Conversion of Waste Agricultural Biomass from Straw into Useful Bioproducts—Wheat Fibers and Biofuels

Abstract

Straw, the primary agricultural waste, constitutes approximately 20% of the total biomass in the EU. Only a small fraction of the material is applied in various products, e.g., animal bedding, mulch, building, and composite materials, while a significantly larger portion is often burned in the field. This practice, while prohibited for several reasons, including the increased risk of fire and the release of carbon dioxide contributing to global warming, is still prevalent. Given the increasingly evident effects of climate change, EU legislation aims to reduce greenhouse gas emissions as much as possible. One of the strategies includes applying the cascade principle in the circular economy. This principle aims to use the entire raw material, in this case, cereal crops, such that the products with the highest added value, like cellulose fibers from cereal straw, are extracted first. The vast potential for utilizing lignocellulosic agro-waste sustainably arises from its status as the most abundant organic compound on Earth. Its significant presence, renewability, and biodegradability make it a desirable source for producing materials in numerous industries. This study examines the potential of wheat fibers, isolated from the straw of two distinct cultivars (Srpanjka represents an old variety, and Kraljica represents the new variety) primarily for application in technical textiles. The following testing methods were applied: determination of wheat fibers and residues yield, fibers tensile properties, length, moisture content/regain, density, morphology, and Fourier transform infrared (FTIR) spectroscopy. The yield of isolated fibers relies on the wheat variety and the climatic conditions affecting plant growth, resulting in fiber yields from 10.91% to 15.34%. Fourier transform infrared (FTIR) analysis indicates reduced peak intensity, which is related to hemicellulose and lignin content, suggesting their improved deposition following the process of chemical maceration. Wheat fiber quality was found to be comparable to cotton fibers regarding its density. However, they showed a significant difference in higher moisture regain (9.72–11.40%). The vast majority of the scientific papers related to wheat fibers did not indicate the length of the individual fibers obtained by chemical maceration nor their strength. Therefore, this paper indicated that both varieties demonstrated sufficient fiber tenacity (greater than 10 cN/tex) and fiber length (2–3 cm), stressing the spinning potential of these fibers into yarns and extending their use to the apparel industry. Moreover, our research underscores the feasibility of adhering to the zero-waste principle. A high percentage of solid waste remaining after fiber extraction (25.3–39.5%) was successfully used for biofuel production, thus closing the loop in the circular economy.

1. Introduction

In the realm of research, sustainable practices have emerged as a fundamental focus, with scientists actively pursuing renewable, sustainable, and biodegradable sources of textile fibers and materials. Environmental consciousness among the people is growing, and so is the increasing pollution of our planet, which is disturbing the equilibrium of its ecosystems. The production of synthetic fibers, which make up approximately 60% of global fiber output (

Table 1. Global fiber production [1].

Fiber	Total Production (%)
Cotton	22
Other plant fibers	5.9
Wool	3
Other animal fibers	0.71
Polyester	54
Polyamide	5
Polypropylene, acrylics, and elastane	5.2
Manufactured cellulosics	64

), has contributed to environmental imbalance, especially with the dominance of polyester (PET) and polyamide (PA).

Based on the textile exchange compilation data, global fiber production has significantly increased from 58 million tons in 2000 to 113 million tons in 2021, and it is estimated that in 2030, total fiber production will grow to 149 million tons [1]. This increase has an enormous effect on the environment, especially on the increase in synthetic fiber production since they are manufactured by using fossil fuel resources. Therefore, its production, distribution, consumption, and waste management process induce greenhouse gas emissions, usage of non-renewable resources, and release of unwanted particles into the environment [2]. Textile materials crafted from manufactured materials possess the potential to release microplastics into the world throughout production and cleaning procedures [3,4], which is a hot topic and has led to a call for more sustainable solutions. As a result, natural fibers have gained popularity as an affordable, wholesome, and ecological option. Environmental sustainability entails striking a harmonious equilibrium between fulfilling human needs and preserving nature, with global efforts focusing on achieving this equilibrium.

Given the magnitude of contamination and waste on the Earth, innovative findings are imperative, driving the exploration of more ecological alternatives for numerous purposes. One of the key solutions involves utilizing biowaste from cereal straws, regarded as second-generation biomass. Wheat (Figure 1), the world’s second-largest grain crop, holds significant intrigue and is widely accessible.

Over the past decade, Europe accounted for more than 30% of the world’s wheat production, while Asia contributed 44%, and North and South America produced 15% [5]. Wheat cultivation is widespread globally, thriving in various climate conditions. Today, the significance of this crop may escalate further thanks to its biowaste potential [6]. Following harvest, substantial quantities of agro-residues, such as wheat straw, are often either abandoned in fields, burned, or left unused, which negatively impacts the environment and endangers ecosystems [7,8,9,10]. Wheat straw, a crucial component of waste lignocellulosic biomass, comprises substantial cellulose content, approximately ranging from 30% to 50% [11,12,13], making it a promising resource of cellulose when compared to other more prominent options (

Table 2. Cellulose content in some plants [14,15,16,17].

