On a recent and somewhat turbulent flight out of Houston TX, I received as my “meal” a barbecued beef sandwich; it seemed appropriate and tasted nasty. The sandwich was sealed in a plastic/foil wrapper and had been dampened by the condensation within. The contents of the sandwich would have been unidentifiable if not for the label. The sandwich had been warmed, which made it slightly less unpalatable.
My question stems from the package. Consisting of some sort of plastic with a metallic finish on the inner surface, it seemed pretty familiar – much the same as many a wrapper of which I’ve carefully and properly disposed in the past. The difference was that the metallic finish to the material appeared to have dissolved where the condensation from heating had touched it.
That’s right. Where the condensation sat on the inside of the bag, the plastic had lost its shine and become transparent. The inner surface had dissolved. Possibly evaporated. I didn’t inhale too close to the mouth of the freshly opened bag.
While the unappetizing sandwich showed no apparent metallic residue, I rather doubt my good judgement is well demonstrated by the fact that I ate the sandwich. A metallic aftertaste might have improved the food, or at least made it more interesting, but this was not to be. Neither did my iron gut flinch at its treatment at my hands. Apparently, whatever it was – and I’m referring to both the wrapper and the sandwich itself here – whatever it was, it was edible.
So my question: what the heck are they putting on the insides of packages that looks metallic, but dissolves or evaporates on heating or wetting? How much food from such packages can I eat before it rots my gut or melts my brain?
Quite sure. The sauce was contained within what can loosely be refered to as “the bread”. The transparent parts were located primarily around the centre of the top and bottom parts of the bun, not near the filling at all. The moisture was clear and colourless; I assume it was water.
Note also that the package was still intact, it was only the inner surface that disappeared. No trace of residue appeared on the surface of the bun.
All that was needed was some acidic residue on the bread or in the “condensation,” and it would have oxidized the aluminum foil. Perhaps the aluminum oxide lost its affinity for the plastic backing and came off after it was oxidized. Or perhaps the acid in the residue which oxidized the foil also ate through the glue sticking it down.
edwino, thank you for the interesting possibilities. I thought I eliminated the possibility of the oxidization of aluminum in my reply to it, in particular by noting the lack of a residue, but you force me to re-examine the issue. Do you think the aluminum oxide might have evaporated away? I suppose it’s possible but I find it unlikely. It may be possible that the condensation contained contaminants such as acids, I hadn’t thought of that.
As to the existence of aluminum or a glue sticking it down, I cannot say. I don’t know what the metallic coating was or how it adhered to the plastic.
If it’s the kind of material I’m thinking of, it’s something like a vapor-deposited aluminum coating, like on the inside of potato chip bags. A thin layer of aluminum oxide is nearly transparent; it’s used to make the conductive traces for LCDs.
I don’t know the answer to the question, but isn’t what we think of as “aluminum” actually aluminum oxide? Aluminum is white and highly reactive. It will oxidize when exposed to air - turning silver in color. The only reason an aluminum can doesn’t blow up is that this Al[sub]2[/sub]O[sub]3[/sub] provides a barrier on the surface so that the elemental aluminum isn’t exposed (and when scratched, it oxidizes so fast and reseals itself so you don’t notice it).
My guess is that the water from the condensation dissolved the glue that held the coating to the paper and the aluminum coating flaked off - you just didn’t notice where it went.
The edges of the transparent patch of plastic were not consistent with flaking. Usually when a material flakes away the edges of the missing surface are slightly flaky themselves. The edges also tend to have a sharp nature to them, indicating that there was a fracture that allowed the flake to separate. The edges of the transparent patch were, instead, smooth and apparently gradual, with a pattern similar to a river’s estuaries. If I remember correctly the transparency gave way gradually to the metallic shine, rather than having a sharp boundary.
Also, I did inspect the sandwich fairly closely looking for signs of metal. Unless the flakes somehow remained entirely within the bag and completely failed to adhere to the sandwich, I’m fairly sure I would have seen them.
Monkey, I don’t know what the material was. If it was some sort of dye, the question still remains: where did it go?
I agree that determining the material itself would be a good start. Q.E.D., I missed your post last time around. Is a thin layer of aluminum (oxide) a standard packaging material? It was very much like a potato chip package.
Aluminized mylar is a common packaging material. It’s not a dye, it’s a metal.
When metallic aluminum (the silver stuff) turns into oxide, it turns transparent. Aluminum oxide is also known as sapphire.
Once while developing a science museum exhibit I had a tank of water filled with aluminum powder and equipped with a pump to keep the stuff suspended. You could see all the turbulent flow patterns, but after several weeks the water turned clear. The opaque aluminum dust turned into aluminum oxide dust which is not transparent. The water looked just the slightest bit milky. Apparently the aluminum oxide particles are almost invisible when wet.
I’ve seen aluminum dissolve quickly under two different conditions: dipped in sodium hydroxide solution, and dipped in ferric chloride solution. Perhaps there was some chemical in the moisture which caused the aluminum to corrode and become Al2O3?
Here’s another possibility:
Controversial nanobacteria cause aluminum corrosion