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Article

Comparative Assessment of Injection and Compression Molding on Soy Protein Bioplastic Matrices for Controlled Iron Release in Horticulture

by
Daniel Castro-Criado
1,*,
Mercedes Jiménez-Rosado
2,
Víctor M. Pérez-Puyana
3,* and
Alberto Romero
1
1
Department of Chemical Engineering, Faculty of Chemistry, University of Seville, 41012 Seville, Spain
2
Chemical and Environmental Bioprocess Engineering Group, Institute I4, University of León, 24071 León, Spain
3
Department of Engineering and Materials Science and Transportation, University of Seville, 41092 Seville, Spain
*
Authors to whom correspondence should be addressed.
Agriculture 2025, 15(12), 1298; https://doi.org/10.3390/agriculture15121298
Submission received: 17 May 2025 / Revised: 12 June 2025 / Accepted: 15 June 2025 / Published: 17 June 2025
(This article belongs to the Section Agricultural Soils)
Browse Figures
Versions Notes

Abstract

:
Conventional horticultural fertilization frequently leads to nutrient loss and environmental contamination, driving interest in biodegradable controlled-release systems. This work developed soy protein isolate (SPI) matrices containing 5 wt.% FeSO4·7H2O using injection. The matrices were evaluated for crosslinking, mechanical properties, water uptake (WUC), soluble matter loss (SML), iron-release kinetics in water and soil, and biodegradability under composting conditions. Injection-molded samples achieved very high crosslinking with moderate rigidity and water absorption and delivered iron rapidly in water, while compression-molded samples exhibited slightly lower crosslinking but greater stiffness, higher WUC, minimal SML, and sustained iron release. Notably, both processing methods yielded comparable iron-release profiles in soil and complete biodegradation within 71 days. Overall, compression molding produces SPI-based matrices with superior mechanical strength and water retention, positioning them as an ideal solution for long-lasting, sustainable nutrient delivery in horticulture.

Graphical Abstract

1. Introduction

As the global population continues to grow at a considerable rate (projected to increase by approximately 35% over the next 40 years) [1], a significant increase in agricultural output will be required to accommodate this expansion. Most of the projected growth of agricultural production is anticipated to result from increasing the amount of food produced on currently farmed land [2]. This could have unfavorable effects on croplands’ biodiversity, as well as on carbon stores in soil and vegetation [3]. Fertilization is the most used technique to increase crop yields; however, its intensification has led to the emergence of new global environmental problems, including nutrient imbalance and the leaching of nutrients from crops to the environment [2]. For these reasons, finding an alternative method that facilitates the introduction of nutrients into plants and enhances their assimilation efficiency is necessary.
To overcome this situation, slow-release fertilizers (SRF) and controlled-release fertilizers (CRFs) have been investigated. The majority of SRF are chemical substances that either degrade slowly due to microbial action or are only barely soluble in water. As a result, the water solubility, chemical hydrolysis, and microbiological degradation of SFR influence the rate at which nutrients are released [4]. Conversely, CRFs are composed of soluble fertilizers coated in substances that restrict the soluble material’s exposure to water and/or limit the nutrient solution by diffusion. In this way, the release rate of CFR compounds is determined by soil water content and temperature [5]. However, traditional materials used in SRF and CRF, although effective, can have negative impacts on the environment in the long term due to the accumulation of non-biodegradable residues and the possible release of unwanted chemicals (i.e., acrylamide) [6]. An emerging trend in the improvement of controlled-release fertilizers is the use of biodegradable and sustainable materials. Biodegradable materials not only mitigate these problems but can also be designed to improve the efficiency of nutrient release [7].
To date, significant research has focused on natural biopolymers such as starch, cellulose, chitosan, alginate, and especially proteins for use in controlled-release systems. Proteins, particularly soy protein isolate (SPI), have shown promise due to their biodegradability, water affinity, and functional versatility. Previous studies have demonstrated their capacity to encapsulate nutrients and release them gradually in soil environments (e.g., Maya et al. [8] and Jiménez-Rosado et al. [9]). However, many of these systems have relied on casting or extrusion techniques, which may limit scalability or produce materials with low mechanical integrity. Moreover, the relationship between processing parameters (e.g., pressure, temperature) and matrix functionality remains underexplored, particularly in relation to micronutrient delivery efficiency and structural robustness. Therefore, further research is required to establish optimal manufacturing conditions and assess the performance of SPI-based systems under agricultural conditions.
In this context, the development of protein-based matrices to incorporate nutrients and release them in a controlled manner based on their biodegradability represents a promising avenue for the avoidance of soil and underground water contamination [10,11]. Furthermore, these matrices can be manufactured from biopolymers with a high water uptake capacity, which enables them to provide plants with a substantial quantity of water when required [12]. Moreover, the usage of proteins can have a biostimulant effect on crops due to the presence of soluble peptides and free amino acids, enhancing root and shoot growth, photosynthesis rate, and crop quality. Thus, proteins also help to regulate biochemical processes that enhance the resilience of crops against biotic stress and stimulate the uptake and efficiency of nutrients [13].
Protein-based matrices can be produced using similar techniques to those employed for synthetic plastics, thus facilitating scalability [14]. One method for developing them is injection molding. In this procedure, the raw material is mixed in the first stage to create a homogeneous blend, which is then subject to injection, compression, and holding. Finally, the sample is cooled. In this case, raw materials are composed of a biopolymeric matrix (i.e., protein) and sometimes additives [15]. Additionally, a plasticizer is required for molding purposes due to its ability to decrease the glass transition temperature and reduce the number of interactions that facilitate the mobility of protein chains and, thus, the molding of the matrices [16]. This method appears to be the most appropriate for processing these matrices due to its efficiency and simplicity, and it offers a high volume of production [17].
Another technique employed in the processing of these matrices is compression molding. This method entails the application of pressure to a blend of particles. The constituent grains of the powder are thereby plastically distorted, thereby producing a surface tension that keeps the grains together in the tablet-like structure [18]. This is a straightforward, inexpensive, and readily industrialized procedure. Consequently, controlled-release tablets are cost-effective and can, therefore, compete with traditional fertilizers in the market [19].
Nevertheless, it is also important to note that there are some potential drawbacks. For example, the correct selection of the injection and compression parameters (temperature, pressure, and time) is necessary to ensure the efficacy of the process and the correct properties of the final product [20]. Extabide et al. [21] highlight the importance of careful parameter selection to balance minimizing the blend’s viscosity and preventing premature protein degradation or cross-linking. Furthermore, Fernández-Espada et al. [22] demonstrated that matrices with high porosity and high water absorption capacity can be obtained by eliminating the plasticizer from processed bioplastics.
In light of this, the main objective of this work was to compare two different processing methods (injection and compression molding) of soy protein-based matrices with iron incorporated as a micronutrient to evaluate which method is more suitable for its future utilization in horticulture. As the pressure in both methods could influence the functionality of these systems, different values (300, 600, and 1100 bar for injection molding and 300, 600, 1100, 1400, and 1900 bar for compression molding) were selected for this study. Physicochemical, morphological, mechanical, and functional properties were evaluated to compare the different processing matrices.