Plant (Stalk/Straw)	Cellulose Content (%)	Reference
Cotton	40.1	[17]
Kenaf	40.2	[17]
Flax	68	[14]
Hemp	70	[16]
Bamboo	48	[16]
Corn	42.2	[15]
Barley	47.1	[15]
Rye	52	[15]
Rice	38.3	[14]

).

Leveraging this resource as a renewable source of cellulose fibers presents a more sustainable strategy for mitigating substantial volumes of agro-residue. This type of biomass stands as a novel discovery in our pursuit of a balanced future and holds immense capability as a valuable raw material for diverse purposes, including the production of pulp and paper, bioplastics, textiles, and thermoelectric power [18,19,20,21,22]. The literature review, presented in

Table 3. Review of literature focusing on the usage of wheat straw.

Description	References
Wheat straw infused with animal glue as reinforcement in composite materials	[38]
Wheat fibers isolated by mechanical process and microbial retting (fiber length is up to 4 mm)	[45]
Wheat straw as reinforcement in composite materials (chopped to the size 18–20 cm, 5 to 15 mm, and 2.5 cm; milled to the particle size up to 1 mm)	[31,32,33,35]
Wheat straw fiber-based hand sheets (unbleached wheat straw fibers, fiber length < 0.58 mm)	[39,43]
Wheat straw fibers as building material (milled and sieved straw, particle size 0.3 mm, and chopped straw up to 32 mm in length)	[23,24]
Milled wheat straw as reinforcement for biocomposites.	[40]
Nanocellulose derived from wheat straw as reinforcement in composite materials	[34,41]
Milled and sieved straw (fiber/particle size 75 to 300 µm) as a soil stabilizer	[25]
Chopped and grounded wheat straw for pellet production	[26]
Wheat cellulose nanofibers as superabsorbent material	[29]
Crushed wheat straw as a source of cellulose fibers	[42]
Wheat nanocellulose as nanoadsorbent for metal ions removal from wastewater	[30]
Cellulose fiber isolation from the dried and milled wheat straw powder (particle size < 1 mm)	[44]
Wheat cellulose pulp for paper production	[37]

, revealed the usage of wheat straw as reinforcement for construction materials [23,24]. Furthermore, it can be used for biofuel production and soil stabilization [25,26,27]. Alternatively, it can serve as a raw material for nanocellulose fabrication, which can then be utilized as a wastewater cleaner [28,29,30]. Wheat cellulose fibers are already used in the composite and paper industry [31,32,33,34,35,36,37] and, upon modification, can be used for very different purposes [38,39,40,41,42,43,44].

Cellulose fibers can be isolated or extracted from the plant stem via various methods, including biological or chemical retting, mechanical and/or physical processes, as well as combinations thereof [46,47,48,49,50]. Chemical retting methods are most often used to isolate fibers from straws, typically followed by mechanical and physical procedures. These methods involve harsh conditions in terms of used chemicals, temperature, and pretreatment time, among other factors [51]. The optimal utilization of short fibers typically lies within the industry of paper. The potential of lengthy fibers is higher in the composite industry, where they can be used as reinforcement. Furthermore, they can be used in apparel if their length meets the spinnability requirements [52]. Within this study, our research group strives to point out a few environmental difficulties, like the overproduction of synthetic fibers, large amounts of agro-waste left on the fields, or insufficiently high production of energy from renewable sources. One of the possible solutions to overcoming these challenges is cereal straw biomass exploitation. It has been proven as a satisfactory renewable source for cellulose fiber isolation and biomass fuel production. As far as the authors know, in the literature [6,10,11,12,15,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44], this is the first study on the full utilization of leading wheat varieties in Croatia. In this paper, a diminished quantity of alkali substance was employed during the retting process to maintain the integrity and quality of the fibers. Furthermore, the comprehensive procedure of isolating long fibers from domestic wheat straw, along with the collection of both firm and fluid waste within the chemical maceration process, gives significant rise to a circular economy. Once the wheat grain is utilized for food purposes, the remaining agricultural waste (straw) serves as a resource for fiber production. Concurrently, the firm remains generated after the fiber isolation procedure, coupled with the evaporated filtrates, are repurposed as input materials for the production of solid biofuel. With the imposition of legal restrictions within the EU prohibiting the burning of agricultural waste in open fields, the utilization of wheat straw as a renewable source of cellulose fibers (bast fibers) emerges as a more sustainable and eco-friendlier alternative. The study delves into the potential of different wheat varieties, namely Srpanjka and Kraljica, in the context of fiber and biofuel production. The efficient extraction of wheat straw fibers (longer compared to the wheat fibers and their length found in the literature) [37,39,43,45] and their subsequent application in the fashion industry or for the creation of technical textiles while concurrently ensuring the total utilization of the residue from the fiber extraction process, introduces sustainable and an innovative bioproducts.

