Kinetic release study of zinc from polylactic acid based nanocomposite into food simulants

Polymer Testing 76 (2019) 254–260

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Kinetic release study of zinc from polylactic acid based nanocomposite into food simulants

T

Mojtaba Heydari-Majda, Babak Ghanbarzadeha,b,*, Mostafa Shahidi-Noghabic, Mohammad Ali Najafid, Perihan Adunb, Alireza Ostadrahimide a

Department of Food Science and Technology, Faculty of Agriculture, University of Tabriz, P.O. Box 51666-16471, Tabriz, Iran Department of Food Engineering, Faculty of Engineering, Near East University, P. O. Box 99138, Nicosia, Cyprus, Mersin 10, Turkey c Department of Food Chemistry, Research Institute of Food Science and Technology, PO Box 91735-147, Mashhad-Quchan Highway, Mashhad, Iran d Department of Food Science and Technology, Zabol University, Zabol, Iran e Nutrition Research Center, Tabriz University of Medical Sciences, Tabriz, Iran b

A R T I C LE I N FO

A B S T R A C T

Keywords: PLA nanocomposite Zn ion release Diffusion Solubility parameter

In the present study, the release of zinc (Zn) from polylactic acid/zinc oxid nanoparticles (PLA/ZnO NPs) nanocomposites to three aqueous food simulants (10% ethanol, 20% ethanol and 95% ethanol) was evaluated (during 15 days at 4, 25, and 40 °C) by atomic absorption spectrometry (AAS). The results indicated that temperature and the simulant type affected the migration of Zn and at a constant temperature; the maximum Zn release was observed in the 95% ethanol simulant. Pseudo-Fickian behavior was observed during the diffusion of Zn and diffusion coefficients (D values) of 0.002–6.08 10−11 cm2 s−1 were obtained in different migration conditions. The obtained partition coefficients (K) were between 21.66 and 0.057 μg g−1. The Zn release data were fitted with the Weibull model, and the b and n model parameters had values ranging from 0.008 to 0.07 and 0.24 to 0.49, respectively. The results of the present study indicated that Zn had little migration from PLA films into food simulants (especially in water at 4–40 °C) which was attributed to the high activation energy (Ea) values for Zn diffusion from PLA/ZnO NPs nanocomposites.

1. Introduction

fillers in polymer matrix, are considered as a modern approach towards improving the barrier, and mechanical properties of packaging materials [10,11]. Nanocomposites can also act as active packaging for controlling spoilage and increase of shelf life in foodstuffs [12]. Metal oxide antimicrobial nanoparticles have several advantages (such as thermal stability, lack of resistancy of microorganisms to them and having effectiveness at low concentration) in comparison to chemical organic antimicrobial compounds [13]. Zinc oxide nanoparticles (ZnO NPs) is one of the metal oxide NPs and having some advantages such as being non-toxic, cost-efficient and having antibacterial properties, nutritive value and relatively simple production processes [14–16]. ZnO NPs are listed by USFDA as generally recognized as safe (GRAS) materials [15]. Recently, incorporation of ZnO NPs into polymers [17,18] and biopolymers [19,20] and its impact on the mechanical, physical and antimicrobial properties of nanocomposites has been investigated by various research groups. Despite improving the properties of food packaging material, nanomaterial incorporation has raised consumer concern regarding the potential release of these compounds, and their possibly adverse effects

Extensive investigations have been conducted on developing novel biopolymers from biodegradable sources. Polylactic acid (PLA) is a commonly used linear aliphatic polymer that is derived from natural, renewable sources including corn starch and other carbohydrates can be used in various food-packaging materials [1,2]. In 1992, the U.S. Food and Drug Administration granted PLA the generally recognized as safe (GRAS) status for use in contact with foods [3]. This biopolymer also has a promising potential in the pharmaceutical and biomedical industries, offering itself as an option in drug packaging and delivery [4–7]. The immense mass of solid waste produced from non-biodegradable packages and containers may be reduced by using PLA. However, certain undesirable characteristics, including poor toughness and relatively high brittleness [8], have limited the practical applications of this biopolymer. Such drawbacks must be overcome in order to increase PLA applications in the packaging industry, biomedical and pharmaceutical fields [9,10]. Nanocomposites which produced by incorporating of nano scale

*

Corresponding author. Department of Food Science and Technology, Faculty of Agriculture, University of Tabriz, P.O. Box 51666-16471, Tabriz, Iran. E-mail addresses: [email protected], [email protected] (B. Ghanbarzadeh).

https://doi.org/10.1016/j.polymertesting.2019.03.040 Received 25 November 2018; Received in revised form 17 March 2019; Accepted 30 March 2019 Available online 31 March 2019 0142-9418/ © 2019 Published by Elsevier Ltd.

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more toughness and homogeneous but agglomeration of particle occurred due to enhancement of particle concentrations above 1.5%. Next, 1.5% (w/w) of ZnO NP (based on PLA dry matter) was mixed with chloroform and then was sonicated (Model VCX 750, Sonics & MaterialsInc., Newtown, CT, USA) for 10 min (at fixed frequency of 40 kHz and 100 W average ultrasonic power). The PLA solution was then added to the pretreated ZnO NP dispersion; the mixture was then magnetically stirred for 2 h at room temperature. After total PLA dissolution, all prepared film solutions were poured into glass Petri dishes, and the solvent was allowed to evaporate over 24 h in an oven set to 35 °C.

