Development of biodegradable starch-based foams incorporated with grape stalks for food packaging

Carbohydrate Polymers 225 (2019) 115234

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Development of biodegradable starch-based foams incorporated with grape stalks for food packaging

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Juliana B. Engela, , Alan Ambrosib, Isabel C. Tessaroa a

Laboratory of Membrane Separation Processes (LASEM) and Laboratory of Packaging Technology and Membrane Development (LATEM) - Department of Chemical Engineering, Universidade Federal do Rio Grande do Sul (UFRGS), Ramiro Barcelos Street, 2777, ZC: 90035-007 Porto Alegre, RS, Brazil b Laboratory of Membrane Technology (LABSEM) - Department of Chemical Engineering and Food Engineering (EQA), Universidade Federal de Santa Catarina (UFSC), Florianópolis, SC, Brazil

A R T I C LE I N FO

A B S T R A C T

Keywords: Cassava starch Grape stalks Foam Biodegradability Applicability test

Biodegradable cassava starch-based foams incorporated with grape stalks were obtained by thermal expansion. The morphology (SEM), chemical structure (FTIR), crystallinity (XRD), and biodegradability of the foams were evaluated. An applicability test was performed in the storage of food. SEM images showed no residue agglomerations and cell structure generally observed in materials obtained by thermal expansion; FTIR analysis verified interactions of foam components. XRD analysis showed native cassava starch characteristic peaks and the loss of crystallinity after the expansion process, with the formation of an amorphous material. Foams were completely biodegraded after 7 weeks, demonstrating that, for the experimental conditions used, the interactions between the starch and the grape stalks did not generate recalcitrant compounds or structural alterations that would impair foam degradation. Furthermore, the foams added with grape stalks presented good properties in the applicability test, showing a promising application in the storage of foods with low moisture content.

1. Introduction For more than half a century, the production of plastic materials has presented continuous growth, currently estimated to be more than 300 million tons per year (PlasticsEurope, 2015). Most of plastics are used for disposable applications, i.e., products that are discarded within a year or less of their purchase (North & Halden, 2013), increasing the critical pollution problem related to this kind of material. Although almost all thermoplastics are recyclable, the separation of the materials presents some limitations, since the process requires selection by resin type (Marsh & Bugusu, 2007). As packaging material, the expanded polystyrene (EPS) is one of the most used plastics due to its versatility and cell structure that provides low density, high impact resistance, and high thermal insulation. However, due to the environmental problems associated to the discard of this material (Bergel, da Luz, & Santana, 2017) and the long time for complete degradation when incorrectly disposed in nature (Henningsson, Hyde, Smith, & Campbell, 2004), consumers are gradually adhering to the idea of using biodegradable packaging (Bergel et al., 2017). An alternative that can reduce carbon footprint, pollution risks and greenhouse gas emissions caused by the use of conventional polymers (North & Halden, 2013) is the use of biopolymers from agro-

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industrial sources that are renewable, abundant and low cost (Davis & Song, 2006). Recent researches have shown that native cassava starch can be used to obtain foams (Chiarathanakrit, Riyajan, & Kaewtatip, 2018; Kaewtatip, Chiarathanakrit, & Riyajan, 2018; Machado, Benelli, & Tessaro, 2017; Sanhawong, Banhalee, Boonsang, & Kaewpirom, 2017) retaining its biodegradable character when converted to a thermoplastic material (Teixeira, 2007). Moreover, starch softens and expands into a foam product similar to EPS (Mariotti, Alamprese, Pagani, & Lucisano, 2006), and this process can be carried out in a molding machine similar to that utilized for EPS (Shey et al., 2007). However, several limitations make the use of this material unfeasible for certain applications, especially for food packaging, because of starch’s high affinity for water (Van Der Maarel, Van Der Veen, Uitdehaag, Leemhuis, & Dijkhuizen, 2002). In this context, residues from agro-industrial activities that are rich in lignocellulosic fibers can be added to the polymer matrix to improve starch foams properties (Machado et al., 2017; Mali, Debiagi, Grossmann, & Yamashita, 2010; Salgado, Schmidt, Ortiz, Mauri, & Laurindo, 2008; Vercelheze et al., 2013). These materials can improve some properties due to their composition, mainly based on cellulose, hemicellulose and lignin (Santos et al., 2012). Sesame cake (Machado et al., 2017), plantain flour and wood fiber

Corresponding author. E-mail addresses: [email protected] (J.B. Engel), [email protected] (I.C. Tessaro).

https://doi.org/10.1016/j.carbpol.2019.115234 Received 22 May 2019; Received in revised form 21 August 2019; Accepted 21 August 2019 Available online 22 August 2019 0144-8617/ © 2019 Elsevier Ltd. All rights reserved.