2. Materials and Methods

2.1. Wheat Variety

Kraljica wheat (The ‘“new” variety) is one of the leading medium early growing winter varieties. The stem is approximately 70 cm long and is resistant to lodging and exhibits a solid and elastic structure. It shows high grain yield potential and adaptability to various production conditions. Moreover, it shows tolerance to the most common diseases [53,54]. Srpanjka wheat (An “old” variety) is a very early-growing winter cultivar with a lower habitus of approximately 64 cm. It is characterized by low height and demonstrates high resistance to both low temperatures and diseases [54,55]. The wheat straw utilized was grown at the premises of the Osijek Agricultural Institute within the 10-month growth period (October to July).

2.2. Pretreatments and Fiber Isolation of Biomass

A chemical retting process was employed, which was a slightly modified version of a process sourced from an existing review of the literature [56]. The wheat biomass was segmented into 12 cm pieces and exposed to a chemical interaction within a bath containing 3% w/v sodium hydroxide (NaOH). The bath was maintained at a ratio of 1:20, and the treatment duration spanned 90 min. Further intricacies of the experiment are elaborated upon in a prior publication [57]. A comprehensive explanation of the experimental process is presented in Figure 2.

2.3. Testing of Fibers

The conducted evaluations included measurements of fiber and residue content after the fiber isolation procedure, tensile properties, fiber length, moisture content, moisture regain, and density. Furthermore, the study examined the morphology of the isolated wheat straw fibers and conducted tests to determine their chemical composition.

Calculation of percentages for fiber and residue yield (Yf% and Yr%) was carried out using the gravimetric method as detailed in Equations (1) and (2) [14]. Measurements were conducted threefold for accuracy and reliability.

$$Yf\%=\left(\frac{mf}{mi}\right)\times 100$$

(1)

$$Yr\%=\left(\frac{mr}{mi}\right)\times 100$$

(2)

In this context, Yf% denotes the percentage yield of fiber, mf symbolizes the weight of wheat fibers isolated from the wheat straw, and mi is an indicator of the initial mass of the wheat straw. Likewise, Yr% is a representation of the percentage yield of the residue, and mr signifies the weight of wheat residues left after fiber isolation.

The mechanical properties related to tensile properties of different varieties of wheat fibers were scrutinized utilizing the apparatuses Vibroskop 500 and Vibrodyn 500 manufactured by Lenzing Instruments. Fifty (50) fibers for each variety were measured using the following parameters: preload (1500 mg), testing velocity (3 mm/min), and gauge length (5 mm).

The lengths of a hundred fibers (individual) from old and new varieties were measured with the help of a scale aligned parallel to the unstretched fibers in their straightened form.

The process of determining both the moisture regain and content began by calculating the mass of the sample after the sample had been air-dried. Afterwards, the sample was placed in a climatic chamber and subjected to standard atmospheric conditions for a period of 24 h. Post-conditioning, the sample’s mass was calculated, followed by a drying period of another 24 h. The moisture regain and content were calculated according to Equations (3) and (4). Testing was carried out in triplicate to ensure precision and consistency.

$$MC\%=\left(\frac{m1-m2}{m1}\right)\times 100$$

(3)

$$MR\%=\left(\frac{m3-m2}{m2}\right)\times 100$$

(4)

where MC% represents moisture content, MR% represents the moisture regain, m1 represents the mass of an air-dried sample, m2 represents the completely dried sample’s mass, and m3 represents the conditioned sample’s mass.

The sample density was evaluated using the Ultrapyc 1200e gas pycnometer, provided by Anton Paar. The density of the wheat fibers was established under atmospheric conditions aligning with the ASTM standard [58]. Nitrogen gas (N₂) of high purity was utilized due to its capability to seep into the minuscule pores, thus augmenting the precision of the measurement. These measurements were carried out threefold to guarantee accuracy.

The structural formation of the fibers was examined using the Mira II LMU Scanning Electron Microscope (SEM), manufactured by Tescan. Before the SEM analysis, the fibers underwent chrome coating to improve the conductivity of the sample. SEM observations were performed at 5 kV electron voltage acceleration.

The surface chemistry of the wheat (straw and fibers) was investigated using a Spectrum 100 FTIR spectrometer, provided by Perkin Elmer, deploying the attenuated total reflection (ATR) method. All spectra were recorded within the range of 4000 cm⁻¹ to 380 cm⁻¹, maintaining a resolution of 4 cm⁻¹ over four scans. The mean value of five individual measurements was applied. Normalization of spectra was performed at 1159 cm⁻¹, which is the area of C-O-C asymmetrical stretching.

3. Results and Discussion

The initial phase of the experiment focused on defining the fiber yield and residue yield for two varieties: Srpanjka and Kraljica. The study explored the impact of climate conditions over a span of two years (2021 and 2022).

3.1. Pretreatment of Biomass

In biomass pretreatment, the most used chemical is NaOH, commonly referred to as caustic soda. It is a multipurpose chemical which finds applications across many sectors, including the textile industry. Indeed, in the textile industry, it is primarily associated with the mercerization process. However, it serves multiple purposes, including printing, dyeing, neutralization, alkaline scouring, and cleaning [59]. Certainly, NaOH is extensively utilized for textile fiber isolation from agricultural (lignocellulosic) biomass. Alkaline treatment with NaOH is renowned for effectively removing hemicellulose and lignin, thereby facilitating the release of cellulose fibers. This process concurrently enhances the physico-chemical and mechanical properties of the cellulose fibers [60,61,62].

