Food colorants: their past,present and future

Whether we are purchasing fresh vegetables from a market stall, ready meals from the supermarket, eating at home or in a five‐star restaurant, we use colour to tell us what to expect in terms of taste, nutrition and safety. This review considers the techniques that have, over the years, been employed to modify the colour of our food, and the interactions of these techniques with issues of safety and nutrition. The demand for brightly coloured food resulted in the incorporation of some questionable inorganic and organic chemistry being used in food products. A limited number of synthetic dyes are still used in food today, but health concerns and the consumer‐driven demand for natural colorants has brought about a change in the way food is coloured. The proliferation of products with labels that state they contain “No artificial colours” on supermarket shelves suggests that the future of azo dyes and their various derivatives is strictly limited. Nature produces an abundance of colours and many of these are extracted and used as natural food colorants; however, they are subject to application limitations and stability problems. Significant research by academia and industry into methods to stabilise and expand the application possibilities for the various approved natural food colorants is ongoing, but most developments that food colour manufacturers proclaim are enhanced vehicles for delivering established natural pigments into food products.     

Stabilisation and detoxification of henna (Lawsonia inermis L.) extract for hair dye cosmetics by spray‐drying encapsulation

Henna (Lawsonia inermis L.) leaves have been widely used as a natural plant colorant for hair dyeing and body art. However, the utilisation of henna dye extract in commercial colour cosmetics is compromised by its potential harmful effects to humans. This study demonstrates that spray‐drying encapsulation using maltodextrin and gum arabic as biocompatible polymeric carriers can be used as a promising strategy to detoxify henna dye extract while maintaining its hair dyeing efficiency for cosmetics. Henna extract‐loaded microcapsules with an average particle size of 4.1 μm were prepared with an excellent encapsulation efficiency (98.4%) and a negative zeta potential (?34.4 mV). The morphology and physicochemical properties of the microcapsules were characterised by comparison with a spray‐dried sample of neat henna extract. The in vitro toxicological results showed that the microencapsulation process significantly reduced the contact toxicity of henna extract towards human epidermal keratinocytes as well as to the eyes and skin. The encapsulated henna extract also exhibited improved storage stability at three different temperatures (4, 25 and 50°C) for 14 days. Furthermore, the microcapsules demonstrated satisfactory hair dyeing efficiency as natural colorants with good colour fastness towards light and washing.     

Extraction of polyphenolic substances from bark as natural colorants for wool dyeing

In Europe, considerable amounts of bark are available from wood‐processing industries such as forestry and timber production. Polyphenolic components can be collected by hot water extraction. The extracted compounds can then be applied as colorants in textile dyeing operations. In this study, a comparative assessment of four different tree species with regard to their colouristic potential for wool dyeing was performed. Aqueous extracts from alder, ash tree, spruce and oak bark were prepared and analysed for their total phenolic content and ultraviolet (UV) absorption at 360–370 nm. The extracts were used for meta‐mordant dyeing by adding iron sulphate mordant (FeSO₄ × 7H₂O). For comparison, iron salt‐based dye lakes were prepared and used in dyeing experiments. For each tree species, a specific correlation between the total phenolic content of the dyebath and the colour depth in terms of K/S and CIELab coordinates was observed, both for the aqueous extracts and the dye lakes. Based on this relationship, standardisation and quality control of raw materials and dye lakes can be installed as important stages in the industrialisation of natural colorants from bark. The preparation of concentrated dye lakes permits formation of a concentrated colorant as dye product, which then can be standardised and delivered to textile dyehouses, similar to synthetic dyes. The preparation of dye lakes offers a relevant route towards achieving the commercialisation of bark extracts as natural colorants.     

