Recovery of Vegetable Fibers from Licorice Processing Waste and a Case Study for Their Use in Green Building Products

Abstract

The present research is aimed at the recovery of vegetable fibers from licorice root processing waste through simple methods that do not involve the use of chemical reagents to guarantee a complete eco-sustainability approach and for their use in the production of fiber-reinforced ecomaterials. The waste was treated through several washing cycles with only water at different temperatures to identify the optimal conditions to obtain clean fibers. The clean fibers and the waste were analyzed and characterized in advance by scanning electron microscopy (SEM), microanalysis (EDS) and thermal analysis (DSC). Subsequently, both the clean fibers and the waste were used to produce fiber-reinforced plaster artifacts. The mechanical properties of the artifacts were measured as a function of % clean fibers or untreated waste. The results obtained showed that it is possible to effectively recover clean vegetable fibers from licorice waste through repeated washing cycles of 30 min with only water. By increasing the temperature, the necessary washing cycles decrease, and a good compromise is five washes at 100 °C. The yield of clean fibers compared to waste is 50%. The creation of prototypes of gypsum matrix panels, which incorporate fibers recovered from licorice processing waste through the methodology tested in this study, has also been successfully realized, representing a significant step forward towards practical applications in the field of eco-friendly construction.

1. Introduction

In recent decades, environmental pollution has become a global problem that threatens human health, biodiversity and ecosystems. The growing sensitivity towards these issues has led several governments to issue laws and regulations to protect the environment and promote strategies for a circular economy [1]. To address environmental pollution, research is moving towards studying strategies and new technologies for reducing the use of toxic substances and promoting clean energy [2,3,4]. Interest is directed to the study and preparation of new materials that can be used for the removal of pollutants from ecosystems thanks to their peculiar properties such as photocatalysis [5] and adsorption [6,7,8]. Ecomaterials represent an important class of materials that respect the principles of environmental balance and are often made using recycled materials. These materials have a low environmental impact and are studied and designed to reduce the impact on the environment during their production and use [9]. Therefore, materials research is increasingly focused on the production of green or bio-based materials and their composites [10,11,12,13].

Lignocellulosic biomass has become one of the preferred raw materials for these new materials since cellulose and lignin are the most abundant biopolymers on the planet. Lignin is a phenolic macromolecule, the main biological source of aromatic structures, with a complex structure that varies depending on the plant species and its isolation process [14]. A significant amount of lignocellulosic waste is produced in various industries, ranging from food to furniture production or disposal of plant waste [15,16,17]. One possibility of reuse [18] particularly concerns the production of fibrous waste that is abundant in the agri-food sector. From these wastes it is possible to extract lignin that allows to increase the possibilities of its use towards the production of new materials [19]. In particular, the root of licorice (Glycyrrhiza) has a very long tradition of use for the extraction of licorice juice. It is processed in various areas of the world, particularly in the Calabria region of southern Italy [20]. It is mainly used in the food industry to produce sweeteners and flavorings, in the tobacco industry and as a source of saponin [21] for its anti-inflammatory action [22]. The uses in both traditional and herbal medicine are particularly linked to the inhibitory effect of the action of Helicobacter pylori, thus functioning as a liver protector, due to the flavonoids present in the licorice root [23]. More recently, the incorporation of licorice extracts into food packaging films with antioxidant activity based on soy proteins has been suggested [24]. Another option explored very recently has been the use of licorice as a source for cellulose nanofiber (CNF) materials: this has shown some potential, although it involves a considerable transformation of the residue and a limited yield, also due to the not very high cellulose content in the raw material [25,26,27] and a large amount of hemicellulose [25,28]. Glycyrrhizin extraction is often performed using an ethanolic solution at high temperatures [29]. The high market value of licorice [30] represents an additional reason to seek the best possible reuse of the waste that comes from its processing.

Santulli et al. reported a study in which fibers were recovered from licorice processing waste, using alcoholic solutions, and used to produce fiber-reinforced materials in epoxy resin matrices [27]. The aim of this research was to study possible strategies for the valorization and use of waste from licorice processing, produced in industries in the Calabria region of southern Italy. In particular, the ideal conditions were sought to obtain clean fibers from the waste through the simplest possible methods that do not involve the use of chemical reagents in order to support a coherent process of eco-sustainability. Finally, both the clean fibers and the as-made waste were used and compared for the production of fiber-reinforced products with a gypsum matrix for possible use in green building.

