Introduction

Plastic and microplastic pollution are a global threat affecting all ecosystems. The increase in consumption of petroleum derivatives (plastic, oil, gasoline, among others) has been related significantly to the increase of greenhouse gas (GHG) and climate change [1]. It requires urgent management to mitigate the problems of final disposal [2]. It is estimated that 275 million metric tons of plastic are produced worldwide annually, and from 4.8 to 12.7 million metric tons are deposited in the oceans. Single-use plastics such as plastic bags and microbeads are the primary contributors to this pollution [3, 4]. In the soil, plastics have significantly impacted agriculture; the principal effects are modifying the microbiota and the mobility of other contaminants that alter the soil ecosystem functionality [5]. For this reason, alternatives have been sought to help mitigate the environmental problems generated by synthetic polymers due to their low or null degradability.

The circular economy based on biological sources requires the correct use of resources [6]. Many commercial products, such as paper, biofuels, and bioplastics, are industrially produced. However, obtaining biopolymers from crops of economic importance increases prices and limits access to these products [7]. For this reason, there is a growing interest in using non-conventional waste, such as those generated in industries and agriculture. The starch, cellulose, pectin, and protein, among other biopolymers, can be isolated from biomass from agro-industrial residues [8]. These biopolymers present mechanical, physicochemical, thermal, and structural properties that make them very similar to those obtained from synthetic plastics [9].

Plastic films or bioplastics can be synthesized from natural organic materials such as polysaccharides, proteins, and lipids, and their biodegradability capacity makes them an alternative to reduce the large volumes of synthetic materials produced annually derived from oil [10].

The proteins used as biopolymers provide multiple molecular interactions (covalent, ionic, and hydrogen bonds) with the biopolymers present in the mixtures, creating materials with improved mechanical properties than bioplastics produced from polysaccharides and lipids [11, 12]. It is necessary to search for biomolecules that can provide firmness and rigidity to biomaterials and offer similar characteristics to plastic made from petroleum derivatives. Some authors have reported the development of biodegradable films using proteins and reinforcing with cellulose nanofiber [13], cellulose nanocrystals [14], and microcrystalline cellulose [15]. However, other compounds could give new alternatives to biomaterial, and they could be found in several agricultural wastes such as 5-HMF, present in the vinasses, a waste of ethanol, and spirit beverage industries.

The ethanol industry generates 10 to 15 L of vinasses per liter of ethanol processed [16]. Similar vinasses are produced in the state of Jalisco, Mexico, from the tequila factories [17]. The Tequila Regulatory Council (TRC) estimates 2288 tons of Agave tequilana Weber var. Azul was used in 2023, generating approximately 5.9 × 109 L of vinasses [18].

The composition of vinasses is complex, presenting a high percentage of water, inulin, fermentable sugars, organic acids, esters, alcohols, lignocellulosic compounds, 5-HMF, furfural and phenolic compounds [19]. Its high organic matter content generates severe problems in agricultural soils due to its irrigation use or inadequate disposal into water bodies [20]. In addition, the treatment for their correct disposal is expensive and not complete by physical, chemical, biological, or combinations [21].

FPCs are mainly composed of furans as 5-HMF, phenols (caffeic, ferulic, 3-hydroxybenzoic, chlorogenic, vanillic, syringic, sinapic, 2,4-di-ter-buthylphenol, salicylic, and cumaric acid) and sugars (xylose, glucose, fructose, and sucrose) from Agave vinasses after two thermal treatments using H2O2 as catalyst [22]. Some molecules in vinasses are precursors of compounds of economic importance due to their ability to form materials with characteristics like synthetic plastics. The 5-HMF is included in the list of the twelve most promising biobased molecules in forming materials biofuels, and as a quality indicator of food, it could be harmful if consumed in excess [23, 24]. Some foodstuffs that contain high concentrations of 5-HMF include coffee (900–1200 mg/kg), breakfast cereals (15–717 mg/kg), corn syrups (47–1655 mg/L), honey (2–232 mg/L), among others [24]. The 5-HMF synthesizes polyethylene furanoate, a material with characteristics like polyethylene terephthalate [7, 25, 26]. Their chemical structure with furan ring and functional groups makes it a material promising for various chemical transformations [27] such as fuels, organic solvents, biologically active molecules, and obtaining plastics [28].

