Study on the Safety of Natural Food Coloring

Color plays a special role in food processing. Because food color is one of the most basic tools for identifying and measuring food freshness [1], consumers often use their perception of food color to identify food freshness and make food choices [2-3]. However, in the modern food processing industry, in order to preserve food for longer, the natural color is often partially lost or changed during the processing. Therefore, most food companies add food coloring to maintain the attractive sensory characteristics of the food during the processing [4]. Food coloring can be divided into natural coloring and synthetic coloring according to their sources. Natural food coloring is a natural coloring substance obtained by physical purification from plant, microbial, or animal materials [5-6].

Literature proves that as early as 1500 BC, humans began to use natural coloring in food. The ancient Egyptians used natural extracts and wine to improve the color of sweets [7]. Chinese documents such as “Historical Records · Biography of Merchants” and “The Essential Techniques for the Common People” also record the extraction and use of Natural Coloring by the ancients during the Eastern Zhou Dynasty and the Northern Song Dynasty[8]. With the development of the Western industrial revolution, the food processing industry also achieved rapid development.

In the 19th and 20th centuries, synthetic pigments appeared and gradually occupied the dominant position of food processing colorants[9]. Compared with synthetic pigments, natural food colors are relatively safe. In recent years, consumers have become increasingly aware of the relationship between diet and health, and they pay more and more attention to food safety. As a result, there is a growing demand for natural food coloring [10]. At present, China is the country in the world that has approved the use of the most natural coloring [5]. The development, application research and safety evaluation of natural food coloring have also become the focus of research and work for relevant scientific researchers and food industry practitioners. This paper provides a preliminary discussion on the classification, characteristics, application and safety evaluation and control of natural food coloring.

1. Classification of natural food coloring

Natural coloring comes from a wide variety of sources, and there are many different types. There are more than 600 types of carotenoids alone [8]. Therefore, the basis for classifying natural coloring varies. According to the source, it can be divided into plant pigments, animal pigments and microbial pigments; according to solubility, it can be divided into water-soluble pigments and fat-soluble pigments; and according to chemical structure, it can be divided into five categories: carotenoids, anthocyanins, etc. [8,11]. Table 1 summarizes the classification of natural food coloring through various literature studies.

2. Properties of natural food coloring

Natural food coloring comes from nature, is diverse, and most of it has no toxic side effects. Compared with synthetic coloring, it has outstanding properties. The advantages of natural food coloring over synthetic coloring include: 1) most natural coloring comes from edible plants and animals, which makes them highly safe, low in toxicity, and not highly rated in toxicological evaluations [15,19-20]; 2) many natural colorings contain nutrients that the body needs or are themselves vitamins or vitamin-like substances [15,19]; 3) some natural colorings have pharmacological effects and have preventive and therapeutic effects on certain diseases. For example, flavonoid pigments have a positive effect on the prevention and treatment of cardiovascular disease. Some pigments also have antioxidant, analgesic, and antihypertensive effects [13,15,19,21]. Natural Coloring has a natural hue and is easy for consumers to accept, and has certain practical and economic value. Some varieties have a special aroma, which enhances the flavor of the food [8,15,19,22-24].

Compared with synthetic pigments, the disadvantages of natural food coloring include: 1) poor stability and unstable hue: mainly manifested in terms of water solubility, thermal stability, and the influence of metal ions [13,25]. Some pigments are only relatively stable at a pH of 2 to 6, such as red globe grape pigments. In addition, metal ions, oxidants and reductants all have a certain damaging effect on pigments. Therefore, natural pigments have low stability and relatively high requirements for the storage environment [13,26]. Many natural pigments have poor solubility, are not easy to mix evenly, and are not easy to color [12]. 2) Poor color retention: Because the pigments are closely bound to other substances when they are not separated, they have a lasting effect on the color of the substance. After separation, they lose the protection of cell membranes and other biological mechanisms and are prone to fading. 3) Narrow application range: Because Natural Coloring has high environmental requirements and is highly specialized, the application range of a single pigment is relatively narrow [13,27]. 4) Low purity: Because Natural Coloring is isolated from plants and animals, it often coexists with other ingredients, making purification difficult and the cost of extraction and purification relatively high [12-13].

