I ate some cooked artichoke stems; some of the outermost skin is tough, so I just chewed it and then spit out the inedible bits. I left those bits in a container on my kitchen counter near the sink, still sealed. A day later, when I dumped the contents, the leftover bits were a bright blue color. What does this mean? Something with saliva enzymes?
We eat artichoke often. We pressure cook them in an old one quart aluminum pot. I have seen the cut stem (not chewed) butt turn and ooze blue from the cut stem when left in the fridge too long. In that condition the stem has a sour fermentation, but no obvious mold. I have eaten it like this thinking, I am getting extra. I like artichoke stem a lot and have bought them with long stem in Oregon, USA. The blue appears to be in the nature inherent with the thistle flower that it is.
I was a chemistry major in college and I suspect this happened because the copper in the artichoke reacted with oxygen in the air creating copper oxide, which is a really pretty blue color in low concentrations. You chewing it may have broken it up, allowing the oxygen easier "access" to react with the copper when you left it out.
It should be a reaction to certain metals, typically iron or aluminum. Not sure why artichoke + iron = blue, but that seems to be how it works.