Sustainable Active Food Packaging from Poly(lactic acid) and Cocoa

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Applications of Polymer, Composite, and Coating Materials

Sustainable Active Food Packaging from Poly(lactic acid) and Cocoa Bean Shells Evie L. Papadopoulou, Uttam Chandra Paul, Thi Nga Tran, Giulia Suarato, Luca Ceseracciu, Sergio Marras, Richard d'Arcy, and Athanassia Athanassiou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b09755 • Publication Date (Web): 02 Aug 2019 Downloaded from pubs.acs.org on August 4, 2019

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ACS Applied Materials & Interfaces

Sustainable Active Food Packaging from Poly(lactic acid) and Cocoa Bean Shells Evie L. Papadopoulou1,*, Uttam C. Paul1, Thi Nga Tran1, Giulia Suarato1,2, Luca Ceseracciu3, Sergio Marras3, Richard d’Arcy4 and Athanassia Athanassiou1,* 1

2 In

3

Smart Materials, Istituto Italiano di Tecnologia, via Morego 30, 16163 Genoa, Italy

vivo Pharmacology Facility, Istituto Italiano di Tecnologia, via Morego 30, 16163 Genoa, Italy

Materials Characterization Facility, Istituto Italiano di Tecnologia, via Morego 30, 16163 Genoa, Italy

4

Laboratory of Polymers and Biomaterials, Istituto Italiano di Tecnologia, via Morego 30, 16163 Genoa, Italy Abstract: Sustainable biocomposites have been developed by solvent mixing of poly(lactic acid) (PLA) with a fine powder of cocoa bean shells (CBS) and subsequent solution casting, using different concentrations of CBSs. The inclusion of CBSs recovers the crystallinity of the initially amorphous PLA films, and improves the physical properties of the composites. Young’s modulus increases by 80% with 75 wt% CBS inclusion, however, the composites maintain plasticity. The barrier properties of the hydrophobic composites were characterised, and the water vapour permeability is found to be ca. 3.5×10-5 g·m-1·day-1·Pa-1 and independent of the CBS content. On the other hand, oxygen permeability is found to depend on the CBS content, with values as low as 10000 mL·μm·m–2·day– 1·atm-1

for 50 wt% CBSs. Furthermore, CBSs confers antioxidant activity to the composites, and

improves swelling properties rendering the composites biodegradable in aquatic environments, reaching 70% of the maximum biodegradability in just 30 days. The above, in conjunction with the low level of migration measured in food simulant, make the PLA/CBS composites a highly promising material for active food packaging. ACS Paragon Plus Environment

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Keywords: PLA, cocoa bean, biodegradation, active food packaging, oxygen barrier, bioplastics, barrier properties

ACS Paragon Plus Environment

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1.


Introduction

Since the half of last century, plastics have become integral commodities in all aspects of our daily lives. The advantages of their physical properties, such as being light weight, made with low temperature manufacturing processes and with a wide range of optical, mechanical, wetting and barrier properties have allowed plastics to dominate applications ranging from agriculture and automobiles, to clothing and high tech industry. The largest application of plastics is packaging where they represent more than 39% of their total volume in EU1, and about half of which is related to food2. Today, plastic food packaging is seen as indispensable for the preservation of foodstuffs, providing several advantages, including extended food lifetime that results in reduced waste1. Common plastics used in food packaging include polyolefins, polyesters, polyamides, polyvinyl chloride etc. 3, all being petroleum derived materials. However, besides the positive impact of the use of plastic in food packaging, packaging represents the highest risk for environmental pollution 4. This problem can be addressed by the development and use of green and biodegradable polymeric materials that today represent less than 1% of the plastics market. One of the most widely used bioplastics in packaging, is poly(lactic acid) (PLA). PLA is an aliphatic polyester, derived from renewable sources, like corn and sugar beet, it is recyclable and biodegradable 5-7.

In addition, it is considered safe and non-toxic8. However, poor mechanical properties, when

compared to some conventional polymers, and low barrier properties restrict its widespread use. The strategy followed to address these limitations by mixing PLA with other polymers or reinforcing fillers. For example, blends with PCL or P(3HB) have been used to improve its mechanical properties9. In addition, talc and montmorillonite have also been used as reinforcing agents and barrier properties enhancers 10-11. In addition to synthetic fillers, natural substances are gaining a lot of interest as additives, especially for active food packaging12-14. Natural additives not only act as reinforcing agents, but they also improve barrier and wetting properties, as well as conferring antioxidant and antimicrobial properties, rendering the polymer matrix an active packaging material15-17. Antioxidant properties are especially ACS Paragon Plus Environment


