Understanding Coffee Roasting Curves


The coffee roasting curve is an important tool that can be used to understand the coffee roasting process. A typical curve displays the roast time along the x axis (usually in minutes) and the roaster temperature on the y axis (in °C or °F). The curves may typically also display rate of rise on a secondary axis for reasons that will become apparent later. The graphs below may look like a bowl of spaghetti to the uninitiated, but when you understand each line and its relationship to the others the curves will make sense and will tell an experienced operator a lot about what is happening inside the machine. Each curve is a graphical representation of what happened during the roast.  

Temperature Curves

Figure 1 shows an ideal roast curve. In a perfect world most roasts will following along this bending line. The curve is defined by a few key variables:

  • The total roast time usually takes 10-12 minutes, although this can vary depending on the desired final temperature.
  • The endpoint, or the final temperature the roast will end at, determines roast degree. This is on a scale of light to dark.
  • The intersection point dictates the where the turning point for the roaster occurs, which will be discussed later.
Figure 1 - Ideal Roast Curve

Figure 1 - Ideal Roast Curve

The most important thing to know about this curve is that it doesn’t increase linearly, it curves. It almost becomes flat towards the end of the roast, but it never completely does. At no point is it straight either, it always decreases. An ideal roast curve will have a declining Rate of Rise and a reason for this will be discussed later.

Figure           SEQ Figure \* ARABIC      2       - Temperature Sensor Positions

Figure 2 - Temperature Sensor Positions

Figure 3 shows this same ‘ideal’ curve with temperatures from the roaster added. These temperatures are taken using 3, k-type, ungrounded 3mm thermocouples. The orange line at the bottom is read using a 10k thermistor. Figure 2 shows the positions of these sensors on the roaster. In relation to Figure 3, The blue line is the environment temperature, the green line is the bean temperature and the top orange line is the exhaust temperature. These probes are placed in strategic positions inside the roaster in order to return specific information

Figure           SEQ Figure \* ARABIC      3       - Roast Curves with Temperatures

Figure 3 - Roast Curves with Temperatures

The bottom orange line displays data from the exhaust stack. This will be ignored for now. The orange line at the top is the Exhaust Temperature. This one is always a fair bit cooler than the other curves. It is the temperature of the exhaust gases as they leave the roaster. At the start of the roast the temperature drops as the cold beans are introduced. It then steadily increases throughout the roast. The exhaust gas does not increase at the same rate as the other curves. This is because a most of the energy is being taken up by the roast rather than being lost to the air.

The next pair of lines are the environment and bean mass temperature probes. They monitor the temperature inside the drum at two points. The probe associated with the green curve is located inside the mass of beans the blue curve is located outside this bean mass towards the centre of the roaster. These lines will drop together as beans are introduced to the hot roaster. They will then level out and then converge. This is because the cold beans will initially resist taking up energy. They will be colder relative to their surroundings. Throughout the roast the beans will become more conductive and increase in temperature. They will therefore approach the temperature of their surroundings. The bean mass curve will level out at the end of the roast and approximate the environment temperature. The mass of the beans will be close to the air temperature inside the roaster. They may even exceed it in darker roasts as the beans undergo an exothermic reaction post first crack.

The last line is the stack temperature. It is largely constant as in the large diameter chimney the air is moving more slowly. This means the hot air stream increases in volume and losses most of its heat to the environment around it. Most energy is being taken up by the beans if the process is efficient so the temperature of the stack should not be expected to increase dramatically.

