Jan., 19511 MONEY AND CHRISTIAN 19 The '' Air Current " Method of Moisture Determination With Particular Reference to Moisture in White Sugars BY R. W. MONEY AND W. A. CHRISTIAN (Presented at the meeting of the Society o n Wednesday, February lst, 1950) For the routine determination of moisture in fine-grain white sugar, vacuum drying at 100" C was found to be inaccurate, the A.O.A.C. method, although valuable as the reference method, was too slow, and the Karl Fischcr method was unsatisfactory owing to the need to maintain a supply of reagent, which deteriorates rapidly, for only three or four determinations per week. Experiment showed that a method in which a preheated current of air was drawn through the sample in a specially designed apparatus gave satis- factory results. The apparatus and the method of operation are described and some typical results are tabulated to show their reproducibility.A comparison of results by the air-flow method with those by the -4.O.A.C. and Karl Fischer methods showed satisfactory agreement. THIS investigation arose from the necessity to determine moisture contents of about 0.05 to 0.3 per cent. in fine-grain white sugar (crystal caster) for use in a manufacturing process requiring freely flowing sugar particles. It was essential that the determination should be made rapidly, i.e., in less than an hour, and should be accurate to within 5 per cent. The rapid method of drying a 1 to 2-g sample in a vacuum oven at 100°C was insufficiently accurate owing to the great significance of errors introduced by slight charring and by the normal experimental errors when such a small moisture content was being determined, while the A.O.A.C.method of drying to constant weight in vacNo at 70" C was too slow for routine purposes, although valuable as the reference method. Consideration was given to the Karl Fischer method and good results were obtained, but the method was not entirely satisfactory as it entailed the maintenance of a supply of reagent, which deteriorates rapidly, for the sake of three or four determinations a week. Various distillation methods were also considered but were unsuitable owing to the very small amount of moisture involved. It was thought that the rapid removal of moisture could best be effected by a current of warm dry air passed through the sugar in such a manner that all the particles were exposed to the air current and kept in motion by the air; the apparatus evolved and the results obtained by employing this principle are described below.A preheated current of air has been used in many methods of moisture determination, but in only a few has the air been passed through the sample. In an oven method described by Spencer,l air is sucked over an electric heating element and through a metal capsule fitted with a metal filter cloth on which the sample is placed; drying periods of up to 20 minutes at 105" C were used for raw sugars. Meade2 used the Spencer oven to determine the moisture content of molasses, syrups and honey by absorbing a dilution on asbestos, which was subsequently dried for a specified time; drying to constant weight was not recommended.Whittaker and Ross3 described an apparatus, the "Moisture Teller,"4 that was similar to the Spencer oven and consisted of a sample dish fitted with a 500-mesh screen as the bottom; the air stream was provided by a blower. The use of this apparatus for "roughages" was described by Monrose and perk in^.^ Since the method described in this paper was devised, Hardesty, Whittaker and Ross,6 and Ross and Love,' have described an apparatus embodying crucibles fitted with sintered glass plates; they used this apparatus to determine the moisture content of fertilisers by heating at 60" C for two hours. In all these methods, the preheated current of air is drawn or blown through the sample in a downward direction so that loss of the finer particles is avoided, but tight packing may be aggravated and this would tend to lengthen the drying time.It was considered that the use of an upward current of air combined with precautions against the loss of fine particles would ensure that packing is minimised, that the particles are agitated and continually mixed, and that an even temperature would be attained.20 MONEY AND CHRISTIAN: THE AIR-CURRENT METHOD [Vol. 76 THE APPARATUS The general principle of the apparatus is shown in Fig. 1, in which the various parts are shown diagrammatically and unassembled. Standard ground glass joints were used. The sample container, S, is formed of two B29 cones fused together, and a sintered glass plate, P, of grade 3 porosity is fused into one cone.The sample container fits into the B29 socket of the heating unit, H, and is closed by the vacuum head, V, placed over the top cone. By the application of a vacuum at the outlet, 0, air previously dried (by passage through silica 0 H V Fig. 1. Apparatus for air current moisture determination gel, calcium chloride, sulphuric acid or other desiccant and filtered through cotton wool) is drawn in at