However, NaOH also has a negative impact on the environment due to its strong alkaline character. Hence, it is advisable to minimize the concentration of NaOH to ensure optimal outcomes in the chemical isolation process of textile fibers [63]. In this study, the concentration of NaOH was decreased to 3% w/v. The findings demonstrated that applying this more environmentally friendly method yielded satisfactory results even now, with fiber yields ranging from 10% wt. to 15% wt.

3.2. Fiber and Residue Yield

This research aimed to examine how distinct wheat sorts affected fiber and residue yield. Technical textile usage required the extracted fibers, while the solid residues were intended for biofuel production, with yields ranging from 25.3% wt. to 39.5% wt. Results presented in Figure 3a,b showed that Srpanjka (the old sort/variety) collected in 2021 had the lowest yield of fiber (10.91% wt.), while its yield of solid residue was 36.70% wt. On the other hand, the same variety collected in 2022 demonstrated the highest yield of fiber (15.34% wt.), albeit with a lower residue yield of 25.34% wt.

For the new variety Kraljica, the trend was the opposite, with slightly higher fiber yield in the 2021 harvest (12.60% wt. compared to 11.55% wt.) and higher residue yield for the 2022 harvest (39.54% wt. compared to 37.23% wt.). Although the variability in fiber and residue yield was highest for the Srpanjka variety from the harvest in 2022, two-way ANOVA analysis indicated insignificant differences among fiber and residue yields within the wheat variety and harvesting year (Fcritical > Fstatistical and p-value > 0.05).

Figure 4 presents the total precipitation recorded from the sowing to harvest period (October to July) based on data published by the Croatian Meteorological and Hydrological Service for the Osijek region. The precipitation value of 459.5 mm of the 2022 harvest, which was characterized by more drought, was significantly lower compared with the 2021 harvest (548.8 mm) [64].

Considering different wheat sorts and climatological data spanning from 2020 to 2022 (as depicted in Figure 4), our analysis showed that the Kraljica variety demonstrated an upwards trend with increased yields of residue and fiber. This trend persisted in spite of the adverse effects of climatic changes, notably the rise in occurrences of dry periods in the climate cycles.

3.3. Tensile Properties

The mechanical properties of various wheat fiber varieties were investigated in terms of tensile properties, which included Young’s modulus, elongation, and breaking tenacity measurements (refer to

Table 4. Mechanical properties of wheat fibers.

	Srpanjka (2021)			Srpanjka (2022)			Kraljica (2021)			Kraljica (2022)
	T [cN/tex]	YM [cN/tex]	E [%]	T [cN/tex]	YM [cN/tex]	E [%]	T [cN/tex]	YM [cN/tex]	E [%]	T [cN/tex]	YM [cN/tex]	E [%]
Average	20.69	396.82	4.81	21.10	273.48	6.00	25.66	460.71	5.35	25.28	405.52	5.52
SD	5.98	221.17	1.25	8.49	185.69	1.19	10.53	328.63	1.21	11.25	287.50	1.55
CV [%]	28.93	55.73	25.94	40.22	67.90	19.88	41.04	71.33	22.65	44.51	70.90	28.14
SE [%]	1.66	61.31	0.35	2.35	51.47	0.33	2.92	91.09	0.34	3.12	79.69	0.43

T represents breaking tenacity, YM is a label for Young’s modulus, E stands for elongation, SD is standard deviation, CV is the coefficient of variation, and SE represents the standard error.

). The Kraljica variety from the 2021 harvest had a higher fiber-breaking tenacity than the Srpanjka variety, with a maximum fiber tenacity of 25.66 cN/tex. According to the literature [52], fibers with a minimum tenacity of 10 cN/tex to 25 cN/tex are spinnable into yarn. Nevertheless, fibers with strength falling within the minimum range must exhibit a robust capability to endure deformation [52], a criterion met by all the tested samples.

The Srpanjka fibers from the 2022 harvest had the lowest Young’s modulus values compared to other tested fibers. However, they remained spinnable owing to their softness, high cohesion forces [65], and rough surface, as proved via SEM analysis. The variability of natural fibers properties, particularly the variation in diameter along their length, has been observed to influence tensile properties [46,66]. Various factors contribute to this variability, including the method of fiber isolation, type of fiber, distribution of pore size, strain rate, gauge length, and number of measured fibers, among others. Consequently, this leads to elevated statistics (e.g., coefficient of variation and standard deviation) and, thus, a high degree of variability in results.

Table 5. Mechanical properties of wheat fibers compared to mostly used natural fibers.

