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ABSTRACT

 

Flax fibre is given importance due to its premium quality in world textile market. Retting technique has a great significance towards fibre quality parameters and inappropriate practice may spoil the whole produce. Despite the fact that water retting produces fibre of the superior quality, it pollutes the environment. Similar to dew retting, which is widespread in European nations, there are significant bottlenecks, particularly with relation to fibre quality. Therefore, the issues associated with traditional retting techniques compel to develop some alternative approaches that will meet fibre quality standards at an economic and environmentally sustainable manner. Several researchers have developed various chemicals (EDTA, sodium tripolyphosphate etc.) and enzymes (pectinase rich mixtures) for retting of fibre flax that are more environmentally friendly and provide high-quality, uniform fibre, although they still need to be refined before being used commercially.   

 

Cite this article:

Pyne S, Roy A, Das A, Kumar S (2022) Alternative approaches for retting of fibre flax. Food and Scientific Reports, 3(8):27-30. 

1.    Introduction

                Flax (Linum usitatissimum L. ssp. usitatissimum) as a fibre crop (Fig. 1) holding an important share in global textile market, composites and paper/ pulp industries. Flax fibre (Fig. 2) is one of the natural fibres that has been extracted successfully from ancient times. It was used in royal attires and therefore regarded as `King`s fibre` in several ancient literatures. Linen fabric has great demand during summer due to its outstanding heat conduction properties and favourable water absorption capacity which gives cooling effect along with its smooth texture (Dey et al., 2022). From recent decades, the demand for linen is increasing that leads to expansion of industrial linen market and economic growth. Globally, linen market had a share of 719.3 billion USD in 2020 which is expected to become 1360 billion USD by the end of 2027 (Anonymous, 2021).           

                On dry weight basis, about 30% of the flax straw biomass is fibre (Easson and Molloy, 1996). Harvested flax straw is allowed for partial degradation i.e. retting that allows removal of binding substances leads to ease in fibre extraction. The fibre yielded after extraction process, can be classified into two broad categories i.e. long fibres, utilized for manufacturing high value linen apparel and short or toe fibres, used for low valued products. In well managed crop, around half to two third of total fibre can be yielded as long fibre. Besides production, quality of fibre plays a crucial role in global market. At present, efficient harvesting methods, proper retting techniques and lack of standards to judge fibre quality parameters are major issues towards successful flax industry. Retting is one of the important post harvest factor deciding the fibre recovery and the type as well as duration of retting also influences fibre quality parameters (strength, fineness etc.) (Dey et al., 2021). Inappropriate retting can result into total wastage of a good harvest. The main objective of this article is to focus on different conventional techniques of flax retting, their pros and cons and also focus on short duration, environmentally sound alternative retting methods that can yield fibre with desirable quality.

2.    Flax retting techniques

                Retting can be defined as partial decomposition of plant stem that loosen fibre from the stalk through dissolution and decomposition of pectins, gums and other mucilaginous substances that leads to ease in mechanical fibre extraction. This process removes the gluey part mainly pectin which binds the fibre bundles to the inner core portion of the stem by the action of enzymes either from microbial source or introduced artificially. The middle lamella of flax is primarily composed of insoluble protopectin that is a complex of polygalacturonic acid chains binds with calcium ions. This polygalacturonide chain is in branched form linked by ester bond with galactose, rhamnose, arabinose and xylose. Breakdown of pectin into soluble galacturonic acid during retting is performed by pectin esterase, pectin lyase and polygalacturonase (Easson and Molloy, 1996). Yield and quality of fibre is highly dependent on type and degree of retting process for example long fibre recovery percentage, freedom from shives during extraction, fibre fineness and fibre colour (Dey et al., 2021). Flax retting can be performed by either (i) biological/ natural means (called traditional retting method) where bacteria (water retting) or fungi (dew retting) acted upon plant stem or (ii) by using chemicals or enzymes has been discussed below.

a. Traditional retting techniques

1. Water retting

                This method is practiced by making bundles of flax stalks after harvest followed by keeping submerged beneath the water surface with the help of load. Stones and dried wooden logs are generally used in the form of load but fresh wooden log should be avoided as the tanin content can cause discolouration of flax fibre. The mechanism of water retting starts with colonisation of anaerobic bacteria (Clostridium felsinium) around flax stem, later fermentation and degradation of pectin and other matrix substances ultimately loosen the bast fibre (Van Sumere, 1992 and Akin 2010). During the entire period of retting, water level should be maintained above the stalk bundles. Water retting is influenced by environmental conditions mainly water quality and temperature (optimum 35 ºC). After completion of retting, the stalk bundles are taken out from water and allows for sun drying before fibre extraction. Duration of retting is very important (4-6 days in sub tropical India). Over retting results into weak fibre. Although the fibre quality is superior and more uniform, but recently water retting is discouraged as this process is not environment friendly and also requires more labour and time as compared to dew retting. A huge volume of clean water is necessary for retting purpose. Meanwhile, water crisis is a burning issue worldwide. Per capita water availability is 1486 m³ per year and it has been estimated that the amount will be reduced to 1228 m³ per year by 2050 (NITI Aayog, Government of India, 2021). Moreover, disposal of untreated retting water containing butyric acid and other fermented product with high BOD value can cause pollution in lakes, streams and ground water and finally threatening towards biosphere. Another limitation is high labour cost involved in drying the retted stems. Therefore, under this circumstance, considering all the issues water retting is not at all sustainable solution and should be replaced with alternative approaches.  

