Roasting in changing weather conditions

It is well known that the weather makes a difference to the roasting process. The change of seasons. A clear sunny day versus rainy weather.

sun-rain-seaAlso listen to podcast episode 13: When the taste change

At first you should think that the temperature was the only factor. But more factors play a role – like humidity.

And each factor may influence different parts of the roasting proces. So it’s complex. And it easily gets confusing. It also depends on the type of roaster and ventilation.

I will not claim that I got hold of all factors affecting your roasting. But here I can mention some of the factors:


At first you should think that the temperature was the only factor. If the surroundings are colder and the air sucked into the roaster during it takes more energy to heat up your beans

Also, what temperature do your beans have before roasting ? Does your bean storage change in temperature ?

Bean storage temperature
If your beans are 25°C and you heat them up to 200°C in 8-9 minutes, that is an average of 20°C per minute. If your beans instead is only 5°C, then there is a 20°C more “road” to heat up till 200°C – so that will take an extra minute to reach.

But temperature is not the only factor – the humidity also affects the roast.



The roaster sucks in air from the outside during the roast. Even though you are roasting indoors the humidity / H2O content in the room is significantly affected by the weather outside.

The level of humidity in the air can work in two ways:

(1) If the air is already heated humid air will transfer heat quicker than dry air. Water molecules transfer heat more efficiently than other air molecules, which are mainly Nitrogen and Oxygen.

So, if you have any airflow in your roaster: Air with higher water content will transfer heat quicker.

Have you tried being in a sauna compared with a steam bath/ Turkish Hamam ? The steam bath is about 40°C, but the sauna is like 80-90°C. That’s because the heat in humid steam bath air will hit you much more efficiently than the dry sauna air.

(2) The roaster sucks in air from the outside during the roast. If the air is not heated up before entering the roasting chamber the humidity will compete with the beans for the heat. So; dry air will make a quicker roast than humid air.

It might also be a factor that drier air gives quicker drying of the beans because it removes water quicker from the beans.


Relative and Absolute humidity

But the air on a rainy day in summer doesn’t have the same water content as a rainy day in the winter.

Lets compare a humid day at 1°C and at 25°C.

Moisture meter shThe humidity is measured with a moisture meter also called a hygrometer. But it will only give the relative humidity.

Relative humidity is the amount of moisture in the air compared to what the air can “hold” at that temperature.


As you see on the graph below; warmer air can hold more water than colder. The x-axis is the temperature in Celsius. The y-axis on the left is grams of water per cubic meters (m3). The lines are levels of relative humidity.

Rel vs abs humindity

When outside air is sucked into the roaster we need to know the absolute amount of water per volume air.

Lets look at high humidity, 100% (rainy weather):
At 25°C the air holds 23 grams water per m3
At 1°C the air only holds 5 grams per m3

So if its 25°C outside and 100% humidity, there will be 4,5 times more water in the air sucked into the roaster – compared to 100% at 1°C.

As you can see on the graph, the difference in water content is much bigger at 25°C than around freezing point. So, in warm weather changes in humidity makes a bigger difference: At 25°C a dry day with 30% humidity has 7 grams water … compared to a humid day at 90% has 21 grams of water.

You can see this on a cold glass of water. On a warm humid day quickly water will condense on the outside of the glass. Thats because the warm air contains a lot of water. But right by the cold glass, it gets cooled down and the reach “dew point” – and condense from water vapor into fluid water.

Dry or humid climate
From the graph you can also see that this has biggest impact at higher humidity. Here in Denmark we got high humidity (often above 80%). But if you live in a low humidity climate (like in the desert), look at the 10% curve on graph: the water content doesn’t change much at different temperatures.


Calculate your absolute humidity
If you want to observe this in relation with your roasting – you can either have a moisture meter in the room, close together with a thermometer … Or get data from an official weather station nearby.

And then calculate the absolute humidity – like with this one
It also takes in Barometric pressure, but it doesn’t have a big influence.

Note: If you use your own moisture meter – good calibration is needed. Moisture meters can easily be 15% wrong.


PRactical experience

How big the influence is depend on how much airflow you use. And remember the coffee beans also loose water during roasting; going from a water content of around 10% to around 2 %.

