Value of Electrical Methods for Estimating the Moisture Content of Wheat H. E. HARTIG, Department of Electrical Engineering, University of Minnesota, Minneapolis, Minn., AND B. SULLIVAN, Russell-Miller Milling Company, Minneapolis, Minn.
T
HE standard method for the determination of moisture
in wheat is the Brown Duvel distillation procedure (4). This method is the only official moisture test recognized under the United States Grain Standards Act for the grading of grain. However, because it is empirical and is affected by many variables, close checking between laboratories is frequently difficult to secure. I n addition, the test is time-consuming and requires the constant attention of the operator, When it is necessary to make many moisture tests for grading wheat or for mill control work, the Brown Duvel method leaves much to be desired, as regards both ac-
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FIGURE1
curacy and speed, Another method which is preferred by many mill laboratories, although it has no official sanction, consists in grinding the wheat and conducting on the ground wheat sample the official method for the determination of moisture in flour ( 2 ) . The difficulty with this procedure is that in the case of wheats with higher moisture contents (above 14 per cent) there may be a loss of moisture by evaporation due to the handling and grinding previous to weighing. In addition, in the case of wet wheats there is often considerable sticking in the burrs of the grinder and the heat generated during the grinding causes a loss in weight. The oven method itself, if one were sure that no gain or loss in moisture had occurred during grinding, gives very consistent results. A quicker method which would eliminate the grinding of samples and still give accurate and closely agreeing results is, therefore, much to be desired. Electrical methods as a means of estimating the water content of grain were suggested several years ago (5, 13). None of these methods, however, found commercial application. Recently several devices have appeared on the market for measuring the moisture in wheat which depend not upon a direct measurement of the water contained in a given sample but upon the effect of the water on certain electrical properties of the wheat. Among these devices are the Burton-Pitt apparatus, the D-K Schnell Wasser Bestimmung Apparat, the Berry dielectrometer, and the Tag-Heppenstall moisture meter (3, 6). Several articles describing the use of such equipment have also appeared (7, 8, 9, 11). These instruments give readings based on the electrical conductivity, the dielectric capacity, or complex combinations of these two properties which, by means of an empirical chart, are correlated with the moisture content of the wheat. Since the measurements are not direct determinations of the amount of water, it is essential to establish whether a given weight of
water added to a given weight of wheat will invariably produce the same changes in the electrical characteristics of the wheat or whether the electrical properties depend also, in a significant way, upon other factors. Such factors might be the variety, class and grade of wheat (for example, whether hard red spring, durum, or soft red winter), the condition of the wheat (whether frosted, bin burnt, or damaged), the length of time between the addition of water and the measurement, the manner in which the wheat sample is placed in the container, the temperature, and numerous other possible variants. The apparatus and experiments now to be described were devised for the purpose of investigating some of the fundamental factors underlying the estimation of moisture in wheat by electrical methods. DESCRIPTION OF APPARATUS A parallel plate condenser, between which a thin, hard rubber cell could be inserted, was connected in one arm of a General Radio shielded capacity bridge as shown a t W in Figure 1. I n the measuring arm of the bridge was connected a General Radio precision variable air condenser a t C, and an 11,000-ohm decade resistance box, R, built integral with the bridge box. In shunt with the parallel plate condenser, W , a 100-micromicrofarad fixed air condenser was connected for the purpose of increasing the current through the bridge and thereby increasing the sensitivity of adjustment for bridge balance. To indicate balance a Western Electric four-stage vacuum-tube amplifier-voltmeter was used. Current for the bridge was supplied by a Western Electric adjustable-frequency vacuum-tube oscillator. Suitable precautions were taken to eliminate the effect of variable stray capacities on the measurements. I
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FIGURE2 The inside dimensions of the flat cell, W , were 1.25 cm. by 7.5 em. by 9.3 em., so as just to accommodate a 50-gram sample of wheat. The cell was fitted by means of guides between the plates of the condenser so as always to occupy the same position after insertion.
PREPARATION OF SAMPLES The samples used in this investigation were made up from the following classes of wheat: hard red spring (Marquis), hard red winter (Kansas), durum (Amber and Red), and western white (Sonora). Most of the samples originally contained between 8 and 10 per cent moisture. All tests by
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ANALYTICAL EDITION
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the Brown Duvel distillation method were made in duplicate on a gas-heated apparatus in strict accordance with the official directions, except that the flame was extinguished a t 190" instead of 180" C., since it has been shown that by making this change the Brown Duvel results compare more favorably with the results obtained by the vacuum-oven test (1).
