About Oily Coffee
There's a common misconception that dark roasted coffees with an oily sheen is bad for your espresso. We're here to tell you that there is no truth to this old wive's tale.
What you should really be concerned about is old, stale coffee.
All coffee beans have varying amounts of oils within the coffee beans. This is perfectly natural, and as long as the coffee is freshly roasted and stored in 1-way valve bags, you have nothing to worry about.
What happens when a coffee is roasted darker, is that the oils migrate from the inside of the bean to the outside of the bean. This oil doesn't magically manifest from nowhere - it's as present in medium roast as it is in an espresso roast, and even in green coffees.
It's therefore irrelevant how much oil is on the surface of a bean.
Regardless of what roast you choose, or how much oil is on the surface, the oils will be exposed to your grinder when the grinder does it's job: breaking the beans into a find grind. This means your grinder will touch all the oil inside of a coffee bean, regardless of whether it's on the surface or not.
Clogged Grinders Caused by Stale Oil
What happens when the oils are exposed to oxygen for long periods of time, is that the oils chemically change, becoming sticky, as well as losing flavour.
The most prolonged aspect of coffee staling is lipid oxidation. This occurs in stages. The first stage is the uptake of the oxygen by the oil and production of peroxides. Any oxygen taken on by coffee can cause this to occur. Like all oxidation processes, two chemicals are formed. Peroxides create breakdown products (highly aromatic undesirable substances), then attack an unoxidized lipid molecule to re-form peroxide. The peroxide acts as a catalyst; the more peroxides present the faster the oxidation. Stale flavor is significant after 2 weeks of storage in the presence of oxygen. The process is one of acceleration: once it begins, products of oxidation increase until all possible paths are exhausted and the coffee is dead stale.
Paul Songer - SCAA
The solution is buy your coffee fresh roasted from a company like BuyCoffeeCanada, as whole bean to limit the amount of oxygen exposure, and not worry about "how much oil" is on the bean.
What's important is to order your coffee fresh roasted, whole bean, and in a 1-way valve bag like the ones we sell our coffee in. These 1-way valves allow natural gases to escape the bag, and prevents oxygen from re-entering.
Choosing a Coffee
You should choose your coffee based on flavour alone. Period.
A coffee will be at peak flavour within 2-4 weeks of being roasted, so buying it fresh is key.
A coffee will retain the most flavour as whole bean, as opposed to ground, so buying whole bean and grinding just before brewing is ideal.
How you store the coffee is important as well - it should come in a 1-way valve bag that allows gases to escape, and doesn't allow oxygen in.
Studies have shown that there's only a 5% difference in the caffeine level of a medium roast and a dark roast coffee, so there's no point in worrying about which roast has the most caffeine.
Finally, whether you buy a medium roast or dark roast, they will have the same amount of oil in the coffee bean. Surface oils are just oils that have migrated from inside the bean to the outside - a medium roast has exactly the same amount of oil as a dark roast.