Experiencing HIGHER humidity making heating during the roasting process more efficient:

Roaster Morten Riiskjær roast 150 tons of coffee a year, both on a 12 kilo Probat roaster using gas – and on a 60 kilo automatic roaster (listen to him roasting by smell in the podcast from december 2016).
He says: “When the weather is both warm and humid – you really have to watch out because the roast quickly runs out of hand”. So he uses lower charge temperature and lower heat around First Crack.

Experiencing LOWER humidity making heating during the roasting process more efficient:

Bob Werby had been roasting a lot on his 1 kilo Bullet R1 for a year – when he changed the humidity levels in his house. Going from 60-65% rel humidity to under 50% using dehumidification Equipment.

This made the roasts quicker. With same settings and batch size he reached first crack at around 7:15 minutes at 47% humidity – compared to an average of 8:50 min to first crack with humidity levels above 60%.


If you would like to explore this in details join the humidity project

Atmospheric Pressure

A roaster told me about this impact on the roasting process:

A fan typically controls the airflow in a coffee roaster. The fan has speed settings. However, the amount of air that the fan moves does not only depend on the fan speed setting, but also depend on the backpressure from the rest of the flow path. So if you have a chaff filter or collector depleted with chaffs it gives more resistance to the fan and the airflow is lower.

And … if the outside air is denser (higher barometric pressure, higher water content) it gives more resistance to the fan and the airflow is lower.

I suppose how much the weather affects the airflow in your roaster  depends on extent of airflow: If you roast with a low airflow it wont affect as much.


Also listen to podcast episode 13: When the taste change


More factors

In general, lower water content in the air gives quicker drying … This should give a faster roast … But the question is if it has any significant importance ?

And roasters living in high altitudes say that affects the roasting too.

Somebody said that the gas burner is affected by weather conditions.

…. So many things influence the roasting process

High heat start

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.

Refshaleoen kursus maj2017

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:

Villars Homa rist 13majIn 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:

Villars Homa 30apr

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.

Repeating a roast profile

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.

Steffens 7 gentagne rist april2017

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.

Roast by color

Coffee roasting is done in different ways. Some stop roast when the beans has reach a certain color.

You can hear about this in my podcast Coffee Roasting Navigated. Episode 3 is about roasting to a certain color. We are at a roasting course with Morten Münchow from CoffeeMind. I have called the episode “Roasting by color”, but Morten focuses on more than the bean color to make a good roast.

Listen at

In Episode 7 roaster Kaya Caretta tell about looking at the color during the roast. Listen at

Fil 12-12-2016 10.14.41Color change during the roast – samples taken out each minute of the roast.

Fil 10-02-2017 09.51.15Roasting class with Morten Münchow (on the right) from CoffeeMind.

Fil 10-02-2017 09.52.47Color and Agtron measurements of different batches.
Here you can see that #6 is darker than #5 – which is reflected in the Agtron values: #6 was 75 and #5 was 84.


A joke about the different roast degrees 😀

With permission from Jon Ferguson from USA. Follow him on Twitter at ADF2050

Episode 8 in the podcast is about the different roast degrees.

Roasting by smell

How do you decide when to stop the roast ? Once, I asked roaster Morten Riiskjær this question. He said by smell. The smell of onion has to be over before stopping.

He has been roasting coffee for 7 years. I visited him in the roastery to try to pick up what he could smell during a roast. He also looks at the bean surface.

Hear Morten talk about it in episode 2 in the podcast Coffee Roasting Navigated

And here a video with an american roaster who also use the smell during First Crack, Eliza from Mill City Roasters:


Morten and his roaster – on the day we recorded for episode 2:








When does First Crack start ?

The onset of First Crack (FC / 1st crack) is an important control point in roasting coffee. Rapid development af the taste takes place from here onward. First crack is when the beans expand and make a noise like popcorn or the crack of a thin branch.

But all the beans do not crack right at the same time. First one single crack, then quiet, then another. But at some point, a lot is cracking. So when should you call the exact time of the start of FC ? Was it a 9:15 or 9:40 minutes ?

Another thing that makes it difficult is the background noise: the noise of the roaster and the beans hitting the drum. So it’s difficult to hear the cracks.

Try listening to this sound bite … Make your own notes before reading on:


I asked in a roaster forum to listen and say when they would say FC started (click on the picture to see a bigger size):


Around 0:15 into the sound bite, I was also shortly wondering: “is it starting now ?” But then it silenced, so no.

Then there is one clear snap around 53 seconds in. But I need more of a roll of cracks to say that it has started.

