The Stability of Natural Colour

Food is essential to humans, affecting their health and survival. People's need for food has not changed, but the types of food have changed significantly. In ancient times, humans hunted animals and gathered plants for food. Nowadays, the new era of humans is inseparable from the demand for industrial food. This demand has also promoted innovation in the appearance, flavor, and shelf life of such goods. The promotion of commodity trade and the improvement of transportation efficiency have also increased the demand for food transported over long distances. In this situation, maintaining the quality of the goods is of paramount importance. Food development As a front end, the use of food additives to reduce food changes is, in most cases, a cost-effective option. There is a wide range of food additives that need to meet market demand and the production of the food industry, while also ensuring compliance with the relevant laws and regulations [1].

A review of the stability of natural colors, including an introduction to the definition of natural colors, their structure, sources, functions, and biological effects, and a brief comparison with artificial colors, introducing their advantages and disadvantages. In addition, natural colors are classified by source, focusing on the enumeration of plant and animal sources to show common sources of natural colors. The discussion of the stability of natural colors focuses on four aspects: factors affecting natural colors, methods for determining the stability of natural colors, methods for improving the stability of natural colors, and the current state of development of natural colors. Existing technical methods and opinions are summarized, and a look to the future is also provided.

1 Overview of food additives

1.1 Legal definition of food additives

The Codex Alimentarius Commission (CAC) defines a food additive as “a substance, which is not normally consumed as food by itself nor as an ingredient of common food, whether or not it has nutritional value. The substance is added to a food because it is a necessary part of the process (including sensory) for the production, processing, preparation, treatment, packaging, boxing, transportation or storage of the food, or it is desired that it or its by-products become a component of the food (directly or indirectly), or it affects the characteristics of the food. The term does not include contaminants or substances added to preserve or enhance nutritional qualities.” In China, food additives are defined as ‘synthetic or natural substances added to food to improve its quality and colour, aroma and taste, as well as for preservation and processing needs.’ The Standard for the Use of Food Additives (GB2760–2014) classifies the functions of food additives into 22 categories, such as anti-caking agents, antioxidants, colouring agents, etc.

1.2 Natural food additives

The use of natural food additives is driven by a number of factors. From an environmental perspective, their use is considered a sustainable approach that meets the needs of industry and society without creating a shortage of food [2]. From a consumer perspective, there is a growing awareness of healthy eating, and with the increasing variety of food choices, there is also a growing demand for “clean labels”. This consumer demand for healthy eating and preference for healthy food is particularly pronounced in economically developed regions, where people are more aware of the risks that industrialised food may pose to their diet and their purchasing habits have been affected accordingly [3]. One of the main drivers for the use of natural food additives in food is consumers' desire for more transparent communication on product labels. This has become an important market force, promoting the use of less processed and more naturally sourced ingredients [4].

1.3 Natural Colour

Color plays an important role in the marketing of products and is essential for consumers to make purchasing choices [4]. Color is a key sensory indicator of food and can have a positive effect on consumers' desire to eat [5]. The choice of colorants used in food production includes synthetic and natural colors. Synthetic colorants are generally preferred in food processing and storage because they are more stable and have stronger coloring power than natural colors, and are also cheaper [4]. However, in recent years there has been increasing concern about the potential health risks of synthetic colors. For example, consumption of more than 50 mg of synthetic color can cause hyperactivity in children, allergic lesions and neurological diseases [6]. In addition, considering regulatory restrictions and consumer demand for natural foods, food companies tend to replace synthetic colors with natural colors.

The advantages of Natural Colour are significant. In addition to providing colour, they can also provide bioactivity to formulated products[7] . Their secondary metabolites exhibit effective biological effects such as antioxidant, anticancer, anti-obesity and neuroprotective activities[8] . They can contribute to human health. In addition, the replacement of artificial colours with Natural Colour can meet the consumer's pursuit of clean labels in modern food purchasing behaviour, as well as the values of naturalness and sustainability that some food companies attach to their products. Despite this trend, replacing artificial colours with Natural Colour is not technically straightforward. There are issues to overcome when applying Natural Colour to formulated products, including processing and shelf life. These issues stem from the fact that Natural Colour is not as pure as artificial colours and contains other ingredients such as proteins and sugars. Higher dosages are therefore required. This has an impact not only on formulation costs, but also on the chemical or physical properties of the food substrate [9].

