Application of synthetic colourants to foods, snacks and beverages has been on the increase within the past 50 years and up to 500% increase has been reported. Consumers of colour foods and beverages have been showing worries on the possible health hazards of such products over time and this has led to a series of reports by researchers on the health implications of synthetic food colourants. This present study took a hard look into variously published reports on a plant-based and commonly used synthetic food colourants. It is revealed that synthetic colourants cause more harm than good to a human and some harmful compounds have been revealed in synthetic colourants. The noticeable side effects of consumption of synthetic colourant include behavioural problems inform of neurobehavioural disorder, aggression, attention-deficit/hyperactivity disorder (ADHD) and inflammatory cascade. In conclusion, synthetic food colourants should be replaced with natural food colourants to avoid further health effect on humans. Also, parents and food producers should be advised on the adverse effect of synthetic food colourants while the government regulates every food production company to ensure proper choice and application of food colourants as well as candid labelling for safety purposes.
Food additives are defined as substances or mixture of one or two substances that are not a basic component of the food, which is introduced to food to check factors such as spoilage, degradation, nutrient loss, decrease or loss of aesthetic properties and functional properties1-3. According to Food and Drug Administration (FDA)4, a food colourant is defined as any pigment, dye, or substance which when applied or added to a drug, food, cosmetic, or the human body, is capable (through reactions with other substances or alone) of imparting colour. In addition to the definition, food additives do not serve as an agent hiding the poor quality of food and food products5. Various food products for example chocolates, chips, dairy products, drinks, fermented products and others attain a high shelf-life level due to the addition of food additives. Food additives such as food colourants and others can either be direct or indirect depending on the purpose for which it is applied and their use is regulated to ensure the safety of that food in which they are added6. Addition of food additives in food products at various stages is done for unique purposes which include; colour improvement, nutritional improvement, prevention of microbial invasion, avoids physicochemical changes, prevents oxidation reaction of any kind, to improve the organoleptic properties of the food such as flavour, taste, texture, mouthfeel, aroma, to extend shelf life via inhibition of; ripening, sprouting and browning. Food additives are of two types; it can be natural or synthetic depending on the source from which it is derived. Salt and wood smoke are ancient natural food preservatives since 900BC, while mauvine is the premium synthetic food colourant7.
Natural colourants are the first food colourants used in the Palaeolithic period before the birth of synthetic food colourants8. In China, the premium record on the use of natural dyes is dated back to 2600BC7. Also, Saffron, as mentioned in the Holy Bible, is an orange-yellow colour dye made from the saffron plant’s stigma and hennas, a reddish plant substance prepared from the dried leaves of Lawsonia inernis was used prior 2500 BC. Wrapping of dead bodies in Egypt was done with coloured cloth5, in the quest for knowledge, a chemical test carried out on the red cloth is seen in king Tutan Khamen’s sepulchre confirmed the presence of alizarin. Alizarin is a pigment, extracted from a herbaceous plant, Rubia tinctorum cultivated for a red-purple dye obtained from the root; it is also known as madder. In the damage of Menhnanjo-daro and Harappan Civilisation in the 3500 BC, a subcontinent of India, dyes were applied and this was confirmed by a coloured garment of cloth and traces of madder dye found at the scene8. Within 1500 BC in Egypt, candy producers do apply natural colour extracts as well as wine to candies in order to improve its aesthetic value. Dyes such as brazil-wood, indigo, madder and dark reddish-purple colour were known in the 4th century which gave the name Brazil to the country because brazil-wood and a blue dye got from the leaves of Isatis tinctoria known as woad was first found there. In Japan, the application of natural colourants in food was traced to the 8th century from the Nara moment text, in shosho which has references to adzuki bean cake and soybean containing coloured substances. The people of Aztec and Maya culture moment of Central and North America were the first to use cochineal dye extracted from edible insects. Mauve is the first synthetic dye, manufactured in the year 1856 by Sir, William Henry Perkin9. Further production and extraction of synthetic food colours sets-in during the early 19th century. They were produced from petroleum-derived products such as aniline. Coal serves as the raw material, which gave it the name “coal-tar”7. The relationship between the art of colouring and civilisation has made scientist dwell in deeper research on food colourants, scrutinising for its safety purposes10.
Consumer’s concern about the toxicity of most synthetic food colourants such as cancer, a hypertensive reaction in children, allergies, asthma, neurobehavioural hazard among others has led this research to critically look into mostly applied synthetic food colourants and natural food colourant as a healthy alternative, to help both the manufacturer and consumer in making a healthy choice always.