So I would also say around 1 minute.


I have made a podcast episode about listening for First Crack and counting Devellopment time from there. It is episode 5 in the podcast Coffee Roasting Navigated. Also in episode 11 we listen to different sounds of First Crack.


Some beans have a more clear FC start than others.

A late FC (like more than 13 minutes) will tend to make a weaker crack. Low density beans tend to make weaker crack.


When you can’t hear the cracking

How well you can hear the cracking also depends on the roasting machine. Some roasters are so insulated that it is impossible to hear.

This is what I have heard that roasters then do:

-> smell the beans in the trier. At first crack, comes a smell of vinegar.

-> look at the beans in the trier. When they crack they jump around.

Listening devices

You can also use some listening device to get the sound. On this photo I listen with a stethoscope. It only really works through air – and not on metal or glass window (because that will transmit all the other noises as well) . On this roaster I took out the trier and fitted the stethoscope on.

Photo by Morten Münchow, CoffeeMind.fil-07-12-2016-11-38-35


Here is another device: earmuffs with a tube. This photo is from the roastery Holy Bean in Denmark. Bo Nielsen got them from CoffeeTools.

Fil 08-09-2017 10.32.02



Sound analysis

Research has been done on the sounds during coffee roasting by P.S. Wilson. In his article “Coffee roasting acoustics”, he concludes:

> Near the end of the roasting process, sounds known as first crack exhibit a higher acoustic amplitude than sounds emitted later, known as second crack. First crack emits more low frequency energy than second crack.

Link to article:

The main problem seems to be to get microphones that can withstand the heat from the roasting process.


Taste preferences

When you get advice from other roasters, it’s important to know their taste preference.

What roast degree do they prefer ? Do they prefer clairity or a richer taste ? What kind of aromas ? How much acidity ? and so on.

itunes-logo1We do not like the same things in coffee. But also; we don’t notice the same things. So much is going on in the taste of coffee – so we can easily get different experiences from the same coffee.

Some people immediately notice bitterness. If the coffee has any bitterness they don’t like it. Maybe they like a light roast with clairity and fruit/flowers aromas. And dislike the burnt flavours.

Others only like coffee with solid burnt flavours, but really don’t pay attention to bitterness (smoking can reduce your ability to taste bitterness). This is in the darker roast range. These people find that the light roast thin and missing what good coffee should taste like.


This is Henrik and me (Therese). He loves very dark roasted coffee – like in the Italian coffee tradition.

I find it tastes like a car workshop and awfully burnt. I don’t like it.


Read more about roast degree in the Coffee Landscape and Light versus Dark.

Normally I roast quite light (from 45 sec to 2 minutes from 1st crack start). But I am not at the lightest end of the scale. Coffee roasters like The Coffee Collective here in Denmark and Tim Wendelboe in Norway are a bit lighter than me – and they like more acidity.

la-cabra-2George Stavrinou from Australia has the same roaster as I (a Bullet R1). He asked me what I was trying to achieve with my roast profiles regarding to taste. This was my answer to him:

My taste preference in coffee
1) Avoiding bad taste
2) Balanced in the basic tastes. No strong acidity, bitterness or sweetness. A little is fine. It should be pleasant in the mouth.
3) A big aroma. In a light roast I get a certain kind of grand aroma that I can’t associate with a specific food/flower/whatever. I like deep aromas that stays as a pleasant aftertaste for a long time.
4) Taste the characteristics of the bean.

But since we don’t percieve taste the same way it is difficult to convey in text. Tasting coffee together is the only sure way to know.

Read more about using your taste sense in the Tasting section.


riste-farveskift-ida-kofodPhoto by Ida Kofod, headroaster at Kontra Coffee.

She took out a sample each minut during a 14 minute roast – to show the color change.

Good and bad tasting of the same bean

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

mt-elgon-great-2016-11-21-05-29-00Taste: 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:

Two roast of the same bean Mt Elgon nov2016

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.


Read more under Roast profiles.

Adjusting profile by control points

beans-trier-sh1It is not that easy to repeat a great roast you did some weeks ago, even though you use the exact same settings on the roaster. That is because the roast is affected by other things: how much the roaster is preheated, the batch size, and the surrounding air (see Changing weather).

The next thing is that the roasting process is slow to respond to changes so you have to make changes in advance. It’s like steering a ship.