1.4 Sources of Natural Colour

Natural Colour sources include plants, animals, microorganisms and minerals [10], and they have significant structural diversity. Most commercially used natural colours are of plant origin. They are extracted from certain parts of natural plants, including stems, roots, flowers, leaves, fruits and fruit peels [11]. Red and orange colors can be derived from turmeric, tomatoes, beetroots, chili peppers, etc. The main components of extracted natural colors are curcumin, β-carotene, capsanthin, lutein, lycopene, astaxanthin, etc.[10]. Blue colors can be derived from green plants and vegetables, generally indigo plants[12], and the main components include anthocyanins, etc. However, blue natural colors are more scarce than red and green ones [11]. According to Siddique et al. [13], this is because the blue part of the visible spectrum provides enough energy to raise orbital electrons to an excited state, which forces the molecule to absorb it, causing the pigment to appear redder or greener. Therefore, there are many more natural plant sources of red, orange, yellow, and green tones than blues.

Animals are also a source of natural colorants. The most common natural coloring agent of animal origin is carmine or carminic acid, which is obtained from dried and crushed female aphids (Dactylopius coccus Costa) [10]. Carmine is a water-soluble anthraquinone that is reddish in color and has a higher stability to light, heat and oxygen than plant-based pigments [14]. However, its production can be subject to regional and seasonal fluctuations, as the content of carmine in aphids is influenced by these factors [15]. Compared to other natural red pigments, it is very expensive [1]. Other animal-derived pigments are astaxanthin, which is isolated from shrimp shells, and echinocrome, which is isolated from echinoderms, and even some marine animals can be used as sources [16].

Microorganisms are described by Manzoor et al. [10] as a promising source of natural color. Compared to plant and animal sources, the raw materials for pigment extraction from microorganisms are more readily available, and the characteristics of easy cultivation allow them to grow quickly, resulting in lower production costs. Jurić et al. [17] share the same view. Microbial natural pigments have already been used in some applications. The red yeast rice fungus can be used to extract orange, red and yellow pigments [18], the methylobacterium can form pink to red colonies on selective isolation media using monocarboxylic and polycarboxylic compounds, and riboflavin from the Candida utilis, from chlorella, astaxanthin, etc. [19]. Manzoor et al. [10] showed that the production of Natural Colour can be maximized by optimizing the fermentation conditions (pH, aeration, temperature, medium composition, etc.). In recent decades, breakthroughs in industrial microbial fermentation and genetic modification have led to the mass production of Natural Colour with many health benefits. Table 1 summarizes common Natural Colour.

2 Discussion

2.1 Factors affecting the stability of Natural Colour

Natural Colour has many known advantages, but due to its natural properties, it also has some disadvantages, and poor stability is one of the most discussed disadvantages. The retention rate of pigment quality is unstable. For Natural Colour, they are sensitive to external interference. Factors that may affect their stability include light, temperature, humidity, oxygen, pH, enzymes, etc., as well as the proteins and metal ions they contain [31]. These influencing factors may come from the previous storage stage of raw materials, the intermediate processing stage of the product, and the later storage stage. Natural pigments of different types or structures are affected differently under the same conditions.

For example, the number and degree of hydroxylation and methylation of different anthocyanins are different, and the stability of anthocyanins is also different. Hydroxylation causes the pigment to turn greenish and reduces its stability, while methylation causes the pigment to turn reddish and increases its stability [32]. Natural colours are also sensitive to different external environments. For example, betalain is more stable than anthocyanin under high temperatures and pH 3–7 [33], so it can be used in low-acid and neutral foods. Generally speaking, the degree of instability and change in color is determined by the internal structure of the pigment and the external environment. For example, the characteristic yellow color of carotenoids may be related to an extended conjugated double bond system, with two cyclic structures on both sides of the molecule, which can absorb light in the shorter wavelength region of the visible spectrum. Therefore, high temperatures and light may affect the integrity of the molecular structure, limiting its biological activity and causing a loss of color. In addition, in applications, the water solubility of lutein, one of the carotenoids, limits its absorption during digestion, thereby reducing its availability in nutritional supplements [32].

In addition to the unstable retention rate of pigments, Natural Colour is also difficult to maintain in terms of consistency in terms of productivity and quality. The availability of Natural Colour from plants is affected by the seasons, and annual production is very limited [11]. Many plants are only harvested once a year. This indirectly increases procurement costs. Even for plants that can be harvested year-round, production is limited to certain regions due to climatic conditions [15]. Natural colourants of animal origin are also limited by their region of origin. For example, carmine is produced in South America, especially in Peru. The quality of carmine is also affected by fluctuations in the content of carminic acid in aphids [34]. In comparison, Natural Color from microbial sources is more stable than that from animal and plant sources. One reason for this is that the production conditions of microorganisms are more controllable, and the stability of the pigment during use is also better [14].