Advantages of using food colourants: We eat with our eyes. An adage says; humans feast/eat with their eyes first. This is true in all perspective. The food colour, either natural or synthetic source, is a very crucial factor to the degree of pleasure man derives from food. Colour and flavour are major factors that make food appealing to the consumer. It is proven that we taste with our eyes; this shows the important role played by colour in life. Colour, flavour, shape and other physical properties of food always affect the eating desire of humans11,12. Information on a food product is mostly dependent on the colour it has. Whenever the colour of a food deviates from the information it is originally meant to convey; a negative psychological effect is being created in human on the physicochemical properties of the food. Colour of food affects its general acceptability due to people are easily moved by colour5.
Additionally, the degree of sweetness and palatability of food is also affected by its colour which deems it right to say that colour and appearance play a vital role in the quality attribute of a food. Colour is as old as mother nature herself and has been used since the onset of recorded history to increase the appeal of foodstuffs. It is a major factor for acceptability of products such as food, cosmetics, textiles and others because naturally, man has a keen interest in colour.
Addition of food colourants to foods, ranging from candy to wine is essential to meet the expectations of consumers. Foods can be coloured due to its effect as in the case of green ketchup by Heinz in 2000. Food colour can affect the flavour of foods because humans relate distinct flavours to distinct colours. Due to variation in colour of foods through the seasons, coupled with the effect of processing and storage, colour addition is employed commercially to retain the preferred and expected colour by consumers-example; adding red colour to glace cherries. Primarily, the reasons for colouring foods include; to bring back the original appearance of the food which may have been affected by the processing operations, to improve the aesthetic attribute of the food, to make the food appealing and irresistible, to enhance the visual characteristics of the food, to apply colour to virtually colourless foods, to ensure uniformity in the colour of a food batch, to enhance the already existing food colour, to provide identity to the food, to protect vitamins and flavours from destruction by light, to mask natural variation in colour, to stimulate appetite, to prevent discouragement and reduction in the desire for the food and beautification/artistic purposes as in cake icing.
Classification of food colourants: Generally, food colourants are divided into two major classes depending on their sources as synthetic and natural.
Synthetic food colourants: Synthetic food colours, also referred to as artificial food colours are man-made food colourants, manufactured through the chemical synthesis of organic coal-tar. In food, pharmaceutical and cosmetic industries, synthetic colourants are mostly used because of the following properties; high colouring ability, brightness, stability, application ease, homogeneity, a variety of colour shades and less expensive, although its health safety issues are alarming13. Examples of the synthetic colourant are sunset yellow, tartrazine, brilliant blue FCF, ponceau 4R, fast green, Allura red, carmoisine. Synthetic colourants are mainly derived from the synthesis of chemical substances of petroleum family such as coal-tar. They are more economical for large scale and industrial use because they have various shades of colour readily available for use, they are affordable, the colours are bright, homogeneity properties and good colour shade, which makes them more popular5. A large number of synthetic colourants, similar to organic aniline dyes with different colours and rainbow shades have been developed and are used in food colouring without knowledge on their safety. Regulators moved into critical research on synthetic food colours due to the significant toxicity of petroleum-based colourants and aniline. The most popular certified food colour is FD and C Red No. 40, followed by FD and C Yellow No. 5. Food colourants with the acronym FD and C show that the Food and Drug Administration (FDA) has approved its application in food, drugs and other products. For application in foods, the certified synthetic colours are; Tartrazine (lemon yellow), Orange B, Brilliant Blue FCF, Fast green FCF, Indigotine, Citrus red AC (orange) with assigned E-numbers14. Synthetic food colourants can further be classified into two major classes: primary and secondary food colourants.
Primary food colourants: These are food colourants that exist basically on their own. This class of food colourants when mixed with another primary colourant yields a desired different colour. Examples include; tartrazine, quinolone yellow, erythrosine, sunset yellow FCF, ponceau 4R, carmoisine, Allura red, amaranth, chocolate brown HT, patent indigo carmine, blue v, black PN, fast green, fast red, red 2G, brilliant blue FCF.
