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T H E JOURNAL OF I N D U S T R I A L A N D ENGINEERING CHEMISTRY
Vol. 14, No. 8
Moisture Absorptive Power of Different Sugars and Carbohydrates under Varying Conditions of Atmospheric Humidity' By C. A. Browne 80 SOUTE ST., N E W YORK,N . Y .
In tests in which dried carbohydrates were exposed to atmospheric ,humidity (60 per cent). starch showed the greatest moisture absorptive power during the $rst hour. Next in order came cellulose and agar. Levulose, which is considered by many the most hygroscopic of the carbohydrates, was eleventh on the list. The lowest absorption was shown by dextrose, mannitol, and sucrose. I n a saturated atmosphere, however, the leoulose-containing materials showed the highest, mannitol the lowest, absorptive power. When the samples were allowed to come to equilibrium under ordinary atmospheric conditions after exposure fo 100 per cent humidity, a11 lost moisture except lactose and rafinose, which showed a slight gain for many months. Sugars which absorbed moisture in excess of the monohydrate form lost moisture to the point of monohydration. Sucrose continued to lose moisture during a period of two years. While the absorptioe power of the substances studied was highest in periods of high humidity and lowest in periods of lowest humidity, nofixed relationship could be established between percentage humidity and moisture content, owing to the rapidity of atmospheric fluctuations and the fag in the absorptioe power of each material.
The sugar which is first in absorptive power during the firgt , hour of exposure is anhydrous maltose, with a gain in weight of 0.80 per cent; then follow anhydrous raffinose with a gain of 0.74 per cent, and anhydrous lactose with a gain of 0.54 per cent. The high absorptive power of these sugars shows the necessity of preserving them in hermetically sealed containers. The substances ranking next in absorptive power are the anhydrous solids of molasses with a gain of 0.46 per cent, honey with 0.44 per cent, glucose sirup with 0.29 per cent, and maltose sirup with 0.28 per cent. The eleventh material upon the list, with 0.28 per cent absorption, is the sugar levulose, which according to general opinion is the most hygroscopic of all carbohydrates. Pure, dry levulose is not excessively hygroscopic a t first. It absorbs moisture slowly but what it does absorb is retained with great tenacity. As the percentage of absorbed moisture increases and the levulose begins to liquefy, the rate of absorption increases rapidly. The substances of lowest absorption during the first hour of exposure were dextrose with 0.07 per cent gain in weight, HE gain and loss in weight of different food materials mannitol with 0.06 per cent gain, and sucrose with 0.04 per under varying conditions of atmospheric humidity cent gain. The results are all given in the first column of was the subject of a paper which the author read before Table I. the Division of Agricultural and Food Chemistry at the The exposure of the substances during the first hour was a t CHEMICAL SOCIETYa temperature of about 20" and a relative humidity of about Philadelphia Meeting of the AMERICAN in 1919. Since then the experiments have been continued 60 per cent. With the varied class of products used in these with R number of different sugars and carbohydrates, singly comparisons some reached moisture saturation before others and in mixtures, and a few general results of the work are SO that the relative order, at the point where there is no reported in the present paper. longer any increase in weight, was different than a t the end The materials used in the experiments were the sugars of the first hour. At the end of 9 days a general condition of dextrose, levulose, rhamnose, sucrose, maltose, lactose, and equilibrium was reached where the percentages of absorbed raffinose; the sugar alcohol mannitol; tho polysaccharides moisture were found to be as given in the second column of starch and cellulose; the hemicellulose agar;. and five com- Table I. The average temperature during this 9-day period mercial food products containing a mixture of sugars, viz., a was 19.6' and the average relative humidity 61 per cent. refiner's molasses, a low-grade honey, a maltose sirup, a TABLEI-PER CENT MOISTURE ABSORBED FROM AIR AT 20° C. glucose sirup, and a commercial invert sugar. 