Measure Color with EasyPlus UV/VIS

Dr. Sushma Rawool

Dr. Hans-Joachim Muhr

Expert Tip Measure Color with EasyPlus UV/VIS

Color is frequently used as a quality parameter for various products in industries such as food and beverage (F&B) and chemicals. EasyPlus™ UV/VIS spectrophotometers provide standard-compliant analytical color measurement workflows that easily and precisely meet quality control requirements.

Relevance for F&B

Food color can influence customer choice and taste perception.

Relevance for chemicals

Color is also an important criterion for monitoring contamination and maintaining quality in industries such as refining and petrochemicals and fine and specialty chemicals.

Relevance for water testing

The presence of metals, soil, organic matter from soil and plants, or industrial waste imparts color to water. Download an Expert Tip from Dr. Sushma Rawool and Dr. Hans-Joachim Muhr and get an overview of color scales available in EasyPlus UV/VIS spectrophotometers (e.g., CIE L*a*b*. ASTM and Gardner) and their application in industry segments.

Color is a physical property of an object determined by three factors: illuminant (light source), object, and observer (whether organic or mechanical). The organic perception of an object occurs when light reflected from the object is perceived by the photoreceptors in the retina. The signal sent by the photoreceptors is then processed in the brain [1]. Color assessments, however, use a spectrophotometer to measure a sample’s transmittance over a range of wavelengths or at a specific wavelength. This measurement is then compared to one or more color standards or used to mathematically calculate the color using information about the observer and illuminant.

Color is an important parameter in many industries because it correlates with product quality. Changes in color can indicate production errors, the presence of impurities, or degradation of reactants or the product itself [2]. Thus, color measurement is an integral part of quality control in a variety of industries such as F&B, chemicals, and water testing to ensure the quality of intermediate and final products.

Relevance for F&B

Food color can influence customer choice and taste perception. The color of products derived from natural ingredients depends on the ingredient source, processing, and storage conditions, e.g., the color of honey depends on its floral source. Moreover, the color of honey become darker when stored. This color change depends on the honey’s original color, its composition - such as acidity, nitrogen, and fructose content - and storage temperature [3].

Different color scales used in the F&B industry are presented in Table 1. CIE L*a*b* color defined by the International Commission on Illumination is commonly used because it assesses color uniformly and most closely matches the perception of the human eye [4]. The American Society of Brewing Chemists (ASBC) and European Brewing Convention (EBC) color scales are used to determine the colors of different beers, such as pale ale, Saison, and imperial stout (Figure 1). The Gardner color scale is applicable to edible oils such as olive, sunflower, and cashew. Iodine color number is used for clear liquids, including oils and fatty acids. The International Commission for Uniform Methods of Sugar Analysis (ICUMSA™)¹ color scale is used to measure the yellowness of sugars, including plantation white sugar, raw sugar, partly refined sugar, and sugar syrups. Yellowness is caused by the presence of residual molasses [5].

Generally, F&B samples are not pretreated prior to color measurement. However, filtering is recommended for turbid or cloudy liquid samples, and crystalline or powdered samples such as sugars need to be prepared in accordance with applicable industry monographs [6].

1 ICUMSA™ is a trademark of ICUMSA Ltd. and Verlag Dr. Albert Bartens KG.

Figure 1. Color variations in beer samples.

Relevance for chemicals

Color is also an important criterion to monitor contamination and maintain quality in industries such as refining and petrochemical and fine and specialty chemicals. The color scales used for chemicals assessment are shown in Table 1.

The American Society for Testing and Materials (ASTM) color scale is utilized for grading petroleum products, such as lubricating oils, heating oils, and diesel fuel oils. Compared to ASTM, the Saybolt color range is lighter and applicable for characterizing fuels including aviation gasoline, jet fuel, and kerosine. The American Public Health Association (APHA) or Pt-Co color scale, also known as the Hazen color scale, is applicable for clear liquids and used to quantify yellowness caused by degradation due to light or heat exposure during processing or storage as well as the presence of impurities [7].

Table 1. Overview of color scales available in EasyPlus UV/VIS spectrophotometers and their application in industry segments [10].