So, it is a good idea to have control points along the way to see where the roast is headed and to do adjustments at an early stage … to better navigate the roast.

kompas kort kop

Here are the control points that I (Therese) use :

Turning point (see definition in Roasting Basics): use the temperature or time to see how fast the roast got off the ground. I note down the temperature. If it is lower than normal, the preaheating was too low. But then apply more heat.

115ºC: I note the time when the BT reach 115ºC. Again here I can see how fast the roast is going. If it is too fast, decrease the heat (P-setting).

From here on I prefer to use the physical changes: color change and the sound of First crack.  I make notes of the time, BT and ROR.

You can also keep using the BT reading. Just be aware that it is also affected by the airflow (fan setting). I initially used the BT reading as a navigation aid, but I found it more reliable to go by the color changes:

udsnit-regneark-analyseNote: this was with an older BT probe that had a lower temperature reading.

Looking at the ROR level tells you how fast the roast is going. If it is higher than I want at this stage, I have to turn the heat down. From around First Crack, you can also increase the airflow (fan setting) to the point where it cools.

I typically aim for a First Crack start between 7 and 9 minutes. But for some espresso roasts I like it to be later – but no later than 11 minutes. Read more under Taste preferences.

You got to find your own control points and choose your aims.

Also see
How Steffen repeated 7 roasts using the PlayBack function in the Bullet software but still needed to adjust along the way.



ProfileTool for the Bullet roast data

Another of the Danish Bullet owner, Steen Hjelmstrand, has made this additional software in 2016.

The purpose was to be able to do more analysis on the data from the Bullet roaster than the RoastTime software from Aillio. But still you have to log the roast with RoastTime and save it in a json-file.


Steen have name it ProfileTool and you can download it for free from his website

Here are two profiles showed in ProfileTool:

1000grams protocol John Plato

Airflow and chaff filter

Its not only the fan setting that determins the airflow in your roast batch. Also the exit passage of the air makes a difference.

→ If it flows easily the airflow is bigger.

→ If the exit passage is somewhat restricted; the same fan setting will give less airflow.

If the filter in the chaff collector is dirty (full of chaff and coffee oils) the passage is restricted. There for regularly cleaning is a good idea.

Back in 2016 the first filter in Bullet roaster looked like this. It quickly got full. Now the filter is much larger.








Here is the cleaning routine for some of the Bullet owners with that filter type:


Read more about fan setting under Roasting basics in the section Fan/airflow

Roasting small batches

In general, you have to adjust the heat and airflow to batch size. Use less heat and fan for smaller batches.

But how little can you roast in a given roaster? It depends on how much you can turn down the heat. And then you have to be aware of what it take to get a fair reading with the Bean temperature probe. To tell what is going on in the beans the probe has to be covered in beans.

Aillio says minimum batch is 350 grams to have the right BT measurement on the Bullet R1.

With smaller batches the BT probe will not be sufficiently covered. But you can still roast smaller batches. Just ignore the BT reading.

Klaus investigated this with 100 grams batches to do sample roasts. Read about his settings here.


Diego Cano in Colombia uses the Bullet roaster to do a lot of sample roasting. He found that it works the best with 250 grams samples.

Learning to roast on the Bullet R1

bullet-sort-hvidThe Bullet R1 is a 1 kilo coffee roaster from Aillio. This website has no commercial relations to the company.

I have just collected experiences with roasting on the Bullet because it was new to the market (Therese talking). I got it in preorder together with a bunch of other guys here in Denmark in 2016. There was no one to ask, so we had to explore together.

This roaster has more setting possibilities and more data output than we were used to – and more than other roasters in this price range. So there was a lot to explore. As people did experiments, I collected them on this website. And then it grew to be about roasting in general.


Learning to roast on the Bullet R1

On this site I have gathered the knowledge and experiences at:

1. The basics of roasting and control functions

2. Roast profiles on the Bullet

Just bear in mind that you most likely cannot precisely replicate a profile settings with the same result. The outcome depends on:
• your surroundings (temperature and humidity)
• the beans (type of beans and temperature)
• your power supply

So you have to try out for yourself. Roasting is not easy.



The Do’s and Don’ts of Roasting on the Bullet R1 from Aillio

Official manual for the Bullet R1 roaster from Aillio


Talk with other Bullet-roasters

You can also find Facebook groups with Bullet owners, like Aillio Bullet R1 User Group

Outside Facebook you can join the Roast world forum at Aillio