Evaluation of bamboo water-retting for fiber bundle extraction

Abstract

Bamboo fiber bundles were successfully extracted from bamboo culms using water-retting, taking advantage of enzymes secreted by microorganisms in the retting liquid. The harvest year and place of origin of the bamboo and the source of water impacted the products of the retting process. One-month-old bamboo was decomposed completely, whereas the one-year-old sample was hardly changed after 24-day retting. Moisture regain and crystallinity varied with the different origins of the bamboo. However, all samples resulted in similar chemical structures and thermal properties. The best operational conditions for water-retting were 3-month-old bamboo from Wuxi incubated in deionized water. Enzyme activities, including cellulase, xylanase, pectinase, and ligninolytic enzymes (lignin peroxidase, manganese peroxidase, and laccase) were monitored during a 24-day retting. Manganese peroxidase was the primary enzyme used to degrade lignin, resulting in absorbance at 294 nm of UV-Vis spectra. In addition, xylanase played a leading role in hydrolyzing hemicellulose, which was consistent with the change in reducing sugar yield. In addition, variations in dissolved oxygen and pH values were also recorded, indicating the changes in bacterial strains and the enzymatic system. The wastewater from bamboo retting showed good biodegradability but a lack of nitrogen and phosphorus. Overall, a manganese peroxidase–xylanase combined enzyme-retting treatment would offer a more environmentally friendly approach for extracting bamboo fibers.

Keywords

Moso bamboo retting ligninolytic enzymes xylanase

Bamboo is a fast-growing, widely cultivated plant common in Asian countries.¹ The most abundant bamboo forest is located around the Yangzi River, China,² where Moso bamboo (Phyllostachys edulis) is planted extensively. As a fast-growing plant, bamboo is a suitable choice for the production of natural fiber composites^3–6 due to its superior mechanical properties⁷ compared with other plant fibers, such as cotton, hemp, flax, jute, sisal, and wood. Other applications^2,8–10 of bamboo focus on furniture, plywood, packaging materials, textiles, paper production, and hygienic materials. In the textile industry, although bamboo pulp fiber is widely used, it loses its natural antibacterial properties due to treatment with alkalis in processing.^6,11 Moreover, as long, fine, and straight bamboo fiber with the original bamboo characteristics is difficult to obtain from bamboo culms, the bamboo industry has failed to maximize the advantages of the material properties, clean production, and economic performance.⁶

Traditionally, natural bamboo fibers were extracted and separated by mechanical^4,5 and chemical treatment.^2,12,13 These methods may lead to strength loss and orientation damage,⁶ which are crucial to the quality of bamboo fibers. The enzymatic process has also been studied and applied to remove non-cellulosic materials and obtain delicate and flexible fiber bundles, usually used in the processing of flax fibers.^14–17 Dew-retting and water-retting are the most widely used enzymatic methods, utilizing pectinolytic aerobic fungi¹⁶ and the bacterial community,^17,18 respectively. The water-retting process provides higher quality fiber bundles^19–21 and reduced environmental pollution.^22–24 Evans et al.¹⁵ modified the water-retting process by using polygalacturonase-containing solutions, which increased the yield of flax fibers. In addition, a radiofrequency-based approach has been adopted to enhance retting efficiency at different stages of the water-retting process.²⁵ Bleuze et al.²⁶ explored dew-retted hemp’s chemical composition and microbial enzyme activities and found a slight increase in lignin content and high pectinase activity. Jankauskienė et al.²⁷ investigated the impact of the retting method, hemp cultivar, and harvest year on the chemical composition and physical properties of hemp fibers. Water-retted stems achieved higher cellulose and hemicellulose content but lower lignin content.

