Exploring the Plasticizing Effect of Yerba Mate Extract and Malic Acid in Potato Starch-Based FilmsAbstract
This study aimed to evaluate the influence of yerba mate extract (YM) (1%, 10%, 20% w/w) and malic acid (MA) (0.5% w/w) on the mechanical response of potato starch-glycerol based films (PS), targeting disposable packaging applications. YM incorporation decreased the Young's modulus and tensile strength of the films proportional to its content, while increasing elongation at break. The effect of MA was found to be synergistic, further reducing rigidity and improving flexibility. DMA analysis indicated that molecular relaxations shifted to lower temperatures with increasing YM content and MA addition, suggesting a plasticizer-like behavior. SEM images revealed surface homogeneity in MA formulations, confirming better additive-matrix compatibility, as well as the presence of nanoparticles at higher YM concentrations. These findings demonstrate that the combination of YM and MA in starch-based matrices is effective in producing flexible films, with MA20YM formulation (0.5% MA and 20% YM) showing the best overall performance.
Keywords:
Starch films; polymeric films; potato starch; yerba mate; malic acid; plasticizer; crosslinking agent
1. Introduction
The consumption of petroleum-based polymeric materials, while indispensable to modern society, poses a significant environmental threat. Their use in rapidly disposable products fails to account for their prolonged degradation time in nature, resulting in persistent waste islands that accumulate and pollute oceans, freshwater bodies and land1,2. Approximately 60% of the conventional plastics market is dedicated to packaging production, a sector whose growth has been driven by a global shift from reusable to single-use packaging3,4. The packaging field remains dominated by polymers such as polyethylene and polystyrene, which exhibit decomposition times of hundreds of years2,3,5,6.
Considering the environmental impacts arising from the production of conventional polymers, the exploration of petroleum alternatives has been extensively studied over the past decades. Consequently, polymers derived from renewable sources, such as starches, chitosan, proteins and lipids, have been employed in the development of environmentally friendly packaging7,8. Biodegradable materials present a promising alternative to conventional packaging polymers, as long as they demonstrate properties suitable for this application9,10. Among these, starch demonstrates great potential as a polymeric film matrix due to its biodegradability, low cost, renewability and edible nature7,8,11. The fundamental component of starch granules, accounting for approximately 99% of their structure, is D-glucose, which exists in two polymeric forms: amylose and amylopectin. The amylose-to-amylopectin ratio plays a crucial role in determining the processing and final properties of the product. For instance, the amylose content influences the tensile strength and elastic modulus of starch films, contributing to increased stiffness12-16.
As a starch source, potato (Solanum tuberosum L.) is a vital food crop in South America, having been consumed for over 8,000 years in the Andean region. Potato starch is readily available in large quantities, making it an attractive raw material for producing biodegradable packaging1,17. Potato ranks as the fourth most produced food crop globally, after rice, wheat and maize18. In Brazil, annual potato production surpassed 4.0 million tons in 2023, with cultivation predominantly concentrated in the South and Southeast regions19. Compared to commercial starches derived from maize and wheat, potato starch produces more flexible films, benefiting from its lower amylose content (18–20%)20-22.
The use of agro-industrial products in the composition of new materials is an appealing alternative for modifying material properties while promoting sustainability and adding value to local products10. Yerba mate (Ilex paraguariensis), a plant native to South America, is one of the most important socio-economic crops in southern Brazil, primarily cultivated by small-scale farmers23-26. Brazil is the world's largest producer of yerba mate, with production exceeding 700,000 tons in 202327,28. Yerba mate is mainly consumed as chimarrão, a beverage prepared by repeatedly adding hot water (70–90 °C) to the plant's dried and crushed leaves28-30. In South America, an estimated 30% of the population consumes more than 1 liter of yerba mate infusions daily31. While yerba mate extracts have been extensively researched over recent decades for its health benefits, very few studies have investigated its use as an additive in polymeric matrices24.
