Metallic taste in cider after kegging

Every winter I get a group of friends together and they all put in some money, and I make cider for them. This year I decided to get fancy and make sparkling cider, so I put on my engineering hat and built myself a keg!

In short it is a stainless steel fire extinguisher to which I have made some modifications. I wasn't sure it was going to work, so I did some pressure test, and much to my surprise it held up.

Very much aware that I was still dealing with an unknown monster here, the first batch of cider I decided to carbonate was the cheapest and easiest one to make (just in case something went wrong).

After fermentation, I tried the cider and it tasted like, well, cider. I put it in the keg and carbonated it and tried some. It tasted very...metallic. Nothing like it had 30 mins earlier.

I washed the keg very thoroughly, both during the testing phase and before carbonating it. I have some brass fittings, but none that are submerged.

The speed at which the metallic taste appeared pretty much rules out an infection. I hope to be able to isolate which component is causing the metallic taste. I will do that by depressurizing keg, taking of top and pouring some cider out. If it tastes metallic then it is coming from either;
a. the CO2 tank (paintball tank)
b. the fire extinguisher itself
c. the tube that connects the nozzle to the bottom of the tank
d. the schraeder valve for the co2

if it doesn't taste metallic then the source is;
a. the nozzle
b. the fittings
c. the tubes coming from the fittings

My suspicion is that it is either the schraeder valve, the tube or the fittings. What are your thoughts? Might it be the co2 tank or co2 tank tubes?

I will get back to you about my findings. If worst comes to worst, and I have to make more cider, this batch was small and was the easiest type to make, so there would be no tragedy, but I would rather not run around in circles nonetheless.

You obviously burst carbed it, which is known to produce a lot more carbonic acid than using the set and forget method, especially for higher carb levels. Many people describe the carbonic acid bite as metallic. Not saying that's it, but it's possible. If that is the case, it should fade over time. What level did you carb it to? Are the extinguisher and diptube aluminum or SS? Is the schraeder valve brass or SS?

Also, there is more than one kind of stainless steel, and not all are food-appropriate (especially for acidic foods). To be 'food-grade' stainless must be certain alloys (low nickel, limits on the chrominum content, etc) and have specific finishes (which help retard bacterial growth & limit surface area). It is possible you have leached some metal out of the steel into the beer, given your extinguisher is unlikely to be food-grade.

Many stainless-steel food containers are lined with ceramic or plastic, to avoid leeching.

Bryan