Colorants based on renewable resources and food‐grade colorants for application in thermoplastics

A series of colorants based on renewable resources and food‐grade colorants have been evaluated for use in polypropylene (PP) and polyvinylchloride (PVC). It has been found that most of these colorants can be processed in PP at 200°C or even 260°C while maintaining good color intensity and color brightness. The colorants evaluated cover a large part of the color spectrum. In PP, the light stability of alizarin (red), carmine (red), indigo (blue), purpurin (red), quinizarin (red), and the aluminium lakes of quinoline yellow (yellow) and indigo carmine (blue) is close to the requirements for indoor applications. The blue colorants indigo and the aluminium lake of indigo carmine are, in principle, sufficiently light stable in PP for indoor applications. A few colorants showed bleeding from PP. Bonding of migrating colorants to the reactive carrier maleic anhydride grafted polypropylene, however, reduced bleeding of the colorant to a large extent. Also after processing in PVC at 200°C, good color intensity and saturation is maintained. Quinizarin, a structural analog of alizarin and purpurin, shows a light stability performance that is close to commercial lead chromate/molybdate orange based colorants. The best performing natural colorants are sufficiently heat and light stable for applications where moderate properties concerning heat resistance and (UV) light stability are required, such as underground PVC water drainage pipes and indoor PP applications. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 2961‐2969, 2004     

Natural dyes in madder (Rubia spp.) and their extraction and analysis in historical textiles

Textiles coloration using extracts from the roots of various madder species (Rubia spp.) has been performed for centuries. To date, 68 anthraquinone colorants have been detected in Rubia spp. used to dye textiles. Many of these dyes are sensitive to hydrolysis and degradation from enzymes, extraction chemicals and processing temperatures, and are often overlooked as colorants in historical textiles. Conclusions in literature of the past 30 years concerning colorants present in planta and, particularly, in madder‐dyed artefacts are being challenged as new analysis methods are developed. The recent advent of ‘soft’ extraction techniques has demonstrated that anthraquinone glycosides and other sensitive molecules, such as carboxylated compounds, need to be preserved; this valuable chemical information embedded in the dye structure may be lost if extraction and analysis is too harsh. Some compounds thought to be present in madder and madder‐dyed artefacts are in fact degradation products resultant from the extraction process, and degradation pathways have been developed to better understand the reactivity and stability of these compounds. Detailed analysis of dyes in textile artefacts can reveal important cultural and heritage information concerning historical textiles relative to the specific dye species, the area of the world where this may have grown, how and where it was dyed, and, perhaps, where it was traded. Understanding the precise molecular structure of these dyes and their chemical reactivity is important to provide knowledge of their interactions with physical substrates, such as textile fibres, which could be used to develop superior techniques for analysis of artefacts.     

Talking about color … and ethics

Ethics is the study of the moral value of human conduct. The ethics of colour use has rarely been featured as a topic of concern to the colour community. A limited exception is that of the use of colorants in food. This has in the past received much attention but legislation has now greatly restricted such usage. However, concern is resurfacing, and there are many other examples of the possible misuse of colour in food and the wider environment. This editorial asks questions in three areas of life: the environment, food, and marketing in general. © 2006 Wiley Periodicals, Inc. Col Res Appl, 31, 87–89, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20204     

Assessment of the dyeing properties of pigments from Monascus purpureus

Monascus purpureus C322 was cultivated on well‐established production media to yield prevailingly red or orange pigment‐rich ethanolic extracts. Once these extracts had been diluted by an overall factor of 50, they were used as such to dye raw wool standard specimens differently premordanted using alum or stannic chloride. Independently of the mordant used, the specimens dyed with the red pigment‐rich extracts showed a pale red colour tending to pink, whereas the specimens dyed with the orange pigment‐rich extracts exhibited a more definite orange colour. By carrying out a few colourfastness standard tests (manual washing at 40 °C, acid and basic perspiration and hot pressing), stannic chloride‐premordanted wool specimens dyed with the red pigment‐rich extracts were found to be less resistant to acid and basic perspiration than their orange counterparts. Since the production of the orange pigment‐rich ethanolic extracts appeared to be more cost‐effective than that of their red counterparts, the former might support the present demand for colorants of natural origin in the textile sector. Copyright © 2005 Society of Chemical Industry     

