The Lab

The Basics: Roasting Coffee

Starting to roast your own coffee can be daunting, but it doesn’t have to be! It can be as simple or complex as you make it. What matters is making coffee that you and your customers enjoy! There are variables you can adjust to manipulate the flavor profile of the coffee to create coffees that you can stand behind. We will talk about what’s happening in the roaster and certain aspects of the roast that you can easily adjust to bring out the best aspects of your coffee.

What’s Happening in the Roaster?

The beginning of the roast sets up the trajectory of the roast curve. After the coffee is added to the drum, the read-out from the bean temperature probe will drop quickly. This is a result of the temperature of the probe cooling to reach the temperature of the (room-temperature) beans that were just added to the chamber. Once the equilibrium is reached and the bean probe is the same temperature of the bean mass, the probe temperature will start to increase with the temperature of the beans. This is known as the turning point. In these first few minutes of the roast, the beans are starting to dehydrate and the chlorophyll is breaking down as the beans change from green to yellow.

Coffee browning is an indicator of caramelization and maillard reactions. Maillard reactions are a complex chain of events that can result in multiple different products. Simply, it is a simple sugar + amino acid reaction that yields flavor compounds. Melanoidins are produced by maillard reactions and they are responsible for the coffee’s brown color. They are perceived as roasty, toffee, malty flavors. Caramelization it is the breakdown of complex carbohydrates and sugars into simpler sugars. These sugars form reactants of further maillard reactions. Both chemical pathways produce other aromatic compounds that impact flavor like ketones, aldehydes, esters, acids or alcohols, but maillard reactions can result in a wider range of flavors through a greater number of reaction pathways. Spending more time browning can result in more aromatic flavor compounds, but only to a certain extent. Eventually these reactions will create unpleasant flavors as sugars burn and acids decompose.

Water will boil as the internal bean temperature rises and vapor pressure increases. The pressure will force the cells to explode and crack audibly. This is called first crack, and it is an important marker in terms of the overall roast profile. How quickly or slowly you reach this point is an indicator of your heat transfer throughout the bean. Faster first crack times are generally better suited for higher density coffees, and later first crack times are better for lower density or higher moisture coffees. Low moisture coffees can reach first crack quickly, but that is not always best depending on the processing style and terroir.

Acids like citric, malic, and phosphoric are generated during maturation of the coffee cherry at origin. They are broken down by the roasting process, especially after first crack when there is very little moisture. Chlorogenic Acid (CGA) breaks down into quinic and caffeic acids. These, along with acetic acid that increases as a byproduct of caramelization, create bitter flavors. CGA is less soluble than Quinic or Caffeic and will only be present in more extracted brews.

What does Development Mean?

Development refers to how evenly the bean was roasted and how long caramelization and maillard reactions occured for. The exterior bean surface will always be more developed than the interior, but we are roasting to control these differences. We want to roast the coffee enough to develop complexity, sweetness, and body without degrading too much of the acidity. Less development will be identified by less refined sweetness, greater acidity, less body and sometimes vegetal flavors. Greater development is characterized by a lack of acidity, over caramelized sugars, and heavy body. Certain coffees will taste best at lower development levels and others will taste best with greater development.

An important tool for tracking development is the RoR or Rate of Rise of the bean temperature curve. The rate of rise is how quickly the bean is heating up, calculated by finding the difference in temperature over a given time period. As the roast progresses, the rate at which this heating occurs should decrease, because we are trying to prevent the exterior bean from developing much faster than the interior bean. If the RoR remains constant, the exterior surface will keep developing at the same rate and the interior will be developing at a consistently slower rate. We need to slow the development of the exterior by decreasing our RoR. If the RoR stalls – drops quickly or below zero – there is no longer heat efficiently flowing towards the center of the bean.

Controlling Development Depends Primarily on Moisture, Density, and Size

Higher moisture coffees will require more time yellowing and more heat to dry them thoroughly.  More moisture being vaporized will create more steam, slowing heat absorption by the bean. Maillard reactions and caramelization reactions will need more time to occur since there is more water absorbing heat. Later first crack target times and applying adequate heat, should generate good development. Lower moisture coffees will be delicate and heat quickly. They generally require gentler heating depending on the bean density.