Rate of Rise

Figure 4 - Rate of Rise

Figure 4 - Rate of Rise

Figure 4 shows the Rate of Rise (ROR) of a roast. It is the derivative, or rate of change in the bean temperature. Different roasters measure this number differently, which is important to consider when comparing curves. It may be in degrees per minute, per 30 seconds or any number in between. Some roasters will also take averages over the period in an attempt to ‘smooth’ their curves. This can be detrimental to the roast because in order to do this, the roaster will have to compare new values with historical readings. The ROR could be bouncing up and down at the roaster will not report this, because it can only look at an average. This may mean in order to correct a crashing or rising ROR, the controller will act too late to save the batch. It is important that this ROR decrease steadily throughout the roast. There are many theories as to why, but at the time of writing, no consensus. There are possible explanations however. One is a declining rate of rise means that the difference in temperature between the outside of the bean and the inside will be less and less as the roast progresses. The beans throughout the roast will change their ability to transfer heat. The beans use a lot of the energy to increase in temperature initially and are not very thermally conductive. The roaster will be set on maximum heating power in order to raise the temperature of the water inside the bean. It is also important to start the roaster at a hot temperature so that some of this excess energy can help increase the temperature from room temperature to over 100°C as quickly as possible. This causes the curve to ‘turn’ usually at the 1:30-2:00 minute mark. As this water vapourises the bean changes size, the internal cells open up and energy is more easily transmitted into the coffee. If this gradient was too high it will cook the outside much faster than the inside, resulting in both over roasting and underoasting. If the rate of rise was zero, or flat the coffee will be held at relatively constant temperature and will lose it moisture to the environment. So although the roast will otherwise appear fine, the bean will taste over roasted, or baked. This is not to say it will necessarily taste ashy or smokey, but it will have a hollow taste.  But if the rate of rise is decreasing it can have the advantage of quickly raising the coffees temperature at the beginning, saving water content then evenly raising the temperature of the coffee later in the roast. I will stress again that there is no consensus on this, this is just opinion. A declining rate of rise seems to give a much larger window to hit between optimum mass loss, and end point of the roast. Drying is a function of airspeed and air temperature. A coffee that spends a large amount of time at high temperature, without that temperature changing, will dry more than a coffee with a declining ROR. Hence the reactions associated with coffee roasting will progress for longer that leads to flavours associated with longer roasting, despite the bean colour being quite light. In extreme cases this can result in a hollow flavour, or a light roast lacking its origin characteristics that it should have.

Heating and Airflow

Figure 5 - Heating and Airflow Settings

Figure 5 - Heating and Airflow Settings

There are two variables the can be manipulated by the roaster after the batch size and charge temperature have been decided. These variables are airflow and heating and are illustrated in Figure 5. This particular roaster uses electric heaters to supply heat to the roaster. Most roasters will not do this. An electric method of heating requires a lot more planning as it takes a long time to change the heat input of the element. It is also much less efficient than gas burners, especially when larger roasters are considered. However the principles are the same. The white line shows the relative output of the element between full power and no power. At the beginning of the roast the computer allows the machine to heat completely and the element is held at 100%. This is because during the beginning of the roast the change in temperature is an artefact of actual temperature readings and much less critical. To elaborate, the cold beans are charged into the roaster at room temperature. The roaster begins the roast a very hot temperature comes down in temperature as thermal energy is absorbed by cold beans. What we see on the graph is a hot probe being cooled by beans. Eventually the probe reaches an equilibrium point where the probe and is the same temperature at the beans around it. This is referred to as the turning point. They then climb away together at the same temperature or similar depending on the thickness of the probe. From the beans perspective they start at room temperature and climb to this equilibrium point and keep going. So a controller is told to ignore this period of the roast. If it was not told to ignore it, the controller would think it was doing something wrong, because despite having the element on full the roaster was decreasing in temperature. With industrial controllers this can cause problems which can cause them act erratically. So sometimes it is better to drop the coffee in manual mode so that the controller doesn’t suffer from issues.  If the controller is programmed correctly it will be looking at the bean temperature throughout the roast and making sure it is rising at the correct rate. For example, if the controller senses that temperature is climbing too quickly, it will begin to throttle back the heating element. This is important because the coffee transitions between an endothermic (heat absorbing) process and an exothermic process (heat releasing). The roaster will not need the amount of heat available to it throughout the whole roast. A good analogy for this is a car on cruise control going over a hill. To go up the hill the controller applies more accelerator to get car up the hill. Once the car is at the top of the hill the computer releases the accelerator to maintain the set speed. This is because the car doesn’t have to climb anymore, so that extra power is no longer required. When the car starts moving down the hill, the accelerator is completely released. The controller is relying on the friction between the road and the car to slow the speed of the vehicle. Most experienced drivers will know they might need to help the car decelerate at this point by applying the brake. The brake on a coffee roaster is the fan. It is used to pull cold air from outside the roaster, past the element, through the roaster and out the stack. This will result in changes in air temperature as hot air is pushed away in favour of cold. The fan also serves the purpose of pulling smoke and chaff outside of the unit. In Figure 5, the red line is representative of the airspeed inside the main stack. For the first part of the roast it is relatively constant and low because it is being told to ignore the first drop (as discussed previously). When the roast is too hot or climbing too quickly, it turns itself up in an attempt to throttle the roast back. When the roast is too hot it throttles itself up, and when it is too cold it turns itself down. Towards the end of the roast the fan should be at maximum to ensure that smoke is being rejected from the roaster and the rate of rise is not increasing. Again this can be done manually but computer control is preferred. One thing changing the airflow in the roaster will do is change the relative humidity in the stack.

