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# Decortication and separation of wet or dry hemp stems - What difference does it make?

Jörg Müssig, Johnny Beaugrand

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Jörg Müssig, Johnny Beaugrand. Decortication and separation of wet or dry hemp stems - What difference does it make?. Industrial Crops and Products, 2025, 223, pp.119965. 10.1016/j.indcrop.2024.119965. hal-05078678

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Submitted on 22 May 2025

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# Decortication and separation of wet or dry hemp stems - What difference does it make?

Jörg Müssig<sup>a,*</sup>, Johnny Beaugrand<sup>b</sup>

## ARTICLE INFO

Keywords:

Hemp decortication

Wet processing

Fibre bundle integrity

Lignin content

Mechanical separation

Fibre damage analysis

## ABSTRACT

Hemp decortication, a crucial step in fibre processing, traditionally uses dry stems with low water content. However, processing hemp in a wet state has gained attention as an alternative method, especially with the development of harvesting techniques that decorticate the stems directly in the field. We investigated the effects of stems' initial moisture content on hemp fibre functional quality using freshly harvested hemp stems from different varieties. The stems were subjected to wet and dry decortication processes to evaluate potential differences in fibre damage and biochemical composition. Our experiments involved utilising a laboratory roller-breaking device for the initial decortication of the fresh hemp stems in their wet state and subsequent mechanical separation to assess fibre bundle integrity. As a comparison, we performed the same procedure on stems that were first dried before processing. We found that wet processing resulted in higher lignin levels in the fibre bundles, containing a higher quantity of middle lamellae, where lignin or another biochemical marker is typically found. The analysis revealed significant damage to the fibre bundles during the wet decortication process, as evidenced by the destruction and fibrillation of the bundles, clearly visible in scanning electron microscope (SEM) images. Interestingly, the dry conditions led to more fractures between fibres, while wet conditions allowed fractures between bundles and the surrounding histological structure. These findings suggest that wet processing may render the material more ductile, resulting in more complex crack modes, including combined torsion and delamination, leading to fracture paths passing through the cross-section of individual fibres. Consequently, the fibre bundles from wet processing may exhibit more structural damage than dry-processed fibre bundles. In conclusion, our study's findings are important as they highlight the severe damage that can occur to the fibre bundles during the decortication and separation of freshly harvested, moist hemp stems.

## 1. Introduction

The influence of harvesting and post-harvesting techniques on the quality of the fibres has been insufficiently considered so far. In the traditional processing form of stems in directional orientation (e.g., in fibre flax; longitudinal flax; compare Müssig et al., 2020), the stems and, thus, the fibre bundles are handled more carefully during the entire process over harvest, post-harvest, separation and processing. In the past, studies on the impact of fibre quality tended to focus on the preparation and processing of the fibres. Here, for example, the work of Ecker (1943) should be mentioned, who dealt intensively with the mechanical influences of the preparation of bast fibres and pointed out the problems of aggressive processing techniques (Ecker, 1943). The methods used for harvesting and post-harvesting the plant stems in the

disordered orientation (disordered hemp) cause a change in the stem orientation and geometry. This can cause damage to the fibres or fibre bundles. As shown in Fig. 1, a correlation between aggressive harvesting techniques and fibre damage could be observed in the first approaches, according to Martens and Müssig (1997) results. Using a combine harvester with a chopper led to damage to the fibre bundles (JD1/Sim) and thus decreased strength in the single element test. Variants 2c, 3c, and JD1 differ only in terms of harvesting techniques under otherwise identical conditions. (Martens and Müssig, 1997). In this context, the work of Gusovius should also be mentioned, who points out the fibre damage caused by aggressive harvesting techniques (Gusovius, 2000). The study by Morgillo et al., (2023) for post-harvest processing reveals that the intensity of separation methods, like scutching and hackling, significantly affects the properties of flax fibres, with more vigorous

<footnote><sup>a</sup> The Biological Materials Group, Biomimetic, HSB – Hochschule Bremen - City University of Applied Sciences, Neustadtswall 30, Bremen 28199, Germany</footnote>

<footnote><sup>b</sup> UR1268 BIA, INRAE Nantes, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: joerg.muessig@hs-bremen.de (J. Müssig).</footnote>
<figure><img src="image_5.png" /><figcaption>**Fig. 1.** Influence of the harvesting technique on the strength of single fibre bundles (adapted from Martens and Müssig, 1997).</figcaption></figure>

processes leading to smaller, more numerous kink bands. These dislocations can alter the fibres' mechanical properties, including reduced cross-sectional area and changes in tensile strength and stiffness.

To be able to investigate the influence of aggressive harvesting techniques in more detail, hemp of the variety Felina 34 was harvested at the experimental site Wehnen (Oldenburg), Germany, of the Weser-Ems Chamber of Agriculture using different harvesting techniques (fertilisation: 100 kg N/ha and sowing rate: 200 seeds/m²) (Martens and Müssig, 1999). The stems were harvested using various techniques, including methods B1 to B5 and C1 (see Fig. 2). The stems were left in the swath on the field. The unretted, dried harvested stems were decorticated in two passages through a lab decorticator (method see: Wang et al., 2018). The mechanically separated fibre bundles were tested for strength in a collective test (Stelometer); results are shown graphically in Fig. 2.

In Fig. 2, variants B1 to B5 do not differ significantly; B3 shows a slightly lower value trend. The results for method C1 are significantly different to all variants. Using an aggressive harvesting technique led to damage to the fibre bundles. In summary, the substantial reduction of the collective strength is primarily due to the damage to the fibre bundles. An additional reduction can be seen in the lower collective efficiency of the C-variant. The results suggest that intensive processing of fresh hemp stems damages the fibre bundles on the one hand and causes inhomogeneous separation on the other.

To investigate this assumption more closely and reproducibly, freshly harvested hemp stems of different varieties were decorticated in a wet and dry state using a laboratory decorticator and then mechanically separated using a coarse separator. The following research questions will be answered within this study:

* How do freshly harvested and dried hemp stems behave in the decortication and separation processes, and what are the differences in mechanical properties of the separated fibre bundles?