2.2 Methods for measuring the stability of Natural Color

Natural color stability can be evaluated from many dimensions. The main function of the pigment is to provide visual color to the product, so color stability is the most important dimension to evaluate. The color value in food is usually determined with the help of a colorimeter or spectrophotometer [35]. It is also supplemented by manual sensory evaluation in many studies [36]. When testing with high-performance liquid chromatography (HPLC), a prepared pigment solution is usually tested at the optimum wavelength first, so that it can be used as a wavelength standard for subsequent experiments to measure the absorbance. Next, a number of solution samples with different gradients are prepared, and some studies place them in different physical environments, such as different temperature conditions, heating times [37] and light conditions [38]. Other studies add different concentrations of metal ions or different chemical additives to the basic solution, or set up multiple pH gradients [39]. Alternatively, colorimetric methods can be used simultaneously, in which case the chromaticity characteristics are obtained in three dimensions: brightness (L*), red or green (a*) and yellow or blue (b*) [38]. Finally, the change in color is measured using the difference colorimetric method [37]. In addition to the two methods mentioned above, some studies use sensory evaluation as a supplement to the results. A sensory evaluation panel usually consists of several trained members [36]. Conducting multiple methods simultaneously can improve the accuracy of the test results.

2.3 Methods to improve the stability of Natural Colour

There are many factors that affect the stability of natural colors, and the food industry is faced with the challenge of finding ways to improve the stability of natural colors in processing techniques and product presentation. The sustainability and healthiness of these pigments is unanimously recognized, and although they are added as colorants, whether artificial or natural, within the limits recognized by regulations, they are considered to be free of health hazards. However, natural pigments undoubtedly minimize consumer concerns and also bring the possibility of reputation for the product itself. To maximise the use of Natural Colour, one of the principles of artificial improvement and processing technology to ensure enhanced bioavailability is to ensure that the molecule does not degrade [40]. For example, in the case of anthocyanins, microencapsulation has been proposed as an effective method to improve the solubility, stability, dispersion and ensure the bioavailability of the pigment [31]. At the same time, emerging green extraction techniques using low-temperature processing during the processing can prevent these Natural Colour from thermal damage and enzymatic degradation.

Research has shown that encapsulating the pigments and combining this technique with other methods can achieve the benefit of stabilizing the color and antioxidant activity of the product [41]. For anthocyanins, Chung et al. [42] found that the use of amino acids and peptides in beverage products containing ascorbic acid has the potential to improve color stability. For lutein, Steiner et al. [43] protected lutein by microencapsulation and used an emulsion-based delivery system to take advantage of its lipophilic bioactivity, prevent its degradation in the food matrix, and thereby improve its bioavailability. Other studies have found that the use of irradiation has a positive effect on pigment retention in red wine [44] and cucumbers [45]. The protection of natural colors has been mentioned in many studies, but for natural colors, which are inherently more expensive than artificial ones, protective measures for stability undoubtedly add to the cost. Therefore, the use of these measures in food industry practice remains to be studied.

2.4 Current development of natural colors

In China, there are vast land resources, abundant products and a wide variety of produce. Common agricultural and sideline products such as corn, sorghum and chili peppers can be found everywhere, while more rare animals, plants and minerals are also distributed in China, especially some raw materials that can be used in traditional Chinese medicine and food with similar properties. China's abundant resources provide raw materials for the development and extraction of natural colors, which is an advantage compared to foreign countries. However, many Natural Colour raw materials are unique to a particular region and are rare, so the cost of importing them is high for any country. Therefore, countries are also actively exploring the availability, extractability and sustainability of raw materials unique to their own countries. For example, Australia is actively exploring the flavours and colours of its native bush foods (Edible native flora or “Bushfoods” of Australia) [46].

Many studies have proposed feasible experimental schemes for the development, extraction and utilization of natural raw materials, but how to apply these technologies in the large-scale food industry remains to be solved. The methods commonly used for the extraction of commercial pigments are cumbersome, time-consuming and material-intensive, such as heating, maceration and grinding. The environmentally friendly processes that have been proposed have to some extent solved the problems of low extraction rate, low utilization rate and unstable color [10], but their low popularity and high technical difficulty are still problems for food manufacturers. In addition, corresponding policies, regulations and usage plans still need to be formulated by governments and industry sectors in various countries, taking into account raw materials, markets, production equipment and technical capabilities. The impact on human health and environmental sustainability needs to be fully considered to truly rationalize and legalize the development of resources.

3 Conclusion

Synthetic colors are widely used in food products, but natural colors are gradually replacing them. Natural colors are considered relatively safe for the human body and environmentally friendly. They add sensory richness to food, and their natural origin gives them high-quality and efficient sensory characteristics. However, natural colors also have obvious disadvantages. Problems such as unstable quality of raw materials, high sensitivity to the environment, low yield, and difficulty in mass industrial production are still before food companies. In the future, while exploring the sources and stability of natural pigments, more attention should also be paid to the regulatory constraints on toxicology and related safety issues. Then, the diversity of their combinations can be explored, more stable natural colours can be used to customize the colours of existing products, and equipment can be developed to enable the mass production of natural pigments, implementing new innovative and industrial solutions.

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