A note about espresso
For espresso training, there are 2 main aspects to consider, the ability to prepare it (barista skills), and the sensory evaluation (tasting).
Here is a brief review of the major variables involved in espresso preparation.
James Hoffman is a former WBC champ, owner of one of the most highly regarded roasters in the world, and a sought after speaker at coffee trade shows and event. This post is at a fairly high entry point, in other words assuming a good grasp of the basic principles of espresso extraction.
There has been a rapid pace of advancement in espresso, in the last 10 years, particularly in the technical consistency of the espresso machines to product stable temperature and pressure, and the knowledge around how to work with the grind fineness, dose (amount of coffee), pressure and temperature to balance the shot and extract the desirable tastes. This involves the use of precision scales and timers for control and consistency. As you go around, look for evidence of this in the espresso bar. Ask about whether they use a timer and a scale to help dial in the espresso grind, whether they are using an ‘on-demand’ or dower-less grinder, and if their espresso machine has temperature adjustment (PID). If they do, you can surmise they are a quality focussed shop.
In some cases, the head barista has learned espresso from a strictly sensory point of view (tasting and experience), and learns to evaluate espresso on the basis of visual cues. Rate of flow, and the start of blonding of the espresso stream as it pours out of the spouts are used to judge when to stop the shot. The adjustment of the grinder and coffee dose is done in an craftsman-like way, rather that scientifically. Either approach can produce a tasty espresso.
As far as tasting goes, the importance of a fine balance between bitter, sweet and acidic components is crucial for a satisfying drink. Mouth feel is also very important. A creamy, smooth feeling in the mouth is a most enjoyable aspect of espresso. Most tasters and bloggers tend to focus on individual taste aspects (chocolate, caramel, cherry, blueberry, etc.) which can be somewhat misleading if you are reading about it, rather than tasting it.
There is a strong trend towards highly acidic (sour) coffees, some of which are used for espresso. This is a result of very light roasts. There is also a trend towards highly overdosed brews (whether coffee of espresso). Traditional Italian espresso uses 14grams of coffee to produce 2 oz. (60ml) of liquid. A modern espresso ristretto (restricted water flow) which has become normal in cutting edge bars is 21g of grounds to produce a scant 1oz (30ml) of espresso. This produces an extremely concentrated espresso.
Both acidic and ultra ristretto espresso should only be attempted by barista who have obsessive control over extraction standards. These are extremely difficult to produce with consistent quality.
A sweet, balanced coffee blended for espresso in suitable for achieving good results from a wide variety of equipment and skill levels.