Tensile Properties	Cotton	Flax	Hemp	Jute	Spanish Broom	Wheat	Barley	Sisal	Coir
Tensile Strength (MPa)	287–597	345–900	300–800	200–800	500–1100	190–380	190–380	100–800	13–220
Young’s Modulus (GPa)	5.5–12.6	27–80	30–70	10–55	15–20	3–7	3–10	9–28	4–6
Elongation (%)	3–10	1.2–1.6	1.3–1.6	1.4–1.8	3–9	4–6	3–7	2–3	15–40
Reference	[67]	[68]			[65]	This study	[57]	[68]

Tensile strength and Young’s modulus values are derived from the mechanical properties and estimated on the wheat fibers’ circular cross-section shape.

outlines the mechanical properties (tensile properties) of representative cellulose fibers in each category, encompassing fibers from seed, stem, leaf, and fruit.

Table 5 shows the tensile properties of different seed, stem, leaf, and fruit fibers, in comparison to the investigated wheat fibers. It was noted that wheat fibers’ tensile strength is in a similar range compared to the other presented plants with the maximum similarity with cotton.

3.4. Length of Fibers

Continuing with the analysis of fiber properties, the study focused on isolating cellulose wheat straw fibers and assessing their suitability for their use in technical textiles manufacturing, particularly biocomposites. This research was carried out as part of project KK.05.1.1.02.0016 BIOCOMPOSITES. The study selected the Srpanjka and Kraljica wheat varieties, which are known for their stems reaching heights of up to 75 cm [69,70]. A chemical retting was employed using a 3% w/v NaOH solution to extract fibers. While existing literature often discusses the production of wheat straw fibers in the form of milled straw [32,33,40] and wheat cellulose pulp for nanofiber production [29,30,34,41,71], there is limited scientific research on the isolation of “long” wheat fibers more suitable for reinforcing composite materials [11,72,73,74].

Natural fibers utilized as reinforcements in composite materials have been classified into two length categories: short (1–5 mm) and long (5–50 mm) [75]. The critical fiber length is a crucial property affecting the strength of fiber-reinforced composites, which differs based on fiber type [76]. Insufficient strength in the composite may occur if the fiber length falls below the critical length, hindering effective stress transfer between two applied components. Moreover, an abundance of short fibers within the composite can lead to a high quantity of free fiber ends, increasing the likelihood of cracks under stress [77]. Conversely, if the fiber length exceeds the critical length and the composite’s strength remains unsatisfactory due to excessive intertwining of “long” fibers during composite manufacturing, it can result in the deterioration of mechanical properties owing to poor fiber distribution within the polymer matrix [75].

The research utilized wheat straw fibers obtained via a chemical retting procedure in an alkaline medium. This process resulted in technical fibers comprised of fiber bundles, which are formed of ultimate (elementary) fibers, as observed in the SEM images. Prior to the chemical maceration process, the straw was cut to lengths ranging from 10 to 12 cm. Among the studied Srpanjka varieties, those harvested in 2021 exhibited the highest fiber length of 2.95 cm, while those harvested in 2022 displayed the lowest average fiber length (2.30 cm), alongside the greatest inconstancy in results (Figure 5 and

Table 6. The lengths of technical fibers, which are isolated from various wheat sorts, gathered over a two-year period.

	Srpanjka (2021)	Srpanjka (2022)	Kraljica (2021)	Kraljica (2022)
Average [cm]	2.95	2.30	2.76	2.38
Standard deviation [cm]	0.86	0.83	0.76	0.78
Coefficient of variation [%]	29.08	35.91	27.49	32.86
Standard error [%]	0.17	0.16	0.15	0.15

). Two-way ANOVA analysis indicated a noticeable length means difference, depending on the harvesting year (Fcritical < Fstatistical and p-value < 0.05), while no significant difference was observed among length means within the wheat variety (Fcritical > Fstatistical and p-value > 0.05). Various factors, such as the plant growth stage, anatomy and composition, its genetic characteristics, environmental conditions, and the pretreatment processes for fiber isolation, can influence the length of cellulose fibers [78]. Based on these findings, it can be inferred that the weather conditions during the growth period, particularly the drier conditions experienced in 2022 compared to 2021, had a negative impact on the fiber length.

3.5. Moisture Regain and Moisture Content

Natural fibers are characterized by their richness in hydroxyl groups, making them highly hygroscopic, which significantly impacts their moisture content and absorption of water [79]. Swelling and water sorption are intricate processes that depend on the biochemical, structural, and morphological features of the fiber, including lumen size, cellulose crystallinity, microfibrillar angle, accessibility to reactive groups, amounts of amorphous regions, and their relative character (hydrophilic or hydrophobic) [80]. The moisture content depends both on the hemicellulose and lignin quantity and the share of amorphous cellulose sections within the fiber [75,81]. It should also be noted that a wide combination of genetic, environmental, and processing factors impact the hemicellulose content in natural fibers [82]. In order to optimize the hemicellulose content in natural fibers for various industrial and commercial applications, it is important to understand and control these factors. High moisture content, as one of the drawbacks of natural fiber properties, can adversely affect the mechanical properties and dimensional stability. Natural fibers are biodegradable, and the increase in the water content can improve their biodegradability [75]. In

Table 7. Moisture content from the aspect of the harvesting year.

