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Abstract: Lead (Pb) release from non-conforming flatwares and artisanal hollowwares was measured by ICP-AES. Influence of acidic beverages (tomato juice, vinegar and lemon juice), food simulants (acetic acid, citric acid, and malic acid) at pH (2.37-5.00), contact time (t) (30 min-2 days), temperature (T) (20 °C-90 °C), and glaze were investigated. Pb release was lowest in tomato juice and highest in lemon juice (t = 24 h; T = 22 °C). In acetic acid 4% (v/v) Pb release was 14% less than in lemon juice. Variability between the flatwares used for experiments in lemon juice and acetic acid 4% (v/v) corresponds to 3.48% and 7.03% respectively. According to the set of experiments where three food simulants were compared, it appeared that acetic acid, citric acid, and malic acid have the same leaching capacity above pH 3, but at pH < 3, citric acid appears to be the strongest extractant. Moreover, the influence of the applied ceramic glaze played an important role in lead release, in certain cases doubling the extracted amount of Pb. It has also been demonstrated that migration kinetics in citric acid is stronger. At 90 °C, after 2 h, the amount of extracted lead is 18% more important than at 20 °C after 24 h in acetic acid 4% (v/v) which may question the capacity of consumer protection of ED 84/500/EEC.[4, 5]. Food and beverage contamination by lead occurs either in a direct way or by contact with packaging, cooking, serving, and/or storing materials. In this study, we were interested in lead migration into food in contact with glazed ceramicwares. Despite studies showing that producing high quality ceramicware without lead is feasible [6], lead oxide(PbO) is still used in the glaze structure. In fact, its excellent properties and wide processing latitude in the glaze structure make it an ideal glaze component over a wide range of ceramicware compositions and firing ranges [7]. When properly processed, the amount of lead oxide extracted by food substances is extremely low, below the established limits set by European Directive (ED) 84/500/EEC, FDA (Food
Table 4). Three food simulants were put in contact with the hollowwares: acetic acid, citric acid, and malic acid, all of them present in important quantities in food and food products. It is recognized that glaze is the main source of Pb migration [25], thus it was straightforward to examine the influence of different, commercially available glazes listed in Table 4 in combination with different acids at different pH. On Figs. 1(a)-(i), Pb release from artisanal, glazed hollowwares in function of pH is represented. The data points are connected with dotted lines for better visibility. Two conclusions can be drawn. First, concerning the glazes, they have a visible influence on Pb release. It appears that 89329, ZrO, FeO, SnO, BaCrO, and CrO release appreciatively 2 times more Pb than Bo, SbO, and AlCr at low pH. On the other hand, above pH 3.0-3.5, differences in lead release become less and less pronounced. Second, it could be noticed that for each glaze citric acid (■ in Fig. 1) appears to be the strongest extractant at low pH, but above pH 3.0-3.5, the difference is negligible. Malic acid (▲in Fig. 1), in the studied pH range, seems to be
as effective as acetic acid (? in Fig. 1). It is important to note that the variability due to the replicates on flatwares considered alike summarized in Table 5 goes from 14% to 46%, making it, in some case, difficult to draw a straightforward conclusion concerning the extraction strength of food simulants.
Ceramic flatwares were used to study and compare the kinetics of migration in acetic acid and in citric acid. For this purpose, leachates of 5 mL were taken at regular contact times and then analysed according to experimental conditions summarized in Table 3. In Figs. 2(a)-(b), the Pb release is represented in function of the contact time for HAc (?) and HCit (□) respectively, as well as the fitted linear trendlines (full lines). Measured data points in Figs. 2(a)-(b) are connected with dotted lines for better visibility. Both
in Figs. 2(a)-(b), the steepest curves correspond to the experiments carried out at lowest pH and the flattest curve to experiments carried out at the highest pH. The sum-of-squares method was used to determine the model describing the most accurately the relationship between the contact time and amount of released lead. For each pH, before attending the plateau, a linear relationship between the contact time and the amount of released Pb was found. The same relationship was found by Gould et al. [26]. For each pH, the slope of the fitted linear model was calculated. This linear relationship is valid for shorter period at lower pH, 240 minutes for both acetic acid and citric acid at pH 2.37 and 2.34, respectively, and longer period at high pH, plateau still not reached after 2 days at pH 3.63 in citric acid, and 4 days at pH 5.0 in acetic acid. These findings are in agreement with Experiment 1. It reinforces the question of contact time to apply in conformity tests in case of ceramic food contact material intended for extended food storage. Migration kinetics is represented on Fig. 2(c). The slopes of the fitted trendlines, calculated from Fig. 2(a) and 2(b), are plotted in function of the pH. The obtained curves allow concluding that the stronger migration kinetics of Pb is observed in citric acid below pH 3.0. This result agrees with the results of the previous set of experiments carried out on artisanal ceramic hollowware suggesting that the nature of the acid does not influence migration above pH 3.0-3.5 (Fig. 1.).
