Application of Oxidation and Copigmentation of Anthocyanins from Purple Sweet Potato to Silk Colouration

Matsubara Takanori *, Inoue Hinata, Satoh Asuka

Department of Mechanical Engineering, College of Industrial Technology, 1-27-1 Nishikoya, Amagasaki, Hyogo 661-0047, Japan

Received 9 May 2019; accepted for publication 10 September 2019

Abstract

The new silk dyeing methods by utilising oxidation and copigmentation of anthocyanins were researched and compared with metal mordanting dyeing. The used colourant was purple sweet potato pigment (PSPP) and its main components were assumed to contain cyanidin and/or peonidin 3-sophoroside-5-glucosides. PSPP dyes silk fabrics purple and the post-treatment changes the colour. The dyeing method were designed two step treatment (post-treatment dyeing). First is dyeing process with PSPP and second is reaction process by using an appropriate material which are Al(III), Cu(II), Fe(II) compound for the metal mordanting dyeing, sodium periodate (NaIO4) for the oxidation dyeing, and polyphenol for the copigmentation dyeing, respectively. The colour of the silk dyed with PSPP is bluish purple for the mordanting dyeing with Al(III) or Cu(II), purplish black for the mordanting dyeing with Fe(II), brown for the oxidation dyeing. The copigmentation dyeing gives deeper purple than the PSPP only dyeing. Their dyeability dependence on the pH of PSPP solution was examined at pH = 2.9 ~ 9.1. The PSPP only dyeing and the copigmentation dyeing gives deeper purple at pH = 4.5 ~ 6.9 and the dyeability of the copigmentation dyeing is higher whole of pH. In the mordanting dyeing with Fe(II), the resulting colour is changed from purplish black to blueish black at pH = 2.9 ~ 7.0, and in the oxidation dyeing that is turned from brown to purplish brown at pH = 2.9 ~ 7.0.

Key Words : Anthocyanins, Silk dyeing, Oxidation, Copigmentation, Purple sweet potato

SHORT PAPERJournal of Textile Engineering (2019), Vol.65, No.5, 73 - 78DOI: 10.4188/jte.65.73© 2019 The Textile Machinery Society of Japan

* Corresponding author: E-mail : matsubara@cit.sangitan.ac.jp, Tel : +81-6-6431-7021, Fax : +81-6-6431-5998

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1.　Introduction

Recently colouration with biobased materials such as natural dyes has been attracting attention for eco-dyeing, and is thought of as one of key technologies for sustainable development. Anthocyanins are one of the largest groups in natural dyes, and they are responsible for pink, red, purple, and blue in flower petals, fruits and vegetables [1]. The water-soluble colourants are able to be extracted from waste of fruit and vegetables such as seed and skin [2][3], and they are high sustainability as dyes. In most cases, the dyeing by using anthocyanins requires metal compounds for increasing colour fastness to light and washing as metal mordanting dyeing and the resulting colour is dull [4][5][6].

Anthocyanins have sugar group(s), and the chromophore structure without sugar group(s) (aglycones) are called anthocyanidins. Most of anthocyanins in cell of plant are stabilised to maintain their colour by complex formation with metal cation and intermolecular association (so-called copigmentation) with the coexisting colourless materials having anthocyanins-like structure such as flavones and flavan-3-ols [1][7]. Also, moderate

oxidation by light, oxygen, or enzymatically stabilises anthocyanins accompanied with changing dull colour in a sense [1][8]. The oxidation occurs catechol (o-dihydroxybenzene), pyrogallol (1,2,3-trihydroxybenzene), and o-methoxyphenol part of B-ring in anthocyanins.

The oxidation and copigmentation may utilise textile colouration as new dyeing method. The oxidation dyeing has been researched by using (+)-catechin having catechol structure and three oxidants (O2 gas, H2O2, NaIO4) for silk colouration [9]. The only NaIO4 oxidises the silk fabrics pretreated with (+)-catechin to colour deep brown. The sodium periodate (NaIO4) is known to oxidise catechol derivatives rapidly to form the corresponding coloured o-benzoquinones [10], also oxidises o-methoxyphenol [11], thus the oxidant was used in this study. On the other hand, little is known about the research of ‘copigmentation dyeing’. In the research on human hair dyeing with anthocyanins from blackcurrant fruit [12], it mentioned that the colour of dyed hair was stable to multiple washings and stabilisation of anthocyanins occurs in hair fibre by ‘copigmentation effect’.