Humidity and Mass Loss

Figure 6 - Humidity and Mass Loss

Figure 6 - Humidity and Mass Loss

Figure 6 shows curves related to Relative Humidity (RH), and the total mass of water in the coffee. The temperature at which water will condense or become a liquid changes based on its temperature. So air will hold a certain amount of water within it, if that air is rapidly cooled, the water will condense. Hot, wet air or air that contains a lot of water will condense at relatively high temperature, think steam condensing out of a kettle. Even on a warm day you will see some “steam” coming out of it as it boils. This isn’t steam however, its liquid water condensing in the air. Simply just blowing on a cold bit of metal will cause some of the water in your breath to fog up the metal. Now, if that same metal is hot, you will not see this same effect because the metal will be too hot for water to condense on its surface, even very warm, humid air may not be able to condense on a hot surface. So if we know the temperature of a gas and its relative humidity we can use a series of equations to work out the total mass of water in the air. If we know how much water is in the ambient air per cubic metre, and how much air is going through the roaster we can cancel out volume based terms. This means at any given moment the result of this calculation will tell you how many grams of water are present in the stack at that specific point. If you start adding those numbers up, you will be able to tell how many grams of water has left the coffee. This is what blue line in the centre represents. It’s the amount of water that is leaving the coffee. At first, before the water hits its boiling point, not much leaves the system. But at the roast progresses and this water starts to boil this line starts declining at a steady rate. The really important thing to know is this rate is dependent on how much water there is in the coffee and how quickly the energy can be transferred to it so it can be boiled. It largely will not matter how hot the roast gases are (within reason) if the energy cannot be transferred into the coffee. So the rate will be constant as the roast progresses. However at first crack this changes. As the bean puffs up and most of the water has been converted to steam this number slightly, but noticeably changes. It picks up speed and water is much more rapidly transferred out of the coffee. The thermal properties of the coffee have changed. This heat sink of water is largely gone the heat can penetrate the bean much more rapidly. This is important when determining final mass of the coffee because once the coffee is past first crack the drying progresses faster. In many areas of food processing there are optimum levels of water content that contribute to flavour. Although we are drying the coffee by roasting it, a coffee roaster is not trying to desiccate the beans. The roaster will want the beans to undergo reactions associated with roasting and will want the roast to be even through coffee. But also a roaster needs to hit an ideal moisture content at the end.  The roaster has largely determined how far the reaction will progress by setting the end temperature.  The roaster must also maintain the flavour of the coffee by ensuring the coffee does not become over roasted. Over roasting can be thought of disproportionately large mass loss despite a low roast end point.

Putting it all together

Figure 7 - Final Graph

Figure 7 - Final Graph

Figure 7 shows every line that has been discussed on one chart. All curves are related to one another. One cannot hope to see the whole exact picture of how heat is being transferred in the coffee and how the reactions are being developed. That is an impossible task because coffee is simply far too complicated. However clues as to what is happening inside the coffee are left by things such as small changes in temperature over time. For a long time, coffee roasters were reluctant to even add probes to their roasters and monitor the roasting process. Temperature probes are a simple low cost way to get some of the picture. The relative humidity of the exhaust gases can also be monitored to further fill out what is happening inside the drum. By adhering to a few simple rules, like charging correctly and controlling for a declining rate of rise and making sure the coffee does not over roast a coffee roaster can dramatically increase their ability to get the most out of their coffee. This is particularly important when roasting specialty coffee where a roaster will try and preserve the origin characteristics of the coffee. What is great and interesting about this method is that it quantifies variables that were otherwise completely qualitative. This is because a computer cannot smell a roast to make sure it is correct. A computer cannot take part in a cupping. But what a computer can do is use this information to make informed decisions about how a roast is progressing, knowing when to add heat and knowing when to remove it. By analysing water leaving the roaster it can even know when a coffee has finished roasting.