Fig. 3 represents the Pb release in function of the pH in acetic acid (?). Trendline following an exponential curve was fitted to the datapoints (r2 = 0.989). The amount of Pb released in different foodstuffs, i.e. datapoints obtained in Experiment 1 for lemon juice, acetic acid 4%, white vinegar, and tomato juice, were plotted on Fig. 3. Lead migrations in lemon juice (○), acetic acid 4% (?), and white vinegar (□) fit well with the results obtained in Experiment 4. Surprisingly, datapoint obtained for lead migration in tomato juice (Δ) sits well above the exponential curve. The observed lead migration in tomato juice exceeds 34 times lead migration in acetic acid at the same pH (4.1). In the tomato juice sample, the amount of lead was below LOQ.
These comparative tests show the importance of glazes applied to ceramicware. Moreover it highlights a trend suggesting that Pb migration above pH 3.0-3.5 at ambient temperature is similar in acetic acid, citric acid and malic acid. At low pH < 3.0, citric acid appears to be more effective than both acetic acid and malic acid. Migration kinetics below pH 3.5 in citric acid was stronger than migration kinetics in acetic acid. Citric acid may be, therefore, an interesting alternative food simulant to use for the standard test method in quantification of lead migration allowing either to shorten the time of the conformity test or setting stricter limits by keeping the same experimental conditions. It is, however, important to interpret the results with precaution considering the variability between ceramicwares sometimes as high as 46% (Table 5).
In Experiment 5, the influence of temperature was investigated using imported flatwares.
Fig. 4 shows the amount of Pb migrated in HAc 4% in function of time at different temperatures. At higher temperatures, Pb migration is more important. Between the mass transfer from ceramic material and the temperature, an exponential relationship was observed, in this case, obeying to Fick’s laws of diffusion [27]. It is interesting to note that at 90 °C, after 2 h, the plateau is reached and the amount of extracted Pb corresponds to 6.17 mg/dm2. It exceeds by 18% the amount of Pb extracted at 20 °C after 24 h(5.1 mg/dm2 in Experiment 1). At 90 °C, after 10 minutes 50% of the extracted Pb (equals to 3.09 mg/dm2) in 120 min. At 20 °C, after 10 minutes, the Pb concentration is still below LOD. At 20 °C, it takes 400 minutes to release 3.09 mg/dm2 Pb, 40 times more than at 90 °C.
As illustrated in Fig. 5, the time needed to extract 0.8 mg/dm2 of Pb (limit set by ED 84/500/EEC for flatwares) is represented in function of the temperature. The exponential relationship is verified(r2 = 0.991). By extrapolating the fitted exponential trendline till 100 °C, it is possible to calculate that 0.8 mg/dm2 Pb is extracted in only 1.42 min, whereas at 20 °C, it takes 102 min to pass the limit of 0.8 mg/dm2 Pb, approximately 70 times more (illustrated on Fig. 5 with the dotted lines).
If we assume that cooking food with low pH (e.g. lemon sauce) in the above studied ceramicware (S = 2.92 dm2) for 30 minutes at 90 °C extracts as much Pb
pH, temperature and nature of the acid present in foodstuff in contact with ceramicware influenced the lead migration. Besides, it was observed that 20% more lead was released after 48 h than after 24 h. After 48 h, at pH 2.37, 3.0, 3.5, 4.0, and 5.0, the amount of the extracted lead is 42%, 36%, 58%, 71%, and 53% more important than after 24 hours suggesting that ceramicware intended to store food for longer periods should be tested under more restricted conditions, or conform to stricter norm values to provide a good protection of the consumer.
The nature of the food simulant seems to play a role in lead extraction, especially in the low pH range. In higher pH though, its importance disappears. Below pH 3.0, it was shown that citric acid is a more efficient extractant than acetic acid. It also displays stronger migration kinetics. In addition, citric acid occurs in abundance naturally in food and frequently used as a food additive (E330). Hence, citric acid (5 mg/L) is proposed as an alternative to acetic acid 4% in standard migration tests.