Compared with flax and hemp fibers, bamboo contains a large amount of lignin and less pectin, so it is more difficult to obtain long, fine, straight fibers directly from bamboo culms for further spinning.^28,29 Cellulase, xylanase, pectinase, and mannanase are able to hydrolyze bamboo powders.^30,31 In addition, the reducing sugar yield and enzymes absorbed on the surface of bamboo were enhanced by using different pre-treatments, including a mild chemical oxidant,³¹ high-temperature heating,^32,33 microwave treatment,³³ and ultrasonic treatment.³⁴ Enzymatic treatments on bamboo effectively removed non-cellulosic materials; however, to the best of the researchers’ knowledge, no studies have investigated which enzyme activities are primarily required in retting.

In the present study, water-retting was carried out under conditions of controlled temperature and humidity. The properties of bamboo fiber bundles such as chemical composition, chemical structures, and thermal behavior were measured. The optimized retting system was selected according to the yield and quality of bamboo fiber bundles produced. Enzyme activities related to cellulase, xylanase, pectinase, ligninolytic enzymes, and reducing sugar yield were monitored. UV-Vis spectra were adopted to analyze the enzymatic hydrolysates from the lignin in the bamboo samples. The wastewater quality of the bamboo retting liquid was also evaluated, including pH, dissolved oxygen (DO), chemical oxygen demand (COD), biochemical oxygen demand (BOD₅), total nitrogen (TN), and total phosphorus (TP). We aim to propose a fast enzyme-retting method and establish a new methodology for extracting bamboo fibers.

Materials and methods

Materials

Moso bamboo (Phyllostachys edulis) samples were harvested from Wuxi, Jiangdu, and Shuyang in Jiangsu Province, China. Bamboo culms were hand-cut into 5 cm-long sections. The dirty-water and soil samples were collected from the same plot as the bamboo (Wuxi). The soil-water was prepared at a ratio of 100 g of soil to 1 L of deionized water. All the materials were stored at 4°C.

Pectin, 2-hydroxy-3,5-dinitrobenzoic acid (DNS), and Azure B were purchased from Sigma-Aldrich (Shanghai, China). 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid (ABTS) was obtained from J&K Scientific Ltd. (Beijing, China). All other reagents were of analytical grade and provided by Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China).

Bamboo fiber bundle extraction

Bamboo culms were incubated in water (deionized-water, dirty-water, and soil-water) with a ratio of 1:10 (w:v) in 250 mL Erlenmeyer flasks and then covered with tin foil and equilibrated at 37°C and 65% relative humidity, with no addition of external microbial strains. The water-retting process took advantage of the pre-existing microorganisms in the bamboo and the different sources of water as described above. Retting liquid samples were taken every 3 days over 24 days to monitor the critical retting enzymes. Samples were kept at 4°C for enzyme and physicochemical analysis until all parameters were determined. After the retting process (day 24), the bamboo fiber bundles were collected, washed, and dried at room temperature for further experiments. The nomenclature used for the bamboo retting system is provided in Table 1.

Table 1.

Nomenclature of bamboo retting system for the different procedures

	Harvest year	Origin place	Source of water
3M	Three-month-old	Wuxi	Deionized-water
1M	One-month-old	Wuxi	Deionized-water
1Y	One-year-old	Wuxi	Deionized-water
3MJD	Three-month-old	Jiangdu	Deionized-water
3MSY	Three-month-old	Shuyang	Deionized-water
3MDW	Three-month-old	Wuxi	Dirty-water
3MSW	Three-month-old	Wuxi	Soil-water^#

Characterization of bamboo fiber bundles

Yield and moisture regain

Bamboo culm (5 cm long) and obtained bamboo fiber bundles were stored and weighed under the same temperature (±25°C) and humidity (65%) conditions before testing recording w_w-b and w_w-f, respectively. The weight of oven-dried bamboo culms (w_d-b) and bamboo fiber bundles (w_d-f) was also noted to calculate yields as

Yield (%) = \frac{w_{d - f}}{w_{d - b}}

(1)

Next, moisture regain was evaluated using

Moisture regain (%) = \frac{w_{w} - w_{d}}{w_{d}}

(2)

following the guidelines outlined in AATCC TM-20A; 2020 Test Method for Fiber Analysis: Quantitative. Bamboo fiber bundles were oven-dried, weighed, and recorded (w_d).