3 Application of natural food coloring

At present, the food processing industry has multi-dimensional potential for the application of natural food colors [28]. Because natural food colors have many beneficial properties, in many cases, the use of natural coloring not only affects the color of the food, but also exerts its pharmacological health effects and adds other effects such as natural flavor [29]. However, the main purpose of using colorants is to help correct natural color deviations in foods or inconsistencies caused by changes in color due to storage, processing, packaging, etc., to bring out the right color effect of the product and ensure an appearance that is more acceptable to consumers. Natural food colors can be widely used in a variety of foods such as beverages, candy, dairy products, pastries, canned fish, meat products, and seasonings [20].

3.1 Beverages

Soft drinks and alcoholic beverages are often colored with pigments to enhance the flavor and make the product more attractive. Due to consumer demand for food safety and health benefits, some natural colorings are used in beverages, such as rose anthocyanins and bilberry pigments. It should be noted that many beverages are packaged in transparent containers, and natural colorings with high light stability are required [27,30-31].

3.2 Sweets and pastries

Hard candies, lollipops, and chocolate-covered candies all have eye-catching colorful coatings to attract consumers. As these products are often exposed to sunlight, water-soluble natural colors that are stable to light and oxidation are required. The use of Natural Coloring in pastries increases safety while also increasing appetite through color

and luster. Opalescent and oil-soluble pigments are often used in pastries.

3.3 Dairy products

Because the milk protein in dairy products can bind with oil-soluble pigments to form a stable, pure texture, the ideal coloring for margarine and butter is oil-soluble Natural Coloring, such as turmeric yellow.

3.4 Fish, meat and canned products

The processing and storage of fish, meat and canned products can cause hemoglobin to change, resulting in obvious discoloration or fading. To attract consumers and restore the original color of the product and maintain its commercial value, the manufacturer will add colorants. Currently, the main natural colorants used in these foods are red beetroot and paprika red. These natural colorings replace synthetic colorants that pose a risk of cancer, improve food safety, and have a good coloring effect.

3.5 Processed vegetables, fruits and seasonings

In order to facilitate long-term storage, fresh fruits and vegetables are often processed into preserved fruits, dehydrated vegetables, seasonings, etc., through methods such as pickling, heating, and drying. In this process, the original bright colors will change and fade. In order to increase the attractiveness of the product, water-soluble natural coloring is often used to help correct the color, such as chlorophyll, paprika yellow, curcumin, barley sprouts, and caramel [31].

4 Safety and risk control of natural food coloring

Natural coloring is generally trusted to be safe because it comes from nature. However, high safety does not mean no toxic side effects. Some natural pigments pose safety risks and require safety evaluation and control.

4.1 Safety risks of natural coloring

Natural coloring is not absolutely safe. There are safety risks in its source, chemical structure and extraction and processing (Figure 1).

4.1.1 Safety risks of the raw materials of natural coloring

Due to the complex composition of natural coloring, some pigment components have not been isolated and identified, and they themselves contain certain hazardous substances, such as gambogenic acid, or toxic molds. Some natural coloring contains certain inorganic pigments, which are generally heavy metals or metal salts, and are generally highly toxic [32-34]. In recent years, humans have used pesticides in crop production, causing pollution of the air, soil and water. These pollutants remain in large quantities in animals and plants through the food chain and are difficult to degrade in nature. Natural Coloring is extracted from these contaminated animals and plants and may also contain excessive pesticide residues, which are harmful to human health [32].

4.1.2 Safety risks during the processing of Natural Coloring

Natural coloring also poses safety risks during the purification and separation process. Currently, most international natural dyes are extracted using organic solvents such as hexane and acetone. The safety of these organic solvents and their residues in subsequent processes can affect the safety of natural coloring. In addition, during the processing and refining of natural dyes, structural changes can occur or impurities can be mixed in, causing safety risks [32, 33-35].

4.1.3 Safety risks of Natural Coloring in food use

The main safety risks of natural coloring in food use are overdose, use beyond the scope, and reactions during the mixing of natural coloring. The key to safety and function lies in the dosage. Substances that are functional at a certain dosage may become toxic if used in excess. The function and safety of any substance are evaluated at a certain dosage. In addition, if natural coloring is used improperly, some adverse reactions may occur between the pigment and the food, which may also pose a hazard to human safety [32].

4.2 Safety evaluation of natural food coloring

The use of natural food coloring in food is related to the health of consumers. There are some safety risks in the production and use of natural food coloring that need to be controlled and reduced. Therefore, the relevant departments have high safety requirements for natural food coloring.

