Per centSolids 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., fromTable 5. The same results may be obtained from the following formula, which was derived fromTable 4:Per centSolids 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 byTable 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 fromTable 5:Per centSolids 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 centSolids in Filtrate= 666(nD- 1.3332) - 20.7(nD- 1.3376).The per cent of total solids in tomato pulp may also be ascertained from the index of refraction of the liquor prepared by filtering the pulp as shown inTable 5; or it may be calculated from the immersion refractometer reading by the following formula, which is derived fromTable 5:Per centSolids in Pulp= 0.289(scale reading of filtrate - 15) - 0.0185(scale reading - 26.4).If the index of refraction of the filtrate has been determined bymeans of an Abbé refractometer, the per cent of solids in the pulp may be calculated by the following formula:Per centSolids in Pulp= 748(nD- 1.3332) - 25.5(nD- 1.3376).It is of interest to note that the relation between the index of refraction of the liquor obtained by filtering tomato pulp and the per cent of solids in that liquid is very similar to the relation between the index of refraction and dissolved solids in beer and wine extract, as shown in the table prepared by Wagner.[12]In the formula given above, as well as inTable 5, it is assumed that salt is absent. If it be desired to calculate the percentage of solids in a sample containing salt from the index of refraction of the filtrate, it is necessary first to determine the amount of salt present and make correction therefor (see p. 34). For this purpose the table of Wagner[13]may be employed. The correction of the immersion refractometer reading amounts to 0.45 for each tenth per cent of salt present.This correction is necessary if the percentage of solids be determined by drying, or calculated from specific gravity.Determination of Insoluble SolidsTransfer 20 grams of the pulp to an eight-ounce nursing bottle, nearly filled with hot water, mix by shaking, and centrifuge until the insoluble matter is collected in a cake in the bottom of the bottle. Transfer the supernatant liquor onto a double, tared filter paper covering the bottom of a Büchner funnel, using suction to facilitate filtration.Again fill the nursing bottle with hot water, stir the cake of insoluble solids so that it is thoroughly mixed with the water, centrifuge, and decant the supernatant liquor on the filter. Repeat the centrifuging and the filtration of the supernatant liquor once more, and then finally transfer the insoluble solids to the filter paper and thoroughly wash with hot water. Dry the paper and insoluble solids, and weigh. The insoluble solids are quite hydroscopic and the weight must be taken quickly.Determination of SugarThe sugar of tomatoes is probably always present as invert sugar. If cane sugar is ever present in the raw product it is doubtless inverted during the concentration of pulp. The per cent of sugar given inTables 2and3was determined by the method of Munson and Walker.[14]Determination of AcidityAccurate results cannot be obtained by the titration of tomato products in the presence of the insoluble solids. If it be desired to determine the acidity in the entire sample of tomatoes or tomato pulp rather than in the expressed juice, the insoluble solids should first be removed by the method given in the determination of insoluble solids or by filtration through filter paper. The per cent of acid given inTables 2and3was obtained by titrating the liquor obtained by filtering the pulp. In products of this nature, the addition of an alkali causes a brownish color which has a tendency to obscure the end point shown by the indicator. To obviate this, the sample should be diluted to at least 200 cc. and a larger amount of indicator employed than is necessary with a clear solution. The following details are suggested.Dilute 20 grams of the filtrate under examination with over 200 cc. of water. Add at ½ cc. of phenolphthalein solution (prepared by dissolving 1 gram of phenolphthalein in 100 cc. of 95 per cent alcohol) and titrate with sodium hydroxide until the end point is obtained. Add 1 cc. of tenth-normal hydrochloric acid, heat the solution quickly to boiling and boil one minute to expel carbon dioxide. Cool the solution quickly to about room temperature, and then add tenth-normal sodium hydroxide until the end point is obtained. The volume of hydrochloric acid added must, of course, be taken into consideration in the final result. The filtrate may also be titrated direct with tenth-normal sodium hydroxide solution with satisfactory results.Determination of SaltThis laboratory has been using the following rapid method which gives results agreeing closely with results obtained by the analysis of the ash:Weigh out 20 grams of pulp, dilute in a volumetric flask to 200 cc., filter and titrate an aliquot portion with standard silver nitrate solution, using potassium chromate as indicator. The acidity of tomato pulp is not sufficient to interfere with this determination.Determination of Specific Gravity[15]The specific gravity of tomato pulp is used as one criterion for establishing the value of pulp that is offered for sale and is also used in connection with the manufacture of pulp to determine the point at which evaporation should be stopped.In the former case there is ample time for making the examination, and conditions may be established which permit a reasonable degree of accuracy in the work.In the determination of the specific gravity of hot pulp during the process of its evaporation speed is essential, and the conditions of a manufacturing plant do not always permit a high degree of accuracy. It becomes necessary, therefore, to consider what methods may give the highest degree of accuracy obtainable under the conditions of the work and at the same time afford quick results.Tomato pulp, owing to its high viscosity, retains a large quantity of air bubbles which increase the volume of the pulp and hence interfere with the accuracy of the determination of specific gravity. In working with cold pulp this air may be eliminated by whirling in a centrifuge. With hot pulp that operation is impossible, and the specific gravity must be determined in the presence of the air bubbles mentioned. Moreover, in working with cold pulp the temperature can be more accurately controlled, and the error caused by variation in temperature can be corrected. With hot pulp these conditions cannot be obtained nearly so well. The determination of specific gravity of hot pulp is therefore only roughly approximate at best. Where time permits it is strongly advisable to cool the pulp under conditions that prevent evaporation before determining specific gravity.The importance of accuracy in the determination of specific gravity in tomato pulp is discussed on page 50.Methods are given below for the determination of specific gravity in both hot and cold pulp.When salt has been added, the amount should be determined and a correction applied by deducting .007 from the specific gravity for each per cent of salt present.(a)COLD PULP AFTER CENTRIFUGING TO ELIMINATE AIR BUBBLESThis method may be employed for pulp of any degree of concentration or for unconcentrated cyclone juice. A specific gravity flask such as is shown in Figure 1 is used together with a “2-bottle” Babcock milk tester (the centrifuge referred to below). The flask may be obtained of Eimer & Amend, Third Avenue, 18th to 19th Streets, New York City, or of Emil Greiner & Co., 55 Fulton Street, New York City, and in ordering it should be designated as “specific gravity flask for tomato pulp of Pyrex glass with a capacity of about 125 cc.” The “2-bottle” Babcock milk tester may be obtained of any dairy supply house. It may also be obtained of any dealer in chemical apparatus by designating it as E. & A. No. 1833.The specific gravity flask may be calibrated as follows:Obtain the weight of the flask after thoroughly cleaning and drying, fill to overflowing with water (preferably boiled and cooled distilled water) and remove the excess water from the mouth of the flask by means of a straight edge. Wipe dry and weigh immediately. If the flask full of water is weighed at any other temperature than 20° C. (68° F.) a correction must be made to obtain the weight at that temperature. These corrections are as follows:TemperatureCorrection