Color Measurement Techniques for Food Products - ACS Publications

Chapter 2

Color Measurement Techniques for Food Products

Downloaded by UNIV OF ROCHESTER on November 4, 2014 | http://pubs.acs.org Publication Date: June 13, 2008 | doi: 10.1021/bk-2008-0983.ch002

Gordon J. Leggett Hunter Associates Laboratory, Inc., 11491 Sunset Hills Road, Reston, VA 20190

The CIE (Commission Internationale de l'Eclairage) offers a well-defined system for the color measurement of opaque and transparent materials. What makes the color of food products especially challenging is that most food products exhibit light trapping or translucency effects which makes them highly dependent on choices of instrument geometry, as well as sample preparation and presentation techniques. This article summarizes the user choices involved in measuring the color of each of the optical categories of food products and provides the best options for color measurement of each.

CIE System of Color Measurement The CIE system quantifies color, as a person perceives it. Human perception of color requires a source of white light, an object that modifies the light and a person who perceives color from the object stimulus. In the CIE system, each element of this triad is represented as numbers. The white light source is representated as a standardized set of numbers called an illuminant. Food products range from transparent to opaque with many falling in between as translucent or light trapping. In all cases, the product measurement will be quantified as a reflectance or transmission spectrum. A person is defined as a standard set of numbers called the CIE Standard Observer. CIE color values are calculated using a mathematical model based on a white light source, object and human observer that represent all colors in terms of L* lightness, a* rednessgreenness, and b* blueness-yellowness.

© 2008 American Chemical Society

In Color Quality of Fresh and Processed Foods; Culver, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2008.

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Figure 1. The CIE System of color measurement defines a mathematical model of measuring color as a person sees it. Instruments with CIE standardized geometries measure the product in transmittance or reflectance which is then used to calculate L*, a*, b* values for all colors. (See page 1 of color inserts.)


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CIE instruments are further standardized in terms of instrument geometry - the arrangement of lamp source, sample place and detector used to measure the object. There are two CIE standard instrument geometries to measure all samples - a bi-directional 45/0 (or its inverse, 0/45), and a diffuse sphere. Both offer advantages with some types of samples. The diffuse sphere has versatility to measure in transmission for transparent samples and reflectance for opaque samples. The bi-directional 45/0 measures in reflectance only; best correlates to visual evaluation of samples; is more robust at measuring translucent samples and can measure a larger area of sample view appropriate for non-uniform food samples (/, 2).

Categories of Food Products and their Color Measurement While food products vary tremendously in their optical characteristics, there is a systematic method of separating all food applications into general categories and presenting them in a manner that is most uniform and consistent for color measurement.

Transparent Solids While few food products are transparent solids, it is recommended that transparent solids be measured in Total Transmission (TTRAN) mode on a CIE sphere geometry instrument. As the transmission spectra and corresponding color measurements are dependent on the path length of the sample, the thickness must be kept constant as a condition of measurement.

Transparent Liquids Transparent liquids have to be made effectively into a solid by measuring them in a glass or clear plastic transmission cell. A sphere instrument is standardized in TTRAN transmission using a clear cell filled with distilled water as a blank, negating the effects of the cell and solvent. The cell becomes a condition of the measurement. The cell path length is selected based on the chroma of the sample - the more chromatic the sample is, the shorter the path length of the cell. Typical cell path lengths are 10-mm for highly absorbing, chromatic liquids; 20-mm as a general case for most colors and 50-mm for near colorless liquids. The scattering of light due to non-soluble particles in the liquid sample is a separate optical phenomenon from the absorbance of the colorant that causes the perception of color, yet can affect the appearance of the food product. The



Figure 2. Transparent materials - solids, liquids, highly absorbing or haze, are measured transmission on a sphere instrument. (See page 1 of color inserts.)


11 hemispherical collector of a sphere instrument collects both regular and slightly scattered signal, negating the effects of any minor scattering inherent even in clear samples. A single measurement using a viewed area of the sample greater than a 15mm diameter is generally sufficient for clear, transparent liquids.


Highly Absorbent Transparent Liquids Samples such as soy sauce are typically so highly absorbing as not to allow any light through using standard path length cells. The compensating technique is to use a samples cell with a very thin path length cell, typically 2-mm or less, that will allow sufficient signal through the sample to separate color differences among lots. Standardize in TTRAN transmission using this "thin" 2-mm path length cell filled with distilled water to negate the cell and solvent effects prior to making sample measurements.

Hazy Transparent Liquids Some beverages may have pulp present that may cause significant scattering. Use of a 10-mm path length cell in TTRAN transmission on a sphere instrument, a large area of sample view and the averaging of 2 - 4 readings with replacement of the liquid between readings ensures a repeatable color measurement of this highly scattering sample. In addition to color, a colorimetric sphere instrument also has the ability to measure relative haze, quantifying the amount of pulp or clouding agents in the beverage.

