Food packaging materials compounded with antimicrobial additives can substantially diminish the incidence of foodborne diseases. Here, poly(L-lactic acid) (PLA) films containing sodium metabisulfite (NaM) were produced by melt extrusion as an attempt to develop a new biodegradable material with antimicrobial properties for packaging. Life cycle assessment (LCA) simulations revealed that the environmental footprints of the PLA film did not change upon NaM addition, and that NaM is more eco-friendly than silver nanoparticles. The PLA/NaM films with NaM content varying from 0.5 to 5.0 wt.% were characterized by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and optical and mechanical properties determinations. The optical properties were sustained after the addition of NaM, but high NaM contents degraded the light transparence of the PLA matrix at some extent. The thermal stability and tensile properties of the PLA film decreased proportionally to the NaM content, while no changes were observed on T
Antimicrobial plastic packaging received much attention recently due to its potential for decreasing the incidence of foodborne diseases [
Poly(lactic acid) (PLA) is a biodegradable polymer with adequate characteristics for food packaging. PLA is a semicrystalline polyester obtained by ring-opening polymerization of lactic acid, which can be produced from corn starch, sugarcane, and other renewable plant-based feedstocks [
Antimicrobial packaging films based on PLA were previously explored by incorporating AgNPs to inhibit the growth of
Sodium metabisulfite (Na2S2O5, NaM) is a food preservative with potential for antimicrobial packaging applications due to its well-known antioxidant properties and wide antimicrobial activity [
Herein, we report on the incorporation of NaM in biodegradable PLA films by melt extrusion aiming at antimicrobial packaging applications. Importantly, the compounding and film casting processes were carried out in a single thermal cycle to minimize thermal degradation effects on PLA and NaM. To extend the sustainable perspective, we first applied the life cycle assessment (LCA) methodology to estimate some environmental footprints (EFPs) of the PLA/NaM films and performed a comparison with their counterparts loaded with AgNPs, which is one of the most used biocidal agents in polymers. The disc diffusion test was used to assess the antimicrobial activity of the PLA/NaM films on
Poly(L-lactic acid) with ~4% D-isomer content (Ingeo™ Biopolymer 2003D) was acquired from NatureWorks (Blair, Nebraska, USA). The polymer was dried at 60°C for 72 h in an air circulating oven prior to use. Sodium metabisulfite (97%) was purchased from Synth (Diadema, São Paulo, Brazil).
The PLA/NaM films were produced by hot-melt extrusion combined with a flake-like masterbach preparation approach. First, 5 g of PLA pellets were vigorously mixed with precise amounts of NaM and were thermo-pressed at 150°C and 3 ton for 1 min. The resulting masterbach was cut into smalls flakes (0.5 × 0.5 cm) and dried overnight at 50°C in an air-circulating oven. The NaM concentration in the masterbatch was varied to result in films with final NaM contents of 0.5 wt.%, 1.0 wt.%, 2.5 wt.% and 5.0 wt.%, after compounding with pure PLA through extrusion.
The extrusion of the PLA/NaM formulations was carried out with a MP19-TC co-rotating twin-screw extruder (L/D = 25, D = 19 mm, B&P Process Equipament and Systems) coupled to a 220 mm-wide rectangular sheet die. The temperature profile was 180–150–150–150–160°C, the screw speed was 160 rpm, and the residence time was 3 min. In a typical run, the PLA pellets and the flake-like PLA/NaM masterbach (m
The environmental footprints (EFPs) of the PLA/NaM films were estimated through a simplified cradle-to-grave life cycle assessment (LCA) according to ISO 14044 standard [
The process units included in the system boundaries were raw material extraction, antimicrobial production, film production (melt extrusion) and disposal in landfill (
The cradle-to-grave LCAs were modeled using the OpenLCA 1.9 software (Green Delta, GitHub). The upstream life cycle inventories (LCIs) were completed by associating the inputs and outputs (materials, energy and chemical emissions) of each process unit with the European ecoinvent life cycle inventory dataset (Ecoinvent v3.3). The EFPs were calculated using the impact assessment method International Reference Life Cycle Data System (ILCD) 1.0.8 2016 midpoint (RLCDS, ILCD, EC-JRC). The EFPs were expressed as the results of the following midpoint impact categories: Global warming potential (GWP) in kg CO2 eq, human health impacts from carcinogenic substances (HHCS) in health comparative toxic units or CTUh, and resources depletion (ReD) in kg Sb eq.