on human health [21,22]. For example, smaller particle can damage the cells, by different routes, e.g. generating the active oxygen species (ROS) within the cells, or direct or indirect toxicity [23]. Such concerns have driven the U.S. National Research Council to emphasize the importance of determining the risks related to the use of such nanocomposites [24]. A theoretical research [25] with a physicochemical view is available on the potential release of nanoparticles (such as: titanium dioxide, iron oxide and synthetic amorphous silica (SAS)-hydrophobic) from packaging to simulants, which indicates that measurable migration takes place in NPs up to approximately 3.5 nm in diameter, but not in bigger nanoparticles. Until now, no standard has been available for the unambiguous detection of small nanoparticles (except titanium nitride) migrating out of plastic food contact materials. Since nanoparticle migration from polymer packaging into food is not expected [25], the quantitative estimation of Zn2+ release from nano-ZnO-PLA composites packaging would allow us to obtain a critical insight in terms of the suitability of those novel materials to be used in food packaging applications. Zn ions were confirmed to release from the biodegradable film by researchers [26–28]. The results of these researchers showed that the released Zn ions more than photocatalytically generated species are responsible for the antibacterial effect. Although few studies have addressed the migration of Zn ions from nanocomposites [26–28], to the best of the authors' knowledge, no attention has been directed towards the release of Zn from PLA. Hence, the determination of the magnitude of Zn ion migration from PLA packaging to foods under different environmental conditions is vital for determining possible health risks. Generally, Fick's second law of diffusion can describe the migration of low molecular mass constituents within packaging systems. The related equations can be used to solve parameters associated with the migration process of these molecules, including the apparent diffusion coefficient (D) (the speed of nanomaterial diffusion) and the partition coefficient (KP,F) (the amount of migrant that diffuses) [29]. The timeand money-consuming procedures related to conventional migration tests can be avoided by using migration modeling, which maintains a high level of accuracy [30]. The goal of the present study was to evaluate the release kinetics of Zn from biodegradable nano-ZnO-PLA composite food packaging into three widely used food simulants (10, 20 and 95% (v/v) aqueous ethanol solutions) at different temperatures (4, 25, and 40 °C). This study can be useful for understanding the interaction between foods and polymer packaging, giving crucial information for the development of the highly progressive, packaging industry.

2.1.2. Migration test of Zn ion in different simulants The kinetics related to Zn ions release from PLA/ZnO-1.5% composite films were evaluated by immersing into different simulant media at various temperatures using a migration cell (a 40 mL amber glass vial with open-top cap and silicon septum) in accordance with ASTM D4754-98 [31]. From each PLA-based biocomposite, a 20 × 20 × 1 mm fractured surface was obtained and placed in a 40 mL tightly sealed vial containing 30 mL of simulant. At three different temperatures (4, 25 and 40 °C in a thermostated incubator), the release of the Zn ions was determined during 15 days (at 48 h time interval). Ethanol 10% (v/v) in aqueous solution (simulant A, as simulants for aqueous food), 20% ethanol (v/v) in aqueous solution (simulant B, as simulants for alcoholic product) and 95% ethanol (v/v) in aqueous solution (simulant C, as a fatty simulant for fats, oil, and fatty foods) were used as the simulants for this experiment [32,33]. Each experiment was conducted with four replicates, and all storage conditions were controlled with a variation of 0.5 °C. 2.1.3. Atomic absorption spectrometry analysis The concentration of Zn2+ ions released into the three simulants was determined using a flame atomic absorption spectrophotometer (Shimadzu, AA-7000; Japan) (AAS) equipped with a HGA-700 atomizer and an autosampler system, in accordance to the study of Marra et al. [34]. The calibration curves were plotted using a stock Zn solution dissolved in 2.5% HNO3. Zn ions concentrations were calculated by correlating the Zn2+ ion concentrations with the elemental contents in the ZnO NPs. All measurements were performed in three replicates. 2.1.4. Mathematical models and determination of key parameters 2.1.4.1. Fick's diffusion model. To assess the mechanism of Zn diffusion from the PLA films to the three simulants, Fick's second law (Eq. (1)) for diffusion in a single dimension was used as follows [35]:

2. Materials and methods

Mt =1− M∞

2.1. Chemicals and samples

n= 1

∑ ∞

2

Dq 2α(1+α) exp ⎛⎜− 2n t ⎞⎟ 1 + α+ α2q2n ⎝ L ⎠

(1)

with: PLA (with code PLA 4032D; Mw = 200 kDa, Mw/Mn = 1.98) was purchased from Natureworks LLC (USA). ZnO nanoparticles (average particle size: 10–30 nm, morphology: nearly spherical and true density: 5.606 gr/cm3) were purchased from the Iranian Nanomaterials Pioneers Company (Iran). Glycerol and Tween 80 were obtained from Merck (Germany), and were used to prepare film-forming dispersions. All other chemicals were of analytical grade or of the highest grade available.

α=

1 VF KP . F VP

(2) 2 −1

where D denotes the diffusion coefficient of the Zn (cm s ), Mt and M∞ denote the simulant's Zn2+ content (μgr) at time t and equilibrium, respectively, L denotes the film thickness (mm), qn denotes the positive roots of the equation tan qn = −αqn, Vs denotes the molar volume of the simulant, Vp denotes the molar volume of the polymer, Kp,s denotes the partition or distribution coefficient of Zn2+ between the PLA polymer and the simulant, and α denotes mass ratio between the amount of Zn in simulants and in the film at equilibrium. At lower concentrations, the Kp,s value can be assumed to be constant, and the ratio of Zn2+in PLA (Cp,∞) to Zn2+in the simulant (Cs,∞) at equilibrium can be calculated using the following equation:

2.1.1. Preparation of PLA films A slightly modified version of the method of Heydari-Majd et al. [12] was used to prepare PLA-based films by solvent casting. PLA is very hygroscopic so the PLA powder was dried beforehand in an oven at 80 °C for 4 h. Then, 2 g of PLA were mixed with chloroform, and 20% (w/w) of glycerol (based on PLA dry matter) was added to the solution as a plasticizer. The pretests in the film production stages showed that 1.5% (w/w) ZnO particles can be well dispersed by solvent casting method in the PLA and the prepared nanocomposite PLA films had

KP,S =

Cp, ∞ Cs, ∞

(3)

In the case that the volume of simulant is considered to be infinite 255

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(α ≫ 1 when VF ≫ VP and/or KP,s < 1), Equation (1) can be simplified as:

Mt 8 =1− 2 M∞ π

n= 1

∑ ∞

(2 n+ 1)2 1 exp ⎡− Dπ2t⎤ ⎥ ⎢ (2 n+ 1)2 4L2 ⎦ ⎣

(4)