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important in food packaging as it protects food from oxidation and spoilage, thereby extending shelflife18. Several natural additives have been investigated as agents for active food packaging, like green tea19, propolis20, soy protein21, ginger and grape seed extras22 or yerba mate23. One material that has emerged as a natural additive for polymeric matrices 24-25 is cocoa bean shells (CBS), i.e. the shell of the cocoa bean, also known as husk. CBS are a by-product of the roasting process during chocolate production26. They are rich in fibres, polyphenols, theobromine and other antioxidants26-27 and have been proposed to be used in food industry as a fibre enhancer28, food antioxidant29 or in food packaging30. Cocoa has a high phenolic content due to which is about 5 times more antioxidant than black tea, 3 times more than green tea and twice more than red wine31. The above characteristics render CBS an interesting material to investigate for food packaging applications. Here, we propose PLA/ CBS composites as an active food and biodegradable packaging material. We use a simple solution-casting method to fabricate the novel composite materials. The resulting composites are characterized in detail for their chemical and mechanical stability and for their possible application as food packaging material, as a function of CBS content. The components of the composites demonstrate a high affinity, manifested in FTIR, XRD and mechanical testing, where the reinforced stiffness is not accompanied by brittleness. In addition, the PLA/CBS composites are shown to have excellent barrier properties, and are hydrophobic, antioxidant and biocompatible. Finally, the PLA/CBS composites are shown to be highly biodegradable in aquatic environment.

2. 2.1

Materials and Methods Materials and Sample preparation

Polylactic acid (PLA 4043D) was acquired from Nature Works. Cocoa bean shells (CBS) were provided by Ferrero S.p.A. (Italy). CBS were mechanically ground to powder by two grinding steps, in order to reduce the powder to microsized granules. Initially, 10 g of the pristine shells were ground for 5 min by Moulinex AR1108 mill at 180 W. Subsequently, the powder was ground a second time


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for 5 min, by a WonderMιll Electric Grain Mill at 1250 W. Figure S1 (in SI) shows a SEM image of the final microsized powder, with average granule diameter ca. 10 μm and the corresponding particle size distribution. Chloroform was acquired from Sigma-Aldrich and used as received. PLA was first dissolved in chloroform at room temperature, to produce 10 wt% polymer in solution,. Once the polymer was dissolved, CBS were added at concentrations 0, 25, 50 and 75 wt% with respect to PLA and bath sonicated at 59 Hz for 7 h, resulting in a homogeneous dispersion of the CBS fillers in the polymer solution. Subsequently, solutions were solution-cast on a glass petri dishes and covered to allow slow evaporation under a fumehood, for at least 10 days. The resulting films were free standing and had a thickness of approximately 200 μm. All composites were prepared under identical ambient conditions.

2.2

Sample Characterization

Morphological: The morphology of the surface and the cross section of the films was studied by scanning electron microscopy (SEM, JEOL JSM-6490LA). Thermal Characterization: It was conducted by thermogravimetric analysis (TGA) in order to evaluate degradation temperature. Samples were heated from 30 °C to 800 °C at a heating rate of 10 °C/min under nitrogen atmosphere at a flow rate of 50 mL/min. Further thermal analysis was performed using a TA Instrument differential scanning calorimeter (Discovery DSC 250). Approximately 10 mg of each sample was weighed and sealed hermetically in aluminum pans, and heated from -20 to 180 °C, at a rate of 10 °C/min. An empty pan was used as a reference. Two heating scans were performed, and the first one was used to determine Tg and Tm of the samples. Chemical Characterization (FTIR): Infrared spectra were obtained with Attenuated Total Reflectance – Fourier Transform Infrared Spectrometer (FTIR, Equinox 70 FT-IR, Bruker; ATR, MIRacle ATR, PIKE Technologies) in the range from 4000 to 400 cm-1, using a diamond crystal, with a resolution of 4 cm-1, accumulating 64 scans.



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Nuclear magnetic resonance (1H NMR) was used to determine the chloroform residue in the samples, after solvent evaporation. Spectra were recorded on 15 mg/mL polymer solutions in deuterated dimethylsulfoxide (d6-DMSO) using a 400 MHz NMR spectrometer (Bruker). 15 mg of the PLA or PLA/CBS composites were mixed with 1 mL of d6-DMSO and pipetted vigorously until the PLA had fully dissolved. In the case of the PLA/CBS composites, a suspension of CBS particles was first formed that sedimented over 15 minutes. 1H NMR was immediately performed after sedimentation of the CBS in a sealed NMR tube. Results were analyzed using Mestrenova (Mestrelab) software. X-ray Diffraction: patterns were acquired on a 9kW Rigaku SmartLab system, with CuKα source operating at 40 kV and 150 mA in parallel beam mode and equipped with D\teX ultra linear detector. A custom-made holder, specifically dedicated to thin film analysis performed in transmission geometry, has been used. TXRD allows optimal management of samples with irregular and creped surface, by avoiding the presence of artifacts in the XRD pattern. In addition, the transmission geometry offers the advantage of keeping the volume of the sample constant, analyzed throughout the entire angular scan. Finally, the use of the linear detector with this configuration, guarantees an excellent collection of signal even in a short time. The PDXL 2.7.2.0 software from Rigaku was used to analyze the diffractograms. The reference structure 00-064-1624 from Cambridge Structural Database (CSD) was used to assign the peaks. XRD in reflective mode was used for the powders. Mechanical Properties: Mechanical properties were measured by uniaxial tension tests on an Instron 3365 dynamometer. Samples were punch-cut in dog-bone shape (length 25 mm, width 4 mm) and tested with 20 mm/min rate until failure. Average results were calculated from 5 repetitions for each sample. Water Contact Angle (WCA): Static contact angle measurements were performed by an automated tension meter (OCAH-200 DataPhysics Germany), using the sessile drop method. A gastight precision syringe with blunt needle (500 µL; inner diameter 0.52 mm) was used to gently place a droplet of 5 µL on the sample and the WCA was measured after 10 s. ACS Paragon Plus Environment

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Water uptake, Wu, was measured following the a previously established protocol, described in

32.

