Sample Presentation

Most instruments treat foods subjected to measurement as having opaque, matte, and uniform surfaces. In practice, foods seldom meet these requirements but deviate from ideal conditions of flatness, uniformity, opaqueness, diffusion, and specularity in ways that affect the measurement. This introduces an element of empiricism into any attempt to instrumentally assess the color of a food and sample preparation, and presentation becomes important. Color measurements are made to obtain repeatable numbers that correspond to visual assessments of the color of the food. Repeatability requires standardized procedures for sample preparation (9).

Many foods require a container of optical material thus introducing an element of gloss in contrast to the ideal matte surface. A uniform sample works best but grinding, mixing, milling, and blending are all processes that affect the light scattering properties of the sample. This may be to the extent that the sample no longer represents visual evaluation of the product, ie, crushed potato chips. The presence of water or other liquids in the mixture decreases the light scattering of the sample. Foods that consist of pieces that do not fit tightly together result in light from the source becoming trapped and thus reducing the light scattering. These foods are best measured with an aperture larger than the incident light beam to maximize collection of the light that has been trapped and diffused beyond the normal-size aperture. Liquids that depend on the transmission of light require optical cells of a thickness that will maximize the color difference. Translucent samples measured by reflection also pose the problem of light being trapped because it is reflected from within the food not from the surface. Light so trapped is not collected to be included in the measurement and a large aperture is useful.

Because most foods do not represent ideal conditions it is recommended that a second measurement be made after turning the sample 90° and the two readings be averaged. In devising a presentation technique that works well for the measurement it is well to remember to check that it retains a true relationship with the visual assessment of the product in its original form.

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