Effect of the Addition of Banana Stem Lignin (Musa acuminata ssp. balbisiana var. Dominico-Harton) on the Physicochemical Properties of Biodegradable Composites Based on Methylhydroxyethylcellulose

Abstract

This study analyses the effect of lignin extracted from the Dominico-Harton banana on the physicochemical properties of biodegradable methylhydroxyethylcellulose (MHEC) composites. Lignin was obtained by grinding and sieving, followed by treatment with sulphuric acid and subsequent separation via centrifugation. Films were developed using the casting method, incorporating lignin and glycerol in a matrix of MHEC dissolved in distilled water. They were characterised according to their physical, barrier, mechanical, optical, and antioxidant properties, using 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid (ABTS). The results showed that lignin provides antioxidant capacity and improves mechanical and barrier properties, while MHEC contributes flexibility and biodegradability. These films have a dense and resistant structure, with potential applications in food packaging, agriculture, and medicine. The research highlights the use of agricultural waste to develop sustainable materials as an alternative to conventional plastics.

1. Introduction

Industrial modernisation has led to the development of new strategies to address the environmental impacts of single-use plastic materials; it has driven the search for new, sustainable, and eco-friendly alternatives [1]. In this context, biodegradable films have emerged as a promising option to reduce plastic pollution [2]. Cellulose and its derivatives have been widely explored as key components in the manufacture of biodegradable films due to their abundance and biodegradability [3]. Furthermore, lignin is one of the complex organic polymers that form important structural materials in the supporting tissues of vascular plants and some algae. In addition to providing rigidity in plant tissues, it confers the ability to grow in height and offers protection against UV radiation and microbial attack. Lignin is highly resistant to degradation due to its chemical and structural properties [4]. It is also a by-product of the agri-food industry and has great potential as a filler and reinforcing agent in such materials, while promoting the valorisation of agricultural waste [4].

The Harton banana (Musa Aab S. var Dominico-Harton) is a variety of banana grown in several regions of the world and is a rich source of lignin [5]. The incorporation of this lignin in films based on MHEC is proposed as a strategy to improve the mechanical properties and biodegradability of such films while promoting the use of an underutilised agricultural resource [6]. Other studies use the pseudostem of the Harton banana (Musa Aab S. var. Dominico-Harton) as a raw material for developing biodegradable films for food applications. This research highlights the use of agricultural byproducts rich in fibres and compounds such as lignin and cellulose, present in this banana variety, to formulate materials with functional properties such as moisture barriers and adequate mechanical capacity [7,8,9]. The following research is based on the need to find sustainable solutions to address the increasing environmental pollution derived from conventional plastics [10]. The exploration of new sources of lignin and its application in biodegradable films opens significant possibilities for reducing the carbon footprint and promoting a circular economy [11].

This study forms part of the search for sustainable alternatives and contributes to the knowledge regarding the extraction and characterisation of lignin from specific agricultural resources, as well as the evaluation of its impact on the properties of biodegradable films based on MHEC. As a polymer derived from cellulose—a natural substance found in the cell walls of plants—MHEC is obtained through the chemical modification of cellulose with methyl and hydroxyethyl groups. This modification process imparts useful properties for a variety of industrial applications, including the manufacture of biofilms, thereby addressing a current and relevant problem and promoting the transition towards more environmentally sustainable materials [12].

Therefore, the main objective is to study the effect of the addition of lignin from banana (Musa Aab S. var Dominico-Harton) on the physicochemical properties of biodegradable MHEC films.

2. Materials and Methods

For lignin extraction and purification, 72% sulfuric acid and 12–13% sodium hydroxide were used, both supplied by Elementos Qímicos Ltda. (Bogotá, Colombia). MHEC was supplied by Shanghai Honest Chem. Co., Ltd. (Shanghai, China), and the glycerol used as a plasticizer was purchased from Productos Químicos Colombia (Bogota, Colombia).

2.1. Obtaining Lignin from the Pseudostem of the Dominico-Hartón Banana

The pseudostem from the Dominico-Hartón banana plant was cut into small pieces and dried by solar drying until reaching a moisture content below 12%. The dried material was then ground using a blade mill Retsch (SM 200 model, Retsch GmbH, North Rhine-Westphalia, Germany) and sieved to obtain a uniform powder with particle sizes retained in a 0.5 mm mesh.

