Abstract
Agricultural residues represent a disposal problem and a biomass source for chemical production. Lignocellulosic composition varies in plants as a function of several factors such as physiological age and tissue function. Banana pseudostem is a large biomass resource that is usually wasted, in spite of the possibility that it can be used as a source of organic compounds such as cellulose and hemicelluloses. The aim of this paper is compare the lignocellulosic content and physicochemical properties of different sheaths of Pacovan banana pseudostems. The trunk was divided into four different fractions, from the outermost sheaths to the core of the structure. There was a significant difference between the lignocellulosic compositions of the fractions. The X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, and differential scanning calorimetry measurements reflected this difference in the sheaths. These results indicate that the Pacovan banana pseudostem cannot be considered to be a uniform biomass, and future approaches to its use as a biorefinery feedstock must consider a preliminary separation of the sheaths prior to chemical extraction of organic components.
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Banana (Musa sp. cv. Pacovan) Pseudostem Fibers are Composed of Varying Lignocellulosic Composition throughout the Diameter
André Luís S. Pereira,a Diego M. do Nascimento,b Men de Sá M. Souza,c Ana Ribeiro Cassales,c João Paulo Saraiva Morais,d Regina C. M. de Paula,b Morsyleide de F. Rosa,c,* and Judith P. A. Feitosa b
Agricultural residues represent a disposal problem and a biomass source for chemical production. Lignocellulosic composition varies in plants as a function of several factors such as physiological age and tissue function. Banana pseudostem is a large biomass resource that is usually wasted, in spite of the possibility that it can be used as a source of organic compounds such as cellulose and hemicelluloses. The aim of this paper is compare the lignocellulosic content and physicochemical properties of different sheaths of Pacovan banana pseudostems. The trunk was divided into four different fractions, from the outermost sheaths to the core of the structure. There was a significant difference between the lignocellulosic compositions of the fractions. The X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetric analysis, and differential scanning calorimetry measurements reflected this difference in the sheaths. These results indicate that the Pacovan banana pseudostem cannot be considered to be a uniform biomass, and future approaches to its use as a biorefinery feedstock must consider a preliminary separation of the sheaths prior to chemical extraction of organic components.
Keywords: Agribusiness; Biorefinery; Fractionation; Agriculture wastes; Byproducts; Pulping; Cellulose
Contact information: a: Engenharia e Ciência dos Materiais, Centro de Tecnologia, Campus do Pici, Universidade Federal do Ceará (UFC), Bloco 714, CEP 60455-900, Fortaleza, Ceará, Brazil; b: Universidade Federal do Ceará (UFC), Centro de Ciências, Campus do Pici, Departamento de Química Orgânica e Inorgânica, Bloco 940, CEP 60455-760, Fortaleza, Ceará, Brazil; c: Embrapa Agroindústria Tropical – CNPAT, Rua Dra Sara Mesquita 2270, Planalto do Pici, CEP 60511-110, Fortaleza, Ceará, Brazil; d: Embrapa Algodão – CNPA, Rua Oswaldo Cruz 1143, Centenário, CEP 58428-095, Campina Grande, Paraíba, Brazil; *Corresponding author: morsyleide.rosa@embrapa.br
INTRODUCTION
Agricultural residues represent a disposal problem because they can lead to environmental pollution and biomass spoilage. Some of these residues are composed of plant waste fibers, an abundant, low cost, and readily available source of lignocellulosic biomass (Oliveira et al. 2007). Many agricultural crops are cultivated in Brazil because of the large area of farmland, fertile soil, and adequate climate conditions. Cotton, sugarcane bagasse, coconut, jute, pineapple, ramie, sisal, and banana are conventional fiber resources in Brazil (Satyanarayana et al. 2007).
Brazil produced ~7 million tons of bananas in 2011. Together with India, China, the Philippines, and Ecuador, it was one of the five largest producers of banana in 2011 (FAOStat 2014). After harvesting the fruit in bunches, around 6.6 million tons of banana biomass are produced (Satyanarayana et al.2007). This biomass is usually left on the plantation soil to avoid erosion, control the growth of weeds, and to act as an organic fertilizer.
The varieties Prata, Prata Anã, and Pacovan are responsible for approximately 60% of the harvested area of banana plants in Brazil (Embrapa 2014). The banana plant of the cultivar Pacovan, Musa AAB, subgroup Prata, is mostly cultivated in the Brazilian Northeast. It is also grown in India, Australia (Queensland), and the Western Pacific islands, where it is known respectively as Pachanadan or Pacha Naadan, Improved Lady’s Finger, and Lady’s finger (Ploetz et al. 2007).
