closed circuit of conjugated double bonds allows them to absorb light. Because of their predominant importance as photoreceptors a considerable number of analytical methods have been developed for the separation and quantitative determination. The analytical methods applied for the measurement of chlorophylls and carotenoids in food products have been reviewed previously [273],... 

The simplest use of pervaporation is for the separation and subsequent determination of a single analyte as a result, most reported methods using this technique involve individual determinations. The determination of acetaldehyde in semi-solid and solid food samples is one salient example of single-analyte determinations where pervaporation avoids time-consuming sample preparation steps such as filtration, removal of dyestuffs and turbidity from the Carrez solution, and centrifugation [175]. One other example is the determination of trimethylamine (an objective parameter for fish quality evaluation that correlates well with sensory evaluation) [176]. The method is based on pervaporation of the analyte and monitoring of the colour change in Bromothymol Blue caused by the basic character of the amine the results are consistent with those provided by the official method for trimethylamine. 

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