Extraction and Characterization of Fiber and Cellulose from Ethiopian Linseed Straw: Determination of Retting Period and Optimization of Multi-Step Alkaline Peroxide Process

Flax is a commercial crop grown in many parts of the world both for its seeds and for its fibers. The seed-based flax variety (linseed) is considered less for its fiber after the seed is extracted. In this study, linseed straw was utilized and processed to extract fiber and cellulose through optimization of retting time and a multi-step alkaline peroxide extraction process using the Taguchi design of experiment (DOE). Effects of retting duration on fiber properties as well as effects of solvent concentration, reaction temperature, and time on removal of non-cellulosic fiber components were studied using the gravimetric technique, Fourier transform infrared (FTIR) spectroscopy and thermal studies. Based on these findings, retting for 216 h at room temperature should offer adequate retting efficiency and fiber characteristics; 70% cellulose yield was extracted successfully from linseed straw fiber using 75% ethanol–toluene at 98 °C for 4 h, 6% NaOH at 75 °C for 30 min, and 6% H2O2 at 90 °C for 120 min.


Introduction
Agricultural crop by-products are considered inexpensive, abundant, annually renewable, and sustainable sources of fiber and cellulose. Finding alternate sources for present natural and synthetic fibers is critical for ensuring sufficient supply of fibers at reasonable rates in the future. Limitations in availability of land, water, and other resources required to grow natural fibers such as cotton, bamboo, sisal, hemp, and kenaf could limit availability and/or raise prices of those fibers, rendering them unaffordable for commodity applications. As a result, attempts to identify alternate fiber sources, particularly from cheap, copious, and renewable lignocellulose wastes, are considered extremely useful [1].

Fiber Extraction and Characterization Methods
A 50 g linseed straw bundle with a 20 cm length and a 3 mm mean stalk diameter was cut from the center section of the stems for uniformity of samples. Next, the bundled stalks were immersed in transparent plastic bottles, each filled with the same amount of water, with a 1:33 solid-to-liquid ratio [28]. After immersion, the bottles were stored at room temperature for 48-264 h, without lids, for retting, as shown in Table 1 [29,30]. About 275 mL of retted water was taken from all samples and tested every 24 h using a calibrated pH meter to find pH measurement [31,32]. Increase in weight of wetted linseed stalks (m w ) from initial dry linseed stalk weight (m d ) due to water absorption from each retting time was recorded, and stalk water-absorption percentage (w a %) was estimated based on Equation (1) [33,34].
Weight of linseed fiber extracted (m o ) from the initial linseed stalk weight (m i ) was recorded for each retting time, and the fiber yield percentage (Y%) was determined using Equation (3) [28,36].
Density of oven-dried stalks amounted to a constant weight [34,37,38]; non-, under-, optimally, and over-retted linseed fiber bundles were measured for distilled water density (ρ w ) using the liquid pycnometer technique. Mass of an empty pycnometer (m 1 ), a pycnometer filled with chopped fibers (m 2 ), a pycnometer filled with water (m 3 ), and a pycnometer filled with water and chopped fibers (m 4 ) was measured and calculated. Density of fibers (ρ f ) was estimated based on Equation (4) [38,39]. Diameter of linseed fibers was measured using an optical microscope. Three samples, from under-, optimally, and over-retted fibers, were prepared; then three equidistant points were marked on a single fiber, and the diameters of these points were measured with two replications. The average diameter was then calculated; this could be considered the mean diameter of the fibers [40].
Fiber samples with different degrees of water retting were kept in the open air at room temperature and 55-65% RH for one week. Then, after drying in an oven to a constant weight, weights of wetted (W w ) and oven-dried (W d ) samples were measured. Moisture content percentage (M C %) of the fiber was calculated using Equation (5) [33,34].
A tensile test of single fibers with different retting durations was performed according to ASTM D 3822-07, using a single-fiber electronic strength tester. The test was carried out at room temperature and 65% RH, with a gauge length of 100 mm, a load range of 5 N, and a test speed of 100 mm/min [41].
A chemical composition analysis (dry-weight basis) of the fiber was conducted to quantify mainly the percentage amounts of cellulose, hemicellulose, lignin, and extractives. This analysis used 1:2 ethanol-toluene for 6 h at 98 • C in a Soxhlet apparatus to determine extractive content [42][43][44][45], 17.5% NaOH at 95 • C for 60 min in a reciprocating water bath to quantify hemicellulose content [46,47], and 72% H 2 SO 4 at room temperature for 120 min; hydrolyzed samples were diluted with distilled water to a 3% acid concentration (adapted from TAPPI T-222 om-02) [48]. Samples were autoclaved for 1 h at 121 • C and cooled for about 20 min at room temperature; the diluted suspensions were centrifuged at 5000 rpm for 15 min and vacuum-filtered. The residues were burned in a muffle furnace at 550°C for 3 h to quantify the amount of ash in the acid-insoluble lignin [41,49], as shown in Figure 1.  Percentage amounts of non-cellulosic constituents (wt.%) were calculated from the difference between initial (wi) and final (wf) fiber weights, using the gravimetric method based on Equation (6) [50,51].
The ash content of dry, chopped raw fiber was determined via burning in a 550 °C furnace for 4 h [48], allowance to cool to room temperature in a desiccator, and weighing (adapted from TAPPI (T 211 om-02)). Finally, the cellulose percentage in wt.% of biomass was calculated using Equation (7), assuming that extractives, hemicellulose, lignin, ash, and cellulose were the main chemical compositions of the linseed fiber [52][53][54]. Percentage amounts of non-cellulosic constituents (wt.%) were calculated from the difference between initial (w i ) and final (w f) fiber weights, using the gravimetric method based on Equation (6) [50,51].

