Affect Of Voltage On The Performance Of A Gene Cafe

An investigation into the affect the voltage supply has on long it takes a UK spec "240V" Gene Café to reach its set roast temperature.

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After moving from an urban to a rural location it was noted that it took significantly longer to reach the higher roasting temperatures i.e. >232 Degrees Celsius. Often the temperature would not even be reached and the roast would take longer than 18 minutes. After measuring the supply voltage it was discovered that at certain times of the day, especially during peak demand in winter, the voltage could drop to circa 220V, occassionally even lower! A variable voltage transformer was purchased so that the voltage supplied to the roaster could be corrected to 240V [1].

To measure the results four batches were roasted. All batches were 250 grams and all the same SO beans in two batches over two days. The first roast of the day was run from ambient (12 degree C) at one voltage and the second was run immediately after the full cooling cycle had completed (60 degree C at the temperature sensor). The following day (ambient 16 degree C) the order of the voltage level was reversed so that each setting had to heat the chamber from ambient and from the level at the end of the cooling cycle. The temperature control was set to 238 Degree C and left until the end of the roast.

The aim of this investigation is to determine how the supply voltage affects the ramp-up of temperature, as measured at the sensor in the exit of the roasting chamber. It is not to determine how the profile affects the taste. Therefore, there are no tasting notes or judgement on which profile is best. The comparison was between a "standard" of 240V and 228V, which is not uncommon and represents a drop of 5%.

Results of the temperature ramp-up with the temperatures averaged from both days.


Results of the temperature ramp-up for all of the roasts.



  1. Even pre-heating the chamber does not offset the performance when roasting at a lower voltage. By four minutes into the roast the "from cold" 240V roast has reached the same exit sensor temperature and reaches the target temperature more than two minutes sooner than the pre-heated 228V batch.
  2. When roasting from cold with 228V the target temperature (238) was never reached (236 was the maximum temperature achieved.
  3. In the "cold" V228 roast the Gene Cafe does not have the power to recover its temperature (as measured at the exit), as the beans expand during 1st crack, for over four minutes.
  4. On average 1st crack is achieved 1.45 minutes earlier and 2nd crack 2.00 earlier.

Roasting with 240V provides the opportunity to profile the roast whereas with lower voltages the time taken to reach the roasting temperature then 1st and then 2nd crack is more dependant on the volume of beans, the ambient temperature and humidity and the type of bean than the machine settings.

[1] Although voltage harmonisation across Europe was supposed to give every country the same 230V in effect the tolerances were simply widened. Therefore, many mainland European countries still supply 220V and the UK providers still supply 240V in most areas.