The first impression that a consumer usually has of a food emulsion is a result of its appearance (Francis and Clydesdale 1975, Hutchings 1994). Appearance therefore plays an important role in determining whether or not a consumer will purchase a particular product, as well as his or her perception of the quality once the product is consumed. A number of different characteristics contribute to the overall appearance of a food emulsion, including its opacity, color, and homogeneity. These characteristics are the result of interactions between light waves and the emulsion. The light which is incident upon an emulsion may be reflected, transmitted, scattered, absorbed, and refracted before being detected by the human eye (Francis and Clydesdale 1975, Farinato and Rowell 1983, Hutchings 1994, Francis 1995). A better understanding of the relationship between the appearance of emulsions and their composition and microstructure will aid in the design of foods with improved quality. This section highlights some of the most important factors which contribute to the overall appearance of emulsions.
9.3.1. Interaction of Light Waves with Emulsions 126.96.36.199. Transmission, Reflection, and Refraction
When an electromagnetic wave is incident upon a boundary between two homogeneous nonabsorbing materials, it is partly reflected and partly transmitted (or refracted) (Figure 9.9). The relative importance of these processes is determined by the refractive indices of the two materials, the surface topography, and the angle at which the light meets the surface (Hutchings 1994).
The reflection of an electromagnetic wave from a surface may be either specular or diffuse. Specular reflectance occurs when the angle of reflection is equal to the angle of incidence (<refiection = <incidence) and is the predominant form of reflection from optically smooth surfaces.
Specular Reflection Diffuse Reflection
FIGURE 9.10 Comparison of specular and diffuse reflectance.
Diffuse reflectance occurs when the light is reflected over many different angles and is the most important form of reflection from optically rough surfaces (Figure 9.10).
When the angle of incidence is perpendicular to the surface of a material, the fraction of light which is specularly reflected is given by the reflection coefficient (R)
where n is the relative refractive index (= n2/nj), and nl and n2 are the refractive indices of the two materials (Hutchings 1994). The greater the difference in refractive index between the two materials, the larger the fraction of light which is reflected. About 0.1% of light is reflected from an interface between oil (n0 = 1.43) and water (nW = 1.33), while about 2% is reflected from an interface between water and air (nA = 1). These reflection coefficients may seem small, but the total amount of energy reflected from a concentrated emulsion becomes significant because a light wave encounters a huge number of different droplets and is reflected from each one of them.
When a light wave encounters a smooth material at an angle, part of the wave is reflected at an angle equal to that of the incident wave, while the rest is refracted (transmitted) at an angle which is determined by the relative refractive indices of the two materials and the angle of incidence: sin(^refraction) = sin^ncidenJ/n.
Absorption is the process whereby a photon of electromagnetic energy is transferred to an atom or molecule (Atkins 1994, Penner 1994a, Pomeranz and Meloan 1994). The primary cause of absorption of electromagnetic radiation in the visible region is the transition of outer-shell electrons from lower to higher electronic energy levels. A photon is only absorbed when it has an energy which exactly corresponds to the difference between the energy levels involved in the transition, that is AE = hv = hc/X, where h is Planck's constant, v is the frequency of the electromagnetic wave, c is the velocity of the wave, and X is the wavelength.
The visible region consists of electromagnetic radiation with wavelengths between about 380 and 750 nm, which corresponds to energies of between about 120 and 230 kJ mol-1 in water (Penner 1994a). Substances which can absorb electromagnetic energy in this region are usually referred to as chromophores (Patterson 1967). The most common type of chro-mophoric groups which are present in food emulsions are attached to organic molecules which contain conjugated unsaturated bonds or aromatic ring structures (Hutchings 1994). Single unsaturated bonds tend to absorb in the ultraviolet rather than the visible region (Pomeranz and Meloan 1994).
Absorption causes a reduction in the intensity of a light wave as is passes through a material, which can be described by the following equation (Penner 1994b, Pomeranz and Meloan 1994):
where T is the transmittance, IS is the intensity of the light which travels directly through the sample, and I0 is the intensity of the incident wave. In practice, IS is also reduced because of reflections from the surfaces of the cell and due to absorption by the solvent and cell (Penner 1994b). These losses can be taken into account by comparing IS with the intensity of a wave which has traveled through a reference cell (which usually contains pure solvent), rather than with I0:
where IR is the intensity of the light which has traveled directly through the reference cell. The transmittance of a substance decreases exponentially with increasing chromophore concentration or sample length (Pomeranz and Meloan 1994). For this reason, it is often more convenient to express the absorption of light in terms of an absorbance (A) because this is proportional to the chromophore concentration and the sample length:
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