Banana pseudostem fiber: A critical review on fiber extraction, characterization, and surface modification

ABSTRACT Banana pseudostems are now recognized as a sustainable raw material with a wide range of applications. Plenty of research attention has been paid to banana pseudostem fiber extraction, characterization, modification, and utilization. Mechanical extraction takes precedence over manual extraction. Surface treatments are employed to modify the surface of a fiber to make it suitable for customized applications. Because of its high cellulose content, good to moderate strength, fineness, fiber length-to-breath ratio, and other properties, banana pseudostem fiber is currently used to make nano and microcrystalline cellulose, activated carbon, green composites, and technical textiles. This review article discusses the manual and mechanical extraction processes of banana fiber, as well as its quality evaluation, morphology, chemical composition, physico-mechanical properties, and various surface modification techniques.


Introduction
The modern scientific world is moving toward a sustainable research environments for the protection of the ecosystem. Different efforts are going on one of them is the utilization of natural fiber in different ways by extraction from the direct nature so that it could be biodegradable material for preservation of the environment (Khan et al. 2021aKumar et al. 2022). All natural fibers are lower in health risks, environmentally safe, recyclable, and nonabrasive for quick processing. Natural fibers have many advantages over synthetic fibers, including wide availability, significantly reduced density, low cost, outstanding thermal insulation, significant endurance, excellent electrical resistance, improved acoustic qualities, and high specific strength (Khan et al. 2021bPremalatha et al. 2021;Sumesh et al. 2022).
Banana has seven recorded wild species viz., Musa. lolodensis, M. maclayi, M. peekelii, M. jackeyii, M. textilis, and M. bukensis, while M. fehi is the cultivated species in many counties like Thailand, India, Indonesia, Malaysia, Philippines, Hawaii, and Pacific islands (Uma, Saraswathi, and Durai 2019). India is the largest producer of banana fruit. Approximately 31.5% of the total banana plants in

Manual and mechanical extraction
One of the critical challenges in banana fiber processing is its extraction. Since banana fiber is not a well-established fiber for apparel textiles, the advanced fiber extraction machineries are not developed yet. Various attempts (Basak et al. 2016;Jayaprabha, Brahmakumar, and Manilal 2011;Paramasivam et al. 2020) have been performed to develop different types of extractors; however, very few are commercially available. The primitive and simple method of extraction is manual extraction done by direct extraction which is carried out using sharp objects like a blade or broken crockery. It is common in villages and rural areas. This technique produces good quality fiber, without much debris on the fiber surface; however, it is laborious and production is very low (Figure 1a). Thus, it cannot be adopted for large-scale production (Gañán et al. 2004). In general, manual extraction produces superior quality of fiber than machine extraction, since mechanical extraction may cause severe damage to the fibers due to high-speed mechanical beating. The key elements contributing to the mechanical qualities of manually extracted fibers are high cellulose content and low microfibrillar angle, which are not disturbed during manual extraction (Jagadeesh et al. 2021).
Decorticator is a general-purpose machine used for the extraction of lengthy natural fibers. It has already been reported for the extraction of many bast and leaf fibers like ramie (Lü et al. 2013), PALF (Jose, Salim, and Ammayappan 2016), and banana (Basak et al. 2016). Though the principle of mechanism is the same for all decorticating machines, it may slightly differ in design according to the manufactures and the type of fiber to be extracted (Jayaprabha, Brahmakumar, and Manilal 2011;Paramasivam et al. 2020). For the fiber extraction, after harvesting the crop, the sheaths of the pseudostems are separated manually. The machine operator holds one end of the long sheaths of pseudostem and inserts it inside the machine. The machine consists of a high-speed rotating drum. Due to high-speed mechanical beating action, the debris associated with the pseudostem is removed and the fibers are getting separated (Figure 1b). The separated fibers are manually combed using a metallic comb to remove the remnants of the pseudostem remains, if any. Finally, the fibers are washed and dried. While inserting the long sheaths inside the decorticator, a major portion of the pseudostem is converted into long fiber ( Figure 2a). In addition to this, the decortication also produces short fiber (Figure 2b), pith, and sap. Although the quality of the fiber is marginally inferior to the manual method, it is well accepted in terms of its high production capacity. Since dried sheaths cannot be processed by a machine, decortication must be done as soon as the crop is harvested. One pseudostem sheet can be processed at a time because the majority of decorticators are made for batch processing. However, in some more modern versions, the decorticator uses a conveyor belt rather than hand feeding (Paramasivam et al. 2020). The majority of decorators are now portable. Table 1 displays a comparison of the quality of banana fiber obtained using various extraction techniques.