Optical parameters (luminous transmittance T%, haze H%, and clarity C%) were determined at 20°C with a Haze-Gard Plus hazemeter (BYK Additives & Instruments) as per ASTM D1003 (2007). The measurements were performed on 50 × 50 cm specimens with six repetitions for each film. Thermogravimetric (TG) and differential thermogravimetric (DTG) curves were recorded with a Q500 thermal analyzer (TA Instruments). Samples (8–10 mg) were placed in a platinum crucible and were heated up to 600°C using heating ramp of 10°C min-1 and synthetic air atmosphere flowing at 60 mL min−1. Tonset was determined from the TG curves as the temperature at which the sample lost 3% of its mass. Tmax was determined from the DTG curves. Dynamic scanning calorimetry (DSC) tests was conducted on a Q2000 calorimeter (TA instruments) using hermetic Al pans, heating rate of 10°C min−1 and N2 atmosphere (50 mL min−1). The samples were first heated from 25°C to 200°C (1° heating scan), then cooled down to 0°C, and heated again to 200°C (2° heating scan). The glass transition temperature (T
The
Data were subjected to one-way analysis of variance (ANOVA). Mean values were compared using the Tukey’s test with a confidence level of 95% (
The potential environmental footprints (EFPs) of the PLA/NaM films were estimated by LCA. The results were compared with the EFPs of hypothetical extruded PLA films loaded with AgNPs to identify possible environmental advantages of NaM as an antimicrobial additive for biodegradable packaging.
The pure PLA film displayed estimated GWP = 14 kg CO2eq kg film−1, HHCS = 3.6 × 10−7 CTUh kg film−1, and ReD = 3.6 × 10−7 kg Sbeq kg film−1. These results point out that the EFP of the PLA life cycle is more significant in terms of carbon footprint, considering the functional unit (1 kg PLA packaging film) used in the LCA simulations. It can be observed that the addition of NaM at 1.0 wt.% had no effect on the GWP of the PLA film but increased by 1- and 3-times the HHCS and ReD EFPs, respectively. In comparison, if 0.5 wt.% AgNPs are hypothetically compounded with PLA, the EFPs become much higher, with 150%, and 3- and 5-times increases in GWP, HHCS and ReD, respectively. The relative contribution of each life cycle stage (process units) from
Regarding the HHCS and ReD, the pure PLA and 1.0 wt.% NaM-loaded PLA films displayed similar EFPs (
Hence, the LCA results suggest that the upstream EFPs of the PLA films did not chance due to compounding it with NaM. Further LCA simulations indicated similar GWP, HHCS, and ReD for the NaM contents of 0.5, 1.0, 2.5 and 5.0 wt.% (data not shown). This means that NaM remains as an eco-friendly antimicrobial additive for PLA films even at high contents. The comparison with the 0.5 wt.% AgNPs-loaded PLA film supports this finding.
The PLA/NaM films were produced in a twin-screw extruder coupled to a sheet die. This allowed PLA to be compounded with NaM under intense distributive and dispersive flow conditions alongside casting the molten into films in a single thermal processing cycle.
