INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1925
1155
Physical Examination of Sugar Juices’ By K. R. Lindfors MICHIGAN SUGAR Co., SAGINAW, MICR.
HE methods a t present used in sugar factory laboratories do not furnish reliable information in regard to filtration effectiveness of different kinds of filter presses or the improvement obtained by treatment with filter aids and sulfitation. The apparent and real purity is practically unaffected, and ash determination is of little value, as most of the material removed is organic. It is here proposed to introduce rapid, uniform methods of physical examination and standards of visible and colloidal turbidity, and to report these, as wel! as the viscosity and surface tension, in terms conveying a definite meaning to the average practical sugar man. With the aid of physical tests it will become possible to check variations in the work, and also to express filtration effectiveness, including results obtained by different methods, in definite figures.
T
Surface Tension
Previous investigations by the author? and the careful work of H. S. Paine and associates3 established the fact that surface tension furnishes the best available means for checking the removal of colloids or gums. Du Noiiy’s surface tension apparatus permits the determinations to be carried out in a few minutes. The precautions ordinarily taken to insure absolutely clean beakers need not be strictly observed in control work, as it is only necessary to rinse out the beaker eight to ten times with the hot juice to be tested. The presence of minute amounts of oil will cause considerable errors, however ; therefore, tlvo samples must always be taken within short intervals, and only if the two check can the result be considered accurate. Although the amount of gums removed by filtration is not regularly proportional to the increase in surface tension, i t is undoubtedly better for practical control work and comparative purposes to express this increase in per cent of gums, even if the relation between the two factors has not yet been accurately determined.
As the depression of the surface tension varies with the composition of the gums, it is probably impossible to prepare a table giving accurately the relation between the two. Nevertheless, even the approximate figures given in the table have several advantages over the report of the result in dynes per centimeter, one of the chief of which is that it conveys a meaning to the practical sugar man, who does not know the significance of the scientific terms. Viscosity Brix and Viscosity Purity
“It is important to note that influence on viscosity is the immediate avenue through which the effect of hydrophile colloids is made manifest during factory operation. Increased viscosity diminishes the rate of diffusion and consequently retards boiling and crystal g r ~ w t h . ” ~The amount of these hydrophile colloids or pectins present in thick juice is ordinarily too small to be detected by the viscosity, unless the work is carried out with a degree of refinement impractical in routine. I n green sirups, second fillmass, and molasses these impurities are concentrated to a point where their influence can be readily observed. When only comparative results are required, a 100-cc. pipet, inclosed in a glass tube through which running water passes to maintain a constant temperature, will serve. A pure sugar solution may be used to standardize the apparatus, the concentration of the sugar solution being determined by polarization. By recording the results in the form of a graph the intermediate points can be directly 65
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Surface Tension and Estimated Per cent of Gums Surface tension Dvnes 75.0 74.8 74.6 74.4 74.2 74.0 73.7 73.3 72.8 69.0 64.0 61.2 60.8 60.6 60.4 60.2 60.1 60.0 59.9
Gums Per cent 0.00 0.01
0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12 0.13 0.14 0.15 0.16 0.17 0.18
Surface tension Dynes 59.7 59.5 59.4 59.2 59.0 58.8 58.5 58.2 57.9 57.6 57.1 56.4 55.7 55.0 54.0 53.5 53.0 52.0
Gums Per cent 0.19 0.20 0.21
0.22 0.23 0.24 0.25 0.26 0.27 0.28 0.29 0.30 0,31 0,32 0.33 0.34 0.35 0.37
PROCEDURE-DiSSO~Ve 26 grams of granulated sugar, thick juice, sirups, and molasses and dilute to 100 cc. Determine the surface tension at 20’ C . The corresponding amount of gums is then obtained directly from the table. Thin juices are tested directly without dilution. 1 Presented before the Division of Sugar Chemistry at the 69th Meeting of the American Chemical Society, Baltimore, Md., April 6 to 10, 1923. a THISJOURNAL, 16, 813 (1924). 8 Paine, Badollet, and Keane, Ibid., 16, 1252 (1924), especially Table VII.
