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.