Fluid dynamics of anaerobic fermentation

Abstract

SummaryIn beer fermentation, yeast cells are kept in suspension, despite their higher density, by natural agitation created by ascending CO, bubbles. Yeast cells are unable to nucleate bubbles but instead release Cot in a soluble form in such a way that the medium tends to become supersaturated.A higher concentration of yeast cells and the presence of solid particles cause the formation of bubbles at the bottom of the fermenter and practically only there. The rising bubbles grow and accelerate by sweeping the COZ formed throughout the fermenter by the suspended yeast cells, thereby creating a fluid regime. A mathematical expression relating the bubble agitation power to the fermentation parameters was obtained and used to design more efficient fermenter shapes.The physical action governing the fluid regime of anerobic fermentation has been discovered. The scope of our work is limited to beer fermentation by yeast but we feel that our observations are also valid for other gas producing anaerobic fermentations. Direct observation of the yeast cells and of the C02 bubbles led to the conclusion that the fluid regime is governed by simple and predictable physical phenomena. Yeast having a greater density than its medium sediments immediately after inoculation. But when fermentation starts the evolution of C02 bubbles induces a definite fluid motion. The pattern of this fluid regime is determined by the geometry of the vessel and specifically by the location of the low point. Here in the cloud of sedimented yeast supersaturation and gas evolution occur well before saturation is reached in the main body

Modeling and simulation of the bubble-induced flow in wine fermentation vessels

“…This was supported by the observed flow pattern. Variations in the geometry led to different flow fields, as the lees kept settling at the deepest point and with that the bubble nucleation sites moved as well [1]. This is in accordance with the general bubble nucleation theory.…”

“…In-situ flow investigation of bubble-driven liquid flow processes in large scale is a challenging and expensive task. Therefore only lab-scale flow pattern analyses were performed on fermenting media in earlier studies [1,2]. Their findings indicated that bubble formation is primarily visible from settled lees.…”

“…50 m 3 of CO 2 is formed. The majority of this CO 2 is released during fermentation as gas bubbles rising to the surface with diameters in the range of 0.05-5 mm [1,16]. Hence, about 10 × 10 10 bubbles/m 3 must are created.…”

Performance characteristics of continuous yeast‐alcohol fermentors with no mechanical stirring

“…The relationships expressed in eqs. (7) and (8) can be used to estimate a fermentor's efficiency as a settling device. At the maximum flow velocity permissible to maintain maximum suspended cell density (F/V),,,ax ( d ) gives a measurement of the settling velocity which cell particles must have to avoid washout.…”

Thermofluidynamics of the multiphase flow inside cylindroconical fermenters with different scales

“…The process is characterizes by different temperature levels and local temperature, velocity and concentration differences. The resulting flow in fermenters strongly influences the heat transfer and the transport of yeast cells [1].…”