Optimization of the application of melanoidin microcapsules from lycium barbarum residue in wool fabric dyeing process

ABSTRACT The extraction of active substances and other nutrient elements from Lycium barbarum and their application in food, medicine, and cosmetics have always been one of the hot topics for scholars in related fields. However, the residue after the extraction still contains melanoidin, which is an important substance that can be used as an environmentally friendly pigment in the coloring of textiles. Therefore, the rational utilization of the by-products after the processing of Lycium barbarum is the key to sustainable development. For this reason, Lycium barbarum residue was used as the raw material to extract textile-grade dyestuff in this work. The phospholipid microcapsules dyeing process of melanoidin dyestuff from Lycium barbarum residue was optimized by the response surface optimization as follows: liquor ratio of 1:35.15, pH of 4.22 and dyeing temperature of 75.8°C. The results showed that the dyeing temperature reduced by 15°C during the dyeing process by using melanoidins microcapsules dyestuff. Dyed wool fabric with biomordant (pomegranate skin) and after-chrome dyeing has high color fastness, antioxidant, and antibacterial property, which greatly increases the added value of the dyed products.


Introduction
In 2020, the total output of Lycium barbarum was 441,200 tons in China, most of which were made into Lycium barbarum juice and capsules (Zhang 2019). This resulted in many by-products, most of which were high added-value compounds (Broeze, Geerdink, and Voogt 2019). However, the residue of Lycium barbarum after fermentation and juice extraction has a high content of crude protein (11.75% after fermentation and 11.54% after juice extraction) and crude fat (12.78% fermentation and 8.70% after juice extraction) in the by-products. One of the high added-value components in the residue is natural dye. They are believed to be safe due to their nontoxic, non-carcinogenic, and biodegradable natures. Some of natural dyes possess antioxidant property and antibacterial properties (Rather, Zhou, and Li 2021). It not only gives the fabrics color but also makes the fabrics biologically active.
Maillard reaction (MR) is a complex reaction of amino and carbonyl groups under certain temperature and humidity, and the main product in the later stage of the reaction is melanoidin (Mesias and Delgado-Andrade 2017). The residue of Lycium barbarum contains a large number of compounds carrying amino and carbonyl groups. Melanoidin is an eco-friendly natural dye with a brown-black color, that is chemically stable with no adverse reactions. It also exhibits excellent antioxidant properties (Cao, Yan, and Liu 2022) and antibacterial properties (Kukuminato, Koyama, and Koseki 2021).
Although metallic mordants can greatly improve the fixation and stability of dyed fabrics, there is still a problem of environmental pollution (Pinheiro et al. 2019). Pomegranate peel is a by-product of pomegranate juice manufacturing industry, which can replace metallic mordants as a biomordant in dyeing process and reduce environmental pollution (Shahamirian et al. 2019). To overcome the hydrophobicity of wool, phospholipid microcapsules consisting of phosphatidylcholine were applied to create a more hydrophilic surface of the wool. The microcapsules can not only enhance the permeability of the natural dye molecules to the wool fiber (Martí et al. 2016) but also reduce the dyeing temperature by 10°C (Kartal et al. 2020).
In this research, melanoidin was successfully extracted and purified from Lycium barbarum residues and encapsulated with phospholipids to dye wool fabrics for the first time. At the same time, it can recycle Lycium barbarum residue, thereby reducing environmental pollution. In the dyeing process, instead of the traditional metal mordant, the eco-friendly biomordant pomegranate peel was used. The response surface methodology was applied to improve the performance of the dyeing process, the binding force between dyestuff and fabric was increased by fixing methods such as adding metal ions, inducing chemical reactions and changing dyeing environment. The color fastness and antibacterial property were also studied. The flow of the studied processes is shown in Figure 1.

