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Food Packaging and Shelf Life journal homepage: www.elsevier.com/locate/fpsl

Migration of metals from ceramic food contact materials. 2: Migration kinetics under various conditions and the influence of conventional thermal heating and microwave heating on migration 1

Yan Li

Department of Geosciences, Environment and Society, Université Libre de Bruxelles, Brussels, Belgium

A RT IC L E IN FO

A B S T RA C T

Keywords: Food contact materials Ceramic ware Migration kinetics Kinetics modelling Cooking conditions

The release of metals from ceramic ware has raised public concerns, especially in the case of lead. The Directive 84/500/EEC monitors the compliance of ceramic food contact articles within the European Union. It sets the migration limits of lead and cadmium, and the corresponding testing conditions. The present research was aimed at investigating the migration kinetics of different metals under a range of testing conditions beyond those stated in the Directive and at evaluating the protectiveness of the Directive testing conditions. This research analyzed the release of metals from ceramic articles under a series of temperatures for a variety of contact times with 4% acetic acid and 5 g/L citric acid. It compared the release of metals under the Directive conditions and under cooking conditions. Overall, the amounts of metals increased steeply with time at the beginning of the migration test, after which the growth was less pronounced and arrived at a plateau. A square root dependence between the amount released and migration time was evidenced, providing an appropriate estimate of migration at low temperatures and short times. The amounts of metals released under the cooking conditions significantly exceeded those released under the Directive testing conditions, indicating that the current Directive testing conditions may not be adequately protective. Testing conditions can be specified on the basis of the use of ceramic ware, and a migration test under cooking conditions is probably required for ceramic cookware.

1. Introduction The release of toxic metals occurs, when ceramic articles including cooking ware, eating utensils and serving ware come into contact with foodstuffs (Sheets, 1998; Belgaied, 2003; Valadez-Vega et al., 2011). The release of metals, particularly of lead, from ceramic food ware has resulted in significant public health concerns. Several cases of lead poisoning have been reported due to the use of ceramic articles for food preparation and storage (Manor & Freundlich, 1983; Matte, Proops, Palazuelos, Graef, & Avila, 1994; Norwegian Scientific Committee for Food Safety, 2004). The use of lead-glazed ceramic ware has increased the level of blood lead. (Avila, Romieu, Ríos, Rivero, & Palazuelos, 1991; Rojas‐López, Santos‐Burgoa, Ríos, Hernández‐Avila, & Romieu, 1994; Azcona-Cruz, Rothenberg, Schnaas, Zamora-Muñoz, & RomeroPlaceres, 2000). As a cumulative toxicant, lead has multiple effects on human body including neurological, cardiovascular, renal, gastrointestinal and hematological effects (World Health Organization, 2010). Children are exceptionally vulnerable to the neurotoxicity of lead, and even relatively low levels of exposure can impair the neurological

1

development (Fewtrell, Kaufmann, & Prüss-Üstün, 2003; International Programme on Chemical Safety, 1995). The safety of ceramics used for food preparation and storage is monitored by most countries. Legislations with respect to ceramic articles are implemented at both the European and worldwide levels. At the European level, the Directive 84/500/EC sets the requirements for ceramic articles intended to come into contact foodstuffs. It regulates the permissible limits of lead (Pb) and cadmium (Cd), and the testing conditions where ceramic articles are filled with 4% acetic acid at room temperature (22 ± 2 °C) for 24 h (Council of the European Communities, 1984). The release of metals to foodstuffs is defined as “the unintentional transfer to food of metal ions from food contact materials and articles” (Council of Europe, 2013). As migration, which is defined as “the mass transfer from an external source into food by sub-microscopic processes” (Castle, 2007), has been commonly used in the Directive 84/ 500/EEC and literature, the term “migration” and “release” are used interchangeably in this article. The release of metals from ceramics is driven by ion exchange and hydrolysis (Hench, Clark, & Yen-Bower,

E-mail address: [email protected]. Current address: Sichuan Institute for Food and Drug Control, No. 8 Xinwen Rd, Chengdu, 610097, China.

https://doi.org/10.1016/j.fpsl.2020.100494 Received 19 February 2019; Received in revised form 11 February 2020; Accepted 15 February 2020