2. Materials and Methods

2.1. Materials

Soy protein isolate (SPI) was used as the biopolymeric matrix of the injected systems and compressed systems (tablets). Protein Technologies International (Supro 500e, Zwaanhofweg, Belgium) supplied it as a yellowish powder with 90 wt.% protein and a grain size between 120–240 µm.
Iron was incorporated as a salt, specifically iron sulfate heptahydrate (FeSO4·7H2O). Panreac Química S.A. (Barcelona, Spain) provided the salt as a blue powder.

2.2. Preparation of Soy-Protein-Based Systems

To prepare the matrices by injection molding, SPI, and distilled water (used as plasticizer) in a 1:1 ratio and FeSO4·7H2O in a percentage of 5 wt.% were mixed in a two-blade counter-rotating mixer, Haake Polylab QC (Thermo Scientific, Waltham, MA, USA) at room temperature and 50 rpm for 10 min. Subsequently, the obtained blends were processed by injection molding using a MiniJet Piston Injection Molding System II (Thermo Scientific, Waltham, MA, USA), resulting in rectangular probes (designated as I-X, where X represents the applied pressure in bar) with dimensions of 60 mm × 10 mm × 1 mm. In this stage, the processing parameters were cylinder temperature (50 °C), mold temperature (90 °C), injection pressure (300 bar, 600 bar, and 900 bar for 20 s), and post-injection pressure (300 bar for 300 s).
The processing method of soy-protein-based tablets was based on the protocol followed by Jiménez-Rosado et al. [23]. Firstly, SPI (95 wt.%) and FeSO4·7H2O (5 wt.%) were manually mixed. This mixture was then compacted using an MP150 lab-scale uniaxial compression system (Maassen Spektroskopie, Ludwigsburg, Germany) at different pressures (300, 600, 1100, 1400, 1900 bar) for 3 min, obtaining cylindrical probes (named as C-X) of 15 mm diameter and 5 mm thickness.
It is important to note that the matrices, due to their polypeptide nature, inherently contain other essential nutrients, such as nitrogen and phosphate. However, due to the complex structure of the protein particles, these nutrients are not readily available for direct uptake by plants. Instead, they primarily act as biostimulants, enhancing plant growth and soil health rather than serving as immediate nutrient sources [24,25].

