Coffee Roasting Series #4- Momentum (& Heat) and Airflow

Today we will discuss the next 2 principles of roasting: momentum and heat application, and airflow.
  1. Momentum and Heat Application


During DRY, use as much heat as your roaster allows without creating defects. Following the soak (if you choose to soak), that is when you will apply the greatest amount of heat to your beans. This is because the beans are at their most resilient and they need to build up momentum and internal pressure, for good flavor development. This can vary by bean. High density beans, kept fresh in green sealed bags, can take a lot of heat. Low density beans, decaf beans, or beans stored in open air for a while will be far more sensitive, regardless of size or original density. This is where lots of experience with your machine will come in, as you need to build some intuition.


If you have roasted lots of quality beans on your roaster, you will notice consistent Turnaround, or Turning Point temperatures. This reading is somewhat arbitrary, but highly useful once understood for a particular machine. If your Turning Point temperature is usually 180 for your favorite washed Colombian bean, and then you notice that your Natural Brazil Turning Point was 190, you will see that the Brazil bean is absorbing heat much more easily. You could have guessed it would, in part because of its likely smaller size and lower altitude of origin, among other things.


Or maybe you get a new Colombian bean, but you notice it has been kept in a burlap bag with no green sealed bag inside. You might expect that it has dried out somewhat, perhaps a lot, between the farm, the warehouse, and you. You will likely need to drop charge temp a bit and use less heat. You can get an even more accurate reading of density by buying special meters, if you wish.


Also note that the less moisture a bean has, the more difficult it will be to develop flavor. Decaf is notorious for this, and the problem applies to exposed or old green beans as well. Despite the coined name "drying phase," coffee beans need moisture in them during MAI for Flavorful Reactions to occur. This is why you cannot be heavy handed when roasting decaf. If you push too hard too fast, you will remove too much moisture too soon, and your coffee will inevitably be flat. Of course, if you push too slowly and lightly, flat coffee can still occur, but for reasons of inner-bean momentum. Use a similarly dense bean as a guide for noting Turning Point and the ROR curve.


  1. Airflow


Although the best roasting guides usually suggest only having one airflow setting throughout the roast, or only changing airflow once, you will quickly find that almost no one does this in real life. Airflow will usually need to be changed for lots of reasons. Airflow’s primary purpose is to facilitate the movement of hot air over the surface of beans for convective heat transfer. Roasting is, depending on the machine, usually a work of both conductive and convective heat transfer. Higher airflow means greater flow of heat energy into the bean.


Airflow can also be used to clear out chaff around Dry End, or the point where you get initial color change from green to a tannish yellow. (After chaff is removed, airflow is usually reduced again.) Airflow can also be used to regulate heat. On many roasters, as the roast progresses into First Crack and beyond, the beans begin to be a source of heat unto themselves, and your heat can get out of control. While you will usually need to reduce heat after First Crack, you may also need to increase airflow to let some hot air out of the system.


Some beans, such as Ethiopian, are prone to the opposite: massive drops in increasing rate of heat after First Crack. Reduced airflow in this case can help keep your momentum going during the development phase. Similarly, if you cut heat in anticipation of a heat increase after First Crack, and find your roast stalling out, reducing airflow can again help avoid that loss of momentum.


Generally, airflow is lowest at charge, increased around Dry end, and increased just before or after First Crack. It can be increased again moving into Second Crack, as this helps remove the additional smoke that the bean is producing by this point. Trapping beans with smoke as they reach these temps leads to a smoky tasting coffee (maybe you want that, maybe you don't).


Thank you for reading another post in this series. The next post will be about principle #4 - heat reduction and ROR curves.