The initial moisture having been carefully determined, 250 grams of wheat were weighed into screw-top, moisture-proof metal sample cans of 1 quart capacity. Basing calculations on the amount of moisture originally present in the wheat, the number of cubic centimeters of water necessary t o increase the
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FIGURE 3 moisture to any given amount was calculated. In each series of experiments, samples were made up ranging in moisture from approximately 8 to 20 per cent in steps of 1 or 0.5 per cent differences. Water was added from a Bureau of Standards buret directly to the sample, the top tightly screwed, the sample vigoroufJY b k e n and then allowed t o stand at room ture for 12 t o 48 hours. At the end of this time determinations were made by the Brown Duvel method and at the same time readings were taken on a separate 50-gram sample using the electrical apparatus. In most instances, the moisture c o n t e n t of the tempered samples as determined by the Brown Duvel procedure checked remarkably well with the theoretical moisture percentages as calculated 3 14 from the amount of water a d d e d to a given $ weight of wheat of known moisture content. j3 In earlier experiments the 130" C. air-oven 12 method was likewise employed to determine the moisture content of the wheats. This method \ /, gives consistently higher results than the Brown Duvel test, the difference between the two methods becoming less a t the higher moisture levels. The authors were particularly interested in wheats having above 13 per cent moisture, since such samples showed more variable readings by all electrical measurements. I n later work the 130' C. air-oven method was abandoned because it necessitated grinding, and the grinding of the wet samples before weighing resulted in a variable loss of water. All the moisture determinations cited in this paper were done by means of the Brown Duvel method.
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frequencies and the total equivalent series resistance, R, and capacity, C (Figure l ) , were recorded. On repeating the entire procedure, including the weighing of a new 50gram charge, the readings of capacity and resistance could be duplicated very closely. When wheat to which water has just been added is placed in the dielectric field of the condenser, the effective capacity and resistance vary with time. The results obtained with one sample are shown in Figure 2. The wheat in this case had originally a moisture content of 9.7 per cent and water was added in sufficient quantity to bring the moisture content to 16 per cent. pour setsof readings were taken, the first on the original sample containing 9.7 per cent moisture, one immediately after water had been added, and the two other readings 1 and 6 hours later, respectively. It will be observed that the increase in resistance immediately after adding water was comparatively small but rose on standing to a maximum and on further standing diminished to a final equilibrium value. The capacity, on the other hand, was largest immediately after adding the water and diminished steadily with time to its final equilibrium value. This variation of measured capacity and resistance with time after adding water was also observed in a great number of wheat samples taken in various stages of the tempering process, prior to milling, in connection with the commercial operation of one of the larger mills of the Russell-Miller Milling Company. Obviously no consistent readings of capacity and resistance can be obtained unless a sufficiently long time has elapsed to establish equilibrium conditions or unless the time when the water was added is definitely known. It was also observed that wheat with a moisture content greater than about 14.5 per cent gave different values of R and C immediately after the sample was poured into the container and after it had stood about 5 minutes. On tapping the condenser, the initial value was restored and on further standing would reach the same final value as before. This procedure could be repeated as often as desired. Figure 3 shows the variation of capacity and resistance of hard winter wheat for three different measuring frequenCieS-1000, 2000, and 3000 cycles per second-taken on samples which differed from each other in moisture content by
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PROCEDURE A 50-gram sample of the wheat to be tested was weighed and transferred by means of a special funnel to the hardrubber flat cell, which was then inserted between the plates of the condenser. The bridge was then balanced for various
FIGURE 4
approximately 1 per cent and in the range from 9 to 20 per cent. All the samples were allowed to stand in moistureproof containers for 48 hours after the water was added and before the bridge readings were taken. It will be noticed that the curves for these frequencies are very similar, merely being shifted with respect to each other. Referring to the curve of equivalent series capacity, it should be emphasized that the wavy character of the curve is actual and not due to uncertainties in measuring the values of capacity. When the entire procedure of weighing the wheat, transferring to the flat cell of the condenser, and balancing the bridge was repeated, the reading could be checked to within less than 0.2 micromicrofarad. The curve bends over rather sharply at about 14.5 per
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INDUSTRIAL AND ENGINEERING CHEMISTRY
cent. This corresponds to a sharp increase in the equivalent series resistance of the sample for this moisture interval. From the practical standpoint of attempting to predict the moisture content from the resistance curves, a glance a t these graphs will show that one must surmount the difficulty of having to distinguish for a given series resistancefor example, 3000 ohms-between three different points on the appropriate curve. This, coupled with the fact that the resistance varies so rapidly and erratically with the moisture content, makes the prediction of moisture from these curves very uncertain. On the other hand, the capacity curves are fairly regular and it remains only to establish whether the curves are valid for varieties of wheat other than the kind for which they were determined. The results obtained with three very different types of wheat are shown in Figure 4. The wheats analyzed were: Sonora, a California white wheat of low protein content; Marquis, a hard red spring wheat of high protein content grown in Montana; and an amber durum wheat of high protein content from South Dakota. I n addition to the above-mentioned wheats, other classes such as red durum and hard winter were also tested and found to give similar results. From the curves given it is clear that both the resistance and the capacity values are very different for each of the varieties of wheat tested. Moreover, the curves for amber durum wheat exhibit the same value of capacity for three different percentages of moisture. No single curve, either of capacity or resistance, was found suitable for predicting the moisture content of all classes of wheat.