Carbohydrate Polymers 225 (2019) 115234

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(Vargas-Torres et al., 2017), sugarcane bagasse and asparagus peel fiber (Cruz-Tirado et al., 2017), malt bagasse (Mello & Mali, 2014), kraft fiber (Kaisangsri, Kerdchoechuen, & Laohakunjit, 2012), sunflower protein and cellulose fiber (Salgado et al., 2008), fish scale waste (Chiarathanakrit et al., 2018), plant proteins, kraft fiber, palm oil (Kaisangsri, Kerdchoechuen, & Laohakunjit, 2014), and cellulose nanofiber (Ghanbari, Tabarsa, Ashori, Shakeri, & Mashkour, 2018) are some examples of residues incorporated into the polymeric matrix to produce starch-based foams. Although some of these studies have identified that the foams produced have potential to be used as packing for low water content foods, none focus on the applicability test nor the biodegradability of the foams. A promising material to be added into starch-based foams is the grape stalk, the lignocellulosic skeleton obtained at the beginning of the fruit processing, in the destemming stage (Garcia-Perez, García-Alvarado, Carcel, & Mulet, 2010). The wine industry contributes substantially to the economy of Brazil and, considering the annual grape production, it is estimated that 37.5 million kg of grape stalks are wasted every year in the country. Although this material is not considered toxic, its high content of organic matter and the high seasonal production can contribute to potential problems of pollution (Spigno, Pizzorno, & De Faveri, 2008), which justifies the importance of its use in applications such as biodegradable packaging. The aim of this study was to evaluate the biodegradability and potential application of starch-based foams incorporated with Cabernet Sauvignon grape stalks for packaging foods with low moisture content, such as English cake. The morphology, chemical structure, and crystallinity of the foams were analyzed to support the results observed.

distilled water:1 g cassava starch and 3.19 g distilled water:1 g grape stalks, according to previous results obtained from the grape stalks water absorption capacity analysis) and added in the first homogenizing step. The resulting mixture was equally distributed in a Teflon coated metal mold (100 mm × 25 mm × 3 mm, length × width × thickness). The mold was closed with a Teflon coated metal lid and set in a heated hydraulic press (SL11/20E, Solab, Brazil). The thermal expansion conditions were set at 70 bar and 180 °C for 7 min. After the foam formation, the samples were stored for 7 days under controlled conditions (55% relative humidity, 25 °C) prior to characterization.

2. Materials and methods

2.2.5. Porosity The porosity of the samples was evaluated relating the bulk density (ρb), determined by the ratio of mass (g) to volume (cm3) of the samples, and the true density (ρt) using Eq. (2.1). The helium gas pycnometry technique (Accu Pyc II 1340, Micromeritics, USA) was used to determine the true density.

2.2.3. Viscosity of the starch pastes The viscosity of the cassava starch-based pastes with and without the incorporation of grape stalks applied to develop the foams was measured in duplicate using a viscometer (Fungilab, CQA Química) at 1 rpm. The results are expressed as the average ± standard deviation. 2.2.4. Morphology The cross-section and surface morphology of the foams incorporated with grape stalks were evaluated by Scanning Electron Microscopy (SEM) (JSM-6060, JEOL, Japan) with an acceleration voltage of 12 kV. The samples were dried at 40 °C for 24 h in an oven, fractured and placed on aluminum stubs with carbon double-sided tape for visualization. Control samples (with no addition of grape stalks and the same glycerol percentage) also had the cross-section morphology evaluated by SEM, with an acceleration voltage of 5 kV.

2.1. Materials Native cassava starch (Yoki, Brazil) containing 28.7 ± 0.4% amylose and 10.7 ± 0.1% moisture (previously determined), guar gum (Exodus Cientifica, Brazil) to avoid sedimentation of solids, magnesium stearate (Exodus Cientifica, Brazil) as releasing agent, glycerol (Dinâmica, Brazil) as plasticizer, and Cabernet Sauvignon grape stalks (11.19 ± 0.07% moisture, 7.3 ± 0.2% ash, 6.0 ± 0.1% protein, 0.60 ± 0.02% lipids, 23 ± 1% lignin, 14 ± 2% cellulose and 11.7 ± 0.1% hemicellulose, previously determined), kindly provided by Salton Winery (Brazil), were used to prepare the foams.

ρ Porosity (%) = ⎜⎛ 1 − b ⎟⎞ × 100 ρt ⎠ ⎝

(2.1)

2.2.6. Chemical structure In order to identify the interactions between the components used to prepare the foams, native cassava starch, grape stalks and the produced foams were analyzed by Fourier transform infrared spectroscopy (FTIR). Foam samples were grinded with a mortar and pestle, dried in an oven at 50 °C for 24 h, and stored in a desiccator containing calcium chloride (30% relative humidity), for 7 days prior to the analysis. The foam, native cassava starch and grape stalk samples were placed directly into the sample holder and compressed. The test were performed using a spectrophotometer (Frontier FT-IR/NIR, Perkin Elmer, USA) in the frequency range of 4000–400 cm−1 and diamond selenide test point.

2.2. Methods 2.2.1. Grape stalks pretreatment Cabernet Sauvignon grape stalks were collected during the destemming stage of the fruit processing in the winery, stored in plastic bags, and kept under refrigeration for not more than 24 h until transportation to the laboratory. Then, the stalks were washed to remove dirt and other impurities, placed in trays and dried in an oven (De Leo, Brazil) at 40 °C for 24 h. After drying, stalks were milled with a knife grinder (MF10 basic, IKA, Germany) and sieved in an 80 Mesh sieve (Ø < 0.18 mm). The milled stalks were placed in bags and stored in a freezer at −18 °C. Before using the stalk, it was re-dried at 40 °C for 1 h to remove any residual moisture.