In this study, the new silk dyeing methods were researched

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utilising these stabilisation reactions, the oxidation as ‘oxidation dyeing’, or copigmentation as ‘copigmentation dyeing’, and compared with the metal mordanting dyeing. Most of anthocyanins are discoloured by light and heating [1]. The colourants from purple sweet potato were used because they contain acylated anthocyanins having relatively high stability to light and heating [13]. The differences in dyeability of each dyeing method, effects of dyeing conditions and colour fastness to ultraviolet light were experimented.

2.　Experimental

2.1　Materials

The white silk fabric (Kinu Habutai, Sikisensya, size: 4 cm × 4 cm, weight: 52.5 g/m2, fabric count: 135 × 98 per inch (warp × weft), plain weave) was used without pre-treatment such as bleaching and scouring. Purple sweet potato pigment (PSPP) was purple powder and was kindly produced from Glico Nutrition. For the metal mordanting dyeing, the metal salts used as mordants were aluminium(III) potassium sulfate dodecahydrate (AlK(SO4)2·12H2O, Wako), copper(II) sulfate pentahydrate (CuSO4·5H2O, Wako), ferrous(II) lactate hydrate (Aldrich). For the oxidation dyeing, sodium periodate (NaIO4, Wako) was used as oxidants without further purification. For the copigmentation dyeing, natural polyphenol solution (PP, hanabirazome-hassyokuzai C, Tanakanao Senryoten) as copigment was used by diluted with water in 5 times. Orange II (Wako) was used as a representive acid dye for the silk dyeing. Hydrochloric acid (HCl, Wako) and methanol (Wako) were used for UV/Vis spectroscopy of PSPP. Phenol reagent (Kishida), gallic acid monohydrate (Wako) and sodium carbonate (Na2CO3, Wako) were used for the quantification of total phenolics of PP. The 0.1 M acetate buffer solution (0.1 M CH3COOH / 0.1 M CH3COONa) for pH = 4.0 and 5.0, 0.1 M phosphate buffer solution (0.1 M NaH2PO4 / 0.1 M Na2HPO4) for pH = 6.0, 7.0 and 8.0, and 0.1 M carbonate buffer solution (0.1 M NaHCO3 / 0.1 M Na2CO3) for pH = 9.0 and 10.0 were prepared and used for pH adjust of the PSPP solution. Needle shaped Marseille soap (Tanakanao Senryoten) was used as detergent for washing fabrics. All of the aqueous solutions were prepared with distilled water.

2.2　UV/Vis spectroscopy

Spectral method of characterising anthocyanins in PSPP and quantification of total phenolics of PP were analysed by using a Shimadzu UV-2600 spectrophotometre at room temperature.

Characterisation of anthocyanins in PSPP was carried out by spectroscopic method referring the previous research [14]. The 0.01 %(w/v) of PSPP methanol solution containing 0.01 %(v/v) HCl was prepared and its absorption spectrum was measured.

Total amount of phenolics was quantified using a Folin-

Ciocalteau method (ISO method) with gallic acid as standard [15]. The 1 mL of samples or standards was mixed with 5 mL of 10 %(v/v) phenol reagent solution and kept for 3 ~ 8 min. Then, 4 mL of 7.5 %(w/v) Na2CO3 solution was added to the mixtures and reacted for over 1 h at room temperature. The absorbance at 765 nm of the obtained blueish solutions was measured and calculated the total amount of phenolics of PP. The amount value was reported in grams gallic acid equivalents per 100 mL of the solution (%(w/v))

2.3　Silk dyeing

Silk fabrics were dyed by one of three kinds of the dyeing methods (a, b and c) by utilising one of the three reactions of anthocyanins (1, 2 and 3). The 1.5 %(w/v) aqueous solution of PSPP was prepared, and the pH of the solution was 2.9.

(1) Metal mordanting dyeing: a) Pre-mordanting dyeing: A piece of silk fabric (4 cm × 4 cm, ca. 90 mg) was immersed in 20 mL of 10 mM Al(III), Cu(II) or Fe(II) solution at 30 °C for 40 min, and then dipped into 20 mL of PSPP solution at 30 °C for 40 min. b) Meta-mordanting dyeing: After mixed between 10 mL of the pigment solution and 10 mL of the metal salt solution at room temperature for 15 min, silk fabric was treated with the mixed solution at 30 °C for 40 min. c) Post-mordanting dyeing: The textile sample was dipped into 20 mL of the dyestuff solution at 30 °C for 40 min, and then immersed in 20 mL of the metal salt solution at 30 °C for 40 min.

