COLOR ANALYSIS

V. COLOR ANALYSIS

Color analysis may be performed through either one of two approaches: the chemical quantification of the colorant compounds or the assessment of the resulting color, the latter being conducted either instrumentally or with human observers. The impetus underlying these two approaches differs greatly, however. In the former case the desire is to closely monitor the concentration of the color additive used (especially in the case of the certified colorants) in order to insure the health and safety of the consumer populace. Though all of the synthetic colorants have been subject to thorough toxicological assessment (and many show minimal toxicity), high concentrations of certain colorants (e.g., FD&C Red No. 40 and FD&C Yellow No. 5) may cause adverse reactions in some persons (Davidson, Color analysis may be performed through either one of two approaches: the chemical quantification of the colorant compounds or the assessment of the resulting color, the latter being conducted either instrumentally or with human observers. The impetus underlying these two approaches differs greatly, however. In the former case the desire is to closely monitor the concentration of the color additive used (especially in the case of the certified colorants) in order to insure the health and safety of the consumer populace. Though all of the synthetic colorants have been subject to thorough toxicological assessment (and many show minimal toxicity), high concentrations of certain colorants (e.g., FD&C Red No. 40 and FD&C Yellow No. 5) may cause adverse reactions in some persons (Davidson,

A. Chemical Analysis

The methods for analyzing the individual colorants follow standard organic analysis tech- niques, the development of more sophisticated techniques having paralleled the develop- ment of new analytical instrumentation (Yeransian et al., 1985). While titrametric and gravimetric methods are allowed for determining pure dye content of color additives (Bell, 1990), spectrophotometric methods have been listed in the AOAC Official Methods of Analysis since 1960. The speed, ease, and efficacy of the spectrophotometric methods make them of particular value, as does their minimal (on the order of ng) sample require- ment (Marmion, 1979).

Spectrophotometric methods often prove inadequate, however, in the analysis of real samples due to the overlapping of spectral absorption maxima (Capita´n-Vallvey et al., 1997). These separation difficulties have been surmounted through the use of specific chromatographic (Puttemans et al., 1981; Maslowska, 1985; Patel et al., 1986; Karovicova´ et al., 1991; Oka et al., 1994), electroanalytical (Fogg et al., 1986; Ni et al., 1996), or absorption (Capita´n et al., 1996) procedures and through the use of multivariate-calibration techniques (e.g., partial least squares regression analysis) (Ni et al., 1996; Capita´n-Vallvey et al., 1997). High performance liquid chromatography (HPLC) in particular has received much attention for the separation of colorants (Puttemans et al., 1981); Gennaro, Abrigo, and Cipolla have reviewed the use of HPLC in the identification and determination of dyes and their impurities (Gennaro et al., 1994).

It is often the case, however, that isolating colorants from the sample matrix is more problematic to the chemist than is the problem of resolving individual colorant components (Greenway et al., 1992). Matrix isolation techniques have typically depended upon one of three general methodologies: leaching, solvent–solvent extraction, or active substrate absorption (Marmion, 1979); the presence of high-affinity binding agents such as proteins, however, necessitates removal of the interfering matrix (e.g., through precipitation) (Putte- mans et al., 1984). As Marmion (1979) notes, no one method is applicable to all sample matrices, thus the chemist requires comprehensive knowledge to optimize conditions on

a sample-specific basis.

B. Visual Colorimetry

Quantification of colorants does not, it should be emphasized, serve as a specification of the color. As Little and Mackinney (1969) noted, ‘‘the measurement of the light-modifying properties of an object does not qualify as a measurement of color.’’ Color is an exclu- sively human perceptual phenomenon; as such any method for assessing color depends at some point upon human response.

Visual assessments of color have typically depended upon the comparison of the sample color to that of reference standards or to a color atlas (Billmeyer, 1988). Fortu- nately for that segment of the population devoid of color vision deficits, color is perceived in a far more uniform manner than are other sensations such as taste or aroma (Clydesdale, 1977). Biases can still affect the assessment, however, either through psychological prefer- ences or through lack of control of the viewing conditions (Mabon, 1993). The latter is

Munsell Book of Color [see Billmeyer (1987) for a survey of the common color order systems], which presupposes that a standard illuminant will be used for viewing (Bill- meyer, 1988). Billmeyer (1988) notes that while color comparison represents a straightfor- ward enough task, the difficulty of control may preclude accurate color assessment.

C. Instrumental Colorimetry

The subjectivity of human response, coupled to the relative constancy of human color perception, make instrumental measures of color both desirable and feasible. These assess- ments depend upon rigorously defined color spaces, notably CIE-LAB and CIE-LUV. However, as Billmeyer (1988) cautions, these color spaces were derived for perceptual matching purposes and not for absolute color specification. A discussion of instrumental colorimetry being beyond the scope of this article, the reader is referred to Mackinney and Little (1962), Francis and Clydesdale (1975), or Hutchings (1994).