FIGURE 5 For the sake of completeness, the variation with frequency of the equivalent series resistance and capacity is shown in Figure 5. Amber durum wheat a t three different moisture levels was investigated. No frequency was found which was better than those used in Figures 3 and 4 for correlating moisture content with the electrical properties.
DISCUSSION Several possibilities suggest themselves as an explanation of some of the observed effects, The variation of effective capacity and resistance may be due, a t least in part, to the presence of polar molecules in the wheat. Water by itself is well known to be electrically polar. The variation of capacity and resistance as a function of the time after water is added suggests very strongly that the electric polarity due to the added water is dependent not only on the amount of water but also upon the extent to which the water is “bound” by the components of wheat, thereby causing a change in the response of the polar molecules to the impressed electric field. The concept of water bound by biological materials and the difference in the physical properties of this bound water as compared to water in bulk has been extensively discussed by Gortner and others (IO). I n an organic
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material as complex as wheat it is very unlikely that the adsorbed water films are held uniformly throughout the structure of the wheat kernel. Moreover, it has been well established that a certain amount of the moisture contained in flour is not liberated by heating it a t the boiling point of water even for relatively long periods of time (1%’). Another electrical mechanism which probably plays a part in the variation of the resistance with frequency is that of dielectric absorption and the migration of ions back and forth through the dielectric under the influence of the external field. I n support of the probability of such a mechanism may be cited the very peculiar behavior of the wheat samples containing moisture in excess of about 14 per cent. The resistance and capacity readings of such samples were altered when disturbed by mechanical shock. It is possible that such a change might be due to a disturbance of the coherence of the wheat particles and of the electric paths between them. It should be emphasized that the resistance and capacity measurements obtained in this investigation and their variation with moisture content describe fundamental properties of wheat and are not dependent on the particular apparatus used to determine them. Other methods for obtaining resistance or capacity of whole wheat kernels would give similar results. It is therefore permissible to draw general conclusions as to the value of resistance and capacity measurements in estimating the moisture content of wheat.
CONCLUSIONS It may be concluded from the results obtained on the measurement of the effective series capacity and series resistance of a condenser in which wheat formed the dielectric that: 1. The capacity and resistance readings depend not only on the amount of water present in wheat but also on whether sufficient time has elapsed to attain equilibrium after the addition of water. 2. Different varieties of wheat with the same moisture content may give different capacity and resistance readings. 3. Three samples of the same kind of wheat containing three different amounts of moisture may give identical readings of capacity or resistance. 4. It is unlikely that any of the devices for determining the moisture content of wheat by electrical means can give more than a rough approximation of the actual moisture content in the range and condition of greatest commercial interest. LITERATURE CITED Am. Assoc. Cereal Chem., Methods of Analysis of Cereals and Cereal Products, p. 15 (1928). Assoc. Official Agr. Chem., Official and Tentative Methods, 3rd ed., p. 166 (1930). Berliner, E., and Ruter, R., 2. ges. Mtihlenw., 5, 168 (1929); 6 , l(1929). Boerner, E. G., Handbook of Official Grain Standards, U. S. Dept. Agr., Bur. Agr. Econ. Form 90, pp. 79-83 (1929). Briggs, L. J., U. S. Dept. Agr., Bur. Plant Ind., Circular 20 (1908). Burton, E. F., and Pitt, Arnold, Phil. Mag., (7), 5, 939 (1928); Can. J. Research, 1, 155 (1929); Northwestern Miller, 6, 323 (1929). Coleman, D. A., Cereal Chent., 8, 315 (1931). Geddes, W. F., and Winkler, C. A., Ibid., 8,409 (1931). Gehman, S. D., and Weatherby, B. B., Phil. Mag., (7), 7, 567 (1929). Gortner, R. A,, Annual Review of Biochemistry, Vol. I, pp 21-51 (1932). Montlaur, M. L., Compt. rend. acad. Agr. France, 16, 931 (1930). Snyder, Harry, and Sullivan, B., IXD. EKG.CHEM.,16, 741 (1924); 16, 1163 (1924); 17,311 (1925); 18, 272 (1926). Zeleny, A., Minn. Engr., 17, 163 (1909). R E C ~ I YSeptember ~D 28, 1932.
