As someone who doesn't just drink espresso, roasting some beans to a point before 2nd crack is a skill that I'm working to acquire. Some coffee just does not give of its best when roasted into 2nd crack. Examples include many of the Centrals and Island coffees such as JBM and Kona. Possibly our Thai coffee falls into this category, but I've not roasted any more yet so I can't be sure.
The difficult thing with roasting light is that you want to aim for some point, between 1st and 2nd crack, where the roast is fully developed but there is no sign of 2nd crack. Tom Owen says you should look for the beans no longer having a "wrinkly" surface. Also you should be looking for the smell turning "sweet".
Now the problem is that I'm running a domestic roaster, not a professional machine. OK it's a Hottop, so it's as good as I can get without building something myself, but there are certain limitations. One is that the smell is masked behind the smoke filter so I have to keep lifting the loading chute lid to have a sniff. Another is that, although you can see the beans in the Hottop, the degree of observation possible is just not enough to detect the surface condition of the bean. The Hottop does not allow you to sample the roast in the way that a professional roaster does. You can check it when you've dumped it into the cooling tray, but not before.
A couple of days ago, I roasted some of my precious KBM (that's Kowali Blue Mountain, JBM beans grown in Kona basically). I noted that 1st crack was over, and I thought that the smell was starting to get a bit sweet, so (not wanting to over-roast these little nuggets of gold) I dumped the roast. At this point I was about 40 seconds beyond the end of 1st crack.
Now I always take a new roast and dump it into a colander, so that I can give the beans a shake and remove most of the chaff that hasn't already fallen into the roaster's chaff tray. To my consternation I noted that the beans were somewhat uneven in colour (probably as a result of some chaff still adhering to the beans?) and horror of horrors, there was a very faint wrinkle on the surface of the beans. This is not something I could have noticed through the Hottop window, but with the beans available for close examination, there it was!
So my guess is that I've over-done the "roast it light" exercise. Previously I've tried to get to this sort of roast by time, doing the first batch to the onset of 2nd crack and then doing subsequent batches slightly lighter, by 10-15 seconds or so. However, being a prat, and also having decided that the roaster can't be totally depended on to give exactly repeatable roasts, I decided that I needed to develop my skills in light roasting - effectively, train my nose.
I'll be tasting this stuff for the first time tomorrow morning. Cross your fingers that I don't have to toss the entire batch. In the mean time, does our resident professional roaster have any comments to make?
Phil