Briefly, the weight difference is calculated after immersing approximately 0.25 g of sample in nanopure water. When the weight of the wet sample stabilises, Wu is calculated by Equation (1): 𝑊𝑢 =

𝑊𝑤𝑒𝑡 ― 𝑊𝑑𝑟𝑦 𝑊𝑑𝑟𝑦

(1)

× 100

where Wwet and Wdry are the weight of the dry sample and the wetted one (g), respectively.

Water Vapour Permeability (WVP): The WVP was measured at 25 °C and under 100% relative humidity gradient (ΔRH%), according to the ATM E96 standard test, following the protocol described in Ref. 33. Briefly, 400 μL of deionised water were inserted in a permeation cell, with 7 mm inner diameter and 10 mm depth. Circular pieces of 7 mm diameter were mounted on the top of the permeation cells, which were then placed in a desiccator containing anhydrous silica beads, with 0% RH, allowing the water to permeate the cell in order to generate 100% RH. The change in weight of the permeation cell due to water evaporation was measured every hour, was plotted versus time and the slope was calculated by linear fitting. The water vapour transmission rate (WVTR) was calculated by Equation (2):

𝑊𝑉𝑇𝑅 (𝑔(𝑚2 ∙ 𝑑)

―1

𝑠𝑙𝑜𝑝𝑒

) = 𝑎𝑟𝑒𝑎 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑖𝑙𝑚

(2)

Then, the water vapour permeability (WVP) was calculated by Equation (3):

𝑊𝑉𝑃(𝑔(𝑚 ∙ 𝑑 ∙ 𝑃𝑎 ―1)) =

𝑊𝑉𝑇𝑅 × 𝑡 × 100 𝑝𝑠 × ∆𝑅𝐻

(3)

where t (m) is the thickness of the specimen, ΔRH is the relative humidity gradient and ps (Pa) is the saturation water vapour pressure at 25 °C (=3168 Pa).



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Oxygen Permeability: The oxygen permeation tests of the composites were performed using an Oxysense 5250i device (Oxysense, USA) equipped with a film permeation chamber. Rectangular pieces (6 cm × 6 cm) of the composites were placed inside the chamber and the permeating oxygen was measured at specific time intervals. The oxygen volumetric flow rate per unit area of the film and time unit (OTR, mL m−2 day−1) was continuously monitored until minimum coefficient of determination (R2) value of 0.995 was reached. The oxygen permeability (OP) of the films was then calculated by Equation (4): 𝑂𝑃 =

OTR × 𝐹𝑡

(4)

p

where OP is the oxygen permeability, OTR is the oxygen transmission rate (mL(m2·d)-1), Ft is the film thickness, and p is the oxygen partial pressure (1 atm). All tests were performed under ambient conditions following the ASTM Method F3136-15. Antioxidant Properties (DPPH•): The antioxidant characteristics (radical scavenging activity, RSA) of the PLA/CBS composites were evaluated by monitoring the decolourisation of a DPPH•/EtOH solution, when the composites are immersed in it, using a UV-vis-NIR spectrophotometer in doublebeam configuration. Radical starting concentration was 0.2 mM and pure EtOH was used as reference. Approximately 0.2 mg of each composite were immersed in 2.0 mL of the DPPH•/EtOH solution and kept in the dark allowing various reaction times. Subsequently, the samples were removed and the absorbance at 517 nm was measured, for the different reaction times (defined as A1). The reference (control measurements) absorbance value was measured with a mixture of 2.0 mL EtOH and 2.0 mL of 0.2 mM DPPH•/EtOH solution (defined as A2). Percentage of DPPH• free radical scavenging activity was calculated by the following formula:

𝐴1

𝑅𝑆𝐴% = [1 ― 𝐴2] × 100


(5)

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Migration Analysis: The overall migration (OM) of molecules from the PLA/CBS composites were performed using Tenax® as dry food simulants, following a previously published protocol34-35. The characterization of the overall migration was done following the EU Technical guidelines for compliance testing in the framework of the plastic FCM Regulation (EU) No 10/2011. Briefly, for each test sample a disc of 20.0 mm diameter was covered with 80 mg of Tenax® (40 mg on each side), placed in a clean glass petri dish, and heated to 70° C for 2 h. These conditions correspond to either food contact during heating up to 70° C for 2 h, or long-term storage at, or below, room temperature or heating up to 100° C for up to 15 min. The samples covered with the food stimulant were kept at 70±0.5° C for 2 h. Subsequently, the samples were removed, cooled down to room temperature and weighed. Blank samples with only 80 mg of Tenax® were used as control, and corrected migration values were calculated by subtracting the blank. The overall migration of the sample surface was calculated by: 𝑀=