Retting
Retting is the simplest method of extraction of natural fiber. Yet, it cannot be adopted in all kinds of fibers. Water retting is the most common retting technique; However, dew retting, chemical retting, fungal retting, and bacterial retting are also common. Retting facilitates the extraction of fiber by removing the cementitious materials such as pectins matter, hemicellulose, wax, and some lignin (Chanakya and Sreesha 2012;Hazarika et al. 2017).
In direct water retting, after harvesting the fruit, the pseudostems are individualized and directly immersed in water. It will take a long time for biological degradation to occur and for the fibers to be extracted. This retted fibers again need further cleaning to remove the attached debris from the  pseudostem if present any. This process is time-consuming and not eco-friendly, as it will contaminate the water resources, thus it is not a recommended method for fiber extraction. The water retention requirement is highly dependent on the quality of the fiber provided by the decorticator. After decortications and subsequent combing, if the banana fiber consists of fleshy remnants of the pseudostem, the fiber may be subjected to water retting to enhance the fiber quality. Although water retting is a slow degradation process by microorganisms, extreme care should be taken up to avoid over retting of banana fiber, which may otherwise result in degradation of cellulose and ultimately reduce the strength and luster of the fiber. The retting time, temperature, pH of water, and micro-nutrients are having a substantial effect on the quality of banana fiber (Subagyo and Chafidz 2018).

Chemical degumming
While facing difficulties for getting individual fibers after decortication or retting, the fiber extraction may be performed using chemicals, and the processing is known as degumming. Since the binding materials (lignin, hemicellulose, fats, and waxes) are acidic, generally mild alkali like soap or sodium carbonate are used for degumming and the process is performed at boiling for 30-60 min (Pandey et al. 2021). If extensive removal is required, then sodium hydroxide can be used (Badrinath and Senthilvelan 2014). Chemicals like magnesium carbonate, calcium hydroxide, sodium bicarbonate were found to be effective in removing hemicellulose, lignin, and other impurities present on the fiber surface (Siva et al. 2019(Siva et al. , 2020(Siva et al. , 2020. The selection of degumming chemicals and treatment conditions are determined by the nature and amount of gum present on the fiber extent of the individualization of fiber entities. A combination of hydrogen peroxide and sodium hydroxide may provide better whiteness to the fiber in addition to fiber separation. A mild acidic treatment is preferred after alkali degumming to remove the excess alkali content present if any. This process may either be performed in the open vessel or in an autoclave. The high-temperature high-pressure process (HTHP) in autoclave results in the partial breakdown of the lignocellulose structure, hemicellulose fraction hydrolysis, lignin de-polymerization, and defibrillation and produces better quality fiber. Proper precautions are required to avoid degradation of fiber during chemical degumming.

Microbial retting
Microbial degradation is a well-established method used for the extraction of natural fibers. It may be either aerobic or anaerobic, however; the latter is much preferred. Selected bacteria and funguses (Pseudomonas, Azotobacter, Clostridium, Bacillus, Clostridiaceae, etc.) at favorable environment can attack the lignocellulosic biomass partially and they may be employed for the extraction of fiber. The mechanically extracted banana fibers were polished with A. niger using anaerobic digestion methods and the results were found encouraging (Sarma and Deka 2016). They also inferred that Aspergillus niger has effectively been used for the extraction of fiber from decorticated banana fiber. Similarly, Jayaprabha, Brahmakumar, and Manilal (2011) attempted anaerobic extraction of banana fiber. They observed that 6 days of anaerobic digestion may give the fibers having almost at par physicomechanical properties as compared to the fibers extracted by the sweaty manual method. The process is less laborious and could be adopted for bulk scale production. To overcome certain inevitable drawbacks such as breakage of some β-glucosidic linkages in cellulose and hemicelluloses during mechanical extraction and brown coloration of fiber during retting, alternative anaerobic digestion in a controlled reactor for the extraction of banana fiber is also reported (Chanakya and Sreesha 2012).