On the other hand, adding inulin to films gelatin-base and cellulose increases the physicochemical properties, lowers water vapor permeability, and improves mechanical properties [29, 30]. The phenolic compounds are considered additives, and their use in forming films could contribute as natural antioxidants [31, 32]. In addition, the development of active, edible, and biodegradable films and coatings has emerged as one of the most significant trends in medicine and food science over the last few decades.

This study proposes an alternative to add value to ethanol distilleries' residues. To our knowledge, this is the first instance where biodegradable films have been developed with the addition of Agave vinasses conjugated (FPC). This study aimed to fabricate a material with good biodegradable characteristics and mechanical, chemical, and structural properties like commercial plastics.

Materials and Methods

Materials

The biopolymers used were gelatin (Duche, Mexico) with 300 Bloom, glycerol (Jalmex, Mexico) used as a plasticizer, and micro cellulose (Sigma-Aldrich™). Dowex Optipore® supplied the SD2 resinore® and the furfural, 5-HMF, xylose, glucose, fructose, sucrose, hydrogen peroxide, sodium chloride, caffeic acid, ferulic acid, 3-hydroxybenzoic acid, chlorogenic acid, vanillic acid, syringic acid, sinapic acid, 2,4-di-ter-buthylphenol acid, salicylic acid and cumaric acid were supplied by Sigma-Aldrich™.

Characterization of Agave Vinasses

Tequila factories provided the vinasses with an artisanal distillery process in Arenal, Municipality of Jalisco, Mexico. The composition of the vinasse original and FPC is shown in Table 1. Analytic methods determined the characterization of vinasses as UPLC-MS (XEVO TQS Micro) (Waters, Milford, USA) for furans, phenolic compounds, glucose, fructose, and xylose, and HPLC-SEC (1220 Infinity LC System HPLC) (Agilent, Alpharetta GA, USA) was used to determine the concentration of inulin according to methods reported by Lorenzo-Santiago et al. [22]. The compounds were quantified using standard reactive with curves in 1, 2.5, 5,12.5, 25 and 50 mg/L for furans, 0.1, 0.5, 1, 2.5 and 5 mg/L for phenolic compounds, 5, 10, 15, 20, 25, and 50 mg/L to xylose, sucrose, glucose and fructose, and 1, 2, 4, 6, 8, 10, 16, and 20 g/L for inulin. The mass spectrometric conditions are shown in Table 2.

Table 1 Conjugates of Agave vinasse characterization
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Table 2 Conditions for mass spectrometric of phenolic compounds, furans, and sugars found in Agave vinasses
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Chemical Oxygen Demand (COD) was determined using vials (20–1500 mg/L) (Hach, Mississauga, ON, Canada). The samples desorbed were prepared at a 1:100 dilution, and 2 mL of sample was added to the vials. A blank sample with deionized water was calibrated to zero the absorbance on the spectrophotometer at 550 nm. They are stirred and placed in the digester at 150 °C for 2 h. Subsequently, they were allowed to cool down and analyzed in the spectrophotometer DR 5000™ UV–Vis (Hach, Mississauga, ON, Canada) were used [33].

Thermal Modification of vinasses Agave to Obtain FPC

The vinasses were used to pH 2.4 (natural acidity) and adjusted to pH 7.0 using NaOH to 0.5 N. The experiment was performed in triplicate for each sample and thermal treatment. The Resin SD2 was previously suspended in water. The resin was eluted from ethanol with 140 mL of 4% HCl, followed by distilled water until the pH of the eluent became neutral. In 500 mL Erlenmeyer flasks, 10 g of resin was combined with 100 mL vinasse. Batch adsorption occurred at 25 °C with agitation at 45 rpm for 24 h. For desorption testing, the supernatant was removed by settling, and the resins were hydrated with 50 mL using 95% ethanol: water (v/v). The desorbed effluents underwent two thermal treatments with 0.5% (v/v) of hydrogen peroxide (H2O2) as a catalyst, heating to 130 ºC for 40 min in an autoclave at 34 psi. The conjugates obtained were denominated FPC, as was reported by Lorenzo-Santiago et al. [22] in previous work.

