[Adapted from an email reply by Ramon Coyle]
Solar Cookers International has received many inquires raising concern about the chemical composition of the underside of lids of jars when the jars are used for solar cooking vessels.
The experts we contacted on this subject, the best advice we could get was to suggest that you contact manufacturers of jars/lids in Taiwan to see what they could tell you about composition and toxicity.
One of our people also contacted a major producers of jars and lids in the US--the maker of "Ball" jars and "Kerr" jars. He was told that it is standard practice, maybe universal practice, to make the lids from coated steel--no latex, aluminum or brass is used. However, there is a thin coating of polyvinyl choride as the coating on the underside of lids. He was told that in normal food processing procedures, the jars are packed and "canned" in a pressure cooker with a temperature of 240 degrees Fahrenheit or 116 degrees Centigrade. The lid maker said that the polyvinyl chloride (PVC) is safe at that temperature--it has tested safely up to 250 degrees Fahrenheit.
Will the polyvinyl coating on the underside of a lid used in solar cooking reach over 250 degrees F? We don't know, but it seems plausible to us. What happens to PVC at that temperature? We still don't know that, either.
One quite reputable source we contacted told us two things about jar lids. First, the gasket portion of the lid is directly in contact with the glass of the jar. The glass of the jar is in ample contact with the food inside the jar. Given this amount of contact, the jar is never going to get much hotter than the food, because as it got hotter the additional heat would be constantly being conducted out of the jar into the cooler food. The food can never get more than a degree or so hotter than 212 degrees F or 100 degrees C as long as it has water in it, because the water will evaporate, which cools the food. Thus the jar will not get much about 212 degrees, and if the gasket had a tendency to get hotter that tendency would be counterbalanced by heat being conducted from the gasket to the cooler glass with which it is in contact. All this was stated to explain that the gasket should be no problem.
That does leave the PVC undercoating. It will tend to reach a fairly high heat, because it will be directly in contact with the lid, which is the hottest thing in the cooker. The same source who said gaskets are not a problem said that the PVC, even if it is heated to the point of becoming unstable, will not cause a problem. This source says that PVC is only a problem if offgasses from it are inhaled into the lungs. Little or no such inhalation can take place with solar cooking because:
- there is very little PVC per jar lid
- if outgassing does occur it will be into the tiny space between the lid bottom and the food in the jar and from there it will leak out of the non-sealed jar into the insulation bag and from there into the atmosphere--far too tiny a quantity of gas to make a difference.
The source adds that if any PVC materials were to actually get into the food, it would not be harmful--that ingested PVC does not present health problems--only inhaled PVC or PVC by-products.
We do not have any independent confirmation of those views. Ramon Coyle personally feels somewhat uncomfortable with PVC being that near his food, especially if it is being heated over 250 degrees F. We can't say that his discomfort is based on science so much as bad past-associations with tales of PVC in the environment.
You might want to take the word of the US jar manufacturer that PVC in jar lids is stable up to 250 degrees F. You might want to guess that a whole nation of food manufacturers would object to pvc being standard in jar manufacturing if they had any doubts at all about its safety and stability under all imaginable conditions. You might want to take the word of our expert who thinks that even an unstable PVC coating on the lids would not cause health problems.