Cellulose Extraction and Characterization Methods
Cellulose was extracted from linseed straw through sequential chemical treatments to remove extractives, hemicellulose, and lignin, as shown in Figure 2.
The ash content of dry, chopped raw fiber was determined via burning in a 550 °C furnace for 4 h [48], allowance to cool to room temperature in a desiccator, and weighing (adapted from TAPPI (T 211 om-02)). Finally, the cellulose percentage in wt.% of biomass was calculated using Equation (7), assuming that extractives, hemicellulose, lignin, ash, and cellulose were the main chemical compositions of the linseed fiber [52][53][54].

Cellulose Extraction and Characterization Methods
Cellulose was extracted from linseed straw through sequential chemical treatments to remove extractives, hemicellulose, and lignin, as shown in Figure 2.  Removal percentage of non-cellulosic components (R%) of linseed fiber in each step of the cellulose extraction process was calculated via taking the weight difference between initial (m i ) and final (m f ) fiber weights, using the gravimetric method based on Equation (8).

Thermogravimetric Analysis (TGA)
In the thermal study, thermal degradation characteristics of fibers (non-retted, retted, extracted, alkalized, and bleached) were analyzed using a thermogravimetric and differential thermal analyzer, from room temperature to 700 • C, at a heating rate of 20 • C min −1 in N 2 atmospheres, and using an 8 mg sample weight [1,58-60].

Statistical Method for Optimization
The selected factors (concentration, temperature, and time) and levels (low, medium and high) for the extraction processes are shown in Table 2. The Taguchi L 9 Orthogonal Array (OA) design of experiments was employed to investigate contribution of selected extraction conditions (concentration of solvents, reaction temperature, and time) to yield removal of extractives, hemicellulose, and lignin, as shown in Table S1.  Many bacteria isolated from bast fibers are capable of promoting retting. The most important phase of this process is hydrolysis of pectic matter that surrounds and cements fibers, thereby loosening fibers from the stem and helping to extract those fibers [31]. Due to absence of pectic matter to be hydrolyzed and utilization of D-galacturonic acid (GA) by bacteria, concentration of GA, which is the end product of bacterial hydrolysis in retting water, began to fall [31,32].

