Bioplastics Made From Starch as Conventional Plastic Substitutes: Evaluation of Physical Properties and Biodegradation

doi:10.21203/rs.3.rs-3783931/v1

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Bioplastics Made From Starch as Conventional Plastic Substitutes: Evaluation of Physical Properties and Biodegradation

https://doi.org/10.21203/rs.3.rs-3783931/v1

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published 23 Sep, 2024

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This work studies the biodegradability of bioplastics (BPs) generated from the combination of starch from the fruit of Sechium edule (SSE) and the starch of Phaseolus vulgaris (SPV) with a plasticizing agent. BPs were obtained using the casting method from filmogenic solutions formed with dispersions of SSE and SPV in concentrations of 1, 3, 5, 6, 7, and 9% w/v individually and in binary combinations at a (1:1) ratio. Those with the best physical properties were chosen to evaluate the biodegradation of BPs in vitro, over-soil (OS), and soil burial (UG) conditions. The results indicated that BPs are transparent and have a high hydrophilic capacity; they also presented high manageability, continuity, and uniformity aspects starting from a concentration of 6% (w/v). The BPs formulated with the starch mixtures (SSE-SPV) show hardness values between 455.20 and 1972.00 gf and an elasticity index greater than 0.86. Under storage conditions of 11.30 and 57.60% RH, the maximum elongation value was reached from the concentration of 6% SSE-SPV, and this property is reduced when the relative humidity of the BPs increases. Under UG conditions, the percentage of biodegradation was 91.02%, with a degradation rate of 7.6854 mg BP/day, the experimental data of the biodegradation process were adjusted to a first-order kinetic model, the biodegradation rate coefficients of BP were 0.1143, 0.0905, 0.6873 days-1 in in vitro, over-soil and soil burial conditions, respectively.

Bioplastic

Biodegradation

Starch

Waste

Soil

SSE-SPV are potential material for application in the food packing.
In bioplastics made with combination of SSE-SPV, elongation percentage is reduced when relative humidity is increased.
The highest biodegradation rate was 7.6854 mg BP/day, with a biodegradation percentage of 91.02% in the burial systems.

Food packaging is essential to preserve the properties of food, ensure its safety, avoid contamination, and extend shelf life. The materials that are conventionally used for these functions are paper, metal, glass, and plastics. Plastic packaging production is cheap, and the packaging generated from this material is highly durable and has good mechanical properties. Due to this, they are widely used in the food industry [1]. However, the chemical structure of plastic packaging is difficult to degrade by natural action [2], and although there are various strategies to collect and recycle this type of packaging, single-use plastic packaging is an important component of the millions of tons of plastic waste that escape collection systems and end up in landfills or rivers [3]. The accumulation of this type of plastic packaging, both in soil and in waterways, is dangerous for living organisms and is one of the most important environmental issues around the world [4][5].

As a primary user and producer of single-use packaging, the food and beverage sector play an important role in transitioning traditional petroleum-based packaging toward sustainable alternatives [3]. One of the innovative solutions for this transition is the generation of bioplastics (BPs) [1]. These act as plastic materials and are classified as biodegradable materials, meaning that when discarded under favorable conditions for their decomposition, they become integrated into nature as carbon dioxide, water, and biomass. BPs are produced from biopolymers extracted from biomass, with or without modifications, such as starch, cellulose, and proteins [6].

Starch is one of the most used biopolymers for the development of BPs. BPs made from starch are obtained after the starch granule breakdown through the gelatinization process. BPs made from native starches present some limitations, mainly related to mechanical properties, thermal stability, and moisture absorption. To improve these properties, mixtures have been made with plasticizing agents such as glycerol, which generates polymers with thermoplastic properties[7][8]. Likewise, hybrid materials or starch mixtures have been used to improve the properties of BPs, which constitute a raw material option to produce biodegradable packaging [9]. Much of the research has been aimed at designing packaging of biological origin and free of petroleum derivatives.

One of the unconventional sources for obtaining starch is the fruit of S. edule. Due to the similarity of the morphology of the granules of this starch, X-ray diffractogram and FT-IR spectrum of the starch from the fruit of S. edule with other starches of industrial relevance and its wide availability, it has been used to obtain microencapsulates of cinnamon oleoresin [10] and for the generation of nanofibers by electrospinning [11]. Porras-Saavedra et al. [11] found that combining S. edule starch with PVA forms highly water-soluble nanofibers, making its application possible for processes where biodegradable systems are required under hydrophilic conditions.

Likewise, another non-conventional starch source is Phaseolus vulgaris, a legume considered an important source of starch (22–45%), with an amylose content of 44.97 to 51.11%. Due to this amylose content, the starch of P. vulgaris is an interesting carbohydrate for developing BPs [12][13]. Betancur-Ancona et al. [14] state that P. vulgaris starch is a by-product derived from extracting other macromolecules of this legume, so finding an application for P. vulgaris starch represents an alternative for comprehensive use. In this research, it is proposed to form BPs from the combination of S. edule starch and P. vulgaris starch.

