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The use of dyes for artistic, social, and religious purposes has been found in every human society since prehistoric times. Nowadays, color still plays a crucial role in the market success of a product. Furthermore, in nature, brightly-colored plants are often poisonous, inedible, and thus avoided by animals. In the same way, color is a visual parameter of food acceptability for humans and can determine the expectation of a pleasant or unpleasant taste. [1] An extensive record exists of prehistoric use of ochre and black pigments obtained by natural rocks and other geological components.
Later societies developed their way to obtain other colors from different sources. Egyptians used to make blue pigment from lapis lazuli. In 500 BC, in China Han Blue was developed from mineral sources, whereas Maya Blue was a mixture of minerals and Indigofera suffruticosa’s leaves. Among organic dyes stands out Tyrian purple, a purple pigment widely produced and commercialized by Phoenicians that was obtained from species of snails Murex brandaris and Murex trunculus. [2]
From the second half of the 19th century, the dying process was marked by a crucial turning point, the introduction of synthetic colorants on the market. The reason for their immediate success was their capacity to dye fabrics such as silk and resist thermal and light degradation and washing. From that moment on many other coal-derived dyes were commercialized and at the turn of the century these unmonitored additives spread throughout the USA and European countries in various foods, such as ketchup, mustard, jellies, and wines. However, soon after their diffusion, the awareness of these colorants’ toxicity spread out and a long journey towards international regulation began. [2]
Nowadays, there is increasing mistrust of synthetic dyes and the attention is moving towards natural ones. However, the extraction of natural pigments has its drawbacks, including cost and environmental impact. The main characteristics, classification, advantages, and disadvantages will be analyzed for both synthetic and natural dyes, exploring the principal sources of the latter. [1], [3]
Synthetic Dyes and Their Regulation
The history of synthetic dyes began with the patent of the dye “Mauvaine” by Henry Perkin in 1856. From that moment on a plethora of synthetic dyes was commercialized. The first ones were aniline-derived compounds obtained from coal. Nowadays synthetic colorants are characterized by a huge variety of structures and auxochromes. Both natural and synthetic colorants are divided into the following classes: azo, anthraquinone, triphenylmethane, nitro and nitroso, indigoid, xanthene, acridine, and phthalein. The main reasons for synthetic colorants’ success are:
- their stability over time,
- being inert to physical, chemical, and biological degradation,
- reproducibility of dyeing process,
- low cost.
The main sectors of their application are the textile, tanning, cosmetic, and food industries.[2] Azo dyes account for 60% of the total use of synthetic dyes due to their low cost, high intensity, and color fastness. Anthraquinones are the second most frequently used dyes due to their accessibility whereas indigoid and triphenylmethane are the most common in the textile industry. [2], [4]
Synthetic colorants’ success has been strongly reduced over the years due to the rapid discovery of toxic effects both on the environment and human health. Regarding environmental pollution, synthetic colorants tend to accumulate in wastewater. Indeed, 20% of the colorants used in the textile industry doesn’t fix on the fiber and disperses into water.
The side effects of these colorants being very stable is that they are also inert and thus recalcitrant, in particular the saturated compounds. They are not only carcinogenic, but increase environmental parameters such as chemical oxygen demand (COD), and they tend to biomagnify, generating high contamination rates at high levels of the trophic chain. [2]
Regarding food colorants, the legislation deeply varies from country to country. The strictest country on synthetic food dyes is India, which allows only 8 of them, followed by the USA and China. EU and Brazil allow a higher amount of artificial colors compared to other countries, but the main difference is that the EU carries out much more frequent reevaluation of authorized dyes than Brazil. The Joint FAO/WHO Expert Committee on Food Additives is the international scientific committee responsible for assessing the risks associated with the consumption of additives and setting an adequate daily intake (ADI) value. [4], [3]
The main health concerns are on azo dyes, due to their reduction to potentially carcinogenic aromatic amines by the azoreductases present in the intestinal bacteria. Data also show the toxic effects of the triphenylmethane group on metabolic activity. Furthermore, this compound absorption rapidly reaches the bloodstream.
A factor that enhanced public mistrust of food colorants is their highly toxic effects on children. [5] Indeed, not only their ADI value is lower due to their lower body weight, but they are also more likely to be exposed to food colorants that are usually contained in food such as sweets and lollipops that are mainly consumed by younger generations. [4], [6]
Natural Hues and Pigment Extraction
Current FDA regulations classify two types of colorants: certified and exempt from certification. This second category generally includes natural pigments, but no legal definition for the term natural has yet been adopted, leading to consumer and industrial confusion. Natural pigments often carry more limitations than synthetic colorants for coloring food.