Chemical composition and crystallinity

The bamboo culms and bamboo fiber bundles were milled into particles small enough to pass through a 1 mm sieve. Wax, water-soluble components, pectin, and hemicellulose were extracted in chloroform, water, EDTA solution, and NaOH + NaBO₃ solution, respectively. Klason lignin was determined using 72% (w/w) sulfuric acid,²⁸ and cellulose content was measured according to TAPPI T-203; 1999 Alpha-, beta-, and gamma-cellulose in the pulp.

A diffractometer model D2 PHASER (Bruker AXS Co., Germany) was applied to investigate the crystallinity of the bamboo fiber bundles produced by the different retting systems. Data were recorded from 5° to 50° with a step of 0.02° at 10 mA and 30 kV, using Cu target. Referring to the Segal method,³⁵ crystallinity was calculated using

Crystallinity (%) = \frac{I_{200} - I_{AM}}{I_{200}} \times 100 %

(3)

where I₂₀₀ is the intensity at 2θ = 22.7° and I_AM^o at 2θ = 18°.

FT-IR spectroscopy

Fourier-transform infrared (FT-IR) spectroscopy of the bamboo fiber bundles produced by the different retting systems was performed using a Thermo Nicolet iS10 spectrometer (Thermo Fisher Scientific Co., Waltham, MA, USA). The spectra were analyzed in a range of 4000 to 1000 cm⁻¹, with a resolution of 8 cm⁻¹ and 32 scans. All the spectra were baseline corrected.

Thermal properties

Thermogravimetric analysis (TGA) of the bamboo fiber bundles was carried out using a TGA Q500 V20.13 Build 39 equipment. The powder samples (mentioned previously in the section on “Chemical composition and crystallinity”) were weighed (5 mg) and equilibrated at 30°C for 1 min, and then raised to 800°C with a heating rate of 10°C/min and nitrogen flow rate of 40 mL/min.

Enzymatic activity determination

The retting liquid was centrifuged at 7200 × g for 5 min; the obtained supernatant was used to measure enzyme activities. Xylanase activity was measured according to an improved dinitrosalicylic acid (DNS) assay.³¹ In brief, 20 µL of retting liquid was incubated with 60 µL of 1% (w:v) xylan solution at pH 7.0 in a water bath at 50°C for 30 min. Afterwards, 60 µL of DNS solution was added and followed by a boiling water bath for 5 min. After cooling, 860 µL of distilled water was added. Absorbances were recorded at 520 nm using a Synergy™ H1. The yield of the product can be calculated from the calibration curve measured using xylose. One unit of xylanase (U) was defined as the amount of xylanase required to liberate 1 μmol of product per min under the above-mentioned assay conditions. The cellulase and pectinase activities were detected using sodium carboxymethyl cellulose and pectin, respectively. The cellulase and pectinase activities were subjected to the same procedure steps as the xylanase activity measurement.

The spectrophotometer (Synergy™ H1) was also used to evaluate the activities of ligninolytic enzymes: lignin peroxidase (LiP), manganese peroxidase (MnP), and laccase (Lac). A volume of 0.05 mL of retting liquid was mixed with 0.2 mL of 0.5 mmol/L substrate solution (Azure B for LiP at pH 4.5, MnSO₄ for MnP at pH 4.5, and ABTS for Lac at pH 3.0). For LiP and MnP measurements, 0.05 mL of 50 mmol/l H₂O₂ was used to catalyze the reactions. The mixture solution was diluted to 1.0 mL using a buffer solution. The absorbance of Azure B, Mn³⁺, and ABTS was measured at 651 nm, 240 nm, and 420 nm and recorded every 1 min for 10 min. One unit of LiP, MnP and Lac (U) is defined as the amount of LiP, MnP, and Lac required to oxidize 1 µmol of substrate per minute.