1980; Clark & Zoitos, 1992). Ion exchange reaction dominates the process at the start of migration; as the migration proceeds, hydrolysis gradually becomes the active process (Clark & Zoitos, 1992; El-Batal et al., 2010; Cailleteau et al., 2008). Crank (1975) has established a simplified diffusion model, which reflects the linear relationship between the amounts of released metals and the square root of contact time, to characterize the beginning of the migration process (Crank, 1975). A number of studies have investigated the release of metals from ceramic articles under carefully controlled conditions. The research demonstrate that the release of metals increases over the rising contact time and temperature, and declines over the rising pH level of food simulants (Beldì, Jakubowska, Peltzer, & Simoneau, 2016; Bolle et al., 2012; Demont, Boutakhrit, Fekete, Bolle, & Van Loco, 2012; Dong, Lu, Liu, Tang, & Wang, 2014; Cakste, Kuka, & Kuka, 2017). Gould, Butler, Boyer, and Steele (1983) and Bolle et al. (2012) reported a linear relationship between the amount of Pb and contact time. In contrast, Dong et al. (2014) established a square root of contact time dependence between the amounts of released metals and the migration time. The release of metals decreased over repeated-use of ceramic ware, and successive migration tests have been suggested to mimic the repeateduse of ceramic ware (Beldì et al., 2016). Performing successive migration tests under the Directive conditions takes long duration. It would be preferable to perform migration tests for a relatively short time e.g. 2 h, and to predict the release of metals for 24 h by a kinetics model. Therefore, the kinetics of metal release is of particular interest. Ceramic articles can be filled with hot liquids such as tea, coffee and soup or be heated by ovens or microwave ovens during the process of food preparation and serving. Although the contact duration of heating in the process of cooking is much shorter than the Directive contact duration, the temperature can be very high. Zhou et al. (2017) showed that the migration of Pb increased over 10-fold under the cooking conditions (boiled at 100 °C for 2 h) comparing with the migration under the normal conditions (soaked at 25 °C for 2 h). Çiftçi and Henden (2016) reported the concentration of arsenic in cooked white bean dish exceeding the concentration of arsenic in 4% acetic acid under the ASTM C 738 standard conditions (25 °C 24 h) by 400-fold. Despite the difference between cooking conditions and the Directive conditions, few studies have investigated the migration under cooking conditions against the migration under the Directive conditions. Whether the testing conditions in the Directive sufficiently represent the real use conditions of ceramic articles is a critical issue since it determines the protectiveness of the Directive. This research analyzed the release of metals (Li, Al, Co, Zn and Ba) from ceramic articles under a series of temperatures for a variety of contact time with 4% acetic acid (AA) and 5 g/L citric acid (CA). It further studied the release of metals under cooking conditions via conventional thermal heating and microwave heating. The objectives of this study are (a) to qualitatively and quantitatively describe the migration kinetics of ceramic ware; (b) to investigate the influence of temperature, simulant and heating manner on migration kinetics; (c) to assess the migration under the cooking conditions against the migration under the Directive conditions.

2. Materials and methods All reagents were prepared with analytical grade chemicals. Ultrapure water (18 MΩ cm) was obtained from Milli-Q water purification system (Millipore, Milford, MA, USA). The RBS concentrate (VWR-Normatom) was diluted with tap water (about 40 °C) into 2% (v/ v) RBS solutions. Ultrapure Nitric acid (60 % v/v, Merck) was used to prepare intermediate solutions. The simulant 4% (v/v) acetic acid (AA) was prepared by adding 40 mL AA (99 % for trace metal analysis; VWRNormatom) into 500 mL Milli-Q water and filling up to 1000 mL with Milli-Q water. The simulant 5 g/L citric acid (CA) was obtained by weighing 5 g CA (99.5–100 %; Merck) and dissolving in 1000 mL MilliQ water. The pH-values of simulants were measured by pH-meter (Thermo Scientific Orion Star A211). The measurements of elements were carried out with an inductively coupled plasma optical emission spectrometer (ICP-OES) (PerkinElmer, Optima 8300, USA). The solutions were pumped by a peristaltic pump from tubes arranged on an auto-sampler (S10, Perkin-Elmer, USA). The analyzed metals included Lithium (Li), Beryllium (Be), Aluminum (Al), Vanadium (V), Chromium (Cr), Manganese (Mn), Iron (Fe), Cobalt (Co), Nickel (Ni), Copper (Cu), Zinc (Zn), Molybdenum (Mo), Cd, Antimony (Sb), Barium (Ba), Pb, Silver (Ag) and Tin (Sn). More details on instrumental conditions and method parameters (limit of detection, calibration curve, repeatability, reproducibility and recovery) refer to the first study “Migration of metals from ceramic food contact materials: 1: Effects of pH, temperature, food simulant, contact duration and repeated use (unpublished)” by the author. 2.1. Sample selection and the migration test under the Directive 84/500/ EEC conditions Samples used in the first migration study were not sufficiently available on the market; therefore, new types of samples were applied in this study. Taking new types of articles can also verify the observations from the first study. Three types of glazed articles with decorations were purchased from the Belgian market. The sample ID was Kafel, Ankara and Moana, respectively. All the samples have over-glaze decorations: Kafel is decorated with multiple colors, Ankara is red colored, Moana is grey colored, as shown in Fig. 1. The test was performed on three replicates of selected samples applied time and temperature conditions set by the Directive 84/500/ EEC. Replicates were washed with detergent and were rinsed with MilliQ water. Being dried, specimens were filled with 4% AA at room temperature (22 ± 2 °C) for 24 h, as shown in Table 1. 2.2. Kinetics under hot filling conditions After the migration test under the Directive conditions, samples displaying a release of a variety of metals at quantifiable levels would be subsequently selected as testing samples for further study. Sample Kafel, releasing multiple elements, was selected in this series of tests. Three replicates were respectively filled with 4% AA or 5 g/L CA. The pH level of 4% AA and 5 g/L CA is identical (measured by pH meter). 4% AA is used in the regulatory migration test for ceramic food contact articles whereas 5 g/L CA is proposed by the Council of Europe as simulant for metals and alloys. The replicates were kept in the oven at Fig. 1. Three types of glazed samples purchased from the Belgian market.