* How can mechanical and morphological changes in the fibre bundles be explained with the help of biochemical evaluation?

* How do decortication and separation impact the fibre bundle's structure integrity?

## 2. Materials & methods

### 2.1. Hemp varieties

Eight different varieties of hemp were grown at Wehnen (Oldenburg, Germany) on the trial site of the Weser-Ems Chamber of Agriculture (fertilisation: 100 kg N/ha and sowing rate: 200 seeds/m²). The stems were harvested manually and were not retted. The stems were stripped of the leaves manually. Half of the stems were processed when freshly harvested. The other half was dried in the laboratory at 18 °C and 45 % humidity indoors and then processed.

The following varieties were investigated (S stands for sample):

S1: Fasamo

S2: Fedrina 74

S3: USO 31

S4: Futura

S5: Felina 34

S6: Fedora 19

S7: Kompolti

S8: Lovrin 110

### 2.2. The moisture content of the hemp stems

**"Wet state"**: Amaducci et al. (2000) showed values for the dry matter content of total biomass for hemp, depending on the harvest date. The mean value of the dry matter was 36.6 % for all harvesting times and showed a dependence on the harvesting time. The dry matter content of total biomass for hemp was 30.7 % at HI (harvest time I; 24 August), 37.6 % at HII (harvest time I; 22 September), and 41.7 % at HIII (harvest time I; 17 October). The following values are obtained from 3 trials with Fibror 79 and Futura 75 in Piacenza, Italy, in 2020 and 2021 and from one trial in Aberystwyth (Wales, UK) in 2021. The water content in hemp stems (average of 32 plots for each trial site - 96 data) was 0.625 ± 0.027 at full flowering and 0.576 ± 0.031 at seed maturity. Stem water content is measured as follows: $W_c = (FW - DW) / FW$ (Blandinières et al., 2023).

**"Dry state"**: After storage of hemp stems at approximately 18 °C and 45 % relative humidity and subsequent oven drying for 3 hours at 105 °C, the average moisture content is about 8–9 %.

**"Re-wetted state"**: After drying for two varieties, the stems were moistened again. They were placed in a water bucket, wrapped in moist cotton towels, and kept wet for 24 hours.

### 2.3. Decortication of hemp stems

The laboratory method for decortication of the stems of bast fibre plants is based on the work of Beckmann (1998). This processing method was adapted from flax to hemp stems. From the stems to be tested (mixed sample of hemp stems from field trial) stems with a mass of

<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Influence of harvesting technique on the strength of fibre bundle collectives (harvest year 1997 / date 1). - Values according to Martens and Müssig (1999).</figcaption></figure>
approx. 100 g each were randomly selected twice. A total of two samples of about 100 g were prepared and weighed accurately before the experiment. The stems were fed into the laboratory breaking unit (Flaksy, Bahmer GmbH, Germany) (speed level 10). After each run, the samples were again fed to the breaker rollers with cautious handling (loss of the shives should be avoided). After the second and sixth runs, the mass loss of the stems was determined gravimetrically.

## 2.4. Separation of decorticated bast fibre bundles

After six passes through the laboratory breaker, the decorticated fibre bundles were prepared for separation. The fibre bundles were formed into collectives that were as parallel as possible. The parallel fibre bundles (6 g/m) were fed to the coarse separator (Müssig, 2001). After the first run, the samples were weighed and separated for a second time. Gravimetric determination of mass loss was also carried out after the second run. The fibre mass after two passes through the coarse separator can be related to the mass of the harvested material and the mass of the fibre bundles after six passages through the laboratory breaking unit.

## 2.5. Airflow measurement

Samples were taken at random from the separated bast fibre bundles. Any shives, dust and other non-fibrous components in the sample were carefully separated. The sample was conditioned for 24 hours in a standard climate (20 °C and 65 % humidity), and three specimens with a mass of 2.5 g were taken. The airflow devices for wool (Wira Instrumentation Ltd, Bradford, UK) work with a differential pressure of approx. 18 mbar and a flow rate of 4–24 l/min. The Airflow device used measures the flow rate at constant negative pressure. The calibration and the creation of the calibration curve of the Airflow device were carried out with climate-controlled wool fibre standards at a pressure of 180 mm water column and a sample mass of 2.5 g. After calibration, the bast fibre specimens were measured at a pressure of 120 mm water column and a sample mass of 2.5 g. Each specimen was measured twice. The mean value, the coefficient of variation and the standard deviation were determined from the readings. The mean value in mm rise height was converted into a wool fibre diameter using the calculated wool calibration, which is given without units as FBAI value (FIBRE Bast Fibre Airflow Index).

## 2.6. FMT shirley

The IIC Fineness & Maturity Tester (FMT) from Shirley (Shirley Developments, Ltd., Stockport, UK), which was developed for measuring the maturity of cotton fibres, showed clear advantages when measuring bast fibre bundles (Musio et al., 2018). Any shives, dust, and other non-fibrous components were carefully separated. The sample was conditioned in a standard climate (20 °C and 65 % humidity) for 24 hours; three specimens with a mass of 4 g were analysed. With the FMT device, the pressure drop due to the fibre bundles of varying fineness is measured at a constant flow rate. A flow rate of 4 l/min is set for the sample compression stage $P_L$ (low pressure). The measuring device is adjusted by setting the flow rate when measuring two cotton samples (internal adjustment samples). After adjustment, the bast fibre samples were measured in the first compression stage $P_L$ (Low Pressure) with a specimen mass of 4 g. Each specimen was measured with two repetitions, and the mean value, the coefficient of variation and the standard deviation were determined. The data is given as a unitless FMT-Shirley $P_L$ value.