Physicochemical characterization of the retting liquid

The reducing sugar yield, pH, and DO were monitored during the retting process. In accordance with Fu et al.,³¹ the reducing sugar yield was measured by the DNS method, as mentioned in the section “Enzymatic activity determination.” DO and pH values were measured in situ using a portable dissolved oxygen meter (ST300D; Ohaus, Parsippany, NJ, USA) and a digital pH meter (ST3100; Ohaus, Parsippany, NJ, USA), respectively. Water quality, including COD, BOD₅, TN, and TP, were analyzed following the standard procedure on day 24. COD was measured by the K₂CrO₇ method,³⁶ BOD₅ by the dilution and seeding method with allylthiourea addition (ISO 5815-1:2003), and TN and TP were measured by peroxodisulfate oxidation.³⁷ UV-Vis spectra were also recorded using a T6 ultraviolet-visible spectrometer and data were collected from 320 to 600 nm with 1 nm path length. Five parallel experiments were conducted for all the parameters.

Results and discussion

Characterization of the bamboo fiber bundles

As described in Table 2, bamboo fiber bundles were successfully obtained using water-retting. The type of products varied by the harvest year and origin of bamboo culm samples. One-month-old bamboo was hydrolyzed completely during the 24-day retting. However, sample 1Y was unchanged in appearance, as no effective microorganism appeared to have multiplied in the deionized water to attack the cuticle layer of the 1-year-old bamboo. Three-month-old bamboo does not mature entirely, and the growth rate of its cell walls varies according to the place of origin of the bamboo. Herein, bamboo fiber bundles were successfully collected except for bamboo harvested from Shuyang. Sample 3MJD exhibited different properties, including higher moisture regain (9.7%), lower crystallinity (33.6%), and lower cellulose content (38.1%). Further, the source of water impacted the yield and quality of the bamboo fiber bundles. Samples obtained using dirty-water retting achieved better dispersion (Figure 1: 3MDW). The yield increased to 51.0% when deionized-water was used, and the fiber bundles were easily separated by hand. Abundant fungi in the soil-water may have further decomposed the cellulose-rich secondary cell walls in the bamboo fiber bundles,^38,39 resulting in a decline of cellulose content and crystallinity. Also, sample 3MSW was rough, and some soil particles (in red circles) can be seen in Figure 1.

Table 2.

Products of retting process and characterizations of the bamboo fiber bundles

	Product	Yield/%	Moisture regain/%	Crystallinity /%	Cellulose/%
3M	Bamboo fiber bundles	51.0	6.0	37.5	45.8
1M	Complete decomposition	--	--	--
1Y	Bamboo culms	--	--	--
3MJD	Bamboo fiber bundles	42.7	9.7	33.6	38.1
3MSY	Complete decomposition	--	--	--
3MDW	Bamboo fiber bundles	30.6	6.1	36.9	44.8
3MSW	Bamboo fiber bundles	31.7	6.2	35.9	39.9

Figure 1.

Pictures of the bamboo fiber bundles produced by retting system 3M, 3MJD, 3MDW, and 3MSW.

The chemical structure on the surface of the bamboo fiber bundles is shown in Figure 2. Absorbances at 3420 cm⁻¹, 2920 cm⁻¹, and 1046 cm⁻¹ were consistent with O-H stretching, C-H stretching, and C-O bending, respectively, which may exist in cellulose, hemicellulose, and lignin. Glycosidic bonds (897 cm⁻¹)⁴⁰ were responsible for linking the reducing sugar to form cellulose and hemicellulose structures. A typical peak of hemicellulose was observed at 1383 cm⁻¹, which was attributed to polyxylose.³¹ Acetyls that may have been present in hemicellulose⁴¹ were noted at 1734 cm⁻¹ except for sample 3MJD, resulting in lower moisture regain (about 6%) among Wuxi samples. The band at 1637 cm⁻¹ was possibly caused by adsorbed water within the cellulose.⁴² For sample 3M, it shifted to a lower wavenumber (1632 cm⁻¹) as the breaking of the hydrogen bonds between cellulose and lignin and a new absorbance appeared at 1604 cm⁻¹, assigned to C=C stretching of the aromatic rings in lignin. In addition, absorption due to CH₂ bending vibration in cellulose, carboxyl vibration hemicellulose, and aromatic ring stretching in lignin was detected at 1428 cm⁻¹.⁴² Phenylpropane, which represents the main structure of lignin, presented a peak at 1514 cm⁻¹. Absorptions at 1320 cm⁻¹, 1255 cm⁻¹, and 1162 cm⁻¹ were related to syringyl, guaiacyl, and p-hydroxyphenyl, meaning the lignin of the bamboo fiber bundles was GSH-lignin, which is similar to that of raw bamboo.⁴³

