EVAPORATION TO THE SPECIFIC GRAVITY DESIRED

Manufacturers of tomato pulp have considerable difficulty in securing a product of uniform concentration and in determining at what point to stop evaporation. Some manufacturers turn off the steam when it is believed that the concentration has gone far enough and make a hasty determination of specific gravity. If it is found the concentration is not as great as is desired, heating is resumed for a time and the specific gravity again determined. Others make but one determination of specific gravity when it is believed that the desired concentration has been reached, and if it is found to be underconcentrated, continue the evaporation for a length of time which experience has indicated to be necessary. Neither of these methods of operating is satisfactory. They involve a great deal of work and the concentration of the product obtained is not sufficiently uniform. Moreover, the determination of specific gravity of hot pulp is very inaccurate (see p. 44).

A method of employing a gauge stick is believed to be simpler and more practicable.

Some manufacturers who desire to work with the simplest possible methods, even at the sacrifice of a high degree of control over the concentration of their products, measure the volume of cyclone juice introduced into the evaporating tank; and when it is believed that the concentration is sufficient, measure the depth of the evaporated product in the tank, the steam being momentarily turned off for that purpose and the measurement being taken after the foam subsides. This method was outlined in detail in a trade paper article published from this laboratory in 1918. The method is somewhat inaccurate, because it is based on the measurement of cyclone juice as it flows from the cyclone and which therefore contains a large amount of air. This air materially increases the volume of the pulp and consequently the amount of finished pulp calculated from the volume of cyclone juice containing this air is greater than can actually be obtained. Some manufacturers of pulp have found the method practicable, however, by making a correction based on factory experience on the amount of pulp which the method indicates should result from the evaporation of each bath. This method also calls for the use of measuring tanks, which many manufacturers do not have and do not care to provide. The method is therefore not repeated here, but the laboratory has a number of reprints of the trade paper article which are available to any who desire more detailed information regarding the matter.

The following method has been found more accurate and more convenient than the one mentioned above. It has the special advantage that it is based on the examination of the cyclone juice after the juice has been heated to a sufficient extent to “break” the foam.

In using this method, the manner in which the cyclone juice is prepared is immaterial. The tomatoes may be broken by steam or mechanical breaker and may be cycloned hot or cold. The steam may be turned into the coils as soon as they are covered and the cyclone juice may run into the evaporating tank until the tank is filled.

Finally, when the last of the cyclone juice is added and the contents of the tank are boiling vigorously, the steam is momentarily turned off. The volume is then determined by means of a gauge stick and a sample is withdrawn, filtered and the specific gravity or degrees Brix determined as described on page 50. The extent to which evaporation must be continued to secure pulp of the desired specific gravity is determined by [Table 9].

This table gives in the first four columns the specific gravity of the partially concentrated pulp taken from the evaporating tank, the per cent of solids of the same, the specific gravity of the filtrate and the Brix reading of the filtrate. In order to use the tables, it is only necessary to make use of one of these columns.

This method of operation can be simplified and more accurate results obtained by equipping each evaporating tank with a one-inch gage glass extending the full height of the tank. The gage glass should be open at the top and connected with the bottom of the tank by a pipe equipped with a valve. Before the tank is filled with cyclone juice the valve is turned off and the gage glass filled with water. Steam is turned on as soon as the pipes are covered and the foam is “broken” quickly without trouble that was experienced in heating the tank filled with cool pulp. The heat is continued while the tank is filled to the desired height with the pulp. The steam is then momentarily turned off and the valve at the top of the gage glass opened to permit the water in the gage glass to equalize in height with the partly concentrated pulp within the tank. The height of water in the gage glass is read by a scale attached, the sample of the pulp taken for examination and the steam again turned on.

There is ample time to determine the specific gravity of the sample of partly concentrated pulp and from its volume as obtained by the gage glass to calculate the volume to which the pulp should be evaporated to secure the desired specific gravity in the finished product. The specific gravity of the sample may be taken by any of the methods described in the chapter on “Determination of specific gravity.” More accurate results can be obtained by pouring the sample of pulp as soon as it is taken into a large loosely stoppered flask and holding the flask with constant agitation in a tub of ice water until it is brought to about the temperature of the room.