Materials
The Lycium barbarum residue is the residual substance after the extraction of carotenoids. Chitosan oligosaccharide (Nanjing Deju Biotechnology Co., Ltd., China), as a catalyst to the reactants in MR, was used in the extraction process. AB-8 macroporous resin (Chengdu Cologne Chemicals Co., Ltd., China) was used in the purification process. Ethanol of analytical grade (Chengdu Cologne Chemicals Co., Ltd., China) was used in the extraction and purification process. Phosphatidylcholine (Xi'an Musen Bioengineering Co., Ltd., China) and soybean lecithin (Xi'an Musen Bioengineering Co., Ltd., China) were used as shell materials for microcapsules, and cholesterol (Shanghai Zhongqin Chemical Reagent Co., Ltd., China) was adopted to improve the capsule stability. Ether of analytical grade (Shenzhen Yongqianxin Chemical Co., Ltd., China) and phosphate buffer saline (PBS, 0.05 mol/L, pH6.8-7.0, Shanghai Ruichu Biotechnology Co., Ltd., China) were used for the preparation of melanoidins microcapsules. 2,2'-Azinobis-(3-ethyl-benzothiazoline-6-sulfonic acid) of analytical grade (ABTS, Shanghai Ruichu Biotechnology Co., Ltd., China) and potassium persulfate of analytical grade (Chengdu Cologne Chemicals Co., Ltd., China) were used to test the antioxidant property of dyed wool fabrics. Acetic acid of analytical grade (Chengdu Cologne Chemicals Co., Ltd., China) was used as a pH regulating solvent for dyeing process. Pomegranate rinds (Haoxing Anbo Pharmaceutical Co., Ltd., China) were used as biomordant for dyeing process. Pure wool fabrics were obtained from Lanzhou Sanmao Industrial Co., Ltd., China. And the specification of wool fabrics are presented in Table 1.

Extraction of melanoidins
2.0 g Lycium barbarum residues and 5% (wt.%) chitosan oligosaccharide were dissolved in 30 ml deionized water. Then, the solution was microwaved in a microwave oven (WG70OCTL2011-K6, Foshan Shunde Galanz Microwave oven Electric Co., Ltd., China) at 700 W for 120 s. Last, the melanoidins crude extract was dried and crushed. Repeat the above steps at least three times.

Purification of melanoidins
The melanoid was purified by macroporous adsorption resin. Adsorption process: 1.5 g of pretreated AB-8 macroporous resin (dry weight) were dissolved in 120 ml, 2 mg/ml melanoidins solution. The mixture was vibrated in a thermostatic oscillator at 40°C for 24 h. Desorption process: the adsorbed AB-8 macroporous resin was dissolved in 120 ml of 60% ethanol (v/v) solution. The mixture was vibrated in a thermostatic oscillator at 40°C for 24 h. Last, dry, and crush the melanoidins.

Preparation of melanoidins microcapsule dyestuff
Melanoidins microcapsules were prepared using a modified multi-emulsification method ( (Martí et al. 2016)). Firstly, a mixture of 1.0 g soy lecithin, phosphatidylcholine and cholesterol (3:3:2, by weight) was dissolved in 30 ml diethyl ether. Then, 2 mg/ml melanoidins dyestuff was added, shell/core materials of 25:1 (by weight), and the mixture was sonicated at 40°C for 5 min until the formation of a stable water-in-oil (W/O) emulsion. 10 ml PBS buffer was added and the mixture was sonicated again until the formation of stable water-in-oil-in-water (W/O/W) multi-emulsion. Finally, the organic solvent was removed from the W/O/W multi-emulsion slowly by using a rotary evaporator (RE-52AA, Shanghai Yarong biochemical instrument Factory, China) at 40°C for 15 min to obtained the final melanoidins microcapsule dyestuff. The SEM images of the melanoidins microcapsule dyestuff are listed in Figure 2. The basic structure of melanoidins microcapsule can be seen from the figure, this indicating that the melanoidins dyestuff molecules were successfully encapsulated by phospholipids.

Process of wool fabrics dyed
According to previous study, the after-chrome dyeing process was determined ((Guo 2021)). The wool fabrics were dyed with melanoidins microcapsule dyestuff by using an electro-thermostatic water bath kettle (HH-4, Wenzhou Darong Textile Instrument Co., Ltd., China). The weight of 1 g wool fabrics and the amount of the melanoidins microcapsule dyestuff 2 mg/ml were added into the dyeing bath at 40°C. Meanwhile, the dyeing bath's pH value was adjusted (according to the Table 2) by the concentration of 6 mol/L acetic acid. And the dosage of 10 mg/ml Pomegranate peel powder were used as a medium to fix the color. Dyeing process was carried out according to the curve shown in Figure 3. Finally, after the dyestuff was cooled to room temperature, the dyed wool fabrics were fully washed and naturally dried.

