October 11, 2019

RoR, Bean Temperature, & More: How to Use Common Roast Data


With just a few temperature probes and a stopwatch, there’s a significant amount of data you can collect while roasting – especially if you also use roast programs capable of performing advanced calculations in real time. 

This data can aid you in developing consistent, controlled, and quality coffee roasts. But without a proper understanding of what the numbers you collect mean, they’re of little use. 

To find out how to use roast data, I spoke with Andrew Tucker, UK & Ireland Sales Manager for Volcafe Select. Volcafe Select is a sponsor of The Roaster’s Forum at this year’s Caffè Culture Show in London from the 28th to 29th October (register before the event for a free trade pass). The Roaster’s Forum will host a series of talks and discussions about the science, craft, and business of roasting. What’s more, before joining Volcafe Select, Andrew ran a specialty roastery in Bristol, UK for almost five years and estimates that he has spent over 4,000 hours behind roasting machines. Who better than Andrew to talk to me about roast data?

So, let’s look at the different types of roast data you can collect and how to analyze them.

Lee este artículo en español Cómo Usar Los Datos de Tueste Del Café: RoR, Temperatura y Más

Tracking roast data

Tracking roast data. Credit: Loring

Bean Temperature

Besides what your senses tell you, this is your fundamental indicator of what’s going on in your roast. You can use bean temperature to understand where the coffee is along the curve of the roast cycle. 

“Bean temperature is crucial,” Andrew tells me. “But in equal crucial measure, it needs to be a clock and stopwatch. Time and temperature is everything.”

Andrew reminds me that the placement and cleanliness of your thermocouples will also affect the readings. This means you should be careful not to over-interpret the bean temperature readings or compare them with readings gathered from other roasters. He tells me, “We don’t have a standardized platform for interpreting roast data because no two roasting machines are the same.”

However, what you can do is note the temperature readings at which you consistently see yellowing and caramelization (indicated by a slight dip in your rate of rise) and first crack. From there, you can compare the data with the reactions you use as checkpoints. This will help you to plan future roasts. 

Andrew says, “The main thing is having a consistent readout. It doesn’t matter if the readout is technically right or wrong from an accuracy point of view. It just needs to be consistent roast on roast, so you’re not getting anomalies or variances within that data.”

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Bean temperature readings

Bean temperature readings. Credit: Zach Latimore

Rate of Rise (RoR)

This reflects how the bean temperature is changing. For previous generations of roasters, there was no way to see this data in real time. However, modern software such as Cropster enables you to track and react to RoR while roasting. Andrew tells me, “Cropster, in my opinion, has probably become the single most important invention or technology advancement in the coffee industry in our history.”

Because RoR curves tell you if your roast is speeding up or slowing down, they give you an insight into the future of your bean temperature curve. Andrew tells me, “It’s an indicator of what you need to do next.” 

For example, if your RoR is rising and then begins to flatten out to form a peak, you know that your roast is about to start slowing down. You can then adjust the roast to compensate or manage this drop-off, depending on your aims.

Likewise, if your RoR is falling and begins to even out into a trough, you know your roast is about to start speeding back up. It’s important to watch these peaks and troughs forming, especially if you have a roaster that is slow to react to changes in heat input or airflow. 

You can also manipulate RoR to affect the chemical reactions in the roast. For example, combining an increasing bean temperature with a gradually falling RoR might allow you to extend the coffee’s time in the temperature range in which Maillard reactions and caramelization occur.

Andrew says, “Without knowing what your rate of rise is, it’s really difficult to know when you become a lot more reactive than proactive. I think that’s when mistakes occur, when you miss crucial timestamps that you need to make variable changes.”

Bean temperature (dark blue) and rate of rise (light blue) readings

Bean temperature (dark blue) and rate of rise (light blue) readings. Credit: Zach Latimore

Environmental Temperature

Your environmental temperature, or air temperature, is a measurement of the environment inside the drum. It strongly correlates with the amount of convective heat energy available to you.

At the beginning of your roast, there will be a large difference between environmental temperature and bean temperature. This is because the beans will be at room temperature. Throughout the roast process, however, your environmental temperature and bean temperature should gradually become more similar.

As with bean temperature, it’s difficult to talk about absolute numbers because thermocouples vary in so many ways. However, it still provides a lot of useful information. If you see a sudden drop in your environmental temperature, for example, you know you have a problem because you’re losing your convective heat potential. 

Andrew says, “It’s an important measurement for me because it’s also going to tell you how aggressive or not you need to be with your controllable variable inputs. You know, if you’re seeing a huge environmental change in air temperature, it’s probably telling you something about your fuel or your heat input going into that environment.”

Bean temperature (blue) and environmental temperature (red) readings. Credit: Zach Latimore

Environmental Rate of Change

Environmental rate of change is to environmental temperature what RoR is to bean temperature: it measures how the environmental temperature is changing. A positive environmental rate of change tells you that the energy in your drum is increasing, regardless of what the environmental temperature is. 

This may be a more useful metric than environmental temperature. “The crucial thing is how quickly it’s changing,” says Andrew. “For me, it matters less about the numbers themselves and more about the rate of change.”