In order to fit the data to Eqs. (1) and (4), Mt/M∞ was plotted vs time t, and D (cm2/s) was calculated by minimizing sum of squared errors (SSE) of the measured and estimated values for each film at the different experimental temperatures. Using MATLAB R2010b (MathWorks, Natick, MA, USA), a non-linear regression (nlin-fit regression) function was utilized to find the best fit for the data [36]. 2.1.4.2. Power law model. Generally, the diffusion mechanism in systems at Mt/MN < 0.6 follows the power law model [35]:

Mt = Kt n M∞

(5)

where Mt denotes the amount of Zn migrant at specific time t, M∞ denotes the amount of Zn migrant at equilibrium, k is a constant which characterizes the macromolecular network system (s−1), t is time (in seconds), and n is the release exponent. The slope and intercept of the ln Mt/M∞ and ln t plots can be used to obtain the values of n and k, respectively. 2.1.4.3. Determination of activation energy. Activation energy (Ea) is the energy required for the molecules of a system to be able to react [37]. To determine the impact of temperature on the diffusion of Zn ions from PLA films into simulants, the activation energy (Ea) was determined using the Arrhenius activation energy model for diffusion [38,39]:

D= D0 exp ⎡ −Ea RT ⎤ ⎦ ⎣

(6)

where D denotes the diffusion coefficient, D0 denotes the preexponential factor of diffusion at infinite temperature (m2/sec), Ea denotes the activation energy (J/mole), R is the universal gas constant (8.3145 J/mole K), and T is the absolute temperature (K). Ea was obtained from the slope of a plot of the reciprocal of temperature (1/T) vs the logarithm of D values (Ea = -slope × 2.303R).

Fig. 1. Amounts of Zn migration in 10% ethanol (a), 20% ethanol (b), and 95% ethanol (c) at 4, 25, and 40 °C.

and 40 °C) is presented in Fig. 1. In all three simulants, the highest and lowest Zn migration was observed at 40 °C and 4 °C, respectively (Fig. 1). For instance, in 95% ethanol, equilibrium was obtained after 264, 216 and 168 h at 4, 25, and 40 °C, respectively (Fig. 1b). Fernandez et al. [41] studied migration behavior of anti-bacterial silver ions from silver zeolites/polylactic acid packaging materials in different simulants, and found that the amount of silver ions that migrated increased with increasing temperature. At high temperatures, equilibrium was reached faster. In all three simulants, the total Zn content increased more quickly during the first 100 h, indicating an initial release of Zn from the PLA films, then increased with a slight slope for the rest of the exposure time, such that Zn concentrations had no obvious increases until the end of the experiment (equilibrium was reached at a certain time). In previous studies, nanoparticles were incorporated into polymers [42–45], and the general finding was that with increasing time and temperature, an increment in nanoparticle and ion migration occurred. These researchers also reported that as the migration time increased, the volume of nanoparticles that migrated increased gradually until equilibrium was reached at a certain time. The time taken to reach equilibrium had an inverse relationship with temperature, with the same trend being observed in all cases. This may be because the increase of temperature accelerates the molecular thermal motion in the food stimulant, such that Zn ions gain extra free energy to overcome the interactions between one another. This allows Zn ions to migrate out of the PLA polymer, resulting in an increased free volume and greater diffusion rate, making it easier for equilibrium to be reached [42].

2.1.4.4. Weibull function. Weibull's model is frequently used to describe release profiles in many biological systems [40], and can be applied for the description of Zn release as follows:

Mt = 1 − exp(−btn) M∞

(7)

where Mt and M∞ denote the total amounts (μg) of Zn transferred at time t (s) and at infinite time, respectively; b denotes the process rate constant (s−1); n is a constant that denotes the shape parameter. Higher b values mean faster rates of initial release. If b = 1, Weibull's model is reduced to first-order kinetics. When b > 1, the release process is governed by a complex mechanism, as indicated by the sigmoid shape of the Weibull function. 2.1.4.5. Statistical analysis. Using MATLAB R2010b software (MathWorks, Natick, MA, USA), sensitivity analysis was performed, parameters were solved with the help of nonlinear regression (nlinfit function), and figures were generated. Statistical significance was indicated at a 95% confidence level by P < 0.05. 3. Results and discussion 3.1. Diffusion of Zn from PLA films The rising content of Zn in the three simulants (10, 20 and 95% ethanol) over time (48 h time interval) at the three temperatures (4, 25 256