Having determined the volume (when heated to the boiling point) of a batch of cyclone juice or of pulp at any stage of its manufacture and its specific gravity (at 68° F.), each of the last five columns of the table gives a factor by which the volume of the partially evaporated pulp may be multiplied to determine the volume of pulp of the specific gravity given at the top of the column. Since both measurements are taken at the boiling point the question of temperature need not be considered.

Table 8.—Corrections for Specific Gravity and Brix[19] Readings at Different Temperatures to 68 Degrees F. (20 Degrees C.)

Corrections to be subtracted from specific gravity or degrees Brix.

Temperature		Corrections
Deg. F.	Deg. C.	Sp. Gr.	Brix.
50	10.0	.0017	.38
51	10.6	.0016	.36
52	11.1	.0016	.35
53	11.7	.0015	.33
54	12.2	.0014	.31

55	12.8	.0014	.30
56	13.3	.0013	.28
57	13.9	.0012	.26
58	14.4	.0011	.24

59	15.0	.0010	.22
60	15.6	.0009	.20
61	16.1	.0009	.18
62	16.7	.0008	.16
63	17.2	.0007	.13

64	17.8	.0006	.11
65	18.3	.0004	.08
66	18.9	.0003	.05
67	19.4	.0002	.03

Corrections to be added to specific gravity or degrees Brix.

Temperature		Corrections
Deg. F.	Deg. C.	Sp. Gr.	Brix.
69	20.6	.0002	.03
70	21.1	.0003	.05
71	21.7	.0004	.08
72	22.2	.0006	.11
73	22.8	.0007	.15

74	23.3	.0009	.18
75	23.9	.0011	.21
76	24.4	.0012	.24
77	25.0	.0013	.28
78	25.6	.0015	.32

79	26.1	.0017	.35
80	26.7	.0018	.39
81	27.2	.0019	.42
82	27.8	.0021	.46
83	28.3	.0023	.49

84	28.9	.0024	.54
85	29.4	.0026	.58
86	30.0	.0027	.62
87	30.6	.0029	.66
88	31.1	.0031	.70

Table 9.—Equivalent Volumes of Pulp of Different Degrees of Concentration

Tomato pulp		Filtrate from pulp		Factor by which to multiply volume of pulp of given specific gravity to ascertain volume of pulp with equivalent solid content and with specific gravity of
Specific gravity at 68° F.	Per cent solids	Specific gravity at 68° F.	Degrees Brix at 68° F.	1.030	1.035	1.040	1.045	1.050
1.0125	2.79	1.0108	2.78	.384	.326	.283	.249	.223
1.0130	2.92	1.0113	2.89	.402	.342	.296	.261	.234
1.0135	3.05	1.0118	3.02	.420	.357	.310	.273	.244
1.0140	3.17	1.0123	3.14	.437	.372	.323	.285	.255
1.0145	3.30	1.0128	3.27	.455	.388	.336	.297	.265

1.0150	3.42	1.0133	3.40	.472	.401	.348	.306	.274
1.0155	3.54	1.0138	3.51	.489	.416	.361	.318	.284
1.0160	3.67	1.0143	3.65	.507	.431	.374	.329	.294
1.0165	3.79	1.0148	3.77	.524	.445	.387	.341	.304
1.0170	3.92	1.0153	3.90	.542	.460	.400	.352	.315

1.0175	4.05	1.0158	4.03	.560	.476	.413	.364	.325
1.0180	4.18	1.0163	4.15	.579	.491	.426	.375	.335
1.0185	4.30	1.0168	4.28	.596	.506	.440	.387	.346
1.0190	4.43	1.0173	4.40	.614	.521	.452	.399	.356
1.0195	4.56	1.0178	4.53	.632	.537	.466	.410	.367

1.0200	4.68	1.0182	4.63	.649	.551	.478	.421	.377
1.0205	4.81	1.0188	4.77	.667	.566	.491	.433	.387
1.0210	4.93	1.0192	4.87	.684	.581	.504	.444	.398
1.0215	5.05	1.0196	4.97	.701	.596	.517	.456	.407
1.0220	5.17	1.0201	5.10	.718	.610	.529	.467	.417