	Moisture Content of Wheat Fibers [%]
	Srpanjka (2021)	Srpanjka (2022)	Kraljica (2021)	Kraljica (2022)
Average [%]	8.93	6.58	9.61	7.21
Standard deviation [%]	0.13	0.02	0.02	0.07
Coefficient of variation [%]	1.43	0.30	0.25	0.98
Standard error [%]	0.14	0.02	0.03	0.08

, the moisture content of isolated wheat fibers is presented, revealing that the moisture content was minimal in 2022 for both wheat varieties, for Srpanjka 6.58% and for Kraljica 7.21%. It is worth noting that a content of moisture below 10% is considered favorable, as it requires less energy for drying and allows for easier storage maintenance [83,84].

Table 8. Moisture regain from the aspect of the harvesting year.

	Moisture Regain of Wheat Fibers [%]
	Srpanjka (2021)	Srpanjka (2022)	Kraljica (2021)	Kraljica (2022)
Average [%]	11.18	9.72	11.40	11.10
Standard deviation [%]	0.21	0.05	0.04	0.19
Coefficient of variation [%]	1.86	0.48	0.39	1.68
Standard error [%]	0.24	0.05	0.05	0.21

displays the standard moisture regain of isolated wheat fibers. The minimal regain was observed in both sorts of wheat fibers—Srpanjka and Kraljica—from 2022, amounting to 9.72% and 11.10%, respectively.

An increase in moisture regain of up to 11.40% was observed for the Kraljica variety from 2021. The values of the wheat fibers met the moisture regain characteristic for natural fibers, as indicated in

Table 9. Physical properties of wheat fibers from this study compared to the most commonly used natural fibers.

Fiber Category	Fiber	Diameter (µm)	Length (mm)	Moisture Regain (%)	Density (g/cm3)	Reference
Seed fiber	Cotton	10–22	12–64	8.5	1.55	[85,86]
Stem fiber	Flax	40–600	5–900	7	1.4–1.5	[65,68]
	Hemp	10–500	5–56	8	1.3–1.6
	Jute	25–200	1.5–120	12	1.4–1.8
	Spanish broom	10–200	5–900	8	1.55–1.6	[65]
Straw/stem fiber	Wheat	10–350	10–60	9–12	1.4–1.6	This study
	Barley	10–350	5–100	10–11	1.4–1.5	[57]
Leaf fiber	Sisal	8–200	900	11	1.2–1.5	[68,85]
Fruit fiber	Coir	10–460	20–150	13	1.1–1.4

. Statistical analysis was performed using two-way ANOVA. The difference between both moisture content and moisture regain mean values is noticeable due to the fact that Fcritical is lower than Fstatistical, while the p-values of both applied variables (variety and harvesting year) are lower than 0.05.

Table 9 demonstrates the physical properties of representative fibers from seed, stem, leaf, and fruit categories, which are characteristic for the classification of plant fibers. The results indicated that the characteristics of wheat fibers closely corresponded to those of stem or fruit fibers. This suggests that wheat fibers possess properties desirable for possible usage for composite materials reinforcement. However, when taking into consideration the utilization of wheat fibers isolated from straw as reinforcements, their hygroscopic nature must be taken into account since it can adversely impact the mechanical properties of the final product. Moisture infiltration into the structure of cellulose, primarily within the non-crystalline regions, can lead to the swelling of fibers and the formation of micro-cracks within the composite [87]. The moisture quantity present in the fibers can also affect the adherence between the hydrophilic fibers and the hydrophobic matrix, thereby detrimentally impacting the mechanical properties of the composite [84,87]. A lower regain is indicative of superior adherence between the two components. Moreover, the moisture regain also influences overall material properties, including thermal effects, dimensional attributes, and electrical properties [88,89].

3.6. Density of Fibers

The density of fiber is a critical property of textile fibers with a significant impact on their application, especially when used as composite materials reinforcements. For instance, in the automotive industry, lightweight products are essential as they contribute to reduced fuel consumption and expenses [90]. The investigated wheat fibers exhibited density values from min. 1.4178 g/cm³ (Kraljica from 2021) to max. 1.5063 g/cm³ (Srpanjka from 2022), all falling within the typical density range of natural fibers, which is 1.2 g/cm³ to 1.6 g/cm³ [91]. Mean values of fiber density from various sorts of wheat gathered from sequential years present noticeable statistical differences (Fcritical < Fstatistical and p-value < 0.05). The tested fibers were ranked according to their weight: Kraljica (2021) < Kraljica (2022) < Srpanjka (2021) < Srpanjka (2022). Following the two-way ANOVA analysis, the statistically noticeable difference was determined in fiber density influenced by variety and year of the harvest (p-value < 0.05).

Figure 6 presents the wheat fiber density which is in direct correlation with fiber volume. The volume of the fibers depends on their chemical structure, which encompasses factors such as the cellulose and other main component content, as well as their crystalline or amorphous nature. It can be inferred that the Srpanjka variety exhibited a reduced fiber volume due to fiber shrinkage stimulated by the more effective extraction of lignin and hemicellulose, a phenomenon not observed in fibers from the new Kraljica variety. The chemical pretreatment of wheat stems under alkali conditions resulted in a reduction and an increase in their volume and density, respectively [66,92,93,94].