Formulation of Biodegradable Films

Biodegradable films were developed by applying modifications to the methods described by Rendón-Villalobos et al. [34] and Jafari et al. [35] particularly in the gelatinization temperature and concentration of biopolymers. The formulation was prepared from gelatin (3% w/v), cellulose (0.1% w/v), glycerol (0.9% w/v), FPC (1, 2, and 3% w/v), and distilled water (96, 95, 94 and 93% v/v). The proposed formulations and the codes utilized for each film are shown in Table 3. The gelatin was hydrated with distilled water and kept at 60 °C for 10 min. Glycerol, cellulose, and FPC were applied until it reached 65 °C for 10 min. The filmogenic solution was cast onto the Petri dishes (90 mm in diameter) and dried at 40 °C for 24 h. The films obtained were peeled off and stored in a desiccator at 25 °C with a relative humidity of 50% provided by a saturated solution of NaCl following the ASTM F1927-20 method [36] up to characterization analysis. Some biopolymers can absorb or lose moisture depending on the surrounding humidity [37]. This can lead to changes in their flexibility, tensile strength and their biodegradability capacity [38, 39]. By conditioning at a standard relative humity of 50%, we ensure that the material reaches a moisture equilibrium, thereby preventing any fluctuations in film properties that could affect test results. In addition, all films were management at same relative humidity condition.

Table 3 Codes and composition (% w/v) for each formulation of control and biodegradable films
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Water Solubility

The films were cut into squares (20 mm × 20 mm). Subsequently, they were subjected to drying for seven days in a desiccator at room temperature. After that, the samples are weighed and placed in a 50 mL falcon tube with 30 mL of distilled water. The tubes were shaken for 48 h at 200 rpm at room temperature. The tube contents were filtered and dried at 110 °C for 24 h. The percentage of soluble matter was calculated using Eq. 1 [40]. The tests were carried out in triplicate in all formulations and compared with the commercial polymer plastic used as control.

$$\text{Solubility }\left(\text{\%}\right) =\left[\frac{\left(\text{Initial dry weight}-\text{Final dry weight}\right)}{\text{initial dry weight}}\right]\times 100$$
(1)

Determination of Mechanical Properties

The films were analyzed following the ASTM D-882-02 method [41]. Films of each formulation with FPC (1–3%), control (gelatin-cellulose), and commercial plastic film (low-density polyethylene (LDPE) bag) were cut (100 mm long by 10 mm wide). The deformation rate used was 30 mm/min until it was broken, with a gap between heads of 50 mm. The mechanical properties analyzed were tensile strength (TS), elongation (%E), and Young's modulus (Ym). These were determined using a BlueHill Lite from INSTRON Model 2519–107 (Massachusetts, USA).

Thermal Characterization by DSC

Thermal characterization was realized using a DSC, TA Instruments DSC 2010 (New Castle, USA). The equipment was calibrated using indium with a melting point of 156.4 °C and an enthalpy of 6.8 cal/g. Each sample (FPC 1–3% and control (gelatin-cellulose)) was carefully weighed between 5 and 10 mg. The sample was used for a heating program that ranged from 20 to 300 °C, with a heating rate of 10 °C/min. During the measurements, 50 mL/min of nitrogen gas (99.9% purity was circulated) was used to maintain an inert atmosphere.

Biodegradability in Soil

Biodegradability analysis by weight loss was performed as per ASTM D5988-12 [42]. Films with FPC (1–3%) and control (gelatin-cellulose) were cut into squares (40 mm × 40 mm), weighed, and deposited in a Petri dish containing 20 g of agricultural soil adjusted to 25% moisture and were incubated at 30 °C. The soil was characterized by texture [43] USDA modified soil texture triangle, pH in water [44], organic C content [45], total N [46], electrical conductivity [47], and total phosphorous [48]. The soil characterization is shown in Table 4.

Table 4 Soil characterization
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The film samples were weighed at 0, 5, 10, 15, 20, and 30 days of soil exposition. A negative control was included in the tests using a commercial plastic, such as a low-density polyethylene (LDPE) bag [34]. The remaining films were cleaned using distilled water until all soil was removed, and they were dried for 24 h at 40 °C. Finally, their weight was recorded. The biodegradability percentage was calculated from Eq. 2. Photographic monitoring was done at 0, 5, 10, 15, 20, and 30 days of soil exposition. Also, a surface analysis was realized by ESEM and structural composition by FTIR at 0 and 15 days, described below.

$$\text{Biodegradability }\left(\text{\%}\right)\left[\frac{\text{Initial weight}-\text{Final weight}}{\text{Initial weight }}\right]\times 100$$
(2)

Structural Characterization by ESEM

Films were adhered to a double-adhesion carbon conductive tape. The characterization was realized by ESEM Carl Zeiss EVO LS 10 (Munich, Germany). Subsequently, they were observed at a voltage of 15 and 30 kV, with a resolution of 3─10 nm; the micrographs were taken in surface and cross-sectional at 100×and 500×magnifications [49].