Doped oxides of cerium as inorganic colorants

The colour of inorganic colorants is based on metal ions such as cadmium, lead, chromium or cobalt, the majority of which are unfortunately potentially toxic. Thus, there is in a need to introduce alternative metal or metal oxides, which would be environmentally friendly and economically viable as a replacement for potentially toxic inorganic colorants. Rare earth-based colorants offer an additional opportunity for the development of optically pure colours. In this study, a replacement for potentially toxic colorants has been sought through the use of a rare earth metal ion, cerium. Appropriate doping of cerium oxides with molybdenum and iron gives an orange–red colorant. The conditions ideal for optimal colour have been standardised and the resultant colorant characterised through powder X-ray diffraction techniques, particle size measurement, etc. The colorants exhibit good hiding power and tinting strength.     

Influence of a Natural Plant Antioxidant on the Ageing Process of Ethylene-norbornene Copolymer (Topas)

In the field of polymer technology, a variety of mainly synthetic additives are used to stabilize the materials during processing. However, natural compounds of plant origin can be a green alternative to chemicals such as synthetic polyphenols. An analysis of the effect of hesperidin on the aging behavior of ethylene-norbornene copolymer was performed. The evaluation of changes in the tested samples was possible by applying the following tests: determination of the surface energy and OIT values, mechanical properties analysis, colour change measurements, FT-IR and TGA analyses. The obtained results proved that hesperidin can be effectively used as natural stabilizer for polymers. Furthermore, as a result of this compound addition to Topas-silica composites, their surface and physico-mechanical properties have been improved and the resistance to aging significantly increased. Additionally, hesperidin can act as a dye or colour indicator and only few scientific reports describe a possibility of using flavonoids to detect changes in products during their service life, e.g., in food packaging. In the available literature, there is no information about the potential use of hesperidin as a stabilizer for cycloolefin copolymers. Therefore, this approach may contribute not only to the current state of knowledge, but also presents an eco-friendly solution that can be a good alternative to synthetic stabilizers.     

Mordant dye application on cotton: optimisation and combination with natural dyes

It is well known that cotton fibres can be dyed through the formation of coordinate bonds involving cellulose chains, mordants such as alum, and natural dyes such as alizarin. Similarly, synthetic dyes known as mordant acid dyes can be used to dye wool fibres. Unlike mordant dyes on wool, the fastnesses of natural dyes on cotton are often low. Although concerns surrounding textile sustainability have sparked renewed interest in the use of natural dyes, extensive replacement of synthetic dyes with natural dyes is neither practical nor fundamentally possible. However, similarities in dyeing methods using mordant and natural dyes raise the possibility of using mordant dyes as alternatives to natural dyes in the dyeing of cotton. Further, the potential for combining suitable dyes from these two classes to expand the colour gamut currently available from natural dyes on cotton seem worthy of exploration. The results of this study indicate that shades comparable with those produced by natural dyes can be obtained on cotton using select mordant dyes following Fe²⁺ and Al³⁺ pretreatments. The best results were obtained using a two‐step/two‐bath process and dyes such as CI Mordant Blue 13 and CI Mordant Orange 6. In evaluations of mordant and natural dye combinations using the two mordant dyes logwood and Osage orange as prototypes, interesting fabric shades were obtained. However, the fastness properties of these dyes must be improved in order to produce commercially viable dyeings.     

Colour in Cosmetics

The discussion is limited to those cosmetics that impart colour to the skin, i.e. make-up. After a brief survey of the use of cosmetics from very early times, legislation covering the use of colorants in cosmetics is discussed. The requirements of make-up are described: to formulate a make-up the technologist has to be aware of the trends in fashion. The two main operations in manufacturing make-up are milling and mixing; these are discussed in relation to the composition of the make-up. The type of skin is important in deciding what type of make-up should be used. Women probably wear make-up to improve their appearance or to be in fashion (or both). Principles of colour harmony are explained. The development of make-up over the past thirty years is traced and related to trends in fashion. Present and future trends in make-up are discussed.     

