VISCOSITY OF STARCHES Determination in a Rotating Cylinder Viscometer H. N. BARHAM, JOHN A. WAGONER, AND G. NATHAN REED Kansas Agricultural Experiment Station, Manhattan, Kans.
A modified rotating cylinder viscometer, suitable for viscosity measurements of starch pastes, has been designed and built. It consists principally of a rotating, doughnut-shapedsolution cup and a “free” cylinder concentric with and suspended into the solution cup. Viscosity is measured by evaluating the restoring torque which must be applied to prevent the suspended cylinder from rotating. The instrument is provided with a micrometer device which measures the effective height of the liquid on the sus-
HE viscosity of starch solutions has been determined by many investigators. Use has been made of various modifications of the capillary-flow, falling-weight, and torsional viscometers. In some instances the work was done a t a constant temperature, while in others the starch suspensions were heated to a desired temperature and then cooled rapidly to a standard temperature before the viscosity was determined. Most workers used water as the suspending medium, but in a few instances other liquids were employed. A fairly complete discussion of this work was given by Blinc and Samec (1). The first thorough investigation of viscosity changes in starch suspensions resulting from changes in temperature was made by Caesar (2, 3 ) . An instrument which he called a. 6‘consistometer”was built; it measured the relative viscosity as a function of the power input required by an electric motor to rotate, a t constant speed, a paddle wheel suspended in the starch suspension. Using this apparatus, the viscosity changes which occurred during the following cycle were determined: 1. Starting with the “starch milk” (a suspension of starch in water), the temperature was gradually increased to slightly below the boiling point of the suspension. 2. The starch pastes were cooked for a definite time at a constant temperature slightly below the boiling point. 3. The starch pastes were cooled to approximately room temperature.
T
Radley (4) describes similar experiments. However, in his instrument the power input to the motor was kept constant, and the change in speed of the paddle wheel was taken as the measure of the viscosity. The curves which Radley obtained were the inverse of those obtained by Caesar. It was believed that such a study involving the viscosity changes with a complete cycle of temperature changes would be valuable for the comparison and evaluation of different starches. However, certain disadvantages in this type of
pended cylinder. This, together with an instrument constant which is independent of the temperature, angular rotation, and properties of the liquid, makes possible the determination of the viscosity in absolute units. The viscometer permits continuous viscosity measurements of one poise or more to be made over a wide range of temperatures, or at a given temperature, at a constant velocity, and for an indefinite period of time. No turbulence was observed within the range of angular velocities employed.
nstrurnent were apparent. The principal objection was the increased mechanical disorganization which the starch might suffer ais a result of the beating action of the paddle wheel. This objection probably occurred t o Caesar also because in correspondence to one of the authors he suggested that a more satisfactory instrument would be one in which the viscosity was measured as a function of torque. With this suggestion in mind, an instrument was designed which should meet the following requirements: (a) It should permit continuous, uninterrupted measurements of viscosity. (h) The measurements should be made in easily intelligible units and, if possible, without the use of arbitrary standards. (c) The temperature of the starch suspensions should be uniform and at the same time be under the control of the operator within the limits of room temperature to 100’ C. ( d ) The starch suspensions should not be subjected to any mechanical action other than the shearing forces which develop between a smooth stationary surface in contact with a liquid in motion. An instrument was built according to the authors’ specifications. Following its calibration with standard oils, the following factors mere examined in order to test the usability of this instrument in the evaluation of starch pastes: effect of angular velocity upon viscosity, effect of concentration upon viscosity, reproducibility of results, behavior of various starches using the same concentration and a definite angular velocity, conversion of viscosity into absolute units, and evaluation of a starch by I measurements. Description of Viscometer Figure 1 is a sectional drawing and description of the principal parts of the instrument. Attention should be directed to the condenser dome 7 , and oil seal, 8. An arrangement of this kind was made necessary not only for eliminating evaporation losses and consequent changes in concentration, but especially for the prevention of caking of the paste a t the 1490
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INDUSTRIAL AND ENGINEERING CHEMISTRY
liquid surface. Solid material adhering to the cylinder surfaces renders measurements unreliable. Control of the rate of temperature change was obtained by the manipulation of three heaters, 3, of different wattage, the largest of which was in series with a variable resistance, and water flow through cooling coil 9. It was found convenient to use the bath temperature as a guide in regulating temperature change. The torque developed actuated a triple-beam balance through a string attached to a small reel, 15, and guided to the lever arm of the balance by ball bearing pulley wheels. The device for measurng the effective height of starch pastes on the cylinder walls consists principally of an electrode which is attached to a micrometer screw, mounted on the top of the tank. The electrode was made of a length of heavy brass wire tipped with platinum and incased in a Pyrex sheath. I n making a measurement, the electrode is adjusted by the micrometer screw so that contact is made with the liquid surface. Contact, which closes a 220-volt alternating-current circuit, is sharply indicated by the glow of a neon bulb. It was hoped that the closing of an a. c. circuit for only momentary intervals would not complicate the changes occurring in the suspensions or pastes through electrolysis or polarization.