to be added with flasks havinga volume of—FahrenheitCentigrade125 cc.400 cc.GramsGrams6920.6.02.057021.1.03.097121.7.05.147222.2.06.207322.8.08.257423.3.09.307523.9.11.357624.4.13.417725.0.15.477825.6.17.537926.1.18.598026.7.20.658127.2.22.718227.8.24.778328.3.26.838428.9.28.898529.4.30.968630.0.321.02If it is desired to use somewhat larger samples and thus secure correspondingly more accurate results, a similar flask, but made somewhat larger (capacity approximately 400 cc.), may be employed. Such a flask, with a diameter of a little over 3 inches, is illustrated in Figure 2. This larger flask will not fit into the Babcock tester, and when it is used a special head for the Babcock tester must be made.Such a head is illustrated in Figure 3, and can be made by any good tinner.The larger flask shown in Figure 2 holds a heavier weight than the Babcock machine is intended to carry, and the advisability of its use is perhaps questionable. In any case, whatever size flask is used, it is important to build a guard around the centrifuge in order to protect the operator when the apparatus gives way, as it eventually will.Fig. 1.Small Specific Gravity Flask.Fig. 2.Large Specific Gravity Flask.Dimensions given are outside measurements. Thickness of walls is about 3/64 in.Fig. 3.Special Head and Flask Receptacles for Babcock Milk Tester.The details of the method of determining specific gravity by the use of this apparatus are as follows:Fill the flask shown in Figure 1 with the sample of pulp and place in the centrifuge (the Babcock milk tester mentioned above). Place a suitable counterpoise[16]in the other receptacle of the centrifuge. Whirl for from one-half to one minute at a speed of about 1,000 revolutions per minute, that is with the handle turning about 100 revolutions per minute. Because of the air bubbles removed by whirling, the surface of the pulp will now be considerably below the top of the flask. Fill the flask and whirl in the centrifuge again. Repeat this filling and whirling until the flask is practically full of pulp after whirling. Ordinarily two or three separate whirlings are sufficient. Then add a few more drops of pulp so that the pulp comes above the top of the flask, and strike off flush with the top of the flask with a straight edge. Wash the outside of the flask, wipe dry, and weigh. Then read the specific gravity of the pulp from a table prepared, giving the weight of the flask full of pulp and the specific gravity of the pulp in parallel columns, or calculate the specific gravity as described below.While the weight is being taken a thermometer may be placed in the pulp remaining in the can or dipper from which the flask was filled. If the temperature varies from 68° F. the specific gravity may be corrected byTable 8. In order to use this method the temperature of the pulp should not bebelow 50° F., or above 86° F.; otherwise, it should be warmed or cooled, as described above.The method is accurate, simple, easily operated and fairly rapid. To calculate the specific gravity from the weights obtained, the weight of the clean, dry flask and of the water it contains at 68° F. are necessary. The weight of the clean, dry flask is then subtracted from the weight of the flask full of pulp to obtain the net weight of the pulp. This divided by the weight of the water the flask will contain at 68° F., gives the specific gravity.A table can be constructed readily for each flask, which will give in parallel columns the weight of the flask full of pulp and the corresponding specific gravity. This greatly simplifies the determination, as it eliminates all calculation. When such a table is employed a balance giving actual weights is practically as convenient as one reading specific gravity directly. It has the very important advantage that the balance, weights, and flask may be tested from time to time.In preparing such a table it is convenient first to draw a curve representing specific gravities of the pulp and corresponding weights of the flask full of pulp of various degrees of specific gravity. The table may then be constructed from the curve.For instance, let us suppose that the flask weighs 56.00 grams and that when full of water at 68° F. (20° C.) it weighs 176.63 grams. The water contained at the temperature mentioned then weighs (176.63 - 56.00) 120.63 grams. Now the specific gravity of the pulp is its weight compared with the weight of an equal volume of water. Having the figures given above we can easily calculate the weight of the flask filled with pulp of any desired specific gravity. We may therefore calculate the weight of the flask plus pulp of two different specific gravities; mark those points on a sheet of coordinate paper with specific gravity of the pulp entered at the bottom and the weight of flask plus pulp at the side, and a straight line drawn through the two points mentioned gives us the weight of the flask when filled with pulp of any specific gravity.For instance, if the flask mentioned above be filled with a pulp of the specific gravity of 1.03, the weight of the pulp is (120.63 × 1.03) 124.25 grams. This added to the weight of the flask (56.00 grams) gives us 180.25 grams. Similarly, if the flask be filled with pulp of 1.04 specific gravity the weight of the contents at 68° F. will be (120.63 × 1.04) 125.46 grams. This added to the weight of the flask (56.00 grams) gives 181.46 grams as the weight of flask pluspulp. Now if a sheet of coordinate paper be prepared with specific gravities entered at the bottom and weight of flask plus pulp at the side, these points may be entered. This is done in Figure 4 and the two points mentioned are each indicated by a circle and are connected by a straight line. A table may be constructed from this line, giving the weights of flask plus pulp in one column and the corresponding specific gravity in another.Fig. 4.Weight and Specific Gravity of Tomato Pulp.As an illustration of this there are given below a series of figures illustrating the beginning of the table that could be constructed from Figure 4. If a large sheet of coordinate paper be taken the line shown in Figure 4 may be extended so that a table may be constructed for pulp of all concentrations.Weight of flaskand pulpSpecificgravity180.251.0300180.311.0305180.371.0310180.431.0315180.491.0320180.551.0325180.611.0330180.671.0335The highest degree of accuracy can be secured by filling the flask and making the weighing at exactly 68° F. This is obviously not practicable under factory conditions, however, and satisfactory results can be secured by taking the temperature of the pulp at the time of weighing and correcting for temperature by the use ofTable 8. This table gives the correction to be added to the specific gravity when the pulp is taken at temperatures between 68° and 86° F., and the correction to be deducted from the specific gravity for temperatures, between 55° and 68° F. As a matter of principle, correction factors should be avoided as far as practicable, and the smaller the correction factor the more accurate the results will be. This table will be found especially useful in determining the specific gravity of the partly concentrated pulp, as is directed on page 50.As stated above, in determining specific gravity by this method it is advisable that the reading be made to the second place of decimals. For this purpose an assay pulp balance is suggested. An assay pulp balance carrying a maximum load of 300 grams is listed by dealers in chemical apparatus at $52.50. This balance may be obtained from dealers in chemical apparatus by designating it as “Assay pulp balance E. & A. No. 292, capacity 300 grams.” The same balance, moreheavily built, and preferable for that reason, carrying a maximum capacity of 600 grams, is listed at $63.A satisfactory set of weights, suitable for weighing a cup similar to that shown in Figure 1, may be obtained from any dealer in chemical apparatus by designating it as E. & A. No. 516, “Metric brass weights in wooden box, 200 grams to 1 centigram.” This is listed at $5.50.For convenience, all of the apparatus necessary for using this method of determining specific gravity is listed below. With the exception of the specific gravity flask this apparatus may be purchased of any dealer in chemical supplies. The specific gravity flasks have not heretofore been available except through this laboratory, which purchased a considerable quantity of them and supplied them to manufacturers of pulp as long as this supply lasted. At the urgent request of the writer, Eimer & Amend and Emil Greiner & Co., both of New York City, have finally stocked this item and stand ready to supply it to those wishing to secure it.Specific gravity flask for tomato pulp, of Pyrex glass, 1½ × 6¼ inches (outside measurements), capacity about 125 cc.Two-bottle Babcock milk tester, with 2 brass holders, E. & A. No. 1883.Assay pulp balance, maximum load 300 grams, E. & A. No. 292.Metric brass weights in wooden box, 200 grams to 1 centigram, E. & A. No. 516.Chemical thermometer, 50 to 212° F.