Is

where a is a constant of proportionality known as the absorptivity, c is the chromophore concentration, and l is the sample path length. The linear relationship between absorbance and concentration holds over the chromophore concentrations used in most food emulsions. The color intensity of a particular chromophore therefore depends on its concentration and absorptivity.

The appearance of an emulsion to the human eye is determined by the interactions between it and electromagnetic radiation in the visible region (Francis and Clydesdale 1975, Hutchings 1994). It is therefore important to measure the absorbance over the whole range of visible wavelengths (390 to 750 nm). A plot of absorbance versus wavelength is referred to as an absorption spectrum (Figure 9.11). An absorption peak occurs at a wavelength which depends on the difference between the energy levels of the electronic transitions in the chro-mophores (^ = ch/AE). Absorption peaks are fairly broad in the visible region because transitions occur between the different vibrational and rotational energy levels within the electronic energy levels and because of interactions between neighboring molecules (Penner 1994b).

9.3.1.3. Scattering

Scattering is the process whereby a wave which is incident upon a particle is directed into directions which are different from that of the incident wave (Farinato and Rowell 1983, Hiemenz 1986). The extent of light scattering by an emulsion is determined mainly by the relationship between the droplet size and wavelength and by the difference in the refractive index between the droplets and the surrounding liquid.

FIGURE 9.11 Absorption spectrum of colored liquids.

Light scattering causes a reduction in the intensity of an electromagnetic wave as it passes through an emulsion, which is characterized by the transmittance (Farinato and Rowell 1983):

where T is the turbidity of the emulsion. In dilute emulsions (<0.05%), the turbidity is proportional to the droplet concentration, but in concentrated emulsions, the turbidity falls below the expected value because of multiple scattering effects (Ma et al. 1990).

Turbidity is a desirable attribute of many food emulsions because it imparts a natural-looking character and appeal (Hernandez and Baker 1991, Hernandez et al. 1991). For example, low concentrations of oil droplets consisting of vegetable or flavor oils are used to provide a turbid appearance to many types of fruit beverage (Tan 1990, Dickinson 1994). The turbidity of these emulsions depends principally on the size, concentration, and relative refractive index of the droplets.

The turbidity of an emulsion is related to the characteristics of the droplets which it contains by the following relationship (Hernandez and Baker 1991):

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