2.2.7. Crystallinity X-ray diffraction (XRD) analysis was conducted to verify crystallinity type of raw materials (native cassava starch and grape stalks) and their conversion to amorphous state after the thermal expansion (thermoplastic starch-based foam). A diffractometer (X'Pert MPD, Philips, the Netherlands) with Kα copper radiation (λ = 1.54184 Å), 40 kV voltage and 30 mA current was used. Assays were performed for 2θ between 5 and 75° with 0.05°/s ramping.

2.2.2. Cassava starch-based foams preparation Based on the total mass, Cabernet Sauvignon grape stalks (7 wt%, Ø < 0.18 mm), guar gum (0.4 wt%), magnesium stearate (0.4 wt%) and distilled water (55 wt%) were mixed for 10 min with a mechanic stirrer (713, Fisatom, Brazil). Then, cassava starch (32%wt) and glycerol (5%wt) were added to the mixture and homogenized for 10 min. The amounts of glycerol, grape stalks and the granulometry of the residue added were determined by optimization of a central composite experimental design developed in previous studies (data not shown). The amount of water incorporated into the mixture was fixed (1 g

2.2.8. Biodegradability test The biodegradability of the foams was analyzed with a modified qualitative test according to the methodology proposed by MedinaJaramillo, Ochoa-Yepes, Bernal, and Famá (2017) and PiñerosHernandez, Medina-Jaramillo, López-Córdoba, and Goyanes (2017). 2

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Vegetable compost (soil) was poured into glass containers, and foams, prepared with native cassava starch and grape stalks, and those only with native cassava starch, were completely buried in the soil. The containers were kept under aerobic conditions at room temperature, and water was sprayed once a day in the soil to ensure moisture of the system throughout the experiment. Samples of each formulation were removed every 7 days and photographed; the degradation was monitored by visual inspection only. 2.2.9. Applicability test Foams were tested in the storage of English cake (19.9 ± 1.7% moisture) purchased in a local market. Moisture content, mass lost and mechanical properties of thermoplastic cassava starch based-foams and foams incorporated with grape stalks were evaluated. EPS trays were also tested for comparison. The foams (10 cm × 8 cm) were prepared and stored for 7 days in a climatic chamber (55% RH, 25 °C). The cake samples were weighed (M214Ai, Bel Engineering, Italy) and placed on the foams. Then the system (food + foam) was wrapped with PVC film. The samples were kept at room temperature in order to reproduce sale conditions of this product in commercial establishments. Temperature and relative humidity of the environment were monitored daily. The test lasted for 9 days (cake’s shelf life) and the analyses of system mass loss, food moisture, packaging moisture, and mechanical properties (flexural tests) were performed at the 3rd, 6th and 9th days of the experiment. The mass loss analysis, performed in duplicate, was determined in a scale (M214Ai, Bel Engineering, Italy) and the results are presented as mean ± standard deviation of the mass loss percentage for each time of analysis. The moisture contents of the English cake, foams, and EPS were determined in duplicate by thermogravimetric method. The samples were weighed and placed on aluminum capsules, then submitted to oven drying at 105 °C for 24 h. After, the samples were cooled in a desiccator and their masses measured. The moisture content was determined by Eq. (2.2), where mi and mf are the initial and final sample mass in grams, respectively. The results are presented as the mean ± standard deviation.

Moisture content (%) =

mi − mf × 100 mi

(2.2)

The flexural tests of the foams and the EPS at each analysis time were performed according to ASTM D 790-03 (ASTM, 2003) using a texture analyzer (TA.XT2i, Stable Micro Systems, United Kingdom) with a 50 N load cell and a three-point bending method with span setting of 4.5 cm. Foams (100 mm × 25 mm) were deformed until break. The stress at break, strain at break and modulus of elasticity were calculated with the data obtained. Six samples of each packaging type were evaluated and the results are expressed as the mean of the measurements. 2.2.10. Statistical analysis Bulk density, true density, porosity, moisture content, mass loss and mechanical properties, before and after the applicability test, were evaluated by Tukey’s mean comparison test (p ≤ 0.05). These analyses were performed using Statistica® v10 software (Statsoft Inc., US).

Fig. 1. Photography and SEM images of the foams. (a) Photography of foam surface; SEM of the (b) surface and (c) cross section of thermoplastic cassava starch-based foam incorporated with Cabernet Sauvignon grape stalks. (d) SEM of the cross section of thermoplastic cassava starch-based foam (magnifications: 25×).

3. Results and discussion 3.1. Morphology

stalks. It can be observed that while the foam presents dense and homogeneous external walls, with small closed cell structure, the interior shows a structure with large open cells, a characteristic sandwichtype structure of thermoplastic starch-based materials obtained by thermal expansion (Soykeabkaew, Thanomsilp, & Suwantong, 2015). This structure is formed because of the water content present in the polymeric matrix that can significantly affect the foaming process. The outer layer of the foams is denser because the polymer matrix dries more rapidly and therefore cannot expand much close to the hot mold.