Figure 2.

FT-IR spectroscopy of the bamboo bundles produced by different retting systems. (a) 3M, (b) 3MJD, (c) 3MDW, (d) 3MSW.

Cellulose and lignin content can influence the thermal behavior of bamboo fiber bundles.⁴⁴ TGA analysis was used to determine the high-temperature degradation behavior of the bamboo fiber bundles under nitrogen atmospheres. Three-step decomposition stages were present in the TGA curves (Figure 3) for all the samples. Water evaporation led to weight loss among the samples, corresponding to the first signal of derivative thermogravimetry (DTG) at around 55°C. Next, the major chemical composition of the bamboo fiber bundles was degraded from 200°C to 380°C, including total decomposition of hemicellulose and cellulose and the partial decomposition of lignin. In this stage, all the samples lost similar weights: 3M: 68.00% loss, 3MJD: 70.56% loss, 3MDW: 70.32% loss, and 3MSW: 69.91% loss (Table 3). The DTG curves displayed a slight shoulder at 298°C and a remarkable peak at about 350°C, attributed to the degradation of hemicellulose and cellulose.⁴⁵ The temperature of their characteristic peaks in the DTG curves was similar to that of bamboo fibers.⁴⁶ However, a characteristic peak for lignin was not detected, due to its wide decomposition temperature.⁴⁵ The third step corresponded with the remaining lignin combustion and char oxidation⁴⁷ (almost 7.00%). In addition, the remaining residues of all the samples accounted for around 16.00%.

Figure 3.

TGA curves of the bamboo bundles produced by different retting systems 3M, 3MJD, 3MDW, and 3MSW.

Table 3.

Weight loss (%) and maximum weight loss temperature (°C) on the different decomposition steps calculated from TGA curves

	Δ weight /% (30–100°C)	T_{d max}/°C (30–100°C)	Δ weight /% (200–380 °C)	T_{d max}/°C) (380–800°C)	Δ weight /% (380–800°C)	Remained /%
3M	6.62	56	68.00	298/348	8.44	15.07
3MJD	5.40	54	70.56	298/352	7.01	16.20
3MDW	5.41	55	69.91	298/352	7.22	15.92
3MSW	6.62	56	68.00	298/348	8.44	15.07

Figure 4.

Enzyme activities of retting liquid from retting system 3M (three-month-old bamboo from Wuxi incubating in deionized water).

Based on the FT-IR and TGA analyses, all the bamboo fiber bundle samples presented similar chemical structures and thermal properties. Overall, 3M provided the optimized conditions with the highest yield and good dispersion of the obtained bamboo fiber bundles. The retting liquid collected from retting system 3M was prepared for enzyme activity analysis and physicochemical characterization.