The powdered pseudostem was mixed with a 20% w/v sodium hydroxide (NaOH) solution (20 g NaOH per 100 mL of distilled water) and stirred at 80 °C for 2 h to promote lignin solubilisation. After the alkaline treatment, the mixture was filtered using warm deionised water to remove the insoluble solids. The resulting liquid extract was centrifuged at 200 rpm to clarify the solution and enhance phase separation. The supernatant, containing the solubilised lignin, was then acidified by slowly adding 72% sulfuric acid until reaching a pH of 2 to precipitate the lignin. The precipitate was washed with warm deionised water until a neutral pH was reached, and then the mixture was centrifuged at 5000× g rpm for 10 min to remove excess liquid. Finally, the lignin was frozen and lyophilised for 24 h to obtain it in powder form

2.2. Characterisation of the Pseudostem Lignin of the Dominico-Hartón Banana

The antioxidant capacity of the films was evaluated following the protocol of described in the literature [13]. DPPH and ABTS tests were used to measure antioxidant activity and free radical inhibition, providing protection against oxidative stress. In the DPPH assay, the decrease in the intense violet colour of the DPPH radical was observed as it reacted with the antioxidants present in the films. The change in absorbance was recorded, and the concentration of the antioxidant needed to reduce 50% of the radical (IC₅₀) was calculated. Similarly, the ABTS radical was formed and reduced by lignin, and the antioxidant capacity was evaluated by measuring the change in absorbance at 734 nm for ABTS. DPPH (2,2-diphenyl-1-picrylhydrazyl) is a stable, purple-coloured free radical, demonstrating that when an antioxidant meets DPPH, it donates an electron to the free radical, causing the purple colour to fade. The ability of a substance to decolorize DPPH is used as a measure of its antioxidant activity. ABTS (2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonic acid)) is another free radical used to assess antioxidant capacity. Like DPPH, ABTS changes colour when it reacts with an antioxidant [14].

2.3. Obtaining Biodegradable Films from MHEC with Incorporation of the Obtained Lignin

The casting methodology standardised by the FP&SL research group was used. The MHEC was dissolved in distilled water at 2% w/w. The appropriate amounts of glycerol and lignin corresponding to each formulation were included. The films were produced according to six formulations, of which the first two served as controls, while the remaining four included lignin, as shown in the factorial design presented in

Table 1. The 2² factorial design parameters of MHEC films with lignin addition.

Formulations	MHEC (p/p)%	Glycerol (p/p)%	Lignin (p/p)%	H2O (mL)
Control 1 (C1)	1.5	15	0	200
Control 2 (C2)	1.5	20	0	200
Lignin 3 (L3)	1.5	15	0.5	200
Lignin 4 (L4)	1.5	20	1	200
Lignin 5 (L5)	1.5	15	1	200
Lignin 6 (L6)	1.5	20	0.5	200

. MHEC was added with respect to water, and glycerol and lignin were added relative to MHEC. Vigorous homogenisation was conducted for 30 min to obtain film-forming dispersions. These were then placed under vacuum to eliminate any bubbles; the dispersions were poured onto Teflon plates and dried at 50 °C until the water evaporated, and dry films could be peeled off. The films were stored in desiccators containing a supersaturated sodium bromide solution (56% R.H.) for one week prior to characterisation [13]. The formulations and experimental design were determined based on preliminary tests.

Lignin Yield

Lignin yield refers to the amount of lignin obtained from a biomass source. The yield can vary depending on the type of plant and the extraction process used. The lignin yield achieved during the extraction process will be determined by the ratio between the amount of lignin recovered and the total amount of raw material used. The yield was expressed in terms of percentage, as shown in Equations (1) and (2).

$$\mathrm{L}\mathrm{i}\mathrm{g}\mathrm{n}\mathrm{i}\mathrm{n} \mathrm{p}\mathrm{u}\mathrm{r}\mathrm{i}\mathrm{t}\mathrm{y}\left(\mathrm{g}\right)=\left({\displaystyle \frac{\mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{p}\mathrm{s}\mathrm{e}\mathrm{u}\mathrm{d}\mathrm{o}\mathrm{s}\mathrm{t}\mathrm{e}\mathrm{m} \mathrm{u}\mathrm{s}\mathrm{e}\mathrm{d}\times \mathrm{W}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{l}\mathrm{i}\mathrm{g}\mathrm{n}\mathrm{i}\mathrm{n} \mathrm{i}\mathrm{n} \mathrm{p}\mathrm{u}\mathrm{r}\mathrm{i}\mathrm{t}\mathrm{y}\mathrm{\%}}{100\mathrm{\%}}}\right)$$

(1)

$$\mathrm{Y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}\left(\mathrm{\%}\right)=\left({\displaystyle \frac{\mathrm{L}\mathrm{i}\mathrm{g}\mathrm{n}\mathrm{i}\mathrm{n} \mathrm{o}\mathrm{b}\mathrm{t}\mathrm{a}\mathrm{i}\mathrm{n}\mathrm{e}\mathrm{d}}{\mathrm{L}\mathrm{i}\mathrm{g}\mathrm{n}\mathrm{i}\mathrm{n} \mathrm{p}\mathrm{u}\mathrm{r}\mathrm{i}\mathrm{t}\mathrm{y}}}\right)\times 100\mathrm{\%}$$