Enzymatic extraction
The action of enzymes is biochemical specific and so, a single enzyme may not be useful for fiber extraction. It is not possible to remove all the cementing materials like lignin, pectin, and hemicellulose using a single enzyme from the fiber. For this, a combination of enzymes is employed. Enzymes are not much effective for the direct extraction of fiber from raw sources. Instead, it is applied for the removal of post retting residual cementing materials after preliminary extraction of fiber. Commonly used enzymes for the extraction of fiber are cellulase, pectinase, hemicellulase, and lignin peroxidase for removal of cellulose, pectinase, hemicellulose, and lignin respectively. The scope of enzyme treatment for the fiber extraction from banana fiber as well as fiber surface modification has not yet been explored systematically. Jacob et al. (2008) prepared a pectinase enzyme applied upon banana pseudostem fiber. Pectinase helps to rupture the middle lamella resulting in fiber separation and increases the smoothness of the fiber due to the removal of debris on fiber surface.

Physico-mechanical properties of banana fiber
The physico-mechanical properties of the banana fiber greatly depend on the method of extraction. The literature (Table 1) gives information about the fiber properties. Length and fineness of the fiber depend preliminary on the variety of plant, sheath length, as well as the method of extraction. It is also observed that hand extraction produces fine fibers as compared to mechanical extraction (Vigneswaran et al. 2015). In many cases, after mechanical extraction, multiple entities are found to be adhered to themselves. The bundle may be opened by mechanical combing. The quality of the combed fiber is further enhanced through better fiber separation and removal of surface debris. Table 1 indicates that the length and diameter of the fiber differ considerably. The variation may be due to the differences in the length of the sheath taken by individual researchers as well as the method of extraction employed. The gauge length and the amount of defects in the fiber are the main determinants of the mechanical properties. Additionally, the strength of the fibers diminishes as the microfibrillar angle raises and the cellulose content falls, while it increases with the number of cells that supply strength (Kulkarni et al. 1983;Mukherjee and Satyanarayana 1984).
A comparative description of the physico-mechanical properties of banana fiber with other lignocellulosic fibers is shown in Table 2. It is depicted that the banana fibers resemble roselle fibers in many aspects like finesses, bundle strength, and moisture content. It is inferior to other popular fibers like flax and sisal. The strength of banana fiber is lower in comparison with jute. Fineness of banana fiber ranges between 3 and 7 tex. The difference may be due to the extent of defibrillation depending on different degumming methods adopted. It can be concluded from the physicomechanical properties that banana fiber can be a potential alternative for the above-said fibers for many applications. Contrary to coir fibers, the structure of banana fibers exhibits an increase in cell number in fibers up to a diameter of 100/m before remaining constant. As a result, so does the strength. There may not be any discernible variations in the mechanical properties with fiber diameter as a result of the lack of considerable variation in the microfibrillar angle and the fraction of cells that renders strength in banana fibers (Geethamma, Joseph, and Thomas 1995;Kulkarni et al. 1983). Table 3 shows the chemical composition analysis of banana fiber reported by different researchers. The major components of the fiber are cellulose (55-65%), hemicellulose (15-25%), lignin (10-15%), and pectin (3-5%). Cellulose being the main constituent of banana fiber provides structural integrity to the fiber. Lignin also plays a crucial role in the plant's mechanical strength (Jústiz-Smith, Virgo, and Buchanan 2008). The yellowish color of the banana fiber is chiefly due to its lignin content. The chemical composition of banana fiber can differ depending on its species, geographical locations, agro-climatic conditions, soil nutrients, plant age, and conditions of extraction. Other bast fibers such as jute, flax and roselle have higher cellulose and hemicellulose content compared to banana. It may be noted that the lignin content of banana fiber is considerably lower than that of jute and sisal. The presence of trace elements such as sodium, calcium, magnesium, aluminum, and silicon in the fiber may increase the brittleness of the fiber (Jústiz-Smith, Virgo, and Buchanan 2008).  (2021)