Denser coffees will transfer heat to the center more quickly because denser materials absorb and conduct heat more effectively. These coffees can withstand high charge temperatures and high heat application, but you may find yourself with too short of a roast if there is not adequate time for maillard reactions and caramelization to develop complex aromatics and supporting sweetness. Low density coffees run the risk of roasting unevenly or scorching. Heat transfer is slow to the center and will require gentle roasting throughout for even development.

Bigger beans have a greater distance to the center and will require more time to allow that heat to transfer. Heat application that is gentler will prevent the outside layer from overdeveloping.

Artisan Roast Profile of a Washed Ethiopian Coffee

How We Control a Roast

Roast Variables

The first variable is charge temperature, or the temperature of the roaster when the coffee is added. Higher charge temperatures set up a faster roast and vice versa. Charging too high may lead to scorching, especially in lower density coffees or with smaller batch size. Charge temperature should remain relatively constant, and it should change primarily with batch size. This will allow for smooth production flow in regard to your in-between batch protocol. Having wildly different charge temperatures for all of your coffees will slow production and will not be repeat-able.

The gas setting at the beginning of the roast has a similar effect to charge temperature. High gas settings will generally set up faster roasts. On some roasting equipment or with certain coffees, you might want to add coffee to the drum with the gas very low or off for 30 seconds to 1+ minute and “soak” the coffee. This would be most commonly used in roasters with large flames directly under the drum to avoid scorching by decreasing the difference in temperature between the hot metal and room-temperature green beans. Lower density coffees could also benefit from soaking since the coffee transfers heat less efficiently and requires more gentle application of heat.

Bigger batch sizes will require higher temperatures and higher gas settings and the opposite is true for small batch sizes. Smaller batch sizes could also benefit from soaking since there is a relatively larger portion of the beans coming in direct contact with the hot drum which could also lead to scorching. Whenever possible, your batch size should remain the same. This will increase predictability and repeat-ability.

Airflow Diagrams of Diedrich and Loring Roasters

The effect of airflow will depend on the source of the air. In convection style roasters like the Loring and Stronghold, the air is the primary source of heat. Higher airflow in this style of roaster will increase the speed of the roast as a result of increased heat and greater friction. For traditional drum roasters, the source for air is cooler and will slow the progression of the roast.  In both styles of roaster, airflow carries smoke and chaff out of the roasting chamber and is important in creating crisp and clear flavors. If a roast tastes smoky but is still light, greater airflow is needed.

It is important to track your roast either using data-logging software like artisan or by using a pencil and paper to record the data manually. The more data you collect, the easier it is to draw conclusions about how changing roast variables impacts flavor. Software can typically collect more information about the roast in real time compared to manual recording. Information like inlet air temperature will help you predict how the roast will progress since the drum temperature directly impacts how the bean temperature will change. The more data you have while roasting, including your senses, the more informed your decisions will be and more accurate your improvisations will be.

When you begin roasting, it is important to change as few variables at a time so that the impact on flavor can be tied to a specific change. This will more clearly illustrates the relationship between the roast variable and flavor. When trying a new type of bean, it may be advantageous to start with small quantities to see if it works with your roasting style. Royal NY offers Fractional Bags for those looking to try new beans without the commitment of purchasing a full bag. You can experiment with different types of processing styles and origins with less risk. The most important part of experimentation is documenting your process and drawing clear conclusions based on the data. For our complete Fractional Bag Offering click HERE.

When roasting coffee, it is important to taste as many of your roasts as possible and record as much data as possible. Tasting coffee from different roasters can help you gauge how effective your roasting strategies are. Roasting more coffee is the best way to become a better roaster, especially if you can roast coffee of different varieties and cultivars at different elevations processed using different methods.

Patrick McKeown

Patrick began his coffee journey on Long Island at a small coffee bar and roasting at home. Since then he has been a barista, manager, and craft roaster in NYC.