Valorisation of Tomato Waste as a Source of Cutin for Hydrophobic Surface Coatings to Protect Starch- and Gelatine-Blend Bioplastics

Abstract

The valorisation of food by-products is an important step towards sustainability in food production. Tomatoes constitute one of the most processed crops in the world (160 million tonnes of tomatoes are processed every year), of which 4% is waste. This translates to 6.4 million tonnes of tomato skins and seeds. Currently, this waste is composted or is used in the production of low-value animal feed; higher value can be achieved if this waste stream is re-appropriated for more advanced purposes. Plant cuticle is a membrane structure found on leaves and fruit, including tomatoes, and is mainly composed of cutin. The main function of plant cuticle is to limit water loss from the internal tissue of the plant. Cutin, which can be recovered from the tomato skins by pH shift extraction, has hydrophobic (water repellent) properties and is therefore an ideal raw material for the development of a novel water-resistant coating. In this study, biomass-based bioplastics were developed. Unfortunately, although these bioplastics have good mechanical properties, their hydrophilic nature results in poor water barrier properties. To mitigate this, a very effective water-resistant coating was formulated using the cutin extracted from tomato peels. The water vapour permeability rates of the bioplastics improved by 74% and the percentage swelling of the bioplastic improved by 84% when treated with the cutin coating. With physicochemical properties that can compete with petroleum-based plastics, these bioplastics have the potential to address the growing market demand for sustainable alternatives for food packaging. Using ingredients generated from by-products of the food processing industries (circular economy), the development of these bioplastics also addresses the UN’s Sustainable Development Goal (SDG) 12, Sustainable Consumption and Production (SCP).

1. Introduction

The valorisation of by-products from the food processing industry is an important step in increasing the food production sustainability worldwide. Using these by-products will reduce the environmental impact and increase the profitability of these raw materials [1]. All by-products from food processing, due to their nature, are typically rich in bioactive compounds. Re-appropriating these by-products for bioactive compound extraction represents a valuable renewable source for functional food additives. These bioactive compounds can be recirculated back into the food production process, benefiting the whole food system [2]. Research on food by-products has increased globally due to its contributions to the circular economy concept. The European Union’s Circular Economy Action Plan [3] targets the entire life cycle of products. It includes objectives related to managing both waste and resources. These objectives are aimed at reducing the amount of waste generated and increasing the proportion of waste that is recycled and reused. This will also promote a more sustainable use of resources such as by-products, tackling UN’s Sustainable Development Goal 12, Sustainable Consumption and Production [4]. As a result, food by-products are now being heavily researched as raw materials for the bioplastics industry. Moreover, the production of alternatives to petroleum plastics is historically associated with significantly higher costs, which can be somewhat alleviated by utilising a food-waste stream [5].

Tomatoes are widely cultivated in Mediterranean countries, mainly for consumption as processed products like tomato paste, sauce, juice, ketchup, among many other forms. Tomatoes in a quantity of 180 million tonnes are grown annually worldwide, and 160 million tonnes are subsequently processed, making tomatoes one of the most widely processed crops in the world. Most of these tomato-based products are seedless and skinless, generating discarded by-products (skins and seeds), which constitute the majority (or 6.4 million tonnes) of the resultant tomato-industry waste [6]. This waste, which represents, on average, 4% of the total tomato harvest weight [7], is not physiochemically damaged during the pulp separation processes and the skins are rich in usable cutin. Cutin, which is a polymer embedded in the plant cuticle, is responsible for the barrier properties of the plant cuticle. It protects the plant cuticle from mechanical and chemical damage, creating barriers against the diffusion of oxygen and water [8]. Cutin can be extracted from a range of plant materials by thermal treatment in alkaline solutions followed by a filtration step to separate the solids from the liquid. Subsequently, the liquid is acidified to precipitate cutin out of the solution [9]. Due to cutin’s hydrophobic properties, it is an ideal raw material for the production of water-resistant surface treatments.

