A Study of the Moisture in Soybeans J
d
A. C. BECKEL AND F. R. EARLE U. S. Regional Soybean Industrial Products Laboratory, Urbana, 111.
T
Treatment of Samples The samples were conditioned by keeping them for more than 3 weeks in a constant temperature-constant humidity room maintained at 25" C. and at a relative humidity of 50 per cent. Except for the whole-bean study, the samples were ground in a Wiley mill with a 1-mm. screen before being conditioned in the humidity room. The sample for each determination was taken from the humidity room in a weighing bottle to prevent an undeterminable loss during the initial weighing of the sample.
HE study of moisture in biological materials by oven methods has been attended by several difficulties, among which were the probability of error due to loss or gain in weight during the process of cooling the sample, the considerable work involved in obtaining a series of determinations, and the unknown effect, on a series of determinations, caused by cooling the oven during the removal of samples. Through the development in the U. S. Regional Soybean Industrial Products Laboratory of the apparatus for the continual observation of changes in weight a t oven temperatures ( I ) , these difficulties and others were eliminated so that a systematic study of the moisture relations of soybeans could be undertaken. It seemed desirable for such a study to compare several types of oven determinations, several varieties of soybeans, a number of samples of the same variety having a range of protein content, a number of samples having a range of oil content, and a number of samples having different moisture contents. I n the portion of the study reported here, four oven methods-namely, the air oven a t 105" and 130" C., and the vacuum oven a t 80" and 105" C. under less than 5-mm. pressure-have been investigated with the apparatus ( I ) ,using
Experimental Results The results obtained for each variety of bean are shown in Figures 1 to 5, with the percentage loss in weight in terms of the entire sample plotted against the time. On this basis, the results of the various methods of analysis on a single variety of bean may be compared. For a comparison of the results obtained by a single method on several varieties of beans, it is necessary to consider the oil content, since the oil is the only major constituent of the bean which is essentially water-free. The presence of this water-free oil will obscure the actual moisture content of the moisture-containing components and cause a displacement of the entire loss in weight curve. Figures 6, 7, and 8 and Table I were prepared from results calculated to the oil-free basis. Discussion
60
I
120
I80
I
300
240 TIME
360
I
420
Before exploring the inferences of the results, it should be recalled that the several methods of analysis subject the samples to somewhat different conditions. The surrounding atmosphere in the vacuum oven methods has a very low humidity, while in the air oven and circulating air oven the surrounding atmosphere has an appreciable moisture content due to external atmospheric conditions. The same relative situation exists in the two types of ovens with regard to the oxygen content of the atmospheres. It might be expected that the difference in humidity in the atmosphere in the ovens would be enough to cause significant differences in the equilibrium moisture content arrived a t by
I
480
IN MINUTES
five varieties of soybeans. The ovens and temperatures are those commonly used, and the varieties of beans were selected because they differ greatly. The hay-type Peking variety is a small black bean which is recognized as having a very tough seed coat, the vegetable-type Giant Green variety is a very large bean; and the other three varieties, Illini, Manchu, and Dunfield, represent the usual grain type of bean. I n addition to the comparison of the oven methods as applied to these varieties, a comparison was made of the effect of the seed coats by studying the loss in moisture of the whole undamaged seed and that of the whole unground seed with the hull removed.
FIGURE 2
ILLIN
e
*HOLE "NBROUND B U N S 105.C ri,P OVEN n OEWLLEO "hG90iW B E I N S l 0 5 . C Ails W E N x CROYND WHOLE BEAN 81.C VbCUUM O W N 0 GROYNO WItOLL @LAN 105.C A I R 0°C"
.
bmum
*MOLL BEAN
0 GROUND *"OLF
120
I80
240
300
360
TIME IN MINUTES
40
BE&*
420
lO5.C l10.C
"PCUUU O V f N AIR W E N
480
January 15, 1941
ANALYTICAL EDITION
41
TABLE I. PERCENTAGE OF MOISTURE CALCULATED TO OIL-FREEBASW -2
105' C. vacuum
4 Hours-
Hours-
130" C.
A 0.01 0.00 0.00
105' C vacuum
Illini 9.58 9.59 Dunfield 9.80 9.80 Manchu 9.71 9.71 Giant Green 9.65 9.72 0.07 Peking 9.72 9.70 0.02 Variation 0.22 0.21 a A 3-gram sample was used which permitted
..
130'C.
A
9.75 9.85 9.78 9.84 9.78 0.10
0.05 0.06 0.08 0.09 0.07
9.70 9.91 9.86 9.75 9.85 0.21
C -
105' C. vacuum
130OC.
A
105' C. vacuum
9.86 9.88 9.87 9.88 9.87 0.02
0.09 0.03 0.03 0.06 0.02
9.87 10.00 9.98 9.98 9.99 0.13
9.77 9.91 9.90 9.82 9.85 0.14
..
..
/ -
loll
i.0 TIME
300 360 8Y MINUTES
420
I
480
I
/ -
/
I
80
70
c
24 HOURI
I
e60
5
50
yi
0
FIGURE 4
I
MAkCHU
IY u
530
20
IO
60
I 120
I 180
1 240
10.81 10.89 11.17 11.12 10.87 0.36
0.10 0.09 0.08 0.16 0.14
..