1.0225	5.30	1.0206	5.22	.737	.625	.543	.479	.428
1.0230	5.43	1.0211	5.35	.755	.641	.556	.490	.438
1.0235	5.55	1.0216	5.47	.772	.656	.569	.502	.448
1.0240	5.67	1.0220	5.57	.789	.671	.582	.513	.459
1.0245	5.80	1.0226	5.72	.808	.686	.595	.525	.469

1.0250	5.92	1.0230	5.82	.825	.701	.608	.536	.479
1.0255	6.04	1.0235	5.94	.842	.715	.620	.547	.489
1.0260	6.16	1.0240	6.07	.859	.729	.633	.558	.499
1.0265	6.28	1.0244	6.17	.876	.744	.646	.569	.509
1.0270	6.40	1.0249	6.29	.894	.759	.658	.580	.519

1.0275	6.53	1.0254	6.43	.912	.775	.672	.592	.529
1.0280	6.65	1.0258	6.53	.930	.789	.685	.604	.539
1.0285	6.77	1.0263	6.65	.947	.804	.697	.615	.549
1.0290	6.90	1.0268	6.78	.965	.819	.711	.626	.560
1.0295	7.02	1.0273	6.90	.983	.834	.724	.638	.570

1.0300	7.14	1.0278	7.03	1.000	.849	.737	.649	.580
1.0305	7.26	1.0282	7.13	1.017	.864	.749	.660	.590
1.0310	7.38	1.0287	7.23	1.035	.878	.762	.672	.600
1.0315	7.50	1.0292	7.35	1.052	.893	.775	.683	.610
1.0320	7.63	1.0296	7.45	1.071	.908	.788	.695	.621

1.0325	7.75	1.0301	7.58	1.088	.924	.802	.706	.631
1.0330	7.88	1.0306	7.70	1.107	.939	.815	.718	.642
1.0335	8.00	1.0310	7.80	1.124	.954	.828	.730	.652
1.0340	8.12	1.0315	7.93	1.142	.970	.842	.742	.663
1.0345	8.25	1.0320	8.05	1.160	.985	.855	.753	.673

1.0350	8.37	1.0325	8.16	1.178	1.000	.868	.765	.684
1.0355	8.50	1.0330	8.27	1.197	1.016	.882	.777	.695
1.0360	8.62	1.0334	8.37	1.214	1.031	.895	.788	.705
1.0365	8.74	1.0339	8.50	1.232	1.046	.907	.800	.715
1.0370	8.86	1.0344	8.63	1.249	1.061	.920	.811	.725

1.0375	8.98	1.0349	8.75	1.267	1.076	.933	.823	.735
1.0380	9.10	1.0353	8.85	1.284	1.091	.947	.834	.746
1.0385	9.23	1.0358	8.97	1.303	1.106	.960	.846	.756
1.0390	9.35	1.0363	9.07	1.321	1.122	.974	.858	.767
1.0395	9.48	1.0368	9.20	1.340	1.138	.987	.870	.778

1.0400	9.60	1.0372	9.30	1.358	1.153	1.000	.881	.788
1.0405	9.73	1.0378	9.45	1.377	1.168	1.014	.893	.799
1.0410	9.85	1.0383	9.57	1.394	1.184	1.027	.905	.809
1.0415	9.97	1.0387	9.67	1.412	1.199	1.041	.917	.820
1.0420	10.10	1.0393	9.80	1.431	1.215	1.054	.929	.830

1.0425	10.22	1.0397	9.90	1.449	1.230	1.067	.941	.841
1.0430	10.35	1.0402	10.03	1.468	1.246	1.081	.953	.851
1.0435	10.47	1.0406	10.13	1.486	1.261	1.094	.964	.862
1.0440	10.60	1.0411	10.25	1.505	1.277	1.108	.976	.873
1.0445	10.72	1.0416	10.36	1.523	1.293	1.122	.988	.884

