Effects of different scouring methods on the catalytic efficiency of pectinase for cotton knitted fabrics

Abstract

The application of bioscouring in cotton knitted fabrics should be improved owing to being a time-consuming process. In order to accelerate the bioscouring rate and shorten the bioscouring process, a repeated padding method is used on cotton knitted fabrics. The effect of the repeated padding with pectinase was studied by using FE-SEM and FTIR, and the catalytic rate was measured at different reaction temperatures, pick-up, and soaking times. The results observed in this study showed that during the bioscouring process for cotton knitted fabrics, the average catalytic rate of the repeatedly padding method was sixfold faster than the impregnation method, and the process time (15 min) decreased by 65 minutes compared with the impregnation method (80 min); the processing time was shortened to one-sixth of the impregnation method. Furthermore, the wettability of cotton knitted fabrics was better than those treated via impregnation. In the repeated padding process, with the soaking time increasing from 5 s to 25 s, the produced reducing sugar concentration increased from 1.71 mg/ml to 3.60 mg/ml in the same process time; with the pick-up increased from 50% to 90%, the produced reducing sugar concentration first increased from 1.99 mg/ml to 3.60 mg/ml, then decreased to 1.86 mg/ml. When the pick-up was about 70%, the catalytic rate of pectinase reached the best value (0.24 mg/ml/min).

Keywords

bioscouring catalytic rate cotton knitted fabric padding pectinase

Enzyme scouring has the advantages of being gentle, a simple process, energy-saving, lower consumption, and being eco-friendly.¹ It is expected to replace the traditional alkaline scouring process.^2–6 At present, the method of enzyme scouring is based on the efficient catalysis and specificity of the enzyme to remove or catalyze hydrolysis of impurities in cotton fibers.^7,8 However, owing to its small accessible area and contact-catalysis mechanism, enzyme scouring cannot carry out more catalysis in the internal structure of fabrics. The biological enzyme impregnation method, which is to soak the fabrics in enzyme solution for a long time, has the defect of strength reduction and being time-consuming; in the biological enzyme cold pad-store method, fabrics are soaked in enzyme solution for 1 h then stored for 4–12 h, which is a lengthy process that is not suitable for industrial application. Some studies have combined ultrasonic, vacuum technology, and plasma pretreatment to improve the performance of biological enzymes and optimize the properties of cotton fabric, but the process is complicated and the cost is too high.^9–11 For the reasons that impurities are not completely removed and scouring time is too long,^12,13 especially in the field of fast fashion, achievement of rapid and continuous cotton fabric bioscouring is an urgent problem to resolve.

The aim of this study is to explore a more efficient method for scouring cotton fabrics, and to discuss the effects of pectinase in the scouring process at different conditions. It is found that bioscouring cotton fabrics by repeated padding (5–15 times, at lower temperature) can greatly shorten the time of enzyme scouring and is an important means to enhance enzymatic efficiency.¹⁴ Through the process of repeated padding with a biological enzyme, we achieve a truly rapid scouring of the cotton knitted fabric, and the process time is greatly shortened to one-sixth of the impregnation method.

Experiment

Materials

Table 1 lists the materials used.
Table 1.
Materials

Material Specification

Acid pectinase Tianjin Aonuo Technology Development Co. Ltd., China, Tianjin

Pure cotton knitted fabrics weight (250 g/m²) Guangzhou Golden Eagle Bio-chemical Co. Ltd., China, Guangzhou

JFC (RO(CH₂CH₂O)nH) Tianjin Hao Yuan Fine Chemical Co. Ltd., China, Tianjin

α-D-Galacturonic acid monohydrate (C₆H₁₀O₇·H₂O, AR, purity > 97%) Hefei Bomei Biological Technology Co. Ltd., China, Hefei

Pectin (AR, purity > 70% from citrus peel) Hefei Bomei Biological Technology Co. Ltd., China, Hefei

Anhydrous sodium sulfate (Na₂SO₃, AR, purity 97%) Tianjin North Day Chemical Reagent Factory, China, Tianjin

