Assessment of colour

The colour of a soft drink or fruit juice may be assessed in a number of ways. If the product is clear it can simply be carried out by measuring the absorbance of the product at one or more wavelengths. The actual values chosen will depend on the particular colour of the product. For a yellow product, such as apple juice, wavelengths of 465, 430 or 420 ntn are often chosen to assess the colour. These values can then be expressed in European brewing convention (EBC) units by multiplication by a factor of 25. The actual Brix value chosen to assess the colour depends on the country however, levels between 11 and 12 are often taken as the norm. If dealing with a red-coloured product, then the assessment is generally carried out at 520 nm. Absorbance values are sometimes also taken at 420 nm in red or black juices to assess the brownness of the product. The two absorbance values are often used to express a colour ratio, which gives an indication of colour versus brownness  

Sometimes the absorbance is also assessed at 580 nm and a ratio calculated for the values at 520 and 580 nm, which is called the blue index. With the red/black juices, where the product may have to be diluted, the intensity of the colour is assessed by dilution in an acidic buffer to ensure that the true colour intensity is recorded. If this is not done the colour can be reduced by the shift in pH affecting the apparent colour of the product. The analysis of the colour in juices is detailed in IFU Handbook method No. 78. 

In cloudy products, colour assessment is more difficult. The particulate material can be removed by filtration in some cases. However, this does not solve the problem in all cases as some colours, particularly the carotenoids and some anthocyanin pigments, are absorbed on to the particulate material/filters and their removal gives a low colour reading. In addition, the particulate material affects the opacity of the product, which affects the visual perception of the colour. It has been known for a number of years that the simple approach of measurement of a product s absorbance values at 420 or 520 nm does not fully address this issue because some products can have similar absorbance values at these wavelengths but will be perceived visually as very different. 

The different instruments (Hunter, Gardner, Instrumental Colour Systems, etc.) process absorbance and reflectance data in slightly different ways, which means that the values obtained can differ slightly from one instrument manufacturer to another. Therefore, a product s defined colour has to be qualified with a statement indicating the instrument used. Notwithstanding this limitation, it is not uncommon to find a tristimulus colour meter in a manufacturer s quality assurance laboratory so that routine quantitative assessment of a product s colour can be made. This is particularly tine for tomato-based products, whose nature makes conventional spectrophotometric assessment meaningless. 

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. 

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Use is made of colour changes resulting from reaction of pollutant and chemical reagents colour intensity indicates concentration of pollutant in the sample. Reaction can take place in solution or on solid supports in tubes or on paper strips, e.g. litmus or indicator paper. Quantitative assessment of colour formation can also be determined using visible spectroscopy. Instruments are calibrated... 

BS (1967). Standard specification for artificial daylight for the assessment of colour. Part 1 Illuminant for colour matching and colour appraisal. British Standard 950 parti. 

Terinte, N., Manda, B.M.K., Taylor, J., Schuster, K.C., Patel, M.K., 2014. Environmental assessment of coloured fabrics and opportunities for value creation spin-dyeing versus conventional dyeing of modal fabrics. Journal of Cleaner Production 72, 127—138. 

When colour is assessed on the basis of reflectance measurements, it is common to consider the three relevant attributes of perception of colour as hue, chroma (or saturation) which is the colourfulness or richness of the colour, and lightness, which refers to the amount of reflected light. These three attributes may be described using the concept of colour space, which shows the relationships of colours to one another and which illustrates the three-dimensional nature of colour, as portrayed in Figure... 

Cohne AJ, Roe FJC. 1991. Review of lead toxicology relevant to the safety assessment of lead acetate as a hair colouring. Fd Chem Toxic 29(7) 485-507. 

Vicki Barwick obtained a first degree in Chemistry from the University of Nottingham. She then joined the Laboratory of the Government Chemist (which became LGC in 1996) as an analyst in the Consumer Safety Group. Vicki was involved with a number of projects to assess the safety of consumer products, including developing test methods for the identification of colourants in cosmetics and the quantitation of phthalate plasticizers in child-care items. 

US Food and Drug Administration (1982) Toxicological principles for the safety assessment of direct food additives and colour additives used in food. US FDA, Rockville, pp 1-130... 

ISO 105-X10 Textiles - Tests for colour fastness - Part XIO Assessment of migration of textile colours into polyvinyl chloride coatings ISO 264 1976 Unplasticized polyvinyl chloride (PVC) fittings with plain sockets for pipes under pressure - Laying lengths - Metric series ISO 265-1 1988 Pipes and fittings of plastics materials - Fittings for domestic and industrial waste pipes - Basic dimensions Metric series - Part 1 Unplasticized poly(vinyl chloride) (PVC-U)... 

Scbramm, W. Jantscbgi, J. Comparative assessment of textile dyeing technologies from a preventive environmental protection point of view. J. Soc. Dyers Colour 1999, 115, 130-135. Naylor, C.G. Environmental fate and safety of nonylphenol etboxylates. Text. Chem. Color. 1995,... 

Gonnet, J.-F., Colour effects of co-pigmentation of anthocyanin revisited — 3. A further description using CIELAB differences and assessment of matched colours using the CMC model. Food Chem., 75, 473, 2001. 

Above all, colour is a major parameter in the assessment of quality, serving at the time of production to standardise the product. Colour deterioration can provide useful information on any quality changes during storage due to temperature fluctuations or microbial spoilage effects, for example. 

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