Quality blues: traditional knowledge used for natural indigo identification in southern China

BACKGROUND

As one of the oldest traditional dyes, people worldwide have used natural indigo for centuries. Local people have unique knowledge about indigo identification, which is crucial for indigo quality control and determining the dyeing effects. However, such traditional knowledge is rarely documented and explained. Therefore, the aims of this study were to document and assess the traditional knowledge used by local people when identifying natural indigo paste as well as quantitatively explore the characteristics and material basis of such traditional knowledge.

METHOD

Three field surveys were conducted between 2019 and 2020. A total of 283 traditional indigo-paste artisans were interviewed in Guizhou, Yunnan, and Fujian Provinces. The frequency of citation, mention index, and fidelity level of each indigo-paste quality criterion were calculated to determine the most commonly used, recognized, and important quality criteria. To explore the characteristics and material basis of the traditional knowledge, we analyzed 21 indigo-paste samples using high-performance liquid chromatography with diode-array detection (HPLC-DAD), pH, and particle size analyses.

RESULTS

Local people possess unique knowledge to identify natural indigo. Based on this knowledge accumulated over thousands of years, four criteria (color, taste, touch, and dyeing ability) were chosen by local people, and using these criteria, nature indigo was divided into five quality grades. The best quality indigo paste was judged according to the following folk criteria: dark blue in color with a purple-red luster; smooth and difficult to wipe off; having a sweet, bitter or spicy taste; and easy cloth dyeing. Additionally, the higher the contents of indigo and indirubin-especially indirubin-the better is the quality of the indigo paste. Within the pH range of 9-12, high-quality indigo-paste was more acidic. There was no significant relationship between particle size and quality.

CONCLUSION

The ancient methods used by local people for identifying natural indigo are comprehensive and unique. By documenting the various folk quality criteria and conducting quantitative analyses, this study revealed the importance of indirubin and pH for assessing the quality of indigo paste. These findings differ from existing quality standards for synthetic indigo. Amid rapid modernization, traditional knowledge remains invaluable as a world heritage of humanity that warrants preservation.

Fungal Pigments: Potential Coloring Compounds for Wide Ranging Applications in Textile Dyeing

Synthetic pigments/non-renewable coloring sources used normally in the textile industry release toxic substances into the environment, causing perilous ecological challenges. To be safer from such challenges of synthetic colorants, academia and industries have explored the use of natural colorants such as microbial pigments. Such explorations have created a fervent interest among textile stakeholders to undertake the dyeing of textile fabrics, especially with fungal pigments. The biodegradable and sustainable production of natural colorants from fungal sources stand as being comparatively advantageous to synthetic dyes. The prospective scope of fungal pigments has emerged in the opening of many new avenues in textile colorants for wide ranging applications. Applying the biotechnological processes, fungal pigments like carotenoids, melanins, flavins, phenazines, quinones, monascins, violacein, indigo, etc. could be extracted on an industrial scale. This review appraises the studies and applications of various fungal pigments in dyeing textile fabrics and is furthermore shedding light on the importance of toxicity testing, genetic manipulations of fungal pigments, and their future perspectives under biotechnological approaches.

Natural Colorants: Historical, Processing and Sustainable Prospects

With the public's mature demand in recent times pressurized the textile industry for use of natural colorants, without any harmful effects on environment and aquatic ecosystem, and with more developed functionalities simultaneously. Advanced developments for the natural bio-resources and their sustainable use for multifunctional clothing are gaining pace now. Present review highlights historical overview of natural colorants, classification and predominantly processing of colorants from sources, application on textiles surfaces with the functionalities provided by them. Chemistry of natural colorants on textiles also discussed with relevance to adsorption isotherms and kinetic models for dyeing of textiles.

Natural colorants from filamentous fungi

In the last years, there is a trend towards the replacement of synthetic colorants by natural ones, mainly due to the increase of consumer demand for natural products. The natural colorants are used to enhance the appearance of pharmaceutical products, food, and different materials, making them preferable or attractive. This review intends to provide and describe a comprehensive overview of the history of colorants, from prehistory to modern time, of their market and their applications, as well as of the most important aspects of the fermentation process to obtain natural colorants. Focus is given to colorants produced by filamentous fungal species, aiming to demonstrate the importance of these microorganisms and biocompounds, highlighting the production performance to get high yields and the aspects of conclusion that should be taken into consideration in future studies about natural colorants.

Crude bacterial extracts of two new Streptomyces sp. isolates as bio-colorants for textile dyeing

Renewed demand for incorporation of natural dyes (bio-colorants) in textile industry could be met through biotechnological production of bacterial pigments. Two new Streptomyces strains (NP2 and NP4) were isolated for the remarkable ability to produce diffusible deep blue and deep red pigment into fermentation medium. Crude mycelial extracts of both strains were used as bio-colorants in conventional textile dyeing procedures avoiding downstream purification procedures. The yields of bio-colorants obtained in this way were 62 and 84 mg per g of mycelia for Streptomyces sp. NP2 and Streptomyces sp. NP4, respectively. Through nuclear magnetic resonance analysis of crude extracts before and after dyeing procedures, it was shown that both extracts contained prodigiosin-like family of compounds that exhibited different dyeing capabilities towards different textile fibers. Polyamide and acrylic fibers were colored to the deepest shade, polyester and triacetate fibers to a noticeable, but much lower shade depth, while cotton and cellulosic fibers stained weakly. These results confirmed that crude bacterial extracts had the characteristics similar to those of ionic and disperse dyes, which was consistent with the identified polypyrrolic prodigiosin-like structures.

Colorimetric Assay and Antibacterial Activity of Cotton, Silk, and Wool Fabrics Dyed with Peony, Pomegranate, Clove, Coptis chinenis and Gallnut Extracts

To investigate the antibacterial functionality of natural colorant extracts, five kinds of natural dying aqueous solutions were obtained by extraction from peony, pomegranate, clove, Coptis chinensis and gallnut using water at 90 °C for 90 min with a liquor ratio (solid natural colorant material/water, weight ratio) of 1:10. The colorimetric assay and antibacterial activity of cotton, silk, and wool fabrics dyed with these natural colorant extracts were examined. It was found that these properties were significantly dependent on the structure of colorant and the kind of fabrics. The hues (H) of all fabrics dyed with these natural colorants were in the range of 6.05YR -1.95Y. The order of value (V) was wool, silk and cotton. The chroma (C) of all samples was found to be at very low levels indicating the natural tone. All the fabrics dyed with the five natural colorants (peony, pomegranate, clove, Coptis chinensis and gallnut) extracts displayed excellent antibacterial activity (reduction rate: 96.8 - 99.9%) against Staphylococcus aureus. However, in the case of Klebsiella pneumoniae, the antibacterial activity was found to depend on the kind of natural colorant extract used.

It must be green: meeting society’s environmental concerns

Consumers today are increasingly demanding goods which not only conform to the public’s image of being ‘eco‐friendly’ and ‘organic’ but of having been produced ‘ethically’. Meeting such high ideals has a down side, both in higher costs and often in that of having to accept more distant suppliers. Present trends in the coloration of foods with natural dyes rather than synthetic ones, increasing consumption of organic products (including fibres) and energy‐saving trends in dye application methods, fuels and lighting, as well as the means of capturing solar energy, are discussed. The discovery of some interesting and historic green colours, the wider use of green (in both senses of the word) products and green chemistry’s future role in producing them are also reviewed.