Gene Cafe - Temperature Control Improvements - By Dave Corbey
The Gene Café coffee roaster is a Hybrid roaster, which uses a tumbling action to evenly roast the beans, this tumbling action reduces the amount of airflow required, important because roasters that use airflow to keep the beans moving (true fluid bed) have a few problems. The main one is speed, the beans have to be maintained at a particular temperature and so if the airflow is high there is the potential for more heat energy to enter the beans and at a faster rate, resulting in a “too fast roast”, the other problem is simply the large quantities of air being used and any oxidation effects.
Contrary to popular belief a commercial drum roaster actually uses a considerable amount of convective heat to roast an depending on design has airflow through the drum. The gene of course has a higher airflow, but its hybrid nature largely negates the problems of the fluid bed roasters.
Having roasted on commercial roasters, I am on a constant quest to have my Gene roasts as close as possible in quality to the commercial roasts, and it’s quite realistic to go a long way towards this goal. I think it’s important here to mention that the Gene is a great little roaster (in my opinion) and when used correctly, superior to the basic Hottop, I am not alone in that view. It is better on quality of roast and certainly design (in fact it's the sort of design that wins awards). The Gene is very modular and incredibly easy to maintain and repair. That said, there is always room for improvement.
The Areas we want to improve
I have tried many different roasting techniques on the Gene to help overcome the main issues that I think when solved make the Gene an even better roaster. There are 2 main areas for improvement:
First - the Steep temperature gradient within the roasting chamber & system of temperature control
This gradient is steeper in summer than in winter and is also affected by the quantity of beans roasted. It exists simply as a result of the laws of physics, it gets steeper as the heating element starts switching on and off when the preset temperature is reached and of course at 1st crack where bean expansion starts limiting airflow a little more.
System of temperature control
The graph shows the exit temperature, but where the airflow enters the roasting chamber the beans will be subjected to the full force of the heating element. This will happen for approximately 70% of the time as the heating element control is purely binary, it’s either off or it’s on. Reducing the temperature as the roast progresses helps to reduce the periodicity of the bursts of heat and mitigate somewhat the effect. It is also why it's easier to get great roasts at cool ambient temperatures, because the colder input air is not heated to such high temperatures by the heating element
The net effect of both these things is that in hot weather or high voltage situations, you need to be careful not to scorch the beans, this is usually done by smaller batch sizes, gradual temperature reductions after 1st crack. Unfortunately in summer, you tend to have higher voltages and warmer input air, so you have to be especially careful. In winter as said before, it’s much easier,
Secondly – Low voltages
This is a particularly challenging area for any roaster, because the low voltages tend to come in winter, usually when you most need a good mains voltage, especially if you are roasting outside. Our mains voltage is as discussed in other areas of the Wiki actually 240V, not 230V, even though we are a 230V country under EU mains voltage harmonisation. Genes initially sold in the UK some years ago (Not models sold by Bella Barista), were bought in with 230V elements, perhaps to offset the effects of low voltages? However, the type of heating element used within the Gene is not going to appreciate having higher than designed voltages put through it and as our mains voltage can often climb to 247V in some areas and late at night is often above 240V….these 230V elements had a short life expectancy. They also actually roasted slower, simply because the Gene has 2 temperature sensors, an exit sensor and an overheat sensor just after the heater box. A 230V element subjected to 240V gets pretty hot and trips this overheat sensor. This causes the “element switching” phenomenon, because this element switching on and off happens long before the preset temp is reached, and also gives you “extra” element switch offs when the preset temp is reached…less heat energy overall is applied to the beans and roast times are lengthened. Unfortunately with a 230V element, when it is on, it gets really hot at 240V, which steepens the temperature gradient and of the likelihood of scorching and overheating the beans described earlier..it could also be a source of early failure of other components within the Gene.
This of course can be solved using a Variac with a 240V Gene Café, to allow the voltage to be boosted at times when the supply voltage is low. Variacs are big dusty, ugly and expensive, certainly not something you want in your kitchen, plugged in at the wrong part of the mains sine wave, the large inrush current can trip breakers, risk of electrocution etc..
We need a solution to both problems and these solutions form stage 1 and 2 of our little project.
Target cost = less than £25
Difficulty = easy
Time to complete = less than 1 hour (it can be done in 20 minutes)
Fully Reversible = yes
It would be of course desirable to have the Gene able to progressively lower the power applied to the heating element, so that rather than switching it on and off, it simply got less hot. This would have the immediate effect of minimising the temperature gradient within the drum and the overall maximum temperatures the beans are subjected to near the inlet end of the roasting chamber
Overall temperatures within the Gene would be reduced extending component life and heating element life, because maximum temperatures would be reduced overall.
Batch sizes can be increased with the ability to prevent scorching and gain an even roast. Batch sizes of 300g are realistic even for very chaffy beans, an exception would be Monsooned Malabar and other aged coffees (as weight for weight they occupy a higher volume).
More accurate profiling of temperature within the roaster at and beyond 1st crack
Limited ability to control the ramp up of temperature
Our roasting results should be more even, not only in colour, but from the surface of the bean to it’s interior. The external portion of the bean will not have been subjected to higher temperatures than desired, and this will especially benefit dry processed coffees. Coffees can be roasted darker without the risk of the chaff charring and overall the number of Divots (pop offs) reduced or eliminated. The taste of beans should be more complex and bitterness reduced (where introduced by too high a temperature).
Stage 2 (simple addition to stage 1)
Target Cost = extra £20
Difficulty = easy
Time to complete = less than 20 minutes
Fully Reversible = yes
This would have all the benefits of stage 1, but in addition would allow for roasting in low voltage situations without the use of a Variac and extend the useful voltage range of the Gene from around 227 Volts to 250 volts or higher.
In addition the ramp up temperature of the gene can be fully controlled, and any desired roast profile achieved.
Thanks to Martin (Mole) for working with me on this project and providing the additional help needed to achieve a successful solution.
Post about your favourite online roasters, roasted coffees and green bean sources
1 post • Page 1 of 1