They are less stable, they are not able to completely match the color characteristics of synthetic hue, and their production often involves a larger amount of raw material and higher production costs than their synthetic counterparts. However, the increasing success of natural colorants is the fact that, contrary to synthetic ones, they are not only safe, but they add beneficial characteristics to food and they can be categorized as nutraceuticals. [1], [7]
Flavonoid Derivatives
Are a group of secondary plant metabolites characterized by a C6C3C6 carbon skeletal backbone. Among these compounds, anthocyanins enclose the most important group of water-soluble pigments. [1] They are responsible for the colors of vegetables such as blueberries, blackberries, purple cabbage, haskap, etc. [7]
They differ from one another for the methoxylation and hydroxylation degree, in nature they are bound to sugars and can be further acylated with aromatic or aliphatic acids. Acylated and non-acylated anthocyanins differ in their stability and the brightness of the hue. Acylated anthocyanins are common in vegetal and floral systems, while non-acylated ones are predominant in fruits.[1]
Anthocyanins are pH-labile, at low pH values they are used as red colorants whereas in alkaline conditions their color turns to purple/blue and they are less stable towards temperature and light. Traditionally, they have been extracted using solid-liquid extraction with acidified organic polar solvents.
More modern methodologies for anthocyanin recovery are extraction with supercritical fluids (SFE) and extraction with pressurized liquids (), as they operate with low temperatures and short extraction times, avoiding the degradation of thermolabile secondary metabolites and allowing the use of non-toxic economic solvents. [7], [8]
Carotenoids
Are isoprenoid derivatives widely distributed in nature, including higher plants, bacteria, fungi, yeast, birds, and insects. They are 40-carbon tetraterpenoids that are divided into carotenes (only polyunsaturated hydrocarbons) and xanthophylls (contain oxygen atoms). These lipid-soluble compounds display colors ranging from yellow to orange to red. Increasing the conjugation system results in redder hues. [1], [7] Lycopene is a typical red natural colorant, found in tomato, watermelon, guava, and pink fruit.
Annatto is a pigment extracted from the tropical tree Bixa aurellana that is commonly used as a yellow colorant. Two pigments are responsible for the hue: bixin, a lipid-soluble carotenoid-type compound, and its saponified form, norbixin, which can be extracted with alkali solution and is thus water-soluble, which represents a major advantage compared to the other carotenoid, whose lipid solubility represents a limitation for industrial applications.[1] Traditionally, carotenoids are extracted with polar (xanthophylls) and apolar (carotenes) organic solvents.
However, the use of novel extraction techniques as alternatives to conventional extraction methods offers several advantages, from extraction efficiency to being environmentally friendly. Among these, ultrasound-assisted extraction (UAE) and extraction with supercritical CO2 are the most frequently used alternatives to traditional extraction. [7], [8]
Chlorophylls
Are symmetric cyclic tetrapyrroles and are present in every photosynthetic organism. They are found in nature with a phytol attachment and centralized magnesium ion. Chlorophylls’ typical color is green, but its tone can vary after extraction. Indeed, chlorophylls are deputed for photosynthesis and plants’ metabolic activity. Being catalysts, these compounds are intrinsically unstable and lose their green color very quickly.
They can be stabilized with cations such as Cu2+ or Zn2+. [1] Extraction of this class of compounds is realized through maceration and traditional extraction with polar organic solvents and sometimes with ultrasound-assisted extraction with organic solvents. This last technique allows to reduce extraction times. Since chlorophylls’ polarity range is wide, attention must be paid during the chromatography purification step, for which reversed-phase columns (C18 and C30) are the preferred stationary phase. [7]
Betalains
Are N-heterocyclic immonium derivatives derived from betalamic acid. They are mainly found in red beet and cactus pear and consist of red and yellow pigment. They are stable over a wide range of pH, but they are prone to light and temperature degradation which is why they are used as dyes in frozen products. Traditional extraction methods are characterized by higher extraction times and temperatures, which carry the risk of betalains degradation. [9] An innovative non-thermal method that reported good extraction yields is pulsed electric field-assisted extraction. Furthermore, betalains can be stabilized using additives such as antioxidants (e.g. ascorbic acid), chelating agents (EDTA, citric acid), preservatives and gums (e.g. pectin, locust bean gum). [1], [9]
Other Classes of Dies
Other important and widely used natural dyes do not belong to any of the above-mentioned classes. Carminic acid, found in cochineal insects from the superfamily Coccoidea, whose color varies from orange to purple increasing the pH value. Crocin, obtained from Saffron, the dried stigma of Crocus sativus flower, is authorized as a yellow-orange food colorant in the US, but not in the European Union where it’s considered a spice. Curcumin, demethoxycurcumin, and bisdemethoxycurcumin are yellow pigments contained in turmeric, a yellow spice from the rhizomes of the herb Curcuma longa.