The banana plant has a very juicy aerial stem, properly denoted as the pseudostem, which is a clustered, cylindrical aggregation comprised of leaf stalk bases of leaf-petiole sheaths (Mukhopadhyay et al. 2009). There is also a subterranean stem, known as the corm, as well as the part that supports the banana fruit, named the peduncle, stalk, or rachis. The leaves and stalk of the pseudostem are disposed of after the fruit has been harvested. Collectively, the waste of banana production is an estimated ~220 tonnes of byproduct per hectare annually (Padam et al. 2012).
The biorefinery concept involves the integral and diversified use of a fractionated lignocellulosic biomass to obtain a wide variety of chemicals, added value products, and bulk chemicals (Arshanitsa et al. 2013; Shi et al. 2013; Fiorentino et al. 2014; Prinsen et al. 2014; Rincón et al. 2014; Zamudio et al. 2014). Many authors who have studied the utilization of banana pseudostem as a bioresource, even using a biorefinery, usually consider the whole trunk as a uniform material (Li et al. 2010; Akpabio et al. 2012; Ho et al. 2013; Gabhane et al. 2014). However, the pseudostem can be separated into several sheaths. The physicochemical properties of each sheath may be different, because they have different physiological ages and functions. So, it could be possible to fractionate the same pseudostem in several sheaths, as a simple physical pretreatment, and each group of sheaths can be used for the most suitable application in accordance to its properties instead of using a mixture of all the fractions and performing chemical treatments to separate each organic fraction.
The objective of this paper is to determine the chemical composition and physicochemical properties of four different morphological regions of Musa sp. cv. Pacovan: core, inner, middle, and outer sheaths.
EXPERIMENTAL
Materials
The banana pseudostem (BPS) samples of Musa sp., cultivar Pacovan, were harvested in January 2011 from a banana plantation at the Experimental Field of the Embrapa Tropical Agroindustry, located in the municipal district of Paraipaba, Ceara State, Brazil. The samples were obtained from mature plants, randomly selected, after cutting the banana bunches. The material was stored at 10 ºC before the separation procedure and analysis.
All reagents were of analytical grade: NaOH 97% (w/w), H2O2 30% (w/w), CH3COOH 99.7% (w/w), and H2SO4 98% (w/w) were supplied by Vetec Química Fina Ltda (Duque de Caxias, RJ, Brazil), and NaClO2 80% (w/w) was provided by Sigma-Aldrich (Saint Louis, MO, USA). The reagents were used as received, with no additional purification.
Preparation of the Fractions
The BPS was manually fractionated into four morphological regions: core, inner, middle, and outer sheaths. The visual aspect of each fraction can be seen in Fig. 1. The regions are differently colored: the outer fraction is composed of green sheaths (Fig. 1a); the middle fraction has a light yellow-pale brown color, a little darker than the inner fraction (Figs. 1b and 1c); the core is white, has no sheath, and contains a smooth and thin fiber bundle with the appearance of a fabric (Fig 1d). Each fresh fraction was weighed to determine its contribution to the whole pseudostem. Prior to analysis, each fraction was cut into parts of about 10 cm length and 3 cm width and dried in an air-circulating oven at 60 ºC for two days. The fractions were then ground in a Willey knife mill (FORTINOX, model STAR FT680; São Paulo, Brazil).
Fig. 1. Macrophotographs of different fractions of Pacovan banana pseudostem: (a) outer sheaths, (b) middle sheaths, (c) inner sheaths, and (d) core
Characterization of the Fractions
Chemical composition
The ash content, extractives, and moisture were determined following the respective TAPPI methods (TAPPI T413 om-93 1993; TAPPI T204 cm-97 1997; TAPPI T550 om-03 2003). The hemicellulose and α-cellulose contents were determined according to the description given by Yokoyama et al.(2002). To obtain the Klason lignin, the TAPPI T 222 om-22 (2002) method with slight modifications was adopted (without overnight settling step).
X-ray fluorescence spectrometry
The ash at 600 ºC, remaining after heating in a QUIMIS Q318M24 microprocessor-controlled oven (Brazil) was analyzed by X-ray fluorescence to identify the inorganic material present. The analysis was performed on a Rigaku spectrometer, model ZSX Mini II (Brazil), with a Pd tube, operating at 40 kV and 1.2 mA.