Ink‐jet printing of cotton with natural dyes

Four natural dyes, annatto, cutch, pomegranate fruit rind, and golden dock, were used as colorants for the preparation of water‐based ink‐jet inks for digital textile printing. The physical and rheological properties (pH, conductivity, surface tension, and viscosity) of the inks were measured over a period of 90 days for the evaluation of ink stability and suitability for ink‐jet printing. The inks were found to be suitable and were used for the digital printing of cotton fabrics. The prints were subjected to wash, light, and rub fastness tests and colour measurements. Colour consistency and fastness results, especially after fixation, are comparable with those of synthetic dyes, which paves the way for the production of environmentally friendly ink‐jet inks using natural dyes for the digital printing of cotton through the formulation of suitable printing techniques.     

Innovative technologies in the control of lipid oxidation

Transition metal promoted oxidative degradation reactions impact quality, shelf‐life, and nutritional content of many packaged foods. When trace metals are present in packaged foods, they can initiate degradation of nutritional compounds such as unsaturated fatty acids, carotenoids, antioxidants, phytosterols, and many vitamins. These reactions occur at metal concentrations naturally occurring in foods (often in the low parts per billion range) therefore complete metal removal is not a practical solution to inhibit these reactions. Chelators such as EDTA can be added to inhibit metal‐promoted oxidation, however there is significant consumer and industry demand to eliminate EDTA from product formulations. Other natural chelators such as citric acid are ineffective in many foods and in some cases can actually increase oxidation rates by increasing metal solubility. An alternate approached to control transition metal reactivity is to utilize active packaging technologies. However, current approaches to antioxidant active packaging can impact packaging film mechanical properties and often exhibit low antioxidant activity. This review article surveys and critically reviews advances in the control of lipid oxidation, paying particular attention to novel advances in both food product formulation (e. g. additives, food microstructure) as well as food packaging. We introduce a new concept of active packaging in which metal ion chelation by non‐migratory active packaging materials may enable removal of synthetic food additives from product formulations while maintaining product quality and shelf‐life.     

Molecular electronic spectroscopy: from often neglected fundamental principles to limitations of state‐of‐the‐art computational methods

The creation of the first synthetic dyes not only stimulated the hunt for new colorants but also drove the search for rules correlating the constitution of organic compounds with their colour. Dye chemistry additionally facilitated the development of molecular electronic spectroscopy as well as theories of molecular electronic structure and electronic transitions. Powerful quantum chemical computational tools are now available for the prediction of the electronic structure and spectroscopic characteristics of organic compounds. Such methods are thus useful in designing new functional colorants and aiding interpretation of their properties. However, without a deep appreciation of the principles and assumptions behind the calculations, one runs the risk of misunderstanding what can be achieved as well as becoming confused about how the outputted electronic and vibronic transition data correspond to observed absorption spectra. This review therefore aims to cover fundamentals of electronic spectroscopy that are often overlooked and enable the dye chemist using modern computational methods to comprehend the subtle differences in the types of transition energy value that such software can generate. In addition, the limitations of these methods in predicting absorption maxima and intensities of real‐world colorants will be discussed in the context of physical influences on absorption band position and shape, for example from the perspective of different forms of the Franck–Condon principle. In essence, the goal of this review is to clarify, in terms that practical dye chemists will understand, what computational methods can predict and how valid these predictions are compared with reality.     

Application of thermochromic colorants on textiles: temperature dependence of colorimetric properties

Commercial thermochromic colorants were applied to a conductive cotton fabric prepared by using nichrome/cotton core‐spun yarns in weft and 100% cotton in warp. The fabrics were pretreated and coloured with thermochromic pigments in isolation, in combination with each other, or in a mixture with a non‐thermochromic pigment. The weft yarns were joined to allow passage of current through the fabric to enable ohmic heat generation to increase fabric temperature. The heat generation and temperature rise could be controlled by monitoring the voltage applied. The colour of the samples changed gradually with an increase in temperature, and significantly so when the temperature of the fabric rose beyond the activation temperature of the thermochromic colorant. The temperature‐dependent properties of commercial thermochromic colorants were determined using a spectrophotometer. Wash fastness of the samples was found to be fair in all cases except with the yellow colorant. Predetermined colour effects, such as camouflage or novel design, can be produced by combining thermochromic colorants with conventional pigments or thermochromic colorants with different activation temperatures.     

