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Enzymes are bio-catalysts and are used in several industrial processes since nineteenth century. Its use in textile industry is an example of industrial or white revolution. Enzymes, due to their non-toxic and eco-friendly characteristics, have gained wide applications in textile industry. Not only they are highly specific, efficient and work under mild conditions but also they help reduce process times, save energy and water, improve quality of product and reduce pollution. As a result they are rapidly gaining global recognition as important requirement for textile industry.
Commercially enzymes can be obtained from three primary sources, animal tissues, plants and microbes. However, these naturally occurring enzymes are not produced in sufficient quantities to be readily used in industrial applications. Hence, microbial strains producing the desired enzyme are cultured and optimised i.e. fermentation, to obtain enzymes in sufficient quantities for their commercial use in textile industry. The enzymes used in the textile industry are amylases, cellulases, pectinases, lipases, catalases, proteases, xylanases etc. and are mainly used for processing of the textiles i.e. preparatory and finishing of the goods. Some of the applications include removing of starch, bleaching, degrading lignin, fading of denim and non-denim, removal of peroxidises, finishing of wool, decolouration of dyestuff, bio-scouring, bio-polishing, wool finishing, etc. (Shenai, 1990; Nalankilli, 1998; Barrett et al., 2003;Cavaco and Gubitz, 2003; Chelikani et al., 2004).
CLASSIFICATION AND PROPERTIES OF ENZYMES USED IN TEXTILE INDUSTRY
Enzymes are very specific toward catalysing the substrate. Hence these are classified on the basis of their catalytic functions in six broad categories:
EC1 Oxidoreductases: catalyze oxidation and or reduction reactions
EC2 Transferases: catalyze transfer of a functional group
EC3 Hydrolases: catalyze the hydrolysis of various bonds
EC4 Lyases: catalyse the cleavage of several bonds by means other than hydrolysis and oxidation
EC5 isomerases: catalyse isomerisation changes within a single molecule
EC6 Ligases: joining of two molecules with formation of covalent bonds
Properties of enzymes exploited for use in industrial application:
Acceleration of the reaction rates by lowering the activation energy of the reaction.
Operating optimally under milder conditions of temperature, pH and atmospheric pressure.
Alternative for polluting, toxic and carcinogenic chemicals
High specificity towards substrate makes easy to control their activity
Biodegradable and do not produce toxic wastes.
Due to their high efficiency, specificity, property of working under milder conditions and biodegradability enzymes are well suited for various industrial applications.
ROLE FO ENZYMES IN TEXTILE INDUSTRY
Use of enzymes in various textile processing processes has greatly benefited textile industry with respect to both environmental impact and quality of product. There are 7000 known enzymes, but only 75 are commercially used in textile industry (Quandt and Kuhl, 2001) and most of them belong to hydolases and oxidoreductases families. The hydrolases family includes amylases, cellulases, pectinases, proteases, catalases and lipases/estarases and oxidoreductase family include laccase, peroxidises.
Amylases actson starch molecules and hydrolyses to give dextrin and small polymers of glucose units (Windish and Mhatre, 1965). These are classified according to the sugars they produced i.e. α-amylases and β-amylases. α-Amylases being produced from filamentous fungi and bacteria are mostly used in industries (Pandey et al., 2000). This enzymes are stable over wide range of pH from 4-11 and optimal activity is related to the growth conditions of the source microorganisms (Vihinen and Mantsala, 1989). In general, α-Amylases show's high specificity towards starch followed by amylase, amylopectine, cyclodextrin, glycogen and maltotriose (Vihinen and Mantsala, 1989).
1.1 Textile Desizing
Size is an adhesive substance used to coat the wrapping threads used in weaving of the fabrics made from cotton or blend. Starch and its derivatives are broadly used to size fabrics due to their easy availability, relative low cost and excellent film forming capacity (Feitkenhauer et al., 2003). Amylases are used to remove this sizing material and prepare the fabric ready for dyeing and finishing (Cavaco-Paulo et al., 2008). Earlier to the discovery of amylases, desizing was done by chemical treatment of fabric with acid, alkali or oxidising agents at high temperature. But, this technique was inefficient in removing the starch which resulted in imperfections in dyeing and also degradation of cotton fibre. Amylases are commercially used for desizing fabric due to its efficiency and specificity and its effectiveness in completely removing the size without affecting the fabric (Cegarra, 1996; Etters and Annis, 1998). Starch is removed during washing in the form of water soluble dextrin and thus reduces the discharge of chemical waste into the environment.
Cellulases are the hydrolytic enzymes that catalyses the breakdown of cellulose to smaller oligosaccharides and finally to glucose. These enzymes are commonly produced by soil-dwelling fungi and bacteria such as Penicillium, Trichoderma and Fusarium (Verma et al., 2007) and shows optimal activity in temperature range from 30°C-60°C. The applicationof cellulases in the textile industry begin in late nineteenth century with denim finishing. It alone accounts for 14% of the world's industrial enzyme market (Nierstrasz and Warmoeskerken, 2003).
