Category: Bullet roaster

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.

Also listen to podcast episode #13 🎧

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.

Results
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

Read more about the Bean Temperature under Roasting Basics.

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.

Different batch sizes

Another indication that airflow is affecting the BT probe … is that the reading at First Crack start depends on batch size.

In general it is established that First Crack (FC) starts around 200ºC. But it is very common that the bean probe on a roaster gives a lower reading. For all types of roasters.

On the Bullet R1 I have seen a range from around 170ºC up to 196ºC for FC start. This is affected by two things: if it’s an old or new probe … and on batch size.

The lower temperature happens when only roasting 400 grams. The Bullet R1 has a capacity of 1 kilo. Below you can see 2 batches Ryan Garnes did with the same bean; 700 grams and 1,000 grams.

700 grams:

1000 gram:

First crack comes higher at 1 kilo:
At 700 gram First Crack sets in at 191ºC.
At 1,000 grams it sets in at 196ºC.

In both cases airflow was around F4. The air moves from the back of the drum – through the beans – and affects the bean probe placed on the front (read about Bean probe sensitivity to airflow). When the air travels through very hot beans it gets heated up before hitting the bean probe. More hot beans will heat up the air more. Therefor higher BT reading at FC.

At yellowing point the 700 grams batch lies higher:
At 700 gram at 149ºC.
At 1,000 grams at 141ºC.

Here the beans are not as hot than at First Crack. So the beans rather cool down the air than heat it.

Taking the trier out

If you got a trier on your roaster … taking it out also effects the airflow. See http://coffeenavigated.net/trier-and-airflow/

Podcast

Also listen to podcast episode #13 in my podcast Coffee Roasting Navigated🎧

Our roasting community here in Denmark have been 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. At home in his own roastery, he is roasting on a Loring. He roast quite light.

Michael got one of our Bullet to try it out. He thought it did well; no burned or smoked taste in the beans. The roaster do well in getting the beans heated quickly. He likes the seperate control of heat and airflow.

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.

Note: 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 also focus on the lenght of the phase from yellow until First Crack start. Here he aims for 3 minutes for this phase – 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 well before the start of FC.

 

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.

New strategy:

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.

Comment from Therese
To me this approach gives a too thin and too acidic taste. I prefer more body and sweetness.

 

Also see Kenyans for a light roasts

 

Note: 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.