Despite the potential of starch-based films, their properties still do not match the performance of traditional polymers. The hydrophilic nature of starch, while crucial to its biodegradability, leads to insufficient mechanical and barrier properties11,32. Consequently, starch alone is unsuitable for most applications and must be modified to enhance its properties for functional use33. In this regard, polycarboxylic acids, such as malic acid, are employed as crosslinking agents to improve starch matrix properties. These acids are non-toxic and safe for consumption34. When incorporated into starch formulations, malic acid promotes the formation of ester bonds between hydroxyl groups, introducing new molecular linkages within the starch molecules and stabilizing their structure35-37.
Given these limitations, this study aims to explore the incorporation of yerba mate extract into potato starch-based films, with the objective of improving their mechanical properties. Additionally, the use of malic acid as a crosslinking agent is investigated to further enhance the films' performance for potential food packaging applications.
2. Materials and Methods
2.1. Materials
Potato starch was supplied by Federal Institute of Rio Grande do Sul (IFRS). Glycerol P.A./ACS (NEON) was used as plasticizer and malic acid (DL) P.A. (Êxodo Científica) was used as crosslinking agent. Yerba mate (Ilex paraguariensis) extract was simulated using commercial yerba mate purchased at a local market in Porto Alegre, Brazil.
2.2. Preparation of the yerba mate extract
The yerba mate extract was prepared by infusing 3 g of sifted yerba mate in 100 g of distilled water at an initial temperature of 80 °C under agitation24,38. After 5 minutes, the liquid was filtered and cooled to room temperature.
2.3. Preparation of the potato starch films
The polymeric matrix was developed by mixing 5 g of potato starch (PS) in glycerol (1 g) and distilled water (94 g). For samples containing yerba mate extract (YM), a fraction of distilled water was replaced by the desired concentration of extract (1 g, 10 g and 20 g), based on the method described by Jaramillo et al.24,38 and Piñeros-Hernandez et al.39. The systems were homogenized under constant agitation at 80 °C until complete starch gelatinization. Subsequently, about 30 g of each filmogenic solution was poured into individual polymeric plates. After casting, the solutions were oven-dried at 30 °C for ~24 h under ventilation in order to form polymeric films. The same method was followed to prepare samples containing 0.5% w/w malic acid (MA), with a proportional decrease in the distilled water fraction. The thickness of the obtained films was (0.11 ± 0.01) mm. Table 1 provides the composition of the samples expressed in mass percentage (% w/w).
Composition of film-forming solutions expressed in mass percentage (% w/w). All samples contained 5.0% potato starch (PS) and 1.0% glycerol. Yerba mate extract (YM) was added at 1–20%, and malic acid (MA) at a fixed 0.5%. Distilled water completed the total to 100%.
2.4. Characterization
2.4.1. Uniaxial tensile tests
Uniaxial tensile tests were performed using an Instron 3367 universal testing machine, with a testing speed of 20 mm/min and an initial grip separation of 25 mm, following the recommendations of ASTM D882-1840. For each formulation, seven samples measuring 80 mm x 25 mm were tested, generating stress-strain (σ vs. ε) curves, along with the determination of the Young’s modulus (E) and tensile toughness values.
2.4.2. Dynamical mechanical tests
Samples measuring 60 mm x 40 mm were subjected to testing on a dynamic mechanical analyzer (DMA 1 STAR System, Mettler Toledo) in tension mode, over a temperature range from -80 °C to 50 °C, with a heating rate of 2 °C/min. A tensile strain of 0.02% was applied to ensure operation within the linear viscoelastic range.
2.4.3. Scanning electron microscopy (SEM)
In order to investigate the morphology of the films, their surface was coated with a thin sputtered gold layer and was examined using a scanning electron microscope JEOL JSM-6510LV in 2000x.
2.4.4. Statistical analysis
Analysis of variance (ANOVA) was performed in all results and the Tukey mean comparison test was applied to evaluate average differences at a confidence interval of 95%.
3. Results and Discussion
3.1. Tensile parameters
Films used for food packaging must withstand external stresses and exhibit adequate flexibility. The main parameters used to evaluate the mechanical properties of these materials include tensile strength (σ), which represents the film's resistance to rupture; strain (ε), which expresses the percentage of elongation relative to the film's initial length; elastic modulus (E), which reflects the material's stiffness; and toughness, which indicates the film's ability to absorb energy and deform before fracturing41,42. The stress-strain (σ-ε) curves obtained under uniaxial tension at room temperature showed the typical behavior of starch and glycerol-based films, regardless of the incorporation of additives: a linear elastic region followed by nonlinear behavior until failure, which occurs with a sharp drop in load at maximum stress, without necking prior to fracture (Figure 1).