Scanning electron microscopy (SEM)
The morphology of the samples was examined using a Zeiss DSM-940A scanning electron microscope (Germany) operating at an accelerating voltage of 30 keV in a secondary electron imaging mode (SEI). The materials were mounted on brass studs and coated with gold in a K 550 Emitec sputter coater (UK).
X-ray diffraction (XRD)
The analysis by X-ray powder diffraction of different dried fractions was carried out in a Panalytical diffractometer, model Xpert Pro MPD (The Netherlands) with a Co tube, operating at 40 kV and 40 mA. The crystallinity index, Icr, was calculated using the following equation (Segal et al. 1959):
(1)
where Iam is the intensity at the minimum of the diffraction related to the amorphous material (21º < 2θ < 22º), and I002 is the intensity at the maximum of the crystalline peak 002 (25º < 2θ < 26º).
Fourier transform infrared (FTIR) spectroscopy
The FTIR spectra were obtained on a Shimadzu FTIR-8300 spectrophotometer (Japan) in the range of 4000 to 400 cm-1 with materials that had been dried in an oven at 90 ºC for 2 h and stored under vacuum. The dried samples were mixed with KBr in a proportion of 3% (w/w) and pressed (3 tons).
Thermal analysis (TGA and DSC)
Thermogravimetric analysis (TGA) of the dried fractions was conducted in a temperature interval of 25 to 700 ºC under a synthetic air atmosphere with a flow rate of 60 mL/min using platinum crucibles. Masses of approximately 8.0 mg were heated at a constant rate of 10 ºC/min. A Q50 Universal V20.10 thermal analyzer (TA Instruments, USA) was used for this study.
The DSC measurements were carried out on a Q20 Universal V4.7A (TA Instruments) differential scanning calorimeter. Samples of around 5.0 mg mass were heated in a temperature range of 25 to 400 ºC, at a constant heating rate of 10 ºC/min. The experiments were performed under a nitrogen atmosphere at a flow rate of 50 mL/min. Platinum crucibles were used.
Statistical analysis
The data was analyzed for statistical significance (p < 0.05) by the Tukey’s test using Sisvar 5.3 software (Federal University of Lavras, Lavras City, Brazil).
RESULTS AND DISCUSSION
Chemical Composition
The outer sheaths corresponded to ~41% of the total mass of the BPS, the middle fraction corresponded to ~27%, the inner section to ~21%, and the core to just ~11%.
The α-cellulose and hemicellulose contents decreased from the outer fraction to the core (Table 1). The same tendency for α-cellulose content to radially decrease inwards has been reported in M. acuminata, M. acuminata x balbisiana, and Nendran type Musa sp. (Aziz et al. 2011; Jayaprabha et al. 2011). The extractives and ash contents, on the other hand, increased radially inwards. The major function of the inner sheaths and core is transport of nutrients, which would certainly contribute to the higher ash and extractives contents (Oliveira et al. 2007; Jayaprabha et al. 2011).
Table 1. Lignocellulosic Composition of Pacovan Banana Pseudostem Fractions on a Dry Weight Basis
The core has a considerable amount of starch, sugars, and minerals (Bhaskar et al. 2011; Padam et al.2012) and is used as a medicine in India (Saravanan and Aradhya 2011; Kumar et al. 2012; Surendar et al. 2013). The pseudostem is a source of fibers for textile, composites, paper, and adsorbent applications (Reis 2006; Sapuan et al. 2007; Satyanarayana et al. 2007; Mohapatra et al. 2010).
The outer sheaths are more suitable as a source of fibers and cellulose, whilst the core and inner sheaths are better used as sources of extractives and ash. The ash content was low, consisting mainly of silicon, which could be troublesome for pulping (Cordeiro et al. 2004). The lignin content may be considered as relatively low, because it was 13% or less (Cordeiro et al. 2004; Oliveira et al. 2007; Jayaprabha et al. 2011).
The main chemical elements in the BPS Pacovan fraction ashes are shown in Table 2. Potassium is the main element for all fractions, followed by chlorine. Calcium was detected only in the outer sheath, while magnesium was absent in the core. Oliveira et al. (2007) obtained a similar result in M. acuminata Colla, Cavendish variety, with potassium as main ash component, and with the presence of calcium, silicon, phosphorus, and magnesium.