Food packaging is widely used to extend the shelf life of food by protecting it from biological or chemical degradation. As a result, food packaging is required to possess good mechanical and chemical resistance properties whilst remaining light-weight and durable [10]. However, this single-use packaging is the biggest contributor to plastic pollution and is responsible for 47% of plastic waste produced every year [11]. Alternatives to these plastics are becoming more popular for some product ranges, such as reusable water bottles, metal, bamboo or silicone straws, and cardboard alternatives, which have been heavily supported by the informed public (Neves et al., 2020a) [12]. Biodegradable bioplastics, as an alternative to traditional plastics, are evolving to service a variety of industries. Starch and gelatine bioplastic blends are home compostable and can be generated from biomass recovered from food waste [5,13]. In a previous study, starch and gelatine bioplastics were described by Stanley et al. (2020) [14] to possess good mechanical properties, such as a tensile strength of 2.52 ± 0.47 MPa. The water barrier properties required improvements as the water solubility reached as high as 60% after 24 h [14]. If these bioplastics are to be successfully adopted as an alternative but sustainable food packaging material, their water barrier properties require improvement.

The research questions of this study are as follows: (i) can the cutin improve the water barrier properties of the bioplastic, (ii) does the method of coating affect the coatings’ performance, (iii) does it affect the degradation of the bioplastic in soil, and (iv) does the coating offer enhanced antimicrobial activity and inhibit the growth of fungi? For this, cutin was extracted from tomato peels, formulated into a coating, and applied to the bioplastics using different methods, after which the water barrier properties of the newly coated bioplastics were tested and quantified. The different treatments used included uncoated bioplastic, a bioplastic surface treated with ethanol, and bioplastics coated by brushing, rolling with a glass rod, and dipping in the coating.

2. Materials and Methods

2.1. Materials

Food-grade piscine gelatine of 200 bloom (Louis Francois, Croissy-Beaubourg, France) and potato starch were used to generate the bioplastic. Tomato peels were sourced from France with the help from French National Interprofessional Tomato Society, SONITO. Topsoil (Westland, Top Soil, Yeovil, Somerset, UK) was used for the soil burial test.

2.2. Preparation of the Gelatine Starch Blend Bioplastic

The bioplastic samples were prepared as previously published [13]. Gelatine was hydrated with water and left to bloom; water and starch were heated until starch had fully dissolved. Then the starch, gelatine, and glycerol were mixed together and heated again until they fully combined and became homogenous. The formulation was left too cool before pouring it into a mould. The bioplastic was then left to dry in a well-ventilated area for 2 days before we removed it from the mould.

2.3. Cutin Extraction

The method used for cutin extraction from tomato peels was adapted from [9]. Tomato waste (skins and seeds) was immersed in water to separate the skins from the seeds using a method where the skins flow to the top while seeds sink to the bottom. The peels were then drained and dried in the oven at 100 °C overnight. Dry tomato peels were then crushed into a flaky powder and immersed in 0.75 M NaOH at a 3% w/v. This was then autoclaved for 2 h at 121 °C. The cooled liquid was then filtered to remove any solids using filter paper Whatman no.1. The filtrate had a pH of 12; this was shifted to pH 5.5 using concentrated HCl. The pH change allowed the cutin to precipitate out of the solution. To separate the cutin from the liquid, the extract was centrifuged for 35 min at 3800 rpm. The liquid phase was discarded and the solid was transferred onto a glass dish and oven-dried for 24 h at 100 °C (Figure 1).

2.4. Coating Formulation

The extracted cutin was in the form of a waxy paste that was dissolved in 96% ethanol in one-to-one ratio in g per ml. The coating was prepared by adding small amounts of cutin at a time, and only adding more when the previous amount had fully dissolved, under continuous stirring, until the coating was homogenous. The solution was covered during the stirring to prevent solvent evaporation.

2.5. Coating Techniques

Brushing, dipping, and rolling with a glass rod were the techniques used to coat the bioplastic with cutin coating; the dimensions of the coated samples were 3 × 3 cm (9 cm²). After applying the coating on both sides, the dry coating was visually inspected under a stereoscope to determine whether satisfying coverage had been achieved. The thickness of the coating was calculated from measurements taken before and after coating application. Bioplastic without coating and bioplastic surface treated with ethanol (96%) acted as controls.

2.6. Coating Thickness

The thickness of the coating was measured by measuring the thickness of the bioplastic before applying the coating and after the coating had been applied. The measurements were taken with a digital calliper (GROZ) in three replicates. The thickness of the coating was calculated by subtracting the before and after measurement and dividing it by 2 as the coating was present on both sides of the bioplastic.