Development and application of a mathematical model to explain fading rate inconsistencies observed in light‐sensitive materials

Light fastness tests conducted on several areas of a light‐sensitive material may sometimes show inconsistent fading rates. These different fading behaviours suggest that colorants are not evenly distributed over the substrate surface or may be attributed to texture variations of the material. A mathematical model has been developed to help explain these discrepancies. Micro‐fade testing, a relatively novel technique, has allowed microspectroscopic detection of variations in the initial colour parameters of a sample, permitting assessment of its diverging fading rates. The method has been applied to fading data of various light‐sensitive materials resulting in more consistent fading patterns. It has been demonstrated that micro‐fading curves obtained for the same material are related by a constant value, which is the ratio of adjusted values of time and ΔE*.     

Industrial organic photochromic dyes

Organic photochromic molecules are well established as colorants in the manufacture of niche products, providing striking colour change effects when irradiated with light. This paper describes the industrially important classes of photochromic dye in terms of their development and chemistries, the applications in which their photochromism is employed commercially, and the technical features behind their success. Photochromic systems form the subject of much industrially funded research into nascent high‐tech applications that have the potential to become the most commercially significant outlets for photochromic dyes. Those types being scrutinised are therefore also discussed along with the directions in which the industrial use of photochromic colorants may take.     

New theoretical and practical aspects of the frying process

The deep‐frying process, normally carried out at 140–200 °C, is a very complex system due to the combination of heat and mass transfer between food and frying medium. The system becomes more complicated as the frying operation proceeds, because the composition of the food being fried and the frying medium is changing continuously due to the progressive deterioration of the frying medium. Apart from a variety of chemical reactions occurring, several changes take place in the frying food, such as gelatinisation of starch, denaturation of protein, and decrease of moisture. These changes bring about swelling of the product, formation of a crusty layer, and the appearance of a golden colour, good texture and taste. The precise control of the fryer enables these physical and chemical changes in the frying of food to convert it into a desirable finished product. This article discusses various types of reactions occurring in the food frying operation, possible mechanisms, a new realistic method – OSET index for measuring heat stability of frying oils – and the protective behaviour of substances that enhance the frying stability of oils.     

Green composites: An overview

The use of natural fibers to reinforce polymers is a well‐established practice, and biocomposites are increasingly used in sectors such as automotive and construction. Green composites are a specific class of biocomposites, where a bio‐based polymer matrix is reinforced by natural fibers, and they represent an emerging area in polymer science. This work discusses the environmental benefits deriving from the use of natural fibers in polymer composites and from substitution of oil‐derived polymers by bio‐based polymers as matrix material. New trends in the selection of natural fibers, that is, from waste rather than from valuable crops are described. Recently developed thermoplastic and thermosetting bio‐based polymers are reviewed, and commercially available green composites obtained thereof are discussed. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Conducting polymers are not just conducting: a perspective for emerging technology

Conducting polymers, such as polyaniline and polypyrrole, are organic semiconductors with mixed electronic and ionic conductivity. In addition to electrical properties, their electrochemical activity opens research opportunities in corrosion protection or energy‐storage devices. Conducting polymers are active in catalysis, the photocatalytic decomposition of dyes or electrocatalysts in fuel cells being examples. In contrast to classical polymers, conducting polymers respond to various stimuli by changes in conductivity, colour or other physicochemical parameters; this is used in sensors. Conducting polymers are good adsorbents of organic pollutants in wastewater treatment and are likely to be applicable for environmental issues. The perspectives of these polymers are briefly outlined. © 2019 Society of Chemical Industry