Despite the prevalence of green hues in natural settings, cool natural dyes are hard to obtain. Apart from chlorophylls, other pigments in the blue-green range of tonalities are phycocyanins, protein-pigment complexes obtained from the cyanobacteria A. platensis, better known as spirulina. However, its use is limited by its poor light stability and sensitivity to heat, and this bright blue-colored compound is mainly used as an ice-cream colorant. [1]
Conclusions
Natural pigments are still characterized by many limitations to substitute synthetic dyes in all of their several applications. However, a coin always has two sides. On one side natural dyes production costs and wastes are still too high to be competitive, on the other both synthetic dyes and and their production wastes are recalcitrant and toxic for health and environment. [2], [4]
Especially in food industry, natural pigments are not only required by consumers because of their believed safety, but they could also improve the nutraceutical value of food thanks to their many biological activities (e.g. antimicrobial, anticancer, cardioprotective activities). On the other hand, natural pigment might confer unpleasant tastes to food as in the case of betalains obtained from red beet. [3], [7]
Undoubtedly, the ambiguity of the legislation on natural dyes strongly disfavors them compared to synthetic colorants. [1] Efforts have to be made both to increase natural pigments regulation and improve their production methods, making them real alternatives to synthetic colorants. The food industry has already set off to a conversion to more natural products, but it is worth asking whether the textile industry should do the same, considering it is the first industry sector for colorant dispersion into wastewater. [2]
References:
[1] G. T. Sigurdson, P. Tang, and M. M. Giusti, “Natural Colorants: Food Colorants from Natural Sources” , 2017, Annual Reviews Inc. doi: 10.1146/annurev-food-030216-025923.
[2] L. D. Ardila-Leal, R. A. Poutou-Piñales, A. M. Pedroza-Rodríguez, and B. E. Quevedo-Hidalgo, “A brief history of colour, the environmental impact of synthetic dyes and removal by using laccases” 2021, MDPI AG. doi: 10.3390/molecules26133813.
[3] A. Burrows, “Palette of our palates: A brief history of food coloring and its regulation” 2009. doi: 10.1111/j.1541-4337.2009.00089.x.
[4] I. G. C. Mota, R. A. M. Das Neves, S. S. D. C. Nascimento, B. L. L. Maciel, A. H. D. A. Morais, and T. S. Passos, “Artificial Dyes: Health Risks and the Need for Revision of International Regulations” 2023, Taylor and Francis Ltd. doi: 10.1080/87559129.2021.1934694.
[5] J. Huff, M. F. Jacobson, and D. L. Davis, “The limits of two-year bioassay exposure regimens for identifying chemical carcinogens” 2008. doi: 10.1289/ehp.10716.
[6] M. Lucová, J. Hojerová, S. PaŽoureková, and Z. Klimová, “Absorption of triphenylmethane dyes Brilliant Blue and Patent Blue through intact skin, shaven skin and lingual mucosa from daily life products”, 2013, Food and Chemical Toxicology, 52, pp. 19–27. doi: 10.1016/j.fct.2012.10.027.
[7] A. K. Molina, R. C. G. Corrêa, M. A. Prieto, C. Pereira, and L. Barros, “Bioactive Natural Pigments’ Extraction, Isolation, and Stability in Food Applications”, 2023, MDPI. doi: 10.3390/molecules28031200.
[8] I. Karimi Sani et al., “Pulsed electric field-assisted extraction of natural colorants; principles and applications,” Oct. 01, 2024, Elsevier Ltd. doi: 10.1016/j.fbio.2024.104746.
[9] R. N. Arshad et al., “Pulsed electric field: A potential alternative towards a sustainable food processing,” , 2021, Trends Food Sci Technol, vol. 111, pp. 43–54. doi: 10.1016/J.TIFS.2021.02.041.
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