Retted Water pH, Stalk Water Absorption and Stalk Weight Loss Analysis
The effect of retting-time duration on the water absorption percentage of linseed stalks showed that water absorption increased rapidly to 172.47% during the first 48 h, then increased slowly to the equilibrium water-absorption percentage of 187.21% at 168 h. After this immersion-time duration had passed, percentage of water absorption became stable, which means less than 1% of variations were observed, and no more weight gain of the wetted linseed stalk was observed during retting-time increments, as shown in Figure 3b. Forty-eight hours after equilibrium water absorption was the optimum retting time for successful fiber extraction.
Water retting occurs when water penetrates the center-stalk section of the plant, swelling the interior cells and shattering the outermost layer to enhance absorption of water and produce bacteria that promotes retting [61,62]. Therefore, retting up to equilibrium water absorption helps with removal of pectin and successful fiber extraction during water retting.
Extending duration of water retting resulted in a considerable increase in weight loss, as can be shown in Figure 3a. When the duration was extended from 48 to 168 h, the weight loss increased from 5.74 to 12.3% due to removal of impurities and pectin components of the fiber. From 168 to 216 h, weight-loss variations were nearly stable or less than 1%, and at the end of this time, 12.67% fiber yield was obtained. However, after 216 h of retting, weight loss was slightly increased due to removal of other non-cellulosic constit- Many bacteria isolated from bast fibers are capable of promoting retting. The most important phase of this process is hydrolysis of pectic matter that surrounds and cements fibers, thereby loosening fibers from the stem and helping to extract those fibers [31]. Due to absence of pectic matter to be hydrolyzed and utilization of D-galacturonic acid (GA) by bacteria, concentration of GA, which is the end product of bacterial hydrolysis in retting water, began to fall [31,32].
The effect of retting-time duration on the water absorption percentage of linseed stalks showed that water absorption increased rapidly to 172.47% during the first 48 h, then increased slowly to the equilibrium water-absorption percentage of 187.21% at 168 h. After this immersion-time duration had passed, percentage of water absorption became stable, which means less than 1% of variations were observed, and no more weight gain of the wetted linseed stalk was observed during retting-time increments, as shown in Figure 3b. Forty-eight hours after equilibrium water absorption was the optimum retting time for successful fiber extraction.
Water retting occurs when water penetrates the center-stalk section of the plant, swelling the interior cells and shattering the outermost layer to enhance absorption of water and produce bacteria that promotes retting [61,62]. Therefore, retting up to equilibrium water absorption helps with removal of pectin and successful fiber extraction during water retting.
Extending duration of water retting resulted in a considerable increase in weight loss, as can be shown in Figure 3a. When the duration was extended from 48 to 168 h, the weight loss increased from 5.74 to 12.3% due to removal of impurities and pectin components of the fiber. From 168 to 216 h, weight-loss variations were nearly stable or less than 1%, and at the end of this time, 12.67% fiber yield was obtained. However, after 216 h of retting, weight loss was slightly increased due to removal of other non-cellulosic constituents [35,63,64]. As a result, the retted water pH, stalk water absorption, and stalk weight loss values obtained can be used to predict optimal retting time.

Effect of Retting Duration on Fiber Properties
Effects of retting duration on physical and tensile properties of fibers that were extracted under different retting durations, shown in Figure 4, were investigated.  Table 3 demonstrates effects of retting duration on physical properties-mainly diameter, density, and moisture-of (R0) non-retted, (R1) under-retted, (R2) optimally retted, and (R3) over-retted fibers. These results revealed that as degree of retting increased, average diameter of the fiber reduced due to removal of surface components via retting [65,66]. Initially, an increment in density values was observed, with increasing retting degree due to removal of less-dense constituents and impurities such as pectin [67,68], but over-retted fibers showed a relative reduction in density as a result of cell-wall decompression [69,70].