2.3. Characterization of Soy-Protein-Based Systems

2.3.1. Fourier Transform Infrared Spectroscopy (FTIR)

FTIR profiles of the different systems were obtained using a Hyperion 100 spectrometer (Bruker, Billerica, MA, USA) using an ATR diamond sensor. The measurements were performed as a mean of 200 scans between 750 and 4000 cm−1 with an opening of 4 cm−1.

2.3.2. Crosslinking Degree

To determine the crosslinking degree, the protocol employed in other works was followed [26,27]. This protocol consists of denaturing the uncrosslinked protein in order to solubilize it and calculate its concentration. Thus, a fresh denaturing solution consisting of a mixture of 0.086 mol/L tris base, 0.045 mmol/L glycine, 2 mmol/L EDTA, and 10 g/L sodium dodecyl sulfate (SDS) at pH 6 was prepared. Each system was immersed in 10 mL of this denaturing solution for 2 h. After this time, the bioplastic was removed, and the amount of protein dissolved in the solution, which corresponds to the non-crosslinked protein of the bioplastic, was evaluated following Lowry’s method [28].
The degree of crosslinking was estimated using the mixture of SPI and iron (0% crosslinking) and the denaturing agent solution without bioplastic (100% crosslinking) as references.

2.3.3. Scanning Electron Microscopy (SEM)

The microstructure of the different systems after water uptake (2.3.5) and further dry treatment in an oven at 50 °C for 24 h were analyzed via SEM. Firstly, a sputter coating with Pd/Au (10 µm) was made to improve the electrical conductivity of the samples and, thus, to improve the quality of the micrographs [29]. Subsequently, the systems were observed with a scanning electron microscope Zeiss EVO (Zeiss, Oberkochen, Germany) at an acceleration voltage of 10/20 kV and at a magnification of ×500.

2.3.4. Mechanical Properties

Dynamic compression tests were carried out to evaluate the mechanical properties of the matrices, as these are the main efforts that the systems will be subjected to during storage and use. These tests were carried out using a RSA III dynamic-mechanical rheometer (TA Instruments, New Castle, DE, USA) with a plate-plate geometry of 8 mm diameter for the systems obtained by injection molding and a plate-plate geometry of 15 mm for the ones obtained by compression molding. First, strain sweep tests were performed at 1 Hz, room temperature, and from 0.002% to 2% strain. The critical strain (εcrit, the last strain in the viscoelastic interval) of the systems was obtained from these tests. Subsequently, frequency sweep tests were carried out between 0.02 and 20 Hz at 65% of εcrit and at room temperature. From these tests, the elastic (E′) and viscous (E″) modulus values were obtained. In addition, the elastic modulus (E′1) and the loss tangent (tan δ1 = E″1/E′1) at 1 Hz were chosen to improve the comparison between the systems.

2.3.5. Water Uptake Capacity (WUC) and Soluble Matter Loss (SML)

The ability of these systems to absorb water is one of their major advantages. The systems can be used as they are capable of storing and releasing water in a controlled manner. To measure this property, each sample was immersed in 300 mL of distilled water for 24 h according to the ASTM D570-98 standard [30]. The water uptake capacity (WUC) was calculated using Equation (1):
W U C % = m 2 − m 3 m 3 · 100
where m 2 is the wet weight of the system after immersion in water and m 3 is the dry weight of the system after water immersion and drying.
Moreover, the soluble matter loss (SML) of each system was calculated to evaluate the stability of the systems upon water absorption. For this, Equation (2) was used:
S M L % = m 1 − m 3 m 1 · 100
where m 1 is the weight of each system before water immersion.

2.3.6. Iron Release in Water

A technique established by Cong et al. [31] and Essawy et al. [32] was used to control the release of fertilizers from different matrices. Thus, the matrices were immersed in 300 mL of distilled water, and the conductivity was measured over time using an EC-meter BASIC 30 (Crison Instruments, Barcelona, Spain) to evaluate the release of iron. The release is estimated to be completed once the conductivity remains constant for more than 1 h.