Chemical Composition by FTIR

The biodegradable films (FPC 1–3%), control (gelatin-cellulose), and commercial plastic from 0 and 15 days (from biodegradability assay) were previously cut into dimensions of 10 mm2. FTIR was employed for the structural characterization of films, utilizing a Perkin Elmer Spectrum 100/100 N model (Shelton, CT, USA). The FTIR analysis was conducted in the range of 4000–650 cm−1 in the transmittance mode, with a resolution of 16 cm−1, and eight scans were performed [19].

Statistical Analysis and Software Used

Statistical analyses were performed with Minitab 20.3 software. All results obtained were subjected to a variance analysis (ANOVA) and post hoc Tukey test with a probability of 5% (p < 0.05) with a confidence level of 95% to analyze the differences between the concentration FPC added in films and the capacity of biodegradation in soil. The abstract graphic for this manuscript was done with BioRender [50].

Results and Discussions

Characterization of Furan-Phenol Conjugates (FPC)

The thermal treatment of residual vinasses, catalyzed by H2O2 and adjusted to a pH of 7.0, significantly reduced the concentration of Chemical Oxygen Demand (COD) by 47%. The treatment also led to a reduction in the concentration of total sugars (58.7%), sucrose (94.62%), fructose (75.37%), glucose (84.79%), and xylose by 40.39%, compared to the original vinasse (Table 1). Notably, the concentration of 5-HMF increased 13-fold, and the phenolic compounds showed a significant increase, i.e., the 3-hydroxybenzoic acid (512-folds), 2,4-di-tert-butylphenol acid (290-folds), salicylic acid (77-folds) and syringic acid (56-fold).

The decrease in sugar concentration is likely due to the increase in 5-HMF, with fructose being the primary promoter of furan formation [51]. The reaction mechanisms of sugars from Agave vinasses in the conversion to furans and phenolic compounds have been previously reported [19].

Water Solubility

Solubility in water is an important parameter to consider, depending on the application of materials. A high solubility is undesirable when the product will be used with food or comes into direct contact with liquids, such as in packaging, bags, plastics for agricultural use, and other applications [52, 53]. However, there are different applications where high solubility might be necessary, such as in edible coatings for fruits and vegetables [54]. Table 5 shows the water solubility of all films evaluated. The positive control presented the highest solubility (48.92%), attributed to gelatin and glycerol's hydrophilic nature[55]. It is necessary to cross-link polymeric chains or use two or more polymers to increase the solubility [56]. The proposed reaction between compounds is illustrated in Fig. 1.

Table 5 Solubility in water and the mechanical properties of films added with Agave vinasse conjugates (FPC) and controls
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Fig. 1
figure 1

Reactions mechanism proposed between major compounds present in films

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Adding FPC decreased water solubility in this study. The FPC-3 film showed the lowest solubility at 39.89%. This decrease could be attributed to the formulation's higher content of 5-HMF. 5-HMF can undergo oxidation to form 2,5-furandicarboxylic acid (FCDA) or hydrogenation to form 2,5-dimethylfuran (DMF); both are used in the production of chemicals and fuels [57,58,59]. These compounds may contribute to higher stability in the materials and lower solubility. All films showed significantly higher water solubility than the commercial plastic (Table 5).

These results are consistent with those reported by Almeida Soares et al. [60] for films made from chitosan adding pomegranate (Punica granatum L.) peel extract. The films showed a solubility of 20% for the lowest concentration of extract added in the formulation and 10% for the highest concentration. The solubility in these films decreased when the concentration of bioactive matter was increased in the formulation. The decreased solubility in films made from biopolymers combined with conjugates could suggest a potential to replace conventional plastics, and FPC could improve mechanical properties.