Calculated Relations Based on Cell Dimensions The absolute viscosity of a substance determined by a rotating-cylinder viscometer possessing but two contact surfaces may be calculated from the equation:
L=- 4rRR:R: R Z - R : W1q where L
=
(1)
torque acting upon cylindrical surface of suspended cylinder
R,, Rz = radii of suspended and rotating cylinders, respec-
tively
angular rotation 1 = height of wall of suspended cylinder exposed t o action of liquid
and the net volume of the liquid above the bottom edge of the torque cylinder by V = a l [ ( r : - T:) (r: - r ; ) ] = 35.251 cc. (6
+
Calibration The viscometer described was built to measure the viscosity changes which take place in starch pastes throughout a cycle of temperatures. To test its suitability for this purpose, it was necessary to determine how i t would respond, in operation, to changes in temperature, angular rotation, and effective height of the liquid on the torque-measuring cylinder. Constant k of Equation 4 is independent of the properties of the liquid, whose viscosity is to be measured, and depends only upon the cylinder radii. Consequently, it will increase with the temperature to an extent governed by the temperature coefficient of the structural material of which the cell is made. On the other hand, the experimentally determined k may be affected by a number of factors such as the relative displacement of the cell parts with changes in temperature and angular rotation, eccentricity of motion of the rotating cylinder, frictional resistance in the torque-measuring assembly, volume coefficient of the liquid in the cup, and end resistance. For the purpose of evaluating the importance of these effects, oils of standard viscosity, designated as N-3 and P-1, were obtained from the National Bureau of Standards. With the oils were furnished their absolute viscosities and densities a t a number of temperatures. Data characterizing: the oils, including I values calculated from the densities and cell dimensions (measured a t 25' C.), appear in Table I. PROPERTIES AND CALCULATED 1 VALUESOF TABLE I. PHYSICAL STANDARD-VISCOSITY OILS
w =
q =
absolute viscosity coefficient of substance under test
Equation 1 assumes that the shearing force on the ends of the suspended cylinder is negligible. For the viscometer employed in these experiments, two coaxial cylinders were joined together at the bottom, forming a rigid solution cup, A third cylinder, coaxial with the other two, is suspended from the top into the liquid to be tested. Accordingly, the torque developed in this case will be the sum of the torques developed by the liquid rotating against the inside and outside walls of the suspended or torque cylinder and may be calculated from the equation:
Since the torque is related to the weight, W , in grams, as measured by the balance and to the reel radius (2.275 cm.) by L = 2.275 X 980.655W (3) Equation 2 reduces to
E = 0.791 = IC1 8
(4)
where 71 = 1.70 cm. r2 = 2.65 cm.
= 2.85 em. r4 = 3.90 cm. w = 6.2832 radians per second r8
Further, the volume of the solution cup is given by V = R ~ ( T :- r:) = 38.70h cc. where h = height of liquid
(5)
1491
l e , Cm.
Poises T ~ ~ Viscosity, ~ . , 0.c. N-3 P-l
... ...
20 12.88 25 8.785 30 6.169 40 3.213 5b:h 60 75 0 5.674 15:42 80 0:2377 5.35 100 a Interpolated values.
...
:
Density N-3 P-1 0.8805 0.8894O 0.8776" 0.8866' 0.8746" 0.88395 0.8685 0.8785" 0.8565 0.8676 0.8476a 0 , 8 5 9 3 " 0.8446 0.8568 0.8326" 0.8461
N-3, 318 grams 8.71 8.74 8.78 8.85 9.00 9.11 9.14 9.30
P-1, 322.12 grams 8.74 8.77 8.80 8.86 9.00 9.10 9.13 9.26
Before using the oils to calibrate the viscometer, a thorough check was made on a possible vertical displacement of the rotating cup while in motion. No measurable change in position could be found. Likewise, no displacement of the suspended cylinder could be detected under any operating condition employed. However, some eccentricity was noted in the motion of the rotating cup. Resistance in the torque-measuring assembly was evaluated by assuming the instrument constant to be 0.79 and calculating weight W in grams a t such a viscosity and effective height that zero pull is registered by the balance. The resistance-weight equivalent was found to approximate 0.6 gram. It was not regarded worth-while t o attempt to correct data by this amount. Although effort was made in each case to weigh against frictional resistance &s well as the torque, this was not always accomplished as evidenced by the relatively large errors in the small weights obtained a t low viscosities. The 1 values of the oils were measured with the caliper device previously described. Because of the low conductivity of the oil, contact a t the surface was not shown by the glow of an electric bulb but rather by the first sign of an oil "streak" on a glazed surface. At the highest oil level, no significant
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differences could be found between the calculated 1 values and below 100" C. and k , values for P-1 oil gives k an average those measured while the cup was not in motion (rotation was value of 0.79 which is equal to that calculated from the cell interrupted only long enough for a measurement to be made). dimensions. Results lead to the conclusion that, within the limits inThis would indicate that the cell dimensions were measured vestigated, the experimentally determined instrument consatisfactorily. The results show further that the volume stant is independent of temperature, angular velocity, and temperature coefficient of the metal is relatively unimportant when compared with that of the liquid. This was exproperties of the liquid whose viscosity is to be measured. With viscosities between 1 and 10 poises, the error of measpected because the volume temperature coefficient of paraffin oil is roughly thirteen times that of brass. urement may be as large as 4 per cent unless correction is made for the eccentric motion of the cup; where the viscosiValues for k are designated in the tables as b and k,. The former was obtained by dividing weight W in grams by the ties are greater than 10 poises, an error of about 1per cent may product of the standard viscosity and calculated I value; k , be expected. If changes are made in the instrument, in accordance with recommendations given in subsequent paragraphs, was found in the same way except that 1 was measured while and if a correction is made for the resistance in the torquethe cup was in motion. Values for 1 were corrected for end measuring assembly, viscosities as low as 0.1 poise should be resistance in all cases before calculatina k; the correction was taken to be equivalent to 0.1 cm. or haif of the wall thickness of the suspended cylinder. No significant differences were noted between k , and experimental k values based on 1 values measured when the cup was not in rotation. Table I1 demonstrates the effects of change in the angular rotation and temperature on k , and k,. From the data on oil N-3, it may be noted that not only does an increase in the angular velocity appear to cause higher 1 values but also that they are significantly higher than those measured when the cup was not in motion. Since there is no vertical displacement of the cup with increased angular velocity or temperature, it must be inferred that the high I values are only apparent and that they are occasioned by the eccentric motion of the cup. Ripples in the surface of the liquid would give fictitious 1 and k , values, when measured by the method employed. This is borne out by the tendency of k, to remain constant between 0.79 and 0.80, except a t those temperatures a t which the viscosity is low. Oil, P-1, being very viscous, is affected but little by the eccentric