Phenol (C₆H₅OH, AR) Tianjin Kermel Chemical Reagent Co. Ltd., China, Tianjin

Potassium sodium tartrate tetrahydrate (C₄O₆H₄KNa, AR) Tianjin Kermel Chemical Reagent Co. Ltd., China, Tianjin

3,5-Dinitrosalicylic acid (C₇H₄N₂O₇, AR) Tianjin Guangfu Fine Chemical Research Institute, China, Tianjin

Methods

Bioscouring process of enzyme by repeated padding

Pure cotton knitted fabrics were wetted at 50℃, and then soaked in biological enzymes for a few seconds before being taken out and passed through the padder. This process was repeated a specified number of times before being removed. The fabrics were washed twice with water at 70℃, then washed twice in cold water, and finally dried at 105℃.

Bioscouring process of enzyme by impregnation

Pure cotton knitted fabrics were wetted at 50℃, and then soaked in biological enzymes for 30 min. Then the fabrics were washed twice with water at 70℃, washed twice in cold water, and dried at 105℃.

Alkali scouring

Pure cotton knitted fabrics were wetted at 50℃, and then soaked in alkali solution and heated and kept boiling for 120 minutes. Then the fabrics were washed twice with water at 90℃ (10 min each time), then washed twice with warm water, then washed twice in cold water, and dried at 105℃.

Analytical methods

Assay for concentration of reducing sugar

The main substance affecting the wettability is pectin, which can be hydrolyzed into reducing sugar by pectinase in the process of enzyme scouring. Therefore, the concentration of reducing sugar obtained after hydrolysis can indicate the effect of pectin removal.

First, the standard curve of α-D-galactose acid was prepared according to the method outlined by Jin and colleagues.¹⁵ We collected 2 ml of the test sample in a 20 ml test tube with a glass stopper, added 1.5 ml DNS reagent and shook well. Next, we heated it for 8 min in a boiling water bath. After removal it was left to cool and then filled with distilled water to 20 ml, covered with a stopper, and shaken well. The absorbance was measured by spectrophotometer at 540 nm. According to the standard curve of α-D-galactose acid, the concentration of reducing sugar was obtained.

Assay for catalytic reaction rate

Assay for catalytic reaction rate of bioscouring process by repeated padding with pectinase

Pure cotton knitted fabrics were wetted at 50℃, and then soaked in pectinase for a few seconds. They were then taken out and passed through the padder. We poured the rolled liquid into the enzyme treatment solution and mixed it evenly. We took 2 ml of biological enzyme treatment fluid to measure the concentration of the reducing sugar. The concentration (C) of reducing sugar was calculated according to the standard curve. The growth rate (V) of reducing sugar concentration reveals the rate of catalysis of pectinase. The growth rate was calculated using equation (1).

Assay for catalytic reaction rate of bioscouring process by impregnation with pectinase

Pure cotton knitted fabrics were wetted at 50℃, and then soaked in pectinase for a specified time and mixed evenly. We then took 2 ml of biological enzyme treatment fluid to measure the concentration of the reducing sugar. The growth rate was calculated using equation (1):
$V = \frac{C}{T}$
(1)
where V is the growth rate of the reducing sugar concentration or the rate of catalysis of pectinase; C is the concentration of the reducing sugar, and T is the time in the impregnating method or the number of padding events in the repeated padding method (one padding is about 1 min).

Determination of scouring effect of cotton knitted fabrics

Bursting strength

The bursting strength of cotton fabric samples was measured using capillary effect tester YG871L (Laizhou Electronic Instrument Co., Ltd., China, Laizhou) according to the method of GB/T 19976-2005: Textiles – determination of bursting strength – steel ball method.

Whiteness of cotton fabrics

The whiteness of cotton fabric samples was measured using whiteness tester DSBD-1 (Beijing Kang Guang Instrument Co., Ltd., China, Beijing). The whiteness value of cotton fabric samples was tested when the fabric was folded twice. Each kind of fabric was tested three times to obtain the average whiteness value.

