Theory and the Principles of the Colorimeter

COLORIMETER

Introduction

Color of food is not a physical characteristic of food but it is an important quality attribute for foods. This is because color affects the acceptability and perception of consumer for the food and even preference and perception of flavor (Lewis, MJ. 1996). The color is determined by the selective absorption of different amounts of the wavelengths within the visible region. Changes of color can occur during food storage, maturation (ripeness), processing and others.

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Colorimetry is the scientific color measurement which used to express color in numerical terms and to measure the color differences between the specimens. The specimens can be paints, textiles, plastics, food and other products that may reflect or transmit color. Colorimeter is an instrument for psychophysical analysis by measuring the amount of light passing through a liquid. This instrument provides measurements that correlate with human eye-brain perception. Besides, the colorimeter is basically like a spectrophotometer but less complex as the spectrophotometer allows selection of any wavelength of light. Colorimeter measures the color through three wide-band filters which corresponding to the spectral sensitivity curves.

A light source creates a beam of light that shines through a sample. The colorimeter then measures the amount of light transmitted or absorbed electronically and provides colorimetric data as tri-stimulus values (XYZ, L, a, b). The design of the Tristimulus colorimetry is about duplicate the response of the human eyes. A light source, three glass filters with transmittance spectra that duplicate the X, Y and Z curves and a photocell are required. This helps to get the reading of XYZ represents the color of the sample. Drawback of XYZ system is not visually uniform, that means one unit of color measurement in one area of the solid was visually different from the same unit in another area. Normally, the values of tristimulus are used to determine the direction and amount of any color difference if a color match is accurate.

The colorimeter provided in this lab is Color Flex colorimeter from the Hunter Lab. Color Flex is a self-contained color measurement spectrophotometer which had been introduced in this lab. It can be used in production or in the laboratory for inspecting raw materials and evaluating the final product. Apart from this, the Color Flex is ideal for measuring powders, granules, pastes, liquids and opaque as it has its port-up or port-forward measurement orientations. The Color Flex require glass sample cup to hold the sample for measure and has a hole to insert the glass sample cup according to its size. Specialized versions of the Color Flex are available for the citrus industry and the tomato industry. These systems include specializes calibration standards and measurement scales appropriate for the industry.

Based on nonlinearly compressed CIE XYZ color space coordinates, a Lab color space is a color-opponent space with the dimension L which for the lightness and the a and b are for the color-opponent dimensions (Hunter, Richard Sewall, 1948). This L, a, b values used in the system are calculated from tristimulus value (X, Y, Z) as the backbone of all mathematical color models. The first system which uses the opponent-color theory is the hunter Lab system (1958). This system states that the red, green and blue cone responses are remixed into opponent coders as they move up the optic nerve to the brain.

Figure 4.0 Tristimulus colorimetr

OBJECTIVES

1. To measure the absorbance of particular wavelengths of light by a specific solution.
2. To determine the concentration of a known solute in a given solution by the application of the Beer-Lambert law which state that the concentration of a solute is proportional to the absorbance.
3. To understand the theory and the principles of the colorimeter.
4. To understand the standard operation procedure to operate the colorimeter correctly.

THEORY AND PRINCIPLE

Color has various degrees of lightness and different values. Opponent-Colors Theory has been developed since the XYZ values are not easily to get understand in term of object. This theory is easier for the scientist perceive color, simplify understanding, improve communication of color differences and can be more linear thought out color space.

Based on the basic of the opponent-color theory, the Hunter L, a, b color space is a three dimensional rectangular, where L (lightness) varies from 0 (black) to 100 (white), a which represent red-green axis with positive (redness) and negative (greenness) values, and b which represent yellow-blue axis with positive (yellowness) and negative (blueness) values. The values of 0 for the a* and b* always represent neutral. Once the L, a, b position of a standard color is determined, a rectangular tolerance box can be drawn around the standard.

Today, there are two popular L, a, b color scales which are Hunter L, a, b and CIE L*, a*, b*. A color still has different numerical values between these two color scales even though these two are similar in the organization. In fact, the Hunter and CIE L*, a*, b* scales are both mathematically derived from the XYZ values. Neither scale is visually uniform, Hunter L, a, b is over expanded in the blue region of color space whereas CIE L*, a* and b* is over expanded in the yellow region.