Physical Properties
The mean density of optimal retted fiber was 1.52 g/cm 3 and the density values of flax fiber reported in previous works of the literature were from 1.40 to 1.55 g/cm 3 [71,72]. These values were obtained with different methods, such as a helium pycnometer (1.54 g/cm 3 ) [73], a gas pycnometer (1.49 to 1.52 g/cm 3 ) [74], and immersion in water (1.54 g/cm 3 ) [75]. Moisture content was reduced with increasing retting degree due to the high amount of cortical parenchyma components remaining on the surface of non-retted and underretted fibers; these fibers may have high water interaction [76].

Tensile Properties
The effect of retting duration on tensile properties-specifically breaking force; breaking elongation; and tenacity of (R1) under-retted, (R2) optimally retted, and (R3) overretted fibers-were tested as shown in Table 3. Initially, mean breaking force and tenacity of each single fiber were enhanced due to removal of a larger amounts of weak substances, such as pectin and other impurities; results were reduced with further increments of retting duration due to cellulose-component degradation resulting in presence of more weak spots and reduction in diameter of the fibers [65,77,78].
These results showed that mean elongation values decreased with an increasing degree  Table 3 demonstrates effects of retting duration on physical properties-mainly diameter, density, and moisture-of (R 0 ) non-retted, (R 1 ) under-retted, (R 2 ) optimally retted, and (R 3 ) over-retted fibers. These results revealed that as degree of retting increased, average diameter of the fiber reduced due to removal of surface components via retting [65,66]. Initially, an increment in density values was observed, with increasing retting degree due to removal of less-dense constituents and impurities such as pectin [67,68], but over-retted fibers showed a relative reduction in density as a result of cell-wall decompression [69,70].

Physical Properties
The mean density of optimal retted fiber was 1.52 g/cm 3 and the density values of flax fiber reported in previous works of the literature were from 1.40 to 1.55 g/cm 3 [71,72]. These values were obtained with different methods, such as a helium pycnometer (1.54 g/cm 3 ) [73], a gas pycnometer (1.49 to 1.52 g/cm 3 ) [74], and immersion in water (1.54 g/cm 3 ) [75]. Moisture content was reduced with increasing retting degree due to the high amount of cortical parenchyma components remaining on the surface of non-retted and under-retted fibers; these fibers may have high water interaction [76].

Tensile Properties
The effect of retting duration on tensile properties-specifically breaking force; breaking elongation; and tenacity of (R 1 ) under-retted, (R 2 ) optimally retted, and (R 3 ) over-retted fibers-were tested as shown in Table 3. Initially, mean breaking force and tenacity of each single fiber were enhanced due to removal of a larger amounts of weak substances, such as pectin and other impurities; results were reduced with further increments of retting duration due to cellulose-component degradation resulting in presence of more weak spots and reduction in diameter of the fibers [65,77,78].
These results showed that mean elongation values decreased with an increasing degree of retting due to removal of non-cellulosic components; this tends to result in fiber brittleness [30,79].

Chemical Composition Analysis
The primary chemical compositions of linseed straw are cellulose, hemicellulose, lignin, and extractives. The chemical constituents of linseed fiber are 68, 20, 5, 4, and 3% of cellulose, hemicellulose, lignin, extractives, and ash, respectively. Cellulose content is comparatively higher than are different lignocellulose biomasses from agricultural wastes, as shown in Table 4. These variations may be due to the type of agricultural crop and the geographic area where the plants were cultivated; the amount of these constituents might vary even among the same plants. Uniquely, all fibers contain the same constituents, but in different percentages, which results in different behaviors [83].