It might be foolish to think that a computer program could replace people in coffee roasting entirely. There may still need to be a coffee roaster who can qualitatively assess the coffee involved in the process. However, it is hoped that program or a program like this could be used to virtually eliminate roast defects associated with machine error. It could also be used to improve consistency in specialty coffee roasting. It could be used to help a specialty roaster best showcase all the farmer’s hard work with the coffee.

Humidity Studies

I’m delighted to say that this article was publish in Roast Magazine in their January 2019 issue. I’ve embedded the article for those interested. My original article is also available for reading in the blog post below. Thanks everyone for your kind words and notes. I’m continuing to work on this theory and a new paper will follow soon…


In this study a coffee roaster was modified in an attempt to measure the mass of water leaving the coffee during a roast. Senors added include, capacitive humidity sensors, an anemometer and thermistors. This was to measure the water content, volumetric flowrate of air and temperature of exhaust gases. A tray of water was placed in the coffee roaster and readings were taken. A batch of coffee was then also roasted to compare the readings observed. The relative concentration of water in the gas stream was calculated using the aforementioned inputs. The integral of this calculation with respect to time was then used to determine the total mass of water that had left the coffee roaster. Results showed a median difference between mass lost calculated compared to actual measured mass loss of 9 grams over 30 roasts. It is therefore concluded that this method of online measurement has the potential to be a viable method calculating mass loss to be used during coffee roasting. This means the relative mass loss during the roast will be known. A coffee roaster will be better able to make decisions about when to release the coffee from the drum of the roaster, in order to achieve specific mass loss targets rather than having to make guesses based on experience. This would move coffee roasting forward towards a fully autonomous roasting system.


Coffee roasting is an interesting and complex set of chemical process that turns a small, dense seed into something beautiful. The Swiss Federal Institute of Technology reports over 800 different volatiles existing in roasted coffee (Rao, 2014). This same amount is also quoted in other texts (Racineux & Tran, 2016). However, this number includes just the identified compounds. There could be many more to consider (Illy & Viani, 2004). With so many complex interactions it can be easy to get lost in detail. This study will only focus on one compound in the gas stream leaving the roaster. This study will focus on water.

Water is an essential component of all organic processes and this is true of the coffee plant. Water sustains the plant while it is growing. It is then driven off during the drying process after the fruit is picked. Water is then further removed throughout roasting. Water is then used as a solvent to extract the coffee during percolation before being consumed. Throughout all these process there are specific windows of water content that must be attained to achieve a palatable product. Green coffee will change during the roasting process from being approximately 10-12% water by mass, to 2-3% (Illy & Viani, 2004). By comparison with other compounds within the coffee, water content undergoes the biggest change (by mass) when looking at the final composition of the coffee (Rao, 2014). This is to say, that most mass losses during roasting are caused by water being driven out of the coffee. Coffee roasting can therefore be modelled as a drying unit operation. 

The latent energy of water in the coffee is a key point to discuss regarding coffee roasting. This is because latent energy drastically changes the energy balance of the coffee roaster. The latent energy is defined as the amount of energy required for a substance to undergo a phase transition. To simplify that terminology this is referring to the heat that is available to the coffee to turn the water inside the bean into steam. So for liquid water inside the bean, it must have enough energy supplied to not only come up to its boiling point but enough energy to transition into steam before increasing in temperature. This is significant because the properties of steam when compared to liquid water are drastically different. Water’s capacity to hold heat, or the amount of energy required to raise the temperature of a given mass of water decreases in this phase transition. Water in the coffee acts almost like a heat sink, soaking up energy, refusing to transmit this heat further into the bean. A large temperature gradient occurs when latent heats are taken into consideration when this process is modeled (Illy & Viani, 2004). The difference between the outside of the bean and the inside of the bean is dramatic because liquid water inside the coffee is soaking up this energy, refusing to increase in temperature. Inside the bean during roasting there is a tug of war going on between water at the hot gases outside the bean. Heat transfer occurs from the hot roast on one side, and the water sinking heat on the other. The bean’s internal pressure builds as this water is converted into steam. Eventually the internal pressure becomes too much as steam is generated and overcomes the internal cell structure of the bean. This fractures the coffee and causes a snapping sound. Coffee roasters refer this point in the roast as ‘first crack’.