(2)

2.4. Characterisation of Biodegradable Films from MHEC with Incorporation of the Obtained Lignin

2.4.1. Mechanical and Thickness Properties

The mechanical properties and thickness tests were developed as established by Moreno-Ricardo et al. (2024) [15]. Elasticity, tensile strength, and deformation. The guidelines established in the ASTM D882 standard were followed to carry out the mechanical property tests [16], and we used a universal testing machine (TX-700, Lamy Rheology, Champagne, France) with a 500 N load cell to carry out the tests.

2.4.2. Colour

The colour test was performed as described and established by Moreno-Ricardo et al. (2024) [15]. A portable colorimeter (Colorimeter CHN Spec CS-10, CHNSpec Technology Co., Ltd., Zhejiang, China)) was used to measure the surface colour of the films. The colorimeter was calibrated using standard D65 lighting conditions and a 10° observation angle. We used the CIE Lab colour system to measure colour parameters. The L* value ranges from 0 (black) to 100 (white); the a * value represents greenish to red tones, and the b * value indicates blue to yellow tones.

2.4.3. Gloss

A gloss meter with a flat surface (3 nh Gloss Meter) was used to measure the gloss of the films. The gloss meter was set to an incidence angle of 60°, in accordance with the ASTM D523 standard for gloss measurement [17].

2.4.4. UV-Vis Internal Transmittance

A test was conducted according to the article by Ortega-Toro et al. (2017) [17]. In the mentioned study, the transmittance test of the films was carried out as follows: Film samples equilibrated under specific relative humidity and temperature conditions were used to ensure the stability of the film properties during the test. A UV-VIS spectrophotometer (Perkin Elmer Instruments, Lambda 35, Waltham, MA, USA) was used to measure the transmittance of the samples [12]. This instrument enabled the analysis of the amount of light passing through the sample within a specific range of wavelengths.

2.4.5. Opacity

The opacity test in this study was carried out using a UV-Vis spectrophotometer at a wavelength of 600 nm [14].

2.4.6. Optical Micrography

This analysis was developed following the methodology of Moreno-Ricardo et al. (2024) [15]. Micrographs of films with and without lignin were analysed to evaluate surface roughness. The results showed that the addition of lignin decreased roughness and increased homogeneity of the films, which could improve properties such as adhesion, barrier, and shelf life.

2.4.7. Moisture Content

Film measurements were made by means of a drying process in a natural convection oven at 65 °C for 12 h. Heating was continued until a constant weight was reached to ensure the removal of all moisture. The percentage of moisture was calculated by dividing the wet weight of the film by its dry weight [14].

2.4.8. Contact Angle

The contact angle was measured using the sessile drop method with a Krüss DSA25 Drop Shape Analyser (KRÜSS GmbH, Hamburg, Germany), following Moreno-Ricardo et al. (2024) [15]. A 5 µL droplet of distilled water was placed on the film surface under controlled conditions (25 °C, 50% RH), and the angle was recorded at 5 s. Due to the hydrophilic nature of MHEC, the angle decreased over time, but only the initial value was used for analysis.

2.4.9. Water Vapour Permeability

We used a gravimetric approach following the procedure established by ASTM E96-95 with certain modifications [18]. Knowledge of water vapour permeability will be essential for the development of packaging that effectively protects products against moisture. It will allow us to optimise film design to control water vapour transfer and maintain the integrity of packaged products. The water vapour permeability test was essential to evaluate the effectiveness of MHEC-based films with lignin incorporation in regulating the passage of water vapour, which will be crucial in packaging and coating applications [15]. The environmental parameters were manipulated to establish a humidity gradient ranging between 52.8% relative humidity and 100% relative humidity at a controlled temperature of 25 °C. Films free of physical imperfections were selected for the water vapour permeability (WVP) evaluations. Payne-type permeation vessels filled with distilled water were used, allowing one film surface to be subjected to 100% relative humidity. These vessels and the films were placed in humidity-controlled cabinets at 25 °C, with the cabinet relative humidity maintained at 52.8% using supersaturated magnesium nitrate solutions. In addition, to improve the applicability of the films to products characterised by high water activity, the free surface of the film was exposed to a lower relative humidity environment during manufacturing. Vessels containing the films were subjected to systematic evaluation using a high-precision analytical balance with a sensitivity of 0.0001 g. After reaching a stable measurement condition, the water vapour transmission rate (WVTR) was calculated based on the slope of the regression line, delineating the decrease in weight over time. This value was then divided by the area of the film. This procedure was repeated three times, yielding results encompassing the mean value and the corresponding standard deviation.