Considering that the yield of extracted licorice juice is approximately 20% compared to the quantity of root used, it is clear that the production of waste is significant given that the production of licorice alone in southern Italy is hundreds of tons per year.

2. Materials and Methods

2.1. Materials Used

The waste used comes from an industry in southern Italy in the Calabria region, which processes the root of the “Glycyrrhiza glabra typica” (Figure 1a,b) for the extraction of pure licorice. The manufacturing processes require that the roots of the plant, once washed, defibrated and chopped, are pressed in water through a boiling process to extract the aromatic components of the licorice. This results in black juice that is strained and, once dried, is ready to be used in confectionery companies and beyond. At the end of the processing process, the waste used in this research remains (Figure 1c).

2.2. Extraction and Cleaning of Fibers Through the Thermal Process

To extract the fibers from the waste, a water treatment process was planned at different temperatures (25; 50; 75; 100 °C) to remove the residual substances of the processing and other impurities. In particular, a waste/distilled water ratio of 30 g/L was used. The system was placed under magnetic stirring and brought to the predetermined temperature and maintained for 30 min. It was then filtered, and 1 L of clean distilled water was added again, brought back to the predetermined temperature and thus starting a subsequent cycle. The cycles were repeated until the water appeared clear. Finally, the fibers were recovered by filtration and dried on absorbent paper for 48 h at room temperature. To untangle and increase the insulation of the obtained fibers, these were subjected to manual carding carried out with brushes with steel nails. The carding process was repeated until well-insulated and soft fibers were obtained (Figure 2).

2.3. Preparation Gypsum Matrix Samples for Mechanical Tests of Flexural Strength

Once the clean fibers were extracted from waste, they were used to make samples of fiber-reinforced products with a gypsum matrix. In particular, the samples were made both directly with the waste, which contains the untreated fibers (waste), and with clean fibers obtained through different washing cycles (see Section 3.2). The dimensions of the samples are 4 × 4 × 16 cm. The initial mixtures were formulated using a constant water/gypsum ratio equal to 0.57 and varying the percentage of waste or clean fibers, equal to 0%, 1.5% and 3% with respect to the total weight (water + gypsum) (

Table 1. Percentage composition of mixtures water/gypsum (W/G) for samples with clean fibers and waste as made.

Mixtures	W/G [%wt]	Clean Fibers * [%wt]	Waste * [%wt]
I	0.57	---	---
II	0.57	1.5	---
III	0.57	3.0	---
IV	0.57	---	1.5
V	0.57	---	3.0

* Compared to the total weight of gypsum + water.

). The choice of these percentages to be used in the formulation of the systems was suggested by preliminary tests: percentages lower than the one chosen (1.5%) presupposed a too small quantity of fibers, while for percentage quantities higher than the one selected (3%) the quantity of fibers or waste was too high to allow adequate workability.

The following

Table 2. Quantity by weight (g) of typical mixtures for samples with clean fibers and waste as made.

Mixtures	Gypsum [g]	Clean Fibers [g]	Waste [g]	Effective Quantity of Water * [g]
I	140	---	---	80.00
II	140	3.3	---	87.75
III	140	6.6	--	95.51
IV	140	---	3.3	90.79
V	140	---	6.6	101.58

* With addition of the amount of water absorbed by the fibers in reference to the previously calculated imbibition coefficient (80 + 235% clean fibers and 80 + 327% waste).

shows the compositions in grams of the mixtures. In calculating the actual quantity of water to be used, the quantity of water absorbed by the clean fibers and the waste was added, in order to keep the workability of the mixture constant. The latter was calculated with reference to the imbibition coefficient of the treated fibers and the waste previously determined (see Section 3.3.2) (Table 2).

2.4. Instruments

The waste and fibers were characterized with different chemical–physical techniques such as scanning electron microscopy, microanalysis (ESEM Quanta 200 FEG + EDS EDAX GENESIS 2000, FEI company, Eindhoven, The Netherlands) and Scanning Calorimetry Thermal Analysis (DSC, Shimadzu, Kyoto, Japan).

