Moisture Determination in
Dehydrated Vegetables VACUUM OVEN METHOD T h e loss of weight that occurs when dehydrated potatoes, carrots, cabbage, and onions are heated in a vacuum oven has been studied as a function of time, temperature, fineness of grinding of the sample, and degree of evacuation in the oven. The loss due to thermal decomposition of the vegetables has been estimated, and the combined results have been used to establish conditions for the determination of moisture in the four vegetables.
0
F the numerous ways of determining moisture in dehydrated vegetables (9),the vacuum oven method is the most important because it is currently used as a reference for the
calibration of other methods. Others described recently are the electrical methods (7), the distillation method (4, 7 ) , and the Fisclier volumetric method (6). The electrical methods are purely empirical and require calibration against a standard method. The distillation and the Fischer methods also require calibrations because usually no definite end point is reached in the determinations. The vacuum oven method used at present by industrial laboratories is identical with t h a t recommended by the Association of Official Agricultural Chemists for dried fruits ( 1 ) . According to this method the material is ground in a food grinder and heated in a vacuum oven a t 70" C. for 6 hours a t a pressure not exceeding 100 mm. of mercury. It has already been pointed out (8) that these conditions for preparation of the sample and for drying are not sufficiently rigorous for dehydrated vegetables t o ensure agreement among different testing laboratories. The present paper shows t h a t the analytical results are not a good measure of moisture content. Results may be too low or too high. The result with carrots, for example, may represent only GO% of the water actually present in the sample (Table 111). A change in procedure must be made for testing dehydrated vegetables, and this new procedure must be varied for different vegetables. When an oven method is used for the determination of moisture, the loss in weight on heating is taken as a measure of the moisture content. With plant tissues loss in weight is a function of time of heating; only rarely does this loss reach a constant value. Loss in weight represents not only the water originally held but also other volatile materials, including volatile products of decomposition a t elevated temperatures (3). It is impossible to tell from a single drying-rate curve when the mechanically held water is completely driven off and when further loss in weight may be ascribed t o chemical changes in composition of the material. There is no sharp demarkation, and chemical changes probably occur a t all stages of drying. T h e over-all loss in weight depends on many factors, such as method of preparation of sample, time of drying, degree of evacuation in the oven, and thermal stability of the material. T h e present paper reports a study of these factors as involved in determinations of moigture in dehydrated carrots, potatoes, onions, and cabbage. The major problem was to determine the time of drying required in order that loss in weight of sample would represent a n accurate measure of moisture content. The moisture content was arbitrilrily defined in this work as the loss in weight, a t equilibrium, when the ground material is
BENJAMIN MAKOWER, SARAH MYERS CHASTAIN, AND ELISABETH NIELSEN Western Regional Research Laboratory,
U . S . Department of Agriculture, Albany, Cali'.
dried in vacuo a t room temperature over a desiccant that permits practically no water vapor pressure. I t was assumed that, at room temperature and in the absence of air, decomposition and oxidation would be negligible. The experimental procedure established in accordance with this definition is referred to as the primary reference method. It was found, however, that a direct application of this definition was not practicabic from a routine standpoint. hIaterials dried in vacuum desiccators over anhydrous magnesium perchlorate required many months to reach constant weight. A secondary reference method was therefore devised for a more rapid determination. This sccondary method, referred t o in this paper as the redrying procedure, was carried out as follows: A sample of the vegetable was dried in the oven for a long time (about 100 hours); a t the end of this time it was assumed to be essentially dry. The sample was then permitted to absorb a known amount of water and the drying was repeated. The time required in the second drying to obtain a weight loss equivalent t o the amount of added water was taken as the correct drying time of the material. Though several assumptions must be made to justify this procedure, the r e ~ u l t s obtained with it were found t o agree reasonably well with those obtained by the primary reference method. From the results obtained by the redrying procedure, definite conditions were set up for the routine moisture determination of each of the four vegetables. I t is recognized that a standardized procedure does not take into account possible variations in drying characteristics of different specimens of the same vegetable. Variations may result from differences in composition or in previous treatment, such as blanching ol: drying. However, the present work indicates that major discrepancies have been eliminated; the analytical results were much more accurate and reproducible than those obtained by the vacuum oven method now in use by the dehydrated vegetable industry. SAMPLING AND REPRODUCIBI LlTY