2.7. Fourier-Transform Infrared Spectroscopy (FTIR)

A Spectrum 65 FT-IR Spectrometer (Perkin Elmer, Hopkinton, Ma, USA) was used to analyse cutin extract. The analysis was performed by placing the extracted paste samples on the FTIR sensor and the analysis was initiated. The test was performed on three samples extracted. The results were recorded as % transmission and the wavelength range used was 750 to 4000 nm.

2.8. Roughness

A roughness tester, SURFCOM130A, with a TS100 sensor (ZEISS, Tokyo, Japan) was used to measure the roughness of bioplastic without and with the cutin coating. The tester was calibrated with a standard of R_a = 1.54 μm before performing the test. The test was performed on 4 replicates per treatment.

2.9. Colour Analysis

A PCE colorimeter (Meschede, Germany) was used to measure the colour of the cutin-coated bioplastics and to assess the evenness of the cutin coating. The measurements were repeated four times for each sample, changing the test area each time. This was performed on bioplastics coated by different techniques; five replicates were tested per treatment. The following values were recorded by the colorimeter: L* = lightness, a* from green (−) to red (+), and b* from blue (−) to yellow (+).

2.10. Absorptive Wetting/Swelling Test

This method was adapted from Shanmathy, Mohanta, and Thirugnanam (2021) [15]. This was performed on bioplastic with five different treatments (uncoated bioplastic, bioplastic surface treated with ethanol, and cutin-coated bioplastic by brushing, rolling with glass rod, and dipping in the coating). The initial weight of bioplastic with dimensions of 3 × 3 cm (9 cm²) was noted. The samples were fully immersed in 50 mL of deionised water; after 120 min the samples were removed from the water and patted dry and the weight was noted again. The test was performed in triplicate at room temperature. The following formula was used to calculate the percentage swelling.

$$\mathrm{P}\mathrm{e}\mathrm{r}\mathrm{c}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{a}\mathrm{g}\mathrm{e} \mathrm{s}\mathrm{w}\mathrm{e}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{g}={\displaystyle \frac{\mathrm{F}\mathrm{i}\mathrm{l}\mathrm{m} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{a}\mathrm{f}\mathrm{t}\mathrm{e}\mathrm{r} \mathrm{s}\mathrm{w}\mathrm{e}\mathrm{l}\mathrm{l}\mathrm{i}\mathrm{n}\mathrm{g}-\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{t}\mathrm{h}\mathrm{e} \mathrm{f}\mathrm{i}\mathrm{l}\mathrm{m}}{\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l} \mathrm{w}\mathrm{e}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t} \mathrm{o}\mathrm{f} \mathrm{t}\mathrm{h}\mathrm{e} \mathrm{f}\mathrm{i}\mathrm{l}\mathrm{m}}}\times 100$$

2.11. Water Contact Angle

Wettability of a solid can be measured using different techniques, one of which is spread wetting. Spread wetting is the wetting of a surface of a solid when a specific amount (droplet) of liquid is placed onto, and then spreads over, the surface. The degree of wetting can be quantified by measuring the contact angle at the solid/liquid interface. If the contact angle is smaller than 90°, the solid is classified as hydrophilic, and if the contact angle is greater than 90°, the solid is classified as hydrophobic [16].

The wettability of the coated bioplastic was evaluated by measuring the water contact angle using a First Ten Angstroms (FTA 200 model) surface energy analyser. This was accomplished by placing 10 µL water droplet on the bioplastic surface using the contact angle metre equipped with a charged-coupled device (CCD) camera, which took pictures of each water droplet. The resulting images were then analysed using the FTA32 Video 2.0 software. This was carried out at an ambient temperature and an average was calculated from measurements of five drops for each sample.

2.12. Water Vapour Transmission Rate

The following method was adapted from Elcometer 5100 permeability cup manual [13]. This was performed on bioplastic with five different treatments (uncoated bioplastic, bioplastic surface treated with ethanol, and cutin-coated bioplastic by brushing, rolling with glass rod, and dipping in the coating). The bioplastic samples were placed on top of permeability cups filled with dry desiccant and silica gel and sealed with the twist caps or clamps. The permeability cups were weighed and placed in a desiccator with a saturated Sodium Chloride (NaCl) at a temperature of 22 °C to maintain relative humidity at 75%. The samples were weighed every 24 h. The water vapour transmission rate of the bioplastic in grams per sq. metre per day (g/(m²·d)) was calculated using the following formula adapted from [17]

$$\mathrm{WVTR}={\displaystyle \frac{(\mathit{W}/\mathit{T})}{\mathit{A}}}$$

Here,

WVTR = water vapour transmission rate (g/(m²·d));

W = weight change (g);

T = time (d);

A = area of the test piece (m²).