SEM analysis

We took a small piece of each fabric from the edge of the 4–5 cm fabric sample, then a scanning electron microscope (Hitachi S4800, Japan, Tokyo) was used to examine it at a magnification of 500× and 2000× times to obtain the overall structure of each fabric and the morphology of single fibers, respectively.

FTIR analysis

The FTIR spectrum (4000–400 cm^–1) of the fabrics was tested by Fourier transform infrared (FTIR) spectroscopy (Nicolet iS50, USA, Waltham), using a resolution of 4. The change in the main absorption peaks of the impurities on cotton fabrics was determined, and the removal of natural impurities from cotton fibers was analyzed.

Results and discussion

Catalytic reaction rate of pectinase by different treatment methods at 50℃

Figure 1 shows the catalytic reaction rate of pectinase in different treatment methods at 50℃. Compared with conventional biological enzyme impregnation treatment of cotton fabrics, for the first 5 min of the process, the concentration of reducing sugar in the repeated padding method increased from 1.35 mg/ml to 1.60 mg/ml. We could conclude that the treatment of cotton knitted fabrics by repeated padding can improve the production rate of reducing sugar, which can improve the removal efficiency of pectin on the cotton knitted fabric. As can also be seen in Table 2, the catalytic reaction rates in the first 5 min were compared between two bioscouring methods, and the repeated padding method was superior to the impregnation method. The concentration of reducing sugar when padding was repeated 15 times was higher than that following 80 min of the impregnation method. In the repeated padding method, one padding is about 1 min. The average catalytic reaction rate of the repeated padding process was about sixfold that of the impregnation method, and the process time (15 min) decreased by 65 minutes compared with the impregnation method (80 min). In the traditional enzyme impregnation method, the reaction between pectin in cotton fabrics and the enzyme is due to free movement and contact with water molecules or enzyme molecules. Pectinase in solution can contact the substrate when the fabric is wetted by water, so as to catalyze and undergo reaction.¹⁶ However, this free movement is slow, and pectinase and pectin on cotton fabrics are difficult to force contact and cause a rapid reaction. After soaking for a long time, the substrates that are not in direct contact with an enzyme or covered with reduced sugar can react only by the enzyme molecule undergoing Brownian movement, and so the rate of late reaction is slow. In contrast, in the repeated padding method in which the cotton fabrics are rolled with a padder, the enzyme solution goes deep into the cotton fabric under the influence of a high-pressure external force to make full contact with the pectin on cotton fabrics.¹⁶ This makes the pectin and pectinase reaction quicker and more efficient, and improves the efficiency of pectin removal. The cotton fabrics were washed several times¹⁷ and the reducing sugar was squeezed out to expose the unreacted pectin to the surface, which then reacted with the pectinase and reduced the feedback inhibition.
Figure 1.
Catalytic reaction rate of pectinase by different treatment methods at 50℃.

Table 2.
Catalytic reaction rate of pectinase by different treatment methods at 50℃

Methods Concentration (mg/ml)
Rate (mg/ml/min)
Total time (min)

First 5 min/ times Final First 5 min/ times Average

Repeatedly padding at 50℃ 1.61 3.60 0.32 0.24 15.00

Impregnation at 50℃ 1.40 3.45 0.28 0.0431 80.00

Catalytic reaction rate of pectinase at different temperatures

Figure 2 shows that the concentration of reducing sugar increased rapidly after 1–5 repeated padding events. With increasing number of padding events, the growth trend of the reducing sugar concentration slowed, but still increased rapidly. This is because more pectin was exposed on the surface or interior of cotton fabrics at the beginning of immersion, and it can react with the pectinase under the wetting effect and high pressure to produce the reduced sugar, so the rate of reaction was more rapid.¹⁶ However, with the padding number increasing, the pectin exposed on the surface of cotton fibers decreased, and the reaction rate also slowed.
Figure 2.
Catalytic reaction rate of pectinase at different temperatures.