The values of the mechanical parameters are detailed in Table 2. The incorporation of yerba mate extract (Figure 1a) resulted in a reduction of the elastic modulus and tensile strength, accompanied by a significant (p ≤ 0.05) increase in the elongation at break. Compared to the PS matrix, these films showed up to 25% lower stiffness and about a 5% reduction in tensile strength. Elongation at break and toughness increased substantially with the rise in extract concentration, reaching values more than 80% higher in the PS20YM film. Similar results were observed by Jaramillo et al.24,43 in cassava starch films with yerba mate extract addition. Other authors also reported similar behaviors with the incorporation of plant extracts into biopolymeric matrices, such as pomegranate peel44, corn stigma45, betel leaves46 and green tea47.
Mechanical properties of starch-glycerol films with different concentrations of yerba mate extract (YM) and malic acid (MA).
The addition of malic acid (Figure 1b) appears to enhance the plasticizing effect of the yerba mate extract, leading to over a 130% increase in elongation at break and toughness in the MA20YM film, compared to the control (PS) (Table 2). Although malic acid modifies the molecular structure through crosslinking, its plasticizing effect has also been reported in starch-based films by Thessrimuang and Prachayawarakorn33, Niazi et al.37,48,49 and Karma et al.50. Studies by Fahrngruber et al.51 and Jiugao et al.52 found that concentrations of polycarboxylic acids up to 0.6% w/w imparted ductility to the films, whereas higher concentrations (above 0.8% w/w) induced excessive crosslinking, limiting starch chain mobility and reducing elongation at break due to acid-promoted hydrolysis51,53. It is worth mentioning that, in this study, the malic acid content was 0.5% w/w, which aligns with the concentrations recommended by these authors as optimal for enhancing film ductility without compromising molecular mobility.
Figure 2 illustrates the comparison of tensile strength and elongation at break values for the films. The general trend observed in the mechanical properties with the addition of yerba mate extract can be attributed to a reduction in the cohesive forces within the starch network, leading to lower tensile strength and a more deformable material. The decrease in elastic modulus and maximum tensile strength with increasing extract concentration is likely due to the incorporation of low-molecular-weight molecules between starch chains, which facilitates molecular mobility under stress24,42. Meanwhile, the crosslinking process induced by malic acid weakens the hydrogen bonds between starch molecular chains, reducing molecular interactions and enabling easier molecular sliding33,51,52,54. Niazi et al.37 propose that, after malic acid crosslinking, glycerol interferes with the remaining hydrogen bonds between starch molecules. Cho et al.55 further correlated the increased elongation at break to a gradual rise in the gel fraction in samples containing MA following starch gelatinization, suggesting that the network remains robust yet sufficiently flexible to accommodate stretching.
Comparison of tensile strength at break (MPa) (in red) and elongation at break (%) (in blue) for films formulated with different concentrations of yerba mate extract (0%-20% YM), with or without the incorporation of malic acid (0.5% MA).
3.2. Dynamic thermo-mechanical properties
Dynamic mechanical analysis (DMA) was conducted to evaluate the material's response across different temperatures and to investigate the molecular mechanisms influencing the mechanical properties observed in the tensile test. The storage modulus (E’), which measures the elastic energy stored by the material during cyclic deformation, serves as an indicator of stiffness: higher values denote greater resistance to deformation, whereas lower values correspond to increased flexibility51,56. Figures 3a, c illustrates the storage modulus curves for all formulations. To profile the films' behavior under various thermal conditions, three specific temperatures were selected: -60 °C, -20 °C, and 20 °C (Figures 3b, d). The results are summarized in Table 3.
Storage modulus (E') as a function of temperature for the PS series (a, b) and MA series (c, d).