(2) Oxidation dyeing: a) Pre-oxidation dyeing: The silk fabric was immersed in 20 mL of 10 mM NaIO4 solution at 30 °C for 40 min, and then dipped into the pigment solution at 30 °C for 40 min. b) Meta-oxidation dyeing: After the oxidation of pigment by NaIO4 at room temperature for 15 min, the silk sample was immersed in the reaction solution at 30 °C for 40 min. c) Post-oxidation dyeing: The textile was treated with the dye solution at 30 °C for 40 min, and further dipped into the oxidant solution at 30 °C for 40 min.

(3) Copigmentation dyeing: a) Pre-copigmentation dyeing: The fabric was dipped into the diluted polyphenol (PP) solution at 30 °C for 40 min, and then dyed with the pigment solution at 30 °C for 40 min. b) Meta-copigmentation dyeing: The PP solution was mixed with the dyestuff solution at room temperature for 15 min. The silk was dyed with the mixture at 30 °C for 40 min. c) Post-copigmentation dyeing: The silk was treated with the pigment solution at 30 °C for 40 min and then immersed in the PP solution at 30 °C for 40 min.

The dyed silk fabrics were washed with 100 ml of the 0.3 %(w/v) soap aqueous solution for 20 min and rinsed twice with 200 ml of distilled water at 30 °C for 15 min. The samples were air-dried under the circumstances of room temperature and humidity.

Also, the silk fabric was dyed with 1 %(w/v) Orange II / 2 %(v/v) CH3COOH aqueous solution at 30 °C for 40 min for the colour fastness test to ultraviolet light. The dyed fabric was washed and dried by the above method.

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2.4　Colour measurements

The colour and spectroreflectance of silk samples were measured by a Konica Minolta CM-2600d spectrocolourimetre employing 10˚-view angle, CIE standard illuminant D65 and SCI (specular component included) mode. All the reflected lights from a sample including the regular reflection are integrated under the SCI mode. The measurements were conducted at three parts of the each sample (measured area: 3 mmφ) and the arithmetic mean values were calculated from the obtained values. The colour was expressed in L*a*b* standard colourimetric system (CIE 1976) [16]. The L* is the lightness index, and a* and b* are the chromaticity coordinates. The positive values of a* indicate red colours and its negative values indicate green ones, and the positive values of b* indicate yellow and its negative values indicate blue. The K/S value is defined from Kubelka-Munk theory [17][18] and calculated as (K/S)λ = (1 – Rλ)

2/2Rλ, where K is the absorption coefficient, S the scattering coefficient and Rλ the spectroreflectance of the light at a wavelength λ (360 ~ 740 nm, every 10 nm). Summation of (K/S)λ of the dyed fabrics, which means colour strength, was obtained and compared as the extent of dyeability.

2.5　Colour fastness test to ultraviolet light

The colour fastness was estimated by the colour difference, ΔE*, for the fabric between before and after under ultraviolet (UV) irradiation. The ΔE* is calculated by ΔE* = {(L*f − L*0)

2 + (a*f − a*0)

2 + (b*f − b*0)2}1/2 where L*f, a*f, b*f, L*0, a*0, b*0 are L*,

a*, b* of silk before and after UV irradiation. The experiments for colour fastness to UV light for the dyed silk were made by using a Sun Energy MDH2501N-02 metal halide lamp. The intensity of the light was 44.6 ~ 61.9 mW/cm2 at 365 nm [19]. The irradiation was performed for 5 h under ambient humidity.

3.　Results and Discussions

3.1　Spectroscopic characterisation of PSPP

PSPP solution shows reddish purplish colour which is due to the chemical structure of anthocyanins contained in the acidic solution. The approximate structure of anthocyanins can be estimated from ultraviolet-visible (UV/vis) absorption spectrum of the methanol solution with 0.01 %(v/v) HCl referring to the preceding study [14]. The spectrum of PSPP acidic methanol solution shows absorption peaks at 528, 329, 297 nm and a shoulder peak at around 250 nm. The PSPP would contains various species of anthocyanins. The peak at 528 nm is delivered from aglycone in anthocyanins. The cyanidin and peonidin derivatives have the peak at 522 ~ 535 nm [14], and PSPP may contain their anthocyanins. The acylated anthocyanins having cinnamic acid part such as caffeic acid and ferulic acid exhibit absorption at 310 ~ 335 nm, and that having

benzoic acid part shows absorption at 250 ~ 260 nm [14]. The spectrum for PSPP have signals at 329, 297, 250 nm, then PSPP contains some species of anthocyanins having cinnamic acid and/or benzoic acid structure. Also, absorbance at 400 ~ 460 nm is lower for cyanidin and peonidin possessing 5-glucoside than that for not having 5-glucoside. The ratio of (absorbance at 440 nm)/(absorbance at maximum wavelength in visible region) is ca. 12 for the 5-glucosides of cyanidin and peonidin [14]. The ratio of PSPP solution is 14.6, and 5-glucosides probably present in PSPP. On the other hand, the qualitative analyses of anthocyanins are enable to measure by HPLC method. The references show that main anthocyanins in purple sweet potatoes are determined cyanidin and peonidin 3-sophoroside-5-glucosides as shown in Fig.1 [20][21][22]. The results based on HPLC are similar with our result of spectroscopic analysis.