## 2.7. OFDA – fibre/fibre bundles width

The method for measuring bast fibre bundles with the OFDA100 instrument (BSC Electronics, Ardross, Australia; Hornik Fibertech, Wald,

Switzerland) is based on the investigations of Drieling and Harig (2000) and Drieling (1999). Any remaining shives, dust and other non-fibrous components were carefully separated from the sample. The sample was conditioned for 24 hours in a standard climate (20 °C and 65 % humidity). A total of five specimens were prepared from each sample for the measurement. The fibre bundles were manually parallelised and cut into approx. 3 mm long pieces with laboratory scissors and evenly distributed on an OFDA slide. The OFDA100 device is adjusted using the internal software before measurement. After adjustment, each slide (n = 5) was inserted into the device and measured in a triple scan (in both directions). Due to this procedure, it must be assumed that some fibre bundles and fibres will be counted several times.

## 2.8. Measurement of the collective strength of fibre bundles using a Stelometer

The sample was conditioned in a standard climate (20 °C and 65 % humidity) for 24 hours. Before each series of measurements, the adjustment of the Stelometer (Spinlab Stelometer, Zellweger Uster, Switzerland) must be controlled. The device is levelled by adjusting the strain indicator. Before the test, the condition of the linings of the Pressley clamps must be checked. After adjustment, collectives of fibre bundles were selected to allow tearing in the mass range ($M_S$) between 2 and 7 kg. The fibre bundles were clamped in a clamping block to insert the fibre bundle collectives into the Pressley clamps. The screws of the clamps were fixed with a torque of 103 Ncm. A test was discarded if clamping fractures were detected or slippage of the fibre bundles in the clamps occurred. After the measurement, the mass of the fibre bundle collective was determined. At least 20 usable measurements were made. The fineness-related maximum tensile strength $R_H$ in cN/tex results from the equation to:

$$R_H = \frac{\frac{M_S}{kg}}{\frac{gF}{tex}} \frac{g}{m/s^2} \frac{1}{10} \quad (1)$$

Where $M_S$ is the measured mass to rupture the collective on the Stelometer, gF is the gravimetric fineness of the tested collective, and g is the acceleration due to gravity. The gravimetric fineness (in tex) can be determined from the quotient of the mass of the tested collective in mg divided by the length of the collective in mm (clamping length plus the width of the Pressley clamp).

## 2.9. Measurement of the collective strength of fibre bundles using an Instron device

For this testing method, the Pressley clamps were attached directly to the Instron 4502 universal tensile testing machine (Instron, Norwood, USA) using unique fixtures without a spacer. A 1 kN load cell was used in the tests. The clamping length is variably adjustable and was 3.2 mm for all measurements, and the testing speed was 2 mm/min. The sampling and sample preparation was almost identical to the Stelometer method. The only deviation relates to the insertion of the fibre bundle collective. The collective was aligned with a preload mass of 18.5 g in the form of a fibre clamp before the clamps were closed.

## 2.10. SEM

The separated fibre bundles were examined using a scanning electron microscope (SEM) type (JSM 6510, Jeol, Eching, Germany) with emission electrons at an accelerating voltage of 5 kV. Before SEM investigations, the samples were sputtered with a gold coating.

## 2.11. Biochemical analysis – monosaccharide analysis

Identification and quantification of neutral monosaccharides were performed with gas chromatography after acid hydrolysis and
conversion of monomers into alditol acetates, according to Lahaye et al. (2020). Peak separation and identification were done thanks to GC Chromatography. External standards (glucose, galactose, mannose, rhamnose, xylose, and arabinose) and inositol as internal standards were used for calibration. Uronic acids (glucuronic acid and galacturonic acid) in acid hydrolysates were quantified using the metahydroxydiphenyl colourimetric method (Blumenkrantz and Asboe-Hansen, 1973). All tests were done in triplicate independent samples.

## 2.12. Biochemical analysis – Lignin analysis

The lignin content was quantified in homogenised powdered particles of samples. The lignin content was calculated by spectrophotometry measurements following the acetyl bromide method (Hatfield and Fukushima, 2005). The sample mass was set to a precise mass of approximately 20 mg per assay. All chemicals were laboratory-grade from Sigma Aldrich (St. Louis, USA), and the analyses were performed with three independent measurements. The lignin content is expressed as a percentage of the dry matter.

## 2.13. Statistical analysis

For both biochemical and mechanical analysis, the equality of variance (Fisher test) was verified before a Student's t-test to compare the samples by pair (same numbers of replicates by samples), with a significance level of probability set at p <0.05 using Excel software (Microsoft, Redmond, USA).

# 3. Results & discussion

## 3.1. Fibre bundle separation; wet vs dry – morphology

Freshly harvested hemp stems of different varieties were decorticated in a wet state using a roller breaker and then mechanically separated using a coarse separator. Stems from the same trial were first dried and then processed in the same way. The fibre bundles of the nine varieties from the freshly harvested and dry separations trials were measured concerning fineness (Fig. 3).

All dry-separation varieties have a similar fineness level. The values measured with the Airflow devices and the OFDA values confirm this. The mean values of the finenesses of the fibre bundles from the freshly harvested separation, measured with the Airflow device, are at a similar level to the values of the dry-separated samples. Samples Fedrina 74 and Lovrin 110 (wet stems) were outside the measuring range of the Airflow device. The extreme change in the mean OFDA widths is particularly apparent.

Purely optically, samples of the wet separation show strong inhomogeneities. While some fibre bundles show almost the entire bark structure and are very coarse, other parts are very strongly separated.

<figure><img src="image_7.png" /><figcaption>**Fig. 3.** Influence of processing on the fibre bundle refinement of freshly harvested and dry hemp stems (mean value with standard deviation as error bar).</figcaption></figure>

One explanation for the differences in values from the three measurement techniques could be that the differences in the sample are not noticeable when viewed integrally during the air flow measurement (Airflow and FMT Shirley). In the samples from the wet separation, the observed phenomenon could be explained by an increased generation of separations of fine fibre bundles or single fibres from a very coarse bundle. An observation of the measurement behaviour of the OFDA100 device confirms this assumption (Fig. 4).