2.3.7. Iron Release in Soil

Analyzing the release of micronutrients in soil could provide a more accurate estimation of the use of the matrices in horticulture. To this end, the protocol proposed by González et al. [33] was followed. Cylindrical glass tubes (40 cm height × 2 cm diameter), open at the bottom, were filled with 30 cm of standard agricultural soil (loamy sand, pH 6.8, 1.5% organic matter), previously sieved and homogenized. One sample of each matrix (approximately 0.7 g) was buried at a depth of 10 cm. Each condition was tested in triplicate. The tubes were placed vertically over a support rack in a room at 25 ± 2 °C and 60 ± 5% of relative humidity and absence of light exposure. The tubes were irrigated with 20 mL of water every 24 h to simulate an intensive irrigation of 20 L water/m2. To analyze iron release, the conductivity of the leachate was measured using an EC-meter BASIC 30 (Crison Instruments, Spain) until no signal was detected (experiment finalization). A blank sample without matrices was prepared to assess the contribution of soil salinity to conductivity. Given the potential interference from other metal ions, the conductivity values obtained from the blank were used to correct those of the samples. Since iron accumulates progressively in each leachate, the daily conductivity values were added cumulatively. Subsequently, each cumulative value was corrected by subtracting the corresponding cumulative blank value, thus minimizing interference from other ions.

2.3.8. Biodegradability

Biodegradability tests were carried out to verify that the matrices had completely degraded in the soil environment and that this process had no negative impact on the ecosystem, as well as estimating how long the degradation process would take. For this experiment, the matrices were buried at room temperature in a composting environment with an 80% water saturation. The composting medium was a 2:1 mixture of soil and compost, carefully formulated to maintain the same percentage of organic and inert matter as specified in ISO 20200 [34]. A sample of each system was buried for evaluation. The degradation process was monitored by periodically uncovering the samples for visual inspection. The test was finished when no piece of the sample (>1 mm) could be unearthed.

2.4. Statistical Analysis

At least three replicates of each measure were carried out. Statistical analyses were performed using a one-way analysis of variance (ANOVA p < 0.05) followed by Tukey’s post hoc test. The mean and standard deviation of each measurement were calculated. Least Significant Difference (LSD) values were also calculated to determine the significant differences in the crosslinking degree between systems.

3. Results and Discussion

3.1. FTIR

The FTIR spectra of some of the most representative systems are shown in Figure 1. The spectra of all the systems were also represented in the Supplementary Information (Figures S1 and S2, for injected and compressed systems, respectively). In addition, the FTIR spectra of the raw materials (SPI and iron sulfate) were also evaluated and included in Figure 1.
The first observations were focused on the distinctive SPI bands, which correspond to the O-H bonds, A and B and amide II (3500–3000 cm−1), CHx (3000–2800 cm−1) and amide I and II (1600–1500 cm−1) [35,36]. On the other hand, in the iron sulfate spectra, a peak is observed between 3500–3000 cm−1, which corresponds to the O-H bonds as the salt is hydrated, and between 1500–1000 cm−1, which corresponds to the vibrations of the SO42− group and the Fe-O octahedra [37]. It is also observed that the matrices obtained by compression molding show similar bands to the SPI, whereas the ones obtained by injection molding are more like the iron sulfate band, but they also exhibit bands related to the SPI. Nonetheless, there are some differences in the intensity of these peaks between the injected and the compressed systems. For example, between 3500–3000 cm−1, the matrices obtained by injection molding show higher peaks due to the presence of water (plasticizer). Moreover, the peaks at 1500 cm−1 are related to the denaturalization of the proteins. Thus, lower peaks mean higher denaturalization. Therefore, it can be observed that the injected matrices are more denaturalized than in the injection process; when applying high temperatures and high pressures, the protein chains break to form the bioplastics, whereas when compressing, the formation of bioplastics is formed by the sintering of the grains without denaturation of the protein [36].

3.2. Crosslinking Degree

The degree of crosslinking (%DC) induced by each processing method is shown in Table 1.
As can be seen, injection molding induces a higher %DC than compression molding. This may be due to the denaturation of the protein during the injection process, which facilitates the formation of covalent bonds within the system. This relationship between denaturalization and the %DC has already been demonstrated in previous articles [38]. However, compressed systems also show a high %DC (>80% with respect to pure protein). This means that, although protein denaturation is not observed by FTIR, the deformation of the grains that occurs during compression also facilitates the generation of covalent bonds. Therefore, compression generates a crosslinked but not denaturalized system. These results suggest that compressed systems can generate a greater biostimulation effect in plants due to their intact structure, which allows them to interact with specific receptors in plant cells that generate this biostimulation [39]. It should be noted that there are no significant differences between the different pressures used in injection or compression. This suggests that the process used is decisive for crosslinking but that the pressure used is not key to modifying it.