Tartrazine: Tartrazine is a lemon-yellow, water-soluble, azo dye synthetic colourant, with a molecular formula C16H9Na3O9S2, CAS 1934-21-0 and molar mass of 534.3 g mol1. The International Union of Pure and Applied Chemists (IUPAC) name is trisodium (4E)-5-oxo-1-(4-sulfonatophenyl)-4-((4 sulfonatenyl) hydrazono) 3 pyrazole-carboxylate. Other names for tartrazine includes E number E102 or C.I 19140, FD and C yellow 5. Tartrazine has 427 = 2nm maximum absorbance when in a liquid solution. According to the European Food Safety Authority (EFSA), tartrazine is mostly applied in cereals, honey and lemon products, sauce, sports drinks, chewing gum, yoghurt, noodles, pickles, fruit squash, potato chips, energy drinks, flavoured corn chips, cake mixes, ice cream, candy and others15,16. Tartrazine is the most commercially used synthetic food colourant due to its yellow colour and also can be used in the manufacturing of green shades when in combination with other synthetic food colourants such as Green S (E142) and brilliant blue FCF, FD, E133 and C blue)15.
Sunset yellow: Sunset yellow FCF is a yellow, water-soluble synthetic coal tar azo dye with E number E110, IUPAC name disodium 6-hydroxy-5-[(4-sulfophenyl) azo]-2-naphthalene sulfonate, a molecular formula of C16H10NaO7S2N2, the molar mass of 452.37 g mol1, CAS No. 2783-94-0 and a melting point of 300°C. It is also known as orange-yellow 5, C.I. 15985; FD and C yellow 6 and E110. It is very common in fermented foods. When applied in fermented foods, the food product should be properly heat-treated16,17. When dissolved in water, sunset yellow FCF passes through a change in phase that moves from anisotropic liquid to a nematic liquid crystal. It is mainly used in food like packet soups, lemon curd, sweets, soft drinks, sweets, apricot jam, orange jelly and squash, bread crumbs, Swiss roll, snacks and other food products that have red, orange and yellow colour15,18. Sunset yellow has been linked with a lot detrimental effect on human such as cancer, hyperactivity in children, allergic reactions, diarrhoea, vomiting, skin irritation and has been repeatedly called to be banned from products.
Allura red AC: Allura Red AC as the name implies is a red azo dye, with the IUPAC name; Disodium 6-hydroxy-5-(2-methoxy-5-methyl-4 sulfophenyl) azo] -2-naphthalene sulfonate; molecular formula: C18H14N2O8S2, Molar mass of 496.42 g mol1, CAS No. 25956-17-6, melting point of >300°C and E number E129. Other names of Allura Red AC are Food Red 17, Allura Red, FD, C.I. 16035 and E129, C Red 40 2 naphthalene-sulfonate acid, disodium salt; 6-hydroxy-5- [(2-methoxy-5-methyl 4sulfophenyl) azo]-2-naphthalene-sulfornate. It has a maximum absorbance of 504 nm in water and is gotten from petroleum or coal tar and is applied in products like children’ drugs, soft drinks, cotton candy as Food and Drug Administrator approved3.
Brilliant blue FCF: Brilliant Blue is a water-soluble coal tar derived synthetic food colourant with the E number E133 and 42090 as a colour index. Brilliant Blue FCF has a maximum absorption of 628 Nanometer in water solution. It has a molecular formula of C37H34N2Na2O9S3 and CAS No. 3844-45-9. Its other names include; patent blue AR, D and C Blue No 4, Antacid Blue FG, Xylene Blue VSG, FD and C Blue No. 1, Erioglaucine Eriosky blue, Acid Blue 9 and Alzen Food Blue No. 1. It appears in a reddish-blue powder form and can give various shades of green colouration when mixed with tartrazine (E102). Mostly, Brilliant Blue FCF is a disodium salt but can also exist as calcium and potassium salts, with 2650-18-2 as its CAS number. Brilliant Blue FCF is commonly present in sweets, canned, processed pear, dairy products and ice-cream17. It has been banned in most countries such as Switzerland, Germany, Italy, Greece, Norway and Austria among others due to its power to stimulate an allergic reaction in asthma patients.