Change from 6 hours 0.95 1.01 1.30 1.24 1.00
..
in weight by this method are less than those obtained by any of the other methods. They are, in fact, less for 24 hours than the losses observed for 1 hour in the vacuum105" C. method, for 1 hour a t 130" C. in the air oven, or in the vacuum-80" C. method for 3 hours. The only other explanation possible is that the high vacuum as such or the temperature of 130" C. caused the volatilization of some slightly volatile material which would lead to high results. This does not agree with the statement of Cox (2) that "considerable work" has shown "that 3 hours at 130" C. in a Freas forced draft oven will produce results agreeing exactly with the toluol method". The possibility of an error in the loss-inweight determination, due to a decomposition caused by elevated temperatures or by a combination of atmospheric oxidation and a high-temperature decomposition, must also be considered since it enters into the loss-in-weight value as determined a t 130" C. in the air oven. That it does not affect the determination before the 6-hour point seems to be convincingly demonstrated by the essentially identical results obtained by this method and by the vacuum-105" C. method. This identity can be seen in Figures 1 to 5, curves V and VI, and in Table I which gives the values obtained for 2, 4, 6, and 24 hours as well as the differences between the values for the two methods. The agreement of results for 2 , 4,and 6 hours is remarkable in that not only was the method different, but the ovens were of different types (the air oven was of the de Khotinsky type,
I
I
11 '-
bu
24 Hours Change f r o m 6 hours 130' C.
increments of 0.07 per cent t o be determined.
I
CO
-
6 Hours-
I I 300 360 TIME I N MINUTES
the several methods, since it has been observed in the Soybean Laboratory on several occasions that samples of very low moisture content were able to remove water from a desiccant such as calcium chloride, when placed over such desiccants in an evacuated desiccator for a period of about 12 hours. Furthermore, it has been observed (3) that in the case of cellulosic materials, the dehydrating power in the dry condition is about equal to that of phosphorus pentoxide. The effect of these humidity conditions and sample behavior would be expected to be most apparent in the case of the moisture determinations in the air oven a t 105" C., and such seems to be the case. The losses
I 420
I
I 480
P ,
I
I
. _,
24
24 HOURS
60
,
120
180
240
,
m TIME IN MINUTES
360
4M
480
I N D U S T R I A L A N D E N G I N E E R I NiG C H E M I S T R Y
42
sume that all carefully conditioned samples would fall within the narrow range of moisture values found in this study. An investigation of a series of samples having a range of nitrogen content is in progress a t the Soybean Laboratory. In ascertaining a practical length of time for the moisture determination, it is essential that the analyst be given some latitude as to the moment for removing the samples from the oven. With this point in view, the rate of loss was calculated and plotted against the time a t which such rates were prevailing (Figure 9). If a consistency of results of less than 0.10 per cent is desired, it is clear that the rate of loss must be less than 0.01 per cent per minute if the interval for removal is limited to 10 minutes or 0.005 per cent per minute if the interval is extended to 20 minutes. For the 105" C.-vacuum method the above conditions are found when the determination has proceeded beyond the 90- and 100-minute periods, respectively, and for the 130" C. method, beyond the 75- and 90-minute periods. For values of the moisture content which are t o be used for the correction of other analytical data to the moisture-free basis or otherwise, it is apparent from Figures 6 and 8 that consistent results will be obtained by selecting for the complete analysis any length of time from 1.5 to 8 hours for the 130" C. method or from 1.5 to 24 hours for the 105" C.-vacuum method. Furthermore, the variation in the results during these periods would be insufficient t o produce a
TABLE 11. NITROGEN, OIL, AND MOISTURE CONTENTS Xtrogen Whole Oil-free basis basis
70
Moistureb (Ground Whole Bean) 130' C., 20 Minutes
Oil
%
%
%
Peking" 6.14 7.30 15.92 7.43 20.79 Giant Green 5.89 Illini 5.83 7.26 19.66 Manchu 6.08 7.56 19.61 Dunfield 5.80 20.52 7.30 a Moisture determined by usual laboratory method: 8.22; 3 hours, 105" C., 7.80. b Determined by usual laboratory method.
7.80 7.35 7.33 7.40 7.50 3 hours, 130' C.,
and the vacuum oven was of the oil-jacketed type), and the measuring devices were separate units. In the case of the Manchu bean the 130" C. determination was allowed to continue until for the 72-hour period the loss of 12.52 per cent indicated that the loss due to decomposition was continuing. The very small loss occurring during the final 18-hour period of the vacuum-105" C. method (Table I) indicates the reliability of the 24-hour value. The Dunfield sample was allowed to continue for an additional 24-hour period, during which there was a loss of only 0.07 per cent. All the values for moisture reported in Table I1 were obtained by the usual laboratory dish method and are the average of two determinations. The 130' C. values were obtained in a Freas circulating air oven and the 105" C. value in a de Khotinsky air oven. The significant point about these values is that they fall exactly on the lossin-weight curves obtained with the loss-inweight apparatus in the other ovens. This 7O C 1 indicates that the results obtained are identical a t 130" C. in an air oven, 130' C. in a forced draft oven, and 105' C. under a vacuum in an oil-jacketed vacuum oven. It is impossible to correlate the results obtained s a t 105" C. in an air oven with those obtained g402 by the other methods.