Microscopic analysis
The longitudinal and cross-section morphology of the banana fiber has been extensively studied using various microscopic methods. The topography of the fiber greatly depends on the method of extraction. Generally, the surface of the raw fiber seems to be slightly rough ( Figure 3a) and may contain certain amounts of debris after manual or mechanical extraction (Jannah et al. 2009). Among various extraction methods, the chemical degumming produces fibers with a smooth surface (Figure 3b) (Kalita et al. 2018). The alkali removes the pectins, hemicelluloses, and a certain amount of lignin from the fiber and ultimately results in a smooth surface. In the cross-section, the banana fiber seems to be irregular and non-spherical in shape, whereas the cell wall is thick and the central lumen is elliptical, narrow, and elongated (Guimarães et al. 2009). The fiber is multiluminar, as revealed from a high number of collapsed dry lumens. Neelamana, Thomas, and Parameswaranpillai (2013) synthesized macro, micro, and nano fibers through steam explosion. The AFM topography (Figure 4) shows the macro, micro, and nano fibers. Multifibrillar structure is more apparent from AFM analysis. The average nano fiber diameter was estimated at about 30 nm. The middle lamella and primary cell walls are removed more efficiently to form micro and nano fibers. As a result, a more cellulose-rich surface and an effective reduction of the fiber dimension to the nano range have been achieved. Chokshi et al. (2022) reported that the primary factor influencing the inherent fiber strength characteristics is the microfibrillar angle. A higher cellulosic percentage and a lower MFA are reportedly necessary for high fiber strength. The lower microfibrillar angle (11°) and comparatively high percentage of crystallinity [08Muk] in banana fibers should be the primary causes of their low elongation (Gassan, Chate, and Bledzki 2001).

X-ray diffractometry
Being lignocellulosic in nature, banana fiber consists of both amorphous and ordered (crystalline) regions. The XRD spectra of raw banana fiber is presented in Figure 5. The spectra showed a sharp  peak at 22.0°Corresponding to the ordered/crystalline region. Though there is a minor difference in the crystallinity of the banana fiber as observed in the reported studies, it is widely accepted as 62-65% crystalline and 35-40% amorphous (Guimarães et al. 2009;Madhushani et al. 2021) which is close to jute (58:42) (Ghosh, Samanta, and Basu 2004), but lower than ramie (63:37) (Song et al. 2019), and cotton (71:29) (Liu et al. 2012). Kiruthika and Veluraja (2009) observed from parallel fiber x-ray diffractometry that crystalline orientation and crystallite size differs within varieties. It indicates that identification of variety(ies) is essentially needed for selective uses of fiber. Chemical treatments like alkaline degumming and bleaching may enhance the crystallinity of the fiber by the removal of noncrystalline components like pectins, hemicelluloses, and lignin (Basu et al. 2015;Taer et al. 2021). It is also reported that the nature of chemicals for the surface treatment has a significant role in determining the crystallinity of the banana fiber (Bar et al. 2021).

Fourier transform infrared spectroscopy
The functional groups present in the banana fiber may be interpreted using FTIR analysis. Figure 6 shows the FTIR analysis of banana fiber. The spectra showed a wide band near the 3335 cm −1 region due to the presence of the -OH group of absorbed water (Pandey, Jose, and Sinha 2020;Rodríguez, Álvarez-láinez, and Orrego 2022). A small band at 2900 cm −1 shows the presence of C-H stretching in alkanes (Pandey et al. 2021). The conjugated >CO stretching of ester and aldehyde groups in hemicelluloses and lignin is observed around 1740 cm −1 (Basu et al. 2015). A slightly wider band present near 1650 cm −1 is the indication of C = O stretching vibration in conjugated carbonyl of lignin . A small bust sharp band at 1314 cm −1 indicates C=C aromatic symmetrical stretching of lignin. The band at 832 cm −1 is attributed to the C -H out-of-plane deformation of lignin in banana fiber (Kalita et al. 2019). The peak at 1160 cm −1 attributes to the C-O-C asymmetric stretching in cellulose I and cellulose II . The strong and sharp band present at 1050 cm −1 is the indication of C-O stretching of hemicelluloses and lignin . A broad region of overlapping bands of absorption (2000-1400 cm −1 ) due to aromatic linkages of C -C, C=C, OH, CO, C -O-C, C -H. (Bar et al. 2021;Rahman et al. 2022).

