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Isolation and Characterization of Iignin from Okra (abelmoschus Esculentus) Fibre and Stick

M. Sarwar Jahan
Pulp and Paper Research Division, BCSIR Laboratories, Dhaka, Dr. Qudrat-i-Khuda Road, Dhaka 1205, Bangladesh.

D. Alam
Department of Applied Chemistry and Chemical Engineering, University of Dhaka, Dhaka 1000, Bangladesh.

M. Mostafzur Rahman
Pulp and Paper Research Division, BCSIR Laboratories, Dhaka, Dr. Qudrat-i-Khuda Road, Dhaka 1205, Bangladesh.

M. A. Quaiyyum
Department of Applied Chemistry and Chemical Engineering, University of Dhaka, Dhaka 1000, Bangladesh.

Abstract/ Executive Summary:

The okra (Abelmoschus esculen tus) plant consists of bast fiber and core (stick). The bast fibre of okra plant was characterized with high α-cellulose (56.7%) and low lignin (12.6%) and stick withlow α-cellulose (34.3%) and high lignin (25.2%) content. Lignin was isolated from the fiber and stick by acidolytic dioxane method and characterized for elemental analysis, methoxyl analysis and FTIR and 1 H-NMR spectroscopy. The C 9 formulas for okra stick and fibre lignin were C₉H_9.61O_4.63 (OCH₃ ) _1.24 and C₉H_8.61O_4.66 (OCH₃)_1.49, respectively. Both of the lignins were of the guaiacyl-syringyl type. The bands of FT-IR spectrum at 1327 cm^-1 , 1122 cm^-1 and 837 cm^-1 associated with syringyl unit was higher in okra fibre lignin than in the okra stick lignin. The structural analysis revealed that the average numbers of proton of β- O-4 (Hα & Hβ) per C₉ unit in okra stick and fibre lignin were 1.53 and 1.20, respectively. The β- O-4 units in these lignins had predominately erythro stereochemistry type.

Keywords: Okra plant; Fibre and stick; Dioxane lignin; Syringyl unit; Guaiacyl unit; Erythro type

Location: Analytical Research Division of BCSIR Laboratories, Dhaka.

Start Date:

End Date:

Program Area: Postharvest and Agro-processing

Commodity: Okra

Objectives:

To isolate lignin by dioxane/HCl mixture from the okra stick and bast fibre, and the isolated lignin characterized by elemental, methoxyl, FTIR and ¹H-NMR spectroscopy.

Methodology:

A farmer supplied okra plants after harvesting the crop in the region of Savar, Dhaka. Subsequently, okra plant was retted for a month and the fibre and stick separated. Afterwards, these were properly sun dried. The stick and fibre were separately ground in a Willey Mill and screened to 40/60 mesh for lignin isolation.Okra stick and fibre meals (40-60 mesh) were refluxed with acetone solvent for 12 h. The acetone extractive free meals were refluxed with acidic dioxane (9:1) solution. The concentration of HCl in dioxane solution was 0.2 N. The dioxane to meal ratio was 8:1. The meal was refluxed with dioxane solution for 1 h at N₂ atm. The N₂ flow was maintained at 50 ml/min. After completing reflux time, wood meal dioxane mixture was filtered in a Buckner funnel using Whatmann filter paper no. 2. The residue was washed with dioxane solution. The filtrate was concentrated in a vacuum evaporator at 40ºC. Then conc. dioxane soln. was added drop wise to deionized water to precipitate lignin. Precipitated lignin was centrifuged and washed frequently until reaching pH around 7 and dried. Dried crude lignin was dissolved in dioxane (9:1), and again precipitated in ether with constant stirring by magnetic bar. The precipitated lignin was dried under vacuum over P₂O₅ and weighed. A mixture of 100 mg purified lignin and 1.5 ml pyridine- acetic anhydride (1:1) was kept for 72 h with frequent stirring. The solution was added to a 10-fold volume of ice-cold water whereupon the acetylated sample was recovered as a precipitate, which was purified by successive washing with water and dried in vacuum desiccators over P₂O₅. Analysis of C and H in okra stick and fibre lignin was carried out by Analytical Research Division of BCSIR Laboratories, Dhaka, and the oxygen was determined by difference. The methoxyl content in lignin was determined in accordance to Japan International Standard Methods. Infrared spectra were recorded by using a Shimadzu FTIR spectrometer model 8201PC.The dried samples were embedded in KBr pellets in the concentration of about 1 mg/100 mg KBr. The spectra were recorded in the absorption band mode in the range 4000-400 cm^–1 ¹H NMR: Spectra of lignin samples (100 mg of acetylated lignin contained in 0.5 ml CDCl₃) were recorded in a Bruker 400 spectrometer. CDCl₃ solvent was used as internal standard (ppm 7.25). For quantification of protons, the signal in specified region of the spectrum was integrated with respect to a spectrum-wide baseline drawn at the level of the background noise, and the results were referred to the signal for methoxyl protons. The average number per C₉ unit was established as described above.

Source of Information: Bangladesh J. Sci. Ind. Res. 50(4), 257-262, 2015

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

The lignins isolated from the okra fibre and sticks are mainly composed with syringyl unit and guaiacyl unit. Okra fibre lignin contains higher syringyl unit compared with stick lignin. The OCH₃/C₉ is 1.49 for okra fibre lignin and 1.24 for stick lignin. From the ¹H NMR spectra, the number of proton of phenolic and aliphatic hydroxyl group were estimated to be 0.41 and 0.79 for okra stick lignin and 0.21 and 1.51 for okra fibre per C₉ unit, respectively. ¹H NMR study showed that the β-O-4 structural unit is higher for lignin of stick. Both lignins contain both erythro and threo configuration, but erythro proportion was higher. Overall the results of this investigation indicate that the okra plant lignin is of guaiacyl-syringyl and significant amount of p-coumaryl type.

Knowledge Management: Journal