3.2　Colour change of PSPP solution by metal chelation, oxidation and copigmentation

The colour of PSPP solution turns from reddish purple instantly into bluish purple, pinkish purple or deep purple by adding Al(III), Cu(II) or Fe(II) solution, respectively. The colour change would be caused by metal chelation between catechol part of B-ring in anthocyanins and metal cation [1].

The PSPP solution instantly turns pale orange by mixing with colourless NaIO4 solution. It found that the signal peak at 530 nm for PSPP aqueous solution changed to broad absorption. NaIO4 oxidises catechol and o-methoxyphenol structure to form the corresponding o-benzoquinones [11]. The colour change of solution may be based on the formation of oxidation products of anthocyanins.

The colour of PSPP aqueous solution also changes reddish colour by adding natural polyphenol (PP) solution. The absorption maximum peak at 530 nm is shifted to longer wavelength (536.5

　

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nm) and the absorbance increases 43 % by mixing them. The PP solution contains 3.0 %(w/v) of polyphenols equivalent to amount of gallic acid, which are obtained from Folin-Ciocalteau method. The spectral changes are typical features of copigmentation and the intermolecular interaction between anthocyanin and the polyphenol contained in PP would form a non-covalently-linked complex [23]. Hydrogen bonding, hydrophobic interactions and electrostatic interactions have been speculated as the main driving force for the copigmentation.

3.3　Silk colouration with PSPP and colour fastness to UV light

The deep reddish purple PSPP solution at pH = 2.9 and 30 °C dyes silk fabric purple. The colourimetric parameters change from L*: 94.7, a*: 0.340, b*: 0.730 (undyed) to L*: 76.7, a*: 15.4, b*: –9.93 (dyed). Further, the oxidation dyeing and the copigmentation dyeing were tried and compared with the metal mordanting dyeing. At first, the optimum treatment order was considered from the effects of pre-, meta-, or post-treatment by the oxidant (NaIO4) and the copigment (PP) on dyeability. The results show that post-treatment of NaIO4, or PP indicates better dyeability than pre-treatment and meta-treatment of that in each dyeing method. For obtaining a better dyeability, the treatment of PSPP should be carried out at first process. The result of the pre-oxidation dyeing is same as that of the PSPP only dyeing, and the pre-treatment of the oxidant don’t affect with the adsorption of PSPP on silk. Also, the dyeability of the meta-oxidation dyeing by using mixture PSPP and NaIO4 is lower. In the copigmentation dyeing, the order of dyeability is pre- < meta- < post-dyeing. PSPP undyes the silk pre-treated with PP (pre-copigmentation dyeing) and the PP pre-treated on silk prevents the adsorption of PSPP.

Table 1 exhibits the photographs and colourimetric values for the silk dyed with PSPP by the post-treatment dyeing. In the mordanting dyeing, the colour of the fabrics dyed with PSPP and Al(III) or Cu(II) is bluish purple, and that with PSPP and Fe(II)

is purplish black. These are typical results for the mordanting dyeing by using anthocyanins. The oxidation dyeing obtains slightly purplish brown. This is the different hue (b* > 0) from the PSPP only dyeing and the metal mordanting dyeing (b* < 0). In copigmentation dyeing, the silk is dyed purple same as PSPP only and the colour intensity slightly increases. The increases would be caused by copigmentation or an interaction between anthocyanins and copigments in silk fibre. Fig. 2 shows the chromaticity diagram for the silk samples undyed and dyed with PSPP. The oxidation dyeing and copigmentation dyeing using PSPP dye silk different colour tone with the mordanting dyeing. The chromaticity

　

　

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coordinates are a* = ‒1 ~ +15 and b* = ‒15 ~ ‒1 for the metal mordanting dyeing, a* = +7 ~ +12 and b* = +11 ~ +23 for the oxidation dyeing (expect for the pre-oxidation dyeing), and a* = +17 and b* = ‒8 for the post-copigmentation dyeing, respectively.