2. Dew retting

                This method is generally followed in European countries. Dew retting is practiced by spreading the harvested flax straw in uniform, thin and non overlapping swaths and allowed for partial degradation. When the straw moisture content comes below 25% microorganisms especially indigenous soil fungi colonize around the stem biomass and degrade pectin substances (Kozlowski, 1992 and Easson and Molloy, 1996). Primarily, saprophytic soil fungi group including species of Fusarium, Aspergillus, Cladosporium, Trichoderma and Rhizopus build up major component of the consortia. But if the retting process is allowed for longer period then secondary colonisation by cellulose decomposing species notably Epicoccus nigrum takes place that will reduce the fibre strength (Akin, 2010). Retting completes within 3-8 weeks depending on the fungi species involved and environmental condition. Occasional light rainfall enhance the retting process and prolong dry period increases the duration. Regular turning of the flax straw in swath is necessary for uniform retting and better quality products. Although, dew retting conserves water but it suffers from various disadvantages. Most important is the quality of fibre which is low and inconsistent than obtained by water retting. Direct contact with soil and fungal growth makes the fibre darker in colour and dirty. Dew retting is very much restricted to geographical regions with suitable moisture and temperature regime required for effective fungal growth. Excessive rainfall as well as dry weather discourages effective retting process. This method also occupies agricultural fields for several weeks. Apart from a number of drawbacks under dew retting, less labour cost and higher fibre recovery makes it more attractive and sustainable to growers.

b. Alternative retting techniques

1. Stand retting

                This is another method of field retting with indigenous fungi. During wet climatic condition, the standing flax crop can be desiccated before harvest by spraying glyphosate which facilitate following retting process (Easson and Long, 1992). It should be applied at about 3 weeks after the midpoint of flowering. This process reduces stem moisture content below 30%. Stand retted flax fibre pre-treated with glyphosate has more fibre strength than dew retted flax but takes longer time in fungal colonisation and retting process. Also time of glyphosate application is very critical. Early application significantly reduces fibre yield whereas later application results in poor effectiveness in glyphosate activity. Therefore, unless the spraying operation is completed between 14 and 21 days after the midpoint of flowering, desiccation of flax stem will be uneven with lower parts of some stems may fail to desiccate. Moreover, this practice is not suitable for lodged plants where the glyphosate spray could not reach all the plants. 

2. Chemical retting

                Research on chemical retting of flax has been undertaken using a variety of chemicals including ethylenediaminetetraacetic acid (EDTA) or other chelating agent at higher pH, strong alkali and detergents. Chelating agent helps to remove Ca²⁺ during retting process. The mechanism is that Ca²⁺ level is high in flax stem where Ca²⁺ binds to carboxylic acid groups of pectin molecules that stabilize pectin molecules and ultimately maintains integrity of plant cell wall and therefore chelating Ca²⁺ with chemicals leads to separate the fibre from stalk. Akin and Rigsby (2002) found that EDTA (8 mM) and sodium tripolyphosphate (50 mM), both with 25 mM NaOH had resulted satisfactory retting of flax stems. Whereas, sodium gluconate, trisodium phosphate and sulfuric acid were ineffective as retting agents.  

3. Enzyme retting

                This method mainly employs the pectic enzymes/ pectinase, which catalyze the degradation of pectic substances, either by depolymerization (hydrolases and lyases) or deesterification (esterases) reactions, applied externally to loosen fibre from non fibre portion. An improved retting method was developed by Kimmel et al. (2001) known as spray enzyme retting (SER) for flax using pectinase rich mixtures. They developed a formulation containing chelators along with enzymes in water at pH 5.0 and sprayed on crimped stems followed by incubating at high humidity for several hours. Chelators increase the effectiveness of pectinase enzyme during retting process. Among the various chelators tested, EDTA performed better in sequestering Ca²⁺ ion. Successful enzyme retting can provide high and consistent quality fibre and also pave the scope to broaden geographic regions of flax production. However, the major issue associated with enzyme retting is maintenance of specific temperature and pH conditions and availability of skilled manpower specially at commercial scale. This might be the possible reason for its confinement at laboratory scale only.

 

3.    Conclusion

Discussion regarding different retting methods of fibre flax reveals that improvement is necessary in traditional retting techniques mainly due to environmental issues (water retting) and lower fibre quality (dew retting). Numbers of experiments were conducted in the last decade on this concerned field and developed some alternate approaches by using chemicals and enzymes but recent study is lacking. Successful enzyme retting can meet the superior with consistent fibre quality. Prior to being used commercially, alternative retting techniques must still be improved in order to increase fibre recovery, meet quality standards, and be both economically and environmentally sustainable.   

 

References

IAkin, D.E and Rigsby, L.L. (2002). Chemical retting of flax straw under alkaline conditions. Textile Research Journal. 72(9): 789-794. 

Akin, D.E. (2010). Flax- structure, chemistry, retting and processing. Industrial Applications of Natural Fibres.

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Easson, D.L. and Molloy, R. (1996). Retting- a key process in the production of high value fibre from flax. Outlook on Agriculture. 25(4): 235-242.  

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