2.13. Soil Burial Test

The purpose of the soil burial test was to test the biodegradability of the bioplastic in the soil and to investigate whether the cutin had any effect on the biodegradability rate. The method was adapted from [18]; the duration and sample size were increased from the original method. The bioplastic samples, coated and uncoated, were cut into 3 × 3 cm squares and buried under 7.5 cm of topsoil; these were then incubated at room temperature for 10 days. The initial weight of samples was noted, and every 2 days, the samples were removed from the soil, brushed to remove any residues of soil, weighed, and returned back to the soil. The percentage weight loss was then calculated using the following formula, where W0 = initial weight and Wf = final weight.

$$\% weight loss={\displaystyle \frac{W0-Wf }{Wf}}\times 100$$

2.14. Fungal Degradation

The purpose of the fungal degradation was to test the effect of cutin on fungi and its ability to degrade the bioplastic. Aspergillus niger was cultivated on potato dextrose agar at 37 °C until spore formation covered the majority of the fungi. The agar plate was flushed with sterile DI water to create spore’s suspension as the stock culture. The spore suspended was diluted by a factor of 100 [18]. Salt agar (SA) (1.8 g K₂HPO₄, 4 g NH₄Cl, 0.2 g MgSO₄·7H₂O, 0.1 g NaCl, 0.01 g FeSO₄·7H₂O, and 15 g of bacteriological agar in 1 L of DI water) [19] was sterilised by autoclaving and poured into petri dishes. Bioplastic samples were cut into 3 × 3 cm pieces. As controls, filter paper, Whatman no.1 (positive control), and polystyrene (PS) plastic (conventional plastic) and agar without any samples (negative control) were used. A quantity of 0.1 mL of the diluted spore suspension of A. niger was spread on the SA media surface and the prepared samples were placed in the centre of the plate. Bioplastic samples and controls were incubated at 30 °C for 10 days.

2.15. Statistical Analysis

One-way analysis of variance (ANOVA) at significance level p ≤ 0.05 was used to perform statistical analysis, and post hoc tests, Tukey’s honest significant difference test followed by Shapiro–Wilk test, were used to determine the normal distribution of the data. The software used to perform the statistical analysis was Excel 2016 (Microsoft Excel 2016, San Francisco, CA, USA).

3. Results and Discussion

The objective of this study was to investigate whether cutin could be used to improve starch- and gelatine-based bioplastics’ water barrier properties without compromising their biodegradability. Cutin was successfully extracted from the tomato peels; the average yield of cutin from tomato peels was 34%. The extracted cutin was in the form of a waxy paste, dark brown in colour with no distinct odour. The resulting coating made from the extracted cutin was thick and sticky in consistency and also dark brown in colour; when spread on a surface, the colour was brown–orange and partially transparent.

3.1. Fourier-Transform Infrared Spectroscopy (FTIR)

FTIR was conducted to identify the substance extracted from the tomato peels. The functional groups of the spectrum of the extract from tomato peels (Figure 2) were identified. The first functional group identified at 3341 cm⁻¹ comprised the stretching vibration of hydrogen-bonded hydroxyl functional groups (O-H···H). The next functional group identified had two well-defined peaks at 2920 and 2850 cm⁻¹ and its stretching vibration of CH₂ was known as the aliphatic stretch; CH₂ is the most repeated structural unit in the cutin polymer. Following this, we could observe the stretching band vibration of ester carbonyl from the ketone group at 1707 cm⁻¹. This was due to the C=O links between different hydroxy and epoxy fatty acids, which formed cross-linkages in the cutin. The C=O also created weak bonds with H (CO···H) in saturated aliphatic acid and carboxylic acids. The 1464 cm⁻¹ peak was responsible for the CH₂ scissoring present in cutin but also for waxes; CH₂ was also responsible for the last peak at 723 cm⁻¹, which was due to C-H bending and C-C out-of-plane bending vibration. A similar FTIR spectrum was found in cutin extracted from watermelon peels [20], confirming that the functional groups identified corresponded to the ones present in cutin samples.