Table 2. X-Ray Fluorescence Analysis of the Main Chemical Elements Detected in the Ash of Pseudostem Fractions of Pacovan Banana
These results indicate that the pseudostem should not be considered as a sole material for extraction of fibers or for other materials, but a rich source of molecules for both technological pathways. This approach is in accordance to the biorefinery concept, where biomass is converted to fuels and chemicals in multi-step and multi-product approaches (Cherubini and Ulgiati 2010; FitzPatrick et al.2010; Kurian et al. 2013).
Scanning Electron Microscopy
The elementary fibers of the outer sheaths were arranged in a compact configuration and a preferential alignment parallel to the bundle axis (Fig. 2a). The same was observed for the inner sheaths (Fig. 2c). The presence of non-fibrous material (denoted by circles in Fig. 2a) can be noted in some regions of the outer sheaths on the SEM micrograph. In the case of the middle sheath fibers, this non-fibrous material covered the bundle (Fig. 2b). Gañán et al. (2004b) and Jayaprabha et al. (2011) reported that the non-fibrous materials encrusted on banana fibers are essentially constituted of hemicellulose, lignins and pectins.
In the inner fraction, close to the fiber bundle covered with non-cellulosic materials, tube-like structures were apparent. Two preferential orientations of fibers have been reported for banana pseudostem by Li et al. (2010) and Gañán et al. (2004b): parallel and perpendicular to the bundle axis. The authors described the perpendicular fibers as narrow fibers. In the core, elementary fibers could not be observed, and only narrow fibers in the form of pipes rather than fiber bundles were evident (Fig. 2d). This structure contributes to the transportation of water and nutrients in the core.
Fig. 2. SEM images of different fractions of Pacovan banana pseudostem: (a) outer sheaths, (b) middle sheaths, (c) inner sheaths, and (d) core. Scale bar equals 1:50 m
X-ray Diffraction
The diffractogram of fractions (Fig. 3) shows a decrease in the relative intensity of the peaks attributed to cellulose from the outer layer to the core. For the CoK radiation, the main peak for cellulose is around 25º to 26º (Bonarski and Olek 2011). The crystallinity of the fractions was 35.3% (outer sheaths), 32.7% (middle sheaths), 12.4% (inner sheaths), and 8.8% (core), in agreement with the α-cellulose percentage in each fraction (Table 1). A crystallinity of 39% was reported for banana pseudostem fibers cleaned with 5% sodium hypochlorite (Guimarães et al. 2009), which was not very different from the value obtained for the outer sheaths (35.3%). The XRD pattern also shows three intense peaks at ~ 21º (except outer fraction), ~ 33º, and ~ 47º, correlated to the salts of inorganic components, as potassium, chloride, calcium, and phosphor, as observed in X-ray fluorescence. Guimarães et al. (2009) observed narrow peaks and correlated them to inorganic substances. The absence of the reflection at ~21º in the outer fraction is an evidence of low concentration of an inorganic fraction, which is in a higher concentration in the other fractions. This is in accordance to the chemical composition (Table 1), with the lowest ash content in the outer sheath.
The results suggest a possible advantage of separating the outer sheaths and the core as a function of the use of each fraction. For example, the processing of the whole pseudostem for extraction of fibers would decrease the mechanical resistance of the components.
Fig. 3. Smoothed X-ray diffraction patterns of different fractions of Pacovan banana pseudostem
Fourier Transform Infrared Spectroscopy
The main organic constituents of pseudostems (α-cellulose, hemicellulose, and lignin) contain –OH groups. For comparison with the intensity of other bands and spectra, the intensity of the band at 3400 cm-1 (–OH stretching) was considered as a standard (Fig. 4). The decrease in band intensities from 1726 cm-1 to 1042 cm-1 was in agreement with the decrease in the total content of organic biomacromolecule material (α-cellulose, hemicellulose, and lignin) from ~54% (outer sheath) to ~15% (core) (Table 1). The bands at 1515 cm-1 (aromatic ring) and 1373 cm-1 (aromatic methoxyl), related to lignin, decreased from the outer to the core and became almost insignificant. The bands around 1254 cm-1 to 1042 cm-1, related to carbohydrate functional groups, as cellulose and hemicellulose, also decreased from the outer sheath to the core, but they were not fully eliminated.