At -60 °C, the MA series samples displayed a tendency toward higher storage modulus, attributed to the high crosslinking of the films, which restricts molecular mobility51. However, this trend reversed at -20 °C, with malic acid-containing films exhibiting a lower modulus compared to non-additized samples. While the PS series formulations showed a gradual reduction in E' with increasing extract content in the temperature range between -60 °C and -20 °C, the MA formulations experienced a steeper decrease, reaching nearly 90%. Notably, although films without yerba mate extract (PS and MA) displayed similar modulus at -60 °C (5448.21 MPa and 5712.65 MPa, respectively), the difference between them grew to 20% at -20 °C and reached 50% at 20 °C, with the MA sample exhibiting lower stiffness. This behavior is further supported by the profile of the curves: while MA samples produced linear curves (Figure 3c), PS films exhibited peaks and plateaus (Figure 3a), suggesting moisture evaporation as the temperature increased, leading to a stiffer material. This effect is evident, for instance, in the PS20YM formulation, whose storage modulus increased by 34.65% between ‑20 °C and 20 °C.
In samples containing yerba mate extract, the difference in storage modulus between the two series was even more pronounced. At 20 °C, the obtained values reflect the behavior observed in the tensile test conducted at room temperature (Table 2). The incorporation of the extract played a crucial role in reducing the rigidity of the materials. The PS20YM and MA20YM formulations showed reductions of 80% and 93% in storage modulus compared to the PS matrix, highlighting the YM extract's influence. Studies by Lozano-Navarro et al.57 support this trend, reporting that the addition of natural extracts to chitosan films reduced the storage modulus due to the introduction of structural discontinuities and a decrease in cohesion between polymer chains, resulting in increased flexibility.
The loss tangent (tan δ) represents the internal friction of the material related to its mechanical damping. An increase in tan δ indicates more viscous characteristics, while lower values suggest greater elasticity. The peaks of tan δ correspond to the temperatures at which the material's structure undergoes molecular relaxations56. Figure 4 shows the temperature dependence of tan δ, with the identified peaks described in Table 4. Three peaks were observed, corresponding to the temperature transitions of the analyzed materials: the first, ranging from -80 °C to 40 °C (T1) in all samples; the second, between -20 °C and 46 °C (T2); and the third, between 18 °C and 50 °C (T3), which was absent in most formulations. The first transition is related to the molecular relaxation of the glycerol-rich phase, typically identified as a glass transition. The second peak corresponds to the relaxation of the starch-rich phase24,51,56. Finally, the third peak may be associated with the yerba mate extract.
Loss tangent (tan δ) as a function of temperature for the developed films. a) PS series; b) MA series.
All transitions shifted to lower temperatures as the concentration of yerba mate extract increased. Thus, within the temperature range analyzed, all three peaks are visible only in the samples with higher YM content: above 10% for the PS series (Figure 4a) and above 20% for the MA series (Figure 4b). This shift to lower temperatures can be attributed to the effect of adding a plasticizing component, indicating weakening of hydrogen bonds and a consequent increase in elongation at break24,51. In the samples containing malic acid, this effect is less pronounced due to crosslinking, which reduces molecular mobility51,56.
Another observed aspect concerns the difference in the intensity of the tan δ peaks between the PS and MA series. While the PS samples exhibited well-defined peaks, indicating clear transitions, the MA samples displayed less distinct peaks, suggesting greater structural homogeneity and better interaction between polymer chains, possibly resulting from the blocking of polar groups by crosslinking. Additionally, the decrease in peak intensity reflects the lower viscosity of the MA samples compared to the PS series51,56.
3.3. Surface morphology
The reduced definition of the transition peaks in the MA series (Figure 4) indicates that malic acid reduces the agglomeration of thermoplastic starch, promoting its dispersion and enhancing the homogeneity of the films51. As a result, the components exhibit better compatibility, a finding also reported by Hernández et al.58 in citric acid-crosslinked cassava starch and chitosan films. To further investigate this, the surface of the samples was analyzed using scanning electron microscopy (SEM). Figure 5 supports this hypothesis, showing that the addition of malic acid (MA series) resulted in a smooth and uniform surface in the films, regardless of the extract content (Figures 5b, d, f, h). Similar effects were described by Thessrimuang and Prachayawarakorn33 and Jiugao et al.52 when incorporating carboxylic acids into starch matrices. The surface smoothing is promoted by the destabilization of hydrogen bonds due to the acidity of the additive, leading to the dissolution and plasticization of starch granules. This restructuring process contributes to a more uniform and flexible profile of the films.