Evaluation and analysis of dyeing process of melanoidins microcapsule
The effect of the three dyeing processing factors (liquor ratio, pH value, and dyeing temperature) on the K/S value of dyed wool fabrics was analyzed by using a Box-Behnken experimental design in this study (Selvanathan, Govindhan, and Dharumadurai 2017). The codes reflecting the values and relationships among the three dyeing processing factors are presented in Table 2. A second-order polynomial equation (1) was adopted to describe the K/S value by using those three independent variables: where R is the encapsulation rate (response), α 0 is the intercept coefficient, α 1 , α 2 and α 3 represent the coefficients of independent factors. α 11 , α 22 and α 33 are the quadratic terms, α 12 , α 13 and α 23 are the interaction coefficients (Karimi, Feilizadeh, and Iliuta 2017). Afterward, the most effective use of those processing factors to optimize the K/S value was conducted by using Design-Expert software (Stat-Ease, Inc. URL https://www.statease.com).

Measurement of fastness properties of dyed wool fabric
The

Measurement of antioxidant property of the dyed wool fabric
The antioxidant property of the dyed samples were measured as per Q/0202LRX010-2020 methods.

Measurement of antibacterial property of dyed wool fabric
The antibacterial property of the dyed samples were measured as per GB/T 20,944.1-2007 methods.

Measurement of chromatism of dyed wool fabric
The chromatism property of the dyed samples were measured as per GB/T 3978 methods.

Characterization of wool fibers
The dyed and the undyed wool fibers were characterized by scanning electron microscopy (SEM, JSM-6700F, Hitachi Co., Ltd., Japan). The scanning acceleration voltage was 5.0 kV and the magnification was 1000.

Response surface optimization
The color strength (K/S) is an important index to evaluate the dyeing performance of the dyestuff, which is defined by the Kubelka-Munk function. Under certain conditions, the higher the K/S value, the darker the surface color of dyed wool fabric, the higher the concentration of colored substances and the better the dyeing performance of the dyestuff (Hosseinnezhad et al. 2020). The effect of the processing factors on the R (K/S) value was evaluated and optimized by a Box-Behnken experimental design. A (Liquor ratio), B (pH), and C (dyeing temperature) during hydration tested ranged from 1:30 1:40, 3 5 and 70 90°C, respectively. Table 3 listed the experimental conditions of 17 tests and their K/S values were determined accordingly.
Regression fitting analysis was performed on the experimental data shown in Table 3 to obtain the K/S value of the dyed wool fabric and the influence of various factors (2): R ¼ 19:87 À 0:65A þ 0:64B þ 0:55C À 0:22AB À 0:045BC þ 0:015BC À 1:65A 2 À 1:59B 2 À 1:88C 2 (2) Analysis of Variance (ANOVA) results are listed in Table 4. The model has a high F value and a low P value. It indicated that the regression is significant and the model prediction is accurate (Seied et al. 2019). Besides, the results suggested that the extent of the effects on K/S value was A (liquor ratio) > B (pH) > C (dyeing temperature). In the model, one-time terms (A, B, C) effect the K/S value significantly (p < .0001). Among the different interaction effects, only the interactions of liquor ratio and pH was significant (p < .05). As shown in Table 4, the most significant effect on the K/S value was shown to be the linear effect of liquor ratio, pH and dyeing temperature followed by the quadratic effect of liquor ratio and pH. As shown in Table 5, the determination coefficient of the quadratic regression equation was 0.9978 (>0.95) and the corrected determination coefficient was 0.9950 (>0.95), indicating that the reference value of equation is high. The response surface plots of the K/S value as functions of three pairs of the independent variables (i.e. liquor ratio, pH and dyeing temperature) are shown in Figure 4(a-c). The results suggested that the figure of pH and liquor ratio is oval, indicating that the interaction is strong. On the contrary, the figure of liquor ratio and dyeing temperature and that of pH and dyeing temperature are round, indicating that the interaction is weak.
In this model, the predicted optimal K/S value was 19.933 under the optimal conditions, with liquor ratio being 1:35.15, pH being 4.22 and dyeing temperature being 75.8°C. Afterward, the optimized processing conditions were applied to the dyeing process and the experimental K/S obtained was 19.901, indicating that the efficacy of this model.
As shown in Figure 4a, the K/S value was initially increased as pH increased (−1-0.5), followed by a decrease at higher pH (>0.5). The dyestuff is in the isoelectric state, and the dyeing effect is the best (Li et al. 2015).
As shown in Figure 4b, the K/S value was increased as liquor ratio, then followed by a decrease at higher liquor ratio. The concentration of melanoidins microcapsule dyestuff in dye bath increased with the increase of liquor ratio, which led to the improvement of the K/S value. However, further increase of cholesterol could increase the concentration of melanoidins dyestuff in dye bath, which influences K/S value. Because the affinity between the shell materials is greater than that between the  shell material and the liposomes in the wool fiber. This might result in microcapsule destruction and dyestuff disclosure. As shown in Figure 4c, the K/S value was increased as dyeing temperature, then followed by a decrease at higher dyeing temperature. The scale layer of wool opened in water above 60°C (Luqman et al. 2016). The extent of the opening of the scale layer in the dye bath increased with the increase of dyeing temperature, which led to the improvement of K/S value. Figure 5 shows the SEM images of the morphology of dyed wool fibers and the undyed wool fibers. There is only slight difference in the scale layer between the dyed wool fiber and the undyed wool fiber. It demonstrated that melanoidins microcapsule dyestuff had almost no damage to the scale layer of wool fibers by low temperature dyeing, which greatly reduced the damage to wool fabric during the dyeing process.