PREPARATION OF THE SAMPLE. Adequate preparation of a
sample includes the problems of sampling and grinding. I n dealing with these problems i t is of primary importance t o recognize t h a t the dehydrated vegetable rnat,erial is not homogeneous in moisture content when i t issues from t h e drier. Not only do the various pieces differ but also a moisture gradient exists within a single piece, the outside portion being drier than the inside portion. When material is stored in a closed chamber, it becomes more homogeneous through diffusion of water among and within the pieces. The rate of diffusion is slow; the time required t o attain moisture equalization decreases with increasing temperature and increasing moisture content, and increases with increasing size of the piece. I n order t o obtain a representative sample of vegetable it was therefore necessary t o grind and then mix the material well. 725
INDUSTRIAL AND ENGINEERING CHEMISTRY The six(, of the sample t o be taken for grindihg dcpentls on ccnsideratioiia tlint are beyond the scope of this paper, but in this laboratory it \\-as the practice to use not less than 100 prmis, systematictilly sampled. The material was first ground coarsely in a JI'iley mill (intcrmcdiate model) equipped with a U. S. 10mesh sieve, and pissed through a quartering funnel. X 25gram portion of the qunrtcred material was then regrouiid in the' same mill tu pans il C . S. 40-mesh sieve and was put away in :i tightly stopiwi~etlcont:riner (bottles with screw cap and n rubbcl. gaskc't). t o IIC u-ecl 1atc.r for t h e det~t~minntiori of moisture. T h e grinding was ra i d , without excessive heating or exposure to air. The particles were limited in size by the sieve opening, and the holdup i i i tlie mill (re-idut. in the grinding chamber) was very small. 1;inc.r grinding proved t o be of no advantage. When a 60-mesh sieve \vas used, the grinding became objectionably long (ivith samples produced commercially at moisture content of 5 to 7 5 ) and, as shown in Table 111, the di,ying time was not shortened appreciably.
0 FIRST DRYING
i
@ SECOND DRYING
e
SECOND DRYING,TRANSLATED
2
2
TIME, H O U R S Figure 1. Drying and Redrying Curves for C a r r o t s at i o o C.
In some laboratories it has been the practice t o sieve the ground material and t o select a certain sieved portion for moisture determination. It was demonstrated in this laboratory t h a t tbjs procedure is not applicahle t o freshly dehydrated vegetables because the moisture content of the sieved portion may differ appreciably from the average moisture cpntent of the whole. I n one test vihere freshly dehydrated diced carrots w r e ground in a \Taring Blendor and sieved through a E. S. 30-mesh screen, it was found t h a t the portion passing through the sieve had a moisture content of 6 . 4 7 ~and ~ t h a t retained, 7.3y0. Two to three weeks were found necessary for equalization of moisture in t h e dices a t room temperature. The grinding procedure described was applicable n i hin a limited moisture range t h a t varied with different vegetables. A t low moisture contents (less than about 2%) errors arose from the absorption of water from t h e atmosphere durin grinding and handling. At moisture contents hirrher than a t o u t 10% for potatoes and carrots, and 7% for c:tbbage and onions, t h e materia!s became rubbery in texture and were difficult t o grind nitliout clogging the mill. In both cases it became necessary to add another step t o the procedure. Prior t o grinding, the very dry materials were a l l o w d t o absorb moisture from t h e air, whereas t h e wet materials were predried t o a moisture level a t which they could be ground safely and conveniently. Predrying is preferably done in a vacuum oven a t 60" C. on coarsely ground material. The change in moisture content can be determined by weighing the unground or coarsely ground materials before and after their respective treatments, and an appropriate correction applied in the final moisture determination. This additional operation is, however, not required with most commercially dehydrated vegetables. OVES CONDITIONR.The ovens used were electrically heated, gravity convection type, similar t o Model 510 of the Precision Srientific Company. The temperature of these ovens was usually read on a mercury thermometer inserted partially into the vacuum chamber. It is important t o know the temperature distribution within the oven because a difference of l oC. gives rise t o a difference of a p roximately 0.170 in apparent moisture content. Temperature fistribution was measured by means of thermocouples attached to various parts of t h e oven shelf. If t h e shelf was iron, a temperature gradient of about 2" C. existed along i t (from front t o rear of oven) when t h e oven was set at 70" c'. (In other ovens in use a t this laboratory, particularly thosc: Nithout an insulated door, t h e giadient was as much as 5 " ( ' . 1 The gradient was reduced to less than 0.5" C. when a
Vol. 38, No. 7