The applied glaze is also an important factor in lead release. Among the nine studied glazes six of them release appreciatively two times more Pb than the three other ones.
Moreover, it was shown that the temperature had an important impact on lead migration. The exponential relationship was proved, as expected form Fick’s laws of diffusion. The lead extracted in acetic acid 4% after 2 h at 90 °C exceeded by 18% the amount of lead extracted at 20 °C after 24 h in acetic acid 4%. It is of particular interest because ceramicwares are often used cooking at high temperatures.
Considering these results, it may be interesting to reconsider some of the conditions laid down by ED 84/500/EEC with special remark on the temperature and the nature of food simulant. Moreover, we conclude that the concentration limits for leachable lead from ceramicware set by ED 84/500/EEC are not strict enough compared to concentration limits of Pb in food. In order to protect the consumer, stricter
Munoz, C. Jimenez, et al., Sources of lead exposure in Mexico City, Environ. Health Perspect. 102 (1994) 384-389.
[24] F. Bolle, K. Parmentier, W. Baeyens, B.J. De, L. Goeyens, Cadmium and lead concentrations in acid food simulants: The values of validation parameters are predominantly affected by interspecific differences of utensils, Food Addit. Contam. 17 (2000) 755-762.
[25] A. Abou, Release of lead from glaze-ceramicware into foods cooked by open flame and microwave, Food Chemistry 73 (2001) 163-168.
[26] J.H. Gould, S.W. Butler, K.W. Boyer, E.A. Steele, Hot leaching of ceramic and enameled cookware: Collaborative study, J. Assoc. Off. Anal. Chem. 66 (1983) 610-619.
[27] R. Franz, Migration modelling from food-contact plastics into foodstuffs as a new tool for consumer exposure estimation, Food Addit. Contam. 22 (2005) 920-937.
[28] M.B. Rabinowitz, Toxicokinetics of bone lead, Environ. Health Perspect. 91 (1991) 33-37.
[29] K.C.H. Blake, G.O. Barbezat, M. Mann, Effect of dietary constituents on the gastrointestinal absorption of 203Pb in man, Environmental Research 30 (1983) 182-187.
[30] H.M. James, M.E. Hilburn, J.A. Blair, Effects of meals and meal times on uptake of lead from the gastrointestinal tract in humans, Hum. Toxicol. 4 (1985) 401-407.
[31] M. Maddaloni, N. Lolacono, W. Manton, C. Blum, J. Drexler, J. Graziano, Bioavailability of soilborne lead in adults, by stable isotope dilution, Environ. Health Perspect. 106 (Suppl 6) (1998) 1589-1594.
[32] M.J. Heard, A.C. Chamberlain, Effect of minerals and food on uptake of lead from the gastrointestinal tract in humans, Hum. Toxicol. 1 (1982) 411-415.
[33] J.C. Carlisle, M.J. Wade, Predicting blood lead concentrations from environmental concentrations, Regul. Toxicol. Pharmacol. 16 (1992) 280-289.
[34] D.A. Cory-Slechta, Lead-induced impairments in complex cognitive function: Offerings from experimental studies, Child Neuropsychol. 9 (2003) 54-75.
Table 4). Three food simulants were put in contact with the hollowwares: acetic acid, citric acid, and malic acid, all of them present in important quantities in food and food products. It is recognized that glaze is the main source of Pb migration [25], thus it was straightforward to examine the influence of different, commercially available glazes listed in Table 4 in combination with different acids at different pH. On Figs. 1(a)-(i), Pb release from artisanal, glazed hollowwares in function of pH is represented. The data points are connected with dotted lines for better visibility. Two conclusions can be drawn. First, concerning the glazes, they have a visible influence on Pb release. It appears that 89329, ZrO, FeO, SnO, BaCrO, and CrO release appreciatively 2 times more Pb than Bo, SbO, and AlCr at low pH. On the other hand, above pH 3.0-3.5, differences in lead release become less and less pronounced. Second, it could be noticed that for each glaze citric acid (■ in Fig. 1) appears to be the strongest extractant at low pH, but above pH 3.0-3.5, the difference is negligible. Malic acid (▲in Fig. 1), in the studied pH range, seems to be
as effective as acetic acid (? in Fig. 1). It is important to note that the variability due to the replicates on flatwares considered alike summarized in Table 5 goes from 14% to 46%, making it, in some case, difficult to draw a straightforward conclusion concerning the extraction strength of food simulants.