STANDARD OPERATION PROCEDURE

ColorFlex Colorimeter

1. The ColorFlex is placed on a flat and stable surface where near an electrical outlet.
2. The system is turned on by pressing the Red (lightning bolt) key and is allowed to warm up for at least two hours before use.
3. Before measuring sample, the instrument must be calibrated. Steps to standardize the colorimeter are carried out as below:

1. The Down Arrow key is pressed until the menu is reached and then the Standardize is selected by pressing the Read key.
2. As instructed, the sample pot should be covered with the black glass first. The black glass is covered at the sample port with the shiny side toward the port and the arrow on the glass should be pointed towards the scientist.
3. The “thunderstorm” button is pressed.
4. The sample pot then is covered with the white tile. Same with the black tile, the sample port is covered with the shiny side toward the port and the arrow on the glass should be pointed towards the scientist.
5. The “thunderstorm” button is pressed and the values are showed on the periphery of the tile.

4. A message which indicates the instrument is ready to read will be displayed when the standardization of the instrument is completed. The value of L*, a* and b* should be 50.87, -25.11 and 14.98 respectively.
5. The sample cup must be cleaned before put the sample into it. Make sure that the sample must be at least fully covered the bottom surface of the cup.
6. After closed the cover, the cup is put onto the sensor to measure the sample and take the reading. The L, a, b value will be show on the screen after the “thunderstorm” button is pressed and wait for a moment.
7. The readings are recorded.
8. Steps 5-7 are repeated for different samples.

DATA

Table 4.0 Standard Tristimulus Values .

Table 4.1 Tristimulus Values of one-third volume of the Samples.

Table 4.2 Tristimulus Values of two-third volume of the Samples.

DISCUSSION

From the demonstration, there are three samples which are green beans, red beans and dried red pumpkin seeds are used to measure by using the colorimeter. These three samples are also measure with different volumes. The L, a, b values are recorded.

For the green bean, the small amount of the green bean sample shows the value of a* as 0.57 while the green bean sample with a higher volume get the value of a* as 0.46. This difference between the reading can be explained as the green bean with higher volume gives more greenness with it’s a* value is nearer to the negative values (low positive values). Moreover, the red bean with the low volume show the reading of the a* as 14.23 whereas the red bean with the higher volume show the reading of a* as 14.61. From this result, we can said that the red bean with higher volume are more redness than that of with the lower volume because it’s a* reading is more positive which indicate more redness. Same situation obtained from the dried red pumpkin seed. The high volume of red dried pumpkin seed has higher reading of a* as 27.11 compare than low volume of dried red pumpkin seed with reading of a* as 27.08.

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Next, green bean has showed that it is lightness with the value of L* as 37.13. It is more brightness than the red bean and also dried red pumpkin seed with the reading of L* as 22.89 and 26.46 respectively. In the demonstration, the value of a* of the green bean is 0.57 which means that are green in color in that sample. Besides, the b* values as 23.71 has showed that the green bean samples consists of more yellow color but less blue if compared to the standard value as it’s b* value is positive value and higher than that of the standard values.

In the demonstration of red bean, it show that it consists of lowest lightness with the value L* as 22.89 if compared to others two samples. Thus, we can conclude that the red bean is the darkest among the samples. The a* value of red bean is 14.23 which means that there are red color in the sample and is more red than the standard which has the negative value of a* as -25.11. Moreover, the red bean has b* values as 8.16 and this showed that it consists of more yellow color but less blue color in it.

Last, the dried red pumpkin seed has the L* value as 26.46 which is lower than that of green bean but higher than that of red bean. Among these three samples, the dried red pumpkin seed has the highest values of a* as 27.11. It showed that it consists of more red color than other samples. For the b* value, it’s b* value is about 13.82 which means that the dried red pumpkin seed also consist of more yellow color but less blue. Compare the results obtained, we can notice that the dried red pumpkin seed has higher values of L* and a* which showed it is more lightness and also more redness than the red bean.

PRE-CAUTION STEP

1. Make sure that the setting of the colorimeter is set as default setup before conduct the experiment to obtain accuracy data.
2. The standard plates must be cleaned and make sure that it is free from dust and fingerprint.
3. After done the calibration, put the black glass and the white tile back to the box to avoid scratching.
4. The volume, size and weight of the sample must be standardized (constant). The amount of the samples must at least cover fully the bottom surface of the sample cup.
5. The sample must be covered with the non-transparent black-coloured cover (light trap) when the readings are being taken. This helps to avoid the light sensitive colorimeter from the disturbance of other light sources.

References

Lewis, M. J. 2006. Physical Properties of Foods and Food Processing System. Cambridge: Woodhead Publishing Limited.

Murano, P. S. 2003. Understanding Food Science and Technology. USA: Wadsworth/ Thomson Learning.

Pankaj B. Pathare, Colour Measurement and Analysis in Fresh and Processed Foods: A Review. 2012. http://works.bepress.com/pankaj_pathare/3/.