Cellulose Extraction, Characterization and Optimization
The cellulose extraction process was conducted in multi-step extraction processes via optimization of extraction-process conditions, including solvent concentration, reaction temperature, and time. The extraction steps mainly focused on removal of extractives, hemicellulose, and lignin using ethanol-toluene, sodium hydroxide, and hydrogen peroxide solvents, respectively, under different extraction-process conditions. The results for a fiber after each step of extraction are shown in Figure 5.
Polymers 2023, 15, x FOR PEER REVIEW cellulose, hemicellulose, lignin, extractives, and ash, respectively. Cellulose con comparatively higher than are different lignocellulose biomasses from agricu wastes, as shown in Table 4. These variations may be due to the type of agricultural crop and the geograph where the plants were cultivated; the amount of these constituents might vary among the same plants. Uniquely, all fibers contain the same constituents, but in dif percentages, which results in different behaviors [83].

Cellulose Extraction, Characterization and Optimization
The cellulose extraction process was conducted in multi-step extraction process optimization of extraction-process conditions, including solvent concentration, re temperature, and time. The extraction steps mainly focused on removal of extra hemicellulose, and lignin using ethanol-toluene, sodium hydroxide, and hyd peroxide solvents, respectively, under different extraction-process conditions. The r for a fiber after each step of extraction are shown in Figure 5. The experimental results showed that the maximum removal values of extra hemicellulose, and lignin were 4.90, 18.10, and 4.00% respectively; these results we served at extraction-variable combinations of 75% at 98 °C for 4 h, 6% at 75 °C for 3 and 6% at 90 °C for 120 min, respectively. However, the predicted mean removal extractives, hemicellulose, and lignin tested under optimum values from the sig noise ratio (SNR) graph was calculated as 4.88, 18.25, and 4.19% at extraction-va combinations of 75% at 98 °C for 8 h, 6% at 75 °C for 90 min, and 10% at 90 °C for 12 with error values of 0.40, 0.82, and 4.53%, respectively. This indicates that the e mental and predicted results were in good agreement, as shown in Table 5.

Statistical Analysis
The experimental results showed that the maximum removal values of extractives, hemicellulose, and lignin were 4.90, 18.10, and 4.00% respectively; these results were observed at extraction-variable combinations of 75% at 98 • C for 4 h, 6% at 75 • C for 30 min, and 6% at 90 • C for 120 min, respectively. However, the predicted mean removal (%) of extractives, hemicellulose, and lignin tested under optimum values from the signal to noise ratio (SNR) graph was calculated as 4.88, 18.25, and 4.19% at extraction-variable combinations of 75% at 98 • C for 8 h, 6% at 75 • C for 90 min, and 10% at 90 • C for 120 min, with error values of 0.40, 0.82, and 4.53%, respectively. This indicates that the experimental and predicted results were in good agreement, as shown in Table 5. The ANOVA results shown in Tables S2-S4 explain the significant level, contribution percentage, and rank of each factor in removal of extractives, hemicellulose, and lignin, respectively. The concentrations of the solvents in removal of extractives, hemicellulose, and lignin were statistically significant (p ≤ 0.05), contributing 97.25, 64.5, and 81.47% to the response, respectively. The reaction temperatures in removal of extractives and hemicellulose were statistically significant, contributing 2.74 and 35.09%, respectively. This value, however, was not significant in removal of lignin (p ≥ 0.05), contributing only 7.48%. The reaction time in removal of extractives, hemicellulose, and lignin was not significant, with no contribution, 0.29%, and 9.26%, respectively.
The ANOVA findings of the linear model equations shown in Equations (S1)-(S4) can appropriately explain removal of extractives, hemicellulose, and lignin within a wide range of operating circumstances, with coefficients of determination (R 2 ) of 0.569, 0.879, and 0.8974 at a 95% level of confidence. The response models examined in this study can explain removal of extractives, hemicellulose and lignin; they contributed 56.95, 87.92, and 89.74% to the response, respectively.

