In the past few weeks we've rolled into our newest Kenya offering: Kenya Gititu! A lot of work goes into releasing a new coffee, so we wanted to take this chance to give a little behind-the-scenes into the roasting process.
When we roast a coffee, we develop what's called a "roast profile": think of it like a recipe for roasting coffee. We change the temperature we start the roast at ("charge temperature"), how much heat we apply at different stages of the roast with an eye on time spent in particular areas, and ultimately when & at what temperature we finish a roast.
We use an innovative, American-made roaster called a Loring that draws heat from a very high strength burner. The body of the roaster contains a drum with rotating paddles (think of a very big, very hot clothes dryer) where the coffee tumbles during the roast. A circulation fan pulls air from that burner into the roast chamber. The actual control for how much heat is being applied is through the speed of that circulation fan.
Unlike other roasters where a burner is applying heat directly to the roasting drum, the fan setup means that we can be very nimble in our roast profile, rapidly speeding up or slowing down to build momentum or draw out the time during different phases.
Evaluating Raw ("Green") Coffee
This coffee was a fun challenge for our roasting team. Over time, we've developed a set of baseline profiles for different types of coffees that we use as a starting point; coffees grown in the same area, of similar varieties, at similar elevations & processing tend to share some similar characteristics for ideal roasting. This means that sometimes developing a profile for a new coffee is pretty straight forward!
Gititu has one major difference than our prior Kenya: screen size. "Screen size" is a measurement of the size of the unroasted, raw coffee bean (we call it "green coffee", because of it's pale green color). In Kenya, you frequently see coffee referred to with a letter-based grade that indicates a certain screen size.
The two most common grades of Kenya we work with are AA & AB. "AA" is the largest screen size of coffee; "AB" is a slightly smaller screen size. We don't find any general rules about coffee quality between AA & AB lots, but for roasting it means a lot!
Roast Profile Changes
Our last Kenyan offering was an AA coffee. Gititu is an AB grade coffee, and is a smaller, denser coffee. Roasted on the same profile as the prior lot, it would have been dull & flat. The difference comes down to internal bean temperature. When we're roasting coffee, we're measuring temperature in the middle of a pile of coffee being agitated & tumbled, but ultimately that only tells us what's going on in the surface of the beans.
The internal temperature of the coffee is different than the surface temperature, just like how you can sear a steak but have the inside be nearly uncooked. In this case, Gititu needs more time to be able to efficiently "soak up" heat and develop the flavors we're looking for.
The result? The most significant tweak is using a longer, drawn out time period, especially during the period at the end of the roast called "first crack" (when the coffee audibly pops as it expands). Looking at the profile, it has a higher temperature at the end of roast than most coffees of a similar roast style, but in the end it's flavor we're after.