This first crack usually signals to the operator that the coffee is undergoing a critical period of transformation and it is time to start monitoring the roaster. This is usually done by looking at the temperature of the bean mass and modulating airflow and heat input into the machine. The water inside the bean has been largely converted to steam due to a changed thermal conductivity of the coffee. This will cause the temperature to change much more readily. This can result in a runaway state where the coffee increases in temperature much more rapidly than desired through this stage. This runaway state can ruin a batch by burning it, or causing the beans to reach a much higher temperature than intended. Some other coffees can have the opposite problem, where water flashing out of the beans condenses inside the roaster, causing the temperature to plummet and potentially 'stalling' the roast. This is another undesriable state which can ruin a batch. 

It would be assumed that water exiting the roaster would be monitored given water’s importance in the coffee roasting process. However there are no coffee roasters on the market that track the humidity levels of flue gas evolved during the roasting process. There have been experiments conducted where flue gas has been condensed and analysed (Dutra, Oliveira, Franca, Ferraz, & Afonso, 2001). But for online monitoring this was deemed to be impractical. It is hypothesised that a capacitive relative humidity probe may be used instead to monitor water throughout the coffee roast. The total amount of water leaving the system could be calculated by multiplying the water concentration by the volumetric flowrate of air, subtracting away background water content and integrating this value over the roast. A coffee roaster could monitor spikes in humidity to determine first crack and also estimate the final development of the coffee using this calculations rather than waiting for audible snaps . This mass loss could also be estimated without having to drop of the coffee out of the machine and measure the batch’s final mass.


  1. Coffee Roaster – The model of coffee roaster was a North Coffee TJ-067 now being sold by Mill City Roasters

  2. Temperature probes – Custom made, 3mm ungrounded probes were used

  3. Data bridge – Custom built circuitry was used to analyse the roaster’s internal temperatures and control air intake and heat input.

  4. Roasting Software – Custom software was written based in Microsoft Excel to monitor the data from the Roaster

  5. Humidity Probe – Novus RHT humidity sensor

  6. Airspeed Sensor – Custom built anemometer to measure airspeed.

The setup of the roaster is explained using the following P&ID. This type of diagram shows the process and how the instrumentation connects to it. It is not supposed to be a literal representation of the unit, just a diagram that shows how the connections work.

Figure 1 - Roaster P&ID

Figure 1 - Roaster P&ID

Coffee enters into the Drum via the hopper. It is mixed inside the drum until roasted. It is then dumped out of the roaster into a cooling pan. Everything else in the diagram shows how heat is added and removed from the process. An exhaust fan is used to draw ambient air from the bottom of the roaster, past the heating element, through the drum then through a cyclone separator and to the exhaust stack. The speed of this fan can be varied automatically by the roasting software to change the airflow as required. The fan causes air to enter the roaster below the 3kW element. The element is heating the roaster and its power level can be varied automatically by the roasting software. Three thermocouples relay temperature information back to the main data bridge. The hot roasting gases exit the cyclone after most particulates have separated and the hot gases cool to approximately 70°C. Halfway up the main exhaust stack there is a humidity sensor that measures temperature and relative humidity at this point. Beyond this sensor another sensor is used to measure four variables. These are airspeed, stack temperature, ambient humidity and ambient temperature. Ultimately the whole process is controlled from a main computer connected wirelessly to the roaster. 

The control thesis for this machine is to use modulated airflow and heat input to change the temperature inside the roaster. Exhaust gases are monitored for their flowrate, temperature and humidity to determine the amount of water exiting the stack. This is compared to the ambient conditions which determine the background water concentration. The mass of water leaving the roaster can be calculated because the background water content is known as well as the airflow and the stack conditions.


On advice from Gods Honest Truth Coffee Roasters, a baseline for the experiment needed to be obtained. The coffee roaster was left to idle at 65% element duty and 42% motor speed. The goal was to achieve equilibrium such that without changing the motor or element settings the temperature and roaster conditions would be stable. Equilibrium was achieved when the relative humidity in the stack and temperature in the stack would remain relatively constant. The ambient conditions observed at the roastery on the day of the experiment were;

Temperature = 17.8°C

Relative humidity = 75%. 

At equilibrium the stack temperature was observed.

Temperature = 74°C

Relative Humidity = 5.3%.

The next step involved converting relative humidity to absolute humidity to check the readings against one another. The following numerical methods were obtained (Vaisala, 2013), Using equations (2) and (3) as per the design document the Saturation vapour pressure was obtained. The water vapour pressure was then calculated using equation (1). Finally, the absolute humidity was calculated using equation (17).