Color fastness of the dyed wool fabrics
As shown in Table 6, the washing fastness rating was found to be between 4 and 5. The perspiration fastness rating was found to be 4 in acidic condition and in between 3 and 5 in basic condition. The rubbing fastness of wet fastness and dry fastness was found in the range of 3-4. The high rating indicated that the dyestuff was attached firmly to the wool fabrics. However, the light fastness rating was found to be 1-2, showing poor result. The UV radiation in sunlight caused photodegradation of auxochromes and chromophores, leading to fading of color (Pandey et al. 2018).

Antioxidant property of the dyed wool fabrics
As shown in Table 7, the scavenging efficiency of the free radical ABTS •+ of undyed wool fabric was 34.11% and that of the dyed wool fabric was 72.88% (calculated using Eq. 3). This indicated that the wool fabric dyed with melanoidins microcapsule had good antioxidant property, because of the melanoidins themself exhibited excellent antibacterial properties (Cao, Yan, and Liu 2022).

Antibacterial property of the dyed wool fabrics
As shown in Table 8, the inhibition rate of Bacillus subtilis, Bacillus licheniformis, Staphylococcus aureus, and Escherichia coli of undyed wool fabric were 17.02%, 16.89%, 18.56%, and 17.39%, respectively, and 92.42%, 90.62%, 91.56%, and 89.39% for dyed wool fabric. This indicated that the wool fabric dyed with melanoidins microcapsule had good antibacterial property, because of the melanoidins themselves exhibited great antioxidant properties (Kukuminato, Koyama, and Koseki 2021).

Chromatism of dyed wool fabrics
As shown in Table 9, the ΔE of dyed wool fabric were basically identical. This indicated that the wool fabric dyed with melanoidins microcapsule had good dyeing uniformity. The dyed and the undyed wool fabric are shown in Figure 6, and the dyed wool fabric was presented a dark brown.

Conclusions
In this work, melanoidins was obtained from Lycium barbarum residue by microwave extraction and and melanoidins microcapsule dyestuff was prepared. Mordant fixation of wool fibers was dyed with melanoidins microcapsule dyestuff and the process parameters optimized by RSM were liquor ratio 1:35.15, pH 4.22 and dyeing temperature 75.8°C. Compared with the traditional dyeing process, phospholipid microcapsules not only increased the affinity between melanoid dyes and wool fabric, but decreased the dyeing temperature, which greatly reduced the energy consumption of dyeing process to wool fabric. Pomegranate peel as a biomordant has better fixation effect and is more environmentally friendly than metallic mordants, which is beneficial to the development and application of ecological textiles. Dyed wool fabric not only has high color fastness, but also has good antioxidant and antibacterial properties, which provided more added values to the dyed products.
In conclusion, Lycium barbarum residue, a by-product from the food processing industry, can be successfully utilized for the dyeing of textiles.

Highlights
• Maillard reaction of wolfberry residue to generate and extract melanoid as a natural dye belongs to waste recycling.
• The melanin microcapsule dye based on the double lipid model was prepared and applied to the dyeing of pure wool fabrics. The performance of the dyeing process was regulated by the response surface method, and the optimized low temperature dyeing process of wool products based on the CMC double absorption model was obtained. • The application of natural mordant in the process of dyeing pure wool fabric with melanoid microcapsules greatly reduces the pollution of dyeing wastewater compared with conventional metal ion mordant, and belongs to the dyeing technique and method for energy saving and emission reduction. • The test results show that this kind of melanin microcapsule dye can dye pure wool fabric to obtain excellent antibacterial and antioxidant properties, and greatly improve the added value of dyeing products.

Disclosure statement
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.