Fast green FCF: Fast green FCF is a synthetic sea-green triaryl methane food colourant with IUPAC name;ethyl-[4-[[4-[ethyl-[3-sulfohenyl]methyl]amino]phenyl]-[4-hydroxy-2-sulfophenyl]methylidene]-1-cyclohexa-2,5-dienylidene]-[[3sulfophenyl]methyl] azanium, molecular formula of C37H37N2O10S3 and the E number E143. Its maximum absorption is 625 nm. Other names for fast green FCF includes; green 1724, food green 3 and C.I. 42053. Fast green FCF has poor absorption by the intestine. This food colourant has been confirmed to cause the growth of tumour and mutagenic effect both in humans and animals. Also, in concentrated form, fast green FCF can lead to skin, eye, respiratory and digestive tract irritation. Its application is found in jellies, fish, sauce, desserts, vegetables, green peas and dry bakery mixes at 100 mg kg1 and has been banned in some countries as well as European Union1.
Secondary food colourants: These are synthetic colourants produced by blending of two primary colours. It is done by the manufacturer to get a desired colour shade and strength to satisfy the consumer. Example chocolate brown, dark chocolate, blackcurrant, coffee brown, pea green, lime green, apple green, egg yellow, yolk yellow, strawberry red, rose pink, raspberry red, grape and violet.
Azo violet: Azo violet is a dark reddish-brown organic chemical substance with the molecular formula C12H9N3N3O4; IUPAC name 4-[4-ni trophenyl] azobenzene-1,3-dol, the density of 1.45 g/cm3 and a vapour temperature of 261.7°C. Azos can be described as compounds derived from diphenyl diazene, azobenzene or diazene which are nitrogen-based13. At times, it is applied as a pH indicator or a dye; it gives different colour shades based on the salts, the amount used and the medium. In a slightly alkaline medium with magnesium, it gives blue colour and in a salt solution of Magnesium will form a violet colour. It is still used for magnesium test.
Chocolate brown: Chocolate brown is a synthetic coal tar diazo dye that is brown with molecular formula C27H18N4Na2O9S2, Molar mass of 652.56 g mol1 and E-number E155. It is applied mostly in chocolate cakes, yoghurts, jams, fish, milk, cheese, fruit products and other products. Its application is banned in some countries such as France, Denmark, Australia, Norway, Sweden, United States and Switzerland due to its ability to trigger allergic reactions in asthmatics people, skin sensitivity and other individuals sensitive to substances such as aspirin2.
Chemistry of synthetic food colourants: In food products, colour additives are available as either dye or lake pigments.
Dye: Dyes are synthetic colourants, soluble in water but insoluble in oil. They are produced in granules, liquid, powders and another form in which they are needed. Dyes are applied in food products like dairy products, baked products, dry mixes, beverages and pet foods. An example includes azo dyes13. Dyes are capable of colouring stool when ingested in a higher amount, which is one of the adverse effects of dyes, unlike lakes.
Lakes: This group of synthetic colourants are manufactured by the mixture of dyes with salts to yield both water and oil insoluble compound but are oil dispersible. Lake colourants are a finely powdered precipitate of aluminium hydrate substrate and are stable and ideal for fats, oils or moisture lacking products3. Dye’s particle size and content determine the tone of the lake's colour. It is applied in cakes, hard sweet, powdered drinks, confectionery, chewing gum and biscuit fillings18. An example includes; lake Amaranth (E123), lake Sunset yellow FCF(E110), lake Tartrazine (E102), lake Carmoisine (E102), lake Ponceau 4R (E124), Brilliant lake blue (E133), lake Allura red (E129). Table 1 below shows the structure, property, application and safety of synthetic food colourant.
Natural colourants: Natural colourants are the group of colourants derived from nature, either from plant source (fruits and vegetables), animal source (insects, shellfish, cow urine), mineral source (iron, archers, umbers) or microbial source (algae, yeast, fungi and bacteria)5. Natural colours are present in our daily diets. Through our daily meals that we eat, we consume chlorophylls, anthocyanins and carotenoids. For food production purposes, they are extracted from their natural source and then applied to foods and food products to improve its aesthetic value and to ensure that identification properties are not defeated. Nowadays, consumers are seriously concerned with their health and what they eat, which makes them move towards natural food products. Natural colours are moderately bright, have an antioxidant effect and also are bioactive. Natural colours have relatively no health effect on human unlike the synthetic colourant, therefore are recently used in food industries to maintain and satisfy their customers19.
Natural food colourants are those substances derived from mineral, plant, animal or any other natural sources that can serve as a colourant for food, pharmaceutical and cosmetic purposes. Natural colourants remain the safest for business purposes due to consumers’ consciousness of health implications caused by synthetic food colourants19,20. A highly purified form of these natural colourants is usually made available to attain good yield, enhance its stability, meet consumers demand and for convenience purposes21. Some factors as pH, temperature, light, storage time and presence of other ingredients are considered in the application of natural food colours. Examples are; Annatto, Paprika, Saffron, caramel, cochineal, saffron, chlorophyll, Anthocyanin, Betanin, betalain, lycopene, carmine.