The mechanical characteristics of the samples were conducted with press (INSTRON 5582, Instrom corporation, Norwood, MA, USA).

3. Results

3.1. Characterization of Waste

3.1.1. Macroscopic Characterization of Waste

The waste is composed of masses of materials of different sizes. The fibers are visible to the naked eye and are mixed with another phase that acts as a glue between them (Figure 1c). The waste placed in cold water does not easily lose its compactness and the resulting system has a weakly acidic pH (Figure 3).

The specific weight of the waste was measured 10 times, and the average value was subsequently evaluated, which was found to be 0.38 g/cm³.

3.1.2. Characterization of the Waste by SEM (Electronic Microscope) and EDS (Energy Dispersive X-Ray Spectroscopy)

Figure 4 shows the images of the waste obtained by observation by SEM scanning microscope.

The images reported in Figure 4a,b highlight that the fibers in the waste are in an interwoven state and not isolated. This behavior can be attributed to the presence of a secondary phase, identifiable as licorice juice residue, which acts as a natural binder between the fibers. This phenomenon clearly reflects the influence of processing methods on the final structure of the waste material, suggesting that targeted interventions are required to recover clean fibers and remove the adhering residues.

Observations conducted at higher magnifications (Figure 4c) reveal additional details about the material’s complexity, highlighting the presence of circular structures. These structures can be associated with residual plant components, such as seeds or pollen, indicative of the biological nature of the waste. Such elements not only add complexity but may also hold potential for exploration in terms of their composition and possible applications, offering a way to maximize the waste’s value.

For a thorough chemical characterization of the material, EDS (Energy Dispersive Spectroscopy) microanalysis was performed on various points of the sample. This technique enabled the construction of a global map and revealed a consistent chemical composition across the sample. Such uniformity suggests that the material is relatively homogeneous from a chemical standpoint. This characteristic is particularly significant for the reuse of the material, as high chemical homogeneity simplifies and stabilizes subsequent processing steps.

A representative EDS spectrum of the sample is shown in Figure 5a. To provide context for the results, a comparative EDS analysis was also conducted on pure licorice extracted during the processing of licorice roots (Figure 5b). This comparison yielded valuable information on the chemical changes introduced during processing and the contribution of plant residues to the final composition of the waste material.

Overall, the SEM observations and EDS analyses provide a detailed understanding of the physical and chemical characteristics of licorice processing waste. This knowledge not only supports the development of effective methods for fiber recovery but also paves the way for optimizing waste valorization processes.

The EDS spectra of the waste and pure licorice compared show similar characteristics with high percentages of carbon and oxygen attributable to the organic nature of both materials. However, in pure licorice a lowering of the calcium and silicon contents is highlighted, to which the nature of the fibers that are present in the waste and not in pure licorice contributes mainly. Other peaks of lower intensity, such as magnesium and phosphorus, present in both samples are attributable to the components of licorice that are also present in the waste as a processing residue.

3.1.3. Thermal Characterization of Waste and Pure Licorice

The following are the DSC thermal analyses carried out on pure licorice (Figure 6a) and on the waste (Figure 6b) obtained during the industrial processing of licorice root.

Comparing the thermal profile of pure licorice (Figure 6a) and that of the waste (Figure 6b), two endothermic peaks below 200 °C and an exothermic peak around 300 °C are highlighted in both cases. These peaks do not correspond precisely between the two samples but those relating to the waste are all shifted to slightly higher temperatures. This can be justified by the presence of the fibers which in some way act with an insulating effect by slowing down the heat transmission and consequently the thermal transformations. The two endothermic peaks present below 200 °C, in both samples, can be associated with losses of volatile components. The exothermic peaks, of 292 °C and 329 °C, respectively, for pure licorice and for waste, are attributable to the combustion of licorice, also present in the waste as residual material from the processing.

3.2. Optimization of the Fiber Recovery Process from Waste

To optimize the operating conditions for the recovery of fibers from waste, the waste/distilled water system with a ratio of 30 g/L was treated at different temperatures and subjected to different treatment cycles. In particular, the following temperatures were taken into consideration: 25 °C, 50 °C, 75 °C and 100 °C. The cycles were repeated until the water was clear, a condition indicative of good fiber cleaning.