Fiber modification techniques
Most natural fibers are inherently hydrophilic in nature due to the existence of the -OH group. The hydrophilic nature of these fibers enables better moisture management properties. However, in certain applications, the connatural hydrophilic nature of the natural fiber is not desired and some hydrophobic properties are required. In this context, meticulous research attempts have been made to impart hydrophobicity to the fibers. Physical methods viz., plasma treatment, gamma, and UV radiation, and chemical methods viz., grafting, enzyme treatment, polymer coating are commonly employed for surface modification. The studies on the physical and chemical fiber modification of banana fiber are described below.

Plasma treatment
When a suitable gas like argon, nitrogen, or oxygen is exposed to the electromagnetic field, plasma is generated. The surface modification largely depends on the nature of the gas used for plasma treatment and it may induce hydrophilicity or hydrophobicity on the fiber surface. This process opens up a scope of eco-friendly surface modification of natural fibers with a short duration of time. Depending on the type and nature of the gases utilized, plasma treatment may be used to create a wide range of surface modification by successively introducing surface crosslinking (Gupta et al. 2021(Gupta et al. , 2022. Whilst this process is effective, however, the plasma-treated fiber gradually loses its acquired surface characteristics after keeping for a prolonged time. Dielectric Barrier Discharge (DBD) assisted plasma treatment was used for the modification of banana fiber. The treatment improved the elastic modulus, tensile strength, surface roughness, and hydrophilicity of the fiber (Oliveira et al. 2012). Hrabě, Müller, and Mizera (2016) modified the fiber with surface plasma treatment. The samples were evaluated immediately for tensile strength soon after treatment and found to be greater as compared to the untreated ones. Vajpayee et al. (2020) modified the surface of the banana fabric with DBD air plasma after coating it with natural leaf extracts. The findings inferred that the treatment improved the hydrophilicity of the fiber due to the enhancement of the polar functional groups. The treated fabric exhibits great tolerance to gram + ve and -ve bacteria.

UV-radiation treatment
Radiation treatments are other important tools for surface modification of natural fibers. The UV rays are having low energy to that of gamma and X-rays. Thus, they cannot diffuse inside the fiber and the interaction remains on the surface only. Very little information is available on the radiation treatments on banana fiber. Zaman, Khan, and Khan (2011) reported the chemical modification of banana fiber under UV radiation for the development of banana/LDPE composites. The results showed improved mechanical properties for modified fiber composites compared to untreated composites. In another attempt, the same group studied the effect of UV radiation on Polypropylene (PP) composite banana fibers. They reported improved mechanical properties of UV irradiated banana fiber and PP composites compared to untreated counterparts. It is also reported that UV radiation treatment improves the tensile strength and elastic modulus of banana fibers (Benedetto, Gelfuso, and Thomazini 2015)

Chemical modification of banana fiber
Chemical modification is a well-established and simple method of modification since it does not need many sophisticated instruments and therefore cost-effective. In most of the chemical modifications of banana fiber, the -OH functional groups of the cellulose are coupled with external chemicals, thus blocks the active reaction site. The common chemical methods of treatments are shown below (Figure 7).

Alkali treatment
Alkali treatment is the most common and wildly adopted chemical treatment. Alkali treatment may be performed either to remove the fiber impurities to make the fiber clean and smooth or to augment the functional performance. Generally, in the first case, mild alkali like soap or sodium carbonate is used, whereas, in the second case, caustic soda is used. Since, non-cellulosic materials viz., hemicelluloses, lignin (partially), pectins are soluble in hot alkaline condition, the method enhances the crystallinity of the fiber. The NaOH treatment at high temperature enhances the whiteness of the fiber by the removal of lignin, and also increases the dye uptake due to the formation of soda cellulose. Benítez et al. (2013) reported that treatment of banana fiber with hot alkali removes the surface impurities and enhances the mechanical strength of the fiber. The alkali treatment also enhances the thermal stability, thermal conductivity, and diffusivity of the fiber (Paul et al. 2008;Twebaze et al. 2022).

