The results by this method are only approximate, but are sufficiently accurate for manufacturing control. [Table 12] gives a comparison of the solids obtained by drying in vacuum at 70° C. with results obtained by the refractometer.
Table 12.—Solids in Ketchup Obtained by Drying in Vacuum at 70° C. and by Abbé Refractometer from Geerlig’s Table
| Solids in tomato ketchup | |
| By drying in vacuum at 70° C. | By Abbé refractometer |
| Per cent | Per cent |
| 29.5 | 29.0 |
| 30.0 | 29.4 |
| 32.8 | 32.4 |
| 28.0 | 27.9 |
| 22.0 | 21.8 |
| 27.7 | 28.0 |
There are several errors in this determination which partially compensate for each other and give results fairly comparable with those obtained by drying. The refractometer of course determines only soluble constituents. Since salt gives a higher refractive reading than the same per cent of sugar, and since tomato solids give a higher refractive reading than the same percentage of sugar, and since any acetic acid of the vinegar is also read as solids on the refractometer, the total increase in reading due to these different factors nearly compensates for the insoluble solids of the ketchup.
The variation of the per cent of solids as obtained by the refractometer from that obtained by drying will depend somewhat on the composition of the ketchup and in using the refractometer it is advisable to also determine the solids by drying on a few samples to obtain the relation between the two figures for that particular ketchup.
Table 13.—Refractive Index and Per Cent Solids in Tomato Ketchup [21]
| Refractive index | Per cent solids | Decimals to be added for fractional readings |
| 1.3484 | 11 | 0.0001 = 0.05 |
| 1.3500 | 12 | 0.0002 = 0.1 |
| 1.3516 | 13 | 0.0003 = 0.2 |
| 1.3530 | 14 | 0.0004 = 0.25 |
| 1.3546 | 15 | 0.0005 = 0.3 |
| 1.3562 | 16 | 0.0006 = 0.4 |
| 1.3578 | 17 | 0.0007 = 0.45 |
| 1.3594 | 18 | 0.0008 = 0.5 |
| 1.3611 | 19 | 0.0009 = 0.6 |
| 1.3627 | 20 | 0.0010 = 0.65 |
| 1.3644 | 21 | 0.0011 = 0.7 |
| 1.3661 | 22 | 0.0012 = 0.75 |
| 1.3678 | 23 | 0.0013 = 0.8 |
| 1.3695 | 24 | 0.0014 = 0.85 |
| 1.3712 | 25 | 0.0015 = 0.9 |
| 1.3729 | 26 | 0.0016 = 0.95 |
| 1.3746 | 27 | 0.0001 = 0.05 |
| 1.3764 | 28 | 0.0002 = 0.1 |
| 1.3782 | 29 | 0.0003 = 0.15 |
| 1.3800 | 30 | 0.0004 = 0.2 |
| 1.3818 | 31 | 0.0005 = 0.25 |
| 1.3836 | 32 | 0.0006 = 0.3 |
| 1.3854 | 33 | 0.0007 = 0.35 |
| 1.3872 | 34 | 0.0008 = 0.4 |
| 1.3890 | 35 | 0.0009 = 0.45 |
| 1.3909 | 36 | 0.0010 = 0.5 |
| 1.3928 | 37 | 0.0011 = 0.55 |
| 1.3947 | 38 | 0.0012 = 0.6 |
| 1.3966 | 39 | 0.0013 = 0.65 |
| 1.3984 | 40 | 0.0014 = 0.7 |
| 1.4003 | 41 | 0.0015 = 0.75 |
| 0.0016 = 0.8 | ||
| 0.0017 = 0.85 | ||
| 0.0018 = 0.9 | ||
| 0.0019 = 0.95 | ||
| 0.0020 = 1.0 | ||
| 0.0021 = 1.0 |
In using [Table 13], find the refractive index which is next lower than the reading actually obtained and note the corresponding whole number for the per cent of dry substance. Subtract the refractive index obtained from the table from the observed reading; the decimal percentages corresponding to this difference, as given in the column so marked, is added to the whole per cent of solids as first obtained.
Correction must also be made for the temperature if above or below 28° C. The temperature correction is obtained from [Table 14]. For instance, suppose the refractive index was 1.3750 and that the temperature was 25° C. The per cent of solids as obtained from the table would be 27.2. The correction for temperature would amount to .14, which would be added to this reading, giving 27.34 as the per cent of solids.
Table 14.—Corrections for Temperature to be Used with Table 13