I, round the heater, through the porous plate and the sample. Temperatures may be taken by the insertion of thermometers fitted with B14 cones into pockets T, and T,; the temperature reached by the sample is referred to again below. Some care is necessary in the design of the heating unit to avoid the risk of volatilisation of metal or metallic oxide from the heating wire on to the porous plate, so causing an increase in weight.It is for this reason that the wire is enclosed in a sealed Pyrex tube, which serves to heat the incoming air. The heater current is obtained from a mains transformer tapped for 12 volts; the heater consumes 24 watts. The rate of air flow is controlled by the vacuum applied and is, of course, dependent on the porosity of the plate and tightness of packing of the drying towers, filter plugs, and so on. The flow should be just sufficient to keep the particles of the sample in motion without violent disturbance. In practice, the vacuum is gradually increased until the sample begins to “dance”; for caster :sugar this occurs when the pressure in S (measured by a manometer connected to T,) is reduced to about 36cm of mercury.AJan., 19511 OF MOISTURE DETERMINATION 21 dry vacuum pump was used to produce the air current in this investigation; if a water pump must be used, suitable traps might be necessary to prevent water vapour passing back to the sample. In order to guard against the possibility of fine particles of the sample being carried away by the air current, a plug of cotton wool is inserted in the upper cone of S, and, as is shown below, provides an effective safeguard against the loss of even extremely fine particles. METHOD OF OPERATION A number of interchangeable containers, S, are fitted with cotton wool plugs, dried in the apparatus, cooled in a desiccator for 20 minutes and weighed, the procedure being repeated until constant weight is reached.The plug is removed, the sample of sugar, 6 to 8 g, introduced, the plug replaced and the weight found by difference. The sample container is then placed in the apparatus previously warmed by means of the heater, and air is passed for 30 minutes. The container is then removed, cooled in a desiccator for 20 minutes and reweighed. In general, for caster sugars, 30 minutes is sufficient time, but as a check, the container is replaced and air passed for a further 30 minutes. During the preliminary work difficulty was experienced in that the moisture content of the sugar used as reference sample was neither constant nor uniform but changed with surprising rapidity with changing temperature, despite storage in a jar fitted with a screw cap and rubber washer.This was undoubtedly due to changes in the distribution of moisture over the surface of the sugar particles and could not be avoided by apparently thorough shaking. The effects were avoided by "conditioning" the sugar in a moist atmosphere (over 85 per cent. relative humidity) at 25" C for various periods of time, the sugar being spread out in very shallow layers and being well mixed at frequent intervals. The well mixed sample was then placed in small specimen tubes, each holding 7 or 8 g, which were completely filled with the sugar before being corked and waxed. The entire contents of a tube were then used for a determination. The method of operation is as follows. TEMPERATURE OF THE SAMPLE The temperature to which the sample was heated in relation to the temperature of the heated air before its passage through the porous plate was measured by modifying the apparatus to allow a thermometer bulb to be immersed in the 7-g sample of sugar in the container.The results are shown in Table I. The temperature of the air was measured by a thermometer placed in T,. TABLE I Duration of air flow, minutes 1 2 3 6 9 12 15 18 21 24 27 30 Temperature of sugar, " C 24.5 29 36 50 57.5 61 64 65.2 66.2 67-0 67.3 67.7 Temperature of air at TI, " C 80 92 101 110 111 112 112 112 112.5 112.5 112.5 112.5 These data showed that during the 30 minutes' heating period the temperature of the sugar did not exceed 70" C. As in practice it would be highly inconvenient to have a thermometer immersed in the sample, a further set of temperatures was taken employing similar conditions of air flow and heating, but taking the temperature of the air above the sample, i.e., by means of a thermometer inserted at T,.Hence, to reach a sample temperature approaching but not exceeding 70" C, it is necessary to attain a temperature of 112" to 114" C below the porous plate (i.e., at TI), and one of 28" C above the sample (i.e., a t TJ. These results are shown in Table 11.22 MONEY AND CHRISTIAN: THE AIR-CURRENT METHOD [Vol. 76 POSSIBLE LOSS OF SAMPLE In order to test whether or not fine particles of sugar might be carried away by the air current despite the cotton wool filter, some experiments were made with icing sugar, which contained about 10 per cent. of particles smaller than 270 mesh (0.053 mm) and so provided a much more severe test than the caster sugar, the particles of which