The bean probe does not only measure the temperature of the beans. It is affected by the metal carcass where it is fixed – and by the air passing by.
The bean probe is quicker affected by the airflow than the beans. Metal absorb heat quicker than the cellulose beans. Think of sitting on a bench in the park on a cold day; the difference of a wood bench and a metal bench. Coffee beans are more like wood than metal.
Here I have tested the bean probes sensitivity to airflow.
This test is on my Bullet R1 roaster (a test roast with old beans, not meant to taste good). The bean probe is a NTC precision type.
Look at the graph after 8 minutes:
In the last 4 minutes of the roast I jumped between a high airflow; F7 and a low; F2 (and once F1). Each time approximatly 30 seconds stretch. I chose F7 because already at F5 the airflow starts to cool rather than increase heat transfer. That is with 400 grams batches – at larger batches it takes higher fansetting.
High airflow made the ROR go negative within 10 seconds.
Low airflow made it go positive within 10 seconds.
During this shifting airflow the Bean temperature (BT) did anyway rise from 187 to 191°C, measured at low airflow. Probably due to heat transfer from the drum, where the heat setting was constant at P3 during these 4 minutes.
Clearly the bean probe is not only affected by the actual bean temperature – but also of the airflow. As we long expected. And as Rob Hoos also wrote in his Roast Magazine article this month (the sep/oct 2017 issue). He also talks about it in this video https://www.youtube.com/watch?v=IQjxCHE10HQ
Note: The first 8 minutes of the roast was because I had just changed the bean probe. At first the test was only to see the difference to the old one. But during the roast I got the idea to test the airflow impact on the BT reading.
Our roasting community here in Denmark is exploring a new approach on the Bullet R1 roaster.
15 of us were on a roasting course with Michael de Renouard from The Factory Roast Lab Copenhagen. He is a roasting consultant and travels the world teaching coffee roasting. In his own roastery he is roasting on a Loring. He roast quite light.
Michael tried out the Bullet roaster. He thought it did well; no burned or smoked taste in the beans. And its possible to get the beans heated quickly. He likes that you can control heat and airflow separately.
He found that you could start out with high heat without the beans got scorched/burned. Preheat 200°C for a 700 grams batch and then P8 – leaving room to step up to P9 at then end of the ROR peak.
But this was for high density beans (high grown). Be aware that lower density beans, like brasil, need a lower preheat temperature. Steffen got a Costa Rica bean which gets burned if the preheat is more than 170°C.
The high heat makes a fast start on the roast. But if it doesn’t scorch the beans, then no problem. Michael focus on the lenght of the phase from yellow until First Crack start. Here he aims at 3 minutes and at the same time aim to bring ROR below 5 for the First Crack phase. To do this the heat must be turned down markedly.
Exploring the difference
Here Thomas Villars used the strategy on a etiopean bean. He thought it tasted better than earlier roasts of the same bean. Despite FC starts as early as 5:23 min.
In general a fast roast like this gives more “clarity” in the taste – and less body. Here the “middle phase” – from yellow to FC start – is 2:15 minutes long. It should give more body to prolong this phase.
Earlier Thomas roasted the same bean like this:
Update 2 months later:
Thomas has continued this new strategy. He like his coffees better now.
There are many approaches in coffee roasting. Don’t take this website as the only way to do it. Just take it as inspiration – and explore for your self what suits your taste.
If you made a good roast, you likely want to repeat it 🙂 But thats not easy to do accurately because so much influence a roast.
Steffen from KaffeRist do it often. Here he made 7 repeated roast of the same bean – trying to make the same roast profile. All done in a row.
You can see how the curves doesn’t overlay completely. And First Crack (FC) starts at different times and temperatures.
He roast on a Bullet R1 roaster. The software got a PlayBack function that repeats the settings of preheat, power and fan during from the roast you want to repeat.
But the same settings doesn’t give the same profile even though it’s the same bean and the same batch size. So during the roast Steffen overrules the automatic settings. The ROR curve is the best way to see where the Bean temperature (BT) is heading – to adjust in time.