[17](b)COLD PULP WITHOUT CENTRIFUGINGA method frequently employed for determining the specific gravity of cold pulp is to fill the cup by pouring, strike off with a straight edge, wash the outside, dry and weigh. As ordinarily practiced, this determination is attended by considerable error. If the balance is arranged for reading specific gravity directly, weights should be at hand for determining the accuracy of the balance and the weight of the flask, and both should be checked from time to time. The pulp on being poured into the flask or cup carries with it air bubbles to such an extent as to materially reduce the weight. Attempts to remove these air bubbles without the use of a centrifuge have not been successful. This is shown inTable 7, in the column headed “Pouring cold and whirling by hand.” The figures given in this column were obtained by weighing the sample after it had been whirled vigorously in the cup shown in Fig. 5 until air bubbles appeared to be eliminated.From 50 to 175 revolutions were given the cup in each of the determinations whose results are shown in this column. Even then it will be noted by comparison with Column 1 that the results are low. As the method is ordinarily practiced in the plant, without any attempt to remove the air bubbles by whirling, the results obtained are likely to be less accurate than those shown in the column just mentioned.(c)SPECIFIC GRAVITY OF HOT PULPMany manufacturers of tomato pulp control the concentration of their product by determining specific gravity when the evaporation is almost completed. They therefore desire the results at the earliest possible moment, and there is no attempt to cool the sample before determining specific gravity, although in that way much more accurate results could be obtained.When necessary to use this method the hot pulp is poured into the specific gravity flask (Fig. 1 or Fig. 2) by means of a dipper until the flask overflows. The top is then “struck off” with a straight edge and the flask placed in a shallow basin of water and the pulp carefully washed from the outside. The temperature of the pulp remaining in the dipper is then determined by means of a chemical thermometer.The flask is then dried with a towel, which operation is greatly facilitated by the heat of the pulp. The cooling of the contents of the flask causes contraction, so that after washing the flask is not entirely full. This should be disregarded, as it is desired to determine the weight of the amount of pulp that filled the flask originally. As soon as the outside of the flask is clean and dry the flask and contents are weighed.The apparent specific gravity of the hot pulp is ascertained from the special table prepared for the flask according to the directions given on page 38, and the correction figure for the temperature of the pulp obtained fromTable 6is added.For example, this method when applied to a certain sample of hot pulp (without centrifuging) indicated a specific gravity of 0.9874. The temperature of the pulp was found to be 201° F. InTable 6we find that the correction .0457 is equivalent to 201° F. Adding this to the apparent specific gravity given above, we have 0.9874 × 0.457 or 1.033 which is as nearly as we can determine from the hot pulp the specific gravity that would have been determined by examining the same sample after cooling by method (a). More accurate results can be obtained by workingwith larger specific gravity flasks. For instance, the specific gravity cup shown in Figure 5 may be made of copper, and may readily be made larger than the glass flasks shown in Figures 1 and 2. All metal flasks will gradually change in weight, owing to the solution of metal by the hot tomato pulp, and their weight should therefore be checked from time to time.Table 6.—Corrections for Specific Gravity of Hot PulpTemp. °F.Correction190.0401191.0406192.0411193.0416194.0421195.0426196.0431197.0436198.0441199.0447200.0452201.0457202.0462203.0466204.0472205.0477206.0482207.0487208.0492209.0498210.0504211.0510212.0515With a materially larger cup or flask (which should be of metal) a heavier balance and heavier weights should be used than suggested on page 40. In using a specific gravity cup similar to that shown in Figure 5 but holding about 1,000 grams of pulp an assay pulp balance with a capacity of 1,500 can be employed, or owing to the increased accuracy of the larger sample a less accurate and cheaper scale such as the “Howard trip scale,” or better a box scale such as is listed as E. & A. 338, may be employed. In working with a cup of this size a set of weights ranging from 1000 grams to 1 centigram is necessary.The determination of specific gravity in hot pulp is attended by considerable error. Even if the flask or cup be carried directly to the kettle, and filled as quickly as possible, the pulp is materially cooled in transferring, and by the time the surface is “struck off” sufficient contraction may occur to increase the weight of the contents of the flask and cause material error.When a pail of hot pulp is carried to another room or building for the determination of specific gravity, the error caused by cooling may be increased. Again, notwithstanding the fact that the pulp is hot, enough air bubbles become incorporated into it in pouring intothe cup to make a considerable difference in the weight. These two errors counter balance each other to some extent, but it is impossible to control the manipulation with sufficient uniformity to secure satisfactory results.Fig. 5.Specific Gravity Cup for Hot Pulp.The figures obtained in the second column ofTable 7(under the heading “Pouring at boiling temperature”) show the error of this method with carefully calibrated apparatus and working under the best conditions. By comparison with the first column, it will be noted that the results are always low, and that the difference between individual determinations is so great that a correction factor cannot be established. It should be borne in mind that these results were obtained by chemists. When the method is employed even by careful operators in the plant, still greater discrepancies may be expected.Table 7.—Comparison of Different Methods of Determining Specific Gravity[18]Sample NumberSpecific gravity by different methods of filling cup or flask.Centrifugingat 68°F.Pouring cold and whirling by handPouring at boiling temperatureDipping at boiling temperature14771.0610...1.0464...Do1.0610...1.0449...Do......1.0600...14841.04231.03301.0380...14851.0347...1.0336...14831.04641.04371.042...Do...1.04201.0446...Do...1.0430......Do...1.0442......14821.04411.04161.0360...Do1.04441.04191.0410...Do1.04471.04241.0413...14811.04491.04101.040...Do1.04491.04101.041...Do...1.04181.0397...1480...1.04301.036...Do...1.04201.040...Do...1.04291.044...Do......1.0407...14961.03401.0330......Do1.03411.0326......Do...1.0326......Do...1.0346...1.0299Do...1.0341...1.0303Do...1.0341...1.0343Do.........1.03315151.0351......1.036Do1.0352......1.03515191.0380......1.0377Do.........1.0383Do.........1.0329Do.........1.035015211.0440...1.04101.0439Do......1.04391.044815221.0500...1.04281.0493Do......1.04551.050815241.0519......1.0504 [46]1524.........1.0529Do.........1.051015261.0519...1.04721.0529Do......1.04631.052515281.0519...1.05091.0514Do......1.04851.051415301.0252...1.02811.0260Do.........1.0264Do.........1.026915311.0291...1.03121.0294Do.........1.0311Do.........1.0313It was thought that better results might be secured by modifying the construction of a cup in such a manner as to permit it to be filled by dipping below the surface of the pulp in the kettle. A bail made of 3/16-inch wire was, therefore, soldered to the opposite side of the cup (see Fig. 5). By means of the bail the cup was lowered into the kettle. After it was filled with the pulp the attempt was made to remove air bubbles by repeatedly giving the bail a quick twist or circular motion with a sudden stop. The cup was then brought