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Elephant grass (Germplasm number: CF052696) was provided by the Tropical Crops Genetic Resources Institute, Chinese Academy of Tropical Agricultural Sciences (TCGRI, CATAS), located in Haikou, China. Stalks of elephant grass were collected during the 2nd (S2), 6th (S6), 8th (S8), and 10th (S10) months of growth. Stalk samples were collected from the same part of elephant grass stalks at different growth stages, with three biological replicates collected at each growth stage.
Choline chloride and oxalic acid dihydrate were procured from Shanghai Macklin Biochemical Co., Ltd., (Shanghai, China) and acetone was acquired from Xilong Chemical Co., Ltd. (China).
Methods
Pretreatment of elephant grass stalks
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The stalks were pulverized using a crusher and screened through a 100-mesh screen to obtain the powder samples. These samples were hermetically sealed and stored in a dry, room-temperature environment.
Isolated of crude cellulose
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Crude cellulose was isolated from elephant grass stalks using a deep eutectic solvent. The mixture of choline chloride and oxalic acid dihydrate (molar ratio 1:1) was heated and stirred at 60 °C for 1 h until a homogeneous solution was formed to obtain deep eutectic solvent. The elephant grass stalks powder (1.0 g) was added to a deep eutectic solvent (20 g) and heated at 110 °C for 3 h. When the reaction was complete, the flask was placed on ice to cool. The reaction mixture was washed with acetone solution (1:1, v/v), filtered and the solid residue was collected. It was then dried at 60 °C to obtain crude cellulose. The solid recovery of crude cellulose was calculated by the Eqn (1):
$ \text{Solid recovery (%)}=\left(\dfrac{{Weight \;of\; crude \; cellulose}}{{Weight\; of\; raw\; material}}\right)\times \text{100} $ (1) Analyses and characterizations of elephant grass stalks and crude cellulose
Morphology characterization
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After gold spraying, the morphology of elephant grass stalks and crude cellulose were characterized using a scanning electron microscope (SEM, Phenom ProX, The Netherlands) at 10 KV.
Determination of water content
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After weighing the freshly harvested stalks, the samples were dried at 105 °C until reaching a constant weight, and then reweighed to determine their dry weight. The water content of stalks is calculated by the Eqn (2):
$ \text{Water content (%)}=\left(\dfrac{{Fresh}\;{weight}-{Dry}\;{weight}}{{Fresh}\;{weight}}\right)\times{100} $ (2) Composition analysis of elephant grass stalks
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The component test methods have been slightly modified according to the references: Cellulose and hemicellulose contents were determined by the phenol-sulfuric acid colorimetric method[18]. Lignin content was determined by the redox method[19]. The crude fiber content was determined by the residual weight method[20]. Acid detergent fiber and neutral detergent fiber were determined using the paradigm washing method[21]. Crude fat extraction was carried out using the soxhlet extraction method[20]. Determination of pectin content was by the carbazole colorimetric method[22]. Nitrogen content was determined by the Kjeldahl method, and crude protein content was calculated using Eqn (3)[20]:
$ \text{Crude protein (%)}={Nitrogen\; content\; ({\text{%}})}\times {6.25} $ (3) Chemical structure analysis of isolated crude cellulose
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The fourier transform infrared spectroscopy (FTIR) of crude cellulose was analyzed using a Tensor 27 (Bruker, Germany) spectrometer. The spectra were collected in the range of 4,000–500 cm–1. The spectra were recorded cumulatively in 32 scans with a resolution of 4 cm–1.
The X-ray diffraction (XRD) spectra of crude cellulose were scanned on a D8 Advance XRD diffractometer (40 kV CuKa) with scanning diffraction angles (2θ) from 10° to 45°. The crystallinity index (CrI) of crude cellulose was calculated using the Segal method[23] and Eqn (4):
$ \text{CrI (%)}=\left(\dfrac{{{I}}_{{002}}-{{I}}_{{am}}}{{{I}}_{{002}}}\right)\times {100} $ (4) where I002 is the maximum diffraction intensity of the 002 crystalline peak at 22°, and Iam is the minimum intensity of the amorphous fraction around 18°.
Thermal gravimetric analysis (TGA)
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TGA analyses of elephant grass stalks powder and crude cellulose were determined using a TG 209 F3 thermogravimetric analyzer (NETZSCH-Gerätebau GmbH). The temperature was increased from 30 to 700 °C with a heating rate of 10 °C min–1 under a nitrogen atmosphere.
Statistical analysis
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Analysis of variance (ANOVA) was performed using SPSS 26.0 by Duncan multiple range test (p < 0.05).
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All data generated or analyzed during this study are included in this published article.
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About this article
Cite this article
Yuan J, Liu G, Liu P, Huang R. 2024. Comprehensive assessment of elephant grass (Pennisetum purpureum) stalks at different growth stages as raw materials for nanocellulose production. Tropical Plants 3: e013 doi: 10.48130/tp-0024-0013
Comprehensive assessment of elephant grass (Pennisetum purpureum) stalks at different growth stages as raw materials for nanocellulose production
- Received: 24 January 2024
- Accepted: 25 March 2024
- Published online: 07 May 2024
Abstract: Elephant grass, a tropical grass plant, serves dual purposes as forage and as a biomass energy source. This study aimed to identify the optimal raw materials for the production of nanocellulose from elephant grass. Elephant grass stalks harvested during the 2nd, 6th, 8th, and 10th months of growth were pretreated with a deep eutectic solvent (containing choline chloride and oxalic acid in a 1:1 molar ratio) for 3 h at 60 °C, to isolate crude cellulose. The results showed that there were significant differences in the content of chemical components in elephant grass stalks at different growth stages. The cellulose content of elephant grass stalks was highest in the 8th month, reaching 27.4%. Meanwhile, there was no significant difference in lignin content among elephant grass stalks harvested in the 6th, 8th, and 10th months. The isolated crude cellulose from elephant grass stalks harvested in the 8th month exhibited excellent thermal stability, high crystallinity, and a high cellulose content. Thus, the stalks harvested in the 8th month were identified as the optimal raw material for preparing nanocellulose. This study lays the foundation for the production of nanocellulose from elephant grass and provides a theoretical basis for its high-value-added utilization.
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Key words:
- Elephant grass /
- Energy crop /
- Nanocellulose