THE ALCOHOL TEXTBOOK THE ALCOHOL TEXTBOOK

Practical management of yeast: conversion of sugars to ethanol 127
resistance to both penicillin and virginiamycin
is increasingly common.
One of the strategies to avoid this risk is to
take advantage of the synergy that occurs when
antimicrobial substances are combined, which
lowers the amounts used. Lactoside, a
combination of antimicrobials without
virginiamycin, has proven very effective for this
reason. A routine maintenance dose of 1-2 ppm
is practical as a precautionary measure. When
there is infection the dosage is increased to 2-5
ppm. There is no evidence of carryover of these
antibiotics into spent grains and solubles.
ALCOHOL LEVELS: A THIRD STRESS FACTOR
High alcohol beers have a tendency to stop
fermenting. Understanding how alcohol levels
act to stress yeast requires a working knowledge
of yeast metabolism and growth. It has been
shown that when yeast are in the reproductive
phase they produce alcohol over 30 times faster
than when not reproducing. Figure 5 shows
typical yeast growth patterns or phases during
fermentation. The first phase is the so-called lag
phase, during which the yeast adapt to the
fermentor environment. During this period there
is little or no yeast growth and consequently little
or no alcohol production. This period can last
from 4 to 12 hrs. Ways of managing and
reducing the length of the lag phase are shown
in Figure 6 in reference to yeast conditioning.
Cell population (millions per ml)
200
160
120
80
40
10
Lag
phase
Adaptation
to mash
Exponential
growth phase
Maximum
growth
Stationary
phase
Total
cell count
Viable
cell count
Decline
phase
12 24 36 48 60 72
Time (hours)
Figure 5. Typical yeast growth curve in distillery mash.
10,000 - 20,000 gallons
Hold for 8 hours
• Good agitation
• Good cooling
• Can be sterilized
Figure 6. Yeast conditioning tanks: A key factor in
maximizing yeast cell numbers and improving alcohol levels.
The second phase is the exponential growth
phase. This is the most important phase where
nearly all of the alcohol is produced. There is a
limited time in which the yeast stay in this phase;
and this is the factor limiting the quantity of
alcohol produced during a given fermentation.
The length of time during which the yeast can
remain reproducing depends on the nutrition
available in the fermentor.
The three major factors that significantly affect
alcohol yield are therefore temperature, acidity
of the mash and the alcohol level produced
during fermentation. Yeast is tremendously
resilient and usually produces efficiently despite
all the negative conditions imposed. Yeast
fermentation will yield well if the temperature is
a few degrees high or if the acidity is a little
high indicating a mild infection. Beers
containing up to 20% alcohol v/v can be
produced without slowing fermentation.
However, when changes in two or more of these
factors happen simultaneously, serious losses in
yield usually occur. For example, in a mash at
37°C contaminated with lactobacilli at 10 million
CFU/ml, it is likely that fermentation would stop
at 8% alcohol, a 33% loss in yield.
MAXIMIZE YEAST PERFORMANCE BY
CONDITIONING
Yeast is the powerhouse of any distillery; and
without healthy yeast, alcohol percentages in the
fermentor and alcohol yield will drop.
Conditioning tanks are a way to ensure good
cell numbers in the fermentor (Table 5). At the
start of batch fermentation there should be a