quickly to the surface of the kettle and “struck off” with a straight edge, the outside of the cup and bail washed quickly with water, dried, and the cup and contents weighed.In using this method the steam is turned off, and as soon as thefoam subsides the cup is sunk well below the surface of the pulp. At this time the heat in various portions of the kettle is of course uniform, by reason of the thorough mixture caused by the vigorous boiling. Owing to the large mass of rather viscous material, and the heat of the kettle itself, the contents of the kettle cool slowly, and even after 10 minutes the temperature does not decrease more than 1° F., except at the very surface of the pulp. As a result of several observations, it was found that a thermometer bulb held 3 inches below the surface of the pulp showed a lowering of temperature of not more than 1° F. in 10 minutes and a lowering of only 0.5° F. in from 5 to 7 minutes.The bail employed was about 6½ inches wide and 8 inches long. There was some difficulty, owing to the pulp spattering on the hands of the operator because of the air escaping from the cup. This might be diminished by the use of a longer bail, or by wearing suitable gloves. When evaporating tanks are used it will probably be necessary to attach the bail to a stick or support of some kind. In addition to permitting this method of filling, the bail has the additional advantage that the cup full of pulp may be handled for washing and conveying to the balance much more conveniently and with less danger of spilling than with the handle on the side of the cup. Again, the bail does not heat when the cup is filled with hot pulp, and for that reason is easier to handle.(d)HYDROMETER METHODHydrometers are of little value in determining the specific gravity of tomato pulp. With cold pulp they cannot be used at all. With hot pulp a relatively slender hydrometer comes to rest and readings can be taken with more or less accuracy. The value of the reading is relative to the specific gravity of the pulp and varies with the shape of the hydrometer and with the character of the pulp. It is necessary therefore to obtain the relation between the reading of the hydrometer in the hot pulp and the specific gravity (obtained by an accurate method) of the same pulp cooled without evaporation. In the hands of a careful operator some manufacturers have found hydrometers (used with hot pulp) helpful in making pulp of uniform specific gravity.The hydrometer gives much more accurate results with the filtrate of pulp. As shown on page 31, there is a direct relation between thespecific gravity of tomato pulp and of the liquor obtained by filtering or straining the same, so that when the specific gravity of the latter is known that of the former may be ascertained readily by means of a table. This method is peculiarly applicable to the examination of cyclone juice and light pulp from which the insoluble solids may be removed quickly by straining through a cloth, and it therefore affords the most rapid method that is available to the average factory for determining the specific gravity of cyclone juice.InTable 8are given a series of corrections making it possible to use this method at any temperature between 50 and 80° Fahrenheit. The more closely the readings are taken to 68° F. the more accurate the results. Moreover, when it is attempted to strain the insoluble solids from hot pulp or cyclone juice, considerable evaporation occurs, causing concentration of the product and producing an error in the results. When hot pulp is handled, therefore, it must be strained as quickly as possible, and more accurate results may be obtained if the pulp is cooled quickly before straining. This may be done by placing in a large can and stirring vigorously while the can stands in ice water, or shaking under water in a large flask.There are several forms of hydrometer which may be used for determining the specific gravity of the filtrate. The ordinary specific gravity hydrometer is the most logical form to use, since it gives the specific gravity directly. Unfortunately, specific gravity hydrometers with the particular marking required for this work are not a stock article, and would, therefore, have to be made to order. For this reason they would be difficult to obtain and not easily replaced if broken.The Brix hydrometer appears to solve the difficulty. This hydrometer has no direct relation to specific gravity, but Brix readings can, of course, be converted to the specific gravity readings by a table arranged in parallel columns.Table 9gives the specific gravity of tomato pulp and the corresponding Brix reading of the filtrate. The Brix hydrometer gives directly the per cent of sugar in a solution of cane sugar, one degree Brix being equivalent to one per cent sugar at the temperature for which the hydrometer was calibrated. This fact and the ordinary purpose for which the instrument is manufactured are of no interest to us in this connection, however. The Brix hydrometer of the range desired for the examination of cyclone juice and pulp is a stock article and can be secured readily.The instrument can be used with the same accuracy as the specificgravity hydrometer, and the results obtained by it, after correcting for temperature byTable 8, are converted into terms of specific gravity by means ofTable 9. The determination of the specific gravity of pulp by means of the hydrometer reading of the filtrate obtained from the pulp has several advantages over the ordinary method of weighing a measured quantity of the pulp. When applied to pulp manufactured from whole tomatoes, the method is reasonably accurate. It is also very rapid and the equipment required is inexpensive. This method is especially applicable to the examination of pulp manufactured from whole tomatoes. It is less applicable to trimming stock pulp, although even with that product the method will be of value, especially for the examination of cyclone juice for the purpose of controlling concentration. With pulp manufactured from trimming stock, the relation of the specific gravity of the pulp to the specific gravity of the filtrate obtained from it will vary according to the nature of the raw material used and also according to the method of manufacture. It seems probable, therefore, that after a manufacturer has determined this relation as applied to his own product, he may be able to use this method with reasonable accuracy even in connection with trimming stock pulp.The method is adapted especially to the examination of cold pulp or cyclone juice.The following apparatus is used in this method:1 Brix hydrometer, graduated at 20.0° C., with a range of 1–10°, graduated in 1/10°.1 Cylinder of heavy glass, lipped, height 12 inches, diameter 2 inches.1 Chemical thermometer, graduated in Fahrenheit system up to 212° F.Since this apparatus is likely to be broken, it is well for each plant that contemplates using the method to equip itself with at least two of each item mentioned above.The Brix hydrometer mentioned above is suggested because it is a stock article handled by all dealers in chemical apparatus and can be secured quickly. It has the disadvantage that it is relatively large, and in order to use it the filtrate must be prepared in much larger quantity than would be required by a smaller hydrometer. By placing orders well in advance with dealers in chemical apparatus special hydrometers may be made with a bulb about one-half inch in diameter and with a total length of five or six inches. Such hydrometers could be used with a cylinder as small as one inch in diameter. They would require much less liquor than is necessaryfor the Brix hydrometer and therefore would enable the analyst to obtain results much more quickly. In securing such hydrometers it would be well to order several at a time, since it would require several weeks to replace any that may be broken.The details of the method are as follows:
Per centSolids in Filtrate= 230 (sp. gr. of filtrate - 1.000).
Per centSolids 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., fromTable 5. The same results may be obtained from the following formula, which was derived fromTable 4:
Per centSolids in Pulp= 257.5 (sp. gr. of filtrate at 20° C. - 1.000).
Per centSolids 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 byTable 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 fromTable 5:
Per centSolids in Filtrate= 0.258 (scale reading - 15) - 0.0165 (scale reading - 26.4).
