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This question is inspired by this answer, which suggests storing simple syrup in a copper vessel. It fairly conclusively demonstrates that copper can keep the simple syrup from growing mold, but comments also raised the point that it may not actually be safe due to copper leaching into the simple syrup.

It's very clear that copper isn't safe for acidic foods: 4-101.14 in the 2013 FDA Food Rules even contact with foods with pH 6 or lower. However, it provides no such limitations for non-acidic foods.

I've also found various sources saying that lined copper is safe, but of course in that case, there's not actually copper in contact with the food, so it doesn't directly say anything about the copper itself (and also doesn't help anyone store their simple syrup). It does suggest that copper itself might not be safe, since the lining is presented as a requirement, but it's not exactly conclusive.

So: is a copper storage vessel, with copper directly in contact with the stored food, safe for long-term storage?

Please provide sources specifically about food. (Notably, the existence of the copper IUD does not prove anything here, since it's not at all clear that copper leaching into food which is subsequently ingested is comparable to an IUD, and in fact I've found at least one study that found that serum copper levels were not altered in copper IUD users.)

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    Not a proper answer because I cannot provide a proper reference, but simple logic suggests that if storing simple syrup in a copper container prevents mold growth in or on the syrup, then the copper has to be leeching into the syrup, because the only scientifically proven mechanism by which this could work is the presence of copper ions in the syrup itself. Whether that will reach a high enough concentration to be an issue for humans or not is a different question, but I think we can say conclusively that leeching of metal ions is happening in that case.
    – Austin Hemmelgarn
    Dec 9, 2022 at 14:53
  • FWIW, if you put symple syrup in a glass jar in the fridge it'll keep for years.
    – FuzzyChef
    Dec 10, 2022 at 0:43
  • @FuzzyChef I think the other question is probably the right place for that kind of answer :)
    – Cascabel ♦
    Dec 10, 2022 at 21:59

1 Answer 1

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Short answer: No.

The quantities of copper you might find in water in the USA are regulated, not by the FDA, but by the EPA (Environmental Protection Agency), and other bodies outside the USA. The EPA commissioned a report on Copper in Drinking Water (2000).

They have clear guidelines for the amounts that are acceptable in drinking water. We can make some assumptions that the same would apply for a simple syrup, as it is mostly water with some dissolved sugar.

The article linked above states

Characteristics of the water that can increase the leaching of copper include low pH, high temperature, and reduced hardness. Electrolysis of copper from pipes can result from using household pipes to ground appliances. The length of time that the water has been sitting stagnant in the pipes can also greatly increase the concentration of copper to several milligrams per liter in the water (EPA 1994).

We can assume that a simple solution or water sitting stagnant in a vessel would also leach copper from the vessel and result in increased concentrations. Now, I'm assuming you're not going to be drinking a lot of simple solution stored in copper vessels at any one time and it is worthy of noting that copper is an essential nutrient, but you only need vanishingly small quantities before it becomes toxic, around 1.5- 3 milligrams per day (for an adult; less for a child).

I also found the following publication: Water quality and risk assessment of copper content in drinking water stored in copper container, which states that water stored for 168 hours (7 days) can have up to 0.823 mg/l copper, while pipes can have between 4 and 70 mg/l with stagnant water (PDF; see page 33860).

Full references:

1: National Academies of Sciences, Engineering, and Medicine. 2000. Copper in Drinking Water. Washington, DC: The National Academies Press. https://doi.org/10.17226/9782.

2: Manne, R., Kumaradoss, M.M.R.M., Iska, R.S.R. et al. Water quality and risk assessment of copper content in drinking water stored in copper container. Appl Water Sci 12, 27 (2022). https://doi.org/10.1007/s13201-021-01542-x

3: EPA (U.S. Environmental Protection Agency). 1994. Drinking Water; Maximum Contaminant Level Goal and National Primary Drinking Water Regulation for Lead and Copper. Fed. Regist. 59(125):33860–33864.

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    Even the max (70 mg/liter) would only end up with 1 mg/tablespoon of syrup, and according to Wikipedia the EPA considers up to 1.3mg/L of copper as safe levels of copper in drinking water. So storage in copper vessels is definitely not dangerous for simple syrup (with no additional acidic ingredients); probably not a good idea for things you eat more of.
    – Esther
    Dec 9, 2022 at 4:54
  • @Esther it looks like the assumption is that people drink 2 litres per day and they based the amount per litre on that (i.e. 2.6 mg per day) so you would only need 3 tablespoons to put you over. I don't know how much simple syrup one might consume in a drink, but I would hazard that 1 per cocktail and 3 cocktails per session might not be excessive.
    – bob1
    Dec 10, 2022 at 4:08
  • Bob1: upvote for the links. But the answer is yes . From your linked source: /From this study, it is clear that trend of leaching copper concentration in water is progressively increasing as time increases, and the rising trend of copper concentration is evidently seen in Fig. 4 and Table 1. Generally speaking, the measured copper concentrations are within the guideline values recommended by both, WHO and USPEA for drinking purposes/ Copper does leach out (and that is no doubt what prevents mold) but even 168 hours resulted in half of what those 2 authorities consider toxic.
    – Willk
    Dec 10, 2022 at 16:12
  • @wilk I disagree - 168 h is only 1 week, so not a long storage - you could keep it in the fridge easily for that duration. I don't know the kinetics of copper into solution in water, but I suspect the upper limit takes quite some time. Having said that no-one's going to be drinking a litre of simple syrup at a time, so probably not too much to worry about, unless you consider that the simple syrup isn't the only source in most people's diets, especially if they have copper water pipes as their plumbing.
    – bob1
    Dec 10, 2022 at 21:05

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