Table 1 Samples and experimental conditions for different tests. Migration tests

Samples & simulant volume (mL)

Directive conditions

Simulants

Conditions

Time intervals

Kafel (490), Ankara (480), Moana (510) Kinetics under hot filling conditions Kafel (490)

4% AA

Kinetics under cooking conditions

Kafel (490)

4% AA, 5 g/L CA 220 °C 4% AA, 5 g/L CA 640 W

3, 5, 7, 10, 15, 20, 25, 30 min 1, 3, 5, 7, 10, 12, 15, 17, 20 min

Migration under cooking conditions

Kafel (490)

4% AA, 5 g/L CA 180 °C 4% AA, 5 g/L CA 220 °C 4% AA, 5 g/L CA 400 W 4% AA, 5 g/L CA 640 W

120 min 60 min 60 min 30 min

4% AA, 5 g/L CA

five different temperatures, i.e., 20 °C, 40 °C, 60 °C, 80 °C and 95 °C. Before filling, the simulants were heated to the desired temperatures and the samples were preheated at the corresponding testing temperatures in the oven for 1 h. Samples were covered with a watch glass during the migration test. Evaporation at higher temperatures was observed after 120 min time interval even with a cover glass. The evaporation at various time intervals was monitored and the concentration of metals was corrected. Aliquots of 5 mL were taken from individual piece at 13 different intervals (5, 10, 15, 20, 25, 30, 60, 90, 120, 180, 240, 360, 480 min). Fresh simulant of 5 mL (4% AA or 5 g/L CA) was added to ceramic articles immediately after the aliquot was taken for analyses. The addition of fresh simulant gave rise to dilution, which was corrected by the following equation:

Cj = Cn +

22 °C

Vt Cj −1 Vl

where Cj is the corrected concentration, Cn is the measured value, Vl is the volume of simulant, Vt is the volume of taken leachate. The testing procedure is listed in Table 1. 2.3. Kinetics under cooking conditions (conventional heating & microwave heating) The kinetics under cooking conditions is investigated by measuring the levels of migrating metals from ceramic articles under the hightemperature conditions via conventional thermal heating and microwave heating. Due to no universal procedure existing for high-temperature migration tests, the literature which have studied the effects of high-temperature heating on cooking ware and the recipes of cooking using ovens and/or microwave ovens were reviewed to determine the high-temperature migration conditions. In this test, a temperature of 220 °C for ovens and a power of 640 W for microwave ovens were used since they could represent the very harsh cooking conditions, being more severe than the conditions in most studies (Bhunia, Sablani, Tang, & Rasco, 2013; Startin et al., 1987; Badeka & Kontominas, 1996, 1999; Castle, Jickells, Gilbert, & Harrison, 1990; Kontominas, Goulas, Badeka, & Nerantzaki, 2006; Poovarodom, Junsrisuriyawong, Sangmahamad, & Tangmongkollert, 2014). Three identical pieces of sample Kafel that released Li, Al, Co, Zn and Ba were placed in the oven, before which the oven was heated to 220 °C, then filled with either 4% AA or 5 g/L CA and covered with a watch glass. An aliquot of 5 mL was taken from the individual piece at 8 different intervals (3, 5, 7, 10, 15, 20, 25 and 30 min). The following steps were the same as those of the kinetics under hot filling conditions. Another three replicates were placed in the microwave oven and were filled with either 4% AA or 5 g/L CA and then covered with a watch glass. The specimens were heated at a power of 640 W, after which an aliquot of 5 mL was taken from specimens at 9 time intervals (1, 3, 5, 7, 10, 12, 15, 17 and 20 min). The subsequent steps followed