2.1 DENIM FINISHING
Denim is high grade cotton and its washing is done in order to give a worn look e.g. stonewashing of denim jeans, in which the denim fabric id faded using sodium hypochlorite or potassium permanganate are used as pumice stones (Pedersen and Schneider, 1998) it resulted in damage to the fabric and machine. Introduction of cellulases have increased the productivity without affecting garment or the machine. Cellulases hydrolyses the exposed surface of dyed (indigo) fabric leaving the interiors intact, partial hydrolyses of the surface results in removal of dye and leaves a light area. Most of the cellulases are produced from fungi, but cellulases from bacterial and actinomycetes origin are now studied with regard to its use in bio-stoning of denim. Cellulases used for washing of the denim can be further classified on the basis of optimal pH required for its maximum efficiency; as neutral cellulases operating at pH 6-8 and acidic cellulases acting at pH 4.5-6.
Washing of other natural (cotton) and man-made cellulosic fabrics, besides denim, such as linen, hemp, rayon and viscose by enzymatic activity of cellulases to improve final appearance is done by bio-finishing or bio-polishing processes (Videbaek and Andersen, 1993). The process helps in preventing the formation of ball of fuzz called pill on the surface of the garment, this formation usually results in unattractive, knotty fabric appearance. Cellulases, hydrolyses the microfibrils protruding from the surface of the fabric which tends to break off leaving a smoother surface.
Bio-finishing may be an optional step for upgrading cotton fibrics, but is very important step in prevention of pilling or fibrillation during finishing of lyocell fabrics (Cavaco-paulo et al., 2008). Similarly, Carrillo et al. (2003) stated that cellulases can be used for viscose type regenerated celloloses like viscose and modal. Yachmenev et al. (2002) showed the use of ultrasound as an efficient way to improve the enzymatic activity in bio-processing of cotton.
These are the enzymes which catalyse the degradation of hydrogen-peroxide (H2O2) to water (H2O) and oxygen (O2). They are synthesised by many bacterias and fungi and most shows optimal activity at moderate temperatures i.e. 20°C-25°C and neutral pH (Mueller et al., 1997). Catalases from animal source are cheap and are economical to produce with special properties such as thermo-stability and pH stability using recombinant strains (Cavaco-paulo et al., 2008).
3.1 REMOVAL OF BLEACHING
Bleaching of cotton with hydrogen peroxidase is a fundamental step involved in the processing of the fabric and is generally done before dyeing step. Hydrogen peroxide is an oxidising agent and presence of its residue on the fabric after bleaching leads to oxidation and or hydrolysis of the dye, which leads to poor uptake of colour by fabric (Tzanov et al., 2001). Washing of the fabric with water and chemical reducers results in decreasing the peroxide residue on the fabric, but large volumes of water is required to remove the bleaching agent which leads to increase in processing time, wastage of water and adds to the cost (Alexandra et al., 2002). Use of catalases to remove the residual bleaching agent not only reduces the cost, water usage and processing time but also it is a more efficient in removing peroxide residue (Schmidt 1995). Alexandra et al. (2002) showed the difference in colour yield of the fabric with and without the treatment of catalases, effectiveness of removing bleach from the fabric was similar to that obtained from use of large volumes of water and chemical redusers, indicating the importance of catalases in saving water, processing time and environment.
Laccases are extracellular and multicopper enzymes belonging to a large family of blue-multicopper oxidase, these catalyses the oxidation of phenols and other aromatic and non-aromatic compounds by using oxygen by a radical-catalysed reaction (Thurston 1994). They are present in plants, insects, bacteria, but are most predominant in fungi (Benfield et al., 1964; Claus 2004; Baldrian 2006). Laccase can react with a broad range of substrate and hence have low specificity towards reducing substrate.
Besides being used in bleaching cotton, laccases are widely used in decolorising textile effluents (Vinod 2001), modifying the surface of fabrics (Zille 2005) and is also used in production of dyes (Setti et al., 1999). Their ability to degrade dyes renders them for the use in dye waste water treatment (Abadulla et al., 2000; Hou et al., 2004).