In another study, cutin from tomato peels was extracted and investigated. The cutin extract was examined by attenuated-total-reflection Fourier-transform infrared spectroscopy (ATR-FTIR). The overall spectra presented by Manrich et al. (2017) [8] resembled the one reported in the present study. The following functional groups were identified: an ester group at 1730 cm⁻¹ and vibration bands of -CH, -CH₂, and -CH₃ at 2950 and 2850 cm⁻¹ [8].

A study investigating different cutin extraction methods from tomato peels reported the FTIR spectra of the extracts. Depending on the extraction method, the spectra had slight variations. The following main band identified as present in all extracts was hydrogen-bonded hydroxyl at 3320 cm⁻¹, followed by the asymmetric scissoring of -CH₂ at 2925 and 2850 cm⁻¹ and bending of -CH₂ at 722 cm⁻¹. The ester group was identified at 1711 cm⁻¹ and it was only present in one cutin extract, extracted by method with an acidification step [21].

3.2. Coating Thickness and Roughness

The bioplastic samples coated with cutin coating using three different techniques (dipping, brushing, rolling) can be observed in Figure 3. The coated bioplastics were brown–orange in colour with a more or less even finish depending on the technique used. The coating technique that provided the most control over the spreading of the coating was brushing. Visually, the bioplastic samples coated by brushing looked the most even in comparison to those coated by rolling and dipping, which were patchier. The drying of the dipped samples was problematic as they had to be hung, which resulted in coating accumulating at the bottom edges of the bioplastics. The rolling technique was most problematic to execute as the pressure applied to the class rod had to be constant to disperse an even layer of the coating.

The surface roughness (R_a) of the bioplastic without cutin coating was 3.2 ± 0.6 µm while the R_a values of the bioplastics with cutin coating applied by dipping, brushing, and rolling were 1.6 ± 0.2, 1.3 ± 0.3 and 1.7 ± 0.3 µm, respectively (

Table 1. Surface roughness (n = 4) and coating thickness (n = 3) values of bioplastic samples coated with different coating techniques. Mean values are shown followed by ±standard deviations; * = statistically significantly different.

Sample Type	Ra (µm)	Coating Thickness (µm)
No coating	3.2 * ± 0.6	N/A
Dipping	1.6 ± 0.2	72 ± 38
Brushing	1.3 ± 0.3	73 ± 26
Rolling	1.7 ± 0.3	60 ± 10

). All cutin-coated bioplastics, regardless of the coating technique used, showed statistically significant decreases in R_a when compared to the bioplastic sample without cutin coating. There was a 51% reduction in surface roughness in the dip-coated bioplastic samples, 58% reduction in the samples coated by brushing, and 46% reduction in the samples coated by rolling compared to the uncoated control.

In comparison, a study investigating a bioplastic made from cutin extracted from fruits of two species of the Solaneum genus reported the R_a to range from 0.0013 to 0.0026 µm. The bioplastic films were standalone films and prepared with a methanol–chloroform solvent. The method used to measure the surface roughness was atomic force microscopy [22].

Another study that investigated edible films from starch and gelatine reported the Ra to be ranging from 0.0107 to 0.0772 µm and the method used was also atomic force microscopy [23].

Therefore, despite the decreases, the R_a values of all cutin coatings in our study were relatively high compared to those of other similar bioplastics reported in the literature. The coating thicknesses of dipping, brushing, and rolling techniques were comparable in thickness: 72 ± 38, 73 ± 26, and 60 ± 10 µm, respectively (Table 1). These were not significantly different. The dipping technique had the highest standard deviation, resulting in the most variable thickness. The rolling technique resulted in the thinnest coating with the lowest variability. These thicknesses are comparable to those reported in the literature for a range of cutin and pectin bioplastic films, which were ranging from 17 ± 2 to 76 ± 4 µm in thickness [8].

