Study on Food Coloring Reference Material

Food Coloring is a food additive that is used to give or improve the color of food [1]. It can be divided into synthetic food coloring and natural food coloring according to its source. Food Coloring can beautify the color of food, increase the pleasing appearance of food, enhance consumer appetite, promote food sales and increase the economic value of food. Some natural pigments are used in the pharmaceutical industry because of their good biological activity. In addition, some Food Coloring can also be used in animal production, environmental protection, food packaging and other fields [2-5]. According to statistics, in 2019, the total production and sales volume of Food Coloring in China reached 472,900 tons, with total sales of 4.347 billion yuan and exports of 112,170 tons. The Food Coloring industry as a whole is showing a steady development trend [6].

However, frequent food safety problems caused by the illegal or excessive use of food coloring, such as the illegal use of sunset yellow in dried meat and excessive use of coloring in beverages [7], have called the safety of food coloring into question. Therefore, accurately detecting the type and content of food coloring is the basis for determining whether food coloring is used in a standardized manner, which helps to ensure food safety and protect the rights and interests of consumers. In order to standardize the detection method for food coloring and ensure the accuracy and reliability of the test results, it is necessary to use reference materials/standards (RM).

Depending on the regulatory agency, RM in China is divided into reference substances (GBW) and reference samples (GSB), which are approved and managed by the national metrology authority and the administrative department in charge of standardization under the State Council, respectively. Both are basically the same in terms of development process and attributes. Internationally, there is no distinction between RM reference substances and RM reference samples [8]. According to the definition of International Organization for Standardization (ISO) Guide 30, a reference material is a material or substance with one or more sufficiently homogeneous and well-characterized values that is used to calibrate measuring devices, evaluate measurement methods or assign values to materials [9]. According to the definition of China's standard JJF 1005-2016 “General Terms and Definitions for Reference Materials”, A reference material is a material with specific properties that are sufficiently homogeneous and stable, and whose properties are suitable for the intended use in measurement or inspection of nominal properties. A certified reference material is a reference material that is accompanied by a document issued by an authoritative body, providing one or more property values with uncertainty and traceability obtained using valid procedures [10]. Certified reference materials can be divided into primary reference materials (GBW numbers) and secondary reference materials (GBW (E) numbers). Primary reference materials have high accuracy and are mainly used to research standard measurement methods or assign values to secondary reference materials. Secondary reference materials are mainly used directly as working standards.

Food Coloring reference materials are materials or substances with one or more sufficiently homogeneous characteristic values that are used in Food Coloring testing to calibrate measuring devices, evaluate measurement methods or assign values to materials. Food Coloring reference materials are an essential material basis for food coloring quality control, research and development of testing methods and standardization work. They play an important role in ensuring the comparability, reliability and traceability of test results, maintaining food safety and market stability, and eliminating trade barriers.

1. Research status of Food Coloring reference materials at home and abroad

1.1 Research status of Food Coloring reference materials in China

GB 2760-2014 “National Food Safety Standard: Food Additive Use Standards” stipulates the types, scope of application and maximum use levels of 67 types of food coloring permitted for use in China. Among these, there are 20 types of synthetic food coloring and 47 types of natural food coloring [11].

At present, the purity of synthetic food coloring produced in China is relatively low, which affects the quality of the coloring products and poses potential hazards to human health [12-13]. The standards for the use of food additives stipulate their scope of application and maximum use. Therefore, purifying synthetic food coloring and developing synthetic food coloring reference materials is of great significance for improving the quality of coloring products, ensuring food safety, and protecting consumer health. According to the information released by the National Standard Material Resource Sharing Platform, as of December 2021, China has developed five types of national first-level standard materials (GBW number) for food synthetic pigments, 23 types of national second-level standard materials (GBW (E) number) for synthetic pigments, and four types of working-level standard materials (NIM-RM number) for instrument calibration. For details, see Table 1.

