Why does minced garlic turns blue/green?

Lately, I find my minced garlic turns blue/green, especially when it is mixed with lemon grass.  The blue/green colour is very bright.  So, I start to wonder whether it is poisonous?.... 

I found a web where somebody has quoted a scientific notation about this matter.  I think it has almost cleared the clouds. 
http://chowhound.chow.com/topics/287019

So, don't have to worry about the colour change matter in future : )