3.3. Colour Analysis

Colour analysis was carried out to compare the coating evenness resulting from the different coating techniques. The results showed no significant difference in the colour values (lightness/darkness) of the bioplastics coated with the different coating techniques. The average perceptual lightness of the coating applied by brushing was 19.9 L*, that applied by rolling was 20 L*, and that applied by dipping was 19.3 L* (Figure 2). This increase in the darkness of the bioplastic was expected as there was a visible colour to the cutin coating itself. Although there was no significant difference between different coating techniques in the lightness/darkness of the bioplastics, there was a difference in colour to the human eye; the samples that had been coated by dipping looked darker with a brown tint when compared to brushed or rolled samples with a more orange tint. This orange–brown tint to the cutin coating was possibly due to lycopene and flavonoids present in the tomato peels. Lycopene and flavonoids are pigments that give tomato fruits their colours. Studies investigating the use of lycopene and flavonoids have successfully extracted these pigments from tomato peels [24]. The cutin-coated samples were significantly darker in comparison to previously reported results from a range of different starch-based bioplastics without cutin coating, with the lowest value of 90.73 L* in a bioplastic made from rice starch [25].

In a study reported by [26], a bioplastic made from cutin extracted from tomato by-products was investigated. The bioplastic samples were prepared from cutin (0.5%) and chitosan (1.5%) and were standalone films. The colour of these films was reported to range from 72.18 to 76.38 L*. These were much lighter in comparison to the cutin-coated bioplastics in this study, but the concentrations used by Simoes 2023 were relatively low and the incorporation of cutin into the bioplastic was different.

Colour is a crucial characteristic when it comes to food packaging as it influences consumer preferences and perceptions (like healthiness) about the food. Colour also plays an important role in capturing consumers attention and allows brands to be more easily recognised by consumers [27,28].

3.4. Absorptive Wetting/Swelling Test

Bioplastics coated with cutin demonstrated a statistically significant decrease in absorptive wetting compared to the uncoated bioplastic (Figure 4), with the uncoated samples presenting an average of 305 ± 33% swelling compared with the coated samples that presented a swelling between 48 ± 5% and 58 ± 2%. The type of coating technique had no influence on the absorptive wetting of the bioplastic; it decreased the percentage swelling by 75 to 79%. Ethanol was applied to the bioplastic by immersing it in the solvent and then allowing it to completely dry before any parameters were measured. Ethanol by itself was used to compare to the coated and uncoated bioplastic, which also significantly decreased, by 20%, the absorptive wetting, but not to the same extent as cutin coating. Therefore, about 55% of the decrease in absorptive wetting was the result of cutin’s water barrier properties. In comparison, a similar result was achieved by Shanmathy, Mohanta, and Thirugnanam (2021) [15] by incorporating 2.5% bentonite into a taro starch bioplastic, decreasing the water swelling of the bioplastic to about 70% water swelling [15].

Reducing the affinity of bioplastic to water was achieved in a study by Manrich et al. (2017) [8], where cutin pectin-blend films were prepared. They reported that the higher the cutin concentration was, the lower the water absorption was. This study used a different technique to determine the water uptake: the films were exposed to relative humidity in the air and the water uptake was calculated from this. Although the technique was different and the results were not directly comparable, the trend and resulting conclusions were similar, showing that the presence of cutin in bioplastics reduces the water absorption of the bioplastics [8].

Reduced water swelling/absorption is beneficial from an application point of view. When food with a high moisture content is packaged, the moisture levels are expected to be maintained in the food by the packaging. The packaging should also provide a barrier from moisture entering from the outside environment. The reduced water swelling of a cutin-coated bioplastic can expand the applicability of the starch–gelatine-blend bioplastic, which otherwise has high water swelling.

3.5. Water Contact Angle (WCA)

The water contact angle did not manifest as expected. The contact angles of samples, with or without cutin deposited on the surface, were all hydrophobic (<90°). There was no statistically significant difference between the treatments as no relevant change in the hydrophobicity was detected (Figure 5). More interestingly, it was found that the dip-coating method caused a decrease in the contact angle, making the samples even more hydrophilic.

This was possibly due, in part, to a reduction in the surface roughness (R_a) of the bioplastic after the application of cutin coating, as previously discussed in Section 3.2. However, although surface wettability is not specifically determined/affected by changes in the R_a value, the time required for a liquid to spread on a surface can be adversely impacted by the additional barriers created by increased asperity morphology and density on rougher surfaces.