Per centSolids 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 centSolids in Filtrate= 666(nD- 1.3332) - 20.7(nD- 1.3376).
Per centSolids in Filtrate= 666(nD- 1.3332) - 20.7(nD- 1.3376).
The per cent of total solids in tomato pulp may also be ascertained from the index of refraction of the liquor prepared by filtering the pulp as shown inTable 5; or it may be calculated from the immersion refractometer reading by the following formula, which is derived fromTable 5:
Per centSolids in Pulp= 0.289(scale reading of filtrate - 15) - 0.0185(scale reading - 26.4).
Per centSolids in Pulp= 0.289(scale reading of filtrate - 15) - 0.0185(scale reading - 26.4).
If the index of refraction of the filtrate has been determined bymeans of an Abbé refractometer, the per cent of solids in the pulp may be calculated by the following formula:
Per centSolids in Pulp= 748(nD- 1.3332) - 25.5(nD- 1.3376).
Per centSolids in Pulp= 748(nD- 1.3332) - 25.5(nD- 1.3376).
It is of interest to note that the relation between the index of refraction of the liquor obtained by filtering tomato pulp and the per cent of solids in that liquid is very similar to the relation between the index of refraction and dissolved solids in beer and wine extract, as shown in the table prepared by Wagner.[12]
In the formula given above, as well as inTable 5, it is assumed that salt is absent. If it be desired to calculate the percentage of solids in a sample containing salt from the index of refraction of the filtrate, it is necessary first to determine the amount of salt present and make correction therefor (see p. 34). For this purpose the table of Wagner[13]may be employed. The correction of the immersion refractometer reading amounts to 0.45 for each tenth per cent of salt present.
This correction is necessary if the percentage of solids be determined by drying, or calculated from specific gravity.
Transfer 20 grams of the pulp to an eight-ounce nursing bottle, nearly filled with hot water, mix by shaking, and centrifuge until the insoluble matter is collected in a cake in the bottom of the bottle. Transfer the supernatant liquor onto a double, tared filter paper covering the bottom of a Büchner funnel, using suction to facilitate filtration.Again fill the nursing bottle with hot water, stir the cake of insoluble solids so that it is thoroughly mixed with the water, centrifuge, and decant the supernatant liquor on the filter. Repeat the centrifuging and the filtration of the supernatant liquor once more, and then finally transfer the insoluble solids to the filter paper and thoroughly wash with hot water. Dry the paper and insoluble solids, and weigh. The insoluble solids are quite hydroscopic and the weight must be taken quickly.
Transfer 20 grams of the pulp to an eight-ounce nursing bottle, nearly filled with hot water, mix by shaking, and centrifuge until the insoluble matter is collected in a cake in the bottom of the bottle. Transfer the supernatant liquor onto a double, tared filter paper covering the bottom of a Büchner funnel, using suction to facilitate filtration.
Again fill the nursing bottle with hot water, stir the cake of insoluble solids so that it is thoroughly mixed with the water, centrifuge, and decant the supernatant liquor on the filter. Repeat the centrifuging and the filtration of the supernatant liquor once more, and then finally transfer the insoluble solids to the filter paper and thoroughly wash with hot water. Dry the paper and insoluble solids, and weigh. The insoluble solids are quite hydroscopic and the weight must be taken quickly.
The sugar of tomatoes is probably always present as invert sugar. If cane sugar is ever present in the raw product it is doubtless inverted during the concentration of pulp. The per cent of sugar given inTables 2and3was determined by the method of Munson and Walker.[14]
Accurate results cannot be obtained by the titration of tomato products in the presence of the insoluble solids. If it be desired to determine the acidity in the entire sample of tomatoes or tomato pulp rather than in the expressed juice, the insoluble solids should first be removed by the method given in the determination of insoluble solids or by filtration through filter paper. The per cent of acid given inTables 2and3was obtained by titrating the liquor obtained by filtering the pulp. In products of this nature, the addition of an alkali causes a brownish color which has a tendency to obscure the end point shown by the indicator. To obviate this, the sample should be diluted to at least 200 cc. and a larger amount of indicator employed than is necessary with a clear solution. The following details are suggested.
Dilute 20 grams of the filtrate under examination with over 200 cc. of water. Add at ½ cc. of phenolphthalein solution (prepared by dissolving 1 gram of phenolphthalein in 100 cc. of 95 per cent alcohol) and titrate with sodium hydroxide until the end point is obtained. Add 1 cc. of tenth-normal hydrochloric acid, heat the solution quickly to boiling and boil one minute to expel carbon dioxide. Cool the solution quickly to about room temperature, and then add tenth-normal sodium hydroxide until the end point is obtained. The volume of hydrochloric acid added must, of course, be taken into consideration in the final result. The filtrate may also be titrated direct with tenth-normal sodium hydroxide solution with satisfactory results.
Dilute 20 grams of the filtrate under examination with over 200 cc. of water. Add at ½ cc. of phenolphthalein solution (prepared by dissolving 1 gram of phenolphthalein in 100 cc. of 95 per cent alcohol) and titrate with sodium hydroxide until the end point is obtained. Add 1 cc. of tenth-normal hydrochloric acid, heat the solution quickly to boiling and boil one minute to expel carbon dioxide. Cool the solution quickly to about room temperature, and then add tenth-normal sodium hydroxide until the end point is obtained. The volume of hydrochloric acid added must, of course, be taken into consideration in the final result. The filtrate may also be titrated direct with tenth-normal sodium hydroxide solution with satisfactory results.
This laboratory has been using the following rapid method which gives results agreeing closely with results obtained by the analysis of the ash:
Weigh out 20 grams of pulp, dilute in a volumetric flask to 200 cc., filter and titrate an aliquot portion with standard silver nitrate solution, using potassium chromate as indicator. The acidity of tomato pulp is not sufficient to interfere with this determination.
Weigh out 20 grams of pulp, dilute in a volumetric flask to 200 cc., filter and titrate an aliquot portion with standard silver nitrate solution, using potassium chromate as indicator. The acidity of tomato pulp is not sufficient to interfere with this determination.
The specific gravity of tomato pulp is used as one criterion for establishing the value of pulp that is offered for sale and is also used in connection with the manufacture of pulp to determine the point at which evaporation should be stopped.
In the former case there is ample time for making the examination, and conditions may be established which permit a reasonable degree of accuracy in the work.
In the determination of the specific gravity of hot pulp during the process of its evaporation speed is essential, and the conditions of a manufacturing plant do not always permit a high degree of accuracy. It becomes necessary, therefore, to consider what methods may give the highest degree of accuracy obtainable under the conditions of the work and at the same time afford quick results.
Tomato pulp, owing to its high viscosity, retains a large quantity of air bubbles which increase the volume of the pulp and hence interfere with the accuracy of the determination of specific gravity. In working with cold pulp this air may be eliminated by whirling in a centrifuge. With hot pulp that operation is impossible, and the specific gravity must be determined in the presence of the air bubbles mentioned. Moreover, in working with cold pulp the temperature can be more accurately controlled, and the error caused by variation in temperature can be corrected. With hot pulp these conditions cannot be obtained nearly so well. The determination of specific gravity of hot pulp is therefore only roughly approximate at best. Where time permits it is strongly advisable to cool the pulp under conditions that prevent evaporation before determining specific gravity.