Activity assay of cellulase, xylanase, pectinase, and ligninolytic enzymes

Retting aims to remove non-cellulosic compounds (mainly lignin and hemicellulose) from bamboo. At the early stage (day 0–9), enzyme activities increased, except for MnP, whose maximum value of 0.45 U was recorded on day 6 (Figure 4). Moreover, ligninolytic enzymes showed higher activity than other enzymes as bacteria primarily acted on the thin-walled cells,²⁶ mainly containing lignin. The lignin structure of bamboo is complex and irregular, consisting of the p-hydroxyphenyl unit, guaiacyl unit, and syringyl unit,⁴³ which are linked via ester and ether bonds. In bamboo lignin, the syringyl unit and ether bonds were predominant structures.⁴³ LiP can attack the non-phenolic lignin network, whereas MnP and Lac were unable to oxidize phenolic lignin substrates. In addition, the Mn²⁺ ion in bamboo was oxidized to Mn³⁺ ion by MnP.⁴⁸ The complex Mn³⁺ ion would not only oxidize non-phenolic lignin in the presence of unsaturated lipids⁴⁹ but may diffuse into the lignified cell wall to oxidize phenolic components in lignin and other organic substrates.⁵⁰ At the middle stage (day 9–15), MnP, LiP, and Lac activities declined, while cellulase, xylanase, and pectinase activities kept growing. However, the activity of MnP was still higher than that of other enzymes, indicating that MnP was the critical enzyme for the first step of lignin degradation in the water-retting of bamboo.

The bacterial community structure in water-retting is non-selective, and some microbes secrete specific enzymes depending on the nutrient substance for their metabolic activities.⁵¹ With the degradation of lignin, pectinase attacked pectin remaining in the primary cell wall and achieved the highest activity on day 21. Furthermore, hydrolysis of the xylan-rich wall, which can protect cellulose-rich fibers,⁵² resulted in a noticeable enhancement of xylanase activity from day 9. Interestingly, xylanase exhibited a higher activity than pectinase. Regarding the high lignin content and low pectin content of bamboo, the primary enzymes required in bamboo retting are different from flax or hemp retting. Meanwhile, cellulase was also secreted to hydrolyze the haphazardly situated cellulosic fiber in the primary cell wall.²⁶ The minimal activity of cellulase is also beneficial for removing non-cellulosic compounds in bamboo, as lignin generates hemicellulose and cellulose clusters.⁵¹ However, the activities of enzymes may be impacted by the retting products, which were used as nutrient substances by bacteria in the water-retting process. Unfortunately, avoiding this shortage is challenging.

For promoting best fiber retting, bacterial consortia or combined enzyme systems were adopted in retting.^53–55 Different commercial enzymes were produced to give consistent fiber quality, restricting fiber damage, and reducing retting wastewater pollution.^54,56 However, most products take advantage of pectinolytic enzymes, xylanase, or a combined pectinase–xylanase system.^57,58 Texazym SER-5 (Inotex) is the only commercial product that involves LiP.⁵¹ Based on our results, it was necessary to add MnP in the first step of lignin degradation, followed by xylanase hydrolysis. Therefore, commercially available products for enzymatic retting might be unsuitable for bamboo retting. The present study’s results indicate that using a combined enzymatic treatment with MnP–xylanase would be a helpful way to extract bamboo fiber bundles.

Physicochemical characterization of the retting liquid

Reducing sugar yield and UV-Vis spectra

The products extracted from bamboo powders were assigned as D-xylobiose and D-glucuronic acid from hemicellulose, and coniferyl alcohol and sinapyl alcohol from lignin. Besides, other compounds involved in the lignin biosynthesis of bamboo along with the lignin monomers were identified as vanillic acid, vanillin, 5-hydroxymethylfurfural, gallic acid, and guaiacol.^34,59

The reducing sugar yield showed a significant increase in the first 3 days (Figure 5(a)), due to the soluble sugars in bamboo culm samples. The reducing sugar mainly resulted from the decomposition of hemicellulose and was consumed by microbial strains. From day 3 to day 18, the reducing sugar yield was slightly raised as the carbon source was over-provided for bacteria. And then less reducing sugar was produced with the decline of xylanase activity, causing a decrease in the reducing sugar yield. At the last stage (day 21–24), the reducing sugar yield raised again because of bacterial death.

Figure 5.

Reducing sugar yield (a) and UV-Vis spectra (b) of retting liquid from retting system 3M (3-month-old bamboo from Wuxi incubating in deionized water).

