Fill a graduated cylinder with water to the 20 cc. mark, and then add
the sample till the level of the mixture reaches the 30 cc. mark.
Close the graduate, or pour the contents into an Erlenmeyer flask,
and shake the mixture vigorously for 15 to 20 seconds. To facilitate
thorough mixing the mixture should not fill more than three-fourths of
the container in which the shaking is performed. For tomato sauce or
pastes, or products running very high in the number of organisms, or
of heavy consistency, 80 cc. of water should be used with 10 cc. or 10
grams of the sample. In the case of exceptionally thick or dry pastes,
it may be necessary to make an even greater dilution.
Pour the mixture into a beaker. Thoroughly clean the Thoma-Zeiss
counting cell so as to give good Newton’s rings. Stir thoroughly
the contents of the beaker with a scalpel or knife blade, and then,
after allowing to stand 3 to 5 seconds, remove a small drop and place
upon the central disk of the Thoma-Zeiss counting cell and cover
immediately with the cover-glass, observing the same precautions in
mounting the sample as given under 28.[8] Allow the slide to stand
not less than 10 minutes before beginning to make the count. Make the
count with a magnification of about 180 diameters to obtain which the
following combination, or their equivalents, should be employed: 8 mm.
Zeiss apochromatic objective with X6 Zeiss compensating ocular, or an
8 mm. Spencer apochromatic objective with X10 Spencer compensating
ocular with draw-tube not extended.
Count the number of yeasts and spores[9] on one-half of the ruled
squares on the disk (this amounts to counting the number in 8 of the
blocks, each of which contains 25 of the small ruled squares). The
total number thus obtained equals the number of organisms in 1/60,000
cc. if a dilution of 1 part of the sample with 2 parts of water is
used. If a dilution of 1 part of the sample with 8 parts of water is
used the number must be multiplied by 3. In making the counts, the
analyst should avoid counting an organism twice when it rests on a
boundary line between two adjacent squares.
[8] This number refers to the section as given in the Methods of
Analysis of the Association of Agricultural Chemists.
[9] Comment by authors: The organisms counted as “yeasts and spores”
are the yeast cell and yeast and mold spores, not bacteria spores.