Any natural coloring must undergo a large number of experiments and provide a large amount of reliable data before being approved. A comprehensive evaluation method is used to evaluate their safety. The safety evaluation of natural food coloring in various countries is mainly based on factors such as the toxicity of animals and plants, toxicological test data, and the chemical structure, properties, purity and stability of the coloring agent used. In 1994, the World Health Organization (WHO) and the Joint Food and Agriculture Organization Expert Committee on Food Additives (JECFA) published the results of toxicological evaluations of certain colorants and proposed reference values for the maximum daily intake (ADI) by humans. The domestic GB 2760-2011 Hygienic Standards for Uses of Food Additives stipulates the safety evaluation standards for various synthetic food colorings. The safety evaluation methods for natural food coloring currently include three aspects: toxicological testing, harmful trace element testing, and hygiene testing [5,36].

4.2.1 Toxicological testing

Toxicological testing includes toxicological dose determination and toxicity experiments. The former measures the ability of a natural coloring to cause harm to the body, while the latter studies the toxic reactions caused by a certain dose entering the body of animals over a certain period of time. It is generally divided into four stages: acute toxicity experiments, genetic toxicity experiments, subchronic toxicity experiments and chronic toxicity experiments (including carcinogenicity experiments). Whether Natural Coloring can be used in food depends mainly on whether toxicity can be controlled under current production and living conditions [37-39].

4.2.2 Testing for harmful trace elements

Natural coloring may be contaminated with toxic trace elements during production, storage and transportation. In order to ensure the safety of natural food coloring and prevent the introduction of toxic substances, it is necessary to test for some common harmful trace elements, and each inspection item must not exceed the standard [37-38, 40].

4.2.3 Hygiene inspection

After passing the toxicology and trace element tests, natural coloring must also pass a hygiene test. This mainly tests for pathogenic microorganisms and pesticide residues. The test must be carried out in strict accordance with standard methods, and the results must comply with national standards before the natural coloring can be used. Generally, the requirements for edible natural coloring are negative for pathogenic bacteria, a general bacterial colony of less than 30 CFU/mL, and no detectable DDT, hexachlorocyclohexane, or Aspergillus.

4.3 Effectively controlling and reducing the safety risks of natural coloring

Since the safety risks of natural food coloring exist at every stage from the source to consumption of the finished product, the control of the safety of natural food coloring must also extend throughout the entire supply chain from source to finished product, identifying, clarifying, and controlling safety hazards at each stage. Therefore, the hazard analysis critical control points (HACCP) and risk assessment (RA) systems are the most effective systems for controlling and reducing the safety hazards of natural food coloring by identifying, assessing, and controlling the hazards that lead to food safety problems [41-44]. Early risk identification is the key to controlling safety risks.

Early risk identification includes establishing a professional database; establishing a framework for identifying and describing the occurrence of risks and a timely reporting system for the occurrence of food safety risks; analyzing trends in food safety alerts; and reviewing and analyzing the characteristics of food safety risks caused by microorganisms, biochemicals, chemicals, and mycotoxins [45]. In addition, literature [46] shows that the control of food safety risks also focuses on some special control research fields, especially the early detection, identification and evaluation of food safety risk occurrences; research and control of microbial risks, chemical risks, biochemical risks and risks caused by global climate change. Therefore, to effectively control the safety risks of Natural Coloring, it is necessary to identify and control each risk from source to finished product, which includes the source control of Natural Coloring, the control during processing and use, and the control of management standards.

4.3.1 Source control

Source control refers to the control of raw materials for natural coloring. There are four main risks associated with raw materials: 1) Natural coloring itself has a complex composition, and the safety of some raw materials has not been verified. The use of these raw materials should be based on historical usage records, and safety evaluations such as toxicological testing, harmful trace element testing, and hygiene testing should be carried out [47]. 2) Raw materials may contain toxic ingredients such as heavy metals. The main methods used to prevent these risks are to label the ingredients and use physical and chemical screening techniques such as TLC to screen for toxic ingredients such as heavy metals and aristolochic acid in the raw materials for coloring [48-49]. 3) The raw material is an active microbial ingredient that produces toxins under certain conditions.

