Encapsulation of colorants by natural polymers for food applications

Product appearance is an important factor for consumers when determining the quality of a product, and colour is one of the most important factors which contribute to product appearance. Currently, the safety and consumer acceptance of some colorants used in food products, such as titanium dioxide and some synthetic colorants, are under discussion. Therefore, new ways to use natural colorants as alternatives to these suspect colorants for future applications are being investigated. A promising method for increasing the applicability of the often sensitive natural colorants is the encapsulation of these colorants in colloidal particles by natural polymers such as carbohydrates, lipids and proteins. In recent years, micro‐ and nano‐encapsulation have increasingly been used for various purposes concerning several food properties such as colour, flavour and micronutrient content. This technique results in improved stability for the often sensitive natural colorants and presents the possibility of entrapping water‐insoluble colorants for improved use in an aqueous system. This paper reviews the main methods that are used for encapsulation by natural polymers, discusses the different types thereof that are used for encapsulation of colorants, and provides a short overview of natural colorants successfully encapsulated in these natural polymers.     

Biosynthesis and synthesis of natural colours

In nature, the detection of colours requires an organism having some type of eye with a retina and two or three types of photoreceptor connected to a nervous system, which can interpret the signals received. Evidence that certain simple organisms were coloured 1 billion years ago, and some more advanced creatures, which could have possessed eyes a few hundreds of million years ago, is exemplified. A vast array of chemicals essential to life are produced by living organisms and their biosynthesis depends upon individual genetic patterns, which determine the enzyme catalysts involved. Plants photosynthesise many pigments, which are essential for them to maximise the absorption of energy from the sun, while others offer protection from any harmful radiation. Such pigments, for example chlorophylls and the carotenoids, flavonoids and betalains, have traditionally been used as natural dyes, food colorants and medicines. This review compares the chemical processes involved in their biosynthesis and the laboratory methods adopted to confirm their chemical structure. Some engineered biosynthetic methods are now used for commercial production of natural colours and these methods may involve the controversial use of genetic engineering.     

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.     

The Coloration of Plastics

Although plastics can be coloured by various techniques, in the main they are mass-coloured with dyes or organic or inorganic pigments. Some of this colouring is done during manufacture, but a growing proportion is now carried out subsequently and even as late as when the plastic is being extruded or injection-moulded into its final form. The way in which colorants are selected for use with plastics and the problems that beset the colourist in preparing economical colour formulations of good performance are discussed.     

Significance of dye research and development for practical dyers

The history of research and development on synthetic dyes stretches back for 150 years. Major contributions to practical dyeing have been the provision of dye ranges for both natural and synthetic fibres of a sufficiently wide colour gamut to obtain a multitude of colours. Dye standardisation, with the associated analytical, testing and evaluation methods, has ensured that products are fit for purpose. Ecotoxicology has been a major issue for several decades, adding to the cost of research but improving the health and safety aspects of dyes. Developments in physical forms of dyes have been designed to improve health, safety, hygiene, ease of handling and accuracy in dispensing. Application research, in conjunction with dye users and machinery makers, has been a long-established feature of the industry. This has resulted in improved, controlled and systematic dye application techniques, leading to shorter dyeing cycles, right-first-time (RFT) processing and a reduction in processing costs. Although dye making is now based mainly in Asia, it is hoped that R&D efforts will continue to ensure that products of the necessary quality and performance remain available.     

Fungal colorants in applications – focus on Cortinarius species

Secondary metabolites in fungi offer an interesting source of bio‐based compounds that could be used as colorants in many applications. From a historical point of view, fungal natural dyes have been used more rarely than plant‐based dyes. This paper investigates the potential of fungal colorants, using Cortinarius species as examples. In our research, fruiting bodies of the fungi Cortinarius sanguineus and Cortinarius semisanguineus were used as sources of anthraquinone dyestuffs. From 10 kg of fresh fruiting bodies, 60 g of anthraquinone powder was obtained, 6% of the dry weight content. The most abundant compounds were emodin, dermocybin and their glucosides, which formed over 90% of the total dyestuff amount. Pure emodin and dermocybin, as well as the crude water extract, were used for the dyeing and printing of natural and synthetic fibres. Conventional mordant techniques and high‐temperature (HT) disperse dye techniques were applied, and light and washing fastness were tested according to International Organization for Standardization standards. Our experiments show that the yields of dye powders extracted from fungi are reasonable compared with the yields of, for example, madder (Rubia tinctorum). Natural anthraquinones produce strong and bright colours on several types of fibres. In particular, for HT disperse dyed polyester, the light and washing fastness properties were excellent. Anthraquinones are common in nature and there are many fungal species which produce them, so there are a variety of possibilities for growing fungi. The use of large‐scale cultures is an interesting perspective for future biocolorant production.     

