The details of the method as given below are reprinted from the Methods of Analysis of the Official Agricultural Chemists as amended in 1921.
Apparatus
(a) Compound microscope.—Equipped with apochromatic objectives and compensating oculars, giving magnifications of approximately 90, 180, and 500 diameters. These magnifications can be obtained by the use of 16 and 8 mm. Zeiss apochromatic objectives with X6 and X18 Zeiss compensating oculars, or their equivalents, such as the Spencer 16 and 8 mm. apochromatic objectives[5] with Spencer X10 and X20 compensating oculars, the draw-tube of the microscope being adjusted as directed below.
(b) Thoma-Zeiss blood counting cell.[6a]
(c) Howard mold counting cell.—Constructed like a blood-counting cell but with the inner disk (which need not be ruled) about 19 mm. in diameter.[6b]
Molds.—Tentative
Clean the special Howard cell so that Newton’s rings are produced between the slide and the cover-glass. Remove the cover and place, by means of a knife blade or scalpel, a small drop of the sample upon the central disk; spread the drop evenly over the disk and cover with the cover-glass so as to give an even spread to the material. It is of the utmost importance that the drop be mixed thoroughly and spread evenly; otherwise the insoluble matter, and consequently the molds, are most abundant at the center of the drop. Squeezing out of the more liquid portions around the margin must be avoided. In a satisfactory mount Newton’s rings should be apparent when finally mounted and none of the liquid should be drawn across the moat and under the cover-glass.
Place the slide under the microscope and examine with a magnification of about 90 diameters and with such adjustment that each field of view covers 1.5 sq. mm. This area is of vital importance and may be obtained by adjusting the draw-tube in such a way that the diameter of the field becomes 1.382 mm. as determined by measurement with a stage micrometer.[7] A 16 mm. Zeiss apochromatic objective with a Zeiss X6 compensating ocular or a Spencer 16 mm. apochromatic objective with a Spencer X10 compensating ocular, or their equivalents, shall be used to obtain this magnification. Under these conditions the amount of liquid examined is .15 cmm. per field. Observe each field as to the presence or absence of mold filaments and note the result as positive or negative. Examine at least 50 fields, prepared from two or more mounts. No field should be considered positive unless the aggregate length of the filaments present exceeds approximately one-sixth of the diameter of the field. Calculate the proportion of positive fields from the results of the examination of all the observed fields and report as percentage of fields containing mold filaments.
Yeasts and Spores.—Tentative
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.