(m𝑜 ― mf)X 1000 S

(6)

where, M is the overall migration into the dry food simulant (mg/dm2); mo is the original mass of the Tenax® (mg); mf is the mass of Tenax® after the migration test (mg); and S is the surface area of the test specimen intended to come into contact with foodstuff (dm2). An average of five measurements was calculated. Biocompatibility assay: The biocompatibility of all samples was assessed using human dermal fibroblasts adult cells (HDFa, Thermo Fisher Scientific) and following a previously established protocol presented in 36. Briefly, extraction medium from the pure PLA and the composite films were prepared according to the ISO10993-5:2009 standard test. The cell viability was determined after 24 h of treatment, via MTS assay (3-(4,5-dimethylthiazol-2yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt; CellTiter 96 AQueous



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One Solution Cell Proliferation Assay, Promega), by reading the optical density of the samples at 490 nm. Three independent experiments were performed, each with triplicates. Biodegradability: Biodegradability studies for the PLA film and its composites with CBSs were performed by the biochemical oxygen demand (BOD) test. This is an indirect and nonspecific test, that evaluates mineralization by measuring the amount of oxygen consumed during a degradation reaction37. Samples were finely minced and 200 mg from each sample were immersed in respirometers of 432 mL that contained seawater from the old harbour of Genoa. The solutions were placed in the dark and were stirred by magnetic stirrer for 30 days, mimicking the pelagic marine environment. Reactions that take place during biodegradation consume oxygen which is recorded at specific time intervals by the sealed OxiTop caps, places on each bottle. Pure seawater was used as reference. BOD tests were performed in ambience ambient temperature. 3.

Results and Discussion

3.1 Solvent evaporation

Figure 1: Annotated 1H NMR spectra of PLA and the PLA/CBS composites in d6-DMSO. Numbers in brackets indicate number of days in vacuum oven at 40oC. Note: CBS constituents sedimented in d6-DMSO and therefore only PLA signals could be recorded by 1H NMR.


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1H


NMR was performed in order to evaluate solvent residues in PLA films, conditioned in different

ways, and the composites, the results of which are shown in Figure 1. Residual chloroform was determined using the integral of its peak at ~8.31 ppm with respect to the integral(s) of PLA at ~1.5 ppm and ~5.25 ppm. PLA peak integrals were normalized using the number of hydrogens per lactide repeat unit (6 or 2 respectively) and the chloroform wt%. was estimated using Equation (7): 𝐼𝐶𝐻𝐶𝑙3 𝑁𝐶𝐻𝐶𝑙3. 𝐼𝑙𝑎𝑐𝑡𝑖𝑑𝑒

(𝑁

𝑙𝑎𝑐𝑡𝑖𝑑𝑒

𝑀𝐶𝐻𝐶𝑙3 𝐼𝐶𝐻𝐶𝑙3

. 𝑀𝑙𝑎𝑐𝑡𝑖𝑑𝑒) + (𝑁

× 𝑤𝑡%(𝑃𝐿𝐴)

(7)

. 𝑀𝐶𝐻𝐶𝑙3)

𝐶𝐻𝐶𝑙3

where I is the integral of the peak (e.g. for chloroform at 8.31 ppm), N is the number of (H) nuclei per peak, per molecule and M is the molecular mass of chloroform (MCHCl3) or lactide repeat unit (Mlactide) of PLA. The results are presented in Table 1 in SI. Chloroform represents ca. 20% of the pristine PLA films weight, which decreased to 9 and 6 wt% by conditioning the PLA films in a vacuum oven at 40 °C (for 1 and 5 days respectively). Interestingly, chloroform represents less than 3 wt% in pristine PLA25 and less than 1% for PLA50 and PLA75. It is evident from these results that complete evaporation of chloroform from pure PLA films is challenging, while even partial removal involves extra, time consuming steps. This is not the case for the PLA/CBS samples, since inclusion of CBS promotes almost complete solvent evaporation. In the following characterization of PLA films, we have conditioned the PLA films in a vacuum oven, at 40 °C for 5 days. No conditioning was necessary in the PLA/CBS samples, since chloroform disappears after CBS inclusion. 3.2 Morphological Characterization



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Figure 2: (a) Photographs of samples with different CBS concentrations. SEM images of the surface (b-e) and cross section (f-i) of pure PLA film (b and f), PLA25 (c and g), PLA50 (d and h) and PLA75 (e and i). The white arrows indicate the CBS agglomerations, while the red arrows indicate the porosity.

In Figure 2a the photographs of the PLA and PLA/CBS composites films studied are shown. Pure PLA forms transparent films. The transparency is lost upon the addition of CBS and the composites


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acquire a brown colour that darkens with increasing CBS concentration. In Figures 2(b-i), SEM images acquired from the surface and the cross-sections of the various samples are shown. The surface of pure PLA is without any characteristic features, as seen in the SEM image in Figure 2b, whereas some roughness appears upon the addition of CBS. Then, on the surface of the films appear faintly distinguishable circular features indicating a small degree of CBS agglomeration. For PLA25, these agglomerations are scarce, as seen in Figure 1c, while for PLA50 (Figure 2d) and PLA75 (Figure 2e) they are somewhat less scarce. The cross-section images (Figure 2f-i) show a compact inner structure, with some porosity present in PLA and PLA75.