Even though all the tested wheat fibers fit the role of being composite material reinforcements, Kraljica variety fibers would result in a vaguely decreased final product mass. This leads to improved energy and economic efficiency, particularly in industries such as automotive manufacturing.

3.7. Fiber Morphology

Yu et al. [95] presented in their research a cross-section of wheat straw where they distinguished main tissues consisting of the epidermis, parenchyma, vascular bundles, and lumen. Vascular bundles are composed of phloem, xylem, and sclerenchyma [96]. In our research, wheat fibers were extracted via a low-concentration NaOH process, resulting in the incomplete removal of lignin and hemicellulose. The study conducted by Chen et al. [97] unveiled the impact of varying NaOH concentrations on the fibers’ microstructure. They found that concentrations lower than 5% had negligible influence on fibers’ microstructure.

The microstructure of wheat fibers isolated from two different varieties of wheat (Srpanjka and Kraljica), using SEM is presented in Figure 7 and Figure 8. Micrographs captured at 2000× magnification compare ultimate fibers isolated from wheat straw of different varieties and harvest years. Figure 7 depicts SEM micrographs of fibers isolated from Srpanjka wheat straw from the 2021 and 2022 harvests, while Figure 8 shows SEM micrographs of fibers isolated from Kraljica wheat straw from the 2021 and 2022 harvests.

The micrographs reveal the morphology and geometrical characteristics of the wheat fibers, such as their non-uniform surface relief and cross-sectional shape. The fibers appear to be relatively smooth, with fine surface features which are representative of most fibers isolated from plant stems. The technical fibers observed in the micrographs consist of ultimate fibers, which are part of the sclerenchyma tissue of wheat straw and are composed of cellulose. The thickness of the ultimate fibers’ cell walls depends on their location within the straw. Ultimate fibers closer to the epidermis have thicker walls (coarse fibers), while fibers located in the middle of the straw have much thinner walls (fine fibers).

Both varieties from 2022 show a more pronounced separation of the elementary fibers within technical fibers. The rough surface caused by cellulose microfibrils is noticeable in all samples, and can be seen as nodes noticeable along the elementary fibers. Those non-uniform surface features are characteristic of bast fibers [82]. Such a non-uniform fiber surface is favorable for composite materials applications where the matrix is reinforced with fibers. The higher quality of adhesion and interlocking between the polymer matrix and the fiber is a very desirable consequence because it decreases the transfer of stress between two composite components [82,98]. The chemistry of cellulose [99] is of great importance since interfacial forces between cellulose and other substances can be adjusted in a way that leads to many industrial applications like the manufacturing of smart composite materials or the design of novel papermaking production strategies [100].

3.8. Chemical Composition

FTIR

The isolated fibers of wheat varieties Srpanjka and Kraljica were analyzed by the ATR-FTIR (Figure 9 and Figure 10) where the impact of maceration with chemicals was assessed by characteristic cellulose, hemicellulose, and lignin peaks [45,57,65,101,102,103].

A broad absorption peak that appears between 3200 and 3400 cm⁻¹ was associated with the -OH group. Peaks ranging from 2918 cm⁻¹ to 2850 cm⁻¹ were associated with the -CH₂ and -CH groups of cellulose, hemicellulose, pectin, fats, and waxes [65,104]. The wheat fibers exhibited a high-intensity peak of the free -COOH groups of polygalacturonic acid at 1743 cm⁻¹ and 1733 cm⁻¹ for Srpanjka and Kraljica, respectively. This peak was associated with pectins, the primary constituent of which is polygalacturonic acid. Hence, the reduction in the intensity of this peak observed within the wheat fibers suggests the extraction of pectin and the beneficial impact of chemical maceration [105,106]. A characteristic peak at an absorption band ranging from 1640 to 1660 cm⁻¹, associated with water adsorbed and arising from -H bonding in the cellulose macromolecules amorphous region, was observed in the wheat fibers [107]. The removal of hemicellulose post-chemical maceration resulted in a decreased intensity of this peak in fibers from the Kraljica variety, changing the organization of cellulose inner chains forming a greater amount of crystalline area and therefore enhancing strength of fibers [65]. Both wheat straws samples exhibited typical lignin bands at 1574–1605 cm⁻¹, 1540–1550 cm⁻¹, and 1505–1515 cm⁻¹, which were absent or had lower intensity in the wheat fibers in Figure 9 and Figure 10. This indicated the removal of lignin after the chemical maceration process [102,103]. Similar results were achieved at lignin bands 1235–1240 cm⁻¹ and 836–840 cm⁻¹, where the peak in the region 1235–1240 cm⁻¹ was shifted to the value of 1263 cm⁻¹ in the case of fibers. The elimination of lignin was confirmed, indicating good efficiency of chemical maceration. After chemical maceration, the fibers exhibited a higher presence of guaiacyl (G) moieties, typically associated with softwood species, whereas syringyl (S) moieties were diminished. This was concluded from the peaks at 1230–1240 cm⁻¹ and 1263 cm⁻¹ in the FTIR spectra [29,96,108].