3.3. SEM

SEM images of the systems made after water uptake are shown in Figure 2. The injected systems (2A–C) present surfaces with increasingly heterogeneous structures as injection pressure varies. Figure 2A,B display compact, homogeneous morphologies with smooth surfaces and a lack of pores, indicating complete fusion between the protein and plasticizer. These features are characteristic of high shear and thermal input during injection molding; they promote the full melting and dispersion of the protein matrix, resulting in a dense and continuous structure [40]. In contrast, in Figure 2C, corresponding to a sample injected at higher pressure (1100 bar), a slight increase in porosity is observed. This unexpected porosity may be due to partial water evaporation occurring during injection under high pressure, especially if local overheating takes place. The water, acting as a plasticizer, can vaporize under extreme conditions, leaving behind microvoids that disrupt the otherwise compact structure [41].
In contrast, the compressed samples (Figure 2D–F) show progressively more porous and granular surface morphologies. These systems, processed without plasticizer, depend solely on the sintering of protein granules. Figure 2D, corresponding to 600 bar, displays moderate porosity with relatively small and uniformly distributed pores, indicating limited but effective sintering. Figure 2E (1100 bar) shows an increase in pore size and heterogeneity, suggesting that while pressure promotes particle contact, the absence of a plasticizer restricts molecular diffusion. Finally, Figure 2F (1900 bar) reveals a highly porous and less homogeneous structure with larger, irregular pores. This result may be explained by the inhibition of particle rearrangement under excessively high pressure, which can prevent efficient sintering. The applied pressure likely compresses the particles to a degree that impedes the necessary mobility for consolidation, resulting in localized defects and pore formation [42].
The differences in SEM appearance between injected and compressed systems are primarily attributed to the combined effects of plasticizer presence, thermal and shear conditions, and the role of pressure in either promoting fusion (in injection) or restricting sintering dynamics (in compression).

3.4. Mechanical Properties

The frequency sweep tests carried out on the different systems are shown in Figure 3. As can be observed, all the systems showed a slight increase in E’ with frequency, indicating a certain instability of the systems with the time of application of a given force. This behavior has already been observed in similar systems made by injection molding [22,43] and compression molding [23,44].
The processing method and the pressure applied greatly influence the mechanical properties (Figure 3B). One-factor analysis of variance revealed statistically significant differences in E′1 values among the different processing systems (F(7, 16) = 20.87, p < 0.001). To identify specifically between which groups these differences were present, a Tukey post hoc test was performed, which showed that the C-1100 system differed significantly from C-300, C-600, C-1900, and I-300 (p < 0.05). These results suggest that this system presents significantly higher viscoelastic properties in terms of storage modulus, which could be related to a higher degree of crystallinity or a more compact structure induced by the compression process. In this way, the injected systems showed a lower elastic modulus than the compressed ones due to the thickness of the matrices. Since the injected systems have a lower thickness than the compressed ones, it seems logical that they would have lower values of E′. Moreover, it can be observed that as the injection pressure increases, E′ also increases, which might be due to the fact that by applying higher pressures, more of the plasticizer is removed, making the system more rigid [45]. The same behavior can be found in the compressed systems. However, it seems that there is a critical pressure above which no improvement in the mechanical properties can be observed, and even a reduction in the values of the elastic modulus is seen, which may be since fragile bioplastics could be obtained with too high pressures as the heterogeneity of the systems increases with increasing pressures as observed in the microstructure in Figure 2 [46].
Finally, all systems showed loss tangent values significantly lower than 1, which means solid-like behavior regarding the processing technique or the pressures applied. This behavior has already been reported in previous works with similar bioplastics in which the loss tangent of the systems made by injection molding exhibited values between 0.1 and 0.2 [47,48]. The loss tangent of the compressed systems showed values lower than 0.1, reflecting the formation of solid tablets.