inch copper shelf was substituted for the iron shelf. The reading of the mercury thermometer agreed u i t h the tempcrature of the shelf only when the oven was at atmospheric pressure. T h e thcrmomcter reading in vacuo ~ a s . to 3 ~ 4' C. loner, w e n though the temperature of the shelf remained essentially unc-hanged. The drop in the thermometer reading was at,tributulile to insuffirient hcat transfer in the evacuated space. Control of t h r oven was therefore guided by thermometer readings when the oven was a t atmospheric pressure. The ovens w r e evacuated by means of rncchanical nil pumps rapable of reducing pressure in a vacuum-tight system to less thav 1 micron. Pressure maintained in the oven may influence both rate and completeness of drying. I t has heen demonstrated, hoa-Ever, in drying experiments n i t h particle of various sizes (8) that rate determination over .most of the drying period gives rate of diffusion of water through the particles. Completeness of drying of such colloidal materials as are found in vegetable tissues must be considered from the following point of view: In drying, the moisture content of the material is lowered to a value t h a t corresponds, at best, t o the equilibrium moisture content for the particular set of conditions of temperaturc and humidity prevailing in the oven ( 5 ) . It is important to note that, for some materials, t h e equilibrium moisture content may be appreciable even at very low humidities. Completeness of drying depends, therefore, not on the t,otal pressure but on the partial pressure of the water vapor in the oven. Pressure of water vapor in the oven can be kept low by effirient evacuation or by the passage of dry air through the oven, :is suggested in the A.0.4.C. method ( 1 ) . I n the present experiments the total pressure maintained in t h e oven varied from 1 t o 5 mm. of mercury. When the total pressure was raised to 50 mm. (by deliberately bleeding humid room air into the oven) the loss in weight a t 70" C. for potatoes was found t o be 0.4Y0 loner, whereas for carrots the change was less than 0.1%. From this observation it was concluded t h a t equilibrium moisture (.ontent of potatoes at the higher pressure (estimated relative humidity, approximately 0.5%) was sufficiently high to make the effect of the humidity change detectable; for carrots the change and probably the absolute value were not high enough t o be detectable. Some supporting evidence for this interpretation is available from d a t a on water vapor pressure (unpublished) for carrots and potatoes. From data a t 35" C. it was estimated t h a t at 0.5% relative humidity the moisture content of potatoes was about 0.770 and t h a t of carrots about O . l ~ o .When the total pressure in the oven was 5 mm., the relative humidity \vas about 0.05%, and the equilibrium moisture content t h a t corresponded t o it (at 35' C.) was less t h a n 0.1% for both potatoes and carrots. Rigorous proof of this point would require careful measurements of equilibrium moisture content at 70" C. and very lon humidities. T h e A.O.A.C. method requires t h a t pressure in the oven be less than 100 mm. of mercury and t h a t dry air be passed through the oven at a certain rate. The effectiveness of the latter practice is subject to certain limitations. I n our experiments no effect on Iveight loss was found when dry air was passed; consequently the practice was discontinued. Under our conditions, with degree of evacuation limited by small leaks of air into the oven, the addition of dry air merely increased the total pressure but did not change the partial pressure of the water vapor in t h e oven. When a vacuum-tight oven is used, however, with a large ca aeity pumping system t h a t is not capable of producing a very Eigh vacuum, the use of dry air in sufficient quant,ity should have a beneficial effect; total pressure will remain nearly the same although the partial pressure of the water vapor will be lowered. OTHEREXPERIMEXTAL DETAILS.Samples t o be dried varied in weight from 2 t o 4 grams and were handled in small glassstoppered yeighing bottles, 20 mm. in diameter and 25 mm. high, ivith an outside ground cap of standard taper. The bottles were placed in the oven in a device t h a t permitted all of them, at t h e t,nd of the drying, to be closed simultaneously and quickly ( 2 ) . Tivo empty bottles were included with each set of samples in t h e oven to serve as blanks. They were p u t through all the operations to check on reproducibility of weighing, on completeness of rooling t o room temperature in t h e desiccators, and on other factors. Special precautions were taken to protect the samples from absorbing moisture from the air, At the end of the drying period the oven was brought to atmospheric pressure with air dried by passage through a calcium chloride tube. The air was introduced slowly for 20 to 30 minutes t o ensure t h a t i t was adequately dried by the calcium chloride and also t o keep t h e finely powdered samples from being blown out of the bottles. Another calcium chloride tube was maintained between the oven and the oil pump. Determinations on a given sample were ,made in duplicate or triplicate; the results of analyses usually agreed within 0.03(7,
July, 1946
INDUSTRIAL AND ENGINEERING.CHEM1STRY
of the weight of t h e sample. This does not, however, represent the esperimental error of the determination; the reproducibility for replicates which were not dried at the same time was only *O.lC,. The error arises principally from variations in the oven temperature and from inequalities of the temperature along the oven shelf. The experimental results are reported on dry basis and are given as averages of the replicates to the nearest
o.170.
