Fermentation Control
In pursuit of a constant fermentation performance, brewers seek
to achieve consistent fermentations, which demands control of
the key variables of yeast quantity and health, oxygen input, wort
nutritional status, temperature, and yeast-wort contact (mixing).
While traditional techniques for counting yeast, such as counts
with a hemocytometer, are still widely applied, there is increasing
use of instrumental approaches, often inserted in-line to achieve
automated pitching control. Devices include those operating on
the basis of assessing capacitance/permittivity and according
to principles of light scatter .
The viability of yeast has long been assessed by staining of cells
with methylene blue; however, other staining approaches have
been proposed . While these techniques inform about
whether cells are alive or dead, they do not gauge the healthfulness
(vitality) of the cells . Diverse procedures have been nominated
for assessing this parameter, but none has been adopted
universally. Techniques include assessments of glycogen , sterols
, ATP , oxygen uptake rate , and acidification
power , as well as modifications of the methylene blue
viability test .
While it has long been recognized that a proportion of oxygen is
needed by all yeast cells to support the production of the sterols
and unsaturated fatty acid components of the cell membranes , there is a less-than-clear appreciation of why different yeast
strains vary considerably in the amount that they demand .
Traditionally, the oxygen is introduced to the wort, although there
have been proposals to pitch unaerated wort with yeast that has
been supplied directly with oxygen . Ensuring contact of all
yeast cells with oxygen when yeast is present at a high density is
important . On the other hand, oxygen represents one of the
stress factors encountered by yeast , while others include ethanol,
which limits the practical alcohol concentrations that can be
achieved in brewery fermentations . Accordingly, there is interest
in the development of yeast strains with greater tolerance of
high-gravity conditions .Areview of all the stresses likely to be
encountered by brewing yeast has been provided by Gibson et al.
. There is extensive use of high-gravity brewing in commercial
brewing , with the attendant osmotic and alcohol stresses.
One major variable that perhaps receives less detailed analysis
and control than others in fermenter control is actually the wort
composition . Most brewers simply regulate the strength
of the wort (degrees Plato) and pitch on that basis, assuming that
the relative balance of the diverse nutrients within the feedstock is
consistent and modulated by the malt selection and how that malt
is processed in the brewhouse. To a first approximation, this
seems to be a reasonable situation on an experiential basis, although
there are two variables that many brewers do seek to regulate
more closely, i.e., the clarity of the wort and the concentration
of zinc ions , although other additions to promote
fermentations, particularly those with higher-strength wort, may
be employed . The presence of insoluble particles in wort
(which are derived in the brewhouse and are present at a level in
inverse proportion to the extent that they are removed in clarification
stages prior to fermentation) promotes yeast action by their
ability to nucleate carbon dioxide, thereby releasing bubbles .
Two effects may be at play, namely, the increased resulting tendency
of yeast to be moved through the fermenter and the impact
that this has on lowering dissolved CO2 levels in the wort from
inhibitory concentrations .
The contact of yeast and wort in fermentation is not inconsequential.
Often, huge fermenters are filled with several batches of
wort, leading to quandaries over precisely when the yeast should
be added to the fermenter and how to ensure homogeneity of
yeast-wort contact throughout the vessel . Mechanical mixing
is uncommon but advocated .
Fermentations may be monitored in various ways, including
measuring the decrease in specific gravity of the wort (including
in-process measurements) , CO2 evolution , the
pH decrease , and ethanol formation , as well as camera-
based observation of events in the fermenter .
At the completion of fermentation, yeast is recovered either for
disposal (commonly to animal feed or production of yeast extracts
or for repitching. For open fermenters, ale yeast is skimmed
from the surface of the vessel, but for closed cylindroconical vessels
the yeast is harvested from the cone. The population of yeast
cells differs in the cone, with stratification such that older cells are
located beneath the younger, more vital ones .
Harvested yeast may either be pumped to the next fermenter filling with fresh wort (cone-to-cone pitching) or stored in either
a pressed or slurry form . It may receive acid washing to kill any
bacteria that may have developed in the slurry . Its collection
from fermenters is often through the use of centrifuges, creating
damage that has implications for subsequent performance (109).
The impact of serial repitching was addressed by Jenkins et al.
(110), who showed that extents of deterioration vary between
yeast cells.