And then he listens for the First Crack (FC) start every time. And aim to make the development from FC start the same every time: regarding time and temperature rise.
If you really want to learn how different roasting profiles affect the taste – then roast 2 batches on the same day with only small differences in the profile. And then taste them side by side in the weeks afterwards.
Here is an example of a good tasting and bad tasting coffee of the same bean. Much in the roast profile are alike, but not all the way. What causes the difference in taste ?
The bean is a washed arabica from Uganda. Fairly high grown at the Mount Elgon mountain on the border to Kenya.
The good tasting, 400 grams, by Therese
Taste: no burn flavour, no bitternes, nice round and big aroma.
I roasted it again 9 days later. First the curves were very close. But after 5,5 minutes the ROR levels differed. Here both curves:
The higher ROR level here gave an earlier FC; at 7 minutes in the second roast where as the first had 8:45 min.
This coffee didn’t taste good to me. It had bitterness and burned taste. And a weak aroma = boring coffee.
What could be causing this difference in taste ?
First the alike elements
The development-% was around 24% for both (the time after FC related to total roast time). And weightloss was about the same: 14,3 and 14,5%
Development time (DT) was 2:44 minutes for the good one. The bad with more burned taste you would expect to be roasted longer, but no, its roasted a bit shorter: the DT was only 2:19 minutes.
Possible explanations for the burnt taste I cant say for sure why they tasted like they did – because several things was different. But here are some thoughts:
(1) A higher temperature rise in the bad one: 12°Celcius (and BT ended on 181°C) … where as the good one only rose 6-8°C (to end-BT at 175°C).
(2) When entering First Crack the bad one had a ROR around 10°C pr min. Whereas the good one was around 5. I have heard a recommandation around 5°C per minute – and not as high as 10°C when entering FC and for the rest of the roast.
(3) The american roaster Rob Hoos talks about the importance of the middle phase: from yellow point to FC start (see his book “Modulating the flavor profile of coffee“). He calls it the Maillard phase. In these two roasts the drying phase up untill yellow point are not that far apart: 4:00 and 3:45. But the lenght of this middle phase is 3 minutes for the bad one, and 5 minutes for the good one.
A roast consultant told me he prefers 3 minutes, so thats not criminal in itself. But maybe it suits this particular bean better with a slower roast; a longer middle phase and lower ROR levels during FC.
(4) They flavours go up and down during the roast – so maybe the bad one here would have been better if dropped earlier or later at than 2:19 minutes from FC start.
Martin Kjeldsen roast of the same bean
Martin Kjeldsen lives in different part of Denmark, but we got the same bean – and the same roaster, the Bullet. The bean is from Uganda from Mount Elgon (on the border to Kenya).
Martin have also roasted the bean several times. This is his best tasting batch:
500 grams, preheat 185°C
Martins bacth was roasted 2 minutes from FC start to end-BT at 192°C. My good one was roasted 2:45 min and to BT 175°C. But our bean probes do not measure alike. Notice the difference in FC starts: Martin at 182°C. Mine at 167°C and 169°C.
The current roast doesn’t necessarily respond the same way to power and fan settings as a previous roast. And the roasting proces is slow to respond on changes so you have to make changes in advance – its like sailing a ship.
So I wanted to find control points along the roast that tells me where the roast is headed – so I can adjust the heat in time. Me = Therese.
I first wrote about adjusting by control points in November 2016. Since I have continued to develop my strategy.
Update October 2017 After having been focusing for a year on the bean temperature profile and ROR – I have now moved it to a secondary position. Because it didn’t really help me make good coffee. And because what happens in the bean probe is not exactly the same as what happens in the beans, see Bean probe sensitivity.
But I still use the BT and ROR as input to how I manage the roast.
In the beginning of the roast I look at these parameters to see how quick the roast got started:
Temperature of turning point (TP)
The ROR peak level
Time when bean temperature (BT) reaches 115 C.