3.3

Thermal Characterization

Figure 3: (a) The thermal weight loss results and (b) their corresponding derivative curves. (c) DSC thermographs of PLA and composites.

The thermal degradation of the composites was studied by TGA and is shown in Figure 3a and b. The degradation temperature of the samples decreases with increasing amount of CBS in the composites. The weight loss is gradual and the remaining material at temperatures higher than 400 °C increases with CBS content (Figure 3a). The PLA film degrades in a single step and the decomposition peak occurs at 360 °C, as shown in the derivative thermogravimetric curve in Figure 3b. Pure CBS powder exhibits two degradation temperatures at 259 °C and 305 °C

14

(not shown here). The PLA/CBS

composites also degrade at a single step, with the degradation temperatures shifting to lower temperatures, namely PLA25 at 312 °C, PLA50 at 307 °C and PLA75 at 302 °C. The two degradation ACS Paragon Plus Environment


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temperatures of CBS are not expected to change in the composite, as it has been previously reported14, and indeed they are detected as they are shadowed by the main degradation peak. The small peak at ~100 °C is most likely due to water evaporation. Overall, the inclusion of CBS led to a decreased thermal stability of the system. Furthermore, DSC thermographs recorded during the first heating run, which best describes the thermal properties of our solution-cast samples, are shown in Figure 3c. Pure PLA films have a Tg of approximately 43 °C. In the presence of CBS, the Tg of the composites is less well defined, and increases up to 57 °C for PLA75. An increase of Tg has been reported to indicate a more crystalline matrix, due to the limited mobility of the macromolecules within the amorphous domains due to crystallisation38. The melting temperature, Tm, is ca. 146 °C and it does not change with the addition of CBS, thus the CBS does not result in changes in the size of the PLA crystallites. The presence of a clear and sharp melting peak in the composites indicates a higher degree of crystallinity. However, during the first heating we did not observe a peak due to no cold crystallization, probably because the sample has reached maximum crystallization during preparation. Furthermore, no crystallization peak is seen during the cooling cycle (not shown here). DSC thermographs indicate that the PLA/CBS composites possess a higher degree of crystallinity than pure PLA films, suggesting that the CBS act as nucleating agents for the PLA, as will be discussed later.

3.4

Chemical Characterization


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Figure 4: (a) The infrared spectra of all samples for 4000-600 cm-1; (b) magnification of the regions 4000-2750 cm-1 where the increase of the symmetric and asymmetric stretching of the CH3 at 2916 cm-1 and 2849 cm-1, respectively is depicted with the increase of CBS; (c) magnification of the region 1900-1500 cm-1, where the Amide I is seen to increase for PLA50 and PLA75, and (d) magnification of the region 1000 cm-1 - 800 cm-1, showing the peaks related to the PLA crystallinity. Red letters indicate peaks coming from CBS, while black letters indicate peaks from PLA. The colours of the spectra follow (a).

The infrared spectra of the PLA films, the CBS powder and the composites, are shown in Figure 4. The typical vibrational peaks of PLA appear at 2996 cm-1 and 2945 cm-1 (asymmetric and symmetric stretching of CH3), at 1451 cm-1 (CH3 bending) and 1359 cm-1 (CH bending) and at 1180 cm-1 (C-OC asymmetric stretching and CH3 rocking). A triplet peak appears at 1127 cm-1, 1080 cm-1 and 1043 cm-1. The central peak at 1080 cm-1 is assigned to the C–O stretching of the –CO–O– group, while the satellites at 1127 cm-1 and 1043 cm-1 are due to CH3 rocking and C–CH3 stretching mode,



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respectively39. Finally, a distinctive vibrational peak appears at 1748 cm-1, assigned to the C=O stretching of the ester bonds 40. The main vibration peaks of CBS are shown at 3305 cm-1 (O-H stretching from polysaccharides), at 2916 cm-1 and 2849 cm-1 (asymmetric and symmetric CH2 stretching from lipids), at 1734 cm-1 (C=O stretching of ester functional groups from phospholipids and triglycerides) and at 1610 cm-1 and 1521 cm-1 (Amides I and II from proteins and nucleic acids)14, 25. Finally, the band at 1025 cm-1 is associated to C-C and C-O stretching from polysaccharides and lignin14, 41. The infrared spectra of the composites exhibit vibrational peaks from both PLA and CBS. As shown in Figure 4a, the main peaks from PLA appear in all composites. Increasing the CBS concentration, the symmetric and asymmetric stretching of the CH2 increase, as shown more clearly in Figure 4b. Similarly, for PLA75, an increase in Amide I at 1610 cm-1 is observed (Figure 4c), while the broad O-H stretching at 3305 cm-1 is not clearly seen at low CBS concentrations, though clearly distinguished for PLA75 (Figure 4b). The C=O stretching at 1748 cm-1 slightly blueshifts with increasing CBS content, reaching 1751 cm-1 for PLA75. This blueshift has also been observed in PLA-curcumin composites, indicating the structural stability of the composite40. However, a blueshift of the C=O vibration is also expected with higher crystallinity 39. Finally, at lower wavenumbers three vibrational peaks appear (Figure 4d) at 956 cm-1, 923 cm-1 and 870 cm-1, that are related to the crystallinity of PLA39, 42. The peaks at 923 cm-1 and 870 cm-1 are related to the α-crystals of the semicrystalline PLA, while the peak at 956 cm-1 is related to the amorphous part of the polymer. Thus, the increment of the 923 cm-1 and 870 cm-1 peaks, with the simultaneous decrement of the 956 cm-1 one, with increasing CBS concentration, indicates that the presence of CBS as a filler promotes the crystallisation of PLA.

