Vacuum Drying of Compressed
Vegetable Blocks W. C. DUNLAP, *
JR.~
Western Regional Research Laboratory, U . S. Department of Agriculture, Albany, Calif.
D
quired only a loosening of Drying of compressed blocks of partially dried potato the stopper in the top of the emergency the comand carrot by the use of vacuum and radio-frequenry jar. As a check on the pression of dehydrated vegeheat at 60' C. was compared with drying in a vacuum method, each sample Ras tables into firm blocks inoven at 60" C. Although the radio-frequency method hac1 weighed both before :md terested military, commerdefinite advantages with respect to speed of drying, the after i t was dried. cial, and research agencies vacuum-oven method produced higher quality. The vacSince high precision was as a means of saving space uum oven set at 70" C. appeared to be roughly equivalent, not required, temperature and container m a t e r i a l s . with respect to both speed of drying and quality, to the control to + l o C. was atMany vegetables can be comradio-frequency method at 60" C. tained by manual control pressed satisfactorily after of the autotransformer ~ U I J they have been dried sufplying primary voltage to the oscillator. Heat may have ficiently t o meet government specifications, but potato pieces been conducted away from the center of the block by the wxes dried to 7% moisture are crushed when compressed. Pieces dried used for the thermocouple leads, since they were of fairly large t o 12-1570 moisture can be compressed without being crushed, diameter (B. &- S. No. 24). Thus the temperature values actually and if blocks thus made could be rapidly dried further, a major problem in compression of dried potatoes might be solved. obtained may have been slightly low, but, because of the low thermal inertia of the theimocouple wire, the error would not be Radio-frequency heating has seemed to be a possible solution. more than perhaps l o C. Robertson ( 1 ) gave a general review of applications of radioInput power to the ovillator for drying potato and carrot frequency heating for processing materials. Sherman (3) conblocks in this work varied from 150 to about 200 watts. No ducted experiments on blocks of carrots, beets, onions, and cabstudy was made of the efficiency of the process. Temperature bage and reported times of the order of 2 hours for drying from measurements with power on and off in general sho~-cdno vnria15 t o 2y0 moisture with radio-frequency heat applied to blocks tion in potentiometer reading with rate of power output from the held in a vacuum.* I n order t o evaluate the radio-frequency oscillator. There \\-ere, however, a few cases in which such a method we compared the vacuum-oven procedure with radiodependence was noted, arid the cause was not found. Frequent frequency heating in vacuum for potatoes and carrots. Since checks were made during each run to guarantee that the cffect 60" C. (140" F.) is commonly used in commercial dehydration, did not occur during the tempelature measuiements. that temperature was used in these experiments. The pressure was measured with a closed-tube mercuiy manometer and was kept constant ordinarily within a few millimeters EXPERIMENTAL METHODS of mercury, although larger variations occurred during short periods. Sufficient ventilation was provided to avoid accumulaThe radio-frequency apparatus used (Figure 1) consisted estion of moisture in the jar. Part of this ventilation came sentially of a 34-megacycle oscillator, by the use of which power through a calcium chloride drying tube and part through leaks was supplied to a pair of plane parallel electrodes (5 X 5 inches, with 2.5 inches of separation). The sample was suspended bein the vacuum system. The samples used had been tween these eleztrodes. B bell partially dehydrated. Some jar and a two-stage mechanof the potatoes had been cut ical vacuum pump were used in the form of julienne strips to maintain a pressure of 10 BALANCEx 3/16 X 3/16 inch); to 50 mm. of mercury around others were cut into half cubes the sample. The sample was X X inch). The suspended from one arm of a carrots had been cut into 3/1torsion balance mounted above inch cubes. the bell jar. The suspending The potatoes used for Samwires (copper and constantan) ples were dehydrated from the also served as a thermocouple raw state to 13-15% moisture, inside the sample. A portaa t which point the radio-freble potentiometer (Leeds & quency (r-f) drying was begun. Northrup) was used to measThe moisture content of the deure temperature. Weighing re1 Present address, General Eleehydrated carrot samples was intric Research Laboratory, Schecreased from 5% t o the value noctady, N. Y. obtaining a t the beginning of 2 A published report of later work r-f drying by keeping them in by V. W. Sherman came to the author's attention after the compleevacuated desiccators over Figure 1. Schematic Diagram of Oscillator and Heating tion of the research reported here Unit suitable concentrations of sul(Proc. Insl. Food Tech., 1944, furic a r i d . T h e p o t a t o T h e vacuum pump and gage are not shown. 87-101).
