Subject: RE: Homebrew Digest for December 19, 1988
Date: 1988-12-21 03:49:00 GMT
I saw your question to Cheryl and thought that I might
respond. I have been a microbiologist for 17 years and a
homebrewer for about a year. I hope I don't step on anyone's
toes by jumping in . :-)
When you pitch your yeast you would like to get about 10
million cells per milliliter final concentration. In the
laboratory, we use a hemacytometer to count microbial cells. A
hemacytometer is a special glass slide with a grided area of
defined size and volume. In order to get accurate counts takes a
little bit of training. You can purchase a hemacytometer from
any good scientific supply house. (If you need a recommendation,
send me a note.) If you don't want to buy a hemacytometer, you
can approximate the amount of yeast you are using by counting the
cells in a single microscopic field. However, you need to
standardize this a little. I suggest the following:
1. Begin a fresh starter.
2. At regular periods (every 8 hours, or even shorter depending
upon your schedule) withdraw a small volume of liquid with a
sanitized (preferably sterile) eye dropper or pipet.
3. Place one drop on a microscope slide and float a cover slip
on it. (One drop will be about 0.02-0.05 ml). You might have to
dilute you sample at later stages, because there will be too many
4. Count all of the cells you see in at least three microscopic
fields. Determine the average.
5. Continue until the fermentation has completed.
6. Graph your results.
I have not performed this experiment with beer wort, but I have
used this method in other systems. What you should see is
clearly defined periods: lag, exponential growth phase, a peak
number of cells, and then a steady decline. The total number,
that is the quantiative results, is not important. When it is
time to pitch your yeast, your starter should be in that
exponential phase of growth that you identified. The beauty of
the system is that yeast only grow to a particular density (about
100 million cells per milliliter depending upon the strain)
regardless of the volume or container they are placed. Once you
standardize your observations you should be able to monitor all
of your yeast preparations. I would be interested in hearing
about your results if you attempt this.
Bacteria (lets hope you don't have any :->) are usually
identified using Gram's Crystal Violet. You can purchase a Gram
staining kit from the same places that sell hemacytometers.
Yeast cells can be stained using a variety of methods. One
method I favor is trypan blue which is excluded by living cells.
This allows you to estimate the number of living cells in your
culture. If you give me a little time I might be able to make
more specific recommendations or send you a reference. Your
public library should have some basic manuals on microbiology
that can help you with some of the techniques. Again send me a
note if you get stuck and I'll see what I can do.
Frankly, I wouldn't bother with the microscope, since all of
that it is not really necessary. If you pitch about 5% of your
batch with actively growing starter (for lagers, use about
10%)....and you should use a fresh starter before the yeast has
completely settled...you should approximate the right amount of
yeast for active fermentation. Your own experience and intuition
is probably all you really need to get good fermentations. I
don't use a microscope except for solving some very special
homebrewing problems. A microscope is useful for tracking down
problems with bacterial contamination. If you get some
experience identifying problem bacteria, you might be able to
diagnose sanitation problems by examining your mash, wort or beer
at different stages. You can also inspect the condition of your
yeast if you are storing it for long periods of time. (Actively
growing yeast have regular, round or oval shapes with multiple
I hope that this has been helpful.
ERIK A. HENCHAL, Ph.D.
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