Fig. 1 shows the surface and cross section morphology of the thermoplastic cassava starch-based foam with (a, b and c) and without (d) grape stalks. It is interesting to notice the homogeneity of the foam surface (Fig. 1a and b) once it has not been detected particle agglomeration. This indicates a good dispersion of grape stalks in the polymer matrix, which was induced by the small granulometry of the residue used (Ø < 0.18 mm). Fig. 1c shows the cross-section of the foam incorporated with grape 3

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3.2. Chemical structure

Water becomes gaseous during the preparation of starch foams and creates bubbles when in temperatures higher than the boiling point, enabling the expansion of the matrix and the formation of the foam (Kaisangsri et al., 2014). The interior of the foam contains mainly larger cells and more open structure due to the large amount of water expelled to the outside of the mold in the form of vapor, causing the rupture of the cells (Shogren, Lawton, Doane, & Tiefenbacher, 1998). Similar cell structures have also been reported by other authors (Machado et al., 2017; Matsuda, Verceheze, Carvalho, Yamashita, & Mali, 2013; Shogren, Lawton, & Tiefenbacher, 2002; Vercelheze et al., 2012). Comparing the structures of foams prepared with and without grape stalks, Fig. 1c and d respectively, it can be inferred that the larger voids present in the former are due to the extra amount of water incorporated into that formulation. The big cells formed in the interior of the foam became so large and absorbed the smaller cells (Rizvi, Park, & Guo, 2008), thus decreasing the amount of cells present in the foam and causing the decrease in cell density (Bergel, Dias Osorio, da Luz, & Santana, 2018). With higher water content in the batter, more steam is produced, leading to a greater number and size of voids inside the foam structure that affects the density (Andersen & Hodson, 1998). The density of the foams is inversely proportional to the expansion ability of the paste (Meng et al., 2019). The results observed in this study corroborate that stated by Andersen and Hodson (1998) in their study about molding of starch items, once the foam prepared with cassava starch and grape stalks presented lower density (0.18 g cm−3) due to its high content of grape stalks (7 wt%) and high water content, attributed to the high water absorption capacity (WAC) presented by the stalks (3.19 g of water per gram of dry sample). Samples prepared only with cassava starch were denser (0.21 g cm−3) and presented smaller voids in the interior structure (Fig. 1d) probably due to the lower amount of water present in the formulation (1 g distilled water:1 g cassava starch). Furthermore, the viscosity of the polymeric matrix can affect the foaming process and therefore, the morphology and the density of the foams. Less viscous pastes cannot hold vapor bubbles as effectively as more viscous pastes. Consequently, the lower the viscosity of the paste, the greater the paste expansion, which generates foams with a thinner outer layer and large inner cells (Pornsuksomboon, Holló, Szécsényi, & Kaewtatip, 2016). The analysis of viscosity of the batters showed that the matrix incorporated with grape stalks, and therefore with higher water content, has a lower viscosity than the pure cassava starch matrix, 53,345 ± 1120 cP and 60,780 ± 1894 cP, respectively. These results support the morphology of the foams (foams with grape stalks presenting a thinner outer layer and inner layer with larger cells) and the lower density of the foams added with grape stalks. The morphology of the foam can also influence its gas permeability, which is greatly affected by its porosity (Ishizaki, Komarneni, & Nanko, 2013). The porosity of the foams and the commercial EPS trays are presented in Table 1. It is possible to observe a significant difference between the porosity of samples; the foam incorporated with grape stalks has higher porosity (87%), followed by the cassava starch foam (84%) and the EPS trays (60%). These results agree with the SEM images (Fig. 1c and d) because of the more open voids.