The importance of accuracy in the determination of specific gravity in tomato pulp is discussed on page 50.
Methods are given below for the determination of specific gravity in both hot and cold pulp.
When salt has been added, the amount should be determined and a correction applied by deducting .007 from the specific gravity for each per cent of salt present.
(a)COLD PULP AFTER CENTRIFUGING TO ELIMINATE AIR BUBBLES
This method may be employed for pulp of any degree of concentration or for unconcentrated cyclone juice. A specific gravity flask such as is shown in Figure 1 is used together with a “2-bottle” Babcock milk tester (the centrifuge referred to below). The flask may be obtained of Eimer & Amend, Third Avenue, 18th to 19th Streets, New York City, or of Emil Greiner & Co., 55 Fulton Street, New York City, and in ordering it should be designated as “specific gravity flask for tomato pulp of Pyrex glass with a capacity of about 125 cc.” The “2-bottle” Babcock milk tester may be obtained of any dairy supply house. It may also be obtained of any dealer in chemical apparatus by designating it as E. & A. No. 1833.
The specific gravity flask may be calibrated as follows:
Obtain the weight of the flask after thoroughly cleaning and drying, fill to overflowing with water (preferably boiled and cooled distilled water) and remove the excess water from the mouth of the flask by means of a straight edge. Wipe dry and weigh immediately. If the flask full of water is weighed at any other temperature than 20° C. (68° F.) a correction must be made to obtain the weight at that temperature. These corrections are as follows:
If it is desired to use somewhat larger samples and thus secure correspondingly more accurate results, a similar flask, but made somewhat larger (capacity approximately 400 cc.), may be employed. Such a flask, with a diameter of a little over 3 inches, is illustrated in Figure 2. This larger flask will not fit into the Babcock tester, and when it is used a special head for the Babcock tester must be made.Such a head is illustrated in Figure 3, and can be made by any good tinner.
The larger flask shown in Figure 2 holds a heavier weight than the Babcock machine is intended to carry, and the advisability of its use is perhaps questionable. In any case, whatever size flask is used, it is important to build a guard around the centrifuge in order to protect the operator when the apparatus gives way, as it eventually will.
Fig. 1.Small Specific Gravity Flask.
Fig. 1.Small Specific Gravity Flask.
Fig. 2.Large Specific Gravity Flask.
Fig. 2.Large Specific Gravity Flask.
Dimensions given are outside measurements. Thickness of walls is about 3/64 in.
Dimensions given are outside measurements. Thickness of walls is about 3/64 in.
Fig. 3.Special Head and Flask Receptacles for Babcock Milk Tester.
Fig. 3.Special Head and Flask Receptacles for Babcock Milk Tester.
The details of the method of determining specific gravity by the use of this apparatus are as follows:
Fill the flask shown in Figure 1 with the sample of pulp and place in the centrifuge (the Babcock milk tester mentioned above). Place a suitable counterpoise[16]in the other receptacle of the centrifuge. Whirl for from one-half to one minute at a speed of about 1,000 revolutions per minute, that is with the handle turning about 100 revolutions per minute. Because of the air bubbles removed by whirling, the surface of the pulp will now be considerably below the top of the flask. Fill the flask and whirl in the centrifuge again. Repeat this filling and whirling until the flask is practically full of pulp after whirling. Ordinarily two or three separate whirlings are sufficient. Then add a few more drops of pulp so that the pulp comes above the top of the flask, and strike off flush with the top of the flask with a straight edge. Wash the outside of the flask, wipe dry, and weigh. Then read the specific gravity of the pulp from a table prepared, giving the weight of the flask full of pulp and the specific gravity of the pulp in parallel columns, or calculate the specific gravity as described below.While the weight is being taken a thermometer may be placed in the pulp remaining in the can or dipper from which the flask was filled. If the temperature varies from 68° F. the specific gravity may be corrected byTable 8. In order to use this method the temperature of the pulp should not bebelow 50° F., or above 86° F.; otherwise, it should be warmed or cooled, as described above.
Fill the flask shown in Figure 1 with the sample of pulp and place in the centrifuge (the Babcock milk tester mentioned above). Place a suitable counterpoise[16]in the other receptacle of the centrifuge. Whirl for from one-half to one minute at a speed of about 1,000 revolutions per minute, that is with the handle turning about 100 revolutions per minute. Because of the air bubbles removed by whirling, the surface of the pulp will now be considerably below the top of the flask. Fill the flask and whirl in the centrifuge again. Repeat this filling and whirling until the flask is practically full of pulp after whirling. Ordinarily two or three separate whirlings are sufficient. Then add a few more drops of pulp so that the pulp comes above the top of the flask, and strike off flush with the top of the flask with a straight edge. Wash the outside of the flask, wipe dry, and weigh. Then read the specific gravity of the pulp from a table prepared, giving the weight of the flask full of pulp and the specific gravity of the pulp in parallel columns, or calculate the specific gravity as described below.
While the weight is being taken a thermometer may be placed in the pulp remaining in the can or dipper from which the flask was filled. If the temperature varies from 68° F. the specific gravity may be corrected byTable 8. In order to use this method the temperature of the pulp should not bebelow 50° F., or above 86° F.; otherwise, it should be warmed or cooled, as described above.
The method is accurate, simple, easily operated and fairly rapid. To calculate the specific gravity from the weights obtained, the weight of the clean, dry flask and of the water it contains at 68° F. are necessary. The weight of the clean, dry flask is then subtracted from the weight of the flask full of pulp to obtain the net weight of the pulp. This divided by the weight of the water the flask will contain at 68° F., gives the specific gravity.
A table can be constructed readily for each flask, which will give in parallel columns the weight of the flask full of pulp and the corresponding specific gravity. This greatly simplifies the determination, as it eliminates all calculation. When such a table is employed a balance giving actual weights is practically as convenient as one reading specific gravity directly. It has the very important advantage that the balance, weights, and flask may be tested from time to time.
In preparing such a table it is convenient first to draw a curve representing specific gravities of the pulp and corresponding weights of the flask full of pulp of various degrees of specific gravity. The table may then be constructed from the curve.
For instance, let us suppose that the flask weighs 56.00 grams and that when full of water at 68° F. (20° C.) it weighs 176.63 grams. The water contained at the temperature mentioned then weighs (176.63 - 56.00) 120.63 grams. Now the specific gravity of the pulp is its weight compared with the weight of an equal volume of water. Having the figures given above we can easily calculate the weight of the flask filled with pulp of any desired specific gravity. We may therefore calculate the weight of the flask plus pulp of two different specific gravities; mark those points on a sheet of coordinate paper with specific gravity of the pulp entered at the bottom and the weight of flask plus pulp at the side, and a straight line drawn through the two points mentioned gives us the weight of the flask when filled with pulp of any specific gravity.