The UV-Vis spectra presented a significant peak at around 294 nm, attributed to non-conjugated aromatic rings and aldehyde groups in enzymatic hydrolyzed products from bamboo.^60,61 With retting, the absorbance decreased and shifted to a lower wavelength because of the multiplication of bacteria and pH changes.

Changes in DO and pH values

The original DO in water and soluble substances in bamboo culms provided suitable conditions for the rapid multiplication of aerobic bacteria, causing a significant decrease in DO and pH values (Figure 6). An aerobic environment can induce microbes to produce ligninolytic enzymes (mainly MnP), whose activity was more stable under acidic conditions. With the consumption of oxygen, microbial community structures in retting liquid changed to anaerobic strains. From day 9 onwards, anaerobic bacteria that can induce xylanase and pectinase^17,18 played a primary role in this process. Hemicellulose and pectin were decomposed into low-molecular-weight carbohydrates, which hardly impacted the pH values. On day 21 of retting, it was possible to separate a few bamboo fiber bundles by hand. Meanwhile, the activity of all the enzymes declined to a low value (<0.15 U). Due to the volatilization of acidic substances, the pH value returned to 6.2 and remained steady.

Figure 6.

DO (a) and pH (b) of retting liquid from retting system 3M (3-month-old bamboo from Wuxi incubating in deionized water).

Water quality of wastewater from retting

Flax retting wastewater is characterized by a high concentration of organic materials;²⁴ to measure this, COD and BOD₅ values of the retting liquid were recorded on day 24. COD describes the total amount of pollution (organic and inorganic materials) present in the wastewater, while BOD provides a measure of the oxygen required for the biochemical oxidation of the organic matter. The results showed that the BOD₅/COD ratio of the bamboo retting wastewater was 0.91, indicating the potential for biodegradation.⁶² Specifically, the ratio of COD:TN:TP was 100:0.54:0.05, while the preferred ratio for biological oxidation is 100:5:1.⁶³ This suggested that both nitrogen and phosphorous need to be added to enhance the efficiency of further biological treatment.

Conclusions

Bamboo fiber bundles could be extracted using the water-retting method. Three-month-old bamboo harvested from Wuxi showed the highest yield (51%) and quality after retting in deionized water at 37°C and 65% relative humidity. At the early stage of the water-retting, aerobic bacterial-induced ligninolytic enzymes (especially manganese peroxidase) showed higher activities. Afterwards, xylanase played the main role in biodegradation. Also, the changes in reducing sugar yield and UV-Vis spectra indicated the decomposing of hemicellulose and lignin. Therefore, a manganese peroxidase–xylanase combined enzyme-retting process could be more efficient in removing non-cellulose components from bamboo. The methodology proposed in the present study may maintain the bamboo fibers’ original characteristics, which benefits further spinning and imparting functions for textile production processes. However, the antibacterial properties of bamboo fibers and the optimized conditions for manganese peroxidase–xylanase retting require further investigation. Subsequent work should also focus on exploring the changes within the bacterial community structure.

Footnotes

Authors Note

Yu Li and Jiajia Fu is now affiliated from China National Textile and Apparel Council Key Laboratory of Natural Dyes, Soochow University, Suzhou, China and Hongbo Wang and Weidong Gao is no longer affiliated from Key Laboratory of Eco-textiles, Jiangnan University, Ministry of Education, PR China.

Declaration of conflicting interests

The author(s) declare that there is no conflict of interest.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (Grant No.31470509), Chinese Foundation Key projects of governmental cooperation in international scientific and technological innovation (Grant No. 2016 YFE0115700), Jiangsu Planned Projects for Postdoctoral Research Funds (Grant No.2018K018A) and Jiangsu Provincial Science and Technology Department Policy Guidance Program-International Cooperation Projects-Innovation cooperation project of “B&R” (No. BZ2020010).

ORCID iD

Jiajia Fu

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