Fourier Transform Infrared (FTIR) Analysis
The FTIR spectra of the non-retted, retted, extracted, alkalized, and bleached linseed fibers shown in Figure 6 were interpreted and discussed according to reported studies regarding the sources of FTIR peaks and their assignments, as shown in Table 6. For every stage of the extraction procedure, FTIR analysis was carried out to identify presence of chemical functional group changes [84]. All samples presented two main absorbance regions: the fingerprint region (700-1800 cm −1 ) and the functional group region (2700-3500 cm −1 ). However, specific absorption peaks can be identified for each particular component [42]. The presence of nearly similar functional groups at 3,425, 2,917, 1,636, 1,114, and 617 cm −1 in all fibers justified preservation of the basic chemical structure of cellulose fiber and water, even after all treatments during the extraction process. It was expected that during the extraction process, non-cellulosic components of the fiber-hemicellulose, lignin, and extractive (pectin, wax) contents-could be completely or partially removed. Therefore, corresponding absorption peaks to these components disappeared or diminished in intensity value. The treatments, on the other hand, increased intensity of bands that corresponded to cellulose [85]. The absorption peaks that corresponded to the extractives were 2917, 2853, 1114, and 1032 cm −1 . The absorbance peak at 2,853 cm −1 disappeared, and others were reduced after retting and extraction due to removal or reduction of pectin and wax. The absorption peaks at 3425, 2917, 1732, 1426, 1383, 1114, 1032 and 901 cm −1 are related to hemicellulose. The absorption peak at 1,731 cm −1 disappeared, and the others were diminished in peak intensity because of hemicellulose removal during the alkalization process. Absorption peaks observed at 3425, 2917, 1731 and 1426 cm −1 are associated with lignin. The absorption peak at 1731 cm −1 disappeared, and the others diminished in peak intensity because of lignin removal during the bleaching process. The presence of nearly similar functional groups at 3425, 2917, 1636, 1114, and 617 cm −1 in all fibers justified preservation of the basic chemical structure of cellulose fiber and water, even after all treatments during the extraction process. It was expected that during the extraction process, non-cellulosic components of the fiber-hemicellulose, lignin, and extractive (pectin, wax) contents-could be completely or partially removed. Therefore, corresponding absorption peaks to these components disappeared or diminished in intensity value. The treatments, on the other hand, increased intensity of bands that corresponded to cellulose [85]. The absorption peaks that corresponded to the extractives were 2917, 2853, 1114, and 1032 cm −1 . The absorbance peak at 2853 cm −1 disappeared, and others were reduced after retting and extraction due to removal or reduction of pectin and wax. The absorption peaks at 3425, 2917, 1732, 1426, 1383, 1114, 1032 and 901 cm −1 are related to hemicellulose. The absorption peak at 1731 cm −1 disappeared, and the others were diminished in peak intensity because of hemicellulose removal during the alkalization process. Absorption peaks observed at 3425, 2917, 1731 and 1426 cm −1 are associated with lignin. The absorption peak at 1731 cm −1 disappeared, and the others diminished in peak intensity because of lignin removal during the bleaching process.

