Background

Among all natural dyes, indigoids (such as indigo, Tyrian, and woad) are often regarded as the most important and the oldest dye used by mankind [1]. Indigo use dates back to at least 6000 years, it is referred to as ‘blue gold’ because of its great value of trading commodity [2, 3]. In addition to as natural dyes, indigo is also used as a food colorant and for medicinal use [2]. Historically, indigo is extracted from indigo-yielding plant species. The indigo-yielding plant species were important cash crops on farmers in India, China, Central America, South Carolina, the southern USA, and Indonesia [4, 5]. Indigo-dyed textiles were widely traded along the legendary Silk Road linking China to Europe [6]. However, in the twentieth century, most commercial natural indigo production declined after the advent of chemically synthetic indigo [2].

Nowadays, synthetic indigo still dominates denim dyeing with the consumption of several thousand tons annually [7]. However, the production of large-scale synthetic indigo presents a serious environmental issues. The synthetic indigo is produced by aniline. Aniline is derived from the petroleum product benzene. It is toxic and the synthesis involves hazardous chemicals [4]. On the contrary, traditional indigo dyes are of plant origin, which exhibits better bio-degradability and more sustainable than synthetic counterparts [8]. Recent years have seen a burgeoning interest in the natural dyes in textile research, eco-friendly fashion, and sustainable-production [4, 6]. Consequently, it is important to find potential indigo-yielding plant species for “greener,” eco-friendly indigo extraction at a larger scale.

One way to find the potential indigo-yielding plant species is to obtain the correct botanical provenance from the previous studies. For example, Cardon [9] recorded about 20 indigo-yielding plant species (7 families and 9 genera) used globally. Han’s study [10] showed that four indigo-yielding plant species (four families and four genera) were mainly used for indigo extraction in Ming (1368–1644 C.E.) and Qing Dynasties (1644–1911 C.E.) in China. However, for some indigo-yielding plant species, apart from a brief mention of use of indigo extraction, there are few contemporary accounts of the indigo extraction methods or how local dyers harvest in China. This knowledge gap needs to be filled before traditional indigo-yielding plant use and extraction methods disappear.

Therefore, the first step is to document the traditional knowledge of indigo-yielding plant species before it is lost. To document this, we chose Hainan Island as the study area. Firstly, Hainan Island is the main part of Hainan Province of China [11]. It is the largest island in the Indo-Burma Bio-diversity Hotspot and has the best-conserved tropical forest in China [2 and 3: (1) harvesting: the fresh materials of indigo-yielding plant species are harvested and transported from the field to their homes; (2) fermentation: the materials were bent and left to steep in a cistern and added cool water to completely immerse the materials. Then removed the materials from the fermentation vat when they rotted. This process usually took 2 to 3 days depending on the amount of soaking materials and the weather condition, up to 1 week; (3) oxygenation: lime powder water was added to the fermentation vat, and the mixture was vigorously stirred for 0.5 to 1.5 h. When the foam was less and the foam color was blue and reddish, the dyers stop the oxygenation process. (4) Sedimentation: after resting for one or two nights, the blue sediment was on the bottom of the cistern and was called lan dian (蓝靛) (translation: indigo paste). The lan dian, compacted in paste, will transfer to a small plastics bucket or pot to keep it moist for storage. Moreover, the five indigo-yielding plant species could be mixed in practical production according to the information of the questionnaire.

Fig. 2
figure 2

Indigo extraction method of Strobilanthes cusia by Hainan Miao dyers. a Strobilanthes cusia harvesting in the home garden. b Removing the rotten aerial parts from the the extraction vat. c Using plastic scoop for aeration. d Ricinus communis seed use

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Fig. 3
figure 3

Indigo extraction method of Indigofera tinctoria by Li dyers. a Removing the rotten aerial parts from the the extraction vat. b Filtering the lime powder. c Agitation for oxygenation. d Sedimentation on the bottom

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The main indigo extraction process is almost the same between Hainan Miao and Li dyers, but there are still some differences in teams of indigo-yielding plant species, part used, harvest season, fermentation duration, amount of lime, oxygenation method tools, and annual production frequency (Table 4). What is most interesting is the Ricinus communis seed use in five Hainan Miao study villages. Ricinus communis L. (Euphorbiaceae), known as gen zong in Hainan Miao language, has two varieties that are distinguished by the color of their stems and leaves (voucher numbers ZLB45, ZLB60). A small number of Ricinus communis seeds were added to the extraction vat at the end of the oxygenation process to help reduce the foam. This is because the Hainan Miao dyers believed that the foam reduced the quality of indigo paste. Commonly 2–5 still moist seeds were crushed on the side of the extraction vat. Some dyers stripped off the seed coat before crushing them. The Ricinus communis seeds were then held while stirring in the extraction vat for 1–2 min before throwing away the seeds at the end of this process. From either variety were used, with no difference in relation to the Ricinus communis variety according to the Hainan Miao dyers.