Mechanical Properties

Table 5 displays the mechanical properties of biodegradable films supplemented with conjugates from vinasses and commercial plastic film. The gelatin-cellulose film (control) exhibited a lower elongation (%E) and Young's modulus than all other films. However, there was no significant difference in tensile strength (TS) between the control and FPC-1, FPC-2, and FPC-3 films. These properties could be attributed to the addition of cellulose in the formulation of the control sample. Cellulose is often used as a reinforcement material due to its mechanical and thermal properties, particularly when the polymer matrix cannot form strong bonds or requires additional stability [15, 49].

The sample FPC-3 presented a significative higher (%E) and Young's modulus than FPC-1, FPC-2, and control films but significantly lower with commercial plastic (Table 5). The FPC-3 film increased 2.14 folds the elongation percentage (63.15%) and 3.62 folds Young´s modulus (82.92 MPa), compared to the control film, suggesting that the addition of conjugates provides a high elasticity and resistance into the films. Using a 3% concentration of FPC resulted in significantly higher elongation values and Young’s modulus in the films. This behavior can be attributed to cellulose, inulin, glycerol, and 5-HMF interaction with the gelatin matrix. These compounds are known to form hydrogen bonds with the amine (NH2) and carboxyl (COOH─) groups of gelatin, thereby enhancing the mechanical properties of the films (Fig. 1) [61, 62].

Some authors propose the addition of high-density or low-density polyethylene with modified composites to improve the mechanical capacities [63,64,65]. However, the degradation time increases, and residual microparticles are not degradable in the environmental matrixes after degradation.

Harussani et al. [66] reported tensile properties and Young´s modulus in cornstarch films added 45 to 60% of glycerol and sorbitol, obtaining 4.49 to 13.61 MPa in tensile strength and Young´s modulus from 11.88 to 61.7 MPa, values like showing in this work. It was reported that adding 30% compound plastifiers (such as glycerol and sorbitol) to films could generate less rigid films [66]. Choi et al. [67] prepared films with possible uses in the food industry using turmeric and gelatin added with tannic acid, caffeic acid, and green tea extract. They reported that low concentrations of phenolic compounds cause a reticulation effect, modifying the chemical properties of biopolymers and forming more resistant materials.

Thermal Characterization by DSC

This study used DSC to investigate the thermal capacity of films with gelatin base and added to Agave vinasse conjugate. All films showed endothermic peaks at various temperature ranges (Fig. 2). The control film showed an endothermic peak that occurred at 115.8 °C. After the addition of conjugates, the thermal capacity of the FPC-1, FPC-2, and FPC-3 films increased by 19.5, 21, and 21.3 °C, respectively, compared to the first endothermic peak of the control sample. According to the report by Nunes et al. [68], the peaks that oscillate at 100 °C represent the evaporation of water. Al-saidi et al. [69] reported that this peak could be related to amine, carboxylic, and hydroxyl groups present in the samples or the evaporation of the free water molecules from the hydrophilic components of the polymers [70].

Fig. 2
figure 2

DSC thermograms with melting transition temperature (Tm) from films (FPC-1, FPC-2, FPC−-3 and control) and the films’ melting enthalpy (∆Hm) at 0 days

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In this work, the melting temperature (Tm) increases with the addition of conjugates, and the enthalpy decreased at 71.5, 99.5, and 91.7 J/g for FPC-1, FPC-2 and FPC-3, respectively, compared to of the control film (263.1 J/g). These results may be attributed to the stability of hydrogen bond chains formed within the polymeric material (Fig. 1) [71]. Additionally, Andrade et al. [66] report that adding phenolic compounds, such as ferulic acid, could promote crosslinking, enhance crystallinity, and increase film thermal capacity.

The second peak of the control film was presented at 218.1 °C with an enthalpy of 7.3 J/g. This enthalpy could be attributed to the addition of cellulose in the formulation [49]. The thermogram showed melting temperatures at 229.8, 235, and 239.2 °C, related to thermal decomposition of gelatin. Similar results were reported by El-Meligy et al. [72] showing a degradation temperature at 230 °C in bovine type B gelatin. The enthalpies were 12.7, 9.5, and 13.8 J/g for FPC-1, FPC-2, and FPC-3, respectively. According to a report by Lorenzo-Santiago et al. [19], the vinasses could be composed of lignocellulosic compounds such as hemicellulose, lignin, and cellulose. Among these, cellulose exhibits a crystalline structure that can enhance the thermal stability of materials [73]. High thermal stability could help produce biodegradable films from the casting or extrusion process [74].