2.4.10. Antioxidant Capacity (DPPH) and (ABTS)

The antioxidant capacity was performed with the same protocol and application as Moreno et al. (2024) [15]. The antioxidant capacity test with (DPPH) and (ABTS) assays was used to evaluate the antioxidant activity or percentage of inhibition to neutralise free radicals and protect against oxidative stress.

2.4.11. Statistical Analysis

The experimental design used in the following research is an entirely random factorial design. All results were analysed using Statgraphics Plus for Windows 5.1 software. An analysis of variance (ANOVA) was performed and then the means were examined using Fisher’s Least Significant Difference (LSD) test with a confidence level of 95%.

3. Results

3.1. Characterisation of Lignin

3.1.1. Lignin Yield from the Banana Pseudostem (Dominico-Hartón)

A total of 10.76 g of lignin was obtained, representing 57.16% of the total mass of the dried pseudostem used in the extraction process. This yield is consistent with values reported in the literature for lignin obtained from Musa Aab var Dominico-Harton, which typically ranges around 60% [12,14]. These results confirm the efficiency of the extraction method and the potential of banana pseudostem as a lignin source.

3.1.2. Antioxidant Capacity

The antioxidant capacity of pure lignin was assessed using DPPH and ABTS assays. The results showed that pure lignin exhibited significant free radical scavenging activity, with IC₅₀ values of 152 ± 3 μg/g for DPPH and 95 ± 2 μg/g for ABTS. These values indicate the amount of lignin required to inhibit 50% of the radical activity in each assay, confirming its potent antioxidant potential. Inhibitory concentration represents the concentration of the substance (in this case, lignin) required to inhibit 50% of free radical activity (DPPH or ABTS). Lower IC₅₀ values indicate greater antioxidant activity, requiring a smaller amount of the substance to neutralise free radicals, while higher IC₅₀ values indicate lower antioxidant activity, requiring a larger amount of the substance to achieve the same effect [19].

In the case of pure lignin, the IC₅₀ values for DPPH and ABTS are relatively low [20]. This suggests that lignin has considerable antioxidant capacity, meaning that it can donate electrons to free radicals, neutralising them and thus preventing oxidative damage to cells. Pure lignin exhibits significant antioxidant activity, which is an interesting and valuable property. This antioxidant capacity is due to the presence of phenolic functional groups in the lignin structure, which can donate electrons and thus neutralise free radicals. Pure lignin could be used as a natural food additive to extend shelf life and improve nutritional quality.

3.2. Characterisation of Biodegradable MHEC Films with Lignin Incorporation

3.2.1. Physical Properties

Table 2. Mean values and standard deviation of thickness (μm), water vapour permeability (WVP, g mm/kPa-h-m²), moisture content (Xw, g water/g dry film), water absorption capacity (Wca, g dry film/g wet film) of the films studied.

Formulations	Thickness (μm)	WVP (g mm/kPa-h-m2)	Xw (g Water/g Dry Film)	Wca (g Dry Film/g Wet Film)
C1	177.8 ± 0.02 c	4.14 ± 0.02 d	0.24 ± 0.04 c	0.147 ± 0.22 a
C2	183.5 ± 0.02 c	4.20 ± 0.06 d	0.11 ± 0.06 d	0.130 ± 0.42 bc
L3	232.8 ± 0.02 b	5.39 ± 0.04 b	0.29 ± 0.03 c	0.131 ± 0.44 d
L4	225.5 ± 0.01 b	5.15 ± 0.08 c	0.65 ± 0.09 a	0.122 ± 0.37 bcd
L5	269.0 ± 0.01 a	6.20 ± 0.05 a	0.23 ± 0.03 c	0.131 ± 0.47 b
L6	224.4 ± 0.02 b	5.85 ± 0.01 e	0.56 ± 0.05 b	0.116 ± 0.43 cd

Different superscript letters indicate statistically significant differences (p < 0.05) between formulations.

presents the mean values and standard deviations of the main physical properties of the films studied, including thickness (μm), water vapour permeability (WVP, g mm/kPa·h·m²), moisture content (Xw, g water/g dry film), and water absorption capacity (WCA, g dry film/g wet film). These parameters are essential for evaluating the performance of the films in terms of their structural stability, moisture resistance, and potential use in packaging applications. A comparison of these values allows for the analysis of how formulation variations affect the barrier properties and water retention capacity of the materials, which is key in the design of functional and biodegradable coatings.