Azeredo et al. [15] describe some of the applications of BPs as primary packaging, that is, those that are directly in contact with the food; for example, BPs can be used as a wrapper for chocolate sweets or breakfast cereal bags (primary packaging) that act as a barrier inside a cardboard box (secondary packaging). In this case, the BPs would not replace the secondary packaging, but they would replace the primary packaging. It is essential to know the biodegradation of BPs to predict their biodegradability and potential use in the food industry. Based on this knowledge, new material proposals will be expected to be developed. Because of this understanding, evaluating the environmental impact of plastic pollution on the environment will be of help [16].

Tai et al. [9] created BPs from a hybrid mixture composed of starch and polyurethane, where the behavior was evaluated during a biodegradation process for 180 days in a soil burial process. A work by Nguyen et al. [17] reported the biodegradation process of a BP obtained from a mixture of low-density polyethylene and starch, demonstrating its degradation after five months.

This work aimed to evaluate the biodegradability of bioplastics made from the combination of starch from Sechium edule and Phaseolus vulgaris with a plasticizing agent to be used as sustainable and environmentally friendly packaging. As well as knowing the physical characteristics of the bioplastic, the biodegradation process, and the morphology through a microscopy analysis.

To obtain the bioplastics (BPs), starch was extracted from S. edule fruits collected in the Mezquital Valley of Hidalgo, Mexico. To extract bean starch, glycerol, and bean seeds of the Flor de Mayo variety acquired from the local market in Mixquiahuala de Juárez, Hidalgo, Mexico, were used (Baker analyzed).

The soil used for the experiments was obtained from the green space of the School of Chemical Sciences of Universidad Veracruzana, based in the Orizaba, Veracruz, Mexico region.

Extraction of starch from Sechium edule (SSE) and starch from Phaseolus vulgaris (SPV)

The extraction of SSE was made from S. edule fruits according to the methodology described by Porras-Saavedra et al [11]. SPV starch extraction followed the methodology described by Betancur Ancona et al. [18]

Obtainment of bioplastic using the casting method

To prepare the bioplastic (BP), the starch dispersion was first prepared in distilled water; then, it was mixed with glycerol and homogenized with constant stirring. This solution was gelatinized on a heating plate at a temperature of 90º C, and once the gelatinization temperature was reached, it was kept like this for one hour. Next, the dispersion was cooled to 60° C and poured into a level square mold (30 x 30 cm); subsequently, it was dehydrated in a drying oven with air circulation (Biobase, BOV-D125, China) at 40° C for approximately 20 hours. Then, the BPs were carefully de-molded and stored in a desiccator until analysis [19]. This procedure was carried out for the BPs generated with concentrations of 1, 3, 5, 6, 7, and 9% w/v of SSE and SPV, individually (Table 1). In the case of binary combinations, a mixture of SSE and SPV was made at a 1:1 w/w ratio for each concentration (Table 1).

Physical characterization of bioplastics (BPs)

Qualitative assessment

The quality of the BPs was evaluated through visual inspection, considering aspects of manageability, homogeneity, and continuity. Manageability informs whether the BP can be manipulated without being damaged, continuity considers the absence or presence of breaks in the BP films. In contrast, homogeneity informs about the uniformity of the film [19].

Thickness

The thickness of the BP was measured with a digital vernier model 3416 (Traceable, USA) with a precision of 0.01 mm. The determination was carried out at five points in the sample [20].

Opacity

To determine the opacity, BP samples were cut to a size of 20 x 8 mm, then placed in a spectrophotometer cell (Genesys, Thermofisher), and the absorbance was measured at a wavelength of 600 nm. An empty cell was placed as a target. This analysis was performed in quintuplicate [21]. The Eq. 1, was used to determine the opacity, where ${A}_{600 nm}$ is the absorbance value measured at 600 nm and $\delta$ is the thickness of the BP measured in mm.

$Opacity=\frac{{A}_{600 nm}}{\delta }$ (Eq. 1)

Water solubility

To determine the affinity of BP for water, the method described by Galus et al. [20] was followed, with some modifications. Five samples (20 x 20 mm) were selected and dried at 100º C for 24 h. Each sample was immersed in beakers with 50 ml of distilled water at 25 ºC and was occasionally shaken. After 24 h of immersion, the water-insoluble sample was removed from the container and dried at 100° C for 24 h. The amount of BP solubilized in water expressed as a percentage was obtained by subtracting the amount of undispersed mass from the initial mass of the bioplastic (Eq. 2) [20].

$Solubility \left(\% S\right)=\left[\frac{Initial dry weight - Final dry weight }{Initial dry weight}\right] * 100$ (Eq. 2)

Swelling

To determine the swelling index, 20 x 20 mm samples were cut, dehydrated, weighed, and immersed in distilled water for 2 minutes at 25° C. Next, excess water was removed from the wet samples with filter paper, and they were weighed. The amount of water absorbed was calculated by the difference in the weight of the wet sample minus the initial weight of the dry sample (Eq. 3) [21].

$Swelling index \left(\%SI\right)=\left[\frac{Ws- Wd }{Wd}\right] * 100$ (Eq. 3)

Where Wd is the dry weight of the BP before entering the water and Ws is the weight of the wet BP.