SECONDARY REFERENCE METHOD
A drying curve was first determined by measuring loss of keight of a sample in a vacuum oven a t various time intervals. Drying was continued long enough (about 100 hours) to ensure that it was essentially complete. The sample was then allowed to absorb water from a humid atnioFphcre until i t regained approxiniately its original amount of water. The exact weight of the atlclcd water was determined from the difference in weights of the dry and the remoistened sample. T o ensure uniform distribution of moisture throughout the material, the remoistened sample was allowed to stand in a c l p e d bottle for about 2 weeks. I t was then dried again in the vacuum oven under the same conditione employed during the first drying. The time required to reduce t h e sample weight by a n amount equal t o that of added water (that is, the time t o reach the dry weight obtained in the first drying) is called "redrying time". Thus, redrying time is not the time rcquired t o remove all the moisture but is the time wlien the sum of the moisture loss and the decomposition loss is equal to the amount of added water. Hence the redrying time is always shorter than the first drying period. The moisture content of the original sample is taken t o be the loss in neiglit in the first drying after a heating period equal t o the redrying time, provided the original and remoistened samples shon the same drying characteristics. Several assumptions are involved in, this procedure. The first is t h a t essentially all the water originally present has been removed a t the end of the first drying run. The factors t h a t support the plausibility of this assumption arc thc long drying period and the very low rate of loss of weight a t the end of the drying-O.005% per hour or less. Sincc tliis loss in weight appears t o be nearly constant in rate, it may lie assumed that the loss is due principally t o decomposition. Furthermore, it was found that the redrying time was unchangctl when the first drying run was conducted for an additional 40 t o 50 hours. The scccnd assumption is that weight loss increases with heating time in the same manner in both drying runs. This is
capable of direct experimental verification by comparison of the weight loss-time curves for the t x o runs. If the initial moisture content and the drying characteristics were the same in the first as in the second drying run, the result,s of the two runs could be represented by one drying curve. Since under the experimental conditions the initial moistures were not the same (becausc of practical difficulties in adjusting the moisture content t o the same value), two different curves are obtained which can be tested for similarity by superposing one on another and showing that they are coincident. T o superpose the two curves, it is necessary t o shift one of them along both the time and the weight loss axes (equivalent to shift of origin). The shift along the time axis corresponds t o the time necessary to dry the sample from the higher to the lower initial moisture level. The shift along the weight loss axis corresponds t o the amount of water removed plus thc weight loss due to accompanying decomposition. Since, in a general case, neither the amount of a a t c r nor the extent of dccomposition is known, the comparison for similarity can be madc only by graphical means (by trial), by manually superposing the two curvcs to dctcrmine whether the curve for loner initial moisture coincides with a portion of the curve for higher initial moisture. T h r problem is, however, greatly simplified for all the cxperiments described in this paper. I t was found that tho shift along the time axis was entirely negligible (less than one hour) h a u s e of small differences in moisture content between the original and,the remoistened sample and because of the rapid initin