Translucent Liquids As the solids content increases, the sample moves from transparent to translucent where a color measurement choice must be made. Translucent samples can be made thinner by measuring in a thin 2-mm cell and measured in transmission, or made thicker and measured in reflectance. If the solids content is high like tomato sauce, the sample becomes effectively opaque if you fill a sample cup to the top and the sample can then be measured in reflectance.

Translucent Liquids in Transflectance Translucent liquid samples like mango juice have a low solids content such that even if the sample cup is filled to full height, this sample is not opaque. A



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ring-and-disk technique can be used to control the sample surround and make this translucent sample effectively opaque for reflectance measurement. A black ring of fxed height, typically 1 0 - 1 5 mm, is inserted into the sample cup. Translucent liquid is poured into the cup to a level above the ring and a white disk is floated down on top of the ring. In a "transflectance" measurement using the ring-and-disk set, most of the reflected signal comes from the sample. However, some measurement light passes through the sample, reflects off the white background of the disk and passes back through the sample again to the detector. This ring-and-disk technique makes a low-solids translucent liquid effectively into a solid for color measurement. It is always recommended that transparent samples be measured in transmission. However, if a reflectance instrument is the only measurement option available, it is possible to obtain comparative measurements using the ring-and-disk technique described above with transparent liquids. The measurements are dependent on the ring-and-disk surround as a condition of measurement.

Translucent Semi-Solids There are a large number of food sauces or purees that are effectively opaque but must be measured in a clear glass container to be made effectively opaque. Measurements can be made on a bi-directional 45/0 (most common) or sphere geometry instrument. If the sample is highly viscous like peanut butter, the sample may need to be pressed slightly to remove any air occlusions.

Translucent Solids Usually translucent food samples tend to be in the form of sheets like pasta, and layering of multiple sheets is usually sufficient to make the sample effectively opaque for reflectance measurements on a bi-directional 45/0 (most common) or sphere geometry instrument.

Opaque Solids While few in number, there are food applications like the color measurement of brick cheese where the sample is effectively opaque, smooth and solid. Measurements are possible on either a bi-directional 45/0 (most common) or sphere geometry instrument. The use of the largest area of sample view and



Figure 4. For opaque food products, a key measurement issue making the sample more uniform for a repeatable color measurement. (See page 2 of color inserts.)

Figure 3. With translucent samples, it is necessary to control the thickness of the sample to make it effectively opaque for color measurement. (See page 1 of color inserts.)


14 the averaging of multiple readings will negate the effects of any non-uniform color across the surface of the sample.


Loose Powders Loose powders trap light between the particles, and can be thought of as sharing some of the optical characteristics of translucent samples. Pressing the powder into a flat, plaque makes the sample effectively into an opaque solid that can be measured on a directional 45/0 (most common) or diffuse sphere instrument. Another option is to measure loose powder like flour through the bottom of a clear sample cup. Tap the cup lightly to tighten up the flour. A large area of sample view 25-mm in diameter or greater is preferred. A single reading per measurement is acceptable, but averaging two readings with replacement of the flour between readings is better.

Particulates Medium-sized particulates or granules such peppercorns trap light in the interstitial cracks among the particles, and must be contained in a sample cell to be made effectively solid and opaque. Crushing or grinding are potential techniques for achieving a more uniform sample and minimizing the light trapping variability. However, this also has the potential to mix exterior (what the customer sees) and interior color, and may distort the requirement of the measurement. Bi-directional 45/0 instruments tend to be preferred over sphere instruments as correlating best to visual assessment of the product. Use of a large area-ofview instrument with a 25 - 50-mm diameter is helpful for area-averaging non uniform color. The preferred measurement technique for particulates is to average multiple readings per measurement with dump-and-fill replacement between readings.

Flakes, Chunks and Large Particulates Flakes, chunks and large particulates require an instrument with a large area of view, preferably 40-mm or larger, combined with averaging multiple readings (suggest 3 - 6 ) per measurement, with sample replacement between readings. With some large particulates such as grapes, a second option is to average multiple readings of single grapes using a small area of view. (2, 3)



Figure 5. As the sample gets larger and more non-uniform, the approach is to average out the variation in single measurement or as a group. (See page 2 of color inserts.)


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Summary By separating food products into optical categories and taking appropriate steps to prepare and present each sample in the most uniform form, a repeatable measurement in transmittance and reflectance can be made and used to quantify color using the CIE system.


References 1.

2. 3.

rd

CIE Publication 15.2004 - Colorimetry 3 Edition; Commission Internationale de L'Eclairage, Kegelgasse 27A-1030, Wien, Austria, NET: http://www.cie.co.at; 2004. MacDougall, D. B. ed.; Colour in Food Improving Quality; Woodhead Publishing Limited: Cambridge, England, 2002; pp 80-110. Hunter, R. S.; Harold, R. W.; The Measurement of Appearance, Second Edition; John Wiley & Sons, Inc.: New York, NY, 1987; pp 316-333.


Color Measurement Techniques for Food Products - ACS Publications

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