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The type of the food simulants significantly affected the release of the Zn. For instance, at 25 °C, the amount of the migrated Zn during the 72 h of storage was 0.06 μg in 10% ethanol, 0.23 μg in 20% ethanol and 0.57 μg in 90% ethanol (Fig. 1). Comparisons of the migration behaviors of the Zn ions in the three simulants showed that the migration value was mainly determined by the polarity of migrating species and the solubility between the film and simulant. Zn release was augmented when the water content of the system fell, which may be due to Zn being slight soluble in ethanol but practically insoluble in water; therefore, the migration values obtained in 10 and 20% ethanol were lower than that of 95% ethanol. Minimum values of Zn release were observed in 10% ethanol (Fig. 1a). In a previous study, Xiao et al. [42] also discussed different types of food simulants that induced different effects on the migration value of nano-selenium particles in nano-selenium packaging materials. Therefore, 95% ethanol had a faster absorption rate than 10% ethanol. Surface desorption, diffusion, ion dissolution, and polymer degradation processes are potential release mechanisms of NPs from polymers. However, some of literature suggest that dissolution of metal nanoparticles into ions is major release mechanism and migration has lower possibility due to larg paticle size in comparision to migration of material in molecular scale [46,47]. The results demonstrated that the Zn was released progressively from the PLA nanocomposite films over 11 days. In general, Zn showed a low tendency to migrate into food simulants. As shown in Fig. 1, the maximum Zn content in the simulants after 360 h was about 1.1 μg, which is far below the National Institute of Health's limit on Zn daily consumption at 40 mg/day [48]. 3.2. Calculation of diffusion coefficient and partition coefficients For each operating condition (three temperatures and three food simulants), diffusion coefficients (D) were calculated from Fig. 2 by using Eq. (1); the obtained values are shown in Table 1. Based on Eq. (1), the values of Mt/M∞ were plotted versus t0.5 during exposure to the simulants, and a strong linear correlation was observed (Fig. 2). The values strongly correspond with experimental data for the three temperatures and simulants shown in Fig. 2. The D value describes Zn migration from the PLA film to a specific stimulant at a specific temperature (Table 1). When storage temperature increased, the D values also increased. For 10% ethanol, D4 °C = 0.002 cm2 s−1and D40 °C = 0.050 cm2 s−1; for 95% ethanol, D4 °C = 0.804 cm2 s−1 and D40 °C = 6.080 cm2 s−1. Sebti et al. [49] and Dole et al. [50] have reported that different factors (including migrant affinity, temperature, and simulant type) affects diffusion. Due to Brownian molecular motion at higher temperatures [51] and gain extra free energy to overcome interaction between Zn and migrate out of the polymer at 40 °C, greater Zn release was expected at with increased temperatures [52,53]. Xiao et al. [42] found that among the four kinds of food simulants (Distilled water, 4% acetic acid, N-hexane, 10% ethanol), with an increase of migration temperature, D values increases in varying degrees, indicating that the higher the temperature is, the larger the diffusion rate. The D values in Table 1 demonstrated that migration into 95% ethanol was fastest at each temperature relative to the other two simulants. The affinity between PLA and the 95% ethanol simulant and low solubility of Zn in water may explain this behavior. Zn is practically insoluble in water [54]. Fernandez et al. [41] reported that the D value for the transport of Ag+ into 95% ethanol at 20 ◦C was 4.5 × 10−18 m2/s. However, the replacement of ethanol by water caused a severe increase in the Ag+ migration capability in the investigated materials. Manzanarez-López et al. [52] found that the release of α-tocopherol and resveratrol from PLA and PLA/starch blends was slower in water than in ethanol. Those authors stated that the plasticization of the PLA film due to the sorption of ethanol could explain their results. Comprehending the binding behavior of an active agent in

Fig. 2. Non-linear regressions of amounts of Mt/Meq versus t0.5: (a) 10% ethanol, (b) 20% ethanol, and (c) 95% ethanol.

polymeric packaging materials and its partitioning between different phases (food/polymer) is highly important for predicting the retention of the active agent by the polymer; a higher portion of migrant in the polymer matrix at equilibrium is indicated by a higher partition coefficient (Kp,s) [55]. Table 1 summarizes the partition coefficient (Kp,s) values of Zn for each simulant and temperature, as obtained from Eq. (3) for the PLA/ZnO films. As seen in Table 1, the Kp,s decreased with a rise in temperature between 4 and 40 °C, indicating that temperature positively affected the release of Zn from PLA/ZnO. The Kp,s for the diffusion process is determined by factors that depend on the nature of the migrant substance, (including its polarity and compatibility with the polymer used as packaging material), and also the solubility of the migrant species in the food simulant [56,57]. Kp,s indicative ratio of active agent in polymer (Cp,∞) to active agent in the simulant (Cs,∞) at equilibrium. In the other words, higher values of Kp,s suggest greater affinity of the active agent with the film material [58]. Molecules with similar polarities exhibit tend to be attracted to each other, leading to a high Kp,s value and a consequently high retention of the active agent in the polymer matrix. Zn is practically insoluble in water and has slight solubility in ethanol [54]. Therefore, at a similar temperature, the obtained Kp,s values were higher for 10% ethanol than for 90% ethanol. For instance, at 25 °C, the Kp,s values were 12.21 μg g−1 in 10% ethanol, 1.97 μg g−1 in 20% ethanol and 0.170 μg g−1 in 90% ethanol (Table 1). Release rates of Zn from the PLA films were higher at 40 °C due to increase of temperature accelerates the molecular thermal motion in the food stimulant. This allows Zn ions to 257

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Table 1 Diffusion coefficients (D, 10−11 cm2 s−1) for transport of Zn from PLA films to simulants, partition coefficients (K, Zn concentrations μg g−1), and mass ratio (α) after sufficient exposure of polymeric films (Cp,∞) and simulants (Cf,∞) for equilibrium to be reached. Food simulant

D

4 °C

α

25 °C

D

0.002 0.200 0.804

21.66 3.920 2.110

D

KP,S

KP,S 10% ethanol 20% ethanol 95% ethanol

α

0.046 0.245 0.473

0.024 1.260 4.070

12.210 1.970 0.170

40 °C

α

KP,S 0.081 0.505 5.803

0.050 1.810 6.080

6.110 1.150 0.057

0.163 0.866 17.309

migrate out of the PLA polymer, resulting in an increased free volume and greater diffusion rate, making it easier for equilibrium to be reached. Comparisons of the behavior of Zn in the three simulants demonstrated that at all three temperature, the Kp,s values of 10 and 20% ethanol were higher than that of the 95% ethanol. Kp,s values obtained from experimental data revealed that Zn had greater affinity for the PLA polymer than for the three simulants in most systems, as Kp,s ≫1 for PLA/ZnO active films, and none of the Zn migrations can be regarded as complete extraction. Xiao et al. [42] reported that the temperature and type of simulant affects the partition coefficient of nano-selenium particles between polyethylene film and simulants. Higher temperatures accelerate the thermal molecular motion of nano-selenium, such that the partition coefficient decreased. Also, among the different simulants, nano-selenium particles have the smallest partition coefficient in Nhexane. Eq. (2) was used to calculate the α values for each operating condition (three temperatures and food simulants), and the obtained values are shown in Table 1. The experimental α values ranged from 0.06 to 21.67. Nevertheless, some interaction of Zn can be assumed in the film since the values of α were not very high. When all the substance contained in the film is released, the value of this parameter tends towards infinite. No mass ratios of Zn in polymers have previously been reported.