Figure 7, Figure 8, Figure 9 and Figure 10 show the washing water after the different heat treatment cycles at different temperatures, after the fibers had been removed.

The images show, as expected, that as the temperature increases, the number of cycles needed to obtain clear washing water decreases. Operating at a temperature of 75 °C, seven washing cycles are necessary, while at a temperature of 100 °C, five cycles are sufficient. Furthermore, it is possible to observe how in both cases the first cycles present many short fibers in suspension. Repeating the washing also allows the removal of residues of broken fibers as well as secondary phases present as residues from the extraction of licorice juice. The optimal extraction conditions are certainly significant and important in an industrial scale discussion where it is necessary to find the right balance between energy costs and operating costs related to the number of processes. In this research, it was decided to work at 100 °C and carry out five washing cycles only for practical reasons. Once the cycles were completed, the fibers were recovered by filtration, dried on filter paper for 48 h and subsequently carded to obtain the untangled fibers (Figure 11).

The fiber yield compared to waste was calculated and is approximately 50%.

3.3. Fiber Characterization

3.3.1. Characterization of Fibers by SEM and EDS Microanalysis

The fibers, obtained from the thermal process, were characterized by electron microscopy and microanalysis. The fibers showed very similar morphological and chemical characteristics when, after the umpteenth cycle, clear wash water is obtained. As seen in the previous chapter, the number of cycles to obtain clear wash water depends on the temperature used. The SEM images show that the treatments applied to the waste were effective in recovering the fibers. It is evident that the surface of the fibers appears completely clean (Figure 12a), and furthermore, the fibers are well separated (Figure 12b). The fiber thickness is predominantly around 100 µm, although there are rare instances of fibers with a higher thickness, approximately 200 µm (Figure 13).

The obtained fibers exhibit non-uniform lengths (Figure 14). The maximum fiber length is approximately 4 cm, while the most abundant fraction has a length ranging between 3 and 4 cm. A small fraction, accounting for less than 5%, consists of fibers shorter than 0.5 cm.

The EDS analysis shows high percentages of carbon and oxygen, the main constituents of the fibers (Figure 15). Unlike the EDS analysis of the waste (Figure 5), the clean fibers no longer exhibit magnesium and phosphorus peaks. These elements, attributed to the residues of licorice juice, were successfully removed through thermal cycles.

3.3.2. DSC Thermal Characterization of Fibers

Below is the DSC analysis of the fibers obtained after five washing cycles at 100 °C (Figure 16).

By examining the thermal profile of the clean fiber (Figure 16), it is evident that the exothermic peak, previously attributed to the presence of licorice juice, is no longer detectable when compared to the DSC analyses of pure licorice (Figure 6a) and the waste material (Figure 6b). This confirms the thorough cleaning of the fibers. In contrast, three distinct, well-defined peaks are observed, associated with endothermic transformations. The DSC analyses thus validate that the thermal treatment, conducted through five washing cycles at 100 °C, successfully removed licorice juice residues, yielding clean fibers.

3.3.3. Absorption Tests

Absorption tests were conducted on both treated and untreated fibers, which were first stored in a desiccator containing silica gel at room temperature for 24 h. Subsequently, known quantities of fibers were placed in distilled water and weighed at scheduled intervals. The absorption coefficient was calculated using the following formula:

$$C_i={\displaystyle \frac{m_{sat}-m_{dry}}{m_{dry}}}\times 100$$

where

m_sat = mass of the fibers after immersion in water (g).

m_dry = initial mass of dried fibers (g).

The mean absorption coefficient was determined by averaging the various coefficients obtained upon reaching saturation:

C_i = ∑_i C_i,_i/i

Figure 17a,b depict the absorption coefficients of the fibers as a function of immersion time.

The data show that, after slightly more than two hours of immersion in water, both treated and untreated fibers reach a saturation condition. The mean absorption coefficient was found to be 2.35 for treated fibers and 3.27 for untreated fibers. Untreated fibers exhibit a higher absorption coefficient compared to treated fibers.