For instance, if the flask mentioned above be filled with a pulp of the specific gravity of 1.03, the weight of the pulp is (120.63 × 1.03) 124.25 grams. This added to the weight of the flask (56.00 grams) gives us 180.25 grams. Similarly, if the flask be filled with pulp of 1.04 specific gravity the weight of the contents at 68° F. will be (120.63 × 1.04) 125.46 grams. This added to the weight of the flask (56.00 grams) gives 181.46 grams as the weight of flask pluspulp. Now if a sheet of coordinate paper be prepared with specific gravities entered at the bottom and weight of flask plus pulp at the side, these points may be entered. This is done in Figure 4 and the two points mentioned are each indicated by a circle and are connected by a straight line. A table may be constructed from this line, giving the weights of flask plus pulp in one column and the corresponding specific gravity in another.
Fig. 4.Weight and Specific Gravity of Tomato Pulp.
Fig. 4.Weight and Specific Gravity of Tomato Pulp.
As an illustration of this there are given below a series of figures illustrating the beginning of the table that could be constructed from Figure 4. If a large sheet of coordinate paper be taken the line shown in Figure 4 may be extended so that a table may be constructed for pulp of all concentrations.
The highest degree of accuracy can be secured by filling the flask and making the weighing at exactly 68° F. This is obviously not practicable under factory conditions, however, and satisfactory results can be secured by taking the temperature of the pulp at the time of weighing and correcting for temperature by the use ofTable 8. This table gives the correction to be added to the specific gravity when the pulp is taken at temperatures between 68° and 86° F., and the correction to be deducted from the specific gravity for temperatures, between 55° and 68° F. As a matter of principle, correction factors should be avoided as far as practicable, and the smaller the correction factor the more accurate the results will be. This table will be found especially useful in determining the specific gravity of the partly concentrated pulp, as is directed on page 50.
As stated above, in determining specific gravity by this method it is advisable that the reading be made to the second place of decimals. For this purpose an assay pulp balance is suggested. An assay pulp balance carrying a maximum load of 300 grams is listed by dealers in chemical apparatus at $52.50. This balance may be obtained from dealers in chemical apparatus by designating it as “Assay pulp balance E. & A. No. 292, capacity 300 grams.” The same balance, moreheavily built, and preferable for that reason, carrying a maximum capacity of 600 grams, is listed at $63.
A satisfactory set of weights, suitable for weighing a cup similar to that shown in Figure 1, may be obtained from any dealer in chemical apparatus by designating it as E. & A. No. 516, “Metric brass weights in wooden box, 200 grams to 1 centigram.” This is listed at $5.50.
For convenience, all of the apparatus necessary for using this method of determining specific gravity is listed below. With the exception of the specific gravity flask this apparatus may be purchased of any dealer in chemical supplies. The specific gravity flasks have not heretofore been available except through this laboratory, which purchased a considerable quantity of them and supplied them to manufacturers of pulp as long as this supply lasted. At the urgent request of the writer, Eimer & Amend and Emil Greiner & Co., both of New York City, have finally stocked this item and stand ready to supply it to those wishing to secure it.
(b)COLD PULP WITHOUT CENTRIFUGING
A method frequently employed for determining the specific gravity of cold pulp is to fill the cup by pouring, strike off with a straight edge, wash the outside, dry and weigh. As ordinarily practiced, this determination is attended by considerable error. If the balance is arranged for reading specific gravity directly, weights should be at hand for determining the accuracy of the balance and the weight of the flask, and both should be checked from time to time. The pulp on being poured into the flask or cup carries with it air bubbles to such an extent as to materially reduce the weight. Attempts to remove these air bubbles without the use of a centrifuge have not been successful. This is shown inTable 7, in the column headed “Pouring cold and whirling by hand.” The figures given in this column were obtained by weighing the sample after it had been whirled vigorously in the cup shown in Fig. 5 until air bubbles appeared to be eliminated.From 50 to 175 revolutions were given the cup in each of the determinations whose results are shown in this column. Even then it will be noted by comparison with Column 1 that the results are low. As the method is ordinarily practiced in the plant, without any attempt to remove the air bubbles by whirling, the results obtained are likely to be less accurate than those shown in the column just mentioned.
(c)SPECIFIC GRAVITY OF HOT PULP
Many manufacturers of tomato pulp control the concentration of their product by determining specific gravity when the evaporation is almost completed. They therefore desire the results at the earliest possible moment, and there is no attempt to cool the sample before determining specific gravity, although in that way much more accurate results could be obtained.
When necessary to use this method the hot pulp is poured into the specific gravity flask (Fig. 1 or Fig. 2) by means of a dipper until the flask overflows. The top is then “struck off” with a straight edge and the flask placed in a shallow basin of water and the pulp carefully washed from the outside. The temperature of the pulp remaining in the dipper is then determined by means of a chemical thermometer.
The flask is then dried with a towel, which operation is greatly facilitated by the heat of the pulp. The cooling of the contents of the flask causes contraction, so that after washing the flask is not entirely full. This should be disregarded, as it is desired to determine the weight of the amount of pulp that filled the flask originally. As soon as the outside of the flask is clean and dry the flask and contents are weighed.
The apparent specific gravity of the hot pulp is ascertained from the special table prepared for the flask according to the directions given on page 38, and the correction figure for the temperature of the pulp obtained fromTable 6is added.For example, this method when applied to a certain sample of hot pulp (without centrifuging) indicated a specific gravity of 0.9874. The temperature of the pulp was found to be 201° F. InTable 6we find that the correction .0457 is equivalent to 201° F. Adding this to the apparent specific gravity given above, we have 0.9874 × 0.457 or 1.033 which is as nearly as we can determine from the hot pulp the specific gravity that would have been determined by examining the same sample after cooling by method (a). More accurate results can be obtained by workingwith larger specific gravity flasks. For instance, the specific gravity cup shown in Figure 5 may be made of copper, and may readily be made larger than the glass flasks shown in Figures 1 and 2. All metal flasks will gradually change in weight, owing to the solution of metal by the hot tomato pulp, and their weight should therefore be checked from time to time.
Table 6.—Corrections for Specific Gravity of Hot Pulp
With a materially larger cup or flask (which should be of metal) a heavier balance and heavier weights should be used than suggested on page 40. In using a specific gravity cup similar to that shown in Figure 5 but holding about 1,000 grams of pulp an assay pulp balance with a capacity of 1,500 can be employed, or owing to the increased accuracy of the larger sample a less accurate and cheaper scale such as the “Howard trip scale,” or better a box scale such as is listed as E. & A. 338, may be employed. In working with a cup of this size a set of weights ranging from 1000 grams to 1 centigram is necessary.
The determination of specific gravity in hot pulp is attended by considerable error. Even if the flask or cup be carried directly to the kettle, and filled as quickly as possible, the pulp is materially cooled in transferring, and by the time the surface is “struck off” sufficient contraction may occur to increase the weight of the contents of the flask and cause material error.
When a pail of hot pulp is carried to another room or building for the determination of specific gravity, the error caused by cooling may be increased. Again, notwithstanding the fact that the pulp is hot, enough air bubbles become incorporated into it in pouring intothe cup to make a considerable difference in the weight. These two errors counter balance each other to some extent, but it is impossible to control the manipulation with sufficient uniformity to secure satisfactory results.