Characterization of texture properties

To measure the texture properties, BP samples with dimensions of 20 x 20 mm were cut and stored until reaching equilibrium with saturated solutions of Lithium Chloride at a relative humidity of 11% at room temperature (approximately 22° C). The texture profile of the BPs was carried out on a texturometer (Brookfield, CT310K, USA). Film support accessories were used for the puncture test. The conditioned samples were placed between two acrylic plates, with a central hole of 11 mm in diameter, the diameter of the probe was 4 mm. The maximum force just before breaking was taken as the rupture strength. The target was 4.0 mm, activation load 7g, return speed: 0.5 mm/s, cycle counter 2.0, speed 1.00 mm/s, with a sampling rate of 50.00 points per second. This analysis was performed in quintuplicate [22].

Deformation

The deformation of the BPs was carried out in a texturometer (Brookfield, CT310K, USA). For this test, BP samples with 7 cm long and 2 cm wide dimensions were stored until reaching equilibrium in saturated solutions of LiCl, NaBr, and BaCl with relative humidities of 11.30, 57.60, and 90.30%, respectively. Once the samples reached equilibrium, they were fixed on the probe (TA-DGA). The goal value was 50.0 mm, activation load 30 g, and test speed 3.00 mm/sec. This analysis was performed in quintuplicate [22].

Statistical analysis of the physical properties

The results of the physical characteristics of the BPs were analyzed with the help of GraphPad Prism v7 Software using a one-way variance analysis with a 95% confidence interval. The means were compared with a Tukey test (p ≤ 0.05).

Biodegradation of bioplastic

Collection of the soil sample

The soil was collected using a previously washed and dried shovel. A total of 10 kg of soil was collected and transported in a previously washed and dried plastic container with a lid. Once in the laboratory, the sample was homogenized manually. During this process, stones, and all other materials other than soil were removed. The soil sample was sieved using a 3 mm pore-size mesh to homogenize the sample.

Subsequently, soil moisture was determined using the dry weight determination technique to maintain constant water content in the biodegradation experiments. After determining the weight loss of the degradation systems, soil moisture was adjusted by adding distilled water.

Adjusting the bioplastic samples

The BP was stabilized in a desiccator, and 2 x 2 cm fragments were subsequently extracted. These were weighed and identified before use in each degradation system used in this work.

Bioplastic biodegradation analysis

The change in the BP’s opacity was determined using a spectrophotometer (Jenway Brand) set to an adjusted wavelength of 600 nm [4].

The percentage of BP biodegradation was calculated by weight loss over time, as previously reported using the Eq. (4) [23] [24][25].

$$\text{%} Biodegradaci\text{ó}n=\frac{{w}_{i}-{w}_{f}}{{w}_{i}}x100 ( \varvec{e}\varvec{q}.4)$$

Where w_i is the initial weight of the sample, and w_f is the final weight of the sample.

On the other hand, the degradation rate was determined according to the following Eq. (5):

$$v=\frac{{w}_{i}-{w}_{f}}{{t}_{0}-{t}_{f}} (\varvec{e}\varvec{q}. 5)$$

Where: w_i is the initial weight of the sample, w_f is the final weight of the sample, t₀ is the initial day of the test and t_f is the final day. The result is expressed in mg of sample/day.

BP degradation kinetics was evaluated using the models presented by Santonja-Blasco [26], with some modifications. The equations explain the decomposition of BP based on first (Eq. 6) and second order (Eq. 7) kinetic models.

${Mn}_{t}={Mn}_{0} x {e}^{(-{k}_{1}t)}+ {Mn}_{f}$ (Eq. 6)

$\frac{1}{{Mn}_{t}}=\frac{1}{{Mn}_{0}}+{k}_{2} x t$ (Eq. 7)

Where: M_nt is the weight of the BP at time t, M_n0 and M_nf are the weight of the BP at the initial and final time; k₁ and k₂ are the first and second-order kinetic rate constants, respectively.

Simulation systems for biodegradation

The degradation in in vitro systems was evaluated using the methodology described by Roy et al.[27], with some modifications. Petri dishes were used with soil extract agar culture medium (250 g of soil, 500 mL of water, and 15 g of agar), where a fragment of the BP or oxyplastic was placed in the center. The dishes were placed in a Felisa brand incubator with a temperature control of 352° C. The conditions of this experiment allow for maintaining humidity and temperature conditions to establish a closed system without humidity losses. The BP and oxyplastic samples were obtained in triplicate, and random sampling was used without replacement every 10 days for a total of 55 days.

Open systems, which consisted of 500 mL glass bottles with 35 g of soil, were used to evaluate the degradation of BP on the soil surface. The moisture content was adjusted to 35 ± 2% because water loss is inevitable in these systems. Within each biodegradation system, a sample of BP was placed on the soil layer, which was previously weighed and recorded. The systems were placed outside the laboratory in an indoor area exposed to a temperature of 19 ± 5°C and relative humidity of 72 ± 13%. Samples were obtained in triplicate, and random sampling was used without replacement every 15 days for a total of 90 days.