The absorption peak at 897 cm⁻¹ represents the cellulose amorphous area [65,109], while the band 1420–1430 cm⁻¹ represents the cellulose crystalline structure. The peaks at 1463 cm⁻¹, 1368 cm⁻¹, 1333 cm⁻¹, 1317 cm⁻¹, and 1203 cm⁻¹ aligned with the bending and stretching of -CO, -CH2, -CH, -OH, and C-O-C cellulose and hemicellulose bonds [109]. The wheat fibers showed peaks between 985 and 1159 cm⁻¹, which corresponded well with the other cellulose fibers. However, the peaks appeared only as shoulders in the wheat fiber FTIR spectra, indicating less development of the secondary cell wall, which has a negative impact on their mechanical performance [65,110].

In general, chemical maceration in a mild alkaline medium induces the dissolution of lignin, hemicellulose, and other extractives, while cellulose is more resistant under alkaline conditions and undergoes minor degradation. After dissolution, hemicellulose is usually more dissolved than lignin. Intermolecular hydrogen bonds between cellulose molecules are broken, causing the swelling of cellulose [111]. At the same time, bonds between lignin monomers or between lignin and other polysaccharides, which are susceptible to alkali, are broken. The ester-linked substituents of the hemicelluloses and phenolic monomers are broken as well. During the first phase of delignification, non-core lignin, which accounts for about 20% of total lignin, is hydrolyzed upon the cleavage of β-O-4 ether bonds in poly-phenolic units or cleavage of ester bonds between the carboxyl group and lignin [111,112]. Therefore, FTIR results have confirmed the satisfactory efficiency of the applied retting process due to the fact that the pretreatment solution with decreased NaOH concentration still belongs to the lowest region of the mild alkaline range required to implement the above-mentioned mechanism.

4. Conclusions

To promote the circular bioeconomy, this study focused on examining the properties of fibers derived from two wheat varieties of straw. Such fibers were extracted via a chemical retting process under mild alkali conditions, employing a low NaOH concentration to promote an environmentally benign process. The objective was to utilize the isolated fibers for the biocomposite materials for technical textile purposes.

The best fiber yield results (15.34%) were achieved via the Srpanjka variety harvested in 2022. Although there was significant variability in the results of the new Kraljica variety, a positive tendency in fiber (11.55–12.60%) and residue (37.23–39.54%) yield was noticed. This is an important consideration, especially in light of the adverse impacts of climate change, such as the escalating occurrences of drought.

The tenacity of the fiber was slightly higher in the Kraljica variety from both harvests (approx. 25 cN/tex) compared to the Srpanjka variety (approx. 21 cN/tex). Both wheat varieties from 2021 and 2022 harvests showed a fiber tenacity greater than 10 cN/tex, suggesting the potential for spinning these fibers into yarn.

The fiber length parameter was notably affected by the wheat harvest year. The fiber lengths of wheat fibers harvested in 2021 (averaging between 2.76 and 2.95 cm) were the longest. Analysis of the length distribution revealed that tested fibers exhibited the best frequency results within the 2–3 cm range, apart from both 2021 varieties, which displayed the most noticeable frequency in the 1–2 cm length range. These fiber lengths demonstrated satisfactory values for utilization as composite materials reinforcements.

The moisture values were found to be significantly influenced by the feedstock variety and the harvest year. The content of moisture within the fibers extracted from 2022 harvest straw ranged from 6.58% to 7.21%, suggesting effective storage conditions and reduced energy expenditure. All measured fibers exhibited a regain of moisture within the 9.72% to 11.40% range, aligning with bast fibers. Fibers demonstrating low moisture regain are more efficient when deployed for reinforcement in composite material due to enhanced adherence between both composite components.

Fiber density ranged from 1.4178 g/cm³ to 1.5063 g/cm³, aligning with typical values for natural fibers. Both wheat variety fibers are suitable for use as composite materials reinforcements; utilizing fibers from the newly introduced Kraljica variety may lead to a slightly lighter final product. This improvement in energy and economic efficiency could be advantageous, particularly in the automotive industry.

Analysis of the fiber morphology disclosed irregular geometric characteristics on their surfaces, a common feature among natural fibers derived from plant stems. Higher roughness of the fiber surface is better when these fibers are employed as reinforcements in composite material, as their textured topography enhances adherence among the natural fibers and polymer matrix.

The composition of the isolated fibers was indirectly examined by FTIR, which suggested a decrease in lignin and hemicellulose peak intensities, indicating their effective elimination by using environmentally efficient chemical pretreatment.

This study forms part of a broader investigation into the potential utilization of lignocellulosic biomass both for producing fibers and biofuels. Therefore, our future actions will have an emphasis on incorporating wheat cellulose fibers as reinforcement into the composite materials, while a significant percentage of the solid waste remaining after fiber isolation will be used to produce biofuels of high quality.