degree of migration data was obtained since the correlation coefficient (R2) of each equation was high. In the present study, pseudo-Fickian behavior was observed for Zn diffusion in PLA at all conditions except at 4 and 25 °C in 20% ethanol (Table 2). Non-Fickian transport (anomalous) occurred in 20% ethanol at 4 and 25 °C, with n = 0.86 and 0.63, respectively. Therefore, pseudo-Fickian behavior was the predominant diffusion mechanism. In pseudo-Fickian diffusion, sorption curves resemble Fickian curves, though a greater amount of time is required to reach the final equilibrium [60,61]. In line with our results, similar published data (with n < 0.5) were observed for polymer packaging systems: nano-selenium particles from nano-selenium packaging to four different food simulants [42]; silvers ions from electrospun AgNP–nylon–6/polypropylene film to four different food simulants [62]. Imran et al. [51] assessed the effect of different temperatures (4 and 40 °C) on the release mechanism of nisin from hydroxypropyl methylcellulose, chitosan, sodium caseinate, and PLA films. They found that the mechanism by which the nisin was released from hydroxypropyl methylcellulose and sodium caseinate films was non-Fickian however; the release of nisin from chitosan and PLA films followed from a pseudo-Fickian pattern. Therefore, the diffusion exponent, n, is an important indicator of the diffusion mechanism of Zn between PLA and simulants.

3.3. Power law model

3.4. Diffusion activation energy (Ea)

In order to investigate the migration mechanism, the power law model was applied to the transport behavior. The values of diffusion exponents (n) were calculated by plotting the Log (Mt/M∞) versus Log t data at the initial part of the release curve (Eq. (4)) for the PLA films containing Zn in various simulants and different temperatures (figure not shown). Afterwards, the values of n were obtained from the slopes of straight lines fitted to the data, and the diffusion constant (k) was obtained from the intercept (Table 2). In this regard, the constant k can be considered as the macromolecular arrangement of the polymer matrix [59]. The diffusion exponent value is different in a variety of cases. Pseudo-Fickian behavior (the time to reach the equilibrium is longer than the fickian release) is observed when n is less than 0.5; a value of n = 0.5 is seen in Case I or Fickian diffusion transport (release proportional to the square root of time); anomalous (non-Fickian) transport is the main mechanism when n is between 0.5 and 1.0 (release depends on both solvent transport rate and polymer relaxation rate); solute transport is directly proportional to time when n = 1.0 (Case II); when n is greater than 1.0, transport is such that the solute is released in the later stages (Super Case II) [28]. According to Table 2, a high fitting

Activation energy may be defined, as the energy required a migrant to overcome the cohesive forces of the polymer and produce an opening between the polymer chains for diffusion to occur [39]. In the present experiment, diffusivities of Zn in all studied systems were closely fitted in the Arrhenius equation (Eq. (6)). The activation energy values (Ea) were calculated from slope of plot of log (D) vs 1/T (Fig. 3) and has been presented in Table 3. The Ea values for the migration of Zn was calculated as 66.1 ± 2.40 kJ mol−1 for 10% ethanol, −1 45.4 ± 1.12 kJ mol for 20% ethanol, and 41.68 ± 1.57 kJ mol−1 for 95% ethanol. This means that less energy was needed for the diffusion of Zn from PLA to 95% ethanol than to 10% ethanol, as Zn is practically insoluble in water and has slight solubility in ethanol [54]. The amount Ea may shows the molecular interactions between Zn and the packaging network; higher values of Ea indicate stronger interactions [63]. The Ea values of the Zn in PLA film were lower in 95% ethanol compared to 10 and 20% ethanol. The low Zn diffusivity in PLA films may be the result of such interactions, suggesting that the release of Zn from PLA films to the three simulants was relatively difficult.

Table 2 Parameters of power law model for Zn diffusion in the active PLA film. Food simulant

K

4 °C

R2

25 °C

K

n 10% ethanol 20% ethanol 95% ethanol

0.0112 0.00186 0.0266

0.2393 0.86 0.4519

R2

K

n 0.98 0.99 0.94

0.0125 0.01221 0.119

258

0.3245 0.6347 0.3535

40 °C

R2

n 0.89 0.97 0.96

0.030 0.07597 0.1232

0.2901 0.3619 0.3923

0.90 0.93 0.97

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frequently encountered in release studies. Values of n > 1 have been attributed to the sigmoid shape of the Weibull function, which indicates that a complex mechanism governs the migration process [66]. Based on Table 4, it seems that Zn diffusion in PLA follows a Fickian diffusion mechanism in all conditions; this results from the predicted values of b for the different conditions, which were below 0.75. However, it should be noted that Weibull's model is not able to distinguish between Fickian and pseudo-Fickian behaviors [37]. Therefore, the results of this section have not contradicted with results of power law equation. In the present study, Zn diffusion in PLA followed Fickian behavior in all conditions. It seems that the dissolution process of the relatively insoluble Zn plays an important role in the release kinetics in combination with other release mechanisms. 4. Conclusion

Fig. 3. Linear relationships between Zn diffusion coefficients (lnD) and temperature (1000/T) in three food stimulants (10% ethanol, 20% ethanol, and 95% ethanol).

Particles may be released from nanocomposites during the manufacturing, usage and disposal of these products. In general, safety concerns may arise from the release of Zn ions. For these reasons, an atomic absorption spectrometry method was developed to identify and quantify the release of Zn from PLA/ZnO nanocomposite films into simulants, and effective diffusion parameters were estimated using Fick's diffusion equation. Statistical analysis demonstrated that the factors of temperature, and food simulant type significantly affected the release of the Zn ions. Temperature had a positive effect on the release of Zn ions, such that the amount of Zn released from the PLA/ZnO into simulants increased with a rise in the storage temperature. In the three simulants, the migration of Zn ionsgradually increased with time prior to reaching a steady state. The amount of Zn ions that migrated into 95% (w/v) ethanol was higher compared to 10% ethanol. The different diffusion coefficients for different food simulants could be attributed to the affinity between the polymer, the Zn ions, and the solvent. The findings indicated that PLA/ZnO films have a high retention of Zn. To the best of the authors' knowledge, this is the first time that the entire release profile of Zn ions from PLA/ZnO nanocomposites has been reported and correlated with interactions between the polymer, the Zn, and the solvent. This information should be useful for assessing the risks associated with exposure to Zn, though further assumptions are required to determine release rates into other food simulants and real foodstuffs.