Using ambient conditions the water content was calculated as 11.4 g/m³

Using stack conditions water content was calculated as 12.2 g/m³

There is a difference in these measurements of 7% which was considered to be negligible.

The tray containing 275g of water was then added at the 13 minute mark (approx.). The results were recorded until the water was depleted from the tray.

The roaster was allowed to reach equilibrium again.

1029g of the Sumatran 'Wanita Gayo' was added to the roaster and data was collected over a 12 minute roast.


Figure 2 - Water Tray Test (control)

Figure 2 - Water Tray Test (control)

The graph above shows a section of background data between 0 and 13 minutes where the roaster is left to best achieve equilibrium. The relative humidity of the stack gases show some temperature variation throughout the whole test, however this is only between 73°C and 77°C. This temperature, although variable within this range, is constant throughout the test.

The relative humidity shows a sharp increase when the water is added. This reaches relatively stable conditions at approximately 17.5% RH at 74°C. Using the same calculations as above, this would indicate 42g/m^3 of water in the stack. This continues until the water is completely driven off at which point the roaster returned to humidity levels that were observed at equilibrium. 

Figure 3 - The integral of the humidity curve

Figure 3 - The integral of the humidity curve

From the chart above the sections where the roaster was at equilibrium was removed. A 5th order polynomial curve was fit to RH(t). The R2 value of this curve was 0.98 which indicates a reasonable fit for the curve. The integral of this curve was then calculated between the end of the experiment and the beginning. This shows the cumulative water content increasing throughout the test. The max value of this curve was 268g comparative to an expected value of 275g.

From the charts above it can be shown that the temperature and humidity sensor in the stack conveys the overall trend of water coming out of the roast well. 

For the next test 1029g of the Sumatran  'Wanita Gayo' was added to the roaster. The roast took just over 12 minutes to complete the roast. Relative humidity and stack temperature was recorded throughout the roast. The relative humidity data was converted to absolute humidity and the background water content was subtracted from this number. The following curve was obtained. 

Figure 4 - Humidity Data, Wanita Gayo, Red Bourbon, Sumatra

Figure 4 - Humidity Data, Wanita Gayo, Red Bourbon, Sumatra

This graph shows a drying curve, much like the control, slowly ramping to equilibrium. However the curve in contrast to the control is not flat throughout the roast. It slowly keeps increasing throughout the roast. It is important to note the ramp in water content in the stack leading into first crack. As first crack occurs a large spike in humidity is observed. A regression curve fit to the Total Water content could not be fit with good tolerance (the best being a 6th order polynomial at R²=0.88), so numerical integration using the trapazoid method was used. Between the start of roasting and the drop at the end the result was 157.9. The mass loss of the sample was 158.7g, with a development of 15.4%.


The DIY coffee guy has a great article on his website. In his article he quotes a statistician as saying,

Essentially, all models are wrong, but some are useful. - George E.P. Box

I think that's important to keep in mind when looking at these results.

I'm also cognisant of Cunningham’s law which says that in order to find the right answer on the internet it is best not to ask the right question, but to post the wrong answer. With that in mind, I'm going to discuss the above results from a brief study into humidity during roasting. 

Figuire 5 - Absolute water content of gas stream and associated integral

Figuire 5 - Absolute water content of gas stream and associated integral

Shown above is the water content from a roast curve and its associated drying curve. I've changed the direction of the curve to better suit how most drying curves appear (as a total amount of water decreasing, rather than water content increasing).  Similar curves to this one were obtained in online flue gas monitoring experiments. (Dutra, 2012). This is to say water content declining steadily throughout the roast. This is also similar to what Mill City Roasters have said in discussion and I think they are correct. In fact when looking at drying operations in general this same behaviour is observed. 

(McCabe, W.L., & Harriot, 2001) proposes an increase to a maximum drying rate, which is the rate at which water can move into the hot air stream. There is a maximum rate at which it can do this based on the material you are working with. Looking at coffee specifically, coffee is observed not being able to efficiently transfer heat throughout the structure of the bean due to the latent heat of water inside the bean. (Illy & Viani, 2004). This means that as the bean heats up throughout roasting, the heat does not transfer as effectively in the begginging of the the roast comparative to the end of the roast.The latent heat is responsible for sinking away heat from the outside of the bean. This culminates in an endothermic flash . (Illy & Viani, 2004), though this has not been observed by the thermocouples in this experiment . At first crack the water has enough energy to be fully vapourised. It is here that roasters will see exothermic behaviour in the roast as this pent up energy is released. Run away behaviour may also be observed if the energy supply to the roast is too great at this point . (Illy & Viani, 2004)