Due to the unique physiological characteristics of these natural substances, safety risk assessments are mainly based on the potential risks of using these active microorganisms. A large number of assessments focus on the analysis of the safety of these raw materials themselves [50-55]. Users should strictly control the types of toxic substances that are prohibited, and strictly label the ingredients of the pigments used. It is also important to understand the relationship between the characteristics and safety risks of active substances in their growth conditions and fermentation media, as well as the resistance of active substances to important antibiotics and gastrointestinal reactions [47, 53, 56]. 4) Raw materials contain residues of harmful chemicals such as pesticides due to contamination. Reducing this type of chemical contamination requires the use of relatively traditional and safe cultivated and farmed raw materials for pigment extraction. The safety of natural and synthetic pesticides used should also be evaluated. In addition, biopesticides also have an impact on the environment and human health. The use of biopesticides and safety assessments should also be considered when selecting pigment raw materials [49,57].

4.3.2 Control during processing and use

During processing and use, some pigments, especially those extracted from natural herbs, may pose safety hazards due to cross-contamination and chemical reactions with other ingredients [48]. Kelly et al. [58] showed that natural substances can react in certain environments (during processing or consumption), for example, by being converted when they come into contact with microorganisms in the mouth or intestines. To prevent this kind of hazard, a database of the chemical reaction hazards and cross-contamination of various related substances should be established, the cumulative dose of substances used should be strictly controlled, and safe operations should be strictly followed to prevent cross-contamination.

In addition, Natural Coloring can also pose safety risks during the purification and separation process. The safety of some organic solvents used and the residues in subsequent processes can also affect the safety of natural pigments. These need to be improved during extraction and purification [59-60]. In order to reduce the residue of harmful substances, new, more environmentally friendly technologies have gradually been applied to the extraction of natural substances, such as the use of carbon dioxide and water instead of some harmful organic solvents. At present, supercritical carbon dioxide extraction technology has become the fastest growing technology because it does not use organic solvents throughout the process, the extract has no residual solvents, and it also prevents the toxicity of the extraction process to humans and pollution to the environment [59, 61-62].

4.3.3 Management and standard control

Many natural substances have been used for food extraction for a long time, but there are no clear standards for acceptable daily intake (ADI) and no observed adverse effect level (NOEL) [63]. At present, the standards for natural coloring used in China only specify the varieties but not the clear dosage, and most of the standards are lower than international standards. Many natural colorings have no detection methods or indicators. Therefore, in order to control the safety risks of natural pigments, strict control must be exercised over the management and standards of natural coloring: the approval process for the use of natural coloring must be strictly managed, including the source of raw materials, name, part of the body from which it is taken, chemical structure of the main component, testing methods, purity of coloring components, ash content, solvent residue, bacterial count, stability tests, toxicological testing, safety levels, etc.[32]. Establish and implement HACCP and good manufacturing practices (GMP) systems in management, conduct safety assessments on natural substances used in food ingredients one by one, and establish a common assessment framework [64-65].

5 Conclusion

Natural food coloring is highly safe and has low toxicity; it contains nutrients that the human body needs or is itself a vitamin or vitamin-like substance. Some natural coloring has pharmacological effects and can prevent and treat certain diseases, while others have antioxidant, analgesic, antihypertensive and other effects. Natural coloring has a natural hue and is easy for consumers to accept. Some varieties have a special aroma that adds to the flavor of the food, and have certain use and economic value. Therefore, consumers are increasingly demanding Natural Coloring. Natural coloring is currently widely used in a variety of foods, such as beverages, candy, dairy products, pastries, canned fish and meat products, and seasonings. However, there are still some safety risks in the development of Natural Coloring raw materials, processing and development, and the application process in conjunction with food. In order to ensure that the long-term use of natural food coloring does not cause any transient or cumulative health hazards, these safety risks must be controlled and reduced.

This requires the relevant departments to do the following as much as possible: 1) Establish and improve the safety, hygiene, and testing standards for natural coloring, and conduct strict safety evaluations and controls on natural coloring. These evaluation methods include toxicological testing, harmful trace element testing, and hygiene testing. 2) Effectively identify and control risks in the production chain of natural coloring, from the source to the finished product. Only by accurately identifying and controlling safety hazards during the development, safety hazards are accurately identified and controlled, and a comprehensive evaluation of the safety of natural food coloring is carried out strictly, the safety of natural food coloring can be effectively guaranteed. 3) Establish a database of hazards and reactions of related substances to avoid hazards. 4) In terms of management, establish and implement the HACCP and Good Manufacturing Practices (GMP) systems, conduct safety assessments on natural substances used in food ingredients one by one, and establish a general evaluation framework.

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