3.5. WUC and SML

Figure 4 presents the water uptake capacity (WUC) and soluble matter loss (SML) values for the bioplastic systems processed by injection (I) and compression (C) molding. Statistically significant differences (p < 0.05) were observed between the two processing methods. In general, the injected systems (I-300, I-600, I-1100) showed significantly higher SML values compared to the compressed systems, with mean SML percentages above 35% for injected matrices and below 10% for compressed ones. Conversely, WUC values were significantly higher in compressed systems, particularly for C-1100 and C-1900, which exceeded 200% and 400%, respectively. Among the injected systems, WUC values remained below 150%, statistically lower absorption capacity compared to their compressed counterparts.
These differences suggest that the processing method has a significant influence on the water interaction behavior of the bioplastics. The higher WUC observed in compressed systems may be attributed to increased porosity, as shown in SEM micrographs (Figure 2), which allows greater water penetration. The trend of increasing WUC with molding pressure also supports this interpretation. However, none of the systems reached the superabsorbent threshold (>1000%) reported for similar soy protein-based matrices [49], likely due to ionic interactions induced by the incorporation of iron sulfate [23,50]. Nevertheless, Messa et al. [51], who found equilibrium values of 192%, claim that these values are adequate for horticultural water supply.
On the other hand, the elevated SML values in injected systems are likely related to the presence of residual water in the formulation and the partial loss of soluble protein and salt during the injection process. Nonetheless, the observed SML values were below the theoretical maximum of 42.5%, suggesting partial retention during processing. In compressed systems, only 1–2% of soluble protein and about 5% of the added salt were released.
These results suggest that systems made by compression molding can act as a water reservoir in soil, helping to maintain local moisture around plant roots and gradually dissolve the incorporated iron. On the other hand, the high SML values of the injected samples indicate that a significant fraction of protein and salt dissolves when first wet, which could lead to a rapid initial nutrient release.

3.6. Iron Release in Water

The salt release in the water of the bioplastics is represented in Figure 5. Thus, the release of the salt was represented over time.
As can be observed, the matrices made by injection molding release the salt much faster than the matrices made by compression molding, showing a greater slope and reaching 100% release in shorter times. Nonetheless, the maximum release time seems to depend only on the processing method, as no big differences are observed with varying pressures. The faster release of the matrices made by injection molding might be due to the presence of water in its composition, which solubilizes and contains the salt and drags it out of the system. Moreover, these matrices have a larger contact surface and lower thickness, contributing to a faster salt release. Similar results were obtained for the systems made by injection molding in other tests carried out by Jiménez-Rosado et al. [52] in which matrices with micro- and nanoparticles were made by injection molding. The profiles shown by the systems made by compression molding were like those obtained by Jiménez-Rosado et al. [23], in which soy protein-based tablets were made with zinc as a micronutrient, with varying heat treatment times.

3.7. Iron Release in Soil

Measurements taken from the leachates in the soil release studies are shown in Figure 6A, and the percentage of release is shown in Figure 6B, where the time taken by the I-1100 and C-1100 to release all the iron incorporated is represented. These two systems were selected because they represent the most significant systems, as they show better properties in general terms.
As shown in Figure 6, both the I-1100 and C-1100 systems exhibited similar release profiles during the first 15–20 days, suggesting comparable iron release kinetics in the early stages. Although C-1100 displayed slightly higher conductivity values (Figure 6A), this may be attributed to its larger mass and slightly higher initial iron content. However, both systems reached nearly complete iron release after 27 days, with I-1100 achieving approximately 100% and C-1100 reaching around 98% (Figure 6B). The slightly delayed release observed in the compressed matrices (C-1100) during the final week represents a sustained supply of iron. This additional 2%, although seemingly minor, can be considered significant in agronomic terms, as it provides an extended-release window that may enhance iron availability in the rhizosphere during critical late growth stages. This feature could be particularly beneficial in iron-deficient soils or for crops with longer nutrient uptake periods, positioning the compressed matrices as potentially more suitable for sustained micronutrient delivery [53].

3.8. Biodegradability

The evolution of the matrices under composting conditions is shown in Figure 7, which includes a photograph taken over the entire study period. This process is made possible by the oxidation of carbon atoms inside carbohydrates and proteins, which results in the cleavage of covalent bonds and the subsequent breakdown of complex organic materials into simpler molecules and elemental components. Typically, organic substances decompose until they are converted into carbon or carbon dioxide. This resulting carbon dioxide can be absorbed by plants to synthesize new complex biomolecules through photosynthesis, thereby reintegrating the process into the biological carbon cycle [54].
As can be observed, the degradation process starts to be noticeable from the eighth day onwards in the case of the matrices made by injection molding, when the matrices begin to show cracks and eventually break into smaller pieces. On the other hand, the biodegradation process in the systems made by compression molding is visually noticeable on day 22, after which the matrices appear smaller. However, the degradation of these systems can be seen on previous days when they were dug up, as their texture is much softer, and they break very easily if they are not removed carefully. This behavior suggests that the systems with better mechanical properties take longer to decompose, which is possible since partitioning into smaller pieces is more difficult, which can be attributed to an increased number of interactions, resulting in a more consolidated structure, and that the systems made by injection molding are more denaturalized [55]. Moreover, the injection molded matrices have higher inherent water content and lower stiffness, which makes them more susceptible to microbial attack and hydrolytic breakdown [56]. Additionally, the thinner dimensions of these matrices increase their surface area for degradation.

