Choosing a Device for Food Color Measurement

The color difference meter can be used to measure the surface quality of rigid solid-state or flexible multi-form food. Rigid solid-state foods such as fruits, potatoes, and legumes have been studied, where researchers used a color difference meter to measure the color of tomatoes, establish a regression equation between fruit color coefficients and lycopene content, and evaluate the lycopene content in fruits. For flexible multi-form foods like meat, flour, and pasta, domestic and foreign studies often use color difference meters to directly measure the color of meat samples. The color of meat mainly depends on the content of myoglobin and hemoglobin in the muscles, thereby obtaining objective quantitative indicators of meat color, which helps in evaluating meat color and grading raw meat, as well as monitoring the freshness of raw meat.

Color difference meters are commonly used to measure the color of regular and irregular meats such as steak, pork, chicken, and tuna, and to monitor color changes in meats during food storage to ensure product quality. They can also measure the surface of pastries like steamed buns, noodles, bread, and noodle sheets to determine the reasonable color deviation of samples, facilitating qualitative and quantitative quality control. Measuring the color of liquid foods with a color difference meter can help understand the purity of the food and judge whether it has deteriorated. Although there are many quality indicators for liquid foods, chemical methods are mainly used, which involve cumbersome detection steps, require a large amount of organic solvents, increase detection costs, and pose hazards to testers. Therefore, using color as one of the quality evaluation criteria for non-destructive testing has been explored. For example, taking heated peanut oil as the research object, a color difference meter was used to establish the relationship between color parameters and chemical indicators; for different bayberry juices, color difference analysis and sensory evaluation were compared, and the results showed a high degree of consistency in evaluating quality. It can also indirectly measure certain components in food, such as amylose, which is the main basis for grading high-quality rice. The absorbance of the color-developing solution is measured by a spectrophotometer, and the amylose content can be calculated by determining the chromaticity value of the sample's color-developing solution, achieving high detection precision.

The Development and Application of Color Difference Meters in Color Measurement

The measurement of color is a new science involving physical optics, visual psychology, and other disciplines. With the continuous development of optical, electronic, and optoelectronic technologies, as well as the wide application of electronic computers, many color measurement instruments have emerged. The commonly mentioned color difference meter is a typical photoelectric integrating color measuring instrument. It uses a standard light source inside the instrument to irradiate the measured object, performs an integral measurement across the entire visible light wavelength range, obtains the tristimulus values and chromaticity coordinates of the transmitted or reflected object color, and gives the color difference between two measured samples through a dedicated microcomputer system. This is an optical analysis instrument with simple operation and low cost, widely used in industrial production, scientific research experiments, quality inspection, commodity inspection, and metrology departments.

The principle of measuring food color with a color difference meter

The color of a substance is a physical phenomenon of selective reflection or transmission of solar visible light (white light) by the substance. The color of visible light after being reflected or transmitted by an object is called object color, and the color of the surface of an opaque object is called surface color. A color difference meter is a photoelectric integrating color measuring instrument that uses a standard light source inside the instrument for illumination to measure transmitted or reflected colors. The color of an object is generally expressed by three scales: hue, chroma, and lightness. 

Hue represents the color characteristics such as red, yellow, green, blue, and purple. 

Chroma uses the visual perception characteristics of equal-lightness achromatic points to represent the shade of the surface color of an object and gives a scale. 

Lightness represents the relative light and dark characteristics of the object surface, which is the scale given to the visual perception characteristics of the object surface based on a white board under the same illumination conditions.

Basic Composition of Food Colorimeter

The photoelectric integrating color difference meter imitates the principle of human eye color perception, using light receptors that can sense red, green, and blue colors, amplifying and processing the respective photocurrents to obtain the stimulus amount of each color, thereby acquiring the color signal. The overall optical conditions of the photoelectric integrating instrument should meet the Luther condition.

The color difference meter mainly consists of four parts: the measuring head, data processor (including display and printer), DC power supply, and accessories. The measuring head of the colorimeter is composed of a lighting source, color filter, silicon photocell, heat-insulating glass, convex lens, light guide tube, baffle, integrating sphere, etc. Among them:

1. The integrating sphere is an important component of the colorimeter, which largely determines the service life, measurement accuracy, and long-term repeatability of the colorimeter.

   
   

2. The data processing system includes amplifier circuits, A/D conversion, central processing unit, display, printing, and other data output components. A microcontroller is used for data processing, and various chromaticity data are displayed via liquid crystal and printed output.

   
   

3. Internal light source lighting system: Low-energy special light-emitting diodes are used, corrected as the standard illuminant D65 for color measurement, ensuring a stable voltage of 220V±22V and 50Hz±1Hz to guarantee the stability of the light source. The internal lighting source of the colorimeter is usually a standard A light source, but in practical applications, it is necessary to measure the chromaticity values of objects under standard D65 and C light sources. Therefore, the D65 light source is simulated to make the overall spectral sensitivity of the instrument meet the Luther condition under D65.

   
   

4. Spectral tristimulus values: Data from the CIE 1964 supplementary observer with a 10° visual field are selected; the instrument's illumination geometry is o/d (object/diffuse).

   
   

5. Observation conditions: Conform to the CIE-specified 0° incidence/45° reception conditions. In terms of spectral conditions, the overall response is equivalent to the tristimulus values X₁₀, Y₁₀, Z₁₀ (abbreviated as X, Y, Z below) under the CIE standard illuminant D65 and the 10° visual field color-matching function. Finally, five colorimetric systems for color measurement can be obtained: CIE Y₁₀x₁₀y₁₀ (1964), CIE X₁₀Y₁₀Z₁₀ (1964), Lab* (1976), LCH° (1976), and Hunter Lab, as well as three color difference calculation systems: ΔEab* (ΔLΔaΔb), ΔEab* (ΔLΔCΔH*), and Hunter ΔE (ΔLΔaΔb). The data measured by different colorimetric systems can be converted by a computer and displayed numerically or printed out.

The advantages of a spectrophotometer not only lie in its ability to obtain the same type of digital data, but also in allowing the observation of the spectral reflectance curve of the color, thereby providing a more comprehensive understanding of color characteristics. Relying on its high-precision sensors and comprehensive illuminance condition data, the spectrophotometer can offer higher measurement accuracy than a colorimeter. In addition, both Threenh's colorimeter and spectrophotometer support non-contact measurement of the food sample to be tested.