It is well known that the weather makes a difference to the roasting process. The change of seasons. A clear sunny day versus humid rainy weather.
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. It takes more energy to heat your beans in colder weather – the surroundings are colder and the air sucked into the roaster during the process is colder.
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.
Humid air will transfer heat quicker than dry air. Water molecules transfer heat more efficiently than other air molecules, primarily 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 a steam bath (also called 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 efficient than the dry sauna air.
The roaster sucks in air from the outside during the roast. Even though you are roasting indoors the water content in the room is much affected by the weather outside.
Relative and Absolute humidity
But the air on a rainy day in summer doesnt have the same water content as a rainy day in the winter.
Lets compare a humid day at 1°C and at 25°C.
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.
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 changing 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 http://planetcalc.com/2167/
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 measure 15% wrongly.
Impact on the roasting process
The factor when high absolute humidity makes heating during the roasting process more efficient:
So you don’t need as high charge temperature → Lower your
And you have to be extra careful around First Crack. In general, when approaching First Crack start you need to lower the heat to avoid to much rise in the Bean Temperature (if you got a ROR curve, you may have noticed a tendency to an increased ROR around First Crack, if you have not turned down the heat).
This goes faster at higher humidity, so you need to lower you heat even more.
Roaster Morten Riiskjær roast 150 tons of coffee a year, both on a 12 kilo gas roaster and on a 60 kilo automatic roaster (listen to him roasting by smell in the podcast from december 2016).
His says: “When the weather is both warm and humid – you really have to watch out because the roast quickly runs out of hand.”
If you would like to explore this join the humidity project http://coffeenavigated.net/humidity-project/
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 ?
Then what about the Barometer pressure ? How does that affect the roast ? Roasters living in high altitudes says that affects the roasting.
…. So many things influence the roasting process