Colour matching for smart and sustainable spinning of coloured textiles

The colours and patterns of coloured textiles are usually obtained via dyeing or printing processes. However, these processes consume large amounts of electricity and cause water pollution, which affects the ecological environment. The hand feel of dyed fabrics is superior to that of printed fabrics. Three‐channel rotor spinning is a highly flexible, adaptable and sustainable method for producing coloured textiles by blending precoloured fibres during the spinning process. Additionally, the process requires approximately half the water required for fabric dyeing or printing. Herein, the colour characteristics, as well as the advantages, of the coloured textiles produced by the new method are demonstrated. Three types of Stearns‐Noechel models are modified to describe the relationship between the blending ratios and resulting textile colours. The colour‐matching accuracy is high. As demonstrated by the results, the three‐channel rotor spinning method can effectively promote coloured textile engineering.     

Synthesis of polycondensable anthraquinone dyes and coloured nylon fibres: III

A series of 1,5-diaryl- and 1,8-diaryl-substituted anthraquinone dyes, which contain −COOH or −NH₂ groups, were prepared. The aryl-substituted anthraquinone dyes possess excellent thermostability and will keep their original colour in polycondensation with caprolactam at 260–280°C. With different aryl substituents the anthraquinone dyes exhibit different colours and can be used to prepare a series of coloured nylon-6 fibres. ©1997 SCI     

Purification and incorporation of the black ink of cuttlefish Sepia officinalis in eye cosmetic products

Synthetic colours used in cosmetics are a point of concern, and biotechnology has, to a certain extent, helped to resolve this problem. The present study aimed to integrate dyes from a natural extract, the black ink of cuttlefish Sepia officinalis, after purification, in black eye make‐up products (mascara and eyeshadow). Analyses showed differences in the chemical composition of treated and untreated forms of sepia black ink. Indeed, remarkable reductions in total mineral load, water content and proteins (79.3, 58.22 and 44% respectively) were noted. Treated sepia black ink had a low content of analytes. Very satisfactory results were obtained in terms of the level of aspect, texture, colour and covering capacity of the formulated cosmetic products. Moreover, the addition of S. officinalis black ink to iron oxide and black bone dyes improved the level of colour, spread and covering capacity of mascara and eyeshadow.     

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.     

Colour combination effects in experimental rooms*

This article deals with the way colours in a room affect each other through contrast effects and reflections. The study presented is based on the visual observations of the colour appearance of three inherent colours in a sequence of experimental situations. My point of departure was a yellow, blue, and red room. The room had distinctly different colour appearances of the same inherent colours when observed at different locations. The effects of both contrasts and reflections were evident. However, reflection effects seemed to dominate the situation. In a study of small experimental rooms, different combinations of the same three inherent colours were systematically tested. Comparisons were made to examine the differences in colour appearance between monochromatic rooms and multicoloured rooms with varying colour schemes. The illumination was kept stable. In the cases studied, it was obvious how the colours in the two‐coloured rooms became more alike or “neutralized each other.” The contrast strengthening between abutting areas on the same level had distinct significance for colour appearance and perception of space. However, not in such a way that the perceived colour differences became larger in the two‐coloured room than between each monochromatic rooms. © 1999 John Wiley & Sons, Inc. Col Res Appl, 24, 230–242, 1999     

The effects of coloured base fabric on electrochromic textile

The colour of electrochromic fabric, and its colour transitions, was explored on the basis of variations in underlying fabric colour. A complete understanding of this phenomenon is essential for the fabric’s use in full‐colour wearable displays or other applications. Previous work in this area utilised only white‐coloured starting materials. Herein, a large colour swathe of fabrics was chosen. They were loaded with the commercially available conducting polymer, PEDOT‐PSS, and were coated with an electrochromic polymer. These all‐organic substrates were then switched between their two coloured states via reversible oxidation and reduction. At every stage, coordinates in the CIE Lu′v′ colour space were measured. It was found that darker colours decrease the overall contrast of the electrochromic, with black being entirely unobservable. More vibrant colours affected the observed colour through a subtractive mixing effect, as expected, but no adverse contrast effects between the two states of the electrochromic system was observed.     