3.5

X-ray Diffraction


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Figure 5: Diffractograms measured by XRD in transmission mode for PLA and PLA/CBS composites. For comparison, CBS and PLA pellets were measured by XRD in reflective mode.

The diffractograms of pure PLA and of the composites measured in transmission mode XRD, are depicted in Figure 5. The pristine PLA pellets and CBS powder were also measured by XRD (in reflection mode). The as received PLA pellets display two intense peaks at 16.6° and at 19.1°, assigned to (200) and (203) planes, respectively, while two smaller peaks appear at 14.7° and 22.3°, assigned to (104) and (211) planes, respectively. This is the characteristic diffractogram of the α-form polymorph of PLA, the most common and stable PLA polymorph, having an orthorhombic unit cell5. The PLΑ films, that form by solution-casting after the dissolution of the pellets in chloroform, are amorphous, exhibiting one broad peak, as seen in the graph. Upon the addition of CBS, the diffractograms of the composites reveal an increased degree of crystallinity, exhibiting a broad halo, indicating the amorphous state, with the main peaks of PLA appearing on this amorphous halo. Calculating the degree of crystallinity from the respective diffractograms, we find that upon CBS addition, crystallinity increases to 15% for PLA25, and saturates at 33% for higher CBS content.



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In this work, the increase of crystallinity when CBS is embedded in the PLA matrix is documented by XRD, FTIR and DSC, as has been discussed in the corresponding paragraphs. We can argue that CBS acts as nucleation agent for the crystallisation of PLA. This is in agreement with the literature, where talc has been shown to act as nucleating agent for PLA10,43. In addition, cocoa shell powder has also been shown to act as nucleating agent in PCL24. In general, both organic and inorganic fillers have been found to result in increased crystallinity of polymer composites44-45.

3.6

Mechanical properties

Figure 6: (a) Stress-strain curves for PLA film and composites, (b) Young’s modulus (left hand axis) and elongation at break (right hand axis), and (c) calculated yield stress (left hand axis) and stress at break (right hand axis).


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The mechanical properties of PLA films and their CBS composites are shown in Figure 6. From the stress – strain curves, shown in Figure 6a, the Young’s modulus, yield stress, elongation at break and stress at break were extracted. However, the absolute values of pure PLA may be affected by the presence of residual chloroform, which was still present in the material after 5 days in vacuum. With this in mind, the addition of CBS in the PLA matrix changes drastically the mechanical response of the composites. Increasing CBS concentration results in a significant increase in Young’s modulus, followed by a corresponding increase of the yield stress and stress at break. On the other hand, elongation decreases. In other words, all composites lose in part their ductile characteristics, while they gain rigidity and yield resistance. It is worth noting that plastic deformation is still appreciable (12% to 54%), meaning that the induced stiffening does not lead to brittleness. The variations of the values for the composites are minimal, with elongation reducing the most, from 54% to 12% as the CBS content increases from 25% to 75%, and Young’s modulus increasing from ~1000 MPa to 1800 MPa, whereas stress at yield and at break do not change significantly. The toughness maintained at such high CBS loading is indicative of the high affinity between the composite materials and suggest the PLA/CBS composites have significant potential as a bio-based, biodegradable alternative to common polymers for food packaging. The above data suggests that PLA might be plasticized by the remaining CHCl3, as discussed in paragraph 3.1. On the other hand, in the CBS composites, where the CHCl3 is significantly lower, it does not seem like the solvent has a significant plasticizing effect. These results can present very good alternatives to more rigid materials commonly used in food packaging, like polystyrene (2-3 GPa Young’s modulus, 30 MPa strength, 4% elongation at break) or polypropylene (1-2 GPa, 30-40 MPa, 100%). 3.7

Composites behaviour in contact with water and barrier properties



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Figure 7: (a) Oxygen permeability (OP) of all samples versus CBS concentration, (b) Radical scavenging activity vs time, and (c) HDFa viability, treated for 24 h with extracts from PLA and composites PLA25, PLA50, and PLA75. Significance is expressed in terms of p < 0.001 (***) obtained via a Student’s t-test for the comparison of all the samples with respect to the control, not-treated cells.

