Ok, after a tiny bit of research I have discovered the transcript of his "The Bulb of the Night" show with this quote:
AB: That's right. That's because the sprout in the middle is still immature, okay? If
you'd looked down there and seen green, that means that the sprout is getting
ready to pop and that means that it would be bitter. [smells it] God, I love
that fragrance.
The entire transcript can be read at this link:
http://www.goodeatsfanpage.com/Seaso...Transcript.htm
I am pretty sure he went over this in another episode too with a bit more information, but this pretty much confirms what I remember.