Fig. 2 shows the FTIR spectra of the foam and the main raw material (native cassava starch and grape stalks) samples. The three spectra present a peak from 3650 to 3000 cm−1, which can be attributed to: (i) the presence of hydroxyl groups (eOH) from alcohols, phenols and carboxylic acids in the grape stalks (Prozil, Mendes, Evtuguin, & Lopes, 2013); (ii) the presence of acetal (CeOeC) and hydroxyl (eOH) groups from the constituent molecules of starch in the cassava starch (Avérous, 2004); and (iii) the occurrence of hydrogen bond-like interactions between the components of the expanded structure during processing of the foam (Marengo, Vercelheze, & Mali, 2013), that may have occurred due to stretching of vibrational complexes associated with free and bound hydroxyl groups (Vercelheze et al., 2012). The peaks observed in the range of 2900 cm−1 correspond to the CeH stretch (Matsuda et al., 2013) and appear in the three spectra, with a higher intensity in the foam spectrum. The peak present at 1602 cm−1 in the grape stalks spectrum may be due to the elongation of C]C bonds and can be attributed to aromatic compounds, possibly lignin or tannins (Farinella, Matos, & Arruda, 2007; Fiol, Escudero, & Villaescusa, 2008). The Cabernet Sauvignon grape stalks spectrum is very similar to that of the grape marc extract (Garrido et al., 2019) and shows mainly the peaks indicating the presence of flavonoids and phenolic compounds, important components present in the grapes and its residues. The low intensity peaks present at 1647 cm−1 and 1618 cm−1 in the native cassava starch and foam spectra, respectively, are associated with the angular bending of the −OH group in water molecules (Mano, 2000), indicating the formation of interactions between water and components of the formulation (Marengo et al., 2013). The most intense peaks present from 1200 to 900 cm−1 are attributed to vibrations in CeOeC bonds, characteristic of starch and other polysaccharides (Wokadala, Emmambux, & Ray, 2014). The higher intensity of this peak in the foam spectrum is an evidence of the occurrence of interactions between the components of the formulation. Overall, the peaks observed in the starch-based foam developed in this study are similar to those reported by other authors (Bergel et al., 2018), and the major starch-related peaks are those found between 3650 to 3000 cm−1 and between 1200 to 900 cm−1, associated with the three hydroxyl groups and the one CeOeC bond per repeating unit of starch (Bergel et al., 2018). 3.3. Crystallinity According to the water content and packaging configuration of the amylopectin double helices (Imberty, Buléon, Tran, & Pérez, 1991), starch may present three main crystallinity types (A, B and C), defined by intensity of X-ray diffraction lines (Cereda et al., 2002). Native cassava starch is generally classified in C-type, consisting of 90% of Atype and 10% of B-type (Schlemmer, 2007). According to the XRD patterns presented in Fig. 3, cassava starch exhibited relatively broad peaks at 2θ = 15; 17 and 22.7°. These peaks consist of a mixture of the A-type (peaks at 2θ = 17 and 22.7°) and the B-type crystallinities (peak at the 2θ = 15°) (Hoover, 2001). Similar results were observed by Machado et al. (2017), Mello and Mali (2014) and Marengo et al. (2013). The grape stalks XRD pattern exhibits a low intensity, but broad peak at 2θ = 21°, which may be related to cellulose residual crystallinity, one of the main components of grape stalks (Vercelheze et al., 2012). This peak was found in cellulose samples in the study conducted by Mulinari, Voorwald, Cioffi, da Silva, and Luz (2009). Due to the gelatinization process that occurs during thermal processing of starch to obtain foams (Marengo et al., 2013), the granular structure is totally or partially destroyed, resulting in an amorphous matrix (Van Soest & Vliegenthart, 1997). This amorphous pattern is evidenced by the diffractogram of the foam, in which the peaks previously present in the cassava starch and in the grape stalks diffractograms are no longer

Table 1 Bulk density, true density and porosity of the cassava starch-based foams and of the commercial EPS trays. Foam sample Starch + grape stalks Starch EPS

ρb (g cm−3)

ρt (g cm−3) b

0.18 ± 0.02 0.21 ± 0.02a 0.031 ± 0.003c

Porosity (%) a

1.447 ± 0.003 1.279 ± 0.002b 0.077 ± 0.002c

87 ± 1a 84 ± 1b 60 ± 5c

Different lowercase letters in the same column indicate significant difference (p < 0.05) between means (Tukey’s test). 4

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Fig. 2. FTIR spectra of native cassava starch, Cabernet Sauvignon grape stalks and thermoplastic starch-based foam incorporated with grape stalks.

starch. The effect of heat, as well as the enzymatic activity of the microorganisms present in the soil shorten and weaken the polymer chains of the starch, causing the degradation process to start (Cerruti et al., 2011). In addition, the moisture from the water that has been sprayed daily on the system may have reacted with the hydroxyl groups of the starch molecules, causing the chains to weaken and, therefore, accelerating the biodegradation process. Hydrogen bonds and molecular interactions between starch molecules were possibly destroyed (Jaramillo, Gutiérrez, Goyanes, Bernal, & Famá, 2016), leading to the macroscopically observable result of polymer degradation (Albertson, 2000). Biodegradability is also influenced by the morphology of starchbased foams. Xu, Dzenis, and Hanna (2005) reported that large cells in the structure of the foams increased accessibility to microorganisms attack, thus increasing the rate of degradation. Stoffel (2015) concluded that starch-based trays that had interiors with larger voids showed a higher surface area of enzyme-substrate contact, accelerating the enzymatic degradation of the material. Therefore, the SEM images presented in Fig. 1 can be taken into account in order to corroborate the good results observed in the biodegradability test. It was possible to observe that foams incorporated with grape stalks developed in this

observed. Because of the crystallinity loss, only a low intensity peak is present at 2θ = 20° in the foam diffractogram, similar to that found in the study conducted by Tavares, de Campos, Mitsuyuki, Luchesi, and Marconcini (2019) and attributed to non-significant residual A-type crystallinity.