Relevance for water testing

The presence of metals, soil, organic matter from soil and plants, or industrial waste imparts color to water [8]. Color scales used to check the quality of drinking water and industrial wastewater are shown in Table 1.

SAC a(254) and UV Abs (254) are used to check the extent of water pollution generated by the presence of humic matter and aromatic compounds. SAC a(436) is used to assess the yellow to yellow-brown colors of a filtered water sample. The APHA color scale is used to determine yellowness in water. According to the National Secondary Drinking Water Regulation in the United States, the permissible color of drinking water is 15 Pt-Co scale [9]. Water quality can be determined using the APHA color scale without any extensive sample pretreatment.

30 color scales included

EasyPlus UV/VIS offers precise color determination. Incorporating 30 different color scales specified by regulating bodies such as the ASTM, ASBC, and ICUMSA. EasyPlus delivers color scale results as defined by standard monographs to satisfy the requirements of a variety of industries. For example, EasyPlus spectrophotometers easily meet the wavelength reproducibility (±0.5 nm) and absorbance reproducibility (±0.003 at 1.0 A) required for sugar color measurement per the ICUMSA monograph [6].

Measuring color with EasyPlus UV/VIS spectrophotometers

METTLER TOLEDO offers EasyPlus UV/VIS spectrophotometers for reliable, error-free, and fast color measurement to maintain product consistency and meet quality standards in different industries (Figure 2).

EasyPlus UV/VIS spectrophotometers offer several advantages:

They include a pre-verified comprehensive list of 30 color scales (UV) and built-in formulas, which makes direct color measurement easy and fast.
They have methods with clear workflow guidance on the touchscreen (Figure 3).
Cuvettes with various path lengths e.g., 10, 16, 20, 30, 40, and 50 mm as well as 1 inch can be used to match both regulatory requirements and sample color intensity. Automatic path length detection ensures workflow security.
It is possible to normalize the results with respect to cuvette path length. E.g., a 10 mm cuvette must be used to determine CIE L*a*b* color scale as per requirements. When working with dark color samples, customers often use shorter path lengths. However, if users do not normalize the measured transmittance/absorbance to 10 mm, the result will be incorrect. Taking the above into account, EasyPlus has defined an optional “normalization” feature so that expected values are obtained.
The compact design of EasyPlus UV/VIS saves precious bench space. Each unit is comparable in size to a piece of paper.
EasyPlus UV/VIS can be connected to EasyDirect™ UV/VIS PC software to acquire data and transfer results for hassle-free documentation.

Figure 2. Easy UV and Easy VIS spectrophotometers.

Figure 3. Color measurement method parameter representation on the EasyPlus touchscreen.

Application

The mentioned attributes are helpful when using EasyPlus UV/VIS to measure the color of beer samples according to the ASBC and EBC color scales (Table 2). The standard deviation (SD) of 0.1 and relative standard deviation (RSD) below 2%, ensures the repeatability and precise measurement of the color of beer samples.

Table 2. Color of beer samples according to ASBC and EBC color scales.

Conclusion

Color measurement is an important aspect of quality control in various industries such as F&B, chemicals, and water testing. EasyPlus UV/VIS spectrophotometers offer easy and fast color measurements using predefined formulas and automatic path length detection for error-free measurement in a compact instrument.

References

[1] METTLER TOLEDO, Guide: Basics of Color Measurement

[2] Hadi, M.H.H., et al, The amber-colored liquid: A review on the color standards, methods of detection, issues, and recommendations, Sensors 2021, 21, 6866.

[3] https://legacy.bjcp.org/mead/color.pdf

[4] Markovic, I. et al, Color Measurement of Food Products using CIE l*a*b* and RGB color space, Journal of Hygienic Engineering and Design 2013, 4, 50-53.

[5] METTLER TOLEDO application editorial, Selected Analytical Methods for Sugar Testing

[6] https://www.bartens.com/product/2906/

[7] https://www.astm.org/d5386-16.html

[8] https://water.mecc.edu/courses/Env211/lab1b.htm

[9] https://www.epa.gov/sdwa/secondary-drinking-water-standards-guidance-nuisance-chemicals

[10] The Pfund scale for honey color and Maple syrup color scale can be determined via an absorption measurement at 560 nm.

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