Fig. 5.Specific Gravity Cup for Hot Pulp.
Fig. 5.Specific Gravity Cup for Hot Pulp.
The figures obtained in the second column ofTable 7(under the heading “Pouring at boiling temperature”) show the error of this method with carefully calibrated apparatus and working under the best conditions. By comparison with the first column, it will be noted that the results are always low, and that the difference between individual determinations is so great that a correction factor cannot be established. It should be borne in mind that these results were obtained by chemists. When the method is employed even by careful operators in the plant, still greater discrepancies may be expected.
Table 7.—Comparison of Different Methods of Determining Specific Gravity[18]
It was thought that better results might be secured by modifying the construction of a cup in such a manner as to permit it to be filled by dipping below the surface of the pulp in the kettle. A bail made of 3/16-inch wire was, therefore, soldered to the opposite side of the cup (see Fig. 5). By means of the bail the cup was lowered into the kettle. After it was filled with the pulp the attempt was made to remove air bubbles by repeatedly giving the bail a quick twist or circular motion with a sudden stop. The cup was then brought quickly to the surface of the kettle and “struck off” with a straight edge, the outside of the cup and bail washed quickly with water, dried, and the cup and contents weighed.
In using this method the steam is turned off, and as soon as thefoam subsides the cup is sunk well below the surface of the pulp. At this time the heat in various portions of the kettle is of course uniform, by reason of the thorough mixture caused by the vigorous boiling. Owing to the large mass of rather viscous material, and the heat of the kettle itself, the contents of the kettle cool slowly, and even after 10 minutes the temperature does not decrease more than 1° F., except at the very surface of the pulp. As a result of several observations, it was found that a thermometer bulb held 3 inches below the surface of the pulp showed a lowering of temperature of not more than 1° F. in 10 minutes and a lowering of only 0.5° F. in from 5 to 7 minutes.
The bail employed was about 6½ inches wide and 8 inches long. There was some difficulty, owing to the pulp spattering on the hands of the operator because of the air escaping from the cup. This might be diminished by the use of a longer bail, or by wearing suitable gloves. When evaporating tanks are used it will probably be necessary to attach the bail to a stick or support of some kind. In addition to permitting this method of filling, the bail has the additional advantage that the cup full of pulp may be handled for washing and conveying to the balance much more conveniently and with less danger of spilling than with the handle on the side of the cup. Again, the bail does not heat when the cup is filled with hot pulp, and for that reason is easier to handle.
(d)HYDROMETER METHOD
Hydrometers are of little value in determining the specific gravity of tomato pulp. With cold pulp they cannot be used at all. With hot pulp a relatively slender hydrometer comes to rest and readings can be taken with more or less accuracy. The value of the reading is relative to the specific gravity of the pulp and varies with the shape of the hydrometer and with the character of the pulp. It is necessary therefore to obtain the relation between the reading of the hydrometer in the hot pulp and the specific gravity (obtained by an accurate method) of the same pulp cooled without evaporation. In the hands of a careful operator some manufacturers have found hydrometers (used with hot pulp) helpful in making pulp of uniform specific gravity.
The hydrometer gives much more accurate results with the filtrate of pulp. As shown on page 31, there is a direct relation between thespecific gravity of tomato pulp and of the liquor obtained by filtering or straining the same, so that when the specific gravity of the latter is known that of the former may be ascertained readily by means of a table. This method is peculiarly applicable to the examination of cyclone juice and light pulp from which the insoluble solids may be removed quickly by straining through a cloth, and it therefore affords the most rapid method that is available to the average factory for determining the specific gravity of cyclone juice.
InTable 8are given a series of corrections making it possible to use this method at any temperature between 50 and 80° Fahrenheit. The more closely the readings are taken to 68° F. the more accurate the results. Moreover, when it is attempted to strain the insoluble solids from hot pulp or cyclone juice, considerable evaporation occurs, causing concentration of the product and producing an error in the results. When hot pulp is handled, therefore, it must be strained as quickly as possible, and more accurate results may be obtained if the pulp is cooled quickly before straining. This may be done by placing in a large can and stirring vigorously while the can stands in ice water, or shaking under water in a large flask.
There are several forms of hydrometer which may be used for determining the specific gravity of the filtrate. The ordinary specific gravity hydrometer is the most logical form to use, since it gives the specific gravity directly. Unfortunately, specific gravity hydrometers with the particular marking required for this work are not a stock article, and would, therefore, have to be made to order. For this reason they would be difficult to obtain and not easily replaced if broken.
The Brix hydrometer appears to solve the difficulty. This hydrometer has no direct relation to specific gravity, but Brix readings can, of course, be converted to the specific gravity readings by a table arranged in parallel columns.Table 9gives the specific gravity of tomato pulp and the corresponding Brix reading of the filtrate. The Brix hydrometer gives directly the per cent of sugar in a solution of cane sugar, one degree Brix being equivalent to one per cent sugar at the temperature for which the hydrometer was calibrated. This fact and the ordinary purpose for which the instrument is manufactured are of no interest to us in this connection, however. The Brix hydrometer of the range desired for the examination of cyclone juice and pulp is a stock article and can be secured readily.
The instrument can be used with the same accuracy as the specificgravity hydrometer, and the results obtained by it, after correcting for temperature byTable 8, are converted into terms of specific gravity by means ofTable 9. The determination of the specific gravity of pulp by means of the hydrometer reading of the filtrate obtained from the pulp has several advantages over the ordinary method of weighing a measured quantity of the pulp. When applied to pulp manufactured from whole tomatoes, the method is reasonably accurate. It is also very rapid and the equipment required is inexpensive. This method is especially applicable to the examination of pulp manufactured from whole tomatoes. It is less applicable to trimming stock pulp, although even with that product the method will be of value, especially for the examination of cyclone juice for the purpose of controlling concentration. With pulp manufactured from trimming stock, the relation of the specific gravity of the pulp to the specific gravity of the filtrate obtained from it will vary according to the nature of the raw material used and also according to the method of manufacture. It seems probable, therefore, that after a manufacturer has determined this relation as applied to his own product, he may be able to use this method with reasonable accuracy even in connection with trimming stock pulp.
The method is adapted especially to the examination of cold pulp or cyclone juice.
Since this apparatus is likely to be broken, it is well for each plant that contemplates using the method to equip itself with at least two of each item mentioned above.
The Brix hydrometer mentioned above is suggested because it is a stock article handled by all dealers in chemical apparatus and can be secured quickly. It has the disadvantage that it is relatively large, and in order to use it the filtrate must be prepared in much larger quantity than would be required by a smaller hydrometer. By placing orders well in advance with dealers in chemical apparatus special hydrometers may be made with a bulb about one-half inch in diameter and with a total length of five or six inches. Such hydrometers could be used with a cylinder as small as one inch in diameter. They would require much less liquor than is necessaryfor the Brix hydrometer and therefore would enable the analyst to obtain results much more quickly. In securing such hydrometers it would be well to order several at a time, since it would require several weeks to replace any that may be broken.
The details of the method are as follows: