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.
VIEW PRESENTATION 236