60 Per cent 60 Per cent 100 Per cent The substances were first dried in a vacuum oven to conANHYDROUS Humidity Humidity Humidity MATERIAL stant weight in the presence of phosphorus pentoxide in order 1 Hour 9 Daw 25 D a w Starch 1.04 12.98 24.3i to remove all traces of moisture. About 1 g. of the powdered Cellulose 0.89 5.37 12.57 Agar anhydrous material was then spread over the bot8tom of a 0.88 20.34 42.98 Si-altose 0.80 6.97 18.35 weighing bottle, provided with a ground-glass stopper, and Raffinose 0.74 12.90 15.91 Lactose 0.54 1.23 1.38 exposed to the air. The absorptive power for moisture was Molasses 0.46 68.921 9.66 Honey measured by weighing the bottles a t regular intervals and 0.44 74.101 10.00 Commercial glucose 47.141 0.29 9.00 noting the increase in weight (Table I). Malt sirup 50.96' 0.28 8.84 Levulose 7.7.391 0.28 0.63 The anhydrous material which showed the greatest absorp- . Commercial invert sugar 76.581 0.19 5.06 Rhamnose tive power for moisture during the first hour of exposure was 0.18 5.00 13.12 Pure invert sugar 0.16 73.961 3.00 the polysaccharide starch, with 1.04 per cent absorption. Dextrose 0.07 14.50 0.07 Mannitol 0.42 0.05 0.06 This result, while perhaps surprising to many, will not be unSucrose 18.351 0.04 0.03 expected by those who have worked with this carbohydrate. 1 Moisture absorption still progressing at end of 25 days. Moisture is absorbed from the air by anhydrous starch with so much avidity that heat is evolved. The substances ranking It will be noted that after 9 days' exposure the hemicellulose next to starch in absorptive power during the first hour of material agar showed the highest moisture absorption with exposure were cellulose with 0.89 per cent and agar with 20.34 per cent, starch being second with 12.98 per cent, and 0.88 per cent absorption. raffinose third with 12.90 per cent. Anhydrous raffinose exceeds any other sugar in its absorptive power; the amount 1 Presented before the Division of Sugar Chemistry at the 63rd Meetabsorbed a t the end of 9 days is still 5 per cent below the ing of the American Chemical Society, Birmingham, Ala., April 3 to 7. 1922.
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T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
theoretical figure for raffinose pentahydrate. The levulosecontaining materials, honey and molasses, had absorbed 10 per cent and 9.66 per cent of moisture, respectively, commercial invert sugar 5.05 per cent, chemically pure invert sugar (made by mixing pure dextrose and levulose in equal proportions) 3 per cent, while pure levulose itself absorbed only 0.63 per cent. Anhydrous dextrose, mannitol, and sucrose are unchanged a t the end of the list, having absorbed no more moisture than a t the end of the first hour,
SATURATED ATMOSPHERE The experiments thus far conducted were made at the comparatively low relative humidity of 60 per cent, and a new experiment was accordingly made to determine how much moisture the materials would absorb from a water-saturated atmosphere. This was accomplished by placing the products in a closed chamber over water for 25 days; although moisture absorption was still progressing a t the end of this time with many of the substances, the experiment was not continued longer owing to the danger of moId formation. The results of absorption from a saturated atmosphere, shown in the third column of Table I, reveal some very surprising results. The highest absorptive power is no longer held by the insoluble polysaccharides, agar and starch (these materials having reached their saturation point a t about the twentieth day), but by the levulose-containing substances. Commercial invert sugar ranks first with 76.58 per cent absorption, honey second with 74.10 per cent, C. P. invert sugar third with 73.96 per cent, pure levulose fourth with 73.39 per cent, and molasses fifth with 68.92 per cent. The presence of levulose has much to do with moisture absorption, but the percentage of water absorbed bears no relation to the percentage of levulose present. Pure levulose, for example, absorbed 73.39 per cent moisture and pure dextrose 14.50 per cent. A mixture of these two sugars in equal amounts might then be expected to have an absorption equal to the average value of 43.95 per cent, but according to experiment this mixture had an actual absorption of 73.96 per cent, which slightly exceeds that of pure levulose. Certain organic and mineral substances may possibly accelerate the moisture-attracting property of levulose, but the mechanism of the process and the formulation of a mixture of highest absorptive power per unit of levulose remain to be worked out. As to the absorptive power of other sugars a t 100 per cent humidity, raffinose absorbed a t the end of 25 days 15.91 per cent moisture, which is still about 2 per cent below that necessary to form the pentahydrate; dextrose absorbed 14.50 per cent, which is 4.5 per cent more than necessary to form the monohydrate; rhamnose absorbed 13.12 per cent, which is over 2 per cent more than necessary to form the monohydrate; lactose, the sugar of lowest absorptive power, attracted only 1.38 per cent moisture, which is still less than that necessary to form the monohydrate. The substance of lowest moisture attracting power at 100 per cent humidity was mannitol which, notwithstanding %itsconsiderable solubility, gained only 0.42 per cent in weight.