The concentration of reducing sugar increased fastest in the reaction system when the reaction temperature was 50℃. This is because 50℃ is the temperature at which pectinase has the highest activity and so ensures the fastest rate of reaction.¹⁸ The fibers’ expansion and enlargement of the fibers’ pores cause enzyme molecules and pectin to react more easily.^19,20 When the temperature is low, the enzyme activity is lower and the fabric expands less, so it is more difficult for the enzyme solution to enter the fibers to make contact with pectin and react. When the temperature is high, enzyme activity decreased with increases in temperature, even though the fabric and pore expansion increases make it easier for enzyme molecules and pectin to make contact.^21,22

Catalytic reaction rate of pectinase with different soaking times

Figure 3 shows the catalytic reaction rate of pectinase with different soaking times. During the repeated padding process, the rate of reducing sugar production accelerates when soaking time is increased from 5 s to 25 s. The produced reducing sugar concentration increased from 1.71 mg/ml to 3.60 mg/ml in the same processing time. Enzymes reacted with pectin on the fabric in the process of impregnation of fabrics in pectinase solution, so the catalytic rate can be improved by prolonging the contact time between enzyme and substrate.²³
Figure 3.
Catalytic reaction rate of pectinase with different soaking times.

Catalytic reaction rate of pectinase at different pick-up

Figure 4 shows the catalytic reaction rate curve of pectinase at different pick-up. The catalytic efficiency of pectinase is different at different pick-up conditions. When pick-up increased from 50% to 90%, the produced reducing sugar concentration first increased from 1.99 mg/ml to 3.60 mg/ml, then decreased to 1.86 mg/ml. When the pick-up was 70%, the catalytic rate of pectinase reached the best value (0.24 mg/ml/min). The reason is that the reducing sugar decomposed by pectinase still attaches to the fabric or the upper layer of pectin, and further hinders pectinase from catalyzing the sublayer of pectin. The roll of the padder has a squeezing action to remove excess enzyme solution, reducing sugar, and impurities. The repeated padding method is beneficial for pectinase to re-contact and react with the substrate on the fabric, resulting in a high reaction rate. When the pick-up is low, the rolled fabric has less enzyme solution, and it is difficult for the pectinase to completely cover or contact the substrate on the fabric, so the catalytic rate is low. When the pick-up is higher, the rolled fabrics have more solution. But it is difficult to squeeze out the excess enzyme solution and the reducing sugar produced by the fabric, and this prevents the pectinase from re-contacting the pectin on the fabric, so the catalytic rate of enzyme is also low.
Figure 4.
Catalytic reaction rate of pectinase at different pick-up.

Table 3 compares capillary effect, whiteness, and bursting strength of cotton knitted fabrics that underwent the three different methods. The strength loss of the repeated padding method is much less than that of the alkali scouring method and the impregnation method. As for its wettability, there is little difference between the repeated padding and alkali scouring methods, but the wettability of products treated by the repeated padding method is slightly better than that treated by the impregnation method. In terms of whiteness it is poor – especially the removal effects of impurities such as cottonseed hulls is not obvious, and there is a gap with the traditional alkaline scouring method.^6,19,24 These differences may be due to changes in the surface morphology and structure of cotton fibers caused by different treatment methods and mechanisms. This can be seen in the following section.
Table 3.
Performance tests after bioscouring by different methods

Capillary effect (cm/30 min) Whiteness Bursting strength (N)

Alkali scouring method 13.25 81.02 397

Repeated padding method 13.20 79.92 457

Impregnation method 13.1 80.16 413

Scanning electron microscope analysis of fibers in different scouring methods

Scanning electron microscope (SEM) was used to observe the surface morphology of cotton fibers treated in different ways and conditions. As shown in Figure 5, raw cotton fibers were wasted circular, and their surface was smooth, with a detailed concave–convex ridge and a layer of impurity.
Figure 5.
Scanning electron micrograph of cotton fibers. (a) Raw cotton fabrics (×2000); (b) cotton fibers impregnated with pectinase (×2000); (c) cotton fibers after repeated padding with pectinase (×2000); (a4) cotton fibers after alkaline scouring (×2000).