BP biodegradation was determined under the soil in open systems built with PVC cylinders, with a length of 10 cm and an internal diameter of 3 cm. The cylinders were previously cut into two vertical sections. Subsequently, they were joined together using elastic bands. Additionally, a base was placed at the bottom to maintain the soil and sample inside the cylinder. Each of them placed a layer of soil of 228 ± 2 g, followed by a BP sample and an additional layer of soil of 228 ± 2 g, with a soil moisture adjustment of 35 ± 2%. The weight of each degradation system was noted to determine the losses due to water loss caused by evaporation. The systems were placed outdoors under a roof and exposed to ambient temperature and humidity at19 ± 5°C and 72 ± 13%, respectively.

Morphological changes of BPs during biodegradation analyzed by Environmental Scanning Electron Microscopy

For analysis of the microstructure of the biodegraded bioplastic, the sample was immobilized on carbon tape in a sample holder and was observed in an environmental scanning electron microscope (XL 30, Philips, USA) operated at 25 KV. Micrographs were obtained at 300X and 1000X magnifications [28].

Physical characterization of BPs

Bioplastics (BPs) with S. edule starch (SSE), P. vulgaris starch (SPV), and a combination of both starch sources (SSE-SPV) obtained by the plate casting method formed matrices with manageable, continuous, and uniform properties (Table 2). The BPs with SSE presented aspects of manageability, continuity, and uniformity from the 6% w/v concentration. In comparison, the BPs made with SPV and SSE-SPV combinations presented these attributes from concentrations of 5% w/v. The obtainment of BPs by the plate casting method depends on the composition of the filmogenic solution, which is made up of SSE and SPV starch in both individual and binary combinations. When it is in excess water and undergoes a heat treatment (90º C), this polymer gelatinizes to form BPs. In this process, the amylose and amylopectin chains form a colloidal network [29]. Another factor observed as determining was the concentration of glycerol, which interacts intermolecularly with starch and reduces hydrogen bonds between amylose and amylopectin. This action that reduces fragility and improves the flexibility and extension of BPs [30]. During pouring, the dispersion of the filmogenic solution acquires the shape of the mold. Likewise, the formation of BPs depends on the time and temperature of solvent evaporation; in this last stage, the intermolecular interactions between the polymer chains are improved and form an adequate microstructure of the BP. However, sufficient time must be considered to evaporate the solvent and prevent the adhesion of the BP to the mold [30].

As shown in Table 2, the opacity of the BPs made with SSE is in the range of 1.39 to 1.67, greater than that of SPV (0.38 to 0.78) and those generated with the SSE-SPV combinations (0.146 to 0.78). 0.44). Opacity is a key point in consumer acceptance. Low values of this parameter indicate the transparency of the BPs; therefore, the BPs generated with starch mixtures can be used in the development of packaging that allows the content of the product to be displayed. The opacity values reported in this work are similar to those found in BPs with potato, corn, wheat, and rice starch at concentrations of 5 and 1.5 wt% of glycerol, with 93.5 wt% of water [31]. Likewise, one of the advantages of starch-based BPs is their excellent barrier property against oxygen due to the formation of the ordered and compact matrix of starch molecules [32].

Regarding the swelling capacity of BPs, values between 4.26 and 10.98% are reported in this work. These values are related to the high hydrophilic capacity of starch and glycerol. This characteristic that allows the formation of hydrogen bonds and, therefore, increases water retention within the polymer matrix. This parameter increases the relative humidity of the BPs and favors ideal conditions for the development of microorganisms. Therefore, the swelling capacity of BPs is related to their biodegradability and potential application in food packaging.

In the BPs obtained with the binary combinations (SSE-SPV), the solubility values are between 25.96–37.89% w/v, with significant differences (p < 0.05). The highest solubility values were observed at concentrations higher than 6% w/v of starch. These values indicate their affinity they have for water, so they can dissolve quickly, causing an increase in the diffusion of the components and favoring the biodegradation of the material [32].

BPs with adequate mechanical properties are required to maintain the integrity of BPs during food transportation, handling, and storage. Dong et al. [22] define hardness as the force per unit area for fracture to occur. In this work, it was observed that BPs made with SSE present hardness values in a range of 41.50 and 59.25 g_f without significant differences between the treatments (p > 0.05). In BPs made with SPV, this parameter ranges between 579.00 and 2102 g_f and increases with the starch concentration. This trend is observed with the BPs made from the starch mixtures (SSE-SPV), which show hardness values between 455.20 and 1972.00 g_f, similar to those obtained with SPV. In the same way, it is observed that the elasticity index in the BPs made with SSE starch is between 0.77 and 1.06 without significant differences between treatments (p > 0.05), while the BPs made with starch mixtures (SSE-SPV) show an elasticity index higher than 0.86. This value is greater than the value of SPV BPs (Table 3). The mechanical properties of starch-based BPs depend on the proportion of amylose and amylopectin [29]. In the starch of various varieties of P. vulgaris, a concentration of 44.97 to 51.11% of amylose has been reported [12], while in chayote starch, a concentration of 26.3% is reported [33]. Based on this information, it is inferred that the difference in the concentration of amylose could be related to the hardness of the films. According to a study carried out on BPs of sweet potato starch (Ipomoea batatas) with different amylose content (21.84–26.34%), starch granules with a more significant amount of amylose molecules form a more rigid polymer matrix per unit of area [32]. This statement coincides with the hardness values of the BPs developed from SPV, SSE, and the BPs formulated with the starch mixtures prepared in the present work.