3.4. Biodegradability test Fig. 4 depicts the thermoplastic cassava starch-based foams biodegradation evolution. In the specific case of starch, biodegradation occurs mainly due to the hydrolysis of the polymer chain, under enzymatic action, with consequent breakage of the α-1,4 bonds of the amylose and amylopectin chains (Oliveira, 2015). The samples showed integrity in shape and size up to the third week of analysis. It was possible to remove the samples easily from the soil and to handle them without causing any damage. In the fourth week, the sample with the incorporation of Cabernet Sauvignon grape stalks lightly adhered to the screen used to facilitate the removal from the soil, and showed cracks in its structure. In the fifth week, both samples strongly adhered to the screen. From this moment on, it was possible to note that the sample with residue incorporation showed faster degradation in comparison to the sample prepared only with cassava

Fig. 3. XRD patterns of native cassava starch, Cabernet Sauvignon grape stalks and thermoplastic starch-based foam incorporated with grape stalks. 5

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Fig. 4. Biodegradability test images of cassava starch-based foams, and cassava starch-based foams incorporated with grape stalks at different times of soil burial.

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samples reached similar levels of moisture, and no significant differences were observed when comparing the results obtained for the same day of analysis. This indicates that the incorporation of grape stalks does not prevent the increase of moisture during storage and that both the environment and the characteristics of the product exert a greater influence on this property. The loss of moisture observed for the cake could have been caused in part by the absorption by the packaging and partly by the lost to the environment through the PVC film, which has permeability to water vapor, as also reported by Machado (2016) and Stoffel (2015). This behavior can be associated to the high affinity that starch has with water; in a ambient with high relative humidity, starch absorbs water causing the material to collapse or disintegrate, losing mechanical strength (Shogren et al., 1998). As a consequence, starchbased foams still have limited use, being appropriate only to be applied as packaging for foods with low moisture content. Table 2 also presents the results for the system mass loss, calculated in relation to day 0. A gradual increase in percentage mass loss of the systems applied in the storage of cake is observed. Only the system composed by the thermoplastic cassava starch-based foam did not present a significant increase in mass loss during the test period. Systems composed of starch-based foams applied on the storage of strawberries in the study developed by Stoffel (2015) showed similar mass loss to that observed in the systems composed by the foams with incorporation of grape stalks applied in cake storage and higher to systems composed by foams prepared only with cassava starch. The flexural properties of the foams, evaluated over the 9 days of the applicability test, are presented in Table 2. Comparing the results obtained for the foams prepared only with cassava starch, it is possible to observe a significant decrease in stress at break between days 0 and 3. However, at the end of the experiment (day 9), the results resembled those obtained at day 0 and the samples stress at break was not significantly different. The strain at break of the thermoplastic cassava starch-based foams increased from day 0 to day 3 and no significant differences were observed between days 3, 6 and 9. The increase in flexibility, as evidenced by the increase in strain at break, was accompanied by a decrease in the stiffness of the samples, observed by the decrease of the modulus of elasticity, probably due to the reduction of internal hydrogen bonding between polymer chains and an increase in molecular space (Gontard, Guilbert, & Cuq, 1993). The values obtained for days 3, 6 and 9 showed no significant differences. The reduction of stress at break observed for the foams incorporated with grape stalks was accompanied by a significant increase in strain at break and, similar to the thermoplastic cassava starch-based foams, there were no significant differences for the strain at break at 3, 6 and 9 days of analysis. The reduction of mechanical strength and increased flexibility of the samples may have occurred due to the presence of the cake as well as to the foam formulation, which contained 5 wt% glycerol and high amounts of water, incorporated in the mixture because of the grape stalks water absorption capacity. Samples with and without the addition of grape stalks, as well as EPS samples, showed significantly similar strain at break at 3, 6 and 9 days, although on day 0 the EPS sample showed higher strain at break than the other samples. Similar to cassava starch-based foam samples, the foams added with grape stalks became less rigid throughout the experiment, as noticed by the significant decrease in the modulus of elasticity. Comparable results were observed by Machado (2016) in cake storage on cassava starch foams incorporated with residue from the sesame processing. Only EPS samples presented increased stiffness, although only significant differences were observed for the modulus of elasticity between days 0 and 9. The EPS stress at break did not change significantly during the analysis and the values were lower than those obtained for samples prepared with cassava starch with and without addition of grape stalks.

