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.
Bacteria.—Tentative
Estimate the number of rod-shaped bacteria from the mounted sample used in 29[10] (yeasts and spores), but before examination allow the sample to stand not less than 15 minutes after mounting. Employ a magnification of about 500, which may be obtained by the use of an 8 mm. Zeiss apochromatic objective with X18 Zeiss compensating ocular with draw-tube not extended, or an 8 mm. Spencer apochromatic objective with X20 Spencer compensating ocular and a tube length of 190, or their equivalents.[11]
Count and record the number of bacteria having a length greater than one and one-half times their width in an area consisting of five of the small size squares. Count five such areas, preferably one from near each corner of the ruled portion of the slide and one from near the center. Determine the total number of the rod-shaped bacteria per area in the five areas and multiply by 480,000. This gives the number of this type of bacteria per cc. If a dilution of 1 part of the sample with 8 parts of water instead of 1 part of the sample with 2 parts of water is used in making up the sample, then the total count obtained as above must be multiplied by 1,440,000. Omit the micrococcus type of bacteria in making the count. Thus far it has proved impracticable to count the micrococci present, as they are likely to be confused with other bodies frequently present in such products.
Determination of Total Solids
1. BY THE EXAMINATION OF THE PULP
The total solids in tomato pulp may be determined by drying in vacuo at 70° C.; by drying at atmospheric pressure at the temperature of boiling water; by calculation from the specific gravity of the pulp; or from the per cent of solids, specific gravity or index of refraction of the filtrate. The solids obtained by different methods on 31 samples of pulp are given in [Table 4].
(a) By drying.—By drying either in vacuo or at atmospheric pressure, it is our experience that after the sample has reached apparent dryness, four hours’ drying gives complete results. From 2 to 4 grams should be taken for the determination, and enough water added to distribute the sample uniformly over the bottom of a flat-bottomed dish at least 2.5 inches in diameter.
The solids as determined by drying in vacuo at 70° C. are about 108.5 per cent of the result obtained by drying at the temperature of boiling water at atmospheric pressure. This figure is the average of the results obtained by the examination of 20 samples of pulp, in all of which the per cent of solids obtained by drying in vacuo agree quite closely with the per cent obtained by drying at atmospheric pressure multiplied by 1.085. In 15 of the 20 samples examined, the difference did not exceed 0.10 per cent, and in only one case did it exceed 0.20 per cent. The results obtained by the subsequent examination of a considerable number of other samples confirm this relation.