Classification of natural colourant: Natural colourants can be classified based on; origin, extraction methods and chemical constituents.
Classification based on origin:
Plant origin: This group of natural colourants are derived from trunks of plants, the bark of trees, fruits, flowers, leaves, dried or wet vegetables of plants. All these mentioned parts of plants produce colour in their unique way which includes purple and blue colour form cornflower which is mainly applied as sugar colouring, tea ingredients and also for confectioneries22, red colour fruit juice from bilberry that can turn blue in basic medium. Brown colour from Camelia tea, the curcumin which is the primary colour pigment that yields yellow-orange colour from Curcuma longa L. (turmeric) mainly used in beverages, cereal, baked and dairy products, green colour from chlorophyll which the green pigment found in plants. Blueberries also yield blue colour which is applied in foods and medicine18 Red colour from Paprika as a result of capsanthin and capsorubin which are carotenoids23. Other examples are madder roots; red, turmeric; yellow, waterlily-blue. Most of these colourants from plants also have other application such as medical purposes22, the colouring of papers and others. Over 500 species of vegetables and plants are found suitable to serve as colourants in India7.
Animal/insect origin: These colourants produced from animal sources is gotten from dried animal/insect bodies and their secretion. The oldest animal dye and most expensive is known as murex. Examples are; sea mollusc secretes a deep violet colour, shellfish, lac insect secretion and cow urine7. Cochineal (Dactylopius coccus) is an insect. The red dye extracted from this insect and its egg exists n form of ceramic acid and is mostly applied in foods such as yoghurt, candy, juices and ice cream. It can lead to an allergic reaction as well as anaphylactic shock in some humans. Sepia Officinalis L (female cuttlefish) gives rich concentrates of an orange-red pigment known as sepiaxnthine22.
Mineral origin: These group of natural colourants unlike that of plant and animal origin is derived from earth or minerals such as umbers, iron, hydrated oxides and oxides of iron and manganese, archers, titanium dioxide24-26. This group of natural food colourant can be applied in food pantries, paints and cosmetics.
Microbial origin: As the name implies, these natural colourants are gotten from microbiological action of algae, fungi, yeast and bacteria and are mostly applied in energy drinks, baby foods, sauces and dairy products27. These natural colourants include yellow from ash by a gossip, brown from bacillus, red from Dunaliella sali. These colourants have both anticancer properties and antioxidant. Monascus purpureus is a fungus that produces a red pigment traditionally used in candy production in some oriental countries28. They are affected by pH, moisture content, temperature and fermentation during production. Several food colourants have been isolated from microbes and applied as food-grade colours, they include; Astaxanthin, Canthaxanthin, Prodigiosin, Violacein, Phycocyanin, Ribo avin, Beta-carotene, Lycopene, Melanin among others27.
Classification based on extraction methods: Natural colourant under this group is manufactured by natural sources such as plant, fungus, microbes, minerals and animals. In the preparation processes which are a very difficult process, the ink is manufactured from the colourant extracted from the natural source using appropriate constituents. There are various methods of extraction of natural colourant and they include; aqueous extraction, ultrasonic and microwave aided extraction, Solvent extraction, Acid/alkali extraction, enzymatic extraction, Fermentation extraction and Supercritical fluid extraction29.
Classification based on chemical constituents:
Under this group falls carajurin which is produced from Bignonia chica
In this group are the red colour dyes, examples are cochineal, lac, Indian madder, Alizarin and morinda
In this group, the presence of the double bond determines the colour. The major natural colourants under this group are Bixin and norbixin from annatto seed and crocin from saffron stigma
This group are replaced dihydropyrans with a flavonoid-like structure
Under this class are the yellow pigments, they include rutin, quercitrin, luteolin and others
This group is responsible for the production of reddish-brown, orange and red shades. Examples are henna and walnut shell
This group is made up of polyphenolic components of tannins. In textile manufacturing, tannin needs mordant and the colour of tannin varies as the mordant varies too. Example; Cutch and Babool
As shown in Table 2 below, the classification of food colourants of natural origin can be extremely complex because of their various unique properties and functional groups they pose. Therefore, distributing various roles at different capacities due to colour shades each provides.