Determination of fermentor shear through empirical and theoretical methods

Yeast and Fermentation Session
Andrew J MacIntosh, Dalhousie University, Halifax, N.S., Canada
Co-author(s): Alexander McKinnon and Alex Speers, Dalhousie University, Halifax, NS, Canada

ABSTRACT: The flocculation of brewing yeast cells within a fermentor is a well-documented phenomenon. It is understood to be influenced by several cell wall factors including zymolectin and hydrophobic interactions, as well as environmental conditions such as metal ions, ethanol, mannose, pH, and the shear forces within the fermentor. Since the late 1980s shear forces have been repeatedly shown to influence the rate and initiation of cell flocculation. However, within industrial fermentors, this parameter has been somewhat difficult to assess without dedicated instrumentation. As well, the current method to calculate shear within an industrial fermentor utilizes a theoretical approach from the 1960s that assumes the evolution rate of carbon dioxide (CO2) and fermentor height are the only influences on shear and flocculation in the process. Until now, it was not possible to easily confirm these average shear rate calculations. Using colloidal aggregation theory we have measured the average shear rate within an industrial fermentor through observation of yeast flocculation behavior in wort samples subjected to various shear rates. Samples taken from industrial fermentors at ~1, 6, 22, 26, 30, 46, 50, 54, 70, 74, and 78 hr of fermentation were subjected to a range of shear conditions within a modified rheometer. At each sample time the rate of flocculation at the shear rate within the fermentor was used to calculate the orthokinetic capture coefficient of the yeast using a modified Smoluchowski equation. The shear rate at which the yeast floc reached an equilibrium size equivalent to that in the industrial fermentor was determined. Further testing within a modified rheometer was undertaken to confirm these findings. An empirically determined shear rate was found to vary from theoretical values by ~5 sec–1. Therefore, while average shear determined theoretically using CO2 evolution and height appears to yield a reasonable approximation, there are likely additional factors that influence fermentor shear, particularly near the beginning of fermentation. This novel empirical assessment technique gives researchers and industry a tool to study the shear within industrial fermentors.

Andrew J. MacIntosh has a Dip. Eng. degree from Saint Mary’s University (Nova Scotia, Canada) and a B.Eng. degree in biological engineering from Dalhousie University (Nova Scotia, Canada). After working in industry for several years he took the opportunity to complete an M.A.Sc. degree in biological engineering and is now pursuing a doctorate in food science. He is near completion of the four-year “Engineering in Training” apprenticeship required to achieve the status of professional engineer. In addition to ASBC, Andrew is also a member of the American Society of Biological Engineers and regularly serves on the council of the Dalhousie Engineering Graduate Society. When not conducting research, Andrew is an avid home brewer. He has made many successful experimental brews and has had the odd (fermenting) catastrophe.