Acoustic condition monitoring of coffee beans, during the roasting process

The research results presented here deal with the acoustic condition monitoring of coffee beans during the roasting process. In particular, the question was investigated whether the current state (roasting level) of the coffee beans during the roasting process can be detected by acoustic monitoring. To answer this question, an experiment was carried out with a sample drum roaster (Hottop KN–8828B– 2K+) in which the airborne sound pressure was recorded for five different roasting methods and then examined. The methods used differed in the inner temperature of the roaster when the coffee beans were inserted (75◦C, 100◦C, 150◦C, 175◦C, 200◦C). The recording of the roasting processes has been made by two microphones placed in front of the drum, with each process being repeated five times. To enable a comparison between the different processes, five points at which the coffee beans had reached a certain temperature were determined for each grating. Five sequences of equal length were then cut from each recording, with the determined time points serving as the starting value of the sequence.

First, the differences in the spectral composition of the signals in the first and second crack were investigated and compared with those in other past publications. Furthermore, the comparison of the behavior as a function of the throw-in temperature has been investigated. For most of the processes analysed, the sound pressure levels can be clearly separated as a function of the bean temperature. In conclusion, it was shown that sound analysis can be used to determine the proportion of cracked beans.