In situ Optical Rotation Measurement for On-Line Monitoring of Brewery Fermentations

Abstract

Fermentation is an important part of brewing with the potential for high variability. Monitoring fermentation progress on-line in real time improves a brewer’s ability to control the process and reduce variability. Several techniques have been standardized by ASBQ to measure ethanol content, but all of them are off-line measurements. Our objective was to evaluate a novel, optical rotation instrument that uses a photoelastic modulator to measure sugar consumption on-line during fermentation. The optical system was developed using commercial, low-cost components. A semiconductor laser beam was polarized and modulated at 50 kHz with a photoelastic modulator. The light was then passed through a sample cell followed by another polarizer acting as an analyzer. Transmitted light was collected using a silicon detector and its electric signal was filtered with a lock-in amplifier. Different sucrose solutions at concentrations from 0.01 to 30% (w/v) were used to calibrate the instrument and determine its sensitivity, linearity, and stability. Continuous sampling of a fermentation was accomplished by circulating fermenting wort via a peristaltic pump to the optical cell of the instrument. To verify and correlate results, additional samples were taken off-line to measure ethanol content by gas chromatography and specific gravity using a precision density meter. A significant linear correlation was found between on-line optical rotation measurements and off-line ethanol and density measurements, thus confirming the feasibility of optical rotation as an in-situ measurement technique for monitoring fermentation progress.