Table 3 Activation energies (Ea, kJ mol−1) for diffusion of Zn from the PLA film to simulants. 10% ethanol

20% ethanol

95% ethanol

66.1 ± 2.40

45.4 ± 1.12

41.68 ± 1.57

Table 4 Weibull model parameters. Food simulant

b (h−1)

4 °C

b (h−1)

n 10% ethanol 20% ethanol 95% ethanol

0.0082 0.0008 0.0183

0.24115 0.9332 0.4722

25 °C

b (h−1)

n 0.0086 0.0070 0.0733

0.3291 0.6627 0.4426

40 °C n

0.0208 0.0440 0.0795

0.2976 0.3961 0.4971

3.5. Weibull's model The Zn release data were also fitted to Weibull's model (Eq. (7)) (figure not shown) and obtained parameters of model (b and n) were presented in Table 4. Higher b values mean faster rates of initial release. For 10% ethanol, as temperature increased from 4 to 40 °C, the experimental b values ranged from 0.008 to 0.29 h−1, and values related to the shape factor (n) ranged from 0.24 to 0.29 (Table 4). Furthermore, in different simulants at 40 °C, the value of b ranged from 0.020 to 0.079 h−1, and the value of Weibull's shape parameter (n) ranged from 0.29 to 0.49. The results demonstrated that Weibull's shape parameter (n) was less than 1 for all simulants and temperatures, indicating a curve with an upward concavity. Bastarrachea et al. [64] investigated the nisin realease kinetics from poly (butylene adipate-co-terephthalate) films. As the temperature increased from 5.6 to 40 °C, the values of b and n increased from 0.02 to 0.98 and from 0.28 to 0.45, respectively. In another study, Mateus et al. [65] modeled the release kinetics of volatile organic compounds from roasted and ground coffee using Weibull's approach. They found that the trend exhibited both upward and downward concavities depending on the stripping conditions, which suggests that the changes in substance behavior may depend on the testing conditions and the medium in which the substance is released. The Weibull model has been predominantly applied for the characterization of changes in food quality parameters and microbial inactivation [37]. Values of b below 0.75 have been attributed to the “Fickian diffusion process” or to “significant external resistance”. For Fickian diffusion, the increase of b reflects the decrease of the disorderliness of the medium. This is while b values between 0.75 and 1.0 indicate a combined mechanism (Fickian and case II transport) that is

Conflicts of interest The authors declare that there are no conflicts of interest. References [1] Z. Javidi, S.F. Hosseini, M. Rezaei, Development of flexible bactericidal films based on poly (lactic acid) and essential oil and its effectiveness to reduce microbial growth of refrigerated rainbow trout, LWT – Food Sci. Tech. 72 (2016) 251–260. [2] H. Zhang, S. Wang, Sh Zhang, R. Ma, Y. Wang, W. Cao, Ch Liu, Ch Shen, Crystallization behavior of poly(lactic acid) with a selfassemblyaryl amide nucleating agent probed by real-time infrared spectroscopy and X-ray diffraction, Polym. Test. 64 (2017) 12–19. [3] F. Sebastien, G. Stephane, A. Copinet, V. Coma, Novel boidegradable films made from chitosan and poly(lactic acid) with antifungal properties against mycotoxinogen strains, Carbohydr. Polym. 65 (2006) 185–193. [4] A. Finne, A.C. Albertsson, Controlled synthesis of star-shaped L-lactide polymers using new diprocyclic tin initiators, Biomacromolecules 3 (2002) 684–690. [5] Z.H. Tang, X.C. Chen, X. Pang, Strontiumbased initiator system for ring-opening polymerization of cyclic esters, J. Polym. Sci. A Polym. Chem. 41 (2003) 1934–1941. [6] Z.H. Tang, X.C. Chen, X. Pang, Stereoselective polymerization of rac-lactide using a monoethylaluminum Schiff base complex, Biomacromolecules 5 (2004) 965–970. [7] M. Jamshidian, E.A. Tehrany, M. Imran, M. Jacquot, S. Desobry, Polylactic acid: production, applications, nanocomposites, and release studies, Compr. Rev. Food Sci. Food Saf. 9 (2010) 522–571. [8] I. Armentano, E. Fortunati, N. Burgos, F. Dominici, F. Luzi, S. Fiori, A. Jimenez, K. Yoon, J. Ahn, S. Kang, J.-M. Kenny, Bio-based PLA_PHB plasticized blend films: processing and structural characterization, LWT - Food Sci. Tech. 64 (2015) 980–988. [9] E. Mascheroni, V. Guillard, F. Nalin, L. Mora, L. Piergiovanni, Diffusivity of propolis

259

Polymer Testing 76 (2019) 254–260

M. Heydari-Majd, et al.

[10] [11]

[12]

[13]

[14]

[15]

[16]

[17] [18]

[19]

[20]

[21]

[22] [23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31] [32]

[33]

[34]

[35] [36]