study had an interior with large cells and a more open structure (Fig. 1c), morphology that may have influenced the more accelerated biodegradation of these samples. Moreover, the predominance of the amorphous pattern of the foam structure, evidenced by the diffractogram presented in Fig. 3, also contributed to the results observed in the biodegradability analysis, since the degradation is initiated in the amorphous phase of the polymer (Amass, Amass, & Tighe, 1998), once this phase is more susceptible to biodegrade than the crystalline region (Abraham et al., 2012). Sanhawong et al. (2017) evaluated the biodegradation of cassava starch-based foams incorporated with cotton fiber and natural rubber latex in a similar manner to the present study, and observed that samples were completely degraded in 8 weeks, a similar result to that found for thermoplastic cassava starch-based foams incorporated with the grape stalks. There was a pronounced change in the thermoplastic cassava starchbased foams color due to contact with the soil. It was not possible to monitor sample mass loss during the biodegradation test, because foams presented soil adhered to the surface, which configures a limitation of this analysis. Even so, it was possible to obtain good results and, within 7 weeks, samples were totally degraded, as it can be seen in Fig. 4. Thus, it can be concluded that foams prepared in this study can be disposed in gardens and flowerbeds, which characterizes a solution that in addition to helping reduce environmental problems, such as pollution from plastic materials, can contribute to the reduction of costs with waste processing, as was also reported by Sanhawong et al. (2017). 3.5. Applicability test The visual aspect of the foams applied on the storage of English cake during 9 days is shown in Fig. 5. During the whole time of the experiment, the relative humidity and average temperature of the environment where the samples were maintained were 55% and 23 °C, respectively. Both the cake and the packages samples showed no development of microorganisms. However, thermoplastic cassava starchbased foams with and without the addition of grape stalks presented visible deformations at the end of the test. The deformations were located mainly in the regions where the cake was contacting the surface of the foam. EPS packaging remained intact throughout the testing period. As shown in Table 2, when the foams faced the cake packaging test (Packaging + cake column) a significant increase in the moisture content of the system foam + cake was observed between the first and 3rd days of analysis, and the highest value was obtained for the sample with cassava starch and grape stalks at 3 days of experiment (sample CS + GSD3, Table 2). In 9 days, the foams prepared with cassava starch, both with and without the addition of grape stalks (samples CS + GSD9 and CSD9), presented no significant differences in the moisture content. The EPS samples had the lowest moisture content (< 2.4%). The cake moisture content decreased significantly over the analysis and this behavior was observed in samples from all the tested packages. At the end of the 9th day, cake samples stored in the different packaging types showed no significant difference in moisture content (samples CSD9, CS + GSD9 and EPSD9). Starch-based coated and uncoated polylactic acid starch trays developed by Stoffel (2015) used in the storage of strawberries had moisture contents higher than those observed in this work. However, it is noteworthy that the strawberries have higher moisture content than the English cake. The tests conducted on the packages without English cake, also wrapped in PVC film in order to reproduce the same experiment conditions, showed that the moisture content of thermoplastic cassava starch-based foams with and without the addition of grape stalks increased throughout the evaluated period, reaching 17%, higher than those observed when the packages contained cake. Although they presented significantly different moisture contents at day 0, these 7

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Fig. 5. Cassava starch, cassava starch with grape stalks and EPS foams applied on the storage of English cake during 9 days.

Table 2 Moisture content, system mass loss and flexural properties of cassava starch, cassava starch and grape stalks and EPS foams applied on the storage of English cake. Moisture content (%)

System mass loss (%)

Flexural properties

Sample

Packaging + cake

Cake

Packaging

Packaging + cake

Stress at break (MPa)

Strain at break (%)

CSD0 CSD3 CSD6 CSD9 CS + GSD0 CS + GSD3 CS + GSD6 CS + GSD9 EPSD0 EPSD3 EPSD6 EPSD9

2.8 ± 0.2e 11.75 ± 0.04b 11.20 ± 0.04b 10.26 ± 0.07c 8.5 ± 0.1d 12.8 ± 0.2a 11.3 ± 0.2b 10.3 ± 0.1c 0.2 ± 0.2g 2.4 ± 0.5e 0.7 ± 0.1f,g 1.1 ± 0.3f

20 ± 2ª 10.0 ± 0.4b,c,d 9.2 ± 0.1b,c,d 7.97 ± 0.09d 20 ± 2ª 12.4 ± 0.2b,c 10.1 ± 0.1b,c,d 8.3 ± 0.1c,d 20 ± 2ª 12.9 ± 0.2b 9.8 ± 0.3b,c,d 8.1 ± 0.1d

2.8 ± 0.2f,g 10.8 ± 0.2b,c,d 13.4 ± 0.1ª,b 17.1 ± 0.7a 8.5 ± 0.1c,d,e 11.1 ± 0.2b,c,d 13.79 ± 0.08ª,b 17.05 ± 0.05a 0.2 ± 0.2g 12 ± 5ª,b,c 6 ± 2d,e,f 4.8 ± 0.8e,f,g

– 5 ± 1e,f 6 ± 1d,e 6.8 ± 0.6c,d,e – 3.9 ± 0.3f 6.3 ± 0.7d,e 8.1 ± 0.8c – 7.3 ± 0.3c,d 10.3 ± 0.3b 12.0 ± 0.2a

2.9 ± 0.6ª 1.3 ± 0.1e,f,g 2.1 ± 0.3b,c,d 2.6 ± 0.4ª,b 2.5 ± 0.4ª,b,c,d 1.5 ± 0.3e,f,g 1.6 ± 0.3d,e,f 1.7 ± 0.5c,d,e 0.57 ± 0.06h 1.00 ± 0.04f,g,h 0.87 ± 0.08g,h 0.96 ± 0.09f,g,h

1.6 3.9 3.5 3.7 1.6 3.8 3.2 3.3 6.4 3.9 3.4 3.6

± ± ± ± ± ± ± ± ± ± ± ±

0.3c 0.4b 0.7b 0.5b 0.2c 0.7b 0.6b 0.5b 0.8ª 0.3b 0.6b 0.9b

Modulus of elasticity (MPa) 202 ± 20ª 57 ± 8c,d 68 ± 19c,d 81 ± 12c 150 ± 15b 63 ± 20c,d 68 ± 20c,d 43 ± 13d,e 23 ± 2e 48 ± 5d,e 45 ± 10d,e 51 ± 13d

Different lowercase letters in the same column indicate significant difference (p < 0.05) between means (Tukey’s test). CS – Cassava Starch foam; CS + GS – Cassava Starch and Grape Stalks foam; EPS – Expanded Polystyrene foam, followed by the day of analysis. 8

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4. Conclusions

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Thermoplastic cassava starch-based foams added with Cabernet Sauvignon grape stalks were successfully developed by thermal expansion. Although further research is needed in order to improve foam’s properties, especially regarding moisture resistance, it was possible to observed that these structures are suitable for the packaging of foods with low moisture content, especially as an alternative for the traditional EPS for short-term or single-use applications, because flexural mechanical properties, at the end of the analysis, were similar to those observed for foams developed with the petroleum based polymer. Regarding biodegradability, the open cellular structure could have facilitated the microorganisms attack and made the biodegradation faster. Besides, the amorphous pattern of the foams may also have contributed to the rapid biodegradation, which is initialized in the amorphous phase of the polymer. Acknowledgements The authors thank Salton winery (Brazil) for the Cabernet Sauvignon grape stalks donation, the Thermodynamics and Supercritical Technology Laboratory (LATESC) from Federal University of Santa Catarina for the support on the porosity analysis, and the financial support received from CAPES (Coordination for the Improvement of Higher Level Personnel, Brazil), CNPq (National Council for Scientific and Technological Development, Brazil) and FAPERGS (Research Support Foundation of the State of Rio Grande do Sul, Brazil). In particular, thanks to the Programa Ciência sem Fronteiras and CAPES CSF-PVE’s Project, process number: 88881.068177/2014-01. References Abraham, E., Elbi, P. A., Deepa, B., Jyotishkumar, P., Pothen, L. A., Narine, S. S., & Thomas, S. (2012). X-ray diffraction and biodegradation analysis of green composites of natural rubber/nanocellulose. Polymer Degradation and Stability, 97(11), 2378–2387. https://doi.org/10.1016/j.polymdegradstab.2012.07.028. Albertson, A. (2000). Biodegradation of polymers in historical perspective versus modern polymer chemistry. In S. H. Hamid (Ed.). Handbook of polymer degradation (pp. 421– 439). (2nd ed.). New York: Marcel Dekker Inc. Amass, W., Amass, A., & Tighe, B. (1998). A review of biodegradable polymers: Uses, current developments in the synthesis and characterization of biodegradable polyesters, blends of biodegradable polymers and recent advances in biodegradation studies. Polymer International, 47(2), 89–144. https://doi.org/10.1002/(SICI)10970126(1998100)47:2<89::AID-PI86>3.0.CO;2-F. Andersen, P. J., & Hodson, S. K. (1998). Systems for molding articles which include a hinged starch bound cellular matrix. US Patent 5.705.203. ASTM (2003). American society for testing and materials. D790-03-Standard test method for flexural properties of unreinforced and reinforced plastics and electrical insulation materials (D 790 - 03). Retrieved from1–11. http://scholar.google.com/scholar?hl=en& btnG=Search&q=intitle:Standard+Test+Methods+for+Flexural+Properties+of +Unreinforced+and+Reinforced+Plastics+and+Electrical+Insulating +Materials#0. Avérous, L. (2004). Biodegradable multiphase systems based on plasticized starch: A review. Journal of Macromolecular Science, C44(3), 231–274. https://doi.org/10. 1081/MC-200029326. Bergel, B. F., da Luz, L. M., & Santana, R. M. C. (2017). Comparative study of the influence of chitosan as coating of thermoplastic starch foam from potato, cassava and corn starch. Progress in Organic Coatings, 106, 27–32. https://doi.org/10.1016/j.porgcoat. 2017.02.010. Bergel, B. F., Dias Osorio, S., da Luz, L. M., & Santana, R. M. C. (2018). Effects of hydrophobized starches on thermoplastic starch foams made from potato starch. Carbohydrate Polymers, 200(June), 106–114. https://doi.org/10.1016/j.carbpol. 2018.07.047. Cereda, M. P., Franco, C. M., Daiuto, E. R., Demiate, I. M., Carvalho, L. J. C. B., Leonel, M., ... Sarmento, S. B. (2002). Propriedades gerais do amido. Série Culturas de Tubérculos Amiláceas Latinoamericanas. V.I. São Paulo: Fundação Cargill 204p. Cerruti, P., Santagata, G., Gomez D’Ayala, G., Ambrogi, V., Carfagna, C., Malinconico, M., & Persico, P. (2011). Effect of a natural polyphenolic extract on the properties of a biodegradable starch-based polymer. Polymer Degradation and Stability, 96(5), 839–846. https://doi.org/10.1016/j.polymdegradstab.2011.02.003. Chiarathanakrit, C., Riyajan, S. A., & Kaewtatip, K. (2018). Transforming fish scale waste into an efficient filler for starch foam. Carbohydrate Polymers, 188(February), 48–53. https://doi.org/10.1016/j.carbpol.2018.01.101. Cruz-Tirado, J. P., Siche, R., Cabanillas, A., Díaz-Sánchez, L., Vejarano, R., & Tapia-

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