were all greater than TABLE I1 Duration of air flow, minutes 1 2 3 6 9 12 13 18 21 24 27 30 Temperature: above sugar a t T,, "C 20 21 21 23 24 25 26 26.5 27 37.5 28 28 Temperature of air a t T,, "C 93 102 106 111 112 113 114 114 114 114 114 114 100 mesh (0.147 mm).The apparatus was prepared as described above wih the exception that the air after leaving the sample container was bubbled through a U-tube containing water. When the sugar had reached constant weight, no sucrose could be detected in the water by colour reactions sensitive to 1 mg. A second experiment was carried out in which five discs of Whatman No. 5 filter-paper were cut to fit the container above the cotton wool. These were dried and weighed and then placed in position over the container.When the icing sugar had reached constant weight, the discs were tested for the presence of sucrose by the method of Partridge and We~tall.~ No sugar was found on any of the discs, although a similar disc on which 1 mg of sugar had been deposited gave a positive reaction. It was therefore concluded that the cotton wool plug was an effective safeguard against the loss of sugar. TYPICAL RESULTS The reproducibility of the results can be judged from the triplicate determinations of the moisture in three samples of sugar shown :in Table 111. The loss of relative precision when the moisture content is very small is due to the increased effects of experimental, sampling and distributional errors. TABLE I11 RESULTS OF TRIPLICATE DETERMINATIONS OF MOISTURE I N SUGAR Moisture I Sample (4 (b) (4 % % % A 0.446 0.437 0-440 B 0.197 0.180 0-187 C 0.055 0.059 0.062 Mean Spread, % 0-441 f l 0.188 &4 0.059 f 7 The moisture contents of ten samples of sugar are shown in Table IV, together with those obtained by drying to constant weight in a vacuum oven at 70°C.The moisture in four of the samples was also determined by the Karl Fischer method. The agreement in general is satisfactory. Whether the observed differences are due to experimental error (in which must be included errors introduced by the uneven distribution of moisture in the samples taken), or to causes fundamental to the methods used, cannot be stated, but the good agreement between the results by the air current method and the chemically determined water of the Karl Fischer method would indicate that the vacuum oven method used was open to doubt for this type of determination.Although the A.O.A.C. method specifies an air vent to the oven, the flow of air is small and, as has been shown by Iles and Sharman,8 it is difficult to ensure a uniform air movement; moreover the sample of sugar on the bottom of a dish is virtually shielded from any air current.Jan., 19511 OF MOISTURE DETERMINATION 23 TABLE IV MOISTURE DETERMINATIONS BY DIFFERENT METHODS Moisture by air current method, 0.415 0.171 0.166 0.164 0.1 21 0.103 0.065 0.064 0.061 0.060 76 Moisture by drying at 70" C, /O 0.399 0-165 0.161 0.164 0.120 0.110 0.075 0.081 0.052 0.080 01 Moisture by Karl Fischer method, 0.420 0.174 % - - 0.067 0.058 - Our thanks are due t o the Directors of J.Lyons & Co., Ltd., in whose laboratories the work was carried out, for permission to publish, and to Dr. E. B. Hughes for his advice and interest. REFERENCES 1. 2. Meade, G. P., Ibid., 1921, 13, 924. 3. 4. 5. 6. 7. 8. 9. THE LABORATORIES Spencer, G. L., I n d . Eng. Chenz., 1921, 13, 70; U.S. Pat. 1,348,767. Whittaker, Colin W., and Ross, William H., J . Ass. 08. Agric. Chem., 1942, 25, 132. Manufactured by Harry W. Dietert Co., 9300, Roselawn Ave., Detroit, Michigan, U.S.A. Monroe, C. E., and Perkins, A. E., Dairy Sci., 1939, 22, 37. Hardesty, John O., Whittaker, Colin W., and Ross, William H., J . Ass. Off. Agric. Chem., 1947, Ross, William H., and Love, Katharine S., Ibid., 1947, 30, 617.Iles, W. G., and Sharman, C. F., J . SOC. Chem. Ind., 1949, 68, 174. Partridge, S. M., and Westall, R. G., Biochem. J . , 1948, 42, 238. 30, 640. J. LYONS & Co., LTD. T nwnnw 1x7 1 A DISCUSSION MR. H. POWERS said that there was a possible loss of dust from pulverised sugar, a small proportion of which may be as small as Some of the coarser conglomerates and grouping of crystals might not dry as soon as the separate particles. MR. MONEY replied that the method was developed for the routine testing of caster sugar of specified particle size that excluded fine dust, but, as shown in the paper, no loss occurred even with icing sugar, all particles of which were less than 0.147 mm. No trouble has been experienced with conglomerates, which were normally absent from the sugar examined, but which have been produced artificially by damping the sugar and breaking up the lumps.MR. ALFRED WRIGHT said that in a very similar method for resins i t had been found that variations in particle size made very large differences to the results, even though resins were amorphous and not crystalline like sugar. Errors also arose from the tendency of the finely powdered portion of the sample t o be blown away. As absolute uniformity of particle size was impossible without sieve grading and its consequent losses, and also since the method required the particle size to be very small for consistency of results and the small size of particle led to losses by blowing away, the method was considered to have too many inherent disadvantages and was discarded.MR. A. L. BACHARACH suggested that figures for the reproducibility of the A.O.A.C. method, together with those reported by the author for his own, should make it possible to assess the significance of the occasional small differences that he found between them. DR. I<. 