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Rust in my pineapple chunks
June 26, 2008
Q. I live in Texas USA and I am a Texas A&M University Graduate. I came upon your website as I was looking for information on material reactions for the inside of canned citrus foods, namely pineapple. I realize I don't have priority over the Industrial Metal Finishing, but I thought I would throw this out there as it may be interesting to someone.
I made my children's lunches this morning and added some pineapple chunks from a pull-top lid can from Del Monte. I left a few pieces of pineapple in the can and when I came home this evening, I noticed that the inside of the can smelled horrible and there was what looks like rust spots and gray spots. Immediately I thought I had poisoned my children w/contaminated food...while up to this moment, they appear to be fine I now would like to know what has caused the 'corrosion'...I don't know what else to call it.
I have searched extensively for aluminum and stainless and coating key words on google and I am not getting the answer I need...have a poisoned my children OR did the can react AFTER I opened it and left the pineapple in the can at room temperature.
Thank you so much for your time if you choose to help me.
- Houston, Texas
June 27, 2008
A. Hi, Gerri. I have noticed that issue with canned pineapple my whole life, but don't know exactly what goes on there. I'd probably guess that what happens is the can is tin plated, which protects the steel as long as the can is sealed (no oxygen), whereas once the can is open the potentials reverse and the tin accelerates the corrosion of the underlying steel. Sometimes you even see small tiny shiny flakes, which are presumeably tin. Tin is non-toxic and rust is too. It will taste metallic and should be discarded, but I am confident that there is absolutely nothing to worry about, both from a lifetime of having done the same thing, and from reading some studies showing that rust is not an ingestion hazard.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
July 2, 2008
A. Assuming your can is not stainless steel, but good old tinplate, there is nothing to worry about. The vast majority of cans are now made of either tinplate or aluminium, although there was a trial in the 1970's in Japan with nickel plated steel cans. Stainless steel cans may be available for certain high value products, but I would very much doubt they are used for canned fruits.
What has happened is that the tin has acted as it is supposed to - a sacrificial anode, just like galvanised steel. When tinplate is produced electrolytically it is flow melted by passing an electric current through it to melt the tin and give it a bright sheen. In doing so, an intermetallic compound (FeSn2) is produced between the tin and the base steel. This compound is dark grey. What you are seeing is the result of the acidic pineapple juice etching the tin until the FeSn2 is exposed; you also may have noticed that your tin layer is showing a nice surface pattern - this is the tin being etched to show its crystal structure. The reason attack happens mostly at the top of the liquid is because it is a redox reaction and one reaction is the reduction of oxygen and the other one is the oxidation of tin. The amounts of tin dissolved are minute and it will not harm anyone. However, it is possible that if the can has been open for some considerable time, that enough metal (tin and iron) will have dissolved in the juice to give it a metallic taste, but it will not be at all harmful.
As the amount of tin on tinplate has been reduced over the years, the thickness of the intermetallic layer has also decreased, so it is not as thick as it used to be; this results in the easier attack of the base steel to produce rust spots on the inside of the can. This corrosion problem is usually overcome by putting a lacquer on the surface, but some solutions are very difficult to protect with lacquers and I suspect that pineapple juice is one of them! As a side issue - if you want to tenderise meat, soak it in pineapple juice for about an hour - there is an enzyme in the juice that breaks down the meat fibres and makes it tender.
Tinplate is an excellent packaging medium and food properly canned can last for many decades. When Captain scott tried to get to the South Pole in March 1912, he left some tins of food as provisions along his route. These were later recovered after his unfortunate death and were opened in the 1960's to find the meats were still perfectly edible.
R&D practical scientist
Chesham, Bucks, UK
July 3, 2008
A. Years ago, the can label used to say not to store it in an opened can. It might have even gone so far as to say a glass or plastic container.James Watts
- Navarre, Florida
March 23, 2009
Q. Is the food still safe to eat if there are blackish discoloration inside the can upon opening the canlui rodrigo
- Manila, Philippines
July 1, 2015
A. Lui, that does not sound safe. I would suggest NOT eating it.Allison Tuinstra
- Missoula, Montana