For packaging applications, the behaviour of the material when it comes into contact with water or water vapour is crucial, since high water content accelerates food oxidation46. Hence, we have characterised our composites with respect to Oxygen Permeability (OP), Water Vapour Permeability (WVP) and Water Contact Angle (WCA). All samples are hydrophobic, with pure PLA obtaining a WCA value of approximately 109°, decreasing slightly to 104° for CBS concentration higher than 50% (Figure S2 in SI). Similarly, water vapour permeability is approximately 3.5×10-5 g·m-1·day1·Pa-1

and it does not change significantly with the CBS concentration (Figure S3 in SI). The stability

of the WVP is expected, as it should follow contact angle. On the contrary, one would expect the WVP to decrease with the increase of film crystallinity 5-6, which in the present case takes place with increasing the CBS content. However, we must take into account the porosity of the polymeric films that increases with the addition of CBS, as seen in the cross-section SEM images, in Figure 2(f-i). Finally, the oxygen transmission rate was measured and the OP was calculated using Equation (4). The results are shown in Figure 7a. The OP of the PLA film is 40100 mL·μm·m–2·day–1·atm-1. Upon the addition of 25% and 50% CBS, OP reduces to 20000 mL·μm·m–2·day–1·atm-1 and 10000 mL·μm·m–2·day–1·atm-1, respectively. The reason for this reduction is twofold. First, it is due to the increase of crystallinity of the composites with the addition of CBS. It has been recently shown that


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increased crystallinity in PLA results in reduced OP values47. This is attributed to the tight chain packing in the α-form PLA polymorph, in conjunction with strong coupling with the amorphous parts of the macromolecules5. Second, the CBS fillers also increase the tortuosity on the path of the oxygen molecules. For PLA75, however, OP increases to 37860 mL·μm·m–2·day–1·atm-1, a value close to pure PLA, due to the high concentration of CBS. As seen in Figure 1e, the dispersion of CBS at such high concentration becomes less homogeneous and small areas with agglomerated CBS are formed on the film. Even though the inhomogeneity of the CBS dispersion in the PLA matrix is small, it is enough to promote the diffusion of the small oxygen molecules and facilitate their transport through the film. In addition, the porosity that is observed at such high CBS concentration can lead to increased OP. It is interesting to compare these results with those obtained for other polymers currently used in food packaging applications. PLA50 has lower OP than both low density and high density polyethylene (LDPE: 42700 mL·μm·m–2·day–1·atm-1; HDPE: 1870×103 mL·μm·m–2·day–1·atm-1), polypropylene (PP: 50000 -100000 mL·μm·m–2·day–1·atm-1)48-49. In addition, it compares well with chitosan (11292477 mL·μm·m–2·day–1·atm-1 depending on RH) and cellophane (263-25470 mL·μm·m–2·day– 1·atm-1

depending on RH)49. On the other hand, the WVP of our samples is higher than that for

conventional plastics, with PP and PE having values in the range of 10-9 g·m-1·day-1·Pa-1 , whereas they compare well with chitosan and cellophane that have WVP values of the order of 10-5 g·m-1·day1·Pa-1 49.

This comparison suggests that our PLA/CBS composites are indeed promising materials for

packaging applications.

3.8 Antioxidant properties Antioxidants are typically added in packaging materials in order to protect food from oxidation, by scavenging undesirable compounds, such as oxygen or reactive oxidative species, that can result in oxidation and spoilage of food18. CBS is known to be a strong antioxidant due to the high flavonoid and polyphenol content 14, 27, 50, we thus expect that the PLA/CBS composites should present strong ACS Paragon Plus Environment


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antioxidant properties. In order to attest the antioxidant activity of our samples, we have determined the radical scavenging activity (RSA), as calculated using Equation (5), which is shown in Figure 7b for all composite films, for up to 24 h. Scavenging activity of CBS powder is shown for comparison. Pure PLA has very low RSA against DPPH·, that reaches ca. 14% after 24 h, whereas RSA of CBS saturates at 95% after 5 h. In presence of CBS, RSA of composites increases and already PLA25 has a scavenging activity of 67%. For PLA50 and PLA75 scavenging activity increases even further, reaching approximately 80% and 85%, respectively.

3.9 Food Migration Analysis Overall migration tests determine the non-volatile substances that may migrate from polymer films into foodstuff. Overall migration tests were carried out using Tenax® as dry food simulant and the results are presented in Table 1. The tests simulates contact with food by using food simulants, according to current legislation Commission Regulation (EU) No 10/2011 that sets a limit of 10 mg/dm2 and demonstrate the safety of the proposed material in the food-packaging field.

Table 1: Overall migration from the PLA/ CBS films in the Tenax® food simulant. Sample

Overall migration (mg/dm2)

PLA

2.2 ± 0.1

PLA25

7.8 ± 2.2

PLA50

5.0 ± 2.1

PLA75

4.9 ± 1.6

After 2 h incubation at 70 ◦C in Tenax®, pure PLA film migration level is around 2.2 ± 0.1 mg/dm2. Increasing CBS content results in higher migration values. The measured overall migration levels are


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lower than the limit of 10 mg/dm2, suggesting a possible application of the designed films in the food packaging field. 3.10 Biocompatibility A cell viability experiment carried out with HDFa cells shows that all samples studied here are biocompatible and the results are shown in Figure 7c. Following MTS assay, cells are metabolically active and a slight but significant increase in their proliferation is seen when they are treated with the extracts from PLA25 and PLA50 (about 4% - 7% viability increase with respect to the control. A slightly lower cell survival is observed when HDFa cells are treated with medium extracts from either pure PLA or PLA75 (about 7%-5% viability decrease with respect to control). Therefore, a lowmedium content of CBS appears to stimulate primary fibroblast growth, setting a window of cytocompatible concentrations.