Ceramic flatwares were used to study and compare the kinetics of migration in acetic acid and in citric acid. For this purpose, leachates of 5 mL were taken at regular contact times and then analysed according to experimental conditions summarized in Table 3. In Figs. 2(a)-(b), the Pb release is represented in function of the contact time for HAc (?) and HCit (□) respectively, as well as the fitted linear trendlines (full lines). Measured data points in Figs. 2(a)-(b) are connected with dotted lines for better visibility. Both
in Figs. 2(a)-(b), the steepest curves correspond to the experiments carried out at lowest pH and the flattest curve to experiments carried out at the highest pH. The sum-of-squares method was used to determine the model describing the most accurately the relationship between the contact time and amount of released lead. For each pH, before attending the plateau, a linear relationship between the contact time and the amount of released Pb was found. The same relationship was found by Gould et al. [26]. For each pH, the slope of the fitted linear model was calculated. This linear relationship is valid for shorter period at lower pH, 240 minutes for both acetic acid and citric acid at pH 2.37 and 2.34, respectively, and longer period at high pH, plateau still not reached after 2 days at pH 3.63 in citric acid, and 4 days at pH 5.0 in acetic acid. These findings are in agreement with Experiment 1. It reinforces the question of contact time to apply in conformity tests in case of ceramic food contact material intended for extended food storage. Migration kinetics is represented on Fig. 2(c). The slopes of the fitted trendlines, calculated from Fig. 2(a) and 2(b), are plotted in function of the pH. The obtained curves allow concluding that the stronger migration kinetics of Pb is observed in citric acid below pH 3.0. This result agrees with the results of the previous set of experiments carried out on artisanal ceramic hollowware suggesting that the nature of the acid does not influence migration above pH 3.0-3.5 (Fig. 1.).
Fig. 3 represents the Pb release in function of the pH in acetic acid (?). Trendline following an exponential curve was fitted to the datapoints (r2 = 0.989). The amount of Pb released in different foodstuffs, i.e. datapoints obtained in Experiment 1 for lemon juice, acetic acid 4%, white vinegar, and tomato juice, were plotted on Fig. 3. Lead migrations in lemon juice (○), acetic acid 4% (?), and white vinegar (□) fit well with the results obtained in Experiment 4. Surprisingly, datapoint obtained for lead migration in tomato juice (Δ) sits well above the exponential curve. The observed lead migration in tomato juice exceeds 34 times lead migration in acetic acid at the same pH (4.1). In the tomato juice sample, the amount of lead was below LOQ.
These comparative tests show the importance of glazes applied to ceramicware. Moreover it highlights a trend suggesting that Pb migration above pH 3.0-3.5 at ambient temperature is similar in acetic acid, citric acid and malic acid. At low pH < 3.0, citric acid appears to be more effective than both acetic acid and malic acid. Migration kinetics below pH 3.5 in citric acid was stronger than migration kinetics in acetic acid. Citric acid may be, therefore, an interesting alternative food simulant to use for the standard test method in quantification of lead migration allowing either to shorten the time of the conformity test or setting stricter limits by keeping the same experimental conditions. It is, however, important to interpret the results with precaution considering the variability between ceramicwares sometimes as high as 46% (Table 5).
In Experiment 5, the influence of temperature was investigated using imported flatwares.
Fig. 4 shows the amount of Pb migrated in HAc 4% in function of time at different temperatures. At higher temperatures, Pb migration is more important. Between the mass transfer from ceramic material and the temperature, an exponential relationship was observed, in this case, obeying to Fick’s laws of diffusion [27]. It is interesting to note that at 90 °C, after 2 h, the plateau is reached and the amount of extracted Pb corresponds to 6.17 mg/dm2. It exceeds by 18% the amount of Pb extracted at 20 °C after 24 h(5.1 mg/dm2 in Experiment 1). At 90 °C, after 10 minutes 50% of the extracted Pb (equals to 3.09 mg/dm2) in 120 min. At 20 °C, after 10 minutes, the Pb concentration is still below LOD. At 20 °C, it takes 400 minutes to release 3.09 mg/dm2 Pb, 40 times more than at 90 °C.
As illustrated in Fig. 5, the time needed to extract 0.8 mg/dm2 of Pb (limit set by ED 84/500/EEC for flatwares) is represented in function of the temperature. The exponential relationship is verified(r2 = 0.991). By extrapolating the fitted exponential trendline till 100 °C, it is possible to calculate that 0.8 mg/dm2 Pb is extracted in only 1.42 min, whereas at 20 °C, it takes 102 min to pass the limit of 0.8 mg/dm2 Pb, approximately 70 times more (illustrated on Fig. 5 with the dotted lines).