Studies reported in the literature have investigated and reported a correlation between bioplastic hydrophobicity and surface roughness [29,30]. Other studies, however, have reported no changes to the WCA. For example, a study by Simões et al. (2023) [26] found no WCA–R_a relationship in cutin- and chitosan-blend films. The water contact angle of the cutin- and chitosan-blend films was reported to range from 93.37 ± 0.31° to 95.15 ± 0.53°; these were >90°, and therefore, the films were considered hydrophobic [26].

The water contact angles in cutin- and pectin-blend films reported by Manrich et al. (2017) [8] ranged from 85 to 93° depending on the specific ratio of cutin and pectin. Overall, this study reported on an increase in the water contact angle when cutin was added to bioplastics in comparison to pectin films. The resulting films were ranging below and above 90°; therefore, depending on their composition, the water barrier could be adjusted [8].

3.6. Water Vapour Transmission Rate (WVTR)

The bioplastics with and without the cutin were assessed for their water vapour transmission rates (Figure 6). The control bioplastic with no cutin treatment was found to have a water vapour transmission rate of 222.03 ± 40.75 g/m²day. Another control was set up with just an ethanol treatment and had WVTR of 233.0917 ± 48.87 g/m²day; the ethanol treatment increased the WVPR by 5% in comparison to the non-coated control. The three bioplastics with the cutin, applied by either brushing, rolling, or dipping, had water vapour transmission rates of 60.88 ± 14.32 g/m²day, 53.77 ± 22.15 g/m²day, and 38.64 ± 5.8 g/m²day, respectively. These were significantly lower than the rate in the non-coated control and resulted in a 73 to 83% decrease in the WVPR in comparison to the non-coated control. The samples with cutin incorporated into the bioplastic and bioplastics directly treated with ethanol and the non-coated sample were not statistically significantly different from each other. The previously reported water vapour transmission rates of other bioplastic materials were 103–195.1 g/m²day for PLA (Polylactic acid) [17] and 73–82 g/m²day for starch–gelatine-blend films [23]. This demonstrates that the water-permeability values of the bioplastics coated with cutin were lower than those of PLA and starch–gelatine blends. The water vapour transmission rate appears to be linked to film thickness (mesoporosity), but this relationship was not addressed in this study.

The water vapour transmission rate is an important characteristic when it comes to material application, when the main purpose of the material is to provide a protective barrier. Materials used for food packaging should have a low WVTR to effectively prevent moisture exchange between the food and outside environment [31]. Therefore, lowering the WVTR of the bioplastic can improve its applicability as food packaging.

3.7. Soil Burial Test

The soil burial test investigates the visual degradation of the bioplastic samples in soil. This test was carried out in our study to determine whether cutin had an effect on bioplastic degradation.

The results of the soil burial test are presented in Figure 7. Initially, the bioplastic samples gained weight as they absorbed moisture from the soil. However, as the experiment progressed, the samples’ mass decreased, indicating the degradation of the bioplastic samples. The test duration was one month, at which stage the samples were unrecoverable from the soil.

Over the initial 72–96 h, the absorption of water occurred at different rates. In addition, the application of the cutin surface treatment did not significantly impact the overall time of bioplastic biodegradation. All of the bioplastics, with or without cutin coating, started to degrade after 96 h (day four) and weight loss (relative to original weight) occurred between days twenty-two and twenty-seven. There was no statistically significant difference between all treatments, and on the first day, a decrease in weight was noted for each treatment; the p value = 0.88 at a 95% confidence level. Moreover, all bioplastics were physically too small to retrieve after thirty days or less of burial. There was no statistically significant difference in weight loss between any of the treatments after 31 days of the test; the p value = 0.21 at a 95% confidence level. This degradation time is very promising for home composting bioplastics, which are expected to be visually degraded within ninety days (Neves et al., 2020) [25].

The starch and gelatine, as naturally occurring materials, are easily degraded by microorganisms found in soil. Starch is rapidly degraded by the hydrolysis of the acetal bonds. This hydrolysis is catalysed by enzymes produced by the microorganisms [18]. Similarly, gelatine can be degraded by a wide range of microorganisms. The microorganisms produce proteases that catalyse the hydrolysis of peptide bonds in the gelatine [32].

3.8. Fungal Degradation

The aim of testing the fungal degradation of bioplastics is to understand how effectively fungi can break down and decompose bioplastics. This type of testing is carried out to assess the biodegradability and environmental impact of bioplastics.