1.0450	10.84	1.0420	10.45	1.540	1.308	1.135	1.000	.894
1.0455	10.96	1.0425	10.57	1.558	1.322	1.148	1.012	.904
1.0460	11.08	1.0429	10.67	1.576	1.338	1.161	1.023	.915
1.0465	11.20	1.0435	10.83	1.594	1.353	1.174	1.035	.925
1.0470	11.33	1.0440	10.93	1.613	1.369	1.188	1.047	.936

1.0475	11.45	1.0445	11.05	1.631	1.384	1.201	1.059	.946
1.0480	11.57	1.0449	11.15	1.649	1.400	1.215	1.071	.957
1.0485	11.70	1.0454	11.27	1.668	1.416	1.229	1.083	.968
1.0490	11.82	1.0459	11.40	1.686	1.432	1.243	1.095	.979
1.0495	11.95	1.0465	11.53	1.705	1.449	1.256	1.107	.990

1.0500	12.07	1.0468	11.60	1.724	1.464	1.270	1.119	1.00
1.0505	12.20	1.0474	11.75	1.743	1.479	1.284	1.131	1.01
1.0510	12.32	1.0478	11.84	1.761	1.495	1.298	1.144	1.02
1.0515	12.45	1.0482	11.93	1.780	1.511	1.311	1.156	1.03
1.0520	12.57	1.0488	12.07	1.797	1.526	1.325	1.167	1.04

1.0525	12.69	1.0492	12.17	1.816	1.542	1.338	1.179	1.05
1.0530	12.81	1.0497	12.30	1.834	1.557	1.351	1.191	1.06
1.0535	12.93	1.0502	12.40	1.852	1.572	1.364	1.203	1.07
1.0540	13.05	1.0506	12.50	1.870	1.588	1.378	1.215	1.08
1.0545	13.18	1.0512	12.65	1.890	1.604	1.392	1.227	1.09

1.0550	13.30	1.0516	12.74	1.908	1.620	1.405	1.239	1.10
1.0555	13.42	1.0520	12.83	1.926	1.635	1.419	1.250	1.11
1.0560	13.55	1.0525	12.95	1.945	1.651	1.433	1.263	1.12
1.0565	13.67	1.0529	13.05	1.964	1.667	1.447	1.275	1.14
1.0570	13.80	1.0534	13.16	1.983	1.684	1.461	1.288	1.15

Table 10.—Specific Gravity and Solids of Tomato Pulp[20]

Specific gravity at 68° F.	Per cent solids in vacuo at 70° C.
1.0145	3.30
1.0150	3.42
1.0155	3.55
1.0160	3.67
1.0165	3.80