If we assume that cooking food with low pH (e.g. lemon sauce) in the above studied ceramicware (S = 2.92 dm2) for 30 minutes at 90 °C extracts as much Pb
pH, temperature and nature of the acid present in foodstuff in contact with ceramicware influenced the lead migration. Besides, it was observed that 20% more lead was released after 48 h than after 24 h. After 48 h, at pH 2.37, 3.0, 3.5, 4.0, and 5.0, the amount of the extracted lead is 42%, 36%, 58%, 71%, and 53% more important than after 24 hours suggesting that ceramicware intended to store food for longer periods should be tested under more restricted conditions, or conform to stricter norm values to provide a good protection of the consumer.
The nature of the food simulant seems to play a role in lead extraction, especially in the low pH range. In higher pH though, its importance disappears. Below pH 3.0, it was shown that citric acid is a more efficient extractant than acetic acid. It also displays stronger migration kinetics. In addition, citric acid occurs in abundance naturally in food and frequently used as a food additive (E330). Hence, citric acid (5 mg/L) is proposed as an alternative to acetic acid 4% in standard migration tests.
The applied glaze is also an important factor in lead release. Among the nine studied glazes six of them release appreciatively two times more Pb than the three other ones.
Moreover, it was shown that the temperature had an important impact on lead migration. The exponential relationship was proved, as expected form Fick’s laws of diffusion. The lead extracted in acetic acid 4% after 2 h at 90 °C exceeded by 18% the amount of lead extracted at 20 °C after 24 h in acetic acid 4%. It is of particular interest because ceramicwares are often used cooking at high temperatures.
Considering these results, it may be interesting to reconsider some of the conditions laid down by ED 84/500/EEC with special remark on the temperature and the nature of food simulant. Moreover, we conclude that the concentration limits for leachable lead from ceramicware set by ED 84/500/EEC are not strict enough compared to concentration limits of Pb in food. In order to protect the consumer, stricter
Munoz, C. Jimenez, et al., Sources of lead exposure in Mexico City, Environ. Health Perspect. 102 (1994) 384-389.
[24] F. Bolle, K. Parmentier, W. Baeyens, B.J. De, L. Goeyens, Cadmium and lead concentrations in acid food simulants: The values of validation parameters are predominantly affected by interspecific differences of utensils, Food Addit. Contam. 17 (2000) 755-762.
[25] A. Abou, Release of lead from glaze-ceramicware into foods cooked by open flame and microwave, Food Chemistry 73 (2001) 163-168.
[26] J.H. Gould, S.W. Butler, K.W. Boyer, E.A. Steele, Hot leaching of ceramic and enameled cookware: Collaborative study, J. Assoc. Off. Anal. Chem. 66 (1983) 610-619.
[27] R. Franz, Migration modelling from food-contact plastics into foodstuffs as a new tool for consumer exposure estimation, Food Addit. Contam. 22 (2005) 920-937.
[28] M.B. Rabinowitz, Toxicokinetics of bone lead, Environ. Health Perspect. 91 (1991) 33-37.
[29] K.C.H. Blake, G.O. Barbezat, M. Mann, Effect of dietary constituents on the gastrointestinal absorption of 203Pb in man, Environmental Research 30 (1983) 182-187.
[30] H.M. James, M.E. Hilburn, J.A. Blair, Effects of meals and meal times on uptake of lead from the gastrointestinal tract in humans, Hum. Toxicol. 4 (1985) 401-407.
[31] M. Maddaloni, N. Lolacono, W. Manton, C. Blum, J. Drexler, J. Graziano, Bioavailability of soilborne lead in adults, by stable isotope dilution, Environ. Health Perspect. 106 (Suppl 6) (1998) 1589-1594.
[32] M.J. Heard, A.C. Chamberlain, Effect of minerals and food on uptake of lead from the gastrointestinal tract in humans, Hum. Toxicol. 1 (1982) 411-415.
[33] J.C. Carlisle, M.J. Wade, Predicting blood lead concentrations from environmental concentrations, Regul. Toxicol. Pharmacol. 16 (1992) 280-289.
[34] D.A. Cory-Slechta, Lead-induced impairments in complex cognitive function: Offerings from experimental studies, Child Neuropsychol. 9 (2003) 54-75.