Kenneth Davids really does know coffee blending! This is a copy of his advice on coffee blending from lucidcafe.com. This website is based on home roasting, but I do not know why it could not be used for commercial roasting as well.
Blends and Blending
Coffee blends are crafted for two reasons: 1) to save money; 2) to produce a coffee that tastes better than (or at least different from) coffees of a single crop or region.
For large commercial roasters the cost issue is paramount. Since their coffees compete in supermarket chains, mass-market blenders attempt to create a decent coffee as cheaply as possible.
Specialty coffee roasters who sell smaller quantities of coffee in whole-bean form to a more demanding clientele also may want to cut costs by blending. But the primary goal of most specialty roasters is to produce a blend that tastes better or more balanced than any of its constituent coffees.
Other blends aim either to mimic characteristics of a famous and expensive coffee (Jamaican Blue Mountain Style Blend, meaning it has no Blue Mountain in it whatsoever), or stretch a costly coffee by mixing it with similar but cheaper beans (Hawaiian Kona Blend).
Blending at Home: Getting Started
For home roasters, subtlety in blending may only be possible after considerable tasting and experimentation. It is probably easier to get a feel for the process by combining very different but complementary coffees; a bright, acidy coffee with a fuller, deeper-toned coffee, for example.
To help that process along, here is a list dividing some well-known coffees into categories according to the particular qualities they might contribute to a blend. Obviously there are numerous ways of categorizing coffees for blending purposes; my list offers only one approach to a complex subject.
Category 1: Big classic coffees. These coffees contribute body, powerful acidity, and classic flavor and aroma to a blend. They perhaps make too strong a statement for use as a base for blends, but are excellent for strengthening and energizing less acidy coffees with softer profiles. I've omitted more expensive coffees like Jamaican Blue Mountain, Hawaiian Kona, and Puerto Rican Yauco Selecto, which given their cost probably should be enjoyed straight.
· Guatemala (Antigua, Coban[AAAa] and Huehuetenango, other good Guatemalan coffees)
· Costa Rica (Tarrazu[AAAu], Tres Rios[AAAi], other good Costa Rican coffees)
· Colombia
· Venezuelan Tachira[AAAFIRSTa], Merida[AAAe]
Category 2: Smaller classic coffees. These are "good blenders"; they establish a solid, unobtrusive base for a blend, and contribute body and acidity without competing with more individualistic coffees. When brought to a darker roast they often confer a satisfying sweetness.
· Mexico (Oaxaca, Coatepec, Chiapas, Tapachula)
· Dominican Republic or Santo Domingo
· Peru (Chanchamayo for more acidity; Northerns for less)
· Brazilian Santos (washed for more acidity, semi-washed for more body and sweetness)
· Panama
Other possibilities are the better coffees from El Salvador, Ecuador, Nicaragua, Haiti and coastal Venezuela
Category 3: East African and Yemen coffees. Their powerful wine-like acidity makes these coffees a poor base for a blend, but excellent contributors of complexity and liveliness. Some, like Kenya, contribute considerable body as well. These coffees should be used with care in blends for darker roasts; they add a sharp bite attractive to many (including me), but may be distracting to others.
· Yemen Mocha (adds richness and body as well as acidity)
· Kenya (ditto above; acidity even more powerful)
· Zimbabwe
· Ugandan Bugishu
· Ethiopian Harar (contributes rough, fruity, exciting acidity, but less body than the above)
· Malawi
Category 4: Asian-Pacific and similar coffees. These add richness and body to a blend, and combine well with other coffees. Their deep-toned acidity will anchor and add resonance to the lighter, brisker coffees of category 2, and balance without blunting coffees in categories 1 and 3.
· Sumatra
· Sulawesi
· Java arabica
· New Guinea
· Ethiopian washed coffees (best are Yirgacheffe and Limu)
· Indian Mysore (unobtrusive; tends to add weight without power)
Category 5: Aged and specially-handled coffees. These add weight and body to a blend, and in the case of aged coffees richness and complexity as well. They are fun to experiment with in blends as a balance to category 1 and 3 coffees.
· Indian Monsooned Malabar
· Any good aged coffee
For more information on Coffee By Country Click Here.
Blending for Taste and Variety
Clearly there are two ways to approach blending for taste alone: by system or by improvisation.
One systematic approach would be to start with a base coffee, as I suggest in the previous section, roast and drink it long enough to really know it, then experiment with adding other coffees to it, keeping notes as you go along. Another approach might be to begin with two coffees that complement one another, like the acidy Mocha and the softer, fuller Java of the original Mocha-Java blend (I'd make them a Kenya and a Sumatra, experiment with the proportions of the two constituents until you learn how they work together, then begin experimenting with adding a third coffee, again keeping notes so that a success can be built upon or duplicated.
Blending for Espresso and Dark Roasts
When blending for espresso cuisine the first question to consider is how you and your guests take your espresso. If you tend to drink it without milk and with very little sugar, you should avoid the big, acidy coffees in categories 1 and 3 and rely mainly on coffees in categories 2 and 4. Italian blenders prefer a base of Brazilian Santos, whereas West-Coast Americans typically rely on Mexican and Peruvian coffees. Good Indonesian coffees make splendid dark roasts, but are relatively expensive. Some Italians like to use high-quality robustas to smooth out their espresso blends.
On the other hand, if you drink your espresso with a good deal of hot milk and/or sugar, you may prefer a more pungent blend. On a base of Brazil, Peru, or Mexico, try adding a coffee from categories 1 or 3, perhaps either a Costa Rica or a Kenya or some of both. Go easy at first, adding a little more of the big, acidy coffee every session, until you achieve a taste you like for the way you drink your coffee. If you know you like an assertive, powerfully twisty espresso, start with a base of Kenya and gradually soften it with increasing amounts of a gentler coffee.
Of course how darkly you roast your espresso blend and what method you use to roast it also profoundly affects flavor.
Blends of Roasts
When I first came into coffee consciousness in the San Francisco Bay Area twenty years ago blends of dark- and medium-roasted beans were common. They are less so today, which is probably a pity. For me one of the most vibrant and exciting ways to enjoy a coffee is to mix darker and lighter roasted beans of the same origin, thus experiencing the coffee in its full range of roast taste.
Try it. Take the same coffee and bring two batches to a medium and to a dark or moderately dark roast, then blend the two. If you enjoy the result try varying the identity of the two coffees: Blunt the acidity of a Kenya by carrying it to a moderately darkroast, then combine one part of the darker-roasted Kenya with two parts of a medium-roasted Indonesian coffee, for example.