This is consistent with phenomena known as 'Case Hardening'. This is noted as being particularly common in food processing (McCabe, W.L., & Harriot, 2001). By monitoring humidity data, the sudden spike in water content is indicative of this energy being overcome and the onset of first crack being imminent. The total water loss throughout the roast can theoretically also be predicted by looking at this drying rate. This would mean that if a coffee roaster had desired water content in mind for a specific product, and the water content of green beans was also known, then by monitoring online humidity data they may be able to use this model to predict when to drop the coffee from the machine and begin cooling the batch. This could also be used to minimise give-away. This would be commercially useful because roasted coffee is sold on a per kilogram basis. There is little sense in driving off water that does not need to be driven off. It costs money in fuel to heat the roaster and will ultimately mean less of a return for the coffee roaster overall when the product is sold. 

Conclusions and Implications

This has been a cursory look at online humidity measurement in coffee roasting. There are numerous holes in the overall picture that need to be filled in. First of all, this study was conducted on an electric roaster. A gas roaster may have differing results as a main product of combustion is water vapour. This may skew the results for a roaster that uses combustion (as most do) to roast coffee. 

Second, as the DIY Coffee Guy mentioned, the roasting process may cause drafts or other issues, effectively sucking more outside air (hence water) into the roaster. The variability of airflow will cause issues with online measurement of water content unless it is accounted for.

It could be a co-incidence that the integral of these two tests happens to match the expected water content of the tests. This has yet to be peer reviewed and may just be conjecture. In application the online monitoring has shown promising results. Over 30 roasts a median difference of 9.5 grams was observed between the expected mass loss and the humidity sensors reported values. The interquartile range of these 30 studies was 37.5g the outlying values were -43.3g at a minimum difference and 48.5g at maximum. One thing to take into account is sensor hysteresis. The sensor can take a while to reach a new humidity value between due to very hot dry air and a sudden drop in temperature . This sensor error can be subtracted away with reasonable reliability however. So far the research seems to indicate a useful, albeit most likely incomplete model. 

There is benefit in understanding what the relative humidity of the exhaust stream means in the context of coffee roasting. This would allow coffee roasters in different climates to take control of their drying rates and achieve overall desired water content of their product. This can mean fewer products ultimately ending up in the atmosphere, rather than in the customer’s hands as previously stated. It would also mean a more consistent product for the roaster. Online humidity measurement could also inform control decisions for the roaster. It could also help when working with older beans when the moisture content has dropped.

You cannot control what you cannot measure. I hope that coffee roasters can better control the water content of their product by measuring humidity throughout the roast.


I'd like to thank the DIY Coffee Guy, Mill City Roasters, God's Honest Truth Coffee Roasters and Gary Davison for their contributions with respect to this article.


Dutra, E., Oliveira, L., Franca, A., Ferraz, V., & Afonso, R. (2001). A preliminary study on the feasibility of using composition of coffee roasting exhaust gas for the determination of the degree of roast. Journal of Food Engineering, 241–246.

Illy, A., & Viani, R. (2004). Espresso Coffee - The Science of Quality. Elsevier.

McCabe, W.L., S., & Harriot, P. (2001). Unit Operations of Chemical Engineering. 6th Edition. Singapore : McGraw-Hill International Edition.

Racineux, S., & Tran, C.-L. (2016). Le café c'est pas sorcier.

Rao, S. (2014). The Coffee Roaster's Companion .

Vaisala. (2013). Humidity Conversion Formulas. Retrieved from Vaisala: https://www.vaisala.com/sites/default/files/documents/Humidity_Conversion_Formulas_B210973EN-F.pdf


How to make espresso

How to make Chemex Coffee

This is a brief tutorial on how to make coffee with a Chemex. A Chemex is a piece of glassware used for pour-over coffee. It makes a sweet beverage with the consistency of tea. Inspired by a chemist it is the winner of multiple design awards and is available in the store. It's a favourite among many and an excellent way to bring out the true character of specialty coffee.

How to make Aeropress Coffee

This is a brief tutorial on how to make Aeropress coffee. An Aeropress is the perfect device for going camping or taking to work. It makes a great cup of coffee on the go and even has a World Championship. So let's get started.