In my last blog in March, which was about small batch roasting, I touched briefly on momentum during a coffee roast. Momentum is, of course, an analogy because heat transfer doesn’t have any inertia. However, it is a great analogy as a roaster works to control the rate of heat transfer during a roast.
A coffee roast starts with room temperature green coffee beans dropped into a preheated roaster. The temperature of the roaster when it is charged creates the initial momentum. During the early part of the roast you are applying heat pretty aggressively, “hitting the gas,” trying to get the roast “up to speed.” Discussion about why this early part of the roast is important for creating volatile aromatics and modulating flavors is something we’ll discuss in another blog. The important part is that we’ve got the roast moving!
The size of the charge, the weight of the green coffee in the roaster, is important in strategizing momentum. Controlling a coffee roast is analogous to driving a boat. The larger the boat the more energy it takes to get it moving. Changing direction of a large boat takes time and anticipation in order to get it to go where you want. I’ve watched barges on the Mississippi and it can sometimes take half a mile to make a relatively minor course adjustment. Additionally, slowing a roast down so that you are transferring heat at the rate that you want toward the end of the roast requires losing momentum.
Now if you’re roasting on a 500g roaster with your charge at or below spec weight, you are talking about driving a speedboat. You can apply energy, “hit the gas,” and get in moving very quickly. Changes in direction can happen nearly instantly. The amount of control is amazing! The drawback is that you will only produce about 420 grams (under a pound) of roasted coffee per roast! However the amount of nuance and flavor modulation, “fun whipping sh*tties,” is increased greatly.
Where is this boat/roasting chat going you are probably wondering? Understanding momentum is one of the keys to roaster control. In specialty coffee world, where you may be roasting an amazing, and pricey, experimental process microlot, roaster control is everything. I often say when explaining to people why being such a nerd about coffee is important, anyone can turn green coffee brown. Put the green coffee in the roaster and apply heat and it will turn brown. But making that coffee taste amazing takes roaster control, along with good green coffee and a solid roasting plan.
So the next time you are enjoying a delicious and complex cup of coffee you can think to yourself, “Wow! This roaster really knows how to control their momentum.” Until next month coffee friends, when I’ll be discussing how roasting coffee has affected the way that I cook in, I believe, a very positive way.