This is very interesting and I am thinking it may work on a few types of beans - well, let's just say it's a hunch I have. According to the theories of espresso extraction, you get a few different stages - an initial couple of seconds of sour (depending upon pre-infusion methods), concentrated sweetness, sweetness and then as blonding occurs there may be bitter taints, then bitterness as the shot continues and there is limited flavor oils remaining in the puck. I've noticed that bitter taints arrive sooner in some beans/blends than others. As an example, Ethiopian's can tolerate quite long extraction times - seems they are more like a small condensed flavor nuggets, just like a long-lasting candy. Some CentAms are less tolerant of extended extraction times and are easily "damaged" by extended exposure to heat and pressure. Sometimes, this comes down to the roast level, roast profile and how the bean was prepared and of course the grind and extraction - the opening of the grind has potentially helped in larger particle size that provides more surface area to resist in the extended volume of extraction. I always have a private, silent chuckle when ordering any "strong" coffee at some of the lesser skilled cafes.......you know they will run a super-long shot that picks up all sorts of taints towards the end of the shot.
