In the coffee roasting process, one of the most important factors to consider is the rate of rise (RoR). Popularised by coffee expert Scott Rao in his landmark work, The Coffee Roasters Companion, RoR refers to the progression of bean temperature per unit time during a roast.
According to Rao, a good roast is achieved when there is a continually decreasing RoR. The beans take on heat quickly at the start of the roast, and more slowly as the roast goes on. However, roasting coffee is a delicate process, and achieving the “ideal” RoR can be a great challenge for specialty coffee roasters.
To find out more about RoR and how understanding it can improve consistency in roasting, I spoke with MTPak Coffee Ambassador Michalis Katsiavos.
What Is Rate Of Rise And How Is It Measured?
RoR is the number of degrees per minute (sometimes measured across 30 seconds) by which the temperature of the beans increases at any point during a roast. Throughout a roast, this data is represented on a graph, which is often known as a roast curve.
In a typical roast, the RoR curve rises steeply before levelling off shortly before the first crack, and then falling rapidly around the time of the endothermic “flash”.
An endothermic reaction is when the beans absorb heat from the drum, with the endothermic flash occurring as a result of the surface-cooling effect of evaporation as large volumes of water vapour escape the beans.
To measure RoR, heat readings are taken using a temperature probe in the roast machine, which are translated onto a moving graph. Experienced roasters use this data to interpret what’s going on inside the machine and make changes accordingly to maintain a consistent roast profile. The three most relevant points for roasters to observe are the max RoR, RoR at first crack, and RoR when the roast has ended.
Michalis Katsiavos is head of quality control at Samba Coffee Roasters in Athens. He’s worked in the coffee industry for more than a decade, and was crowned the Hellenic Barista Champion in 2018. He tells me that the aim with RoR is to create a smooth curve.
“RoR is measured over a specific period of time, usually between 30 or 60 seconds,” he says. “For example, a RoR of 5 over a period of 30 seconds means that the actual bean temperature is rising by 5°C per 30 seconds.”
“In general, the aim is to create a smooth curve,” Michalis explains. “This is a sign that things are going well.
“Coffee beans require a more gentle RoR curve during the first stages of roasting. If we observe spikes on the graph, that’s when we should start worrying,” he adds.
A spike, or “flick”, on the RoR graph means that the beans are taking on heat too quickly. Coffee roasting software company Cropster suggests that a sudden rise in temperature at the end of the roast can cause the beans to lose complexity and take on a charred flavour. The beans become scorched, resulting in an uneven roast.
Conversely, if the heat of the roast begins to reverse and the beans start getting cooler, then it can produce what’s known as a “baked roast”, which results in coffee that tastes flat and bland.
For that reason, RoR is important for roasters in maintaining consistency throughout the roasting process. By observing the data points on the graph, you will be able to determine what may have gone wrong in the roasting process, and make the appropriate changes to the next batch.
The Impact Of Density, Bean Size, And Moisture Content
Before roasting, it is important for specialty coffee roasters to know the coffee’s density, bean size, and moisture content. This will ultimately dictate how much heat must be applied to the coffee to achieve the desired roast profile.
Factors such as shade, altitude, latitude, sunlight, and post-harvest processing all have the potential to influence these measurements.
For example, coffee trees cultivated at a high altitude tend to yield denser beans than those grown at low altitude due to a higher nutrient content in the cellulose structure of the bean. Torch Coffee Company state that a coffee bean is “basically like a honeycomb” with a “cellulose structure similar to wood”. This structure is designed to hold nutrients for the embryo, but some beans are only partially full, which affects their size, flavour, and density.
Washed coffees also tend to be more dense, which means roasters need to add more heat to the beans to penetrate the structure, and bring out the distinct flavours within.
In natural or dry processed coffees, higher levels of the cherry’s sugars remain in the beans. This tends to make them slightly more susceptible to taking on heat, which in turn, affects the amount of heat that will be required during roasting.
If a roaster has a good grasp of the density, bean size, and moisture content of the beans they are roasting, they will be able to control the RoR more effectively, resulting in a more consistent and balanced roast profile.
Roasting coffee beans requires close attention to detail. It is a sensitive process in which the smallest of changes can have a dramatic effect on the outcome of the coffee. Too much or too little heat at the wrong time will create an imbalance of flavour and could lead to batches of roasted coffee being wasted.
RoR allows roasters to study precisely what’s going on during the process, and make changes accordingly. By presenting the process in the form of data points on a graph or roast curve, roasters can understand the points they need to focus on to produce a consistent roast profile.
At MTPak Coffee, we work with many different specialty coffee roasters, and understand the time and effort that goes into roasting high quality coffee. We provide coffee packaging solutions that reflect your hard work by showcasing the quality of the product inside the bag, while protecting the coffee’s freshness.
For more information on our range of coffee packaging solutions, contact our team.
Photo credits: Cris Flores