Surface Morphology by ESEM

The morphology from the film's surface and cross-sectional were observed in Figs. 3a and 3b, respectively. Micrographs illustrate the morphology of the films at the initial stage (before degradation in soil). The control film presented a corrugated surface, revealing the structure of the gelatin and glycerol, which were not well distributed. The cross-sectional view displayed low integration and fractures, with porosity visible on all surfaces.

Fig. 3
figure 3

ESEM Micrograph of (a) surfaces (100×) (red circle indicates the lack of homogeneity on the surface of the films), and (b) cross Sects. (500×) from control, FPC-1, FPC-2 and FPC-3 films at 0 days (red circle indicate presence of pores and blue arrow the integration of materials), and (c) surface ESEM images (500×) of films after of 15 days of biodegradation test (red circle indicate presence of pores and blue arrows the presence of microorganisms)

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The film FPC-1 displayed varying coloration and an irregular surface, which may be attributed to uneven dispersion of the conjugates within the matrix, leading to discontinuous regions or cavities in the film. However, the cross-sectional view revealed a reduction in porosity. The films FPC-2 and FPC-3 presented a compact surface, smooth and flat, suggesting that films with a higher content of conjugates can achieve better dispersion and compatibility within the gelatin-cellulose matrix. Also, it is attributed to the presence of the carboxyl and amine groups and the formation of hydrogen bonds between the compounds, which aligns with the observed results for the mechanical and thermal properties in films (Fig. 1). Similar results were reported for pea starch/pulp cellulose nanofiber composite biodegradable films added with apple polyphenols [75].

Chemical Characterization by FTIR

The films were compared with the structural composition of a plastic bag and a film without conjugates (Control gelatin-cellulose) (Fig. 4a).

Fig. 4
figure 4

FTIR spectrum in (a) commercial plastic, control (gelatin-cellulose film), FPC-1 (1% conjugates), FPC-2 (2% conjugates), and FPC-3 (3% conjugates) films at 0 days, and (b) after 15 days of biodegradability in soil

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The polymer plastic spectrum displayed absorption bands at 2920 and 2845 cm−1, indicating C–H stretching. Another peak was observed at 1740 cm−1, corresponding to C = O stretching, while peaks at 1460 cm−1 and 720 cm−1 were attributed to CH2 bending and CH2 rocking, respectively, characteristics of high-density polyethylene [76]. The FPC 1–3% of conjugate and control film presented a strong signal to 3280 cm−1, corresponding to the O–H stretching vibration. This peak is associated with water molecules, alcohols, carboxylic acids, sugars such as inulin, and other organic compounds [77]. The increase of this peak was related to the increase in vinasse conjugate, with the film FPC-3 having the most extended peak.

Other characteristic peaks were phenolic compounds. The signal 2930 cm−1 indicated the asymmetric vibration of the aliphatic structures, and 1446 cm−1 the antisymmetric bending in the CH3 bond [78]. On the other hand, the signals for 5-HMF were observed in 1648 cm−1, a region that expresses the vibrations stretching of the aldehyde group (C = O), and in 1522 cm−1 the stretching vibrations of the ring (C = C) [79, 80].

The range 1084 to 800 cm−1 region has been assigned to the C–O stretching vibration, CH2–OH and C–O–C. Functional groups in this region are associated with the compounds with structures, typical structures of carbohydrates such as glucose, xylose, fructose, sucrose, and inulin [77, 80, 81].

Biodegradability of Films by Weight Loss in Soil

The weight loss methodology is the most utilized index for elucidating the degradation rate across varying treatments [34, 79, 82]. The biodegradation process of all samples in the soil can be observed through photographic analysis (Fig. 5), and the percentage of biodegradability by weight loss from films is shown in Fig. 6.

Fig. 5
figure 5

Photographic tracking in biodegradability test in soil under laboratory conditions

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Fig. 6
figure 6

Films biodegradability percentage recorded in soil. Different capital letters indicate significant differences between films on the same day, while different lowercase letters indicate significant differences in the same film between all times. Tukey's test was used with a significance level of p < 0.05. Bars represent the standard error

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The polymeric plastic does not present changes after 30 days in soil. The exposition in the soil is not sufficient to present a structural modification or weight loss in polymeric plastic.

