Of the 33 samples shown in [Table 2], the result obtained by multiplying the per cent of solids in the filtrate (obtained by drying in vacuo) by the factor 1.12 is very nearly identical with the per cent of solids in the pulp (obtained by drying in vacuo). In 22 of the 33 samples the difference between these two figures is less than 0.1 per cent. In 17 samples it is less than 0.06 per cent, and in 13 samples it is less than 0.05 per cent. In only two samples does it exceed 0.17 per cent.
(b) By calculation from the specific gravity of the filtrate.—The specific gravity of the filtered liquor may be determined by means of an ordinary pycnometer. From the specific gravity at 20° C., the per cent of solids in the filtrate as determined by drying in vacuo at 70° C. may be obtained from [Table 5]. It may also be calculated by the following formula, which was derived from the same table:
Per cent Solids in Filtrate = 230 (sp. gr. of filtrate - 1.000).
The per cent of solids in the pulp may also be ascertained from the specific gravity of the filtrate at 20° C., from [Table 5]. The same results may be obtained from the following formula, which was derived from [Table 4]:
Per cent Solids in Pulp = 257.5 (sp. gr. of filtrate at 20° C. - 1.000).
It is of interest to note that the table suggested by Windisch for the determination of extract in wine (Bureau of Chemistry, U. S. Dept. Agri., Bull. 107, revised, Table V) may be employed to determine solids in tomato pulp from the specific gravity of the filtered liquor from the same. If the specific gravity of the liquor be determined at 20° C., the figures in the adjoining column, under “Extract,” correspond very closely to the per cent of total solids in the original pulp. A still closer agreement is obtained if the figure 0.05 be deducted from the percentage of extract given in the table.
(c) By calculation from the index of refraction of the filtrate.—The index of refraction of the liquor obtained by filtering tomato pulp may be determined by means of either the Zeiss-Abbé refractometer, or the immersion refractometer at the temperature of 17.5° C. The latter is preferable as it permits of much greater accuracy. The corresponding percentage of solids in the filtrate and the percentage of solids in the pulp from which it is prepared may be ascertained from the index of refraction by [Table 5]. The per cent of solids in the filtrate may also be calculated from the scale reading of the immersion refractometer at 17.5° C. by the following formula, which is derived from [Table 5]:
Per cent Solids in Filtrate = 0.258 (scale reading - 15) - 0.0165 (scale reading - 26.4).
If the index of refraction has been determined by means of an Abbé refractometer, the per cent of solids in the filtrate may be calculated by the following formula:
Per cent Solids in Filtrate = 666(nD - 1.3332) - 20.7(nD - 1.3376).