This is not about philosophy; it’s about knowing origins, their taste and aroma, and having a solid experience in combining them and in interpreting your target taste.
First step is being able to know where you want to get with your blend. A series of question arise from this: who is going to drink this blend (Italians, Southern Europeans, Northern Europeans, Americans, People who know coffee, people who don’t…)? How is this blend going to be drunk (i.e. pure espresso, with milk, short, tall…)? Who is going to brew this blend (trained professionals, fast food crew…)? What kind of machine is brewing this coffee (traditional, bean to cup, vending…)? And, last but not least, how much must this blend cost?
In general we must say that each blend comes from an analysis of factors and guidelines that are difficult to put together. The good blend is in general a balance of these factors, perfectly satisfying all of the given characteristics we were expecting.
Another important factor which is in general underestimated by small roasters and home aficionados is the availability and consistency of the ingredients you choose for your blend. A once in a time creation can be excellent but useless because you may be unable to get hold of the same ingredients you used for your first preparation. This brings it down to stocking green beans: historic blends need to be stable during the years, so that drinkers can feel the consistency of flavor and taste during years. To achieve this you need to have your ingredients ready when you need them and this means you need to build up a serious stock of green beans. This will allow you to source your ingredients with a decent anticipation. If, for instance, you’re using a particular Costa Rican from a certain “Finca”, and this gets unavailable for any reason; you need to have a sufficient stock that allows you to find an equivalent product from a similar other producer. This may take months, or even one year, and this is the minimum time your stock needs to guarantee.
Back to roasting and blending.
It is very important to know origins before blending them. Every green coffee gives its best at a certain roasting degree range, and it is within that range that it should be roasted. This depends on beans size, humidity and age.
Basically a blend should contain beans roasted at the most homogenous degree, this because brewing water has the same temperature and infusion time once you are preparing your blend, and this gives the correct extraction for each origin. In general the lighter the roast, the hotter the brewing water needs to be, and if the roast is dark, you won’t be running the risk of overcooking the blend by using water that is too hot. So it is mandatory to blend origins roasted at the same degree.
What are known as “harlequin” blends (the ones in which you find beans with different tones of brown, going from light beige to carbon black beans) are not a very good idea, because the water you are using to brew is only suited to have the best extraction for one of the degrees of roasting in your blend.
A good quality indicator for any blend is the homogeneity of the tones of brown in the same blend.
Attaining the same roasting degree on different origins may require different roasting times and temperature profiles, requiring single blend roasting and post-roasting blending; but if we are blending origins that are very close in their pre-roasting conditions, we can roast a pre-blended batch with excellent results. It is very important though to be very aware of what we are doing. A note on roasting time: quality roasting requires time. An espresso blend requires at least twelve minutes per batch, and some times up to sixteen, depending on what you are roasting; this is made at a relatively low temperature. Naturally you can achieve the same external look on beans by roasting for a short time at higher temperature, and this is what some industrial roasters do, but the quality in fact is absolutely different: a shorter roast gives beans that are roasted correctly only outside, while remaining raw inside. And the taste and aroma are deeply and negatively affected.
A rule of the thumb tells that delicate blends are more indicated to be drunk as straight espresso, a way to emphasize all of the flavors and fine aromas of the blend. These blends, predominantly made by Arabica origins should not be roasted too dark, in order to preserve the original characteristics of the beans. Some high quality Robustas may be used to correct the excessive acidity of the Arabicas and to give some more body to your blend.
If you’re thinking to a blend that is intended as a base for milk beverages, you may need something stronger, something which cuts through milk and this is obtained by roasting your blend darker, so using origins that give their best at a higher roasting degree. In general a higher amount of Robusta is tolerated in these blends because Robusta beans better stand deep roasting.
Knowing your consumers is as much important as knowing the destination of your blend: If they are used to deep roasts and are looking for it, it is useless to impose a perfectly balanced light roast blend, a dark “full city” is a way better, so using beans that better go with such a degree of roasting is a must. And so on, adjusting your blends to your goals.
I tried not to give a recipe of how to do the job, but to give guidelines that have to be evaluated while starting to work with origins and blends. Experience is essential in this job and you never have enough of it. Continuing to test origins, sample blending, sample roasting and tasting is the way to go. And this requires a lot of time, sessions are long and can get confusing if you examine too many qualities at a time. Work needs to be done methodically and with a straightforward approach. Confusion and mess lead to nothing.