URIPU'G the recent war
T=E&
1250
December, 1946
INDUSTRIAL AND ENGINEERING CHEMISTRY
1251
samples had been held in cold storage for several weeks after dehydration. Nearly all samples were pressed a t 160" F. for 30 seconds with a n approximate pressure of 300 pounds per square inch. It is unlikely that there was appreciable variations in moisture throughout the piece as a result of this treatment. It is also of interest that tasting panels were unable t o distinguish the quality of potato pieces after pressing from that of the pieces before pressing. # The compressed blocks were approximately 3 X 2 X 1inch in size, and their density was about 60 pounds per cubic foot. This bulk density was sufficiently low to warrant the assumption that compression had little effect'upon drying rate-that is, the compreqsion was small enough t o cause the pieces merely t o stick together in a block without adhering in a uniform mass. A bulk density of the order used is suitable for acceptable reconstitution of the material. I n general the blocks used were similar to those used in commercial practice, except for size. Data on initial moisture contents were somewhat indefinite, because of an unknown loss of moisture in the preparation of the block. Hence moisture determinations (16-hour drying in the vacuum oven a t 70" C. of half the block, ground to pass a 2-mm. screen) were made upon the finished product. The original moisture content was then calculated from the final moisture content and from the known loss in weight during the experiment. DRYING RATES FOR CARROTS AND POTATOES
With the radio-frequency vacuum method, the drying obtained can be attributed t o the combined effect of two factors: the h i a t imparted t o the material by the radio-frequency fields in the sample, and the low pressure prevailing around the sample. I n vacuum-oven drying, a low pressure also prevails around the sample, but the heat inside the sample must be acquired by radiation, conduction, and convection from the walls of the oven. Thus increases in drying rate obtained with the r-f vacuum method can be attributed to the superiority of the r-f method of applying heat as compared to the processes in a vacuum oven. Figure 2 shows the r-f drying curves for potatoes (julienne strips) and carrots (s/(-inch cubes) at 60" C. (140' F.) and a pressure of 35 mm. of mercury, compared wiBh curves A and B, which represent the vacuum oven a t 60' C. (140"F.) and a pressure of 12 mm. of mercury. The air pressure, a t least between 12 and 50 mm., has little effect upon drying rate, so that the conditions here are comparable as far as pressure is concerned. Curves B and B' were obtained with samples wrapped in cheesecloth, tied with twine, and !aid on the metal shelf of the oven. The oven used was a standard laboratory-vacuum oven of the air heated type. Curve A represents a block wrapped in heavy copper gauze and pressed against the shelf by a weight. The rates shown by curves A and B are nearly the same and yet represent fairly well the extremes obtained with good and poor contact of the blocks with thc shelf. Most of the samples dried in a vacuum oven were wrapped in cheesecloth; if coppergauze wrapping were used, the vacuum oven presumably would dry the samples somewhat faster, as in the case presented. The most important conclusion t o be drawn from Figure 2 is that the initial drying rates were considerably greater with r-f vacuum than with the vacuum oven for beth potatoes and carrots. Seconaly, in spite of this advantage, 6 hours were required to dry the potato blocks from 15 to 7% by the r-f method. For the same reduction in moisture with the vacuum oven, 9 t o 10 hours were required. Thus for potatoes the 60" C. r-f method requires about 30% less time for drying to 7% than does the vacuum-oven method, under the conditions employed. As is shown later, however, this advantage is obtained a t the expense of quality. It is of interest that the superiority in rate of the r-f vacuum method, as compared with the vacuum oven, was greater for carrots than for potatoes. Whereas the drying rate (or slope)
.
DRYING TIME- H O U R S
Figure 2. Drying Curves for Potato and C a r r o t Blockr Obtained by Use of 60" C. r-f V a c u u m M e t h o d and 60" C. V a c u u m Oven
at 8% moisture for potatoes was I1/2 times greater by the r-f vacuum method than by the vacuum oven method, in the case of carrots the slope was 3 times greater by the r-f method than by the oven method. Calculation of the slopes showed t h a t this relation remained about the same for all moistures used-that is, 7y0and above. Thus i t is clear that, because of the variation i n thermal and in dehydration properties, the r-f method may have considerably greater advantage with some vegetables than with others. EFFECT O F PARTICLE SIZE IN CARROTS
To determine the effect of particle size upon drying rate of carrot blocks, blocks of a type similar to the potