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

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.

Contents lists available atScienceDirect

Carbohydrate Polymers journal homepage:www.elsevier.com/locate/carbpol

Development of biodegradable starch-based foams incorporated with grape

stalks for food packaging

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 L E I N F O

Keywords:

Cassava starch

Grape stalks

Foam

Biodegradability

Applicability test

A B S T R A C T 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 ag-glomerations 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

gradu-ally 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-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 thermo-plastic 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 ma-chine 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 ac-tivities 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 mate-rials 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), plantainflour and wood fiber

https://doi.org/10.1016/j.carbpol.2019.115234

Received 22 May 2019; Received in revised form 21 August 2019; Accepted 21 August 2019

⁎Corresponding author

E-mail addresses:julianaengel@gmail.com(J.B Engel),isabel@enq.ufrgs.br(I.C Tessaro)

Available online 22 August 2019

0144-8617/ © 2019 Elsevier Ltd All rights reserved

T

(Vargas-Torres et al., 2017), sugarcane bagasse and asparagus peelfiber

(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

na-nofiber (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

ob-tained at the beginning of the fruit processing, in the destemming stage

(Garcia-Perez, García-Alvarado, Carcel, & Mulet, 2010) The wine

in-dustry contributes substantially to the economy of Brazil and,

con-sidering 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

po-tential 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

crys-tallinity of the foams were analyzed to support the results observed

2 Materials and methods

2.1 Materials

Native cassava starch (Yoki, Brazil) containing 28.7 ± 0.4%

amy-lose 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

2.2 Methods

2.2.1 Grape stalks pretreatment

Cabernet Sauvignon grape stalks were collected during the

des-temming stage of the fruit processing in the winery, stored in plastic

bags, and kept under refrigeration for not more than 24 h until

trans-portation 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

2.2.2 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

gly-cerol (5%wt) were added to the mixture and homogenized for 10 min

The amounts of glycerol, grape stalks and the granulometry of the

re-sidue 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

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 homo-genizing 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 condi-tions (55% relative humidity, 25 °C) prior to characterization

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 visuali-zation 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

2.2.5 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 sam-ples, and the true density (ρt) using Eq.(2.1) The helium gas pycno-metry technique (Accu Pyc II 1340, Micromeritics, USA) was used to determine the true density

= ⎛

⎝

⎠

× Porosity (%) 1 ρ

b

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 di-rectly 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

2.2.7 Crystallinity X-ray diffraction (XRD) analysis was conducted to verify crystal-linity 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

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

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

mon-itored 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 PVCfilm

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

, 6thand 9thdays 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

percen-tage for each time of analysis The moisture contents of the English

cake, foams, and EPS were determined in duplicate by

thermogravi-metric 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 miand mfare the

initial and final sample mass in grams, respectively The results are

presented as the mean ± standard deviation

Moisture content (%) m m

Theflexural 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)

3 Results and discussion

3.1 Morphology

Fig 1shows the surface and cross section morphology of the

ther-moplastic 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

agglom-eration 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

stalks It can be observed that while the foam presents dense and homogeneous external walls, with small closed cell structure, the in-terior shows a structure with large open cells, a characteristic sandwich-type 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 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×)

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

cor-roborate 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

ma-trix, 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 inTable 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

3.2 Chemical structure

Fig 2shows 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 be-tween 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−1correspond 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−1in 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 pre-sence offlavonoids 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−1are 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, asso-ciated 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 A-type and 10% of B-A-type (Schlemmer, 2007) According to the XRD patterns presented inFig 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 crystal-linity, one of the main components of grape stalks (Vercelheze et al.,

2012) This peak was found in cellulose samples in the study conducted

byMulinari, 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 dif-fractogram 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 ρ b (g cm−3) ρ t (g cm−3) Porosity (%)

Starch + grape stalks 0.18 ± 0.02 b 1.447 ± 0.003 a 87 ± 1 a

Starch 0.21 ± 0.02 a 1.279 ± 0.002 b 84 ± 1 b

EPS 0.031 ± 0.003 c 0.077 ± 0.002 c 60 ± 5 c

Different lowercase letters in the same column indicate significant difference

(p < 0.05) between means (Tukey’s test)

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 byTavares, de Campos, Mitsuyuki, Luchesi, and

Marconcini (2019) and attributed to non-significant residual A-type

crystallinity

3.4 Biodegradability test

Fig 4depicts the thermoplastic cassava starch-based foams

biode-gradation evolution In the specific case of starch, biodebiode-gradation

oc-curs mainly due to the hydrolysis of the polymer chain, under

enzy-matic 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

de-gradation in comparison to the sample prepared only with cassava

starch

The effect of heat, as well as the enzymatic activity of the micro-organisms 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, accel-erating 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 starch-based 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 en-zymatic degradation of the material Therefore, the SEM images pre-sented inFig 1can 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 Fig 2 FTIR spectra of native cassava starch, Cabernet Sauvignon grape stalks and thermoplastic starch-based foam incorporated with grape stalks

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

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.

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

sus-ceptible to biodegrade than the crystalline region (Abraham et al.,

2012).Sanhawong et al (2017)evaluated the biodegradation of

cas-sava 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

starch-based 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

lim-itation 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 andflowerbeds, 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

ex-periment, the relative humidity and average temperature of the

en-vironment where the samples were maintained were 55% and 23 °C,

respectively Both the cake and the packages samples showed no

de-velopment of microorganisms However, thermoplastic cassava

starch-based foams with and without the addition of grape stalks presented

visible deformations at the end of the test The deformations were

lo-cated mainly in the regions where the cake was contacting the surface

of the foam EPS packaging remained intact throughout the testing

period

As shown inTable 2, when the foams faced the cake packaging test

(Packaging + cake column) a significant increase in the moisture

con-tent of the system foam + cake was observed between thefirst 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 9thday, 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

ob-served 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 PVCfilm in order to reproduce the same experiment

con-ditions, showed that the moisture content of thermoplastic cassava

starch-based foams with and without the addition of grape stalks

in-creased 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

samples reached similar levels of moisture, and no significant differ-ences 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 PVCfilm, which has permeability to water vapor, as also reported byMachado (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, starch-based foams still have limited use, being appropriate only to be applied

as packaging for foods with low moisture content

Table 2also 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 Sys-tems composed of starch-based foams applied on the storage of straw-berries in the study developed byStoffel (2015)showed similar mass loss to that observed in the systems composed by the foams with in-corporation of grape stalks applied in cake storage and higher to sys-tems composed by foams prepared only with cassava starch

Theflexural properties of the foams, evaluated over the 9 days of the applicability test, are presented inTable 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

sig-nificantly 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 ac-companied 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% gly-cerol 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

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 andflexural 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 (%) Modulus of elasticity (MPa) CSD0 2.8 ± 0.2 e 20 ± 2ª 2.8 ± 0.2 f,g – 2.9 ± 0.6ª 1.6 ± 0.3 c 202 ± 20ª

CSD3 11.75 ± 0.04 b 10.0 ± 0.4 b,c,d 10.8 ± 0.2 b,c,d 5 ± 1 e,f 1.3 ± 0.1 e,f,g 3.9 ± 0.4 b 57 ± 8 c,d

CSD6 11.20 ± 0.04 b 9.2 ± 0.1 b,c,d 13.4 ± 0.1ª ,b 6 ± 1 d,e 2.1 ± 0.3 b,c,d 3.5 ± 0.7 b 68 ± 19 c,d

CSD9 10.26 ± 0.07 c 7.97 ± 0.09 d 17.1 ± 0.7 a 6.8 ± 0.6 c,d,e 2.6 ± 0.4ª ,b 3.7 ± 0.5 b 81 ± 12 c

CS + GSD0 8.5 ± 0.1 d 20 ± 2ª 8.5 ± 0.1 c,d,e – 2.5 ± 0.4ª ,b,c,d 1.6 ± 0.2 c 150 ± 15 b

CS + GSD3 12.8 ± 0.2 a 12.4 ± 0.2 b,c 11.1 ± 0.2 b,c,d 3.9 ± 0.3 f 1.5 ± 0.3 e,f,g 3.8 ± 0.7 b 63 ± 20 c,d

CS + GSD6 11.3 ± 0.2 b 10.1 ± 0.1 b,c,d 13.79 ± 0.08ª ,b 6.3 ± 0.7 d,e 1.6 ± 0.3 d,e,f 3.2 ± 0.6 b 68 ± 20 c,d

CS + GSD9 10.3 ± 0.1 c 8.3 ± 0.1 c,d 17.05 ± 0.05 a 8.1 ± 0.8 c 1.7 ± 0.5 c,d,e 3.3 ± 0.5 b 43 ± 13 d,e

EPSD0 0.2 ± 0.2 g 20 ± 2ª 0.2 ± 0.2 g – 0.57 ± 0.06 h 6.4 ± 0.8ª 23 ± 2 e

EPSD3 2.4 ± 0.5 e 12.9 ± 0.2 b 12 ± 5ª ,b,c 7.3 ± 0.3 c,d 1.00 ± 0.04 f,g,h 3.9 ± 0.3 b 48 ± 5 d,e

EPSD6 0.7 ± 0.1 f,g 9.8 ± 0.3 b,c,d 6 ± 2 d,e,f 10.3 ± 0.3 b 0.87 ± 0.08 g,h 3.4 ± 0.6 b 45 ± 10 d,e

EPSD9 1.1 ± 0.3 f 8.1 ± 0.1 d 4.8 ± 0.8 e,f,g 12.0 ± 0.2 a 0.96 ± 0.09 f,g,h 3.6 ± 0.9 b 51 ± 13 d

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

4 Conclusions

Thermoplastic cassava starch-based foams added with Cabernet

Sauvignon grape stalks were successfully developed by thermal

ex-pansion 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

tradi-tional EPS for short-term or single-use applications, becauseflexural

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

fa-cilitated 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

fi-nancial 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

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