films into a fatty food simulant, Food Res. Int. 54 (2013) 873–880. [37] M.A.J.S. Van Boekel, Kinetic modeling of food quality: a critical review, Compr. Rev. Food Sci. Food Saf. 7 (2008) 144–158. [38] J.A. Garde, R. Catala, R. Gavara, R.J. Hernandez, Characterizing the migration of antioxidants from polypropylene into fatty food simulants, Food Addit. Contam. 18 (2001) 750–762. [39] W. Limm, C. Hollifield, Modelling of additive diffusion in polyolefins, Food Addit. Contam. 13 (1996) 949–967. [40] M.J.S. Varma, A.M. Kaushal, S. Garg, Influence of microenvironmental pH on the gel layer behavior and release of basic drug from various hydrophilic matrices, J. Control. Release 103 (2005) 499–510. [41] A. Fernandez, E. Soriano, P. Hernandez-Munoz, R. Gavara, Migration of antimicrobial silver from composites of polylactide with silver zeolites, J. Food Sci. 75 (2010) 3. [42] X. Xiao, X. Zhang, S. Yang, C. Zhao, Study on migration of nano-selenium particles of nano-selenium packaging marerials in food simulants, Dig. J. Nanomater. Biostruct. 13 (2018) 427–437. [43] G. Artiaga, K. Ramos, L. Ramos, C. Cámara, M. Gómez-Gómez, Migration and characterisation of nanosilver from food containers by AF4-ICP-MS, Food Chem. 166 (2015) 76–85. [44] H. Song, B. Li, Q.-B. Lin, H.-J. Wu, Y. Chen, Migration of silver from nanosilverpolyethylene composite packaging into food simulants, Food Addit. Contam. 28 (2011) 1758–1762. [45] Q.-B. Lin, H. Li, H.-N. Zhong, Q. Zhao, D.-H. Xiao, Z.-W. Wang, Migration of Ti from nano-TiO2-polyethylene composite packaging into food simulants, Food Addit. Contam. A 31 (2014) 1284–1290. [46] M. Jokar, R. Abdul Rahman, Study of silver ion migration from melt-blended and layered 27 deposited silver polyethylene nanocomposite into food simulants and apple juice, Food Addit. Contam. A 31 (2014) 734–742. [47] S.-A. Ntim, T.-A. Thomas, T.-H. Begley, G.-O. Noonan, Characterisation and potential migration of silver nanoparticles from commercially available polymeric food contact materials, Food Addit. Contam. A 32 (2015) 1003–1011. [48] https://ods.od.nih.gov/factsheets/Zinc-HealthProfessional. [49] I. Sebti, D. Blanc, A. Carnet-Ripoche, R.E. Saurel, V.E. Coma, Experimental study and modeling of nisin diffusion in agarose gels, J. Food Eng. 63 (2004) 185–190. [50] P. Dole, A.E. Feigenbaum, C.D.L. Cruz, S. Pastorelli, P. Paseiro, T. Hankemeier, C. Papaspyrides, Typical diffusion behavior in packaging polymers-application to functional barriers, Food Addit. Contam. 23 (2006) 202–211. [51] M. Imran, A. Klouj, A.-M. Revol-Junelles, S. Desobry, Controlled release of nisin from HPMC, sodium caseinate, poly-lactic acid and chitosan for active packaging applications, J. Food Eng. 143 (2014) 178–185. [52] F. Manzanarez-López, H. Soto-Valdez, R. Auras, E. Peralta, Release of [alpha]-tocopherol from poly(lactic acid) films, and its effect on the oxidative stability of soybean oil, J. Food Eng. 104 (2011) 508–517. [53] H. Soto-Valdez, R. Auras, E. Peralta, Fabrication of poly(lactic acid) films with resveratrol and the diffusion of resveratrol into ethanol, J. Appl. Polym. Sci. 121 (2011) 970–978. [54] EFSA Panel on Food Contact Materials, Enzymes, Flavourings and Processing Aids (CEF).Scientific Opinion on the safety evaluation of the substance zinc oxide, nanoparticles, uncoated and coated with [3-(methacryloxy)propyl] trimethoxysilane, for use in food contact materials, EFSA J. 13 (4) (2015) 4063. [55] X. Yining, R. Maria, A. Rafael, Modeling of surfactant release from polymer-clay nanocomposites into ethanol, Polym. Test. 50 (2016) 57–63. [56] E.A. Tehrany, S. Desobry, Partition coefficients in food/packaging systems: a review, Food Addit. Contam. 21 (2004) 1186–1202. [57] R. Franz, A. Störmer, Migration of plastic constituents, Plast Packag, Weinheim, Germany, Wiley-VCH Verlag GmbH & Co. KGaA, 2008, pp. 349–415. [58] D. Chung, S.E. Papadakis, K.L. Yam, Release of propyl paraben from a polymer coating into water and food simulating solvents for antimicrobial packaging applications, J. Food Process. Preserv. 25 (2000) 71–87. [59] C.M.P. Yoshida, C.E.N. Bastos, T.T. Franco, Modeling of potassium sorbate diffusion through chitosan films, LWT - Food Sci. Technol. (Lebensmittel-Wissenschaft -Technol.) 43 (2010) 584–589. [60] K. Lee, K. Cho, I. Park, B.H. Lee, D. Rana, S. Choe, Migration phenomena of surfactants in polyethylene film, J. Polym. Sci., Part B: Polym. Phys. 37 (1999) 1387–1395. [61] R.S. Khan, D. Rousseau, Hazelnut oil migration in dark chocolate–kinetic, thermodynamic and structural considerations, Eur. J. Lipid Sci. Technol. 108 (2006) 434–443. [62] T.-H. Cheng, Sh-B. Lin, L-Ch Chen, H.-H. Chen, Studies of the antimicrobial ability and silver ions migration from silver nitrate-incorporated electrospun nylon nanofibers, Food Packag. Shelf Life 16 (2018) 129–137. [63] A. Teerakarn, D.E. Hirt, J. Acton, J.R. Rieck, P. Dawson, Nisin diffusion in protein films: effects of film type and temperature, J. Food Sci. 67 (2002) 3019–3025. [64] L. Bastarrachea, S. Dhawan, Sh-S. Sablani, J. Powers, Release kinetics of nisin from biodegradable poly(butylene adipate-co-terephthalate) films into water, J. Food Eng. 100 (2010) 93–101. [65] M. Mateus, C. Lindinger, J.C. Gumy, R. Liardon, Release kinetics of volatile organic compounds from roasted and ground coffee: online measurements by PTR-MS and mathematical modeling, J. Agric. Food Chem. 55 (2007) 10117–10128. [66] A. Bunde, S. Havlin, R. Nossal, H.E. Stanley, H. Weiss, On controlled diffusionlimited drug release from leaky matrix, J. Chem. Phys. 83 (1985) 5909–5913.