Fig. 4 illustrates the problems of measuring very coarse fibre bundles with many branches of fine bundles or single fibres. These finer structures are measured more preferentially in the OFDA100 measurement process than the coarser ones. In the case of very coarse, inhomogeneously separated fibre bundles, the branches of fine bundles or single fibres are mainly measured, while the coarse structures are not or only partially taken into account by the OFDA analyser (Müssig, 2001). This is because of the OFDA's measuring optics, which are designed for wool fibres. Simor (1965) points out similar problems when measuring the fibre bundle fineness with microscopic techniques. More recently, Hamdi et al. (2015) showed that the relationship between fibre diameter and cutting length is complex; larger fibres or bundles tend to show an increase in average diameter with longer cutting lengths, and different analysis techniques (2D vs 3D) can lead to varying interpretations of this relationship.

## 3.2. Fibre bundle separation; wet vs dry – mechanics

In a collective test, the fibre bundles of the wet and dry decortication were tested for their strength (Fig. 5).

As seen in Fig. 5, there are apparent differences between the values of the collective strength of the fibre bundles from the wet and dry separation. The dry-separated varieties are at a similar level for all varieties. The slightly higher values of the Felina and Fedora 19 varieties could be due to higher collective efficiency (lower collective fineness). The apparent differences in collective fineness in the Stelometer test indicate damaged fibre bundles from the wet separation. Stelometer measurements should show values between 2 kg and 7 kg. In the case of strong fibre bundles, as in the case of dry separation, only a few individual bundles are sufficient to reach the lower value with a low collective fineness. If fibre bundles are fragile at a similar fineness, a much higher mass is needed to exceed the lower force limit of the Stelometer. These observations suggest that the fibre bundles from the wet separation are significantly more damaged, confirming the optical observations (Fig. 4). One needs to consider that the higher number of fibre bundles in the collective reduces the collective efficiency. The work of Suh et al. (1994) clearly shows the influence of the number of fibres on the collective strength of cotton fibres. While the collective force increases with an increasing number of fibres in the collective, the collective efficiency decreases. Collectives with a lower mass were measured to check whether the significant decrease in the collective strength of the

<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Snapshot of an OFDA100 measurement of fibrillated fibre bundles (adapted from Müssig, 2001). The white lines show the points where the software has measured the fibre bundles in the present image section.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Influence of processing on the collective strength of the fibre bundles of wet and dry stems measured with the Stelometer. A symbol * means the same variety is significantly different when comparing wet and dry stems (p<0.05) for both collective strength and collective fineness values.</figcaption></figure>

varieties from the wet separation could be explained by the damage or the collective influence. For this evaluation, three samples with a low collective fineness (from values displayed in Fig. 5) were measured using a universal tensile testing machine (see Fig. 6).

The comparison of Figs. 5 and 6 results highlights that despite a substantial reduction of the collective fineness, the collective strength remains similarly low in wet-processed varieties. Thus, it can be considered proven that intensive processing of freshly harvested, wet stems severely damages the fibre bundle structure.

When freshly harvested stems are processed, very coarse, inhomogeneously separated fibre bundles with many branchings of fine bundles and single fibres are produced. The number of splits produced is many times higher than in the separation of dried stems. It was also shown that intensive processing of freshly harvested stems severely damages the fibre bundles. The assumption that the decrease in collective strength is due to a lower collective efficiency could be rejected. It became clear that when testing fibre bundle collectives, the collective fineness and the number of fibre bundles in the collective must be considered when discussing the results.

### 3.3. Fibre bundle separation: wet vs dry vs re-wetted

Trials were carried out on two varieties to investigate whether mechanical separation only causes damage to freshly harvested wet stems or re-wetted once dried stems also shows the same effect. For this purpose, the stems were moistened after drying and treated with the roller breaker and the coarse separator. Subsequently, the samples were examined using a Stelometer. The results are summarised in Fig. 7.

As seen in Fig. 7, the strength values for the variety Futura are below the values of the dry-separated samples but not at the level of the freshly harvested wet-separated stems. For variety Felina 34, the results of the re-wetted samples are more comparable with the samples from the wet separation. Fig. 8 shows that fewer fibre bundles are also needed for an acceptable Stelometer test. The values here are more on the level of the

<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Collective strength of fibre bundles from wet separation measured with the Instron universal tester using Pressley clamps.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Collective strength (Stelometer) of fibre bundles from different forms of separation; wet vs dry vs re-wetted (mean value with standard deviation as error bar). The same letters mean that the values for the same variety are not significantly different at a 95 % confidence level.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>**Fig. 8.** Collective fineness (Stelometer) of fibre bundles from different forms of separation; wet vs dry vs re-wetted (mean value with standard deviation as error bar). The same letters mean that the values for the same variety are not significantly different at a 95 % confidence level.</figcaption></figure>

dry-separated samples.

Overall, it can be stated that re-wetting of already dried stems also favours lighter damage to the fibre bundles during decortication and separation. However, this effect is less pronounced than with freshly harvested wet stems.

### 3.4. SEM

A selection of the SEM examinations is shown in Fig. 9. The severe damage to fibre bundles and individual fibres caused by the separation process of freshly harvested wet stems is striking. The decortication and separation of dry stems lead to coarse fibre bundles with significantly less damage.

### 3.5. Biochemical analysis – Monosaccharide analysis

The total monosaccharide content of the samples can vary significantly according to both the variety and the condition (wet or dry) (Fig. 10). Regarding the variation of monosaccharide levels between varieties, our results are confirmatory with literature (Cappelletto et al., 2001); one can visualise the variable amount between varieties; see, for instance, the Fasamo variety with about 75 % monosaccharide when Fedora displays 85 % of the dry matter.

Looking into the effect of the wet or dry process, there is an apparent effect since 5 of the 8 tested varieties show a statistical increase in their monosaccharide contents for stems processed in a dry state (see Fig. 10). After dry processing, the Fasamo, Futura, Felina 34, Fedora 19, and
<figure><img src="image_13.png" /><figcaption>Fig. 9. SEM micrographs for two selected varieties, Futura and Felina 34; damages of fibre bundles and fibres after wet, dry and dry/re-wetted processing.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 10. Results of the total monosaccharide content of the different hemp samples (mean value with standard deviation as error bar). The symbol * indicates that dry and wet conditions in the same variety are statistically different at the 0.05 confidence level.</figcaption></figure>

Lovrin 110 varieties have higher total monosaccharide levels than wet-processed stems. The lowest total monosaccharide level is shown in the Fasamo sample, which contains only 65 % of the dry matter. Interestingly, three varieties do not display significant variations of total monosaccharide content between the processing of wet or dry stems, namely Fedrina 74, USO 31 and Kompolti, albeit processing in dry condition leads to higher total monosaccharide content, but not significantly at a confidence level of 95 %. The glucose monomer is the predominant monosaccharide in the hemp bundle (see Fig. 11) and is often assimilated to the cellulosic content in bast fibres.