NaM (wt.%) | Transmittance (%) | Clarity (%) | Haze (%) |
---|---|---|---|
0.0 | 95.8a ± 0.1 | 99.5a ± 0.1 | 1.3a ± 0.1 |
0.5 | 92.5b ± 0.1 | 90.0b ± 0.2 | 6.7b ± 0.1 |
1.0 | 92.6b ± 0.1 | 81.1c ± 0.1 | 11.8c ± 0.2 |
2.5 | 91.7b ± 0.1 | 70.1d ± 0.1 | 20.2d ± 0.1 |
5.0 | 92.0b ± 0.1 | 68.7e ± 0.1 | 21.0d ± 0.1 |
*Mean values ± standard deviation (n = 8). Mean values in the same column bearing the same letter are not statistically different according to the Tukey’s test (
Thermogravimetric analysis was performed in O2-rich atmosphere (
NaM (wt.%) | Tonset (°C) | Tmax (°C) | T |
T |
X |
||
---|---|---|---|---|---|---|---|
0.0 | 297 | 359 | 0.6 | 0.0 | 57 | 142 | 2 |
0.5 | 274 | 338 | 0.9 | 0.3 | 58 | 142 | 2 |
1.0 | 269 | 328 | 1.1 | 0.7 | 59 | 148 | 1 |
2.5 | 264 | 322 | 1.2 | 1.7 | 59 | 147 | 1 |
5.0 | 254 | 309 | 1.9 | 3.3 | 59 | 143 | 3 |
Pure NaM | 119 | 171 | 72.5 | 66.3 | – | – | – |
*
**X
It was possible to ascertain the composition of the PLA/NaM films by comparing the theoretical and experimental residue contents at 600°C. It can be seen that except for the 2.5 wt.% and 5 wt.% NaM-loaded films, the obtained residue contents were close to the nominal values, taking into account the residue left by the PLA matrix (~0.6 %). This indicates that PLA was properly compounded with NaM at the targeted contents through the one-step melt extrusion process. The result also indicates that NaM endured the thermal processing, considering that its Tonset was lower than the used processing window (T = 150–180°C). This can be explained by the fact that the residence time (t = 3 min) was not sufficiently long to induce the thermal decomposition of the antimicrobial agent, and that no other heating step was used to cast the films. Additionally, it can be observed from
The thermal transitions of PLA were assessed by DSC to gain insights into the thermal behavior of the PLA/NaM films. DSC curves relating to the second heating scan are shown in
It is observed that the pure PLA film displayed T
NaM (wt.%) | E (GPa) | σT (MPa) | εB (%) |
---|---|---|---|
0.0 | 3.2a ± 0.1 | 56.3a ± 2.9 | 3.5a ± 0.6 |
0.5 | 3.0a ± 0.2 | 44.1b ± 3.7 | 2.3b ± 0.3 |
1.0 | 2.6b ± 0.1 | 36.6c ± 2.6 | 2.1b ± 0.3 |
2.5 | 2.6b ± 0.3 | 44.1b ± 4.7 | 3.5a ± 0.9 |
5.0 | 2.5b ± 0.2 | 38.3b,c ± 5.8 | 2.6a,b ± 0.8 |
*Mean values ± standard deviation (n = 8). Mean values in the same column bearing the same letter are not statistically different according to the Tukey’s test (
The reduced tensile properties of the PLA/NaM films may be explained by the fact that the NaM particles acted as stress concentration factors within the glassy PLA matrix. This may be due to the hydrophobicity of PLA, while NaM is a water-soluble salt, thus the miscibility between PLA and NaM was already expected to be low. However, it is worth noticing that the mechanical properties of the PLA/NaM films were still high enough to cover most packaging applications, even for the highest NaM content (5 wt.%). Furthermore, the chemical incompatibility between NaM and PLA allows one to expect that the NaM particles can be released from the PLA films, exerting an antimicrobial effect through migration.
The results of the agar diffusion tests are illustrated in
A possible explanation for the antimicrobial inactivity of the PLA/NaM films is that NaM did not migrate to a large extent from the PLA matrix to the agar medium. This is because the T
An eco-friendly packaging material was successfully developed by melt compounding PLA with NaM, a traditional food preservative. Using a twin-screw extruder coupled to a sheet die allowed for uniform distribution of MNa within the PLA matrix and film formation with no significant thermal degradation. As a result, the PLA/NaM films displayed important physical properties, including tensile strength as high as other synthetic polymer like PP and PE, high optical transparency, and thermal stability up to 250°C, which may be suitable for rigid food packaging applications, such as bowls and standup pouches. Furthermore, the antimicrobial activity of the films was not significant due to the high T
The authors thank DEMa/UFSCar and Embrapa Instrumentation for the support to this work.