1.0170	3.92
1.0175	4.05
1.0180	4.18
1.0185	4.30
1.0190	4.43

1.0195	4.56
1.0200	4.68
1.0205	4.81
1.0210	4.93
1.0215	5.05

1.0220	5.17
1.0225	5.30
1.0230	5.43
1.0235	5.55
1.0240	5.67

1.0245	5.80
1.0250	5.92
1.0255	6.04
1.0260	6.16
1.0265	6.28

1.0270	6.40
1.0275	6.53
1.0280	6.65
1.0285	6.77
1.0290	6.90

1.0295	7.02
1.0300	7.14
1.0305	7.26
1.0310	7.38
1.0315	7.50

1.0320	7.63
1.0325	7.75
1.0330	7.88
1.0335	8.00
1.0340	8.12

1.0345	8.25
1.0350	8.37
1.0355	8.50
1.0360	8.62
1.0365	8.74

1.0370	8.86
1.0375	8.98
1.0380	9.10
1.0385	9.23
1.0390	9.35

1.0395	9.48
1.0400	9.60
1.0405	9.73
1.0410	9.85
1.0415	9.97

1.0420	10.10
1.0425	10.22
1.0430	10.35
1.0435	10.47
1.0440	10.60

1.0445	10.72
1.0450	10.84
1.0455	10.96
1.0460	11.08
1.0465	11.20

1.0470	11.33
1.0475	11.45
1.0480	11.57
1.0485	11.70
1.0490	11.82

1.0495	11.95
1.0500	12.07
1.0505	12.20
1.0510	12.32
1.0515	12.45

1.0520	12.57
1.0525	12.69
1.0530	12.81
1.0535	12.93
1.0540	13.05

1.0545	13.18
1.0550	13.30
1.0555	13.42
1.0560	13.55
1.0565	13.67

1.0570	13.80
1.0575	13.92
1.0580	14.05
1.0585	14.17
1.0590	14.29

1.0595	14.42
1.0600	14.54
1.0605	14.67
1.0610	14.79
1.0620	15.03

1.0630	15.27
1.0640	15.52
1.0650	15.77
1.0660	16.02
1.0670	16.27

1.0680	16.52
1.0690	16.77
1.0700	17.02
1.0710	17.27
1.0720	17.51

1.0730	17.76
1.0740	18.00
1.0750	18.25
1.0760	18.50
1.0770	18.75

1.0780	18.99
1.0790	19.24
1.0800	19.48
1.0810	19.72
1.0820	19.97

1.0830	20.22
1.0840	20.47
1.0850	20.72
1.0860	20.96
1.0870	21.21

1.0880	21.46
1.0890	21.70
1.0900	21.95
1.0910	22.20
1.0920	22.45

1.0930	22.70
1.0940	22.94
1.0950	23.18
1.0960	23.43
1.0970	23.68

1.0980	23.93
1.0990	24.18
1.1000	24.43
1.1010	24.67
1.1020	24.92

1.1030	25.18
1.1040	25.42
1.1050	25.67
1.1060	25.91
1.1070	26.16

Illustration: Suppose that when the cyclone juice is all added to the tank the contents of which are vigorously boiling so that they are doubtless of uniform composition, the volume is found by the gauge stick to be 815 gallons. A sample of this partially evaporated pulp is withdrawn and filtered and the filtrate is found to have a Brix reading of 6.90. Let us suppose that the product is to be evaporated to a pulp having a specific gravity of 1.035 and the operator desires to know at what point to turn off the steam. By referring to [Table 9] in the column headed by the figure 1.035, we find opposite the Brix reading 6.90 the factor .834. Multiplying the volume of the pulp (815 gallons) by this factor, we obtain 680 gallons. It follows, therefore, that the steam should be turned off when the evaporation has reached such a point that the gauge stick shows the volume of pulp to be 680 gallons.

[Table 9] may be used in the same way for calculating the volume of any pulp, hot or cold, of any specified specific gravity equivalent to a certain volume of pulp of any other stated specific gravity when held at the same temperature. For instance, the illustration given above serves equally well to illustrate how the relative value of two finished pulps of different specific gravities may be calculated. It also shows directly the relative value (based on tomato solids alone) of the same volume of two pulps of different gravity.

Illustration: Suppose a shipment contains 1,000 cases of No. 10 cans of pulp thought to have a specific gravity of 1.040 but found on examination to have a specific gravity of 1.0365. What is the value of the pulp in comparison with pulp of specific gravity of 1.040? Turning in [Table 9] to the figure 1.0365 in the left-hand column we follow the horizontal line containing that figure to the column headed by specific gravity 1.040. Here we find that .907 is the factor by which to multiply the volume of pulp of a specific gravity 1.0365 to obtain the equivalent volume of pulp of specific gravity 1.040. The answer to our question therefore is 1000 × .907 = 907. In other words pulp of a gravity of 1.0365 judged by the tomato solids it contains, has 90.7 per cent of the value of pulp of the gravity of 1.040.

[Table 9] is based on the results obtained from a series of samples of whole tomato pulp and cyclone juice varying in specific gravity from 1.02 to 1.05. The table was extended by calculation to give corresponding values for more dilute cyclone juices and more concentrated products. The lower portion of the table has been repeatedly confirmed by results obtained in the examination of cyclone juice and pulp, but the figures in the higher portion of the table are based on calculation from lower concentrations.

This table is only applicable to pulp to which no other substance, such as salt, has been added. Salt if present to the extent of more than 0.25 per cent can be recognized by the taste. The amount of salt, when any has been added, may be determined by the method given on page 33 and the specific gravity corrected by subtracting from the apparent specific gravity 0.007 for each per cent of salt present. This gives the specific gravity of the salt-free pulp and the corresponding per cent of solids may be obtained from [Table 9].