Seconds
found. The concentration or Brix corresponding to the number of seconds required to empty the pipet can be read without further calculation. For practical purposes it is suggested that this figure be directly reported as viscosity Brix, and all reference to centipoises be omitted. Thus, in the diagram a time of 140 seconds would correspond to 62 viscosity Brix. The polarization divided by the viscosity Brix gives the viscosity purity. As the viscosity purity is largely independent of concentration it furnishes a means of comparing sirups of different periods and from different plants. Important deductions can also be drawn from the relation of viscosity purity to the apparent purity. Normally the two purities should be almost identical: a higher viscosity purity indicates the presence of salts lowering the viscosity and comparative absence of gums; a lower viscosity purity shows that abnormal amounts of gums have accumulated in the sirup. Hence a
I N D U S T R I A L A X D ENGINEERING CHEMISTRY
1136
decided increase in the viscosity purity on filtration is a proof of good performance. It is recommended that all viscosity determinations be made a t 20” C. or calculated to that temperature. A small variation in the temperature causes a large difference in the result; hence, it is necessary to use a thermometer graduated to 0.2” C. As the sugar factory laboratories are ordinarily not equipped to furnish a current of water of exactly 20.0” C., it is suggested that a temperature correction table be prepared for every 2 per cent of concentration by observing the results a t IS’, 19’! 20°, 21°, and 22’ C., and the differences for each 0.2’ C. be obtained by interpolation. Turbidity
Turbidity that can be observed by ordinary light is here referred to as “visible turbidity.” The cloud appearing when a tube of the juice is subjected to the action of a strong narrow ray of light is called “colloidal turbidity.” REAGENTS-standard Turbidity Solution. Rub 0.5 gram of powdered anhydrous bentonite4 with several portions of cool water in a mortar until uniformly mixed. Add 0.2 to 0.4 gram of gum arabic, make up to 500 cc., place in a shaking bottle, and shake vigorously for 10 minutes. Pour into a 500-cc. graduate cylinder and let stand for 24 hours. Adjust a siphon so that the inside leg just reaches the 250-cc. mark and draw off the supernatant liquid without disturbing the lower half. The solution drawn off is standardized on the Jackson turbidimeter. Caramel Solution. Weigh out about 4 grams of granulated sugar in a platinum or fused silica dish. Heat over a small flame until the sugar is completely caramelized, digest repeatedly with small portions of water on water bath until the caramel is completely dissolved, make up t o 200 cc., and mix. If the solution, diluted with ten parts of water, shows turbidity with Horne’s turbidiscope, i t is filtered through a thick asbestos mat until absolutely clear.
DETERMIXATIOK OF VISIBLE TvRsrnITY-The
determination is carried out in the Jackson turbidimeter. A 75-cm. glass tube (preferably inclosed in a brass tube of the same length) is used. The sirup is introduced gradually along the side of the tube by means of a large pipet until the flame of the candle underneath the tube can no longer be distinguished. The reciprocal of the depth of the liquid in centimeters multiplied by 100 gives the turbidity. Thus, a depth of 100 cm. corresponds to a turbidity of 1.00; 50 cm. to 2.00; and a depth of 13.7 om. to a turbidity of 7.30. For the determination of the turbidity of juices too clear t,o be read directly, the following method is proposed: ( a ) Pipet 100 cc. of the standard turbidity solution into a 500-cc. flask, fill up to mark with a 2 N (52.0 grams to 100 cc.j solution of cube sugar (filtered through asbestos if not absolutely clear), mix, and determine turbidity as above. ( b ) Pipet 100 cc. of the standard turbidity solution into a 500-cc. flask, fill up to mark with solution to be tested, mix, and determine turbidity as before. If the turbidity of granulated sugar is t o be determined a concentration of 52.0 grams sugar per 100 cc. is recommended.
To obtain the turbidity of the juice, deduct the turbidity found under (a) from the turbidity of the juice mixture ( b ) , and add one fifth of the difference (as correction for the dilution) to the difference. To illustrate: Suppose the turbidity of the diluted standard (a) t o he 1.905 (52.5 cm. depth), that of the juice mixture ( b ) equal t o 2.075 (48.2 cm.); then the turbidity of the juice is l / 6 X 0.170 or 0.170 0.034 = 0.204. 0.170
+
+
DETERMINATION OF COLLOIDAL TURB~DITY-W.D. Horne recently designed a simple turbidiscope5 consisting principally of a black sheet-iron cylinder, inclosing a powerful electric bulb (200 to 250 watts). At the level of the luminous wire 4 Alexander, THISJOURNAL, 16, 1140 (1924). tained from the author of this article. 6 THISJOURNAL, 16, 626 (1924).
Bentonite can be ob-
Vol. 17, No. 11
in the lamp there is a slit about 5 mm. wide pasbing all around the cylinder. A circular test tube rack is attached to the cylinder. When a test tube with juice is placed in the rack, the powerful electric light makes the colloidal particles in the juice appear like a luminous cloud, like dust in a ray of sunshine. Dilute a part of the standard turbidity solution until it has a turbidity of exactly 5.00 (20 cm. depth in the Jackson turbidimeter). One cubic centimeter of this solution diluted to 50 cc. will then have a turbidity of 0.1. Prepare a set of standards by placing 1, 2, 4, 6, 8, and 10 cc. in 50-cc. Nessler tubes, add sufficient caramel solution to impart approximately the same color as that of the juice under examination, mix after filling up to mark, and compare with the sample to be tested.
Thin and thick juices are tested without dilution; of dark liquors such as green sirup and molasses the normal solution (26 grams to 100 cc.) is taken and the result multiplied by 4. If the uniform slit were replaced by one of varying width, so that the openings in front of the tubes would be 1-14-2-2-%3-344-5-5 mm., it would be possible to compare illuminated fields of varying thickness and thus check the relation to the standard found by dilution. Furthermore, such an arrangement would make it possible to compare a sirup before and after filtration and roughly determine the proportion of colloidal turbidity eliminated. It saves time t o prepare a 200-cc. solution of double normal weight when testing wash sirup, green sirup, or molasses. Half of this volume can then be used for polarization by Horne’s method, 30 cc. for surface tension, and 50 cc. for determination of colloidal turbidity. Application
Kith the aid of the foregoing methods the improvement obtained a t a filter-press station can be definitely expressed as is shown in the following typical report: SURFACE TENSIOS: Dynes Per cent gums VISCOSITY: Brix Purity TURBIDITY: Visible Colloidal
Before filtration
After filtration Improvement
54.4 0.33
62.1 0.11
65.1 57,s
59.1 88.2
5.00 2.4
2.38 1.1
7.7 0.22 0.4
2.62 1.2
By comparing the improvement obtairied with different types of filter presses, cloths, or filter aids, their relative value can rapidly be found. The rate of flow must also be taken into consideration, however, especially in judging the effectiveness of filter aids. In Steffen house work it is necessary to discharge the mo,lassea a t intervals, ba5ed on the aiiiount of raffinose present. Paine and Balch6 have shown that the ordinary method of determination of raffinoee is unreliable on account of other optical substances present in the molasses. This difficulty can he overcome, either by the use of the enzyme method, as recommended by t,hese authors, or by means of surface tension apparatus. It will be fouiid that surface tension, and, to some extent viscosity purity, furnish the most reliable information as to the accumulatioii of gummy substances in t,he molasses, and thus a better indication of the time for change, than any raffinose determination. -illthough further investigations of the tests herein described still remain to be carried out. this paper is now published in the hope that it’may lead to a discussion out of which officially recognized methods, especially for determination of turbidity, will be developed. 111 the meantime the tests will aid in controlling the work in refineries as well as in beet sugar factories, and also help in solving perplexing problems of filtration. 8
THISJ O U R P A L , 17, 240 (19231