motion of the cup. Consequently there is slight deviation between k , and k,. Table I11 compares W/Z ratios obtained a t 60 r. p. m. a t three different levels in the cup over a range of temperatures. Portions of 137, 228, and 318 grams of K-3 oil were used. It was found that, a t any selected temperature, the ratio was practically constant, indicating that the cylinders were quite uniform in cross section. The above experiments were performed 1-1 to determine the reliability of measurements made in this viscometer, especially upon 1 liquids other than those used in its calibration. Of the two oils, P-1 more closely approximates the general behavior of starch FIGURE1. SECTIONAL DRAWING OF VISCOMETER pastes over the temperature range from 60' The oil bath is a half section, the drive mechanism shows a quarter section while the aolution cup and the suspended cylinder are shown with one third sectionlremoved. to 100" C. From numerous observations on starch pastes it may be expected that, 1. Oil bath 12. Solution cup support 2. Bearing housing with oil seal 13. Suspended (torque) cylinder as long as conditions requisite to flow are 8. Individually controlled heat14. Steel shaft of suspension maintained, they will shorn no greater tending coils (100, 300, 500 assembly watts capacity) 15. Reel ency to ripple than oil P-1. Not only is k 4. Asbestos lagging thermo16. Chromel-alumel 5. couple independent of temperature and angular 6. 17. Sprocket wheels (angular rorotation for oil P-1 but, if I measurements 7. tation varied by changing s i z e of spraokets) 8. may be corrected for the effect of eccen9. 18. Spiral gear t o drive shaft 10. 19. Ball bearing mounting f o r tficity of cup motion, it is also constant for 11. drive shaft oil N-3. Averaging k, values for 1 - 3 oil
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1493
veloped. Further stirring is unnecessary as the OF VARIATION OF TEMPERATURE AND ANGULARROTATION starch will remain in uniform suspension. TABLE 11. EFFECT W Woo, Im, IC, During the first or heating stage the temperaT;m$:, R.?.M. Grsma Grams Cm. Cm. Im - IC lo* ko ture is raised about 0.5" per minute until the oil 318 Grams of N-3 Oil bath reaches 98" C. During the second or cook60 91.5 91.5 8.86 8.71 0.15 0.79 0.81 20 ing stage the oil bath is kept constant for 30 90 135.5 90.3 0.80 8.88 8.71 0.17 0.78 91.0 8.89 8.71 120 182.0 0.18 0.79 0.80 minutes a t 98" and that of the starch paste a t 62.7 8.91 8.74 0.17 0.79 60 62.7 0.81 25 93" * 0.5" C. During the last or cooling stage 62.5 8.96 8.74 0.22 0.79 90 93.7 0.80 the temperature is allowed to fall at a rate of 62.5 8.97 8.74 120 125.0 0.23 0.78 0.81 about 1" per minute to a final reading of 43.5 8.96 8.78 60 43.5 0.18 0.78 0.79 30 40 75 100
60 80 100
90 120 60 90 120 60 90 120 60 90 120
60 120 60 120 60 120
64.4 87.5 22.0 33.6 45.6 4.0 6.2 8.3 1.7 2.3 3.2
42.9 43.8 22.0 22.4 22.8 4.0 4.1 4.1 1.7 1.5 1.6
322.12 Grama 400.0 400.0 401.0 802.0 113.5 113.5 228.0 114.0 40.0 40.0 79.5 39.8
8.97 8.99 8.99 9.08 9.08 9.19 9.28 9.29 9.37 9.40 9.41
8.78 8.78 8.85 8.85 8.85 9.11 9.11 9.11 9.30 9.30 9.30
0.19 0.21 0.14 0.23 0.23 0.08 0.17 0.18 0.07 0.10 0.11
0.77 0.78 0.75 0.76 0.77 0.76 0.78 0.78 0.76 0.68 0.71
of P-1 Oil 9.00 8.99 9.00 9.01 9.15 9.13 9.13 9.17 9.25 9.26 9.27 9.26
-0.01 0.01 0.02 0.04 -0.01 0.01
0.79 0.79 0.80 0.80 0.80 0.79
measurable and greater accuracy should result. But as the instrument now stands, it is quite suitable for measuring in absolute units the viscosities developed by starch pastes over a cycle of temperatures. Suggested Refinements While the results so far obtained with this apparatus are fairly satisfactory and have led to the conclusion that the instrument is capable of accurate measurements, certain changes would be desirable. These changes would, it is believed, increase the accuracy and utility of the instrument: 1. The tubin t o be used in the construction of the solution cup and suspencfed cylinder should be machined accurately to avoid the irregularities of drawn tubing. 2. The ball bearings, 19, mounting the drive shaft should be precision, self-aligning bearings to prevent the eccentric motion of the solution cup. 3. The torque-measuring device should operate continuously and with greater accuracy. The use of a torsion balance would be an improvement. 4. The method of measuring the temperature of the liquid under test should be modified. An integrated series of thermocouple junctions in the wall of the suspended cylinder would give accurate average temperatures.
0.78 0.80 0.77 0.78 0.79 0.77 0.79 0.79 0.76 0.69 0.72
30" C.
During the heating stage, simultaneous readings were made a t ,l-minute intervals of the weight and the temperatures of the bath and paste; for the remainder of the experiment, readings were recorded a t 5-minute intervals. I n some cases the height of the paste on the suspended cylinder, 1, was also measured. The data collected are presented in graphic form. The weight readings, V ,are used as the 0.79 0.79 ordinates. The units along the abscissa depend 0.80 upon the stage of the experiment. For the heat0.80 ing and cooling stages, degrees centigrade repre0.80 0.79 sent the temperature of the starch suspensions; for the cooking stage the units represent minutes. I n this manner a composite grauh is constructed which illustrates 'the chinges in viscosity occurring throughout a complete cycle of temperature changes. The curves have been drawn through all points, which accordingly do not appear in the plots. Effect of Angular Velocity on Viscosity Determinations The effect of the variation of angular velocity was examined for the purpose of finding the minimum permissible angular velocity rather than to confirm the generally recognized lack of direct relationship between angular velocity and viscosity. Irish and sweet potato starches were selected as representatives of large- and small-granule starches. Ten per cent suspensions were pasted at 30, 60, and 120 r. p. m. Only with Irish potato starch a t 30 r. p. m. was there any difficulty in obtaining uniform, reproducible viscosity curves. I n this case, however, there were distinct irregularities in the viscosity during the heating period, the effect of which was noted throughout the remainder of the temperature cycle. That the irregularities are due to rapid settling of the relatively large granules is definitely indicated by the formation of clots within the paste. Obviously, the angular velocity
Viscosity of Starch Pastes The required amount of water (sufficient with the starch to make a total of 400 grams) is measured a t 20' C. and heated to within about 4 ' of the lower limit of the gelatinization temperature of the starch used. At the same time the oil bath is heated to the same temperature. The starch sample is carefully weighed, after its starch content is determined by some accepted analytical procedure. It is stirred into the water, using just sufficient agitation to get the starch into suspension, and poured immediately into the solution cup which is in motion. For this purpose a funnel is placed in a hole through the top of the apparatus. After the funnel is removed a glass stirring rod is suspended in such a manner that the bottom of the rod is held about 1 nun.from the bottom of the solution cup. By means of this stirring rod the starch is kept from settling to the bottom. The rod is removed for about 15 seconds while each reading is taken, and the stirring is continued until a pull of about 50 grams is de-
TABLE111. EFFECTOF VARIATIONOF OIL LEVELIN CUP (60 R.P. M.) Temp O C." 20 25 30 40 75 100
N-30il Grama' 137 228 318 137 228 318 137 228 318 137 228 318 137 228 318 137 228 818
W
Grak 30.8 61.8 91.5 21.5 42.0 62.7 14.6 29.6 43.5 8.1 16.7 22.0 1.2 2.6 4.0 0 1.2 1.7
Zm.