Based on the accumulated evidence and to evaluate the effect of relative humidity (RH) on elongation, BPs made with the combination of SSE-SPV starch in the different concentrations studied in this work were chosen and stored under controlled conditions of 11.30, 57.60 and 90.30% RH.

In BPs stored at 11.30% RH, the percentage of elongation peaked in BPs coded as 6% SSE-SPV with significant differences between treatments (p < 0.05), and it is even higher in 7 and 9% SSE-SPV BPs (Fig. 1). These results coincide with what Lauer and Smith [34] reported, who indicate that a high percentage of amylose tends to form more rigid films and is reflected in reduced elongation of the BPs.

In the storage conditions of 57.60% RH, the maximum elongation value was reached from the concentration of 6% of SSE-SPV. This parameter is similar to that reported in BPs made with 4% w/w of chayotextle starch or potato starch, 0.5% w/w of cellulose, and 2% w/w of glycerol [35]. In Fig. 1, it is generally observed that the elongation percentage is reduced when RH is increased, which is attributed to the fact that an increase in RH around the BPs also increases their water content. In other words, when the BP is wet or absorbs too much moisture from the environment, molecular mobility is promoted in the polymer network, resulting in swelling of the film, which can spontaneously give way to the deformation and partial solubilization of the BP matrix, as well as the hydrolysis of unstable bonds of the polymer [31]. These characteristics could help explain the biodegradability of the BPs analyzed in this study.

Derived from the hydrophilic capacity of BPs made from SSE and SPV starch and the effect of RH on the elongation percentage, its application to produce primary packaging containing low-humidity foods stored in dry environments can be considered. Based on the physical characteristics of the BPs analyzed in this work and to study their biodegradation, the BPs made with the 6% SSE-SPV mixture were selected.

Biodegradability

Three different methodologies were used during the biodegradation tests of the selected BP: in vitro, over-soil, and soil burial, to compare the biodegradation capacity between two different materials: the BP proposed in the present work and a commercial sample of an oxyplastic. Figure 2 shows the images obtained during the evaluation of the biodegradation process at different times. In the in vitro systems (Fig. 2A), bacterial colonies were formed in the systems with BP from day 5 to day 55. However, the formation of bacterial colonies was not observed in the oxy-plastic samples. This may be due to the composition of BP, made up of starch, an important source of bioavailable carbon for microorganisms. Likewise, it was observed that the humidity of the soil extract agar contributed to the swelling of the BP, and, therefore, to its bioavailability to microbial attack. The cause of this, as mentioned above, is that when the BP absorbs too much water, molecular mobility is promoted in the polymeric network, generating its deformation. This meant that the bioplastic sample could not be removed from the petri dish because it was observed to have fractures. On the other hand, the oxo-plastic did not present apparent changes due to the effect of humidity.

During the evaluation of BP biodegradation in the over-soil systems (Fig. 2B), microbial colonization on the BP was observed from day 15 and with the passage of time it was more visible until day 90. This could be observed by the change in color from a light tone to a very dark one, where the microbial colonization was completely visible. Conversely, the oxo-plastic resisted the microbial attack because, in the first 30 days, microbial colonization was not visible. However, by day 45, microbial growth was observed on the surface of the oxo-plastic, which increased without covering its total surface. During biodegradation, the BP changed its initial shape until it formed a cylindrical structure. This was probably due to low humidity and the structural changes of the BP because of the microbial attack.

Finally, during biodegradation in soil burial systems (Fig. 2C), microbial colonization on the BP was observed from day 3. Starting on day 9, irregular edges and a change in color in the physical appearance of the BP were observed. On day 15, the partial degradation of the BP was visualized, and fragments of the BP were obtained during the recovery of the sample from the soil burial systems. For its part, the oxo-plastic did not present structural changes during the total time of the experiment; only microbial colonization was observed on its surface.

During a BP biodegradation process, the main factors that determine the speed of this process are temperature, humidity, the enzymatic activity of the microorganisms, and the composition of the soil, as has been reported in similar studies [36] [37].

Microscopic analysis of biodegradation

The samples obtained at different times of the biodegradation processes in the over-soil and soil burial systems were analyzed by scanning electron microscopy (Fig. 4) using 300 and 1000X. In the BP samples obtained from the over-soil biodegradation systems, hyphae and spores were observed during the first 15 days (OS-15). As time went by, the presence of spores (OS-60) increased, which colonized the surface of the BP. These observations are consistent with other works, where the same structures are observed and described, which were identified as fungal spores [38] [39].

On the other hand, during the biodegradation process of BP in the soil burial system, the presence of fractures was observed at the beginning of biodegradation (SB-03), while the presence of bacteria was observed on day 9 (SB-09), which increased until the bacteria colonized the entire surface (SB-18). The colonization of BP made mainly of starch has been previously observed by Ruggero et al.[40].

The difference in the colonization of the BP surface between the different biodegradation treatments could be associated with humidity, temperature, and enzymatic microorganism activity, as previously mentioned [36] [37].