Silane treatment
Silane coupling is generally performed to replace the -OH group of natural fiber with the organosiloxy group, as a result, the fiber becomes hydrophobic. The most common silane coupling agents are tetraethyl orthosilicate (TEOS), 3-amino propyltriethoxysilane, and vinyl triethoxysilane. In general, the coupling agents hydrolyze to silanol and form an effective ester bond/linkage with carboxylic groups of fiber. Apart from this, there might be chances of the formation of other bonds like hydrogen or ester linkage, which highly dependent upon the availability of functional groups such as -NH 2 or -OH. The silane treatment increases the surface energy and lowers the surface wettability. Silane modifications are mainly performed on banana fibers before the preparation of composites. It is reported that the treatment of banana fiber with 3-amino propyltriethoxysilan and vinyl triethoxysilane enhances the hydrophobicity of fiber (Jandas et al. 2011;Kannan and Thangaraju 2022;Pothan, George, and Thomas 2002).

Acetylation
An alternative method of imparting hydrophobicity to natural fiber is grafting with acetic anhydride and the process is known as acetylation. In this process, the -OH group of the fiber is replaced with the acetyl group. This process improves the hydrophobicity of banana fiber. Although the acetylation of some natural fibers has been reported, attempts on the banana fiber are scanty. Teli and Valia (2013) used acetic anhydride and N-Bromosuccinimide (NBS) for solvent-free acetylation on banana fiber to improve its oil absorbency. Acetylation on banana fiber contributed to a notable increase in acetyl groups. Therefore, the modified fiber was found to have better hydrophobicity and thermal stability.
The acetylated banana fiber may have better adhesion with synthetic resins for composite preparation and it can also find application in oil spilled water bodies.

Enzyme treatment
Enzymes are biological molecules and are having specific activity. In the textile industry enzymes are employed for various stages of wet processing viz., desizing, scouring, bio finishing, etc. Pectinase, cellulase, laccase, transglutaminase, protease, xylanase, lipase, amylase, and peroxidase are the most commonly used enzymes for textiles. The main objective of enzyme treatment on natural fiber is to remove the surface impurities like fat and waxes, and in certain cases, the enzymes can be used to target lignin and cellulose. The process is eco-friendly and it ultimately results in the smooth fiber surface and enhancement of dye uptake. In a reported study, biopectinase has been employed for the surface modification of banana fiber. The prolonged duration of enzyme treatment more than 6 h was not found effective. With the removal of pectins, and hemicelluloses, the biopectinase treatment enhanced the surface smoothness and thermal stability of banana fiber; however, a marginal decrease in the mechanical properties was observed (Ortega et al. 2016). The treatment of banana fiber with laccase resulted in the removal of lignin, while xylanase was found to be effective in the removal of hemicellulose (Vishnu Vardhini and Murugan 2017). The higher concentrations of enzyme treatment may adversely affect the mechanical strength of the fabric.

Conclusion
This review article discussed the significance of banana pseudostem fiber, extraction procedures, characteristics, and various surface modification methods. Banana fiber is primarily composed of cellulose, hemicellulose, lignin, and pectin, with cellulose serving as the primary component. The banana fiber exhibits structural and physico-mechanical characteristics that are very similar to jute and roselle. The mechanical extraction through decortications method is preferred for fiber extraction than manual methods. Retting/degumming is an optional operation to improve the quality of the fiber. The fibers are often modified with chemicals or enzymes as per the requirement such as for the preparation of composites or getting better process ability. The surface treatments remove the noncellulosic materials from the fiber as a result the fiber achieved improved properties.

Highlights
• Various methods of fiber extraction from banana pseudostem are reviewed.
• Various surface modification techniques of banana pseudostem fiber are discussed.
• Microscopic, spectroscopic, and thermal characterization of the fiber are described.
• The chemical composition and physico-mechanical properties of the fiber are enumerated.

Disclosure statement
No potential conflict of interest was reported by the author(s).