ABSORPTION AFTER EXPOSURE TO 100 PERCENT HUMIDITY The substances after a 25-day exposure in a moisture-saturated atmosphere were allowed to stand again under ordinary atmospheric conditions until equilibrium was once more reached. The results of approaching this equilibrium from the anhydrous state and from a more or less moisture-saturated state are not, however, identical. All of the products lost moisture upon taking from the moisture-saturated atmosphere, except lactose and raffinose, which continued to
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show a slight gain for many months. This was to be expected, as these two sugars had still not absorbed the necessary amount of moisture to complete the formation of the stable lactose monohydrate and raffinose pentahydrate. The slowness of final complete absorption in case of these sugars is probably due to occlusion of unchanged anhydride within aggregates of the hydrate. Sugars which had absorbed moisture in excess of the stable monohydrate forms, such as dextrose and rhamnose, lost moisture to this point of monohydration, and there remained practically constant, dextrose with about 10 per cent absorption and rhamnose with about 11 per cent. Maltose also lost moisture after removing from a saturated atmosphere and went back to approximately the monohydrate forms. Levulose decreased from 73.39 to. 11.19 per cent, which is slightly above that required for the monohydrate combination. TABLE 11-RANGE
IN MOISTURE ABSORPTION DURING THE YEAR UNDER ORDINARY ATMOSPHERIC CONDITIONS FOR MATERIALS PREVIOUSLY EXPOSED TO 100 PER CENT HUMIDITY Lowest Highest Range MATERIAL Per cent Per cent Per cent Agar 21.00 30.74 9.74 15.77 16.24 Rzffinose 0.47 Starch 12.29 18,41 6.12 Commercial invert sugar 34.73 22.92 11.81 Molasses 33.00 11.51 21.49 Commercial glucose 23.30 11.46 11.84 Malt sirup 11.28 24.36 13.08 Levulose 36.31 11.19 25.12 Rhamnose 12.29 10.85 1.44 34.92 Honey 10.52 24.40 Dextrose 10.57 0.69 9.88 Maltose 5.40 9.37 .3.91 Cellulose 10.89 5.06 5.83 Lactose 0.36 1.69 1.33 Mannitol 0.52 0.22 0.30
The range in moisture content of the different materials during the year, after previous exposure for 25 days to 100 per cent humidity, is indicated in Table 11. The lowest average interior relative humidity for the climate of New York is about 50 per cent for an average temperature of about 18.5"C. and occurs late in January; the highest average interior relative humidity for this climate is about 70 per cent for an average temperature of about 80" C. and occurs late in July, The general variation in moisture content of the materials studied was a decrease to a minimum in late January and an inorease to a maximum in late July. Of the substances examined, lactose and mannitol showed the lowest absorptive power and the lowest range between minimum and maximum. The substance of highest absorptive power a t low humidity was found to be agar with 21.00 per cent water, and the substance of highest absorptive power a t high humidity was levulose with 36.31 per cent moisture. I n a previous experiment conducted in the summer of 1919, the percentage of moisture absorbed by levulose from the air during a protracted period of rain reached 47.3 per cent (Table V). The average moisture absorptive power of levulose for different months of the year is shown in Table 111. TABLE 111-ABSORPTIONOF WATERBY LEVULOSE FOR DIFFERENT MONTE~S
MONTH January February March April May June
J AUL :u :st September October November December
OII THE YEAR ROOM TEMPERATURE RELATIVE ABSORPTION O c . HUMIDITY PERCENT 19.5 53.9 13.7 19.7 47.5 11.9 20.4 56.7 12.8 20.6 58.9 13.2 61.8 21.9 18.3 21.2 25.3 71.5 25.7 28.4 70.0 24.2 24.2 72.1 23.2 22.2 71.4 19.9 69.0 22.6 20.0 19.2 60.1 56.0 13.5 19.2
The experiments of Table 11, as previously mentioned, were conducted upon products which had previously absorbed a high percentage of moisture from a saturated atmosphere.
