Peace Coffee

Joseph Maurey is Head of Coffee Roasting & Quality Control at Peace Coffee. He brings 21 years of experience in the specialty coffee industry, leading roasting operations, quality control and production teams. Maurey is committed to maintaining coffee quality, consistency and operational excellence throughout the roasting process.

As a process, coffee roasting is thermal, thermodynamic, and irreversible. Seeds of the coffee plant, at this stage called green, are transformed from a raw material into a usable product known as coffee beans. In the specialty coffee segment of the industry, where value is placed on coffee with distinctive attributes, the basic logic of coffee roasting is that the experience of the final cup is ultimately determined by the quality and characteristics of the green coffee. It is the role of a coffee roaster to maximize that potential through the controlled and strategic application of heat, airflow and agitation.

For roasters, what makes this challenging - and interesting - is that there is no one single approach. There are certainly a wide range of roasting systems and capabilities, roasting styles, roast levels (light, medium, dark, for example), and customer preferences. In other words, for a single green coffee, there is a range of possible expressions. At the same time, no two green coffees are quite the same. This is the result of a confluence of factors such as plant species and variety, soil, climate, topography, and agronomic practices. During the harvest cycle, once the fruits of the coffee plant, known as coffee cherries, are picked, post-harvest processing commences, followed by milling, grading and sorting. Each step in the process has a profound impact on the quality and character of the green, so it is not surprising that coffee professionals and coffee lovers often talk about terroir, the sense that, in the final cup, the overall flavor perception evokes a sense of place or process.

The Science behind Every Roast

The roast curve - called such for its characteristic shape when plotted on a graph - is a record of temperature over time, as well as gas and airflow settings during the roast. A lot of the discourse within the specialty coffee roasting community is about the process of landing on an optimal roast curve for a particular coffee. This is known as roast profile development, which, at the end of the day, is more about how the coffee tastes than anything else. The curve is just the path.

In specialty coffee, the craft of roasting and the quality of the coffee are always first and foremost. For the purpose of this discussion, let us assume the coffee is specialty grade and that the blend recipes and roast curves are dialed in. You are happy with the way the coffee is tasting. Let’s instead focus on the coffee roasting process from a manufacturing perspective. Specifically, what can the roast curve tell us about manufacturing specialty coffee at scale?

“Coffee production is a multi-step process ripe with variability at all stages.”

Because quality is the primary consideration, the roast duration is variable depending on the roast curve. Once the roast is finished, there is some period of time before the roaster can start the next batch. The period, known as BBP (between batch protocol), is entirely dependent on two factors, one being the minimum length of time it takes for the roasting machine to thermally reset, the other being the requirements of the operator’s workflow.

For maximum efficiency, the target BBP should equal the minimum duration required to thermally reset the system. A significant variance between actual and target BBP could indicate bottlenecks in production, gaps in processes, opportunities for improving workflows, inadequate resource utilization, or issues with thermal management. In order to get a true average BBP per run, all idle periods, whether planned or unplanned, must be accounted for and removed from the equation. A byproduct of this is the ability to measure resource utilization per roasting line. A practical way to calculate average BBP with data from roast logs uses the following formula:

Average BBP per Batch (minutes) = ((End Time of Last Batch - Start Time of First Batch) - (Total Roast Duration + Idle Time during Roast Session)) / (Total Number of Batches - 1)

The batch duration is the total of the roast plus BBP:

Batch Duration (minutes) = Roast (minutes) + BBP (minutes)

It’s now possible to calculate a theoretical daily run rate per roast curve using the target roast and BBP durations, and compare this tos the actual run rate:

Daily Run Rate (Batches per Hour) = 60 (minutes) / Average Batch Duration (minutes)

Scaling Without Compromising Quality

The target run rate is not the same as the machine’s rated capacity, although the roast curve represents a theoretical capacity based on each individual coffee and its roast curve. Minimizing variance relies upon the roaster’s skill to roast according to the curve and accurately manage BBP. Routines are critical, as production roasters are responsible for sequencing three batches simultaneously - one batch is queued in the loader, one batch is in progress, and a third is cooling. Production roasting routines will vary depending on the level of automation. If the system skews manual, with limited or no data automation, then production roasters may spend an unnecessary amount of time making calculations and verifying roast curves, colors, and yields are in spec. It is possible, using just a spreadsheet and the basic principles of statistical process control (SPC), to configure real-time pass/fail responses based on whether or not the data point captured by the operator falls within the tolerance range relative to that which is being measured.

Improving overall run rate to build capacity is possible and is one of the primary concerns for the head roaster. After all, a higher run rate means the potential for more pounds per hour. Like the roast duration and BBP, pounds per hour is variable in that different roast curves mean different yield percentages. And while I feel we barely scratched the surface in terms of understanding the relationship between the roast curve and manufacturing specialty coffee at scale, perhaps a good place to leave all this for now is to emphasize that coffee production is a multi-step process ripe with variability at all stages. Small errors at one stage may compound, ultimately having a more consequential effect than known or anticipated. This sort of snowball effect, known as a variability cascade, may result in wasted time or worse, coffee. One of the most impactful ways to avoid these sorts of problems is to begin measuring the roasting system as comprehensively as possible. This allows you to compare overall performance against a target, identify variances, solve problems, and continuously improve.