3.4. Characterization of Gypsum Matrix Samples

3.4.1. Flexural Strength Tests

Several samples with a gypsum matrix were prepared, using either raw waste material or clean fibers obtained from waste through a process involving five cycles of washing in water at a temperature of 100 °C. Flexural strength tests were conducted on these samples to gather preliminary insights into the potential advantages of using raw waste material compared to clean fibers. In parallel, the benefits of using raw waste or clean fibers were also analyzed relative to artifacts without any additives, consisting exclusively of gypsum matrix. This comparison is critical for understanding whether the integration of recycled materials can truly improve mechanical and functional properties over traditional solutions based solely on gypsum. The tests conducted at this stage are purely preliminary, aimed at comparing the various samples and providing an initial knowledge base. Standardized testing according to regulatory guidelines was deferred to future studies, as the primary goal of this work was limited to comparing the samples to derive preliminary, non-definitive data. The decision to compare raw waste material with clean fibers stems from the need to balance two complementary requirements. On the one hand, the use of raw waste offers the advantage of a simpler, more cost-effective production process, promoting greater environmental sustainability and significantly reducing waste. On the other hand, clean fibers obtained through an accurate washing process might ensure better mechanical performance, making them more suitable for applications requiring higher quality standards and durability. Additionally, the comparison with artifacts made solely from a gypsum matrix allows us to establish whether recycled materials, either raw waste or clean fibers, can represent a real improvement over traditional construction solutions. This analysis is crucial for determining the effectiveness of integrating such materials into the production of innovative and sustainable artifacts. The choice to use a gypsum matrix for this experimentation was guided by the prospect of future applications, such as the possible development of insulating and fiber-reinforced panels. These applications represent a highly relevant sector in the field of eco-construction, where the combination of high mechanical performance, sustainability, and low environmental impact is an essential added value. Figure 18a,b, respectively, present the flexural breaking load and flexural breaking times of gypsum matrix samples with varying amounts of clean fiber and waste.

The results illustrated in Figure 18 reveal a significant phenomenon regarding the influence of fibers on the gypsum matrix. Samples prepared without the addition of clean fibers or raw waste material exhibit higher flexural strength, but shorter breaking times compared to samples with added clean fibers or waste material. This behavior highlights two distinct characteristics of the analyzed materials.

The samples without additives demonstrate greater flexural strength, enabling them to withstand higher loads. However, their internal structure is more fragile, leading to rapid breakage once the maximum load limit is exceeded. This occurs due to reduced plastic deformation capacity and energy absorption, making these materials effective under high loads but prone to sudden failures.

In contrast, samples enriched with fibers or waste material display lower flexural strength but significantly longer breaking times. This indicates a more ductile or viscoelastic behavior, with the ability to tolerate prolonged deformation before reaching the critical breaking point. These characteristics make such materials particularly advantageous for applications requiring resilience and the ability to absorb stresses over time.

The data clearly show that, for both clean fibers and raw waste material, increasing the quantities added to the gypsum matrix results in a linear decrease in strength and a corresponding increase in breaking times. This phenomenon suggests that higher fiber content alters the material’s internal structure, directly influencing its breaking mechanisms and enhancing its adaptability.

A comparison between clean fibers and raw waste material reveals distinct differences: samples containing clean fibers exhibit longer breaking times compared to those with raw waste material, while flexural strength remains relatively constant between the two material types. This behavior highlights the role of clean fibers in improving ductility and deformation capacity under load, whereas raw waste material has a less pronounced impact. These findings are particularly relevant for the design of internal panels or insulating applications, where mechanical properties must be balanced with functional and durability requirements. Panels with clean fibers, having longer breaking times, demonstrate greater capacity to withstand prolonged stresses, making them ideal for internal environments where resilience is key.

Panels with raw waste material may be better suited for decorative or covering applications, where temporary load resistance is sufficient without requiring superior durability. Furthermore, the use of raw waste material provides an ecological and sustainable option, promoting the reuse of recycled materials with acceptable performance.

The choice of material depends on the application’s objectives. If long-term durability, resilience, and adaptability to environmental stresses are priorities, panels made with clean fibers are the most advantageous option. Conversely, for less demanding applications emphasizing sustainability or moderate strength, panels containing raw waste material offer a viable alternative.