Fig. 3 exhibits the a* and b* for the silk samples dyed by the four colouration methods at differing pH of PSPP solution (the pH of post-treatment solution is constant). As the pH increasing, the colour of the silk dyed with PSPP only is deepened, is deeper purple at pH 4.5 ~ 6.9, and is pale purple at the higher pH. The colour of PSPP solution at pH = 2.9 ~ 9.1 changes from reddish purple to blueish purple as increasing the pH. However, the colour phase of each dyed fabrics is unchanged. The ionic interaction between anthocyanin and silk surface may be dominant at the adsorption process. Anthocyanins have red flavylium cation under acidic condition (pH < 4), neutral quinoidal base at pH = 4 ~ 7, or blue quinoidal anion at above neutral pH (pH > 7) [24]. The surface of silk is positive-charged at pH < 4, uncharged at pH = 4, or negative-charged at pH > 4 as a whole because of the ca. 4 of isoelectric point of the silk fibre (3.6 for fibroin and 4.3 for sericin [25]). Thus, the anthocyanins may be relatively harder to access to silk surface by electrical repulsive force at the pH < 4 and pH > 7.

In the mordanting dyeing with Fe(II), the obtained colour is changed according the pH. This changes from black to blueish black with increasing the pH and the dyeability decreases at above pH 7.0. The colour of the silk dyed by the oxidation method turns from reddish purple to bluish purple with an increase of the pH (a* increases and b* decreases). The results mean that the structure of anthocyanins adsorbed at the first PSPP treatment is probably determined by the pH condition, and Fe(II) ion and NaIO4 would keep the structure accompanied with the metal chelation and the oxidation at the post-treatment.

Also, the trends of the pH effect on the dyeability for the copigmentation dyeing is observed same as the dyeing with PSPP only. Therefore, the colour for the copigmentation dyeing

is deeper (higher a* and b*) than that for the PSPP only dyeing. The adsorption behaviour is similar to PSPP dyeing, the higher dyeability of copigmentation dyeing may be due to increase the surface absorption property by copigmentation. Moreover, it is assumed that the post-treatment of the polyphenol stabilises the colourants in the silk fibre and decreases the desorption amount of the colourants at the washing process due to copigmentation.

On the other hand, the stability to light of anthocyanins is generally weak [1][13], and the colour of the fabric dyed by anthocyanins was expected to lose easily. Then, colour fastness to ultraviolet (UV) light for the silk dyed with PSPP was examined and was compared with that for the fabric dyed with Orange II, which is representative mono azo acid dye (Table 2). Judging from the colour difference ΔE*, the order of colour fastness is PSPP only (30.9) ~ Copigmentation dyeing (30.3) < Orange II (24.1) < Oxidation dyeing (15.3) < Metal mordanting dyeing (12.3). The post-oxidation treatment improves the stability to UV, but the post-copigmentation treatment have no effects of the improvement.

　

　

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4.　Conclusions

In this study, the silk dyeing with purple sweet potato pigment (PSPP) containing anthocyanins by the oxidation dyeing and the copigmentation dyeing was attempted and the dyeability for these dyeing methods compared with the metal mordanting method. The resulting colour is purple for PSPP only, bluish purple or black for the mordanting dyeing with Al(III) or Cu(II), black for the mordanting dyeing with Fe(II), reddish brown for the oxidation dyeing or deeper purple for the copigmentation dyeing, respectively. The chromaticity coordinates obtained by the oxidation method (a* = +7 ~ +12 and b* = +11 ~ +23) are differed from that by the metal mordanting dyeing (a* = ‒1 ~ +15 and b* = ‒15 ~ ‒1) and PSPP only dyeing (a* = +15.4 and b* = ‒9.93). The copigmentation dyeing gives deeper purplish colour than the PSPP only dyeing. The colour of the silk samples is changed from black to bluish black for the Fe(II) mordanting dyeing or from reddish brown to bluish brown for the oxidation dyeing with an increase of the pH of PSPP treatment solution (pH = 2.9 ~ 9.1). The dyeability of the PSPP dyeing and the copigmentation dyeing is higher at pH = 4.5 ~ 6.9 Also, the colour fastness for only PSPP dyeing and copigmentation dyeing is inferior to that for Acid Orange II, and that for the oxidation and the metal mordanting dyeing is superior.

AcknowledgementsThe authors would like to thank Glico Nutrition Co. for providing

us purple sweet potato pigment powder. This study was financially supported partly by the Japan Society for the Promotion of Science (JSPS) as Grant-in-Aid for Early-Career Scientists (Grant Number 18K13027).

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