If the roast is going too slow for what I want → I will use more heat (P). If it goes too fast → I will use less heat.
What happens in the beginning of the roast very much determines the direction of the rest of the roast – and more so the larger the batch is. It is very difficult to change direction later on.
From there on I note the physical change: color and start of First Crack.
Time of yellow point – and note temp and ROR
Time of medium brown – and note temp and ROR
Time of start dark brown – and note temp and ROR
Time of First Crack start – and note temp and ROR
I have in mind approximately how fast I want to reach First Crack, the length of the middle phase (=from yellow to first crack, MAI) and the ROR level around first crack. I adjust the heat according to how the roast is progressing and how I know the roast behave in the different stages.
I don’t worry about getting the exact same BT and ROR curves as an earlier good roast of the same bean. I don’t worry about have a perfectly nice declining ROR curve.
My earlier approach:
Post from November 2016
I have found a new approach to decide adjusting power and fan during the roast on the Bullet R1. Until now I have just tried to do what worked earlier. And then hoping it was right for the current roast.
When it didn’t work out, it was too late to do anything about it. Like a ship the roast takes time to change direction.
How the batch develops from a given setting is much affected by type of bean, batch size, ambient temperature, how clean the filter is and so on.
My hopes for this roaster were to be able to design the profile: ROR levels, time of 1st crack (FC), yellowing and so on … And to know what I am doing during the roast. Not having to repeat the same roast many times before I get to where I want.
So I got the idea to find control points that tells me where the roast is headed. So I can adjust the heat in time.
When the roast has reached BT= 115 C at a certain time, I know if its on time or ahead or behind – and I can adjust the heat to regulate.
ROR levels are the speed of the roast and predict the time to reach FC start and so on.
I analyzed a bunch of my previous roasts. Both the successful ones and the unsuccessful (acording to how I want to roast).
Successful ones = when FC start around 7-8 minutes
Unsuccessful = FC starts later than 10,5 minutes
I picked milestones along the roast: bean temperature and yellowing and FC. Then noted the time and ROR.
There was a clear pattern. The unsuccessful roasts had lower ROR, naturally. Not so clear at 115 C. But from 130 C and onwards.
With this list I then roasted 3 batches. What a difference it made! Now I had the feeling of knowing where I was headed and could adjust heating in time.
All 3 batches got a FC on 8-9 minutes. I got the ROR levels more where I wanted them.
I am thrilled 😀
I have posted my list under Roast Profiles. But its only a start. Surely it will be developed as I go.
Be aware: Our bean probes are different: so you can’t use my list if your FC onset isn’t around 170 C. Make your own list.
This is for high-density beans = high grown (I haven’t investigated the threshold for this, but something like above 1.500 meters) and a light roast.
My bean probe may be different than yours, so you cant use the degrees directly – but this can serve for inspiration to make your own guidelines.
A high heat start → higher preheat than earlier
To prevent to big a drop in ROR after the initial peak (at somewhere around 1-3 minutes), increase heat at this point
Already at 150°C; aim for ROR level around 3-5 at FC start (typical around 170°C with my bean probe) by slowly decreasing Power
Settings Preheat for 400 grams: 185°C – but if its the first batch 190°C
Start at P6. At turning point go to P7. During the ROR peak rise to P8. Exactly how this is done depends on Turning point temperature, the ROR-level of the peak and the time for reaching 115°C and 125°C
Fan at P2 until yellowing point or just as browning starts then F3.
After yellowing point, around 150°C decrease power step by step. I adjust this according to:
Aim for ROR around 10 at 150°C . And at 160°C ROR at 8 and power stepped down to P4.
At First Crack start have power at P2 and have ROR around 5. During FC the ROR has a tendency to drop. Don’t worry too much about it. But try and keep ROR between 2 and 4. If ROR is to high try with fan at F5 or even F6.
If ROR is higher than 5 after FC start, then do a shorter time to drop.