Cm. 2.98 5.90 8.86 3.00 5.92 8.91 3.06 5.94 8.96 3.09 6.02 8.99 3.20 6.21 9.19 3.41 6.33 9.37
le,
Cm. 2.88 5.81 8.71 2.89 5.83 8.74 2.91 5.87 8.78 2.94 6.91 8.85 3.05 6.09 9.11 3.13 6.23 9.30
-
20 0.10 0.09 0.15 0.11 0.09 0.17 0.15 0.07 0.18 0.15 0.11 0.14 0.15 0.12 0.08 0.28 0.10 0.07
Em
w
I. 10.3 10.5 10.3 7.2 7.1 7.0 4.8 5.0 4.9 2.6 2.6 2.4 0.4
8:::
... 0.2
0.2
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 34, No. 12
of 30 r. p. m. is too low for large granule starches even with supplemental stirring. On this basis 60 r. p. m. is regarded as approaching the lower limit of permissible angular velocity for starches in general. 'Effect of Concentration on Viscosity Determination Using an angular velocity of 60 r. p. m., the viscosity was determined for 5 , 7.5, 10, 11, 12.5, and 15 per cent Irish potato starch suspensions and for 5, 10, 15, and 20 per cent sweet potato starch suspensions. The effects noted are qualitatively the same for any starch and may be illustrated by the Irish potato starch data of Figure 2. Here again the granule size plays an important part. With excessive concentrations, irregularities in viscosity develop during the heating period due to nonuniformity of the pastes. Clot formation during pasting influences the viscosity throughout the remainder of the cycle and is most evident a t the end of the cooling curve, The upper concentration limit is 10 per cent for Irish potato starch and 15 to 20 per cent for sweet potato starch. The concentration of 10 per cent was selected for further experiments both because it is suitable for both large and small granule starches and because greater detail results, the higher the concentration. Decreases in the temperatures a t which the viscosity first becomes measurable and at which the initial maximum appears as the concentration is increased confirms a similar finding by Caesar (8). The continuation of the downward slope of these curves, as with others, into the cooling stage is assumed to be caused by a temperature lag between the surface and body of the paste. Many duplicate determinations were made on 10 per cent Irish and sweet potato starches. I n most cases the resulting curves were practically superimposable; where deviation occurs, the greatest variation a t any part of the cycle falls within 0.4 per cent. Viscosity Behavior of Various Starches Selecting the conditions of 60 r. p. m. and 10 per cent concentration as the most desirable for starches in general, viscosity determinations were made on Irish potato, sweet potato, tapioca, corn, and Blackhull kafir starches. The sweet potato and one Irish potato starch were prepared in this laboratory; the Blackhull kafir starch was supplied by the Department of Chemical Engineering of Kansas State College; a second Irish potato and the corn and tapioca starches were commercial products supplied by Stein, Hall and Company, Inc. The curves of Figure 3 emphasize individual differences in the pasting properties of miscellaneous starches. Comparisons may be made with respect to the temperature range of the first rise in viscosity (this range may or may not correspond to the gelatinization temperature as measured by the disappearance of anisotropy), magnitude and position of the initial viscosity maximum, thinning during cooking, and viscosity developed during cooling. The final maxima of two of the starches do not appear in Figure 3; cornstarch attains a value of 1550 grams a t 35' C., Irish potato starch, 2000 grams a t 35" C. Conversion of Viscosity into Absolute Units By using the micrometer attachment described earlier, the heights of the Irish potato starch suspensions upon the suspended cylinder, 2, mere measured a t frequent intervals throughout the temperature cycle. This quantity, together with the weights and the instrument constant (0.79), is suf-
FIGURE 2 (Above). EFFECTOF CONCENTRATION O N VISCOSITY OF IRISH POTATO STARCH PASTES FIGURE3 (Below). CHARACTERISTIC VISCOSITYCURVESFOR VARIOUSSTARCHDS
ficient for the calculation of the absolute viscosities from Equation 4. Figure 4 compares curve I, in which the viscosity is represented by the weight in grams, with curve 11,which expresses the viscosity in absolute units. Units along the ordinate were so placed that the nearness to superimposition might be judged. Deviations of one curve from the other are relatively small. For many purposes, therefore, a direct proportionality between torque and absolute viscosity may be assumed. There will be other starch investigations which will require that viscosities be expressed accurately in absolute units. The authors believe that the viscometer here described, especially after some modification, furnishes a means for such measurements. Differentiation between Starches by I Measurements The starch curves of Figure 5 were prepared by plotting measured values of I against the temperature (and against