For the following discussion, we assume that all the glucose monomers are cellulose, even if it is a rough approximation. This simplification seems acceptable as it is known that there is a positive relationship between mechanical properties and glucose content (Nuez et al., 2020) and that cellulose is the polymer of reinforcement of the cell wall (Fratzl, 2003). Here, we have cellulose that is predominantly between 60 % and 65 % of the dry mass, with lower values found in Fasamo wet with values as low as 55 %. The highest values are monitored for Felina 34 and Fedora 19 dry samples, with cellulose contents of about 70 % of the dry matter mass.

The cellulose content in fibre bundles from wet-processed stems is lower for 5 of the 8 varieties examined, the same is shown in Fig. 10 for the total monosaccharide content. It is evident that cellulose is the major source of content variation between monosaccharide samples.

Fig. 12 shows the difference in “non-cellulosic” polymer by removing the cellulose (Glc) that masks the others from the graph. In Fig. 12, rhamnose, galactose, and glucuronic acid do not vary between dry or wet processing conditions from the same varieties, seemingly not impacted by the processing moisture content arabinose, which is around 1–1.3 % of the dry matter mass varies significantly for two varieties, USO and Fedora respectively (see the * mark in Fig. 11). Regarding mannose, one can see that 3 varieties are impacted in their content between conditions, Futura, Felina and Lovrin. To close the analysis, two monosaccharides, xylose and galacturonic acid, vary significantly for 5 of the 8 varieties according to dry or wet processing. Fasamo, Felina, Futura, Fedora and Lovrin are the most impacted varieties.

Previous reports monitored a relationship between xylose and glucose in hemp shives, which in turn impacts nanoindentation modulus (Beaugrand et al., 2014); for glucose, so cellulose, the role in mechanical properties seems obvious, whereas xylose is hypothesised to be involved
<figure><img src="image_15.png" /><figcaption>**Fig. 11.** Distribution of monosaccharides of the different hemp samples. The symbol * means a significant variation of the glucose (Glc) content at a confidence level > 95 %.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>**Fig. 12.** The presentation of the cumulated monosaccharide content of the hemp samples focuses on the monomer distribution without the glucose, which is highly predominant. The symbol * means that the monosaccharides close to the symbol are statistically different at the 0.05 confidence level for the same variety from dry and wet conditions.</figcaption></figure>

in the structural polymer matrix that embedded the cellulose fibril. The fact that those two monosaccharides are identified to be significantly impacted by moisture level during processing suggests that the moisture content impacts the fibre's cell wall ultrastructure differently. More damage is visible on SEM pictures with wet processing (see Fig. 9).

Mannose and rhamnose have been reported to correlate to nanoindentation modulus in hemp bast fibre thanks to a high Person coefficient correlation (Padovani et al., 2019); here, this monosaccharide does not evidence any correlation at the bundle scale.

Galacturonic acid oscillates between 3 % and 4 %; this uronic acid in bast fibre, often referred to belongs to the pectic type I rhamnogalacturonan (RGI) (Gautreau et al., 2022; Zhang et al., 2013) decorated with β (1–4) galactan side chains. Those pectins are known to be located both in the secondary wall (Gorshkova and Morvan, 2006) and the compounded middle lamella (Richely et al., 2021; Chernova et al., 2018), which give the cohesion of bundles. Interestingly, five wet proceeded samples displayed higher pectin content (galacturonic acid) than the dry ones. Given rise to the hypothesis that when moistened, the fracture location more often passes through the secondary cell wall (rich in glucose and xylose, Fig. 10) instead of breaking the middle lamella, rich in galacturonic acid, which could mean that more middle lamellae

remain in the wet state than in the dry state.

Two varieties were retained for an extra investigation where the dry stems were re-wetted before decortication and separation, namely, Futura and Felina 34 (Fig. 13). The total monosaccharide (sum of all monosaccharide) is significantly different for the three conditions wet, dry, and dry/re-wetted (see Fig. 13 upper letters, B or C) of the same variety at 95 % confidence.

In detail, the glucose content of the dry and re-wetted samples is closer to that of the wet conditions samples. In contrast, dry samples show significantly higher glucose in bundles, 60–67 % for Futura and 62 and 70 % for Felina 34 (see Fig. 13, lower letters). Two monosaccharides exhibit different contents in Futura and Felina34 re-wetted conditions compared to the two others, namely glucose and galacturonic acid (see letter "c" in Fig. 13). In addition, Futura shows a third monosaccharide content that significantly differs from the dry or the wet conditions, namely mannose.

### 3.6. Biochemical analysis – Lignin analysis

Besides the monosaccharides, lignins are polymers well known and investigated in lignocellulosic biomass due to their protective role at the
<figure><img src="image_17.png" /><figcaption>**Fig. 13.** Comparison of each monosaccharide of three conditions: wet, dry, and dry/re-wetted. A different upper case letter (A, B or C) means that the total monosaccharide amount is significantly different for the same variety at 95 % confidence, and lower case letters (a, b or c) mean that each monosaccharide differs from the wet content, for Futura and Felina 34 respectively.</figcaption></figure>

scale of the plant and the hardness of the cell wall (Lu et al., 2022) and cross linkages with polysaccharides (Terrett and Dupree, 2019) that impact stress transfer to the cellulose reinforcement part (Spies et al., 2022), possibly during processing as we did in this study.

Even a relatively low amount of lignin is reported in hemp bast bundles (Placet et al., 2017) compared to woody shives; however, the lignin location is related to the compounded middle lamellas that connect single fibres in bundles, as galacturonic acids did. The lignin content of the extracted bundles from wet and dry processed stems is shown in Fig. 14, with values in agreement with the literature.