Analyzing Table 1, these 32 standard substances for synthetic food colors only cover 7 types of synthetic food colors, namely amaranth, carmine, lemon yellow, sunset yellow, brilliant blue, erythrosine and allura red. The main developers are the National Institute of Metrology, China, Tianjin Center for Disease Control and Prevention, Tianjin Bio-Technology Development Co., Ltd., Beijing Food Safety Monitoring and Risk Assessment Center and Beijing Coast Hongmeng Standard Material Technology Co., Ltd. In addition to the national standard substances that have been released, some researchers are also working on the development of standard substances for edible synthetic colors. He Guihua and others have developed standard substances for azo carmine with a purity of (99.23±0.38)%, new red standard material with a purity of 99.59%, and indigo standard material with a purity of 98.26% [14-16]; Su Xihui et al. developed a standard material for lemon yellow with a purity of 99.87% and a standard material for brilliant blue with a purity of 99.87% [12, 17]. However, there have been no reports of standard substances for synthetic food coloring such as titanium dioxide, red yeast pigment, caramel color (produced with ammonia), caramel color (caustic sulfate), caramel color (ordinary method), caramel color (ammonium sulfite method), quinoline yellow, iron oxide, and copper sodium (potassium) chlorophyllin.

Since 1992, China has advocated the preferential use of natural pigments [18], but at present, the amount of natural pigments used in food in China is less than 20% compared with synthetic pigments [19]. In addition to the function of coloring food, some natural pigments also have the functions of lowering blood lipids, anti-cancer, and anti-oxidation [20-22]. As of December 2021, China has only promulgated one national secondary standard material and one working-level standard material for edible natural pigments. For details, see Table 2. They are GBW (E) 100269 Riboflavin (Vitamin B2) Purity Standard Material and NIM-RM3601 Riboflavin (Vitamin B2) Purity Standard Material, respectively. The development and declaration of national standard substances for food natural pigments is still in its infancy. At present, most of the standard substances for food natural pigments used in China are self-made by scientific research institutes or purchased from domestic and foreign reagent companies. However, some standard substances for natural pigments have problems in the development process, such as low extraction efficiency, difficulty in separation and purification, instability of pigments, and high price.

1.2 Research status of Food Coloring reference materials in the UK

In 2011, the European Union issued Regulation (EU) No. 1129/2011, which stipulates the Food Coloring allowed to be used in food, including 24 synthetic food colors and 16 natural food colors. Currently, the UK still complies with this regulation [23-24]. The Laboratory of the Government Chemist (LGC) is the main institution in the UK engaged in the development of reference materials and the world's largest supplier of reference materials [25-26]. According to the LGC website, it can provide 25 types of Food Coloring reference materials, 13 of which are edible synthetic pigment reference materials, including Tartrazine, Quinoline Yellow, Sunset Yellow FCF, Azorubine, Amaranth, Cochineal Red A, Allura Red AC, Patent Blue V, Indigotine, Brilliant Blue FCF, Brilliant Black BN, beta-apo-8′-carotenal (C 30), titanium dioxide. The 12 natural food coloring reference substances are curcumin, riboflavins, cochineal, chlorophylls, lycopene, lutein, canthaxanthin, anthocyanins, calcium carbonate, aluminium, silver and gold.

1.3 Research progress of American Food Coloring reference materials

According to the United States Code of Federal Regulations, Title 21, American food coloring is divided into exempted Food Coloring and certified Food Coloring. Among them, exempted Food Coloring is usually natural pigments of plant or mineral origin, a total of 39 kinds, and certified Food Coloring is synthetic pigments, a total of 9 kinds [27]. The United States is the first country in the world to develop reference materials, and the national institution responsible for developing reference materials is the National Institute of Standards and Technology (NIST) [28].

According to the NIST website, of the 48 Food Colorings allowed for use in the United States, NIST has developed three types of natural food coloring reference materials: ultramarine blue, calcium carbonate, and curcumin. See Table 3 for details. Among them, SRM 324, SRM 915, SRM 915a, and SRM 915b are purity standards used for standardized analysis and evaluation of the methods used. SRM 3300 is for the detection and analysis of turmeric root. In addition to the government standards and measurement agency NIST, the American commercial company Sigma-Aldrich provides most of the Food Coloring reference materials and has played an important role in the construction of the Food Coloring reference material system in the United States.

