I've been often asked "Hey, why are my beans all oily?"
Well, there are a few reasons for this, but before I get to the science of it, here's some basics about coffee:
Coffee beans essentially have three components to them (note this is a HIGHLY simplified explanation): the fibrous woody material, fats and protiens, and moisture (which comes from the woody material). When a bean is unroasted or green it is mostly this dense woody material with all the oils and moisture trapped inside. As it is roasted, the cells break down and release the moisture and oils.
That's the basics, but here's the details (I do believe that's where the devil might be) of what happens when you roast a bean. As the bean heats up, the cellular material of the bean breaks down. This releases the coffee oil (sometimes called coffeol - I don't know why) and leaves the woody material. Now, it takes a lot of heat over a longer period of time to pull the oils to the "top" of the bean, and that's why the darker roasts seem oily. As a bean is roasted even further, the sugars are caramelizing, or worse yet cooked right out of the bean (these dark roasts are called Vienna or Italian or the particularly dark and flat, Spanish roast). All that you are left with is essentially a flat tasting oil and wood.
This is why I don't do Italian or Spanish roasts - there are very, very few beans (arguably, perhaps none) that will take that dark a roast and remain tasty.
Interesting side note to all this, as you roast a bean darker, it cooks the caffeine out of it too. So, a darkly roasted "espresso" bean will have LESS caffeine than a properly roasted lighter brown espresso bean.
Let's dig a little deeper into the science of this to better explain why the perfect roast is so important.
When a bean is put in the roaster, the first thing that happens is the roaster heats the bean up past the boiling point of water, that cooks the moisture out of the bean , and starts to change the color of the bean. As the temperature inside the bean climbs from about 170 to 200 Celsius, the sugars in the bean start to caramelize. This reduces the sweetness of the bean, as uncaramelized sugar is much sweeter tasting than caramelized sugar. As the bean caramelizes, the sucrose (sugar) is essentially cooked brown inside the bean. When the bean hits 205C (by the way, that's 401 Fahrenheit), it has its "first crack" - which is just what it sounds like, a cracking sound as the cells of the beans are rupturing (city or city+ roast level).
From here, the longer the bean is roasted, the more the sugars are caramelized reducing the sweetness. However, at the same time, the more the you roast, more of the bitter tasting compounds are broken up and cooked out of the bean. The key to the perfect roast is a balance between not too sweet, but not too bitter either. There is a "second crack" as the bean breaks down further which occurs when the bean hits about 220-230C (Full City or Full City+). We should all think of this as the limit for a bean. Typically, RR won't roast past the second crack (about 225-230C) - this is a good French roast level and is pretty dark.
This is particularly true of espresso. When roasting espresso, many companies tend to focus on roasting the beans very dark to remove the bitterness of the espresso.
But... (now let's see if you have been paying attention) What happens when roast a coffee bean longer?
You're right! The sugars caramelize and remove all sweetness from the bean, leaving a flat, almost stale-tasting, uninteresting espresso.
(Note that a lot of the specifics of this info came from coffeeresearch.org)
I've heard it a thousand times, customers tell me that they prefer my beans (hopefully!) or X brand (unfortunately) coffee to Starbucks because "Starbucks tastes burnt." Why is that? There are two things going on here to give it that burnt taste. First, Starbucks tends to roast their coffees on the dark side - some of them take this very well, many don't, but all seem to be roasted the same regardless. Second, when they make their espresso, they scald the grounds which burns them further. The final step is to add milk or creme and/or sugar to put the sweetness back in, but if you just order espresso it lacks that sweetness. In Starbucks defense (I can't believe I'm defending them!), they did just announce that their baristas were not as focused on the product and customer as they were in their heyday (duh. Ya' think??). Management will close every Starbucks to do a training session with their baristas - so who knows, perhaps their espresso will be good again. I refuse to believe they'd be as big as they are if their espresso was always as craptastic as it is now.
Well, there you go, the science behind coffee roasting and how that relates to Starbucks (who knew this post would end up there!).