When yeast sits in the bottom of a fermentor for a period of time, the alcohol will kill some of the yeast. But how much yeast is killed?
Yeast has a some what varying tolerance to alcohol, but it is not as black and white as I thought. For example, WLP862, Cry Havoc, is tolerant to 5-10% alcohol by volume, but it took about two weeks to reduce the yeast to 50% viability.
The alcohol tolerance listed by yeast manufactures is slightly different that what is presented here. Yeast manufactures want to ensure that the yeast will complete fermentation of a wort that reaches a specified alcohol level. This graph shows the percentage reduction in viable cells over time when exposed to alcohol. This is analogous to how you might expect yeast to survive in a fermentation vessel after primary fermentation has completed.
Being able to estimate the viability of the slurry based on time and alcohol content should give you a reasonable estimate of the viability of a harvested slurry. In addition the amount of cell death might give you an idea of the amount of autolysis related flavors that may be added to your beer.
Five different mixtures containing the same amount of yeast, and varying levels of alcohol were allowed to sit at ambient air temperature (approximately 60°F) The cells were stained with methylene blue and the number of live cells were counted. The graph shows the number of live cells compared to the initial number of live cells counted. Because there was no growth, the numbers above 100% will give you an idea of the error associated with the cell counts. No method of viability testing is perfect, but these results should get you in the ball park.
The top two sets of data, for 0% ABV and 4% ABV show very little drop in viability over time, but notice at about day ten there seems to be significant drop. This corresponds to the time I also started noticing mold cells growing with the yeast. Unfortunately, daily measurements created dozens of opportunity for contamination.
Cell death was roughly exponential. At 16% ABV the death rate was approximately 70% death per day, where at 8% ABV the death rate was only 6% per day. This performance give a window into what you might expect for viability cells removed from a slurry. From the data collected here the death rate based on time and percent alcohol can be extrapolated.
Across the top, each column is for percent ABV of the finished beer
Down the left, each row is the number of days at that ABV level stored at ambient.
|
1%
|
2%
|
3%
|
4%
|
5%
|
6%
|
7%
|
9%
|
11%
|
13%
|
15%
|
17%
|
1
|
100%
|
100%
|
100%
|
99%
|
99%
|
98%
|
97%
|
94%
|
86%
|
70%
|
44%
|
15%
|
2
|
100%
|
99%
|
99%
|
98%
|
98%
|
96%
|
95%
|
88%
|
74%
|
49%
|
19%
|
2%
|
3
|
99%
|
99%
|
99%
|
98%
|
97%
|
95%
|
92%
|
82%
|
64%
|
35%
|
8%
|
0%
|
4
|
99%
|
99%
|
98%
|
97%
|
95%
|
93%
|
90%
|
77%
|
55%
|
24%
|
4%
|
0%
|
5
|
99%
|
98%
|
98%
|
96%
|
94%
|
91%
|
87%
|
73%
|
47%
|
17%
|
2%
|
0%
|
6
|
99%
|
98%
|
97%
|
96%
|
93%
|
90%
|
85%
|
68%
|
41%
|
12%
|
1%
|
0%
|
7
|
99%
|
98%
|
97%
|
95%
|
92%
|
88%
|
83%
|
64%
|
35%
|
9%
|
0%
|
0%
|
8
|
98%
|
97%
|
96%
|
94%
|
91%
|
87%
|
80%
|
60%
|
30%
|
6%
|
0%
|
0%
|
9
|
98%
|
97%
|
96%
|
93%
|
90%
|
85%
|
78%
|
56%
|
26%
|
4%
|
0%
|
0%
|
10
|
98%
|
97%
|
95%
|
93%
|
89%
|
84%
|
76%
|
53%
|
22%
|
3%
|
0%
|
0%
|
11
|
98%
|
96%
|
95%
|
92%
|
88%
|
82%
|
74%
|
49%
|
19%
|
2%
|
0%
|
0%
|
12
|
97%
|
96%
|
94%
|
91%
|
87%
|
81%
|
72%
|
46%
|
16%
|
1%
|
0%
|
0%
|
13
|
97%
|
96%
|
94%
|
91%
|
86%
|
79%
|
70%
|
43%
|
14%
|
1%
|
0%
|
0%
|
14
|
97%
|
96%
|
93%
|
90%
|
85%
|
78%
|
68%
|
41%
|
12%
|
1%
|
0%
|
0%
|
21
|
96%
|
93%
|
90%
|
85%
|
78%
|
69%
|
56%
|
26%
|
4%
|
0%
|
0%
|
0%
|
28
|
94%
|
91%
|
87%
|
81%
|
72%
|
61%
|
46%
|
17%
|
1%
|
0%
|
0%
|
0%
|
35
|
93%
|
89%
|
84%
|
76%
|
66%
|
53%
|
38%
|
11%
|
1%
|
0%
|
0%
|
0%
|
42
|
91%
|
87%
|
81%
|
72%
|
61%
|
47%
|
32%
|
7%
|
0%
|
0%
|
0%
|
0%
|
49
|
90%
|
85%
|
78%
|
69%
|
56%
|
42%
|
26%
|
4%
|
0%
|
0%
|
0%
|
0%
|
56
|
89%
|
83%
|
76%
|
65%
|
52%
|
37%
|
22%
|
3%
|
0%
|
0%
|
0%
|
0%
|
63
|
87%
|
81%
|
73%
|
62%
|
48%
|
32%
|
18%
|
2%
|
0%
|
0%
|
0%
|
0%
|
70
|
86%
|
80%
|
70%
|
58%
|
44%
|
28%
|
15%
|
1%
|
0%
|
0%
|
0%
|
0%
|
The relationship is exponential.
The power of the exponent is related as follows: p = 0.0014*e^(0.4251*ABV)
And the viability is related as follows: v = e^(-d*p)
where p is the power of the exponent, v is the viability, d is the time in days. e is approximately 2.718281828459