4. Conclusions

This study demonstrates that compression-molded soy protein isolate (SPI) bioplastic matrices with incorporated iron significantly outperform injection-molded matrices in the context of controlled-release fertilizers. Compression-molded tablets exhibit higher stiffness and water uptake capacity, which allows them to better withstand soil pressures and retain moisture. These structural advantages result in a more gradual and prolonged iron release profile, effectively matching the nutrient delivery to the crop’s growth cycle. For example, compression-molded tablets released their full iron content over approximately 35 days, a period closely aligned with the growth duration of many vegetable crops, whereas injection-molded samples released their iron much more rapidly. Importantly, both systems are fully biodegradable under composting conditions (complete degradation in approximately 71 days), but the enhanced integrity and water retention of the compression-molded matrices make them especially suited for sustained fertilizer release.
Overall, these findings suggest that compression molding is a more effective fabrication method for SPI-based controlled-release fertilizer matrices in horticulture. The improved mechanical integrity and water-holding capacity of compression-molded systems should enable growers to reduce irrigation and minimize nutrient leaching, as micronutrients are released steadily throughout the entire cultivation period. Thus, employing compression–molded SPI–iron matrices can enhance nutrient use efficiency and improve environmental safety in crop production. In practical terms, adopting compression-molded bioplastic tablets for micronutrient delivery can provide farmers with a sustainable tool that aligns fertilizer release with plant needs, ultimately enhancing crop nutrition while limiting waste and pollution.
Building on these results, further research to enhance applicability in horticulture would be interesting, such as using other agro-waste or by-products to valorize them and improve the circular economy or using other micronutrients, such as copper or manganese, among others, to increase the versatility of these matrices.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agriculture15121298/s1, Figure S1: FTIR profiles of the systems made by injection molding; Figure S2: FTIR profiles of the systems made by compression molding.

Author Contributions

Conceptualization, M.J.-R. and A.R.; methodology, M.J.-R. and V.M.P.-P.; software, D.C.-C. and M.J.-R.; validation, M.J.-R. and A.R.; formal analysis, D.C.-C.; investigation, D.C.-C.; resources, V.M.P.-P. and A.R.; data curation, D.C.-C. and M.J.-R.; writing—original draft preparation, D.C.-C.; writing—review and editing, M.J.-R., V.M.P.-P. and A.R.; visualization, D.C.-C. and M.J.-R.; supervision, A.R. and M.J.-R.; project administration, M.J.-R. and A.R.; funding acquisition, A.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research is part of the project PID2021–124294OB-C21 funded by MCI/AEI/10.13039/501100011033/FEDER, EU. In addition, this research is part of the applied research and innovation project: “Desarrollo de matrices proteicas para la liberación controlada de nutrientes y agua en horticultura” (SOL2024-31712) cofounded by UE—Ministerio de Hacienda y Función Pública—Fondos Europeos—Junta de Andalucía—Consejería de Universidad, Investigación e Innovación.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author due to ongoing research.

Acknowledgments

Authors would like to acknowledge CITIUS for granting access to the Microscopy and microanalysis services.

Conflicts of Interest

The authors declare no conflicts of interest.

