Andrew MacIntosh (1), Bruno Barazania (2), Ted Hubbard (2), Stephan Warnat (2); (1) Dalhousie University Process Engineering and Applied Science department, Halifax, NS, Canada; (2) Dalhousie University, Mechanical Engineering Department, Halifax, NS, Canada

Yeast, Fermentation, and Microbiology

The mechanical properties of individual yeast cells were characterized using micro-electro-mechanical systems (MEMS). Samples were taken throughout two controlled fermentations conducted as per ASBC Yeast-14, one utilizing ale yeast (Saccharomyces cerevisiae), while the other utilized a lager strain (Saccharomyces pastorianus). Samples of cell populations were collected at the beginning, middle, and end of fermentation and diluted to ~1 × 106 cells/mL. Approximately 4 µL of each cell suspension was added to the MEMS microchip and then diluted with filtered water. Individual cells were moved and placed between the MEMS actuator and a reference spring by micropipette aspiration. At least 5 cells of each fermentation stage (beginning, middle, and end) and species (ale and lager) were tested. Cell compression was induced and measurements of displacement were made using optical microphotographs with a FFT-based image analysis algorithm (with a precision of ~10 nm). The entire procedure was recorded for each cell as a series of 30 images taken over ~40 sec; these were used to create videos of the cell rupture. The failure of each cell was similar: the cell would undergo minor deformation until a visible burst occurred, followed by significant cell shrinkage. Using the actuator and reference spring displacements, the rupture force and stiffness of each cell were determined. Ale cells were found to rupture under an average force of 0.28 ± 0.05 µN across all fermentation stages, while lager cells burst at 0.47 ± 0.10 µN. The average stiffness at the midpoint of fermentation was found to be 4.8 ± 1.0 N/m and 5.3 ± 0.9 N/m for ale and lager samples, respectively. The post-rupture stiffness was also determined and found to be ~5× lower than the pre-ruptured stiffness for both species and all three different fermentation stages. The results showed in this study can be used to better select process parameters in order to increase brewing fermentation efficiency and assist in the design of novel yeast handling technologies.

Andrew MacIntosh is an assistant professor with the Department of Process Engineering and Applied Science at Dalhousie University, Halifax, Canada. Between core chemical engineering courses, Andrew teaches advanced brewing science as a chemical engineering technical elective and conducts fermentation-related research, much of which is published in the JASBC. The reported work is a collaboration between Andrew and Dr. Ted Hubbard of the Dalhousie Mechanical Engineering Department, where the mechanical engineering graduate student Bruno Barazani has spent many hours learning to isolate and crush individual yeast cells.