C1 and C2 present relatively low thickness values (177.8 ± 0.02 and 183.5 ± 0.02 μm, respectively), suggesting that the absence of lignin results in thinner biofilms. In contrast, films with lignin (L3, L4, L5, and L6) exhibit significantly higher thickness, with values ranging from 224.4 ± 0.02 to 269.0 ± 0.01 μm. This increase in thickness indicates that lignin acts as a filler, contributing to the overall volume of the films. Although the increase in volume suggests a less compact structure, further density measurements would be required to confirm any change in the density of the polymeric matrix. Regarding water vapour permeability (WVP), it is highlighted that control films 1 and 2 present significantly lower values (4.14 ± 0.02 and 4.20 ± 0.06, respectively) compared to the formulations containing lignin (L3, L4, and L5), whose permeabilities were 5.39 ± 0.04, 5.15 ± 0.08, and 6.20 ± 0.05. These results suggest that MHEC may confer greater protection against water vapour permeability, probably due to its ability to form a more homogeneous and less porous matrix, which limits vapour diffusion [21]. On the other hand, film 6, which also contains lignin, presented an intermediate permeability (5.85 ± 0.01), which reinforces the idea that the combination of MHEC, glycerol, and lignin in different proportions directly influences the regulation of water vapour permeability. This behaviour can be explained by the hydrophobic nature of lignin, which, at higher concentrations, could alter the polymer structure, generating greater variability in the vapour barrier. These findings underline the importance of a precise adjustment in the formulation to optimise the performance of these polymers. Modifying the ratio of MHEC and lignin could allow the development of materials with tailored barrier properties that are suitable for specific applications in food packaging or the preservation of moisture-sensitive products. Thus, understanding the interaction between these components not only adds value to the formulation of biopolymers but also opens opportunities for the improvement of sustainable coatings with optimised functional properties. On the other hand, the moisture content (Xw) of the studied films varied among formulations. Lignin-free films, C1 and C2, exhibited relatively low moisture contents, with 0.24 ± 0.04 g water/g dry film and 0.11 ± 0.06 g water/g dry film, respectively. Formulations containing lignin, such as L3 and L6, showed higher moisture contents, reaching 0.29 ± 0.03 g water/g dry film and 0.56 ± 0.05 g water/g dry film, respectively. This increase in moisture content may indicate a higher water-holding capacity of the lignin-containing films. Although higher moisture contents could be seen as a challenge in food packaging applications, they could also be beneficial in creating edible coatings, where the additional moisture could contribute to increased flexibility and improved moisture barrier properties. Films with higher moisture levels could be helpful for foods that need to stay hydrated, such as fresh fruits or produce, while also acting as protective barriers against oxidation and quality loss during storage [22,23]. The films, both with and without lignin, exhibit similar moisture values within a narrow range, suggesting that the processing conditions were effectively controlled. MHEC, as a polymer, can form hydrogen bonds with water, influencing the moisture content of the film. At low concentrations, MHEC can promote water retention through intramolecular sorption and subsequent swelling, contributing to the structural stability of the material [21].

3.2.2. Mechanical Properties

Young’s modulus (EM) is a mechanical property that measures the stiffness of a material, indicating its ability to resist elastic deformation under tension or compression. This modulus refers to the ratio of applied stress to the resulting deformation in the elastic region of the material. Tensile strength (TS) measures the resistance of the film to stretching before breaking; a higher tensile strength indicates a stronger film. On the other hand, strain capacity (E) refers to the ability of a material to undergo deformation before reaching the fracture point or undergoing plastic deformation. In summary, while Young’s modulus focuses on resistance to elastic deformation, strain capacity encompasses the overall behaviour of the material until failure [24].

Table 3. Mean values and standard deviation of the mechanical properties (Tensile Strength: TS), Elastic Modulus (ME), and deformation capacity (E) of the studied films of MHEC with and without lignin.

Formulations	TS (MPa)	ME (MPa)	E (%)
C1	11.5 ± 0.3 b	295 ± 3 ab	38.2 ± 0.5 c
C2	10.9 ± 0.2 a	287 ± 5 a	45.3 ± 0.4 a
L3	12.1 ± 0.5 b	312 ± 3 c	36.7 ± 0.4 d
L4	15.4 ± 0.2 c	318 ± 2 d	37.2 ± 0.2 cd
L5	17.1 ± 0.4 d	325 ± 5 e	34.4 ± 0.8 e
L6	11.8 ± 0.5 b	301 ± 4 b	40.5 ± 1.5 b

Different superscript letters indicate statistically significant differences (p < 0.05) between formulations.

shows the mechanical testing for the six (MHEC) formulations with and without lignin.