1.4 Research progress of Food Coloring reference materials in Japan

Japan classifies food additives into four categories: “designated additives”, “existing food additives”, “natural flavorings” and “substances used as food additives” [29]. At present, there are 103 kinds of food coloring in Japan, including 22 kinds of “designated additives”, such as β-carotene, brilliant blue and aluminum lake; 47 kinds of “existing food additives”, such as gardenia red and gardenia blue; and 34 kinds of “substances used as food additives”, such as cherry red and strawberry red [30]. The main national institution for the development of Japanese reference materials is the National Metrology Institute of Japan (NMIJ). According to the NMIJ official website, NMIJ has not yet developed Food Coloring reference materials. Kanto Chemical Co., Ltd. and Fuji Film Wako Pure Chemical Corporation are well-known comprehensive reagent manufacturers in Japan, which can provide more than 30 kinds of Food Coloring reference materials for pigment composition analysis and scientific research.

2. Current status of Food Coloring standard material applications

2.1. National standard testing requirements

Since the consumption of synthetic food coloring is harmful to human health, and the amount of natural food coloring added to food is not necessarily beneficial, China's GB2760-2014 “National Food Safety Standard Food Additive Use Standards” [11] has certain regulations on the amount of various Food Coloring added. Testing Food Coloring in food is of great significance for the supervision of food safety in China. Currently, China has issued national standards for the detection of some food colorings, as shown in Table 4. As can be seen from the analysis of Table 4, China has currently formulated national standards for the detection of 26 food colorings, and the test samples cover foods, health foods, meat products, feed, and cooked fruit products.

Different testing methods are used according to the nature of the different food colorings, including high-performance liquid chromatography, inductively coupled plasma-atomic emission spectrometry, diazobenzylmethane colorimetry, high-performance liquid chromatography-mass spectrometry, fluorescence spectrophotometry, spectrophotometry, colorimetry, and paper chromatography. Among these methods, high-performance liquid chromatography is the main method for detecting food coloring because of its fast analysis speed, high separation efficiency, high sensitivity, and wide application range [32]. These detection methods provide technical means for the detection and analysis of food coloring, meeting the needs of China for the detection of some food coloring. These standard methods require the use of food coloring standard materials. Food coloring standard materials are used in the detection process for qualitative judgment and quantitative analysis, which can ensure the accuracy and reliability of the results, provide traceability standards, control food quality, ensure food safety and market stability.

2.2 Development and validation of analytical methods

Food Coloring standard substances can also be used to develop effective analytical methods for food coloring and to verify the differences between different testing methods [33]. Li Xing et al. [34] established an analytical method that can accurately quantify 10 synthetic food colors in food using synthetic food colors developed by Dr. Ehrenstorfer GmbH, a subsidiary of the UK Government Chemist Laboratory (LGC). Fu Dayou et al. [35] optimized the analytical conditions using the national standard substances of lemon yellow, amaranth red and carmine in China, and established a detection method suitable for the illegal addition of synthetic pigments in wine.

Song Xin et al. [36] used five kinds of synthetic food coloring solution standard materials developed by the National Standard Material Research Center to explore the application effect of the oscillographic polarimetric method for the determination of synthetic food coloring, and compared it with the HPLC method. There was no significant difference between the two determination methods. Guo Yuanheng et al. [37] used carmine acid developed by Sigma in the United States as a standard sample, and measured the peak area by high performance liquid chromatography and the absorbance by spectrophotometry, and quantitatively analyzed 10 pigment samples using the two methods. The results showed that there was no significant difference between the two methods.

3 Conclusion and Prospect

There are few certified Food Coloring reference materials available from developed countries abroad, and most are provided by large commercial companies. The development of reference materials for synthetic and natural food colors is a work in progress. At present, the development of Food Coloring reference materials in China is generally lagging behind, and the choice of reference materials is still very limited. The certified Food Coloring reference materials developed in China involve only 7 types of pigments, and the Food Coloring reference materials developed in China involve only 1 type of pigment. In the future, China should be guided by the scientific research and testing needs of food coloring, overcome the difficulties of developing natural food coloring, which is difficult and costly, and continue to enrich the types of food coloring reference materials to meet the demand for reference materials for food coloring analysis and testing, so as to promote the development of food coloring testing technology and methods and strengthen the construction of China's standard system.

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