Thermogravimetric Analysis (TGA)
Decomposition of lignocellulose materials mainly shows three stages of degradation; due to differences in chemical structures between extractives, hemicellulose, cellulose, and lignin, they usually decompose at different temperatures [77,88]. Degradation of linseed straw fiber went through three phases, as shown in Figure 7: light-component drying and evaporation, hemicellulose and amorphous cellulose decomposition, and crystalline cellulose and lignin decomposition. Decomposition of lignocellulose materials mainly shows three stages of degradation; due to differences in chemical structures between extractives, hemicellulose, cellulose, and lignin, they usually decompose at different temperatures [77,88]. Degradation of linseed straw fiber went through three phases, as shown in Figure 7: light-component drying and evaporation, hemicellulose and amorphous cellulose decomposition, and crystalline cellulose and lignin decomposition. Weight loss, decomposition temperature ranges, and residue contents of fibers in each degradation stage are summarized in Table 7. The thermal stability of the raw, extracted, alkalized, and bleached fibers was 179, 205, 250, and 269 °C, respectively. All extraction steps resulted in thermal stability improvement of the fiber due to retention and improvement of the structural order, as well as reduction in amorphous content [89].
Each fiber showed dissimilar weight losses and temperature ranges in all degradation stages. Weight loss (%) in the first stage was higher for raw fiber compared to that of extracted and alkalized fibers due to presence of extractives and higher moisture content. It was reduced after removal of extractives and hemicellulose, which are responsible for moisture absorption of fibers, and on the other hand, the bleached fiber showed the highest weight loss (%) due to more moisture-absorption properties of the fiber after removal of lignin, which is naturally hydrophobic [90].
An increase in weight loss was observed in the second degradation stage after the extraction process due to the removal of extractives, which increased the proportions of cellulose and hemicellulose [77]. However, weight loss decreased after the alkalization Weight loss, decomposition temperature ranges, and residue contents of fibers in each degradation stage are summarized in Table 7. The thermal stability of the raw, extracted, alkalized, and bleached fibers was 179, 205, 250, and 269 • C, respectively. All extraction steps resulted in thermal stability improvement of the fiber due to retention and improvement of the structural order, as well as reduction in amorphous content [89].
Each fiber showed dissimilar weight losses and temperature ranges in all degradation stages. Weight loss (%) in the first stage was higher for raw fiber compared to that of extracted and alkalized fibers due to presence of extractives and higher moisture content. It was reduced after removal of extractives and hemicellulose, which are responsible for moisture absorption of fibers, and on the other hand, the bleached fiber showed the highest weight loss (%) due to more moisture-absorption properties of the fiber after removal of lignin, which is naturally hydrophobic [90].
An increase in weight loss was observed in the second degradation stage after the extraction process due to the removal of extractives, which increased the proportions of cellulose and hemicellulose [77]. However, weight loss decreased after the alkalization and bleaching processes due to removal of hemicellulose and lignin. The decomposition temperature range decreased after each extraction step in all stages, since degradation of non-cellulosic components occurred over a low, broad temperature range due to presence of low molecular weight components [91]. Finally, residues that corresponded to ash content decreased during the extraction steps as a result of removal of non-cellulosic matter, which is responsible for ash content [92,93].

Conclusions
The linseed plant is a dual-purpose crop. Even if it is first and foremost cultivated for its seeds, its straw can be also useful, possibly contributing to an additional source of income for farmers as a source of fiber and cellulose due to its comparable bast-fiber and cellulose contents. This study reported optimum extraction of fiber and cellulose, as well as characterization from linseed straw. According to the experiments and analyses performed, pH, stalk water absorption, and weight loss were found to be good indicators for termination time of the water-retting process and optimum retting time. Effects of retting-time duration on tensile and physical properties of the fibers were tested, analyzed, and discussed. At the recommended optimum retting time (216 h), fibers with a density of 1.52 g/cm 3 , a diameter of 104.65 µm, and a moisture content of 8.32% had a mean breaking force of 278.4 cN, a breaking elongation of 2.06%, and a tenacity value of 59.1 cN/tex. The chemical composition of the optimum retted fiber had content of 68% cellulose, 20% hemicellulose, 5% lignin, 4% extractives, and 3% ash. Cellulose was present at the highest levels; therefore, extraction of cellulose from linseed straw is feasible and a promising sustainable cellulose source for different applications, such as packaging, filtration, composites, implants, paper, and pulp. Cellulose is extracted through successful optimization of multi-step extraction-process parameters for linseed straw. The recommended optimum cellulose extraction conditions for linseed fiber were identified as 75% ethanol-toluene at 98 • C for 4 h, 6% NaOH at 75 • C for 30 min, and 6% H 2 O 2 at 90 • C for 120 min, for successful removal of non-cellulosic constituents.