compounds in Polylactic acid polymer for the development of anti-microbial packaging films, J. Food Eng. 98 (2010) 294–301. L-zh Liu, H-j. Ma, X.-S. Zh, Preparation and properties of polylactide/nano-silica in situ composites, Pigment Resin Technol. 39 (2010) 27–31. H. Zhou, T.J. Shi, H.L. Wang, L.F. Zhai, J.Z. Wang, Gaofenzi Cailiao Kexue yu, Preparation and characterization of polylactide/SiO2 organic-inorganic hybrid material, Polym. Mater. Sci. Eng. 22 (2006) 226. M. Heydari-Mjd, B. Ghanbarzadeh, M. Shahidi-Noghabi, M.-A. Najafi, M. Hosseini, A new active nanocomposite film based on PLA/ZnO nanoparticle/essential oils for the preservation of refrigerated Otolithes ruber fillets, Food Packag. Shelf Life 19 (2019) 94–103. E. Hoseinzadeh, P. Makhdoumi, P. Taha, H. Hossini, J. Stelling, M. Amjad Kamal, A review on nano-antimicrobials: metal nanoparticles, methods and mechanisms, Curr. Drug Met. 18 (2017) 120–128. I. Shahabi-Ghahfarrokhi, F. Khodaiyan, M. Mousavi, H. Yousefi, Preparation of UVprotective kefiran/nano-ZnO nanocomposites: physical and mechanical properties, Int. J. Biol. Macromol. 72 (2015) 41–46. S. Shankar, X. Teng, G. Li, J.W. Rhim, Preparation, characterization, and antimicrobial activity of gelatin/ZnO nanocomposite films, Food Hydrocolloids 45 (2015) 264–271. Y. Xie, Y. He, P.L. Irwin, T. Jin, X. Shi, Antibacterial activity and mechanism of action of zinc oxide nanoparticles against Campylobacter jejuni, Appl. Environ. Microbiol. 77 (2011) 2325–2331. I.S. Elashmawi, N.A. Hakeem, L.K. Marei, F.F. Hanna, Structure and performance of ZnO/PVC nanocomposites, Physica B 405 (2010) 4163–4169. A. Emamifar, M. Kadivar, M. Shahedi, S. Soleimanian-Zad, Evaluation of nanocomposite packaging containing Ag and ZnO on shelf life of fresh orange juice, Innov. Food Sci. Emerg. Technol. 11 (2010) 742–748. L.H. Li, J.C. Deng, H.R. Deng, Z.L. Liu, X.L. Li, Preparation, characterization and antimicrobial activities of chitosan/Ag/ZnO blend films, Chem. Eng. J. 160 (2010) 378–382. D.S. Vicentini, A. Smania, M.C.M. Laranjeira, Chitosan/poly (vinyl alcohol) films containing ZnO nanoparticles and plasticizers, Mater. Sci. Eng. C 30 (2010) 503–508. A.R. Badireddy, M.R. Wiesner, J. Liu, Detection, characterization, and abundance of engineered nanoparticles in complex waters by hyperspectral imagery with enhance darkfield microscopy, Environ. Sci. Technol. 46 (2012) 10081–10088. G.V. Lowry, K.B. Gregory, S.C. Apte, J.R. Lead, Transformations of nanomaterials in the environment, Environ. Sci. Technol. 46 (2012) 6893–6899. Victor-G-L. Souza, A.-L. Fernando, Nanoparticles in food packaging: biodegradability and potential migration to food-A review, Food Packag. Shelf Life 8 (2016) 63–70. National Research Council, A Research Strategy for Environmental, Health, and Safety Aspects of Engineered Nanomaterials, [Internet] National Academy Press, Washington (DC), 2012 [cited 2012 Jan 25]. Available from http://www.nap.edu/ catalog.php?record_id=13347. P. Simon, Q. Chaudhry, D.J. Bakos, Migration of nanosized layered double hydroxide platelets from polylactide nanocomposite films, Food Nutr. Res. 47 (2008) 105–113. Y. Cao, T.-T. Chen, W. Wang, M. Chena, H.-J. Wang, Construction and functional assessment of zein thin film incorporating spindle-like ZnO crystals, Royal Soc. Chem. 7 (2017) 2180–2185. O.Y. Popadyuk, M.V. Melnyk, Experimental study of releasing nano zinc oxide from wound healing biodegradable polymer film “Biodep-Nano, Archiv. Clin. Med. 22 (2016) 1. M.-L. Kaariainen, C.-K. Weiss, S. Ritz, S. Putz, D.C. Camerona, V. Mailanderb, K. Landfester, Zinc release from atomic layer deposited zinc oxide thin films and itsantibacterial effect on Escherichia coli, Appl. Surf. Sci. 287 (2013) 375–380. J. Bott, A. Störmer, R. Franz, Migration of nanoparticles from plastic packaging materials containing carbon black into foodstuffs, Food Addit. Contam. A 31 (2014) 1769–1782. M. Jokar, G.-A. Pedersen, K. Loeschner, Six open questions about the migration of engineered nano-objects from polymer-based food-contact materials: a review, Food Addit. Contam. A 34 (2017) 434–450. ASTM, Standard D 4754-98. Annual Book of ASTM Standards vols 08–02, ASTM, Conshohocken, PA, 2006. Commission Directive, Commission Directive 2007/19/EC of 2 April 2007 Amending Directive 2002/72/EC Relating to Plastic Materials and Articles Intended to Come into Contact with Food and Council Directive 85/572/EEC Laying Down the List of Simulants to Be Used for Testing Migration of Constituents of Plastic Materials and Articles Intended to Come into Contact with Foodstuffs, (2007). K. Bhunia, Sh-S. Sablani, J. Tang, B. Rasco, Migration of chemical compounds from packaging polymers during microwave, conventional heat treatment, and storage, Compr. Rev. Food Sci. Food Saf. 12 (2013) 523–545. A. Marra, A. Boumail, S. Cimmino, P. Criado, C. Silvestre, M. Lacroix, Effect of PLA/ ZnO packaging and gamma radiation on the content of Listeria innocua, Escherichia coli and Salmonella enterica on ham during storage at 4 °C, J. Food Sci. Eng. 6 (2016) 245–259. J. Crank, The Mathematics of Diffusion, Oxford University Press, Oxford, U.K., 1975. C. Colín-Chávez, H. Soto-Valdez, E. Peralta, J. Lizardi-Mendoza, R. BalandránQuintana, Diffusion of natural astaxanthin from polyethylene active packaging

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