3.11

Biodegradability



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Figure 8: (a) Oxygen consumption (specific to our system) versus time, (b) Water uptake of PLA and composites after 6h immersion in a water bath, and (c)-(f) Surface SEM images of the samples after the BOD test for PLA, PLA25, PLA50 and PLA75, respectively.

One of the biggest pollutants of the oceans is plastic, hence biodegradation studies in aquatic environment is of crucial importance. Here, we have studied the Biochemical Oxygen Demand (BOD) of our samples for 30 days, presented in Figure 8a. For the course of the experiment, no biodegradation of pure PLA takes place. On the other hand, PLA/CBS composites degrade and their degradation characteristics depend on the CBS concentration. Degradation of the composites starts faster as CBS concentration increases. For PLA25 degradation starts after approximately 5 days, for PLA50 it starts after 2.5 days and finally PLA75 starts degrading after 2 days. Oxygen consumption after 30 days in seawater depends on the CBS concentration, with PLA75 reaching oxygen


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consumption of 28 mg/L. Interestingly, after 30 days, oxygen consumption from PLA25 had already reached more than 40% of the maximum value possible for our system (maximum O2 consumption is 40 mg/L), demonstrating the synergistic effect of CBS in the degradation of the PLA polymer by seawater. PLA is a biodegradable polymer, however it only degrades under specific conditions51. It is not home compostable4 and it does not biodegrade in seawater in natural conditions4, 52. More specifically, PLA is very resistance to microorganism attack in ambient conditions, and biodegradation only starts after hydrolysis has taken place8 where the water molecules attack the ester bonds. However, hydrolysis can be initiated at 50 °C, while the alkyl groups hinder the attack of water molecules, resulting in long half-life of hydrolysis8 and very slow degradation. In short, PLA is a rather stable polymer that does not readily (bio)degrade under natural conditions, and this fact has led to some scepticism as to its potential advantages over more conventional polymers53. In the present work, it is seen that the addition of CBS in PLA renders the polymer readily (bio)degradable. This is attributed to the higher swelling properties of the composites, as manifested by the water uptake experiment, depicted in Figure 8b. PLA does not uptake water, whereas the composites are able to swell due to the hydrophilic nature of CBS. PLA25 and PLA50 have maximum water uptake after 24 h of immersion of the samples in water, whereas PLA75 presents the maximum water uptake after 6 h, probably due to its porosity, as manifested in Figure 2. Even though one can argue that swelling might be a limiting factor for food packaging, it also confers biodegradability, which is an significant advantage, considering the plastics waste problem2. Hence, the increasing concentration of CBS facilitates the penetration of water molecules into the bulk of the polymer matrix, rendering the polymeric chains more susceptible to hydrolysis, followed by microorganism attack. SEM images taken after the course of the 30 days BOD test (Figure 8c-f), show that the surface of PLA film remains intact, while the surface of the composites, after 30 days in aquatic water, is worn out, cleaved and with high degree of porosity.



4

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Conclusions

In this work, we have prepared and characterised new composite materials based on PLA and CBS. Once dissolved in the solvent and solution-cast, PLA becomes considerably amorphous, however, the addition of CBS increases the crystallinity of the composites up to 33%, resulting in improved barrier properties. The addition of CBS also enhances mechanical properties of the composites, resulting in stronger materials with high plasticity. The mechanical properties, in conjunction with the results from infrared spectroscopy, indicate the blend of the two components leads to improved physical properties. The presence of CBSs in PLA, improves swelling properties, rendering the composites biodegradable in aquatic environment. The materials are biocompatible, as expected regarding the biocompatibility of both PLA and CBSs, but the inclusion of the CBS fillers improves the antioxidant activity of PLA by more than 60%. Finally, the overall migration of the proposed bioplastics to foodstuffs upon contact was tested using a dry food simulant and was found to be significantly lower than the 10 mg/dm EU limit, making the composites safe for food contact. The above results suggest that the PLA/CBS composites can be considered as new materials. This, in conjunction with the facile fabrication, can make these materials attractive to a broad range of applications of bioplastics, including active food packaging. ASSOCIATED CONTENT

Supporting Information. SEM of the CBS powder; wetting properties; water vapor permeability.

AUTHOR INFORMATION

Corresponding Author


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*E-mail: [email protected]

*E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT Ms Lara Marini is acknowledged for the TGA and the DSC measurements.

ABBREVIATIONS PLA: polylactic acid; CBS: cocoa bean shells; TGA: thermogravimetric analysis; TXRD: x-ray diffraction in transmission mode; NMR: Nuclear magnetic resonance; FTIR: Fourier-transform infrared spectroscopy; WVP: water vapour permeability; OP: oxygen permeability; RSA: radical scavenging activity; BOD: biochemical oxygen demand



Page 28 of 31

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Sustainable Active Food Packaging from Poly(lactic acid) and Cocoa

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