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INDUSTRIAL AND ENGINEERING CHEMISTRY
1495
those prepared from data obtained by using the electrical device. The point of significance here is that there are usually abrupt volume changes during the pasting of a starch which could yield information of value in differentiating between starches. Summary A modified rotating-cylinder viscometer suitable for the viscosity measurements of starch pastes has been described. It is believed that this instrument has the following advantages : 1. Continuous, uninterrupted measurements of viscosity may be made over a wide range of temperatures or at a given temperature for an indefinite period of time. 2. It provides for the measurement of viscosity in absolute units. This removes the need of setting up arbitrary standards and places viscosity studies of starches on a sounder theoretical basis. . 3 . The viscosity ran e is broad. As long as the liquid does not channel, viscosities o? one poise and more are easily measured. 4. No evidence of turbulent flow has been observed. OF VISCOSITYINTO POISES FIGURE 4. GONVERSION
time during the cooking period). For comparison, the volumes of the Irish potato suspension and the standard oil were taken as identical a t 30’ C. and from that point 1 values of the oil were calculated from specific volumes, It is apparent from the plots that changes in density in Irish and sweet potato starch pastes respond to the same causative factors that effect viscosity changes. The two particular starch samples shown here follow each other as closely as any that have been tested and, therefore, give an idea of the minimum deviations to be expected between starches from different sources. Among the other half dozenstarches tested in this way, some show pronounced differences. There is no intention to imply that the method employed for making I measurements is accurate in the sense that the impressed alternating current is without influence upon the behavior of the suspension or paste. Actually the current acts through electrolysis or polarization to accentuate the volume changes and to cause a slight shift in the viscosity curves. Although I measurements which are made manually on starch pastes are less precise for a number of reasons, the curves prepared from them have the same general form as
Experiments with the instrument indicate that 60 r. p. m. is the lowest permissible angular rotation and 10 per cent is the most satisfactory concentration for starches in general. Viscosity measurements may be reproduced with considerable accuracy. Viscosity curves throughout a complete cycle of temperature changes are given. These curves show characteristic differences between starches and should be of value in their characterization. The plotting of experimentally determined I values, which reflect changes in density, against the temperature differentiates sharply between starches and should contribute also to their characterization. Literature Cited (1) Blinc, M., and Samec, M. M., Congr. intern. tech. chim. ind. agr., Compt. rend. Vd Congr., 2, 214-49 (1937).
(2) Caesar, G.V., IND.ENG.CHEM.,24, 1432-5 (1932). ( 8 ) Caesar, G.V., and Moore, E. E., Ibid., 27, 1447-51 (1935). (4) Radley, J. A., “Starch and Its Derivatives”, New York, D. Van Nostrand Co., 1940. PREBINTED in part before the Division of Agricultural and Food Chemistry at the 102nd Meeting of the AMERICAN CHEMICAL SOCIETY, Atlantio City, N. J. Contribution 267, Department of Chemistry, Kansas State College.
Condensation of Vapors from Noncondensing Gases-Correction It has come to the author’s attention that a consistent error exists in the calculation of the required surface area in the above article, which appeared on pages 1248-52 of the October issue. The mass velocity, 0,for point 2 (page 1250, column 1) was calculated as follows: G = 20,000
- (24.2 X 88 X 0.4333) = 19,079
It should read: G
DEPTHOF FIGURE5. VARIATIONSIN EFFECTIVE WITH TEMPERATURE
5
20,000
- [(24.2 X 88)/0.4335]
15,080
This error introduces errors in the calculated values of Re, h,, and K,and considerably increases the surface area required. It does not, however, altersthe relative results of the two solutions and in no way invalidates the proposed modification in the method of design. On page 1249, column 1, under NUMBER OF TUBES the value SOLUTIONS of 8760 for the rate .of flow should read 8670. JULIAN C. SXITH