4.1 COTTON BLEACHING
Bleaching of cotton is done to remove the decolourizing natural pigments such as flavonoids , giving cotton fibres its white colour (Ardon et al., 1996). Hydrogen peroxide is the most common industrial chemical agent used for bleaching of cotton and requires alkaline pH and high temperature (100C) for its action. But this condition of temperature and pH damages the fabric and large volumes of water are required to remove hydrogen peroxide residues. Laccases have shown to improve whiteness of cotton fabrics by oxidative removal of flavonoids and when used as a substitution or in combination with chemical bleach have resulted in less fabric damage and conservation of water (Tzanov et al., 2003a). Laccases isolated from Trametes hisuta have been shown to remove flavonoids morins, luteolin, rutin and quercetin and pretreatment of cotton fabric with these laccases increases the whitness (Pereira et al., 2005). Use of ultrasound have also shown an synergistic effect on the activity of laccases (Basto et al., 2007). Some of the industrial applications of laccases in denim finishing are DeniLite (Novozyme) and Zylite (Zytex). Use of Laccases for coating natural and synthetic fibre is under study.
Pectinases are the complex enzymes used in the degradation of pectic substances (present in the plant cell walls). They are naturally produced by saprophytes and plant pathogens (Bateman 1996; Lang and Dorenberg 2000). Pectinases accounts for 10% of the industrial enzymes (Semenova et al., 2006). Pectinases can be divided into two groups; acid pectinases and alkaline pectinases. Acid pectinases are used in degumming of jute, hemp, flax and cotton fibres and are also used in enzymatic scouring of cotton (Bruhlmann et al., 1994).
Natural untreated cotton contains many impurities such as pectins, mineral salts, hemi-celluloses, waxes present in cuticle and primary cell wall (Batra 1985; Etters et al., 1999). These impurities are responsible for hydrophobic nature of the fabric which interferes with the aqueous chemical processes of dyeing and finishing (Freytag and Dinze 1983). Therefore it is essential to remove these impurities to increase the efficiency of dye binding and this is achieved by pre-treatment of raw cotton with chemicals such as sodium hydroxide; called as scouring. Apart from removing the impurities, this chemical adversely affects the fabric by attacking the cellulose and reducing the strength and fabric weight.
The quality of fabric can be improved with the use of pectinases for pre-treatment of raw cotton to remove the impurities; called as Bio-scouring or enzymatic scouring. Pectinases makes the surface of raw cotton more hydrophilic by removing the impurities without causing much damage to the cellulose and thus helps maintain strength and fabric weight (Li and hardin 1998), it increases softness of the cotton as scouring takes place at neutral pH and also reduces the water consumption by 25%.
Proteases catalyse the hydrolysis of peptides, amides, esters, thio esters and thiono ester bonds. There are three different types of proteases; acidic proteases, alkaline preteases and neutral proteases. Alkaline proteases (serine proteases and cysteine proteases) are most widely used in the textile industries for ezzymatic treatment of wool and degumming of silk. The optimum temperature of alkaline proteases ranges from 50C-70C.
6.1 ENZYMATIC TREATMENT OF WOOL
Natural raw wool is hydrophobic due to presence of impurities such as fatty acids and wax and grease. Alkaline scouring method is generally adopted to remove these impurities; it is done using sodium carbonate, pre-treatment with potassium permanganate, sodium sulphite or hydrogen peroxide. Wool fabric, on wet treatment gets shrink and the harsh chemicals adds to further degradation. Many chemical methods have been developed to overcome these drawbacks such as Chlorine-Hercosett process (Heiz 1981), plasma treatment at low temperature (Kan et al., 1998). However, these methods were not widely commercialised because of cost, compatibility and capacity problems. Levene et al.(1996) reported improved anti-shrinkage properties, no impurities and increased dyeing affinity of wool fibres after pre-treatment with proteases. But, a report by Shen et al.(1999) stated due to small structure of enzyme it penetrates into the fibre and causes destruction of inner parts of the wool structure. To render use of enzyme in treatment of wool, Silva et al.(2004) stated a solution i.e. attaching polymers such as PEG to increase the size of enzyme, this can reduce the penetration of enzyme and reduces the weight and strength loss.
6.2 DEGUMMING OF SILK
Degumming is a process to remove a proteinaceous substance covering the fibre, and is usually performed by use of soap solution whic also affects fibrin structure. Its enzymatic removal is widely done using Papain, an cysteine protesase, that removes cocoons and degumming of silk by boiling.. Acidic protesases and neutral proteases can also be used but have shown to be less effective in removing sericin and improving surface properties of fibre such as shine, texture and handling, as compaired to alkaline proteases (Freddi et al., 2003; Arami et al., 2007).
CONCLUSION AND FUTURE PROSPECTS
These are few applications of enzyme technology in textile industry, however much such potential have to be explored. Enzymes are been used as substituent for the chemical process because they are eco-friendly and reduces the cost and process times. There are many successful practical applications of enzymes in textile industry such as amylases- desizing, cellulose and laccases- denim finishing etc. But many more processes need to be developed for commercialization of enzymes. High cost of enzymes is the main hindrance to its wide acceptance in the textile industry so; new methods or improvement in existing methods to increase the yield of enzyme will always be appreciated. It can be done by using new improved biotechnological techniques to develop new high efficient strains of micro-organisms to produce enzymes.