The analysis of the mechanical properties of the films, presented in

Table 4. Mean values and standard deviation of the brightness (GU) and colour parameters (luminosity (L*), red/green (a*), yellow/blue (b*), chromaticity (C), and hue angle (h, °) of the films studied.

Formulations	Gloss at 60°	Colour Parameters					ΔE
		L*	A*	B*	C	h
C1	8.2 ± 0.9 c	82.7 ± 3.2 a	−1.8 ± 0.6 ab	3.4 ± 1.5 a	3.8 ± 1.6 abc	117.9 ± 2.3 a	-
C2	8.6 ± 0.9 c	82.5 ± 1.4 a	−2.3 ± 0.4 b	3.6 ± 0.4 a	4.2 ± 0.5 a	122.2 ± 4.6 a	-
L3	6.0 ± 0.8 d	85.2 ± 1.2 a	−1.5 ± 0.2 ab	2.0 ± 0.4 b	2.5 ± 0.4 c	126.5 ± 1.9 a	4.1 ± 0.8 a
L4	13.3 ± 1.7 a	83.1 ± 0.2 a	−1.9 ± 0.1 ab	3.4 ± 0.6 a	3.9 ± 0.5 abc	119.2 ± 4.2 a	2.9 ± 0.8 a
L5	13.8 ± 0.3 a	83.4 ± 1.3 a	−2.0 ± 0.6 ab	3.4 ± 0.1 a	4.0 ± 0.3 ab	119.9 ± 6.5 a	2.6 ± 0.2 ab
L6	11.3 ± 1.8 b	85.5 ± 1.8 a	−0.9 ± 1.5 a	2.4 ± 0.5 ab	2.8 ± 0.6 bc	121.7 ± 7.6 a	4.8 ± 0.5 a

Different superscript letters indicate statistically significant differences (p < 0.05) between formulations.

, reveals the impact of lignin on their strength and stiffness. Films containing lignin exhibit a higher tensile strength compared to those without lignin, suggesting that lignin acts as a structural reinforcement, improving the mechanical strength of the polymeric matrix [24,25]. Likewise, an increase in the elastic modulus is observed in films with lignin, indicating greater stiffness. This behaviour can be attributed to the ability of lignin to form a three-dimensional network within the polymeric matrix, which restricts molecular mobility and strengthens the structure of the material [26].

On the other hand, the deformation capacity tends to be slightly lower in films with lignin, suggesting that the greater stiffness provided by this compound limits the flexibility of the material. The reduction in deformability is due to the restriction of molecular mobility within the polymeric matrix, which decreases the film’s ability to adapt to mechanical stresses without fracturing [27]. These findings reinforce the idea that lignin not only improves the mechanical properties of biofilms but also positively influences their thickness and stability. Its interaction with the polymer MHEC could favour the formation of a more complex and resistant intermolecular network, which contributes to less deformation and greater structural integrity in the developed films [28]. On the other hand, a study evaluated the effect of glycerol on cellulose films and observed that its addition improves tensile strength and flexibility because glycerol acts as a plasticiser, facilitating film elongation. Its presence reduces interactions between polymer chains and promotes more significant molecular attraction, which increases elongation capacity [29].

3.2.3. Colour

Table 4 presents the values of the colour parameters and gloss obtained for the biofilm formulations expressed in the CIE Lab* colour space. These parameters provide a quantitative description of the colour appearance of the samples, allowing the analysis of the impact of lignin on their chromatic characteristics.

C1 and C2 show very low gloss values, reflecting considerable variability in the results obtained. This variation could be related to factors such as the amount of glycerol used in the formulation, since this compound can increase the flexibility of the film and, consequently, influence the formation of a smooth surface, thus affecting the reflection of light [30]. On the other hand, tests L3, L4, L5, and L6 also show reduced gloss values, with a range that varies between 6.0 ± 0.8 GU and 13.8 ± 0.3 GU. These results suggest that the incorporation of lignin may contribute to the decrease in gloss in biofilms, probably due to the promotion of irregularities on the surface. In addition, the chromophore nature of lignin could be influencing this reduction in gloss, as pointed out by Ban (2007) [31]. In terms of lightness (L*), no clear difference is observed between the formulations with and without lignin. However, there is a slight tendency towards higher L* values in the formulations containing lignin, with samples 3 and 6 standing out, with lightness of 85.2 ± 1.2 and 85.5 ± 1.8, respectively. The similarity between these values suggests that lignin could influence light reflection, possibly due to its interaction with the polymeric matrix [32]. In terms of hue, no significant differences are evident between MHEC films with and without lignin. However, on the −a* axis, negative values indicate a tendency towards greenish tones, while on the +b* axis, positive values reflect more yellow tones. In this sense, a variation is observed between the formulations, with sample 3 (with lignin) presenting a value of 2.0 ± 0.4 and sample 2 (without lignin) reaching a value of 3.6 ± 0.4, suggesting that lignin could slightly influence the intensity of the yellow tone. On the other hand, formulations with lignin tend to show slightly lower chroma (C*) values, indicating lower colour saturation. Formulations 1 and 2 also present low values in this parameter, suggesting that lignin does not introduce a significant variation in colour vividness. In the case of the hue angle (h*), no clear trend associated with the presence of lignin is observed, indicating that its impact on colour perception is limited. Overall, the results suggest that the incorporation of lignin in MHEC films has a moderate effect on the colour appearance of biofilms. This could be explained by the similarity in the hue of the polymers used, which tend to be yellowish, and by the minimal variation in the concentration of MHEC, glycerol, and lignin, factors that can influence colour perception. Although slight differences in saturation and lightness are identified, these are not statistically significant in all cases.