If one looks at the processing condition, 6 of the 8 wet-processed stems display a higher lignin content in their bundles than the dry conditions; only Fasamo and Fedrina do not follow this trend and are identical. It should be recalled that the stems were not retted, so the amount of lignin could differ significantly according to this bio-processing step (Placet et al., 2017). To explain the trend of the lignin amount in the wet-processed samples being higher than in dry conditions, we can arguably hypothesise that because lignin is reported to be mostly concentrated in the middle lamella, this thin layer made of amorphous polymers can be differently preserved during wet or dry processing. Here, the higher lignin values (6 of the 8) suggest that more middle lamella remain after processing of wet-processed bundles. This is arguable, as the monosaccharide examination suggests more damage in individual fibre cell walls than in compound middle lamella (CML). In

Fig. 3, the comparison of fibre bundle fineness between the Airflow and Shirley methods shows minimal differences, suggesting that both techniques yield similar results in terms of fineness. However, the Optical Fibre Diameter Analyser (OFDA) measurements indicate the fibrillation of fibre bundles following wet decortication and separation. This observation suggests that the fibre bundles likely lose lignins as dust during the decortication and separation of the dried stems. In contrast, when separating wet stems, the fibre bundles undergo a different process where the lignins adhere to the fibrils, fibres and bundles. This indicates that the processing method (dry vs wet) notably impacts the fibre structure. Finally, the fractography observation also suggests more breakages occurring in individual fibres than in the CML.

One must recall that after harvesting, the stems were dried in the laboratory and re-moistened, decorticated, and separated in a moist state. Looking at the lignin content in the dry stems of two varieties, namely Futura and Felina 34, that were dry and then re-wetted before decortication and separation, one can see in Fig. 15 that oppositely to monosaccharides (Fig. 12), there is more lignin content in re-wetted samples compared to dry processed stems samples. The lignin content here is higher than in the stems decorticated and separated freshly harvested for both Futura and Felina. However, re-wetted Felina bundles extracted harboured even more lignin than in the wet condition. It appears that less lignin is removed by decortication and separation for wet, dried and remoistened stems. The same considerations can be made

<figure><img src="image_18.png" /><figcaption>**Fig. 14.** Lignin content in the hemp bundle samples, mean value with standard deviation as error bar is expressed as a dry matter percentage. The symbol * means a significant variation of the lignin content between wet and dry stems processing at a confidence level > 95 %.</figcaption></figure>
<figure><img src="image_19.png" /><figcaption>**Fig. 15.** Comparison of lignin content of the three conditions: wet, dry and rewetted. Bars are mean values with standard deviation as error bars expressed as a dry matter percentage. The symbol * means a significant variation of the lignin content between wet and dry stems processing at a confidence level > 95 %.</figcaption></figure>

here as for the wet-processed stems.

Another explanation that can run concomitantly is that the non-lignin component, here monosaccharide, might have been removed from bundles to a greater extent during processing for dry conditions. Here, the fractographic examination (see Fig. 9) shows qualitatively a similar degree of surface damage between Futura and Felina re-wetted when the bundles obtained by dry conditions are less opened and less damaged. The severe damage to the fibre bundles and the fibrillation of the fibre bundles from the wet-processed stems can be seen in Fig. 16.

Wet processing has changed the local mechanical properties of the CML and the fibre cell wall in such a way that cracks and fractures (delamination between fibres in bundles by the CML and fragmentation by the fibre wall) occur. The higher the positive correlation, the higher the CML should be. Fig. 17 shows a correlation between lignin and the galacturonic acid reflecting pectin (RGI).

## 4. Conclusion

Table 1 summarises the most important findings of this investigation

<figure><img src="image_20.png" /><figcaption>**Fig. 17.** Lignin and galacturonic acid contents in the hemp fibre bundles. Each dot corresponds to a wet, dry, dry, then rewetted hemp sample analysis.</figcaption></figure>

in a simplified manner to facilitate the analysis.

This study has analysed how different processing methods, specifically wet and dry processing, impact the structural and biochemical properties of hemp fibre bundles. The findings demonstrate that dry processing produces more uniform and less damaged fibre bundles with higher cellulose content and greater mechanical integrity. In contrast, wet processing, including re-wetting after drying, significantly compromises the strength and structural cohesion of the fibre bundles, primarily due to increased fibrillation and retention of lignin and pectin within the fibre bundles.

The structural damage observed in wet-processed fibre bundles, as confirmed by Scanning Electron Microscopy (SEM), highlights the severity of the impact, with cracks and fractures occurring more frequently within the fibre cell walls and the compound middle lamella (CML). The glass transition temperature of the different polymers is undoubtedly involved in these differences, as the Young's modulus of the polymer in a glassy or rubbery state can be significantly reduced below or above the T<sub>g</sub> (Salmén, 1990).

The differential preservation of biochemical components such as lignin and galacturonic acid further supports the notion that wet

<figure><img src="image_21.png" /><figcaption>**Fig. 16.** SEM micrographs for two selected varieties, Fedrina 74 and Fedora 19; damages of fibre bundles and fibres after wet and dry processing.</figcaption></figure>
<table><caption>**Table 1**
Summary of the investigations and interpretation, on average, of the 8 varieties tested.</caption><thead><tr><th rowspan="2"></th><th colspan="2">Mechanical investigations</th><th>Fractography</th><th colspan="3">Biochemical investigations</th></tr><tr><th>Fibre bundle fibrillation</th><th>Fibre bundle collective strengths</th><th>Fibre bundle surface damage</th><th>Monosaccharide content<br>Cellulose<br>(fibre marker)</th><th>Pectins (CML marker)</th><th>Lignin content<br>(CML marker)</th></tr></thead><tbody><tr><td>Dry as reference</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td><td>-</td></tr><tr><td>Wet</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td></tr><tr><td>Re-wetted</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td><td>✓</td></tr></tbody></table>

processing alters the mechanical properties of the fibres, leading to greater susceptibility to damage. Biochemical investigation reveals that varieties are also affected, with various intensities depending on the variety. Cellulose content is lower after decortication and separation in wet conditions. Interestingly, as a marker of the fibre secondary wall, this reinforcement polymer is lower, suggesting more damage generated in the bundle during decortication and separation in wet conditions. In contrast, galacturonic acid, related to the pectin of the CML that cemented the fibres together in bundles, is higher. Altogether, this suggests that wet decortication and separation destroy and lead to the loss of more fibre structure rather than breaking the CML when wet. A second marker of CML indicates that a higher amount of CML is lost during wet processing. The rate or extent of variation in the bundle fibrillation and bundle strength seems higher than the delta observed in biochemical components.