Light-induced colour changes in dyes and materials

The present knowledge of the properties and reactions of photoexcited states of dyes and materials is presented and discussed. It is shown that the most probable result of the absorption of daylight by the majority of coloured products in practical use is the decomposition of the colourless substrate or additives, or their photooxidation. In both reactions, radicals are produced which can convert dyes or coloured pigments into compounds that are colourless or possess a different colour. The conclusions drawn may be useful in the interpretation of anomalous light stabilities of coloured products, and help develop products with improved lightfastness.     

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.     

Evolution and human's attraction and reaction to colour: Food and health

For continued good health, we are urged to design our meals around the colours of the rainbow. To answer the question “Why?” requires consideration of two evolutionary stories. These broadly are natural evolution, and evolution driven by the activities of human beings. For the first, we need to go back to the start of life on earth and think about pigments rather than colour. There are two major pigment families, carotenoids and those based on the porphyrin ring, without both of which life on earth would not exist, and which still play major roles in our make‐up and well‐being. Other pigments, edible and poisonous evolved as pressures on plant life increased through predation and their own need for survival. The second story commenced with the arrival of human beings and their eventual ability to control fire leading to development of food preservation, processing and more recently, ultraprocessing. The rainbow rule does not apply to such foods in which colour is an indicator of process. Heavily processed foods include use of coloured, cheap, poor quality, and unhealthy ingredients contributing to the present health crisis. For this intervention by humans we must switch from consideration of pigment to that of colour, because it is colour that is a major factor in our selection of the food we eat today.     

Random Errors in Dyeing

The differences in colour from the target of a dyed material which are due to known weighing errors are calculated using computer match-prediction techniques. The results, which are presented in the form of a colour map, show that the sensitivity is dependent on the target colour and the colours of the constituent dyes in the recipe.     

Colour meaning and context

This study compares semantic ratings of colour samples (chips) with those of the same colours applied to a variety of objects. In total, 25 participants took part in the colour‐meaning experiment, and assessed 54 images using five semantic scales. In Experiment 1, simplified images (coloured silhouettes) were used whereas in Experiment 2 real images were used. In this article, the terms “chip meaning” and “context meaning” are used for convenience. Chip meaning refers to the associated meanings when only isolated colour chips were evaluated while context meaning refers to colour meanings evaluated when colours were applied to a variety of product categories. Analyses were performed on the data for the two experiments individually. The results of Experiment 1 show relatively few significant differences (28%) between chip meaning and context meaning. However, differences were found for a number of colours, objects, and semantic scales i.e., red and black; hand wash and medicine; and masculine‐feminine and elegant‐vulgar. The results of Experiment 2 show more significant differences (43%) between chip meaning and context meaning. In summary, the context sometimes affects the colour meaning; however, the degree to which colour meanings are invariant to context is perhaps slightly surprising. © 2016 Wiley Periodicals, Inc. Col Res Appl, 42, 450–459, 2017     

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.     

Revival of natural dyes in Asia

Increasing environmental awareness has persuaded some of the major Asian countries to reassess the feasibility of natural colorants. As one of the most agriculturally rich regions of the world, the emphasis has been on finding new colour sources which have the potential to yield high quality dyes. Syed Ishrat Ali argues that a detailed programme of R & D into the capabilities of natural dyes is of vital importance.     

An approach to sustainable coloration of lyocell fabrics by screen printing using extracts of leaves and bark from eucalyptus

This paper presents an initial study from a more extensive programme of research at the design/technology interface that is focused on the use of design methodology as a means to lower the environmental impact of fashion clothing. The leaves and bark that are byproducts of the responsibly farmed eucalyptus used in the manufacture of lyocell fibres were extracted with water to provide a source of natural colour. Lyocell fabric, which required minimal preparation, was screen printed with an aqueous paste containing only the extracted colour and a natural thickener to provide attractive golden‐yellow prints. The printed fabrics demonstrated surprisingly good fastness towards light, washing and rubbing. A mordanting pretreatment of the lyocell with alum modified the colour of the naturally coloured prints to become stronger and duller, but provided no advantage in terms of technical performance. The outcome presents a potentially useful model for the development of sustainably coloured fashion textile products.