Table 4 Comparison of the indigo extraction methods between Hainan Miao and Li dyers on Hainan Island, China
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Discussion

Indigo-yielding plant species in use

In the previous study [9], few brief mentions of Wrightia laevis use for indigo but no detail is given in any of these accounts of how local dyers harvest. In this study, the detailed indigo extraction method of Wrightia laevis was first documented (Table 4). Although Wrightia laevis was the least favored (PR = 2.95) for its low indigo paste quality, it was still as a supplement because this species is easy to find around the villages (A1 = 1.48, A2 = 1.01). For Hainan Miao dyers, the most cited and favored species is Strobilanthes cusia. The respondents gave the reason that Strobilanthes cusia provide the highest yield of indigo paste according to their experience, which is consistent with Chanayath’s result that Strobilanthes cusia gave more indigo than Indigofera tinctoria (the main indigo source in the world) in the ratio of 4:3 [37]. Further, it is easy to survive by cuttings and become the main source of indigo paste in Hainan Miao villages. In China, apart from the indigo paste source, Strobilanthes cusia is also a frequently used Chinese herbal medicine (TMC) that recorded in “People’s Republic of China Pharmacopoeia 2015” [38]. For example, its roots are known as “Nan-Ban-Lan-Gen (南板蓝根),” commonly used to prevent and treat virus-related respiratory diseases such as influenza virus infection [39]. Its leaves and stems are produced as “indigo naturalis (青黛)” to treat chronic diseases such as psoriasis [40]. During the outbreak of severe acute respiratory syndrome (SARS) in 2003, Strobilanthes cusia has been listed as one of the eight major anti-SARS medicines [41, 42]. Moreover, prior ethnobotanical surveys have shown that the tender stems and leaves of this species are also as an edible vegetable in ** methods we documented on Hainan Island were also reported in Indian, Japan and the south of China [9, 46,47,48]. However, the stee** method practiced by Hainan Miao and Li dyers differs from those used by other minorities. For example, the parts used of Indigofera tinctoria and Indigofera suffruticosa were different. A previous study [1] showed that the indigo compounds from the two species are extracted by stee** leaves, but we found the stems, leaves, and ripe fruit were used in Li villages. This is because Li dyers believed the high-quality indigo paste needed the fruit ripening period according to the experience. So, they usually sow in March to April and harvest from July to September.

In addition, the use of a few Ricinus communis seeds after the oxygenation process is first documented in this study. The Ricinus communis seeds were used at the end of the oxygenation process by Hainan Miao dyers but Li dyers did not use them. This is because Hainan Miao believed that Ricinus communis seed use could reduce foam to improve indigo paste quality. This introduced plant species are commonly found in all five Hainan Miao villages, which might be one reason for its use. However, the mechanism of Ricinus communis seed use to improve indigo paste quality is still unknown. Legrand [49] described another plant species used in the stee** method in Santiago Niltepec of Mexico, the pulpy fruit of a local plant known as “gulavere” is added to the oxygenation vat to help accelerate sedimentation. Nevertheless, there is no information on which species the local name gulavere represents or on its possible chemical function in assisting indigo sedimentation.

Chemical pigments formation of indigo extraction

Indican (indoxyl-3-O-β-D-glucoside), a second plant metabolite, is the most prominent indigo precursor [50]. Stored in the vacuole of indigo-yielding plant species, indican was hydrolyzed and produce indoxyl after stee** in the water, and thereby indoxyl molecules produce a chemical reaction to form indigo pigment [51]. In most common situations, indigo is blue color and indirubin is red [52]. If there were fewer impurities in the indigo paste, the blue and red color would be more obvious. Namely, to a certain extent, the color implies the purity of chemical pigments of indigo paste. This may explain the folk knowledge that Hainan Miao and Li respondents believed those with dark blue and reddish indigo paste were of good quality.

Production of indigo paste at the village level has some disadvantages compared to synthetic indigo for two main reasons. Firstly, in addition to water, the actual indigo content in the indigo paste could vary from 2 to 70%, the rest were lime, indirubin, and other organic materials [47]. Yet, synthetic indigo owns consistently high purity, which has always exceeded 90% [1]. To improve the indigo purity of traditional indigo-extracting methods, more attention should be paid to the indican metabolism since the indigo pigments come from the precursor indican as discussed above. Secondly, the method of synthetic indigo production is controllable because it follows specific chemical reactions [6]. However, the indigo yield of the traditional method is so hard to control that even a senior dyer cannot guarantee the same indigo yield every time. The water-to-biomass ratio, fermentation duration, fermentation temperature, lime quality, pH, and dissolved oxygen concentration are directly or indirectly related to indigo yield [11]. Therefore, a better understanding of these physicochemical parameters may contribute to adopt novel technologies to help standardize the traditional indigo extraction methods.

Conclusion

Historically, the traditional use of indigo as a pigment took place across many societies because of its high value as a trading commodity. Despite the process of modernization and urbanization, some Hainan Miao and Li dyers have still maintained the traditional indigo extraction methods. We found that Strobilanthes cusia and Indigofera tinctoria are the best potential candidates for sustainable indigo production. In addition, the detailed use of Wrightia laevis as an indigo source and Ricinus communis seed use during oxygenation were first documented in this study. More attention should be given to adopting novel technologies to make natural indigo as a compatible and sustainable alternative to synthetic colorants.