After being impregnated with pectinase, a lot of irregular mottled protrusions appeared on the fiber surface, and some of the areas also contained granular materials. The detailed concave–convex ridges became blurred. This is because the pectinase has specificity and selectivity to pectin on the surface of cotton fibers. Apart from the pectin, wax, ash, and other impurities on the surface of the cotton fibers are closely intertwined and adhered to each other. Due to its poor accessibility, pectinase is unable to fully contact the pectin substances that are covered by some underlying substances and produced reducing sugar. In addition, part of the pectin substance still adhered to the surface of the fiber, and the connection structure between the wax and other impurities was not significantly damaged, thus showing layered and irregular granular impurities on the surface of the cotton fiber.

Compared with the impregnation method, the surface of the cotton fiber after repeated padding with pectinase is obviously different. Most of the impurities on the fiber have been removed and the surface of the fibers has become smooth.²⁵ The results show that the natural impurities in cotton fibers have been removed greatly by the repeated padding method.²⁶ The possible mechanism includes three elements. First, because of the specificity of the enzyme, the pectin substances will be effectively removed by pectinase in the available areas; second, the rolling force can push the pectinase solution into the cotton fabric, expand the accessible area and increase the catalytic decomposition efficiency^20,27,28; third, because cotton wax and ash are mainly adhered to the surface of cotton fibers through the adhesion of pectin substances, the pressure exerted by the repeated padding can ensure the decomposition products of the pectin on the surface of the cotton fabrics is removed promptly.¹⁶ This may be just like the portion of the impurity adhering to the pectin substance being repeatedly washed away from the knitted fabric by the roll extrusion fluid. Therefore, the surface of the cotton fiber is smooth, the irregular granular material is reduced, and the wettability of the cotton knitted fabric is improved. Consequently, the repeated padding with pectinase for cotton knitted fabrics is beneficial to effectively destroy the connective structure between impurities. Most of the impurities can be removed by the rolling action.

The fiber surface after alkali scouring is distinct from that of pectinase scouring. The morphology and structure of the fiber surface changed quite clearly after alkali scouring. The original parallel ridge peaks became rough and irregular concave–convex surfaces, and the primary cell walls were severely damaged. This may be attributed to reacting for a long time with alkali at high temperatures, which could destroy the primary cell wall layer of the cotton fiber surface. The rough, irregular concave–convex surface is formed by denudation of alkali to the primary cell wall layer. This denudation will not only destroy the primary cell wall of the fiber, but also affect the internal structure of the fiber and damage the strength of the fiber if it is not properly controlled.

FTIR-ATR spectra analysis of fibers by different scouring methods

Figure 6 is the FTIR-ATR spectra of cotton fibers treated with different methods. A mixed material absorption peak appears around 1634.9 cm^–1 in the FTIR-ATR spectra of raw cotton fibers. It is mainly caused by the presence of pectin in the form of COO^– and the carboxylic ester group of cotton wax in cotton fibers. The symmetric and asymmetric stretching vibrations of these COO^– will present two relatively obvious peaks near 1499.6 cm^–1 and 1634.9 cm^–1 in the spectra.²⁶
Figure 6.
The FTIR-ATR spectra of cotton fibers treated with different methods. (a) Raw cotton fibers; (b) cotton fibers treated with alkali; (c) cotton fibers treated with pectinase by the impregnation method; (d) cotton fibers treated with pectinase by the repeated padding method.

So it can be analysed the removal of the main impurities on cotton fibers by observing the absorption peak intensity at 1634.9 cm^-1. That is, the lower the peak intensity of the fiber spectrum is, the more the removal of pectin is, and vice versa.