3.2.4. UV-Vis Internal Transmittance

The films exhibit notable internal transmittance, as observed in Figure 1, which illustrates the amount of light that passes through them and exits the other side. The graph reveals that all six films, both with and without lignin, allow light to pass through in the ultraviolet (UV) and visible (Vis) ranges. An increasing trend in transmittance is observed as the wavelength increases, indicating that the films are more transparent to visible light than to ultraviolet light. This behaviour is particularly evident in films containing lignin, where the change in transmittance is more significant, suggesting that the incorporation of lignin influences the interaction of the film with light [33].

Lignin-free films (C1 and C2) exhibit a slightly higher transmittance in the visible region of the spectrum compared to some of the films containing lignin. This suggests that the absence of lignin could confer greater transparency to visible light [34]. Lignin, being an aromatic compound, can absorb light in certain regions of the spectrum, especially in the ultraviolet (UV). This phenomenon may explain why films with lignin show a lower transmittance in this region. Indeed, the incorporation of lignin in the films significantly reduced the transmittance in the UV region (200–240 nm), achieving a complete blocking of ultraviolet light compared to films without lignin. This variability in transmittance could depend on factors such as lignin concentration, its type, and its interaction with other film components [34]. L4 exhibits an irregularity in its transmittance compared to the other formulations. In this case, the transmittance varied from 0% to 63% in the range from 240 nm to approximately 900 nm, suggesting that its composition influences its optical behaviour. One possible explanation for this phenomenon is the higher concentration of glycerol and lignin in the formulation, which could be the cause of its lower transmittance. As mentioned, lignin has a strong absorption in the UV region, and a higher concentration could increase this absorption, thus reducing the amount of light passing through the film. In addition, the interaction between glycerol, lignin, and MHEC could generate a more compact structure or one with a higher degree of cross-links, making it even more difficult for light to pass through [35].

3.2.5. Opacity

Figure 2 shows that the opacity of biodegradable films varies depending on the formulation, with an increase in some cases due to the incorporation of lignin, although not consistently. Factors such as MHEC concentration, preparation method, and drying conditions also influence this parameter. Lignin dispersed in the polymeric matrix can act as light scattering centres, reducing transparency, and its effect on opacity depends on its concentration and distribution [36].

Films with higher opacity can be useful in applications that require light barriers, such as the packaging of sensitive products. These results allow formulations to be optimised by adjusting the concentration of lignin and other components to achieve the desired opacity [35].

3.2.6. Contact Angle

The contact angle was used to evaluate the wettability of the surfaces of the MHEC-based films with and without lignin, as shown in

Table 5. Mean values and standard deviation of the water contact angle (CAw, °) of the films.

Formulations	CAw
C1	58.333 ± 0.8 c
C2	55.000 ± 0.5 d
L3	52.000 ± 0.5 e
L4	80.333 ± 2.5 a
L5	70.000 ± 2.2 b
L6	69.333 ± 2.1 b

Different superscript letters indicate statistically significant differences (p < 0.05) between formulations.

. A low contact angle (<90°) indicates good wettability, suggesting that the surface is hydrophilic, while a high angle (>90°) indicates low wettability and a hydrophobic surface.

Contact angle analysis shows that MHEC films are mostly hydrophilic, with variations in their wettability that may be crucial for specific applications. According to the data in

Table 6. Antioxidant capacity test DPPH IC₅₀ (μg/g lignin) and ABTS IC₅₀ (μg/g lignin) of the film formulations.

Formulations	DPPH IC50 (μg/g Dry Film)	ABTS IC50 (μg/g Dry Film)
C1	---	---
C2	---	---
L3	720 ± 2 a	501 ± 2 a
L4	495 ± 2 b	342 ± 5 b
L5	504 ± 5 b	347 ± 2 b
L6	715 ± 6 a	498 ± 3 a

Different superscript letters indicate statistically significant differences (p < 0.05) between formulations. “---”—The antioxidant capacity of these samples was not determined.

