Sumatra coffees are famous for their peculiar flavor profile, low acidity, thick body, and rustic flavors that can often be described as earthy. Much of the flavor comes from the way Sumatras are processed, the wet-hull method , not to be confused with wet-processed coffee. The flavor of typical wet-hull Sumatra is polarizing among buyers. Some love it, but they must bracket this type of flavor profile because it would be considered unacceptable from any other origin besides Indonesia. Each coffee drinker has to discover if this type of flavor is right for them, or not; whether it's a go-to daily drinker, an occasional diversion, or flat-out unacceptable.
On a cupping table of well-processed Central American coffees, a Sumatra would immediately be thrown out. The earthy and foresty flavors - herbal, sometimes mossy or even mushroomy - would be attributed to processing errors, and the coffee labeled defective. So why this schism in the way the coffee trade treats wet-hull Indonesia coffees, and Sumatra in particular?
It comes down to taste: If a Sumatra supplier can consistently provide the same coffee, processed the same way, be it fruity or earthy, there are buyers who see this as a uniquely different flavor profile, and a welcome break from the Central America, Colombia or Kenya coffees. And of course the bottom line is that their customers like it. Those who like minimally-processed wines, or those wines with complex flavors of leather, peat moss, fir, cedar, humus, tannins, will see something in the Sumatra flavor profile.
We look for sweetness in the cup, an expanded definition of sweetness than one might use when cupping other origins. This could be raw sugar, like muscavado, or molasses. It could be unique syrups like brown rice syrup, or sorghum syrup. In any case, a coffee with no sweetness is rarely, if ever a good coffee. We look at the rustic elements to distinguish gross flavors like dirt from positive clean-earth, humus or other positive and relatively clean natural scents and tastes. While slight green herb and mossy is good, vegetal notes that are too bittering hint at poor processing or under-ripe fruits. In our lab we also check the defect count, ultra-violet appearance of the coffee, water activity, humidity and density of the bean. These tell the story of the coffee, but ultimately we find that cupping reveals the truth just as well.
There is a tendency to over-roast Indonesians. The reason is that they don't show as much roast color, and have a mottled appearance up until 2nd crack and even a bit into it. Don't let this make you think you have to roast them dark (although they can be nice this way too). Great Indonesians will be wonderful roasted just to the verge of 2nd crack but NOT into it at all. So ignore the weird beans you see green, and ignore the mottled appearance of lighter roasts, and only focus on the what you get in the cup.