The results of the fungal degradation test can be seen in Figure 8. A. niger growth on the bioplastics became visible after 48–72 h. After 48 h, the non-coated control bioplastic had the most fungal growth, covering up to 100% of the bioplastic surface. No growth was visible on the positive control (filter paper) and negative controls without the sample or on the polystyrene plastic sample. After 48 h, fungal growth was visible on all bioplastic coated with cutin. The dipping technique experienced the least coverage after 48 h. This changed after 72 h; at this stage, rolling technique experienced the least coverage by fungi. After 120 h, all bioplastic samples were fully covered by fungal growth. The controls and PS plastic did not show any fungal growth after 120 h. After 192 h (9 days), some growth started to be visible on the positive control while no growth was visible on the negative control or PS plastic. Both the coated and uncoated bioplastic samples showed no resistance to A. niger; the potato starch and gelatine in the bioplastics provided a rich source of carbon for the fungi, which could rapidly grow on the bioplastics. Cutin-coated bioplastic samples resisted the fungal attack for a little longer, but in the end, they were degraded by the fungi. The polystyrene plastic had high resistant properties against A. niger as there was no fungal growth; this was because the fungi could not degrade this plastic. Overall, the test demonstrated that the bioplastics generated in this study with or without cutin coating were rapidly biodegraded by fungi. The cutin coating does not offer any additional antifungal properties for these bioplastics. It also does not affect the degradation time of the bioplastics by fungi. Therefore, the bioplastics will easily be composted in home composting facilities and will not require industrial treatments to be degraded. In the literature, other bioplastics were also reported to be highly biodegradable by A. niger.

A study by Masitoh et al. (2019) [33] investigated a cassava starch and carbon nanotube bioplastic. This study reported that more than 60% of the surface of the bioplastic was covered by A. niger within 7 days. This heavy growth was an indication that the bioplastic was biodegradable according to ASTM G21-70 [33].

Similar results were observed in a study by Nissa et al. (2019) [18], in which cassava starch bioplastic was used; by day ten, 80% of the bioplastic’s surface was covered with A. niger [18]. Furthermore, these results were in line with published data on the ability of A. niger to utilise bioplastics as a carbon source [34,35]. In a review article, A. niger was reported to have the ability to degrade polyethene plastics such as linear low-density polyethylene torque blended using starch, high-density polyethylene films, plastic cups, and polythene bags [35].

In a study where hydroxypropyl starch bioplastic was infused with Calophyllum inophyllum extract to improve antimicrobial properties, this bioplastic also did not exhibit antifungal properties towards A. niger as a clear zone of inhibition was not detected [36].

4. Conclusions

Cutin, as the basis of a hydrophobic thin-layer surface treatment, has the potential to substantially improve the water barrier properties of bioplastic and possibly other mesoporous materials. FTIR has provided enough data to identify the extracted sample but more in-depth analysis using GC-MS or NMR should be carried out in further studies to characterise the cutin further. In this study, FTIR was redeemed sufficient as the application of the cutin and not characterisation was the main focus of this study. It was demonstrated that cutin can decrease the water swelling and water vapour transmission rates of the selected starch–gelatine-blend bioplastics. These properties are indicators of materials’ hydrophobicity and although water contact angle was not improved, according to the water vapour transmission rate and water swelling, the hydrophobicity improved to a certain extent. The water contact angle can be influenced by surface roughness, camouflaging the improvement in the hydrophobicity of a material. There were no statistically significant differences in the performance of the coated bioplastics regardless of the coating method used. The cutin treatment also delivered substantial improvements in water ingress resistance properties without compromising the biodegradability of the biomass-based materials, which is an important aspect for biodegradable or compostable materials. With improved hydrophobicity, a bioplastic is more feasible in applications such as food packaging when it would directly come in contact with moisture from the foodstuff. Finally, by repurposing the selected tomato waste streams and extracting cutin from the discarded tomato skin, the process will address the sustainability and circular economy objectives of the EU’s circular economy action plan and the UN’s Sustainable Development Goal 12. However, more studies need to be conducted on improving the coating deposition process, which should focus on improving the deposition-thickness controls, homogenising the final colour, and addressing industrial scalability requirements. Studies investigating the mechanical properties of cutin-coated bioplastic should also be carried out in future studies to assure the coating does not negatively impact these.