Huang et al. [83] reported that the exposition of polyethylene films in the soil for 50 to 100 days could change the microbiota in the soil, but the biodegradability is not representative.

At 5 days in soil, all samples presented a biodegradation lower than 17%, with the FPC-1 (16.2%) film having the highest biodegradability, followed by the control (15.9%) (Fig. 5). However, there are no significant differences between both.

At 10 and 15 days in soil, the control film exhibited the highest biodegradation, reaching 45% and 63%, respectively, while the FPC-1 films showed 29.3% and 51.7%. Similar results were reported by Susmitha et al. [84] with a degradation of 50% after 15 days of exposition in the soil of edible films of corn starch-gelatin integrated with mango and pineapple. In contrast, FPC-2 and FPC-3 demonstrated lower biodegradation, with 16.3% (10 days) and 33.9% (15 days) and 16.5% (10 days) and 35.5% (15 days), respectively (p > 0.05). Both films presented significative differences (p < 0.05) with FPC-1 and control films.

The control film exhibited the most significant weight loss after 30 days in soil, demonstrating a 98% biodegradability. In contrast, FPC-1 showed a slightly lower rate of 95.1%, while FPC-2 and FPC-3 reported 87.9% and 82.7%, respectively, indicating less degradation in films formulated with 2% and 3% concentrations of Agave vinasse conjugates. Statistically significant differences were observed between the four samples. These findings align with previous research on biodegradability, such as a study on a fiber-reinforced starch biocomposite film that achieved complete biodegradability within 30 days [85], as well as a study on a corn starch active biocomposite film used in bread packaging, which demonstrated 100% biodegradability within 35 days in soil [86].

Figure 3c shows the surface characteristics of all samples after 15 days of exposure to soil, as observed through ESEM micrographs. The control film exhibited a high porosity level, with visible fibers on the surface. This surface structure was attracted by microorganisms from the soil. FPC-1 showed the growth of septate mycelium and porosity on the surface. In contrast, FPC-2 exhibited septate and non-septate mycelium, bacteria (coccus), and tiny pores on the surface. Finally, the surface of FPC-3 displayed minimal mycelium and did not present porosity. The presence of phenolic compounds by Agave conjugates in the biodegradable films could potentially inhibit the adaptability of microorganisms [87].

Figure 4b shows the spectrum of all films after 15 days of exposition in soil. The spectrum showed a reduction in the signal at 3280 cm−1 compared to films in the soil at 0 days (Fig. 4a). This change could be due to dehydration during soil exposition and the degradation of organic matter. The same occurs with the signal at 2930 cm−1, peak characteristics by phenolic compound [78].

The peak at 1648 cm−1, which indicates the stretching vibrations of the aldehyde group (C = O) of 5-HMF, significantly reduces in intensity. This reduction in intensity could indicate the degradation of 5-HMF by the soil microorganisms that use HMF as a carbon source [79, 88].

The region between 1084 cm−1 and 800 cm−1, corresponding to carbohydrates, exhibits low signals, indicating the sugars’ degradation in the conjugates [77, 80, 81]. These results suggest that after 15 days in soil, fungi, and bacteria colonized the surface and thoroughly adapted to compounds in the films. In FPC-3 less microbial presence was observed on surface films, possibly by high amounts of phenols and 5-HMF, which done slow biodegradation due to their antimicrobial properties. After 30 days, the samples reach their highest biodegradability in soil (Figs. 5, 6). The properties reported here are better or comparable to the yields obtained from other films using gelatin-cellulose as a base for forming biodegradable materials (Table 6).

Table 6 Previously reported gelatin-cellulose films
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The search for materials with biodegradable characteristics becomes a primary need due to pollution problems from the high volumes of single-use plastic waste generated annually.

Conclusions

The films enhanced with conjugates from Agave vinasses could be used in various industries as an alternative to reducing the production of single-use plastics. The increase in thermal capacity, improvement in mechanical properties, and natural antimicrobials such as phenols and furans make them suitable for packaging perishable fruits, as agricultural coatings in postharvest applications, and as biodegradable mulches in agriculture. However, it is necessary to conduct microbial inhibition analyses on these materials using microorganisms that negatively impact the food and crops of economic importance. Additionally, other applications for these films should be researched.