From Figure 6, the differences of absorption peak intensities are not obvious among the three treatments. However, compared with the raw cotton, there was a significant difference in peak intensity. The same results can be observed near 1499.6 cm^–1. It was proved that this method can achieve the same effect as the traditional alkali and impregnation methods in the removal of pectin. The results of this experiment are basically consistent with those presented from the scanning electron micrograph. This shows that pectinase can be used to refine the cotton fabric, which has a limited ability to remove impurities from the fiber surface. In addition, spectral data show that pectinase can remove natural impurities such as pectin and cotton wax. However, there is still a gap in their removal capacity compared with alkaline scouring.

Conclusions

During the bioscouring process of cotton knitted fabrics, the average catalytic rate of the repeated padding method was sixfold faster than the impregnation method, and the process time decreased by 65 minutes compared with the impregnation method. Furthermore, the wettability of cotton knitted fabrics treated by the former method was better than that treated by the latter. In the repeated padding process, with the impregnating time increasing from 5 s to 25 s, the produced reducing sugar concentration increased from 1.71 mg/ml to 3.60 mg/ml in the same process time. When the pick-up was about 70% for the repeated padding method, the catalytic rate of the pectinase reached the best value (0.24 mg/ml/min).

FTIR-ATR spectra test and SEM analysis of cotton knitted fabrics treated with different methods show that in the repeated padding method, roller pressure can promote permeability and enlarge the area accessible to the pectinase. The removal efficiency of scouring is better than that of the impregnation method, and it can reduce the damage caused by the traditional alkali scouring of the fiber. It has the characteristics of high efficiency, mildness, and being environmentally friendly. The refined cotton product has a good feel and strength.

Material	Specification
Acid pectinase	Tianjin Aonuo Technology Development Co. Ltd., China, Tianjin
Pure cotton knitted fabrics weight (250 g/m²)	Guangzhou Golden Eagle Bio-chemical Co. Ltd., China, Guangzhou
JFC (RO(CH₂CH₂O)nH)	Tianjin Hao Yuan Fine Chemical Co. Ltd., China, Tianjin
α-D-Galacturonic acid monohydrate (C₆H₁₀O₇·H₂O, AR, purity > 97%)	Hefei Bomei Biological Technology Co. Ltd., China, Hefei
Pectin (AR, purity > 70% from citrus peel)	Hefei Bomei Biological Technology Co. Ltd., China, Hefei
Anhydrous sodium sulfate (Na₂SO₃, AR, purity 97%)	Tianjin North Day Chemical Reagent Factory, China, Tianjin
Phenol (C₆H₅OH, AR)	Tianjin Kermel Chemical Reagent Co. Ltd., China, Tianjin
Potassium sodium tartrate tetrahydrate (C₄O₆H₄KNa, AR)	Tianjin Kermel Chemical Reagent Co. Ltd., China, Tianjin
3,5-Dinitrosalicylic acid (C₇H₄N₂O₇, AR)	Tianjin Guangfu Fine Chemical Research Institute, China, Tianjin

Methods	Concentration (mg/ml)	Rate (mg/ml/min)	Total time (min)
Repeatedly padding at 50℃	1.61	3.60	0.32	0.24	15.00
Impregnation at 50℃	1.40	3.45	0.28	0.0431	80.00

	Capillary effect (cm/30 min)	Whiteness	Bursting strength (N)
Alkali scouring method	13.25	81.02	397
Repeated padding method	13.20	79.92	457
Impregnation method	13.1	80.16	413

Footnotes

Acknowledgements

The authors greatly acknowledge Tianjin Noao Science and Technology Development Co., Ltd. We would like to thank Mr. Liu Minyao (Noao) and Ms. Liu Jing (Noao) for providing materials for this study.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the project of the National Science and Technology Supporting Plan (2014BAE01B02) and the Project of Innovation Fund for Technology Based Firms of Tianjin (13ZXCXSY08100).

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Methods	Concentration (mg/ml)		Rate (mg/ml/min)		Total time (min)
Methods	First 5 min/ times	Final	First 5 min/ times	Average	Total time (min)
Repeatedly padding at 50℃	1.61	3.60	0.32	0.24	15.00
Impregnation at 50℃	1.40	3.45	0.28	0.0431	80.00