3.4.2. SEM Characterization

Figure 19 shows SEM images of the samples obtained with varying clean fiber and waste percentages in gypsum matrix samples.

The analyzed SEM images reveal, overall, a strong adhesion of fibers to the gypsum matrix, applicable to both clean fibers and those derived from waste. This aspect represents a notable strength, highlighting a significant interaction between the reinforcement and the matrix. Additionally, in all cases, crystal nucleation is observed on the fiber surfaces, a phenomenon that suggests active chemical interaction between the matrix and the fibers, contributing to reinforcing the overall bond.

However, a more in-depth analysis unveils substantial differences between the two types of fiber. Clean fibers exhibit a uniform and regular distribution within the matrix. This uniformity, attributable to their linear structure and greater controllability during preparation, promotes more effective integration into the matrix. This feature accounts for the superior mechanical performance of materials reinforced with clean fibers, as demonstrated by higher flexural breaking times, as previously discussed.

On the other hand, waste-derived fibers display limitations related to their intrinsic nature. The presence of tangled fibers complicates their homogeneous distribution within the matrix, creating areas of heterogeneity that could compromise the material’s mechanical properties. This heterogeneity explains the lower flexural breaking times compared to materials reinforced with clean fibers, reinforcing the correlation between reinforcement quality and the composite’s overall performance.

A particularly significant result of this work is the creation of gypsum matrix panels incorporating fibers recovered from licorice processing waste through the methodology tested in this study (Figure 20).

This represents a tangible example of how these fibers can find practical applications in the construction sector, offering a sustainable and innovative solution.

The process of integrating licorice waste fibers into gypsum panels highlights not only the technical feasibility of this application but also their crucial role in the design of advanced materials characterized by lightness and efficiency. These panels embody innovation in the field of eco-friendly materials and demonstrate the potential of vegetable fibers as a strategic element for more environmentally friendly construction.

4. Conclusions

The experimental work has primarily achieved two key results: identifying a simple and cost-effective method for recovering fibers from licorice processing waste and preliminarily exploring the use of these fibers in the production of eco-friendly building materials. The overall results allow for meaningful conclusions, highlighting the potential of vegetable fibers recovered from licorice processing waste.

The proposed recovery process is characterized by its simplicity, environmental sustainability, and cost-effectiveness. It does not require the use of chemical reagents and relies solely on water at 100 °C for five washing cycles. This methodology not only ensures effective fiber cleaning but also provides a practical and replicable approach, reducing environmental impact and promoting the principles of the circular economy.

An advantageous aspect is that the starting material, namely licorice waste, is already partially processed during production. This simplifies fiber recovery compared to using untreated raw materials. The direct use of waste instead of fresh raw materials leads to a significant reduction in energy and operational costs while also minimizing the environmental impact associated with the extraction and processing of natural resources.

Initial experimental tests on the use of recovered fibers in eco-sustainable materials for bio-building applications confirm their potential. The fibers integrated into gypsum matrices have shown good adhesion, with the ability to tolerate prolonged deformation before reaching the critical breaking point. Furthermore, the incorporation of fibers allows for an overall reduction in the weight of materials, making them lighter and easier to handle. This feature is particularly advantageous in certain sectors of sustainable construction, where lightness is a fundamental requirement. The fibers can also act as fillers, reducing the amount of raw materials required for material production, thereby improving economic and environmental efficiency.

These studies represent only a starting point. It is hoped that future research will expand the scope of applications for licorice vegetable fibers by studying their interactions with various matrices, such as cement-based or polymeric ones. Integrating these fibers into more complex matrices could open up new opportunities for developing high-performance yet sustainable materials.

The adoption of vegetable fibers recovered from industrial licorice waste aligns perfectly with the principles of the circular economy. This innovative approach not only adds value to materials otherwise destined for disposal but also creates new opportunities in the eco-sustainable construction sector. Future research could focus on optimizing fiber integration processes into matrices, analyzing the physical–mechanical properties of the resulting composites, and exploring new fields of application. In summary, the described method proves effective for fiber recovery and promising for the development of innovative and sustainable composite materials, marking an important step towards a more ecological and technologically advanced future.