In the over-soil systems, only one of the faces of the BP was in contact with the soil surface. Therefore, the relative humidity was lower than the soil burial system, so fungal growth was favored under these conditions. On the other hand, there was a greater effect on biodegradation in the soil burial systems, which is attributed to the fact that both surfaces of the BP remained in contact with the two layers of the soil and, therefore, the moisture content was larger, a factor that favored the biodegradation of BP mainly due to bacterial action. As observed in the images obtained by SEM (SB-09 and SB-18).

Evaluation of the Biodegradation of the BP

During the experiments, the change in the mass of the BP was an important parameter that allowed us to compare the biodegradation process in the different treatments used in this work. The weight variation allowed us to determine the percentage of biodegradation, the specific biodegradation rate for each system, and the biodegradation parameters for the first and second-order kinetic models (Table 4). According to the results obtained from the changes in weight of the BP, the highest percentage of biodegradation was observed in the soil burial system (91.02%), followed by the in vitro system (71.97%), and the lowest percentage (57.89%) was observed when over-soil biodegradation was simulated. The BP was degraded due to its starch-rich composition, with starch being a hydrophilic molecule that is completely biodegradable [9]. Consisting mainly of glucose, it is a monosaccharide easily metabolized by microorganisms as a carbon and energy source. It is to be oxidized until total mineralization, that is, to be transformed into CO₂ and water.

In determining the calculation of the specific biodegradation rate, a relationship was observed with the percentage of biodegradation of BP. The highest rate was 7.6854 mg BP/day, with a biodegradation percentage of 91.02% in the burial systems; this indicated that the biodegradation rate was three times higher than the other evaluated systems. Therefore, it can be said that BP degraded in greater proportion and with greater speed in the soil burial system compared to the other two systems evaluated in the present work.

Determining the kinetic parameters for BP biodegradation was evaluated by applying of first- and second-order kinetic models previously reported by Santonja-Blasco [26]. The results of adjusting the experimental data of BP biodegradation in the different in vitro treatments, over-soil, and soil burial, demonstrated a greater adjustment to the first-order kinetic model according to the correlation coefficient obtained (Table 4). Considering the quality of the adjustment of the first-order model in the three systems, the biodegradation rate coefficients were 0.1143, 0.0905, and 0.6873 days^− 1 for the in vitro, over-soil, and underground biodegradation systems, respectively. The results obtained in the present work are consistent with those reported by Gil-Castell et al. [41], where the experimental data of the biological degradation process of the biofilm based on polylactic acid obtained from corn were adjusted to a first-order kinetic model.

In a biodegradation study of polylactic acid-based BP presented by Santoja-Blasco et al. [26], a k with a value of 7.12x10-4 days^− 1 was obtained. This value of k is lower than the results obtained in the different experiments carried out in the present work.

The combination of starch from Sechium edule and Phaseolus vulgaris in a final concentration of 6% w/v forms bioplastics with physical properties that allow their application to preserve of foods with low moisture content that requires transparent BPs. BPs are characterized by their high swelling and solubilization capacity in water, as well as the reduction of elongation at high RH. The BPs generated with the SSE-SPV starch combination degrade more rapidly under soil burial conditions.

The following statements should be included under the heading "Statements and Declarations" for inclusion in the published paper. Please note that submissions that do not include relevant declarations will be returned as incomplete.

Acknowledgements. The first author aknowledges to the Tecnológico Nacional de México/ITSOEH. The authors would like to thank Laboratory of Gestion y Control de la Contaminación Ambiental de la Facultad de Ciencias Químicas (Orizaba) de la Universidad Veracruzana.

Author contributions. All authors had read and agreed with the published version of the manuscript. JPS, MCC: Conceptualization, Methodology, Data curation, Investigation, Writing-original draft. All authors read and approved the final manuscript.

Conflict of Interest. The author declares that he has no conflict of interest.

Data availability. Not applicable

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Table 1

Different Starch combinations from the fruit of Sechium edule (SSE) and the starch of Phaseolus vulgaris (SPV) with a plasticizing agent to obtain different bioplastic.
Sample identification	Starch Sechium edule (% p/v)	Starch Phaseolus vulgaris (% p/v)	Glycerol (p/v)
SSE 1%	1	-	1
SSE 3%	3	-	3
SSE 5%	5	-	5
SSE 6%	6	-	6
SSE 7%	7	-	7
SSE 9%	9	-	9
SPV 1%	-	1	1
SPV 3%	-	3	3
SPV 5%	-	5	5
SPV 6%	-	6	6
SPV 7%	-	7	7
SPV 9%	-	9	9
SSE-SPV 1%	0.5	0.5	1
SSE-SPV 3%	1.5	1.5	3
SSE-SPV 5%	2.5	2.5	5
SSE-SPV 6%	3.0	3.0	6
SSE-SPV 7%	3.5	3.5	7
SSE-SPV 9%	4.5	4.5	9