Wet processing has changed the local mechanical properties of the CML and the fibre cell wall in such a way that cracks and fractures (delamination between fibres in bundles by the CML and fragmentation by the fibre wall) occur. Overall, the moist fibre bundles behave more ductile than the dry, and more complex crack shapes with possibly more combined torsion/delamination lead to fracture paths running through the cross-section of individual fibres, e.g., at the interface of constitutive sub-layers. The moist decorticated and separated fibre bundles show significantly more severe damage and fibrillation. While wet processing may offer certain advantages in fibre separation, the trade-off regarding reduced fibre strength and integrity cannot be overlooked. The rewetting process, while less detrimental than fresh wet processing, still results in noticeable fibre damage, suggesting that moisture content plays a crucial role in determining the final quality of the fibres.

Overall, this research emphasises the importance of selecting appropriate processing methods based on the desired application of the hemp fibres. For applications where high-strength fibres are essential, dry processing remains the preferable method. However, wet processing may be refined and optimised in future research to minimise fibre damage and harness the full potential of hemp fibres for diverse industrial applications.

In conclusion, while traditional dry processing is currently the best method to produce high-quality hemp fibres, ongoing research is essential to develop improved wet-processing techniques that could offer a viable alternative without compromising fibre quality.

### CRediT authorship contribution statement

**Johnny Beaugrand:** Writing – review & editing, Writing – original draft, Visualization, Validation, Resources, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Jörg Müssig:** Writing – review & editing, Writing – original draft, Visualization, Validation, Resources, Methodology, Investigation, Formal analysis, Data curation, Conceptualization.

### Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

We want to thank Reent Martens for providing the hemp samples and carrying out the cultivation trials. We thank Jonas Baumann for his support with the SEM images. The authors thank Méline Calatraba and Lèna Brionne (INRAE, BIA) for their skilful assistance in biochemical analysis.

### Data availability

Data will be made available on request.

### References

Amaducci, S., Amaducci, M.T., Benati, R., Venturi, G., 2000. Crop yield and quality parameters of four annual fibre crops (hemp, kenaf, maize and sorghum) in the North of Italy. Ind. Crops Prod. 11 (2-3), 179-186.

Beaugrand, J., Nottez, M., Konnerth, J., Bourmaud, A., 2014. Multi-scale analysis of the structure and mechanical performance of woody hemp core and the dependence on the sampling location. Ind. Crops Prod. 60, 193-204.

Beckmann, A. (1998). Methoden zur Messung physikalischer Eigenschaften von Industriefaser-Lein (Flachs) und damit verstärkten Kunststoffen. PhD thesis at the RWTH Aachen, Faculty of Mechanical Engineering, Aachen, Germany 1998, Forschungsbereich Agrartechnik des Arbeitskreises Forschung und Lehre der Max-Eyth-Gesellschaft Agrartechnik im VDI (VDI-MEG) 326 (ISSN 0931-6264), in German.

Blandinières, H., Croci, M., Impollonia, G., Marcone, A., Gay, A., Winters, A., Amaducci, S., 2023. Multi-environment assessment of a yellow hemp (Cannabis sativa L.) cultivar's eco-physiology and productivity under varying levels of nitrogen fertilisation. Ind. Crops Prod. 195, 116360.

Blumenkrantz, N., Asboe-Hansen, G., 1973. New method for quantitative determination of microgram quantities of uronic acids. Anal., Biochem 54, 484-489.

Cappelletto, P., Brizzi, M., Mongardini, F., Barberi, B., Sannibale, M., Nenci, G., Pasini, P., 2001. Italy-grown hemp: yield, composition and cannabinoid content. Ind. Crops Prod. 13 (2), 101-113.

Chernova, T.E., Mikshina, P.V., Salnikov, V.V., Ibragimova, N.N., Sautkina, O.V., Gorshkova, T.A., 2018. Development of distinct cell wall layers both in primary and secondary phloem fibers of hemp (Cannabis sativa L.). Ind. Crops Prod. 117, 97-109.

Drieling, A. (1999). Probleme bei der Charakterisierung von Festigkeit, Feinheit und Länge von Bastfasern unter Berücksichtigung verschiedener Aufschlussverfahren. Faserinstitut Bremen – FIBRE –, Bremen, Germany, Research report, FK Textil e.V. / AiF 11083, 1999.

Drieling, A. and Harig, H. (2000). Methoden zur reproduzierbaren Feinheitsmessung an Bastfasern. 6. internationaler Fachkongress für nachwachsende Rohstoffe, NAROSSA 2000 (Magdeburg, Germany, 5. - 6. June 2000), proceedings.

Ecker, J., 1943. Betrachtungen zu den mechanischen Einflüssen bei der Aufbereitung, von Bastfasern. Allg. Text. -Z., 1. Jahrg. (1943), Heft 7, S 207-212.

Fratzl, P., 2003. Cellulose and collagen: from fibres to tissues. Curr. Opin. Colloid Interface Sci. 8 (1), 32-39.

Gautreau, M., Durand, S., Paturel, A., Le Gall, S., Foucat, L., Falourd, X., Beaugrand, J., 2022. Impact of cell wall non-cellulosic and cellulosic polymers on the mechanical properties of flax fibre bundles. Carbohydr. Polym. 291, 119599.

Gorshkova, T., Morvan, C., 2006. Secondary cell-wall assembly in flax phloem fibres: role of galactans. Planta 223, 149-158.

