Optical detection of liquid-surface displacement associated with gas-bubble activity

Yeast produces carbon dioxide during fermentation, forming gas bubbles that immediately collapse and emit sound within the medium. This acoustic phenomenon has traditionally been monitored within the human hearing range to assess fermentation progress. Beyond the audible range, however, the bubbles also produce ultrasonic acoustic emissions, a fact that has been confirmed using hydrophones. I hypothesize that this information can be extracted by measuring vibrational motion at the liquid surface using a contactless sinusoidal phase-modulating laser interferometer. The constructed vibrometer exhibited a measurable frequency range of up to ∼200 kHz. By focusing the laser beam on the surface of the medium during yeast fermentation, intermittent displacement responses were optically detected. After 4 h of incubation following sugar addition, broad mechanical oscillations between 0 and ∼70 kHz were observed, suggesting that acoustic emissions occur inside the liquid. After 6 h, under both pH conditions (3.8 and 7.9), the frequency bandwidth of these spectra, defined by the background noise levels, was significantly higher than that measured in the controls. In particular, at pH 7.9, the bandwidth after 6 h was significantly greater than that after 4 h. These findings are potentially applicable to the development of monitoring systems for fermentation processes in the brewing industry and to studies investigating acoustofluidic phenomena, bubble dynamics, and soft matter behavior in liquid-phase systems.