Micrographs obtained by SEM (magnification 2000x) of the surface of the films: a) PS; b) MA; c) PS1YM; d) MA1YM; e) PS10YM; f) MA10YM; g) PS20YM; h) MA20YM.
On the other hand, in the samples without malic acid (PS series), increasing the yerba mate extract content leads to a rough texture on the surface (Figures 5a, c, e, g). This behavior is consistent with the observations of Piñeros-Hernandez et al.39, Jaramillo et al.43, Ceballos et al.59, and Miranda et al.60, who reported that the incorporation of plant extracts enhances the surface roughness of films.
It is observed that, at higher concentrations of extract, both formulations display yerba mate particles with nanometric dimensions on their surface (120 - 340 nm), as shown in Figure 5g (PS20YM) and Figure 5h (MA20YM). It is evident that surface homogeneity was maintained in the MA series, while in the PS series, the particles tend to aggregate, forming clumps. Estevez-Areco et al.61 and López-Córdoba et al.62 also reported the appearance of nanoparticles in starch films with increasing plant extract content. This effect was attributed to the hydrophobic nature of certain antioxidants, which can cause the precipitation of insoluble components and promote morphological changes in the films. Additionally, Costa et al.63 and Shapi'i et al.64 reported an increase in the deformability of biopolymeric films upon adding nanoparticles, which is due to the strong intermolecular interaction between them and the matrix. This is attributed to their high surface area, which facilitates the movement of polymer chains under tension. The dispersion and dimensions of the particles can be seen in Figure 6.
SEM micrograph (magnification 10,000x) of the surface of the PS20YM formulation, highlighting the presence of yerba mate nanoparticles.
It is well known that the addition of plasticizers significantly influences the final morphology, as it penetrates the molecular structure, reducing the attractive forces between polymer chains, thereby increasing the material's flexibility65. In conjunction with the results from the uniaxial tensile test and dynamic mechanical analysis, the information obtained from SEM indicates that the incorporation of yerba mate extract and malic acid into the starch-glycerol matrix may impart plasticizer properties to the films, with the MA20YM formulation (0.5% MA and 20% YM) standing out as the most effective combination.
4. Conclusions
The incorporation of yerba mate extract (YM) and malic acid (MA) into starch-based films significantly affects their mechanical and morphological properties. The uniaxial tensile test revealed a reduction in the Young’s modulus and tensile strength, while elongation at break increased with higher extract content, indicating a more flexible material. Dynamic mechanical analysis (DMA) showed that the molecular relaxations shifted to lower temperatures as the yerba mate extract and malic acid concentrations increased, suggesting a plasticizer-like effect. Scanning electron microscopy (SEM) further confirmed the morphological changes, with the presence of nanoparticles in the films, particularly at higher extract concentrations. The combination of yerba mate extract and malic acid produced a synergistic effect, enhancing the flexibility and reducing the rigidity of the films. This effect was most evident in the formulation containing 0.5% MA and 20% YM (MA20YM). These findings highlight the potential of yerba mate extract and malic acid as effective additives in starch-based matrices, making them promising candidates for applications in food coatings and other biopolymeric materials.
5. Acknowledgments
The authors wish to thank CAPES-PROEX and CNPq for supporting the research, the Laboratory of Polymeric Materials (LaPol) and the Sustainability Hub (NS) at the Federal University of Rio Grande do Sul (UFRGS) for providing the infrastructure, and Feevale University for conducting the DMA and SEM analyses.
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Data Availability
Raw data supporting the findings of this study are available from the corresponding author upon request.
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Edited by
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Associate Editor:
Leonardo Gondim de Andrade e Silva.
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Editor-in-Chief:
Luiz Antonio Pessan.
Raw data supporting the findings of this study are available from the corresponding author upon request.
