At the foundation of every great cup of coffee is an amazing raw ingredient, but there is a wide array of roast variables, styles, and approaches that unlock a coffee's flavour potential. So how do we get there? A detailed recipe of course! Just like any raw ingredient, the recipe and the way you cook it will dictate the final flavour.
In earlier posts, we learnt all about the different stages of the roast, how the roast progresses and the sensitive chemical reactions we carefully monitor to bring out the flavours we love in a coffee and avoid the ones we don’t. In this entry, we’re finishing up our 3 part series by looking at a coffee’s roast profile, and how our roasting team monitors and manipulates different variables during the roast to unlock each coffee's ideal flavours on each and every roast.
What is a Roast Profile?
A ‘roast profile’ is a specific record of roasting variables, including the batch size and all the data logged by thermocouples that we use to roast a specific coffee. The profile is the ideal roast curve of a coffee that is chosen based on the desired flavours and overall roast degree of the coffee. Our goal as coffee roasters in production is to follow the roast profile as exactly and consistently as possible every time we roast a coffee.
How do we follow the profile?
We use a roast curve. A roast curve is the plotted relationship between time and temperature of the coffee in the roaster (represented by readings from thermocouples) as a result of heat application. We use a program called Cropster for this that displays the temperature of each of the thermocouples as well as the rate of temperature change (often referred to as RoR or rate of rise) of thermocouples in real time.
What are the 4 main variables we use to ensure the roast will follow its profile?
1. Batch Size is the amount of green coffee that is charged (introduced) into the roaster at the beginning of the roast. The batch size is chosen based on things like the attributes of the green coffee, roasting machine capacity, desired roast approach, and order volume. Larger or smaller batch sizes will greatly impact the overall roasting process and will impact both charge temperature and gas application.
2. Charge temperature records the ambient temperature of the various thermocouples inside the roasting system right before the coffee is charged into the roaster. We use the word ‘charge’ as a reference to the amount of thermal energy that is being stored in the roasting system. Charging a coffee at the same temperature as the desired roast profile is extremely important for executing a roast to specification and taste consistency.
3. Heat Application refers to the amount of heat that is being applied during the roasting process. This is displayed in a roast profile so that the roaster operator is aware of how much gas to use at different points during the roast. Many different factors can affect the decision of how much gas to apply when in any given roast such as moisture content and density of the green coffee, desired roast approach, batch size, charge temperature, and even external factors like the temperature and humidity of the warehouse and ambient temperature of the green coffee.
4. Temperature is measured by thermocouples that are placed in different areas inside of the roasting machine to give a real time reading of the temperature of their location. The roaster operator monitors these carefully and uses their temperatures as a reference to charge batches appropriately, adjust heat application, and execute roast profiles consistently. One of the most important thermocouples is located within the bean mass while the coffee is roasting. We call this the ‘bean probe’ and it measures the temperature of itself while buried inside of the coffee during the roast. These readings are able to give us the most direct and accurate indication of the temperature of the beans inside the drum and helps us to drop the coffee out of the roaster at precisely the right temperature.