Temperature and pH affect copper release kinetics from copper metal foil and commercial copperware to food simulants

Abstract

Copper (Cu) metal and alloys are used in cookware and other food contact surfaces due to their desirable properties for various applications. However, Cu metal can ionise and subsequently transfer to food and beverages under certain conditions. Here, we tested how pH and temperature affected Cu release kinetics using model systems utilising Cu metal foil and commercially available copperware. Cu foil and copperware were exposed to food simulants composed of 3% (w:w) aqueous solutions of citric acid, malic acid, acetic acid, or deionised (DI) water at temperatures ranging from 4°C to 60°C. An additional pilot experiment tested how simulated long-term cleaning affected subsequent Cu release from lined and unlined copperware to 3% citric acid. Food simulants were then analysed by ICP-MS for total Cu. After 180 min, incubation of Cu metal foil with acid-containing food simulants at 4°C resulted in Cu release ranging from 8.7 - 14.0 µg cm^-2, while 21.5-38.1 µg cm^-2 was released at 60°C. In contrast, Cu transfer from metal foil to DI water was relatively low, with <0.6 µg cm^-2 released after 180 min at 60°C. With citric acid food simulant, lined copperware released between 0.6 and 3.0 µg Cu cm^-2 over 180 min at the set temperatures, while unlined copperware released approximately 25-45 fold higher amounts of Cu (26.9-74.6 µg cm^-2) over this same time period. In contrast, use of DI water food simulant resulted in Cu release of <0.1 µg cm^-2 for the lined copperware and <2 µg cm^-2 for the unlined type. No significant effect of simulated long-term cleaning on Cu release from copperware was observed. These data indicate that Cu release is affected by temperature and pH, and that specific steps can be taken to limit Cu metal release from food contact surfaces to foods and beverages.

Keywords: Copper; ICP-MS; acrylic coatings; copperware; food simulants; organic acids.

Figure 1.. Effect of temperature and pH on Cu release kinetics from sheet metal foil to food simulants.

Cu release (μg·cm-2) at 4, 30, and 60 °C to food simulants composed of (A) DI water, (B) 3% citric acid, (C) 3% malic acid, and (D) 3% acetic acid. Values are plotted as means ± SD.

Figure 2.. Effect of temperature and pH on Cu release kinetics from two types of copperware to food simulants.

Cu release (μg·cm-2) at 4, 30, and 60 °C from (A) unlined copperware to DI water, (B) unlined copperware to 3% citric acid, (C) lined copperware to DI water, and (D) lined copperware to 3% citric acid. Values are plotted as means ± SD.

Figure 3.. Effect of simulated long-term cleaning on Cu release kinetics from two types of copperware.

Cu release (μg·cm-2) to 3% citric acid at 60 °C from (A) unlined copperware, and (B) lined copperware. Values are plotted as means ± SD.

Figure 4.. Comparison of Cu release kinetics from sheet metal foil and unlined copperware model systems.

Cu release (μg·cm-2) to (A) DI water at 4 °C, (B) DI water at 30 °C, (C) DI water at 60 °C, (D) 3% citric acid at 4 °C, (E) 3% citric acid at 30 °C, and (F) 3% citric acid at 60 °C. Values are plotted as means ± SD.

Figure 5.. Characterization of lined copperware coating.

(A) ATR/FT-IR spectra of (a) PMMA powder, (b) copperware coating film, and (c) copperware coating extract as a dry powder. (B) DSC curves indicating the glass transition temperatures of copperware coating extract and PMMA under N2 at a temperature rate of 10 °C·min-1. (C) TGA curves for the thermal decomposition of copperware coating extract and PMMA under N2 at a temperature rate of 20 °C·min-1.