4. WILLIAMS said that the principle described by the authors had been applied to other materials. In particular, he mentioned that in 1918 the Ministry of Food adopted as standard a method put forward by their Committee of Analysts for determining moisture in oils, and this was very similar to that now put forward. It consisted in spreading the oil over a wad of asbestos contained in a tube and driving off moisture contained in the oil by means of a stream of dry inert gas a t about 50" C.In the early form of the method the moisture removed from the oil was caught on calcium chloride or sulphuric acid and weighed. Later, however, it proved simpler to ascertain the loss of weight of the tube containing the oil on asbestos. Some care was necessary in choosing asbestos suitable for the test as some specimens had proved to lose moisture slowly and contirfuously when dry gas was aspirated over them, and these were clearly useless. inches.24 MONEY AND CHRISTIAX [Vol. 76 MR. R. A. FINCH said that Mr. Money had referred to the determination of moisture in dehydrated vegetables by his air flow method. It was well known thak there was, as yet, no accurate means of measuring moisture other than by drying in vacuum for many hours. This was of no practical value where a large number of samples was being examined daily, and a method of drying dehydrated vegetables to constant weight in a short time, preferably in their unground form (dice or strip) would be very useful.He would be interested t o know how long the author’s method took, and what vegetable he had used. The amount of work done was very small, but drying did take some hours, and the indications were that the method was not as satisfactory as with sugar particles where the moisture was all on the surface. MISS E. IRENE BEECHING asked the author whether he had used the method he described for the determination of moisture in high-boiled sweets, milk powders and chocolate crumb. MR. MONEY replied that the method gave good results with chocolate crumb, but had not been tried for milk powders.Boiled sweets were effectively a glass, and the method could not be used unless the sample were pulverised, during which procedure it would probably absorb a certain amount of water, and even if it were possible to devise a method of pulverising the substance in absence of water, the result would probably depend on the particle size of the powdmer. Much work was needed on the determination of moisture in high-boiled sweets, before a satisfactory method could be devised. MR. L. G. BECKETT drew attention to a statement in the paper that difficulty had been experienced in maintaining known moisture conditions in sugar retained in a Kilner jar closed b y , a rubber ring and screw-on cap. He said that it was not generally realised that rubber was permeable to water vapour.Considerable difficulty in this respect was experienced in the packaging of dried pharmaceutical preparations. Greavesf and Flosdorf2 both referred to this permeation. It was possible that the variation in sugar moisture was due to water vapour flow through the diaphragm. Mr. Beckett asked whether the author had had any experience of back diffusion of water vapour from a water jet pump when used for aerating a sugar sample as described in the paper. He would not have expected back diffusion with the through-put of air necessary to maintain a pressure of 34 cm of mercury. In view of the long time necessary for com- pletely drying sugar under vacua by the method described, had the author any experience of tumbling sugar under high vacuum conditions and the consequent effect upon the time required ? Had Mr. Money considered equating his shorter drying time by air current methods to fundamental considerations of the kinetic theory of gases ? MR. MONEY replied that it was only a rubber washer, not a rubber diaphragm, that was used and therefore the joint was practically glass to glass. He agreed that back diffusion of water was extremely unlikely, but he preferred to take that precaution. The sugar was virtually tumbled but not a t a very high vacuum and he had had no experience of the effects of tumbling under such conditions. MR. MONEY replied that he had tried tomato powder and potato powder. REFERENCES TO DISCUSSION 1. Greaves, R. I. N., in Keynes, G. L., “Blood Transfusion,,’ Simpkin Marshall, Ltd., London, 2. 1949, p. 686. Flosdorf, E. W., ‘‘ Freeze-Drying,” Reinhold Publishing Corp., New York, 1949, pp. 150-164.