Table 2

Physical characteristics and qualitative evaluation of the bioplastics of different starch combinations from the fruit of Sechium edule (SSE) and the starch of Phaseolus vulgaris (SPV)
Sample	Qualitative evaluation			Opacity	Swelling (%)	Solubility (% p/v)
Sample	Handleability	Continuity	Uniformity	Opacity	Swelling (%)	Solubility (% p/v)
SSE 1%	No	No	No	---	---	---
SSE 3%	No	No	No	---	---	---
SSE 5%	Yes	No	Yes	1.67 ± 0.21 ^b	6.20 ± 1.02 ^a	20.40 ± 0.79 ^a
SSE 6%	Yes	Yes	Yes	1.53 ± 0.38 ^a	7.63 ± 0.50 ^a	23.22 ± 0.37 ^a
SSE 7%	Yes	Yes	Yes	1.39 ± 0.15 ^a	6.18 ± 0.52 ^a	26.75 ± 0.39 ^b
SSE 9%	Yes	Yes	Yes	1.42 ± 0.14 ^a	10.98 ± 2.70 ^b	30.27 ± 3.43 ^c
SPV 1%	No	No	No	---	---	---
SPV 3%	Yes	No	Yes	0.42 ± 0.14 ^a	4.26 ± 1.19^a	29.20 ± 2.69^c
SPV 5%	Yes	Yes	Yes	0.63 ± 0.22 ^a	6.09 ± 0.87^b	18.11 ± 1.06 ^a
SPV 6%	Yes	Yes	Yes	0.44 ± 0.17 ^a	7.03 ± 0.94^b	19.56 ± 0.67^ab
SPV 7%	Yes	Yes	Yes	0.38 ± 0.11 ^a	6.73 ± 0.55^b	20.98 ± 2.77^ab
SPV 9%	Yes	Yes	Yes	0.78 ± 0.28 ^a	6.69 ± 0.84^b	23.13 ± 2.08 ^b
SSE-SPV 1%	No	No	No	---	---	---
SSE-SPV 3%	No	No	Yes	1.46 ± 0.38 ^b	4.92 ± 0.30 ^a	25.96 ± 1.05 ^a
SSE-SPV 5%	Yes	Yes	Yes	0.98 ± 0.23 ^b	6.19 ± 0.53 ^a	27.19 ± 1.87 ^a
SSE-SPV 6%	Yes	Yes	Yes	0.97 ± 0.29 ^b	9.13 ± 1.62 ^c	35.79 ± 2.09 ^b
SSE-SPV 7%	Yes	Yes	Yes	0.44 ± 0.29 ^a	6.78 ± 0.90 ^ac	35.97 ± 2.23 ^b
SSE-SPV 9%	Yes	Yes	Yes	0.53 ± 0.09 ^a	8.36 ± 1.28 ^bc	37.89 ± 4.45 ^b

The data shown represent the mean ± SD, different letters in the same colum denote significant differences (p < 0.05). --- Bioplastic is not obtained.

