Roasting Fundamentals: Decafs
The classic decaf flavors that most people think of are the overwhelming maltiness and in the worst cases the wet cardboard flavors of both aged and damaged coffees, but these flavors are generally either the results of the original quality of the coffee itself or the intensity of the decaf processing. When the right coffee is selected and the process is carefully monitored, a good deal of the coffee's characteristics should survive. A really great decaf shouldn't taste all too differently than before it was processed for decaf.
Mike Strumpf of Swiss Water Process states:
"In our process, a decaffeinated coffee should taste like the original green coffee and nothing else. After each decaffeination run we sample roasts and cup the before and after decaffeination samples side by side, focusing primarily on any differences in cup qualities between the two. This clarity means that an exceptional coffee will make an exceptional decaffeinated coffee, and that is what most of us are looking for."
Strumpf says that physical bean characteristics are important in selecting coffees for decaffeination as well. At Swiss Water coffees are analyzed for their moisture content (percentage of the bean that is water), water activity (the state of energy of the water in the bean), and density (mass/volume). These three aspects of green beans are an important trifecta for both roasters and decaffeinators. Knowing the relationship between those three physical characteristics can indicate whether or not a green coffee is or is not viable for decaffeination, and as long as a coffee is fresh and sound there are generally not problems.
The same holds true for how the coffee behaves in the roaster, for the most part. A well processed decaf Ethiopia should behave more or less like a regular Ethiopia, except that the decaffeination process does affect the coffee's density and so you of course want to be aware of that and sensitive to how you use your energy input during the roast, especially during the initial drying stage and after the 1st crack as really started to roll which you can end up really flying through if you're not careful. Basically the more you process a decaf, the more you break it down, and also if a coffee is already in poor shape, you're going to break it down even more.
The major difference in roasting a decaf is in the color change indications. Color change is a big part of monitoring roast development in regular coffees, but because the decaffeination process alters the color of the raw coffee so drastically, the same color change indicators simply aren't there when roasting decafs. One of the areas where this is the most problematic is at the very end of the roast, where decafs can appear to be much darker than what their actual roast levels are, and even begin to sweat some oils as the cellular structure has been weakened in the decaffeination process as well. Just because the coffee color is darker and some oils may be present, this doesn't mean that you've engaged in dry distillation or are developing roasty flavors. This is one more reason why when we talk about roast level that the conversation has to be about more than just roast color.
Other physical and chemical changes are for the most part similar in decafs as in regular coffees, such as bean expansion, the 1st and 2nd cracks, as well as aroma indicators. The initial pops of 1st crack might seem to be a little softer, but any well developed roast should have a distinctive finish to 1st crack, and the roast aromas during and after the 1st crack are some of the most telling indicators of roast development during this period. You should move past the cereal and bready aromas and begin to smell some pungency, almost vinegar-like aromas, but with sweetness to it. Timing past the end of 1st crack is also crucial here, use it along with your aromas to tell you when the coffee is at a level that you desire.
If the roaster you're using doesn't allow for the opportunity to check the roast aromas, then timing becomes even more crucial. If you roast the regular version of that coffee to an end point of 20 seconds after the end of first crack, then do the same with the decaf version. In lesser quality decafs, the coffee has been altered enough through the break down of density and moisture content that this is the stage of the roast that they can really take off on you, so as I stated before, be aware of achieving a stop to the 1st crack and not allowing the roast to run right into 2nd crack. Again and Again, great decafs should taste and roast fairly similarly to the regular counterparts.