24 h

20, 40, 60, 80, 95 °C

5, 10, 15, 20, 25, 30, 60, 90, 120, 180, 240, 360, 480 min

the steps of the kinetics under hot filling conditions as well, as shown in Table 1. 2.4. Migration under cooking conditions The migration under cooking conditions was explored through thermal and microwave treatment, which represents the frequently used cooking methods in real life. Two series of tests including the medium-harsh heating and harsh heating were performed. For the medium-harsh series, three identical pieces of the selected sample (Kafel) were heated with the oven at 180 °C for 120 min (boiling of the simulant occurred approx. at 70 min) or with the microwave oven at 400 W for 60 min (boiling of the simulant occurred approx. at 28 min). For the harsh series, three replicates were heated with the oven at 220 °C for 60 min (boiling of the simulant occurred approx. at 42 min) or with the microwave oven at 640 W for 30 min (boiling of the simulant occurred approx. at 19 min). The testing conditions are shown in Table 1. 3. Results and discussion 3.1. Migration test under the Directive conditions and sample selection All the three types of sample released a variety of elements, as shown in Table 2. Apart from Li, Al and Zn, Kafel released Co and Ba, Ankara released Mn, Fe, Cd and Pb, and Moana released Ba. The levels of Pb and Cd from Ankara were below the Directive permissible limits (0.8 mg/dm² for Pb and 0.07 mg/dm² for Cd), but exceeding the levels of discussion starting values (DSVs) (2.0 μg/dm² for Pb and 1.0 μg/dm² for Cd) proposed by the European Commission Directorate-General Health and Food Safety (DG SANCO) (European Commission, 2017). In support of lowering the permissible limits of Pb and Cd, based on the initial risk management, the DG SANCO provides the DSVs as the target Table 2 Concentration (mean and standard deviation) of elements released from the purchased samples. Elements

Li Al Mn Fe Co Zn Cd Ba Pb

-: Below LOQ.

Kafel

Ankara

Moana

μg/dm²

μg/dm²

μg/dm²

0.53 ± 0.03 2.6 ± 0.5 – – 6.7 ± 2.5 1.7 ± 0.2 – 0.64 ± 0.07 –

0.73 ± 0.01 11.4 ± 0.6 0.65 ± 0.01 0.63 ± 0.05 – 1.3 ± 0.4 2.4 ± 0.5 – 13.2 ± 0.3

0.81 ± 0.07 3.10 ± 0.02 – – – 3.1 ± 0.2 – 1.78 ± 0.04 –

Fig. 2. a: Migration (mean and standard deviation error bar) of Co until 480 min at 20 °C, 40 °C, 60 °C, 80 °C and 95 °C with 4% AA; b: Migration (mean and standard deviation error bar) of Co until 480 min at 20 °C, 40 °C, 60 °C, 80 °C and 95 °C with 5 g/L CA. The black line represents the amount of Co under the Directive conditions.

values for verifying the analytical method at lower levels rather than a proposal of regulatory limits. The migration of Pb and Cd from ceramic articles has been extensively studied; this study focuses on the kinetics of other elements. The first migration study (Effects of pH, temperature, food simulant, contact duration and repeated-use) has found that Co, Al, Zn and Ba can be released at high levels; the migration behavior of these metals is thus of interest. Kafel released multiple elements, particularly Co and Ba, at quantifiable levels, and thereby was selected as the target sample for further migration tests.

3.2. Kinetics at a series of temperatures (hot filling conditions) The quantities of metals released at 20 °C, 40 °C, 60 °C, 80 °C and 95 °C were plotted as a function of time. The release of, for example, Co at five temperatures in 4% AA and 5 g/L CA solutions as a function of time is reported in Fig. 2. The profiles of 20 °C, 40 °C, 60 °C and 80 °C were similar: the amount of Co grew markedly at the beginning of migration, after which the growth declined and became marginal (plateau) at the end of the measured time. The kinetic curve of 95 °C, however, appeared different from those at relatively low temperatures. In contrast to the marginal growth from 180 min to 480 min, the amount of Co increased continuously which probably results from network dissolution. The release of Li, Al, Zn and Ba generally complied with the kinetics of Co, which has been presented in Table S1a-1 j. The migration rate of Co at 20 °C, 40 °C, 60 °C, 80 °C and 95 °C was calculated, as shown in Table 3. The migration rate of Co decreased along with contact time. By the continuance of reaction, it declined to a relatively stable value, for example, at respectively 360 min, 240 min and 180 min for the migration tests performed at 20 °C, 40 °C and 60 °C in AA solutions. The higher the temperature, the faster the migration rate declined. The contact time for the stable reaction rate corresponds

to the time when the release of Co arrives at the plateau in Fig. 2. The migr...