Table 3

Mechanical properties of the bioplastic obtained from different starch combinations from Sechium edule (SSE) and the starch of Phaseolus vulgaris (SPV)
Starch from Sechium edule
Parameter	Starch concentration (% p/v)
Parameter	3	5	6	7	9
Hardness	-	43.80 ± 2.58 ^a	41.50 ± 10.15 ^a	59.25 ± 9.74^ab	51.67 ± 3.78^ab
Deformation according to hardness (%)	-	3.22 ± 1.74 ^a	3.90 ± 0.14 ^a	3.98 ± 0.04 ^a	2.84 ± 1.55 ^a
Adhesive Strength	-	3.20 ± 0.44 ^a	2.80 ± 1.48 ^a	3.20 ± 1.09 ^a	3.20 ± 1.09 ^a
Adhesive	-	0.00 ± 0.00 ^a	0.08 ± 0.10 ^a	0.22 ± 0.27 ^a	0.14 ± 0.31 ^a
Resilience	-	0.37 ± 0.46 ^a	0.70 ± 0.30 ^a	0.93 ± 0.21 ^a	0.53 ± 0.45 ^a
Fractureability	-	8.60 ± 1.14 ^a	7.00 ± 0.81 ^a	7.33 ± 2.08 ^a	7.20 ± 1.48 ^a
Chewyness	-	---	23.25 ± 18.84 ^a	35.67 ± 19.55 ^b	55.50 ± 42.75 ^b
Chewy index	-	48.67 ± 2.88^c	32.80 ± 8.58^b	---	39.40 ± 9.88^b
Hardness cycle 2	-	43.67 ± 4.72 ^a	47.50 ± 4.20 ^a	47.75 ± 4.19 ^a	49.67 ± 2.08 ^a
Elasticity index	-	1.06 ± 0.11 ^a	0.77 ± 0.10 ^a	0.94 ± 0.71 ^a	1.00 ± 0.55 ^a
Starch from Phaseolus vulgaris
Hardness	579.00 ± 165.40 ^a	1269.00 ± 193.50^b	1667.00 ± 155.^60b	1297.00 ± 103.20^b	2102 ± 73.58^c
Deformation according to hardness (%)	8.98 ± 6.02^a	19.30 ± 0.97^ab	20.00 ± 0.00^ab	20.00 ± 0.00^ab	22.62 ± 5.85^b
Adhesive Strength	1.500 ± 1.732^a	4.250 ± 2.872^a	2.20 ± 1.78^a	2.66 ± 2.08^a	2.80 ± 2.16 ^a
Adhesive	0.10 ± 0.22^a	0.16 ± 0.25^a	0.00 ± 0.00^a	0.13 ± 0.23^a	0.80 ± 0.10^a
Resilience	0.022 ± 0.08^a	0.16 ± 0.08^b	0.27 ± 0.04^c	0.29 ± 0.01^c	0.27 ± 0.01^c
Fractureability	579 ± 165.4^a	1609 ± 487.80 ^bc	1558 ± 279^b	1521 ± 394.80^bc	2198 ± 157.70^c
Chewyness	0.325 ± 0.75^a	0.733 ± 0.75^a	43.42 ± 7.23^b	48.33 ± 14.26^b	68.48 ± 5.74^c
Chewy index	13.50 ± 12.02^a	748.5 ± 40.31^b	886.80 ± 146.60^b	986.30 ± 285.00^b	1403 ± 105.20^c
Hardness cycle 2	253.8 ± 163.00^a	1366 ± 112.40 ^b	1471 ± 250.30^b	1455 ± 386.90^b	2097 ± 162.70^c
Elasticity index	0.74 ± 0.16^a	0.65 ± 0.09^a	0.79 ± 0.05^a	0.83 ± 0.04^a	0.81 ± 0.04^a
Starch combination from Sechium edule and Phaseolus vulgaris
Hardness	455.20 ± 24.77^a	689.4 ± 61.19^a	1972 ± 42.64 ^c	1552.00 ± 233.50 ^b	1563.00 ± 250.60 ^b
Deformation according to hardness (%)	13.30 ± 0.00^a	15.92 ± 3.72 ^a	13.30 ± 0.00 ^a	13.30 ± 0.00 ^a	13.23 ± 0.15 ^a
Adhesive Strength	1.00 ± 1.00^a	1.67 ± 2.08 ^a	1.20 ± 0.83 ^a	3.00 ± 0.70 ^a	1.00 ± 0.81 ^a
Adhesive	-1.98 ± 4.48 ^a	0.12 ± 0.32 ^a	0.00 ± 0.12 ^a	0.00 ± 0.07 ^a	-0.02 ± 0.05 ^a
Resilience	0.30 ± 0.01 ^a	0.30 ± 0.01^a	0.22 ± 0.00 ^b	0.22 ± 0.01 ^b	0.32 ± 0.00 ^a
Fractureability	455.2 ± 24.77 ^a	689.4 ± 61.19 ^a	1921.00 ± 119.30 ^c	1614.00 ± 245.80 ^bc	1361.00 ± 452.40 ^b
Chewyness	13.02 ± 0.57 ^a	19.76 ± 2.41 ^a	52.60 ± 3.83 ^c	39.60 ± 6.98 ^b	36.93 ± 10.42 ^b
Chewy index	332.00 ± 14.05 ^a	520.00 ± 39.78 ^a	1345.00 ± 97.16 ^c	1092 ± 236.70 ^bc	951,80 ± 268.40 ^b
Hardness cycle 2	417.80 ± 18.25 ^a	643.80 ± 62.77 ^a	1791.00 ± 92.62 ^c	1502.00 ± 228.3 ^b	1453 ± 220.20 ^b
Elasticity index	0.90 ± 0.02 ^a	0.91 ± 0.02 ^a	0.87 ± 0.01 ^a	0.86 ± 0.08^a	0.88 ± 0.04^a

The data shown represent the mean ± SD, different letters in the same row denote significant differences (p < 0.05).

Table 4

Fitting model and parameters for the modelling of average mass for bioplastics using three biodegradation conditions.
Biodegradation System	First order model			Second order model		Degradation parameters
Biodegradation System	M_hf	k₁ (days^− 1)	R²	k₂ (days^− 1)	R²	Rate (mg BP/d)	(%)
In vitro	0.1955	0.1143	0.9192	0.3997	0.8982	2.5582	71.97
Over Soil	0.1929	0.0905	0.9961	0.2020	0.9476	2.1437	57.89
Soil Burial	0.1934	0.6873	0.8171	2.7283	0.7898	7.6854	91.02

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Bioplastics Made From Starch as Conventional Plastic Substitutes: Evaluation of Physical Properties and Biodegradation

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Abstract

Figures

Highlights

Introduction

Material and methods

Obtainment of bioplastic using the casting method

Physical characterization of bioplastics (BPs)

Qualitative assessment

Thickness

Opacity

Water solubility

Swelling

Characterization of texture properties

Deformation

Statistical analysis of the physical properties

Biodegradation of bioplastic

Collection of the soil sample

Adjusting the bioplastic samples

Bioplastic biodegradation analysis

Simulation systems for biodegradation

Morphological changes of BPs during biodegradation analyzed by Environmental Scanning Electron Microscopy

Results And Discussion

Physical characterization of BPs

Biodegradability

Microscopic analysis of biodegradation

Evaluation of the Biodegradation of the BP

Conclusion

Declarations

References

Tables

Supplementary Files

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