Optical Nanomotion Detection Reveals Nanomechanical Vitality of Brewer’s Yeast During Cold Storage

Yeast vitality during storage is essential for maintaining consistent fermentation performance. This study compares the physiological responses of top- and bottom-cropped Saccharomyces cerevisiae stored at 4 °C for 20 days and evaluates Optical Nanomotion Detection (ONMD) as a rapid, label-free vitality assessment tool. Classical assays (FUN-1, methylene blue, propidium iodide, glucose acidification power, glycogen content, and ethanol tolerance) were used to monitor metabolic activity, membrane integrity, and stress resilience. Bottom-cropped yeast retained metabolic activity, membrane stability, and energy reserves longer than top-cropped cells. ONMD revealed distinct single-cell nanomotion signatures and detected mechanically active subpopulations even when traditional vitality indicators declined. Analysis of nanomotion slopes showed an increasingly negative trend in the decline over storage time in top-cropped cells, indicating reduced temporal stability of nanomechanical activity during the 180 min recordings. Ethanol-challenge experiments confirmed the vitality dependence and stress-sensitivity of the ONMD signal. Together, these findings demonstrate that ONMD resolves cold-storage-induced changes in yeast nanomechanical vitality and provides complementary information beyond conventional vitality and viability assays. ONMD offers a fast, reagent-free method for monitoring brewing yeast physiology and represents a promising basis for future development toward brewery quality-control applications.