Gusovius, H. 2000: Zum Einfluss von Ernteverfahren und Witterung auf ausgewählte Qualitätskennwerte von Faserhanf. Veranst.: Faserinstitut Bremen, Vortrag zum Seminar Qualitätssicherung in der gesamten Wertschöpfungskette - (Bremen: Baumwollbörse 8. Juni 2000), im Rahmen des EUADAPT Projektes Marktinnovation Hanf, MIH-Fachvorträge.

Hamdi, S.E., Delisee, C., Malvestio, J., Da Silva, N., Le Duc, A., Beaugrand, J., 2015. X-ray computed microtomography and 2D image analysis for morphological characterization of short lignocellulosic fibers raw materials: a benchmark survey. Compos. Part A: Appl. Sci. Manuf. 76, 1-9.

Hatfield, R., Fukushima, R.S., 2005. Can lignin be accurately measured? Crop Sci. 45 (3), 832-839.

Lahaye, M., Falourd, X., Laillet, B., Le Gall, S., 2020. Cellulose, pectin and water in cell walls determine apple flesh viscoelastic mechanical properties. Carbohydr. Polym. 232, 115768.
Lu, C., Wang, C.H., Li, C.H., Tong, J.F., Sun, Y.F., 2022. Structural and Mechanical Properties of Hemp Fibers: Effect of Progressive Removal of Hemicellulose and Lignin. J. Nat. Fibers 19 (16), 13985-13994.

Martens, R., Müssig, J., 1999. Integration heimischer Pflanzenfasern in technische Anwendungen – Konzepte eines Qualitätsmanagements. Vortrag im Innovationszentrum „Ingenieur-Werkstoffe“, Werkstoff- Forum, Hannover-Messe 1999. Düsseldorf: Springer-VDI-Verlag GmbH 1999, S. Sonderpublikation, pp. 21–22.

Martens, R., Müssig, J. 1997: *Qualität von Hanffasern auf der Basis unterschiedlicher Beerntungs- und Bergetechniken*. NRW-Hanfttag 1997 / Hanf & Co. Rohstoffe für eine nachhaltige Entwicklung (Bad Sassendorf 28. August 1997). Landwirtschaftskammer Westfalen-Lippe, Projektbüro Hanf.– Tagungsdokumentation S. 21 – 33.

Morgillo, L., Brionne, L., Melelli, A., Ouagne, P., Scheel, M., Weitkamp, T., Bourmaud, A., 2023. Elucidating links between the mechanical performance of flax fibres and their structural defects. Ind. Crops Prod. 206, 117722.

Musio, S., Müssig, J., Amaducci, S., 2018. Optimizing hemp fiber production for high performance composite applications. Front. Plant Sci. 9, 1702.

Müssig, J., Amaducci, S., Bourmaud, A., Beaugrand, J., Shah, D.U., 2020. Transdisciplinary top-down review of hemp fibre composites: From an advanced product design to crop variety selection. Compos. Part C: Open Access 2, 100010.

Müssig, J. (2001). Untersuchung der Eignung heimischer Pflanzenfasern für die Herstellung von naturfaserverstärkten Duroplasten - vom Anbau zum Verbundwerkstoff. VDI Verlag GmbH, Düsseldorf, DE, (Fortschritt-Bericht VDI, Reihe 5, Grund- und Werkstoffe / Kunststoffe, Nr. 630), (ISBN 3-18-363005-2).

Nuez, L., Gautreau, M., Mayer-Laigle, C., d'Arras, P., Guillon, F., Bourmaud, A., Beaugrand, J., 2020. Determinant morphological features of flax plant products and their contribution in injection moulded composite reinforcement. Compos. Part C: Open Access 3, 100054.

Padovani, J., Legland, D., Pernes, M., Gallos, A., Thomachot-Schneider, C., Shah, D.U., Bourmaud, J., Beaugrand, A., 2019. Beating of hemp bast fibres: an examination of a hydro-mechanical treatment on chemical, structural, and nanomechanical property evolutions. Cellulose 26, 5665-5683.

Placet, V., Day, A., Beaugrand, J., 2017. The influence of unintended field retting on the physicochemical and mechanical properties of industrial hemp bast fibres. J. Mater. Sci. 52 (10), 5759-5777.

Richely, E., Durand, S., Melelli, A., Kao, A., Magueresse, A., Dhakal, H., Guessasma, S., 2021. Novel insight into the intricate shape of flax fibre lumen. Fibers 9 (4), 24.

Salmén, L., 1990. On the interaction between moisture and wood fibre materials. MRS Online Proc. Libr. (OPL 197, 193.

Simor, P. (1965). Faserfeinheits- und Spaltbarkeitsmessung an Flachs mit dem Micronaire-Apparat. Spinner, Weber, Textilveredelung, (ISSN 0038-7541), (1/ 1965), p. 29 – 33.

Spies, P.A., Keplinger, T., Horbelt, N., Reppe, F., Scoppola, E., Eder, M., Rüggeberg, M., 2022. Cellulose lattice strains and stress transfer in native and delignified wood. Carbohydr. Polym. 296, 119922.

Suh, M.W.Cui, X.Sasser, P.E. (1994). New Understanding on HVI Tensile Data based. on Mantis Single Fiber Test Results. Beltwide Cotton Conferences (January 5 - 8). San Diego, CA, USA, 1994.- Proceedings Vol. 3 of 3, p. 1400 – 1403.

Terrett, O.M., Dupree, P., 2019. Covalent interactions between lignin and hemicelluloses in plant secondary cell walls. Curr. Opin. Biotechnol. 56, 97-104.

Wang, S., Gusovius, H.J., Lühr, C., Musio, S., Uhrlaub, B., Amaducci, S., Müssig, J., 2018. Assessment system to characterise and compare different hemp varieties based on a developed lab-scaled decortication system. Ind. Crops Prod. 117, 159-168.

Zhang, J., Pakarinen, A., Viikari, L., 2013. Synergy between cellulases and pectinases in the hydrolysis of hemp. Bioresour. Technol. 129, 302-307.