Sensor array for the combined analysis of water–sugar–ethanol mixtures in yeast fermentations by ultrasound

For the sugar fermenting industry, it is important to know the exact concentration values of sugar and ethanol in the process fluid during the alcoholic fermentation as an indicator for the fermentation progress. In this study, an ultrasound-based method for determining the concentration of sugar and ethanol simultaneously is presented. The ultrasound velocity in various binary (water–saccharose, water–ethanol) an ternary (water–saccharose–ethanol) mixtures in dependance of the temperature was measured with a pulse-echo method. The sugar and ethanol fraction in aqueous solutions cause an additive, but nonlinear component to the sound velocity of pure water dependant on the concentration and the temperature. A calibration model for the ultrasound velocity in water–sugar–ethanol mixtures was found by establishing a polynomial approach based on the experimental results. The coefficients of this polynom are obtained with a linear regression method by inserting the measurement points. The derived calibration function is continuous, monotone and relates one value of sound velocity to one certain set of concentration values and temperature unambiguously. By means of these mathematical properties, it can be shown that it is possible to determine the sugar and ethanol concentration values in water–sugar–ethanol mixtures only by measurement of the sound velocity at two different temperatures. The method is evaluated by determining the sugar-, respectively ethanol concentration in standardized water–saccharose–ethanol mixtures and in commercial beer probes with an accuracy of approximately 0.5 g/100 g up to 0.01 g/100 g.