Fig. 4. FTIR spectra of different fractions of Pacovan banana pseudostem
Table 3. FTIR Analysis of the Main Chemical Elements Detected in the Ash of Pseudostem Fractions of Banana Variety Pacovan
Thermal Analyses
TGA/dTG
Four thermal events can be clearly identified (Fig. 5 and Table 4). The first weight loss at temperatures up to 120 ºC was due to moisture evaporation (water adsorbed to fiber). Hemicellulose, which has an amorphous structure, was the first substance to be thermally decomposed. Cellulose decomposes after hemicellulose, due to its crystalline structure, lack of branches, and high molecular weight. Lignins are not polysaccharides and are highly cross-linked, properties that lead to the material having a higher stability compared with the polysaccharides of fibers (Sathasivam and Haris 2012). The three events at Tmax of 195 to 200 ºC, 280 to 293 ºC, and 427 to 433 ºC corresponded to hemicellulose, α-cellulose, and lignin thermal decomposition, respectively.
Guimarães et al. (2009) analyzed banana pseudostem and obtained a shoulder at around 190 ºC, along with two main peaks (321 and 445 ºC) in the DTG curves obtained in air. The shoulder at around 190 ºC could be due to the low content of hemicellulose (0.8%). These results are in agreement with those obtained in the present study.
Fig. 5. TGA (continuous line, scale on the left) and dTG (dashed line, scale on the right) curves of pseudostem fractions of Pacovan banana cultivar in synthetic airflow
Table 4. TGA/DTG Curves of Pseudostem Fractions of Banana Variety Pacovan
DSC
Strong endothermic peaks at around 100 ºC due to water evaporation can be seen for all fractions of the DSC thermograms for the banana pseudostem fractions (Fig. 6). The minimum temperatures increased from 102 ºC (outer sheaths) to 115 ºC (core), indicating an improvement in the moisture retention property, which was probably related to higher hydrophilicity in the central part of pseudostem.
Fig. 6. DSC thermograms for pseudostem fractions of Pacovan banana in nitrogen flow
The two endothermic peaks between 190 and 240 ºC are caused by partial dehydration of the –OH primary groups and the decomposition of minor constituents in the fraction, as suggested by Khan et al. (2006) for jute fiber. Another possible scenario is proposed by Gangopadhyay and Ghosh (1999), who attributed these peaks to the slow pyrolytic decomposition of the sisal fiber. Probably, these minor constituents are sugars and carbohydrates that occurs as extractives in the lignocellulosic analysis (Table 1). Paul et al. (2013) also found sucrose and KCl salts in banana pseudostem. Sucrose has two main endotherm events, related to the carbohydrate fusion, at ~188 ºC and ~229 ºC (Beckett et al. 2006; Eggleston et al. 1996; Hurtta et al. 2004). Mineral components can dislocate the temperatures of the thermal events in the DSC analyses, as stated by Beckett et al. (2006). As the extractives content is higher than the biomacromolecule contents, the extractives events may be overlapping the lignin, hemicellulose, and cellulose events, until the exothermic event at ~330 ºC.
A sharp exothermic peak was present in all DSC thermograms at around 327 and 337 ºC. Guimarães et al. (2009) found that weaker intermolecular interaction (amorphous region) in the banana pseudostem fractions promotes a reduction in the temperature of decomposition These peaks were attributed to the decomposition of α-cellulose, hemicellulose, and lignin. The decomposition of α-cellulose in nitrogen gas was observed at 360 ºC for the sisal fibers (Gangopadhyay and Ghosh 1999). Lignin and hemicellulose present char formation peaks at, respectively, ~350 to 450 ºC and ~200 to 350 ºC (Kemp 1999; Yang et al. 2007). Therefore, the exothermic event is a sum of the biomacromolecule exothermic peaks, caused mainly by pyrolysis reactions.
CONCLUSIONS
- There were significant differences (p<0.05) in the four analyzed lignocellulosic fractions of Pacovan banana.
- The outer sheaths were more suitable for α-cellulose extraction, while the core was more suitable for extractives extraction.
- Macromolecule content (α-cellulose and hemicellulose) increased radially from the core to the outer sheaths, and extractive and ash contents increased from the outer sheaths to the core.
ACKNOWLEDGMENTS
The authors are grateful to Prof. José Marcos Sasaki for the X-ray analysis, INCT-INOMAT and CNPq for the financial support, and also CNPq and FUNCAP for the scholarship.
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Article submitted: March 28, 2014; Peer review completed: September 14, 2014; Revised version received and accepted: October 21, 2014; Published: October 31, 2014,