, significant differences are highlighted based on the incorporation of lignin and glycerol. Films with lignin (L4, L5, and L6) present higher contact angles (80.33, 70.00, and 69.67°), suggesting that higher amounts of lignin and glycerol increase this value, conferring more hydrophobic characteristics. L4 shows a more pronounced behaviour in this direction. On the other hand, films without lignin (C1 and C2) and with reduced amounts of lignin (L3) have lower contact angles (58.33 and 55.00°, respectively), highlighting the inverse relationship between the amount of lignin and wettability. These results suggest that choosing the appropriate formulation according to the specific requirements of each application is essential to obtaining the desired results. Furthermore, increasing the amount of lignin and glycerol could optimise the surface properties of biofilms, since lignin, as a polymeric molecule, can form a protective barrier that affects surface wettability [33].

3.2.7. Optical Micrography

In Figure 3, surface optical micrographs (10 X) of the lignin-free MHEC control films are shown, revealing notable differences in surface morphology compared to the films containing lignin.

C1 and C2 exhibit higher roughness, while the formulations with lignin show a smoother and less rough surface, indicating that the incorporation of lignin contributes to a more homogeneous distribution in the polymeric matrix, as observed in Figure 3. This greater surface homogeneity suggests that the lignin content improves the stability of the films, as also pointed out by Parit et al. (2018) [32]. These images suggest that formulations containing lignin have considerable potential for improvement, which could contribute to the development of biofilms with improved wetting and label adhesion properties, as well as offering barrier properties against fats and moisture, reducing the penetration of contaminants and, consequently, prolonging the shelf life of foods [21].

3.2.8. Antioxidant Capacity (DPPH) and (ABTS)

Table 6 presents the 50% Inhibitory Concentration values obtained for films containing lignin (MHEC), glycerol, and distilled water using the methods (DPPH and ABTS). These values indicate the amount of film needed to inhibit 50% of the free radical activity (DPPH and ABTS), which is also shown in the work [19].

The antioxidant activity of formulations L3, L4, L5, and L6 shows a significant capacity to neutralise free radicals, indicating that the presence of lignin in these films contributes to improving this property. Although there are variations between the formulations with lignin, all of them present antioxidant activity, being necessary to inhibit 50% of the free radical activity (DPPH) with a value of (495 ± 2). Film 3 shows the highest inhibition capacity, with a value of (720 ± 2), evidencing differences in the antioxidant capacity between the formulations, especially in films 5 and 6. This suggests that the amount of lignin influences the final antioxidant capacity [19]. Both methods, DPPH and ABTS, show consistent results, as reflected in Table 6, confirming the antioxidant capacity of the films. However, slight differences in the 50% inhibitory concentration values are observed between the methods, which could be due to the different characteristics of the free radicals and the test conditions. When comparing these results with those of pure lignin, lower inhibition was observed (DPPH: 152 and ABTS: 95) compared to films, suggesting that the differences between the inhibition of pure lignin and films could be related to the composition and processes to which the film matrices are subjected [20]. A study developed a chitosan film with banana peel extract to evaluate its antioxidant activity. The results showed that it improved the postharvest quality of apples, which was also attributed to carotenoids, biogenic amines, and ascorbic acid [8]. Therefore, the incorporation of lignin in the formulations significantly increases the antioxidant capacity of the films, making them attractive for applications where protection against oxidation is required. These lignin-containing films have great potential to be used in the packaging of food, cosmetics, and other oxidation-sensitive products, acting as protective coatings.

4. Conclusions

This study has demonstrated the potential of lignin extracted from the Dominico-Harton banana, combined with glycerol and MHEC, to develop biodegradable films with improved properties. With its significant antioxidant capacity, lignin provides mechanical and barrier properties, while MHEC provides flexibility and biodegradability. Glycerol, acting as a plasticiser, enhances the tensile strength and flexibility of the films by facilitating polymer chain mobility. The synergy between these components improves the mechanical characteristics and homogeneity of the films, highlighting their potential as a sustainable alternative to conventional plastics. The resulting films have a denser and more resistant structure, favouring their application in various areas, such as food packaging, agriculture, and medicine. These films can be used as edible coatings, active packaging materials, or supports for controlled drug release, broadening their application field. In addition, incorporating lignin into MHEC matrices represents a promising avenue for developing sustainable materials, taking advantage of agricultural waste and contributing to sustainability. In short, MHEC is an ideal polymer for designing biodegradable and sustainable materials, with properties adaptable to various needs.