One practical way roasters can use this data is around first crack. Most roasters try to considerably slow the roast down at this point, but it is easy to overdo it. Around first crack, you need to keep this number positive. A positive environmental rate of change means that you have enough energy in the drum to continue developing the coffee, even while the temperature curve flattens out.

If your environmental rate of change becomes negative, on the other hand, you know you are losing heat from the drum. Your bean temperature may continue to increase, but this is because conductive heat from the drum and paddles is doing most of the work, resulting in uneven development. 

Andrew also stresses the importance of knowing your roaster. Depending on its size, material, and technology, the amount of time you will need to effect changes in the environmental rate of change will vary. If you are not aware of how your roaster reacts to heat and air, then you will have less control over what is happening inside the machine.

Environmental temperature, environmental rate of change, bean temperature, and rate of rise readings. Credit: Zach Latimore

Inlet Air Temperature

This provides insight into the convective heat coming into your roaster, and so allows you to predict what might happen next.

 “[It’s] important because it tells me a lot about perhaps where I need to be charging my coffee, or how steep the drop-off will be from when I charge the coffee to the turn at the bottom,” says Andrew. 

“You know if your inlet temperature is low… you’re going to have to basically get the roaster warmer in order to prevent a potentially destructive drying phase.”

Exhaust Temperature

Just like inlet air temperature, Andrew tells me that it’s useful to know your exhaust temperature. “They’re… important indicators for me about what’s going on, sometimes of course with the coffee, sometimes just with the machine itself.”

He jokes, “If your exhaust air temperature is going through the roof then, for me, that’s when I start to get slightly scared.” Never forget that you’re playing with intense heat, and fires can happen.

Checking roast development

Checking roast development. Credit: Loring

Weight Loss

Different bean densities and differing moisture levels within the beans will mean subtle but significant variances in weight loss across multiple coffees. If your weight loss is higher, you know you need to shorten the time or lower the temperature at which you drop the coffee. 

Andrew says that this is crucial both for consistency and profits. “It’s a direct cost to your business,” he stresses. “And so, from a numbers point of view, if you’re keeping a pretty tight ship in terms of your profit and loss account for a roastery, then knowing what you’re averaging as a weight loss across your inventory over the course of a year is really important to figuring out costs. 

“If you’re roasting a million pounds of coffee a year and you think you’re only losing 15% of moisture, but you’re actually losing 19%, then 4% of your total weight of coffee over the course of the year adds up to a lot of money.”

He adds, “A good Head Roaster, for me, should absolutely be controlling or interested in those kind of numbers as well as what’s actually happening to the coffee in the roast cycle.”

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Development Time Ratio 

This is the time elapsed from the beginning of first crack to the end of the roast, as a percentage of the total roast time. But why is the time after first crack so important? “From the moment those coffee beans open up, they become incredibly volatile,” Andrew tells me. “So, they’re taking on heat much faster post-first crack than they are at any other point in the roast, as far as I’m aware.”

Organic acids rapidly burn off after first crack, while bitter compounds begin to emerge. This means the taste balance in your coffee after first crack shifts from predominantly sweet-sour to predominantly sweet-bitter. Unless you are sample roasting, you probably want at least a little of all three of these primary tastes. Knowing your development time ratio will help you to consistently fall in the right range. 

Andrew stresses that it’s important to only consider development time ratio as a percentage of the final roast time. “If you take, for example, a big batch of espresso,” he explains. “On a hot day, it might typically roast over 14 minutes and you may work to 1 minutes 35 development time. But if that roast gets stretched out to 16 minutes on a super cold day and you stick to one minute 35 development time, then as a percentage of total roast, that’s gone way down. So you could potentially be looking at a completely underdeveloped coffee even though your development time is the same.” 

Your target end temperature and development time ratio together will give you a pretty narrow window on where to end your roasts, helping you to consistently achieve a balanced coffee profile.

Graph showing a development time ratio of 27.5%, indicating a relatively dark roast. Credit: Zach Latimore

Having data won’t replace old-fashioned roasting skills. You still need to be familiar with how your green beans react to heat, to understand the sounds and aromas emanating from the roaster, and learn as much as you possibly can – be it from books, courses, or events such as The Roaster’s Forum

However, data can be a valuable support in allowing you to achieve consistency – even when factors outside your control fluctuate. Andrew emphasizes the impact that environmental changes can have on a roast from one season to the next. The more data you have about previous successful batches, the easier it is to replicate the process. Without knowledge such as development time ratio percentage, he tells me, “you would struggle to consistently replicate a profile that captures as much of the potential in that coffee as possible.”

Bean and environmental temperature and rate-of-change curves are the primary data you can use to guide each batch along a specific profile. Inlet and exhaust temperature add a little more detail to the picture of what’s going on inside your machine. Measuring weight loss will help you track your consistency and costs. Development time ratio, together with time and temperature, will help you dial in the end of your roast.

You can use data to roast proactively and more consistently. You can also use it to make predictions about how you might improve a roast curve and troubleshoot any issues. And when it is so easy to collect, there is no good reason to not start tracking and analyzing roast data.

Please note: This article has been sponsored by Caffè Culture. Caffè Culture 2019 will be held on the 28th and 29th October in London. Register before the event for a free trade pass and to take part in The Roaster’s Forum.

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