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Agriculture 15 01298 g001
Figure 1. FTIR spectra of the raw materials (SPI and iron sulfate) and the bioplastic matrices processed by injection molding (I-1100) and compression molding (C-1100). Characteristic absorption bands corresponding to functional groups are highlighted to illustrate the interactions and structural differences among the materials.
Figure 1. FTIR spectra of the raw materials (SPI and iron sulfate) and the bioplastic matrices processed by injection molding (I-1100) and compression molding (C-1100). Characteristic absorption bands corresponding to functional groups are highlighted to illustrate the interactions and structural differences among the materials.
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Figure 2. Scanning electron micrographs (×500) of bioplastic systems produced by injection molding at 300 (A), 600 (B), and 1100 bar (C); and by compression molding at 300 (D), 600 (E), 1100 (F), 1400 (G), and 1900 bar (H). All scale bars represent 10 µm.
Figure 2. Scanning electron micrographs (×500) of bioplastic systems produced by injection molding at 300 (A), 600 (B), and 1100 bar (C); and by compression molding at 300 (D), 600 (E), 1100 (F), 1400 (G), and 1900 bar (H). All scale bars represent 10 µm.
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Figure 3. Frequency sweep tests on bioplastic matrices processed by injection (I) and compression (C) molding. (A) Storage modulus (E′) and loss modulus (E″) values across a frequency range (0.1–20 Hz). (B) Comparison of E′, E″, and loss tangent (tan δ) values at 1 Hz for each system. Error bars represent standard deviations. Different superscript letters indicate statistically significant differences among systems for each parameter (p < 0.05). Error bars represent standard deviations.
Figure 3. Frequency sweep tests on bioplastic matrices processed by injection (I) and compression (C) molding. (A) Storage modulus (E′) and loss modulus (E″) values across a frequency range (0.1–20 Hz). (B) Comparison of E′, E″, and loss tangent (tan δ) values at 1 Hz for each system. Error bars represent standard deviations. Different superscript letters indicate statistically significant differences among systems for each parameter (p < 0.05). Error bars represent standard deviations.
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Figure 4. Water uptake capacity (WUC) and soluble matter loss (SML) values of bioplastic systems produced by injection (I) and compression (C) molding. Different superscript letters indicate statistically significant differences among systems for each parameter (p < 0.05). Error bars represent standard deviations.
Figure 4. Water uptake capacity (WUC) and soluble matter loss (SML) values of bioplastic systems produced by injection (I) and compression (C) molding. Different superscript letters indicate statistically significant differences among systems for each parameter (p < 0.05). Error bars represent standard deviations.
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Figure 5. Release profiles in water of bioplastic systems manufactured by injection (I) and compression (C) molding under different processing pressures. The percentage of cumulative release is plotted as a function of time (min).
Figure 5. Release profiles in water of bioplastic systems manufactured by injection (I) and compression (C) molding under different processing pressures. The percentage of cumulative release is plotted as a function of time (min).
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Figure 6. Conductivity during the iron release of the systems I-1100 and C-1100 over time (A) and the corresponding percentage of iron released over time (B). All values were normalized by subtracting the conductivity measured in a blank test (soil without bioplastic matrix). Error bars represent standard deviations.
Figure 6. Conductivity during the iron release of the systems I-1100 and C-1100 over time (A) and the corresponding percentage of iron released over time (B). All values were normalized by subtracting the conductivity measured in a blank test (soil without bioplastic matrix). Error bars represent standard deviations.
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Figure 7. Visual evolution of the degradation process over time for all bioplastic matrices produced by injection (I) and compression (C) molding under different pressures. Scale bars indicate the real measure of samples.
Figure 7. Visual evolution of the degradation process over time for all bioplastic matrices produced by injection (I) and compression (C) molding under different pressures. Scale bars indicate the real measure of samples.
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Table 1. Degree of crosslinking (%DC) and Least Significant Difference (LSD) values of the bioplastic systems processed by injection (I) and compression (C) molding at different temperatures. Different superscript letters within the same column indicate statistically significant differences (p < 0.05).
Table 1. Degree of crosslinking (%DC) and Least Significant Difference (LSD) values of the bioplastic systems processed by injection (I) and compression (C) molding at different temperatures. Different superscript letters within the same column indicate statistically significant differences (p < 0.05).
System%DCLSD
I-30098.44 a1.73
I-60099.26 a1.73
I-110098.12 a1.73
C-30089.84 bc1.73
C-60088.98 bc1.73
C-110088.54 b1.73
C-140091.51 c1.73
C-190089.52 bc1.73
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MDPI and ACS Style

Castro-Criado, D.; Jiménez-Rosado, M.; Pérez-Puyana, V.M.; Romero, A. Comparative Assessment of Injection and Compression Molding on Soy Protein Bioplastic Matrices for Controlled Iron Release in Horticulture. Agriculture 2025, 15, 1298. https://doi.org/10.3390/agriculture15121298

AMA Style

Castro-Criado D, Jiménez-Rosado M, Pérez-Puyana VM, Romero A. Comparative Assessment of Injection and Compression Molding on Soy Protein Bioplastic Matrices for Controlled Iron Release in Horticulture. Agriculture. 2025; 15(12):1298. https://doi.org/10.3390/agriculture15121298

Chicago/Turabian Style

Castro-Criado, Daniel, Mercedes Jiménez-Rosado, Víctor M. Pérez-Puyana, and Alberto Romero. 2025. "Comparative Assessment of Injection and Compression Molding on Soy Protein Bioplastic Matrices for Controlled Iron Release in Horticulture" Agriculture 15, no. 12: 1298. https://doi.org/10.3390/agriculture15121298

APA Style

Castro-Criado, D., Jiménez-Rosado, M., Pérez-Puyana, V. M., & Romero, A. (2025). Comparative Assessment of Injection and Compression Molding on Soy Protein Bioplastic Matrices for Controlled Iron Release in Horticulture. Agriculture, 15(12), 1298. https://doi.org/10.3390/agriculture15121298

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