July 3, 2020

Using Roast Curves To Guide The Coffee Roasting Process

Keeping track of the variables that impact a coffee roast can give roasters valuable insight into how the roasting process works. As heat is an important part of roasting, this makes temperature a variable worth tracking. Creating a roast curve to track these temperatures can help roasters better predict a roast’s outcome, or adjust their roast temperatures for better results.

Coffee roasting software creator Cropster and green coffee importer Balzac Brothers recently launched the RoastID competition – See the Curve, Match the Curve to test the knowledge of roasters worldwide. This online challenge featured three rounds, where participants had to analyze five roast curves to progress to the next round. Participants had to match each roast curve to its corresponding cupping score in order to predict how the temperatures plotted in each curve would impact the coffee’s score and its qualities.

The competition was supplemented with educational webinars and Q&A sessions by various experts, discussing how each curve would impact the roast it described. Here’s what these experts had to say, and how following different curves impact the coffee being roasted.

Lee este artículo en español Cómo Guiar la Tostión Del Café Usando Las Curvas de Tueste

What Are Roast Curves & Why do They Matter?

Roast curves help monitor roasts by tracking the heat in a roast drum at key times throughout the roasting process. They can help roasters create a guideline for reproducing a roast profile by demonstrating how applying the same amount of heat to a batch, at the same time, can yield consistent results. 

Roast curves can help identify and track the phases that coffee goes through during the roasting process such as the drying, Maillard, and development phase. Each phase’s length can impact a coffee’s taste. For example, the duration of the bean’s drying phase will impact its acidity and body, while the duration of the bean’s Maillard phase will impact its sweetness and caramelisation. Roast curves also monitor whether sufficient heat is applied during each roast phase.

Often, a roast curve is used to form a reference curve. Following this reference can help roasters recreate a desired roasting profile, by matching the temperatures and times of an ongoing roast to the reference’s temperatures and times. For example, a reference curve for roasting Kenyan coffee might include a higher charge temperature, as Kenyan beans tend to be dense. Alternatively, a reference curve for a low acidity Brazilian coffee might include a longer Maillard phase, to increase the coffee’s sweetness and body.

How Coffee Processing Methods Impact a Roast

In round three of the RoastID competition, management executive for List + Beisler Joe Marrocco mentioned that a roast curve will be impacted by how the coffee was processed.

He says that as natural coffees are dried with their cherries attached, the beans spend more time in a dark and moist environment, encouraging them to ferment and break down before roasting begins. As a result, these beans will need to be roasted slowly, to preserve their inherent sweetness. A washed coffee would need to be roasted faster, to preserve its inherent acidity.

A table used in the RoastID competition to detail some green coffee qualities that are important to note when deciding how to roast a coffee and what the cup quality will be.

Choosing the Correct Charge Temperature

When following a roast curve, adhere to the charge temperature indicated, which indicates how much energy is inside the drum for the start of the roast. Start at a lower or higher charge temperature than on your curve and you risk starting your roast incorrectly or lose control of the roast later on.

During the first webinar, Anne Cooper, Roasting Consultant at Equilibrium Master Roasters, mentioned that starting with a too low charge temperature can leave you behind the roast curve. This could force you to speed through the Maillard phase to catch up with the curve, resulting in an underdeveloped roast. When the roasting curve for an underdeveloped coffee appeared in the competition, the flavor notes indicated that coffee roasted this way will develop grass or hay-like notes.

Anne also said if you start with a too high temperature, you’ll likely still be able to follow your roast curve. However, in the process you could create too much heat in the drum, scorching your beans and creating burnt, smoky, and spicy flavours.

A roast curve used during the RoastID Competition indicating an underdeveloped roast. Here, the roast’s starting temperature was too low, causing it to fall behind the reference curve present.

Managing The Maillard Phase

A roast’s Maillard phase takes place when its heat causes a reaction between the bean’s carbohydrates and amino acids. This reaction produces Melanoidin molecules, browning the beans and generating flavour and body. A good practice to achieve the desired cup profile is adhering to the duration of the Maillard phase outlined in a roast curve.

Rob Hoos, the owner of Hoos Coffee Consulting, explains that rushing through or overextending your roast’s Maillard phase could negatively impact the coffee’s cup profile. Overextending the phase can create overly savory flavor characteristics, which might not be the desired result

A table comparing the duration of time and temperatures experienced at different phases of five different roast curves The notes above indicate the changes found in each curve (compared to the baseline) during the competition.

Handling First Crack, Development Time & Turning Points

First crack takes place after the Maillard phase. It describes what happens when the coffee beans expand and pop open due to heat and pressure, releasing the steam and carbon dioxide trapped inside them. A roast curve that reaches first crack quickly (compared to the reference curve) and then is dropped out soon thereafter, may develop light, bright, and vegetative flavours.

The phase that starts directly after first crack and ends when the roast finishes is called the development phase. A roast curve that has an extended development time will usually have a shorter Maillard phase, creating the perception of more acidity and sweetness.

In the final phase of the roast, the development phase, the time in which the bean spends after first crack has a noticeable influence on caramelization, acidity, sweetness, and overall flavors. The shorter the time, the brighter and less carmelized the flavors may be, whereas the longer the time, the more “developed” or browned, caramelized, and less bright the flavors may be. 

Stalling is one issue that may occur during this phase, which was discussed by Anne and Shelby Williamson. Shelby is the reigning US roast champion and head roaster for Huckleberry Roasters. This occurs when not enough heat is applied and the bean rate of rise drops too quickly, or drops below zero. It is most common to see this at the end of the roast when heat application is often the lowest.

A roast curve used during the RoastID Competition, indicating under roasted beans. This kind of roast curve is characterized by heat loss during the bean’s development time and a lower final temperature (compared to the reference).

Every roaster should follow their own roast curves, and tailor these curves to their machine, green coffees, and customers’ desired roast levels and flavors. Following a customized curve will ensure that these roasters keep producing roasted coffee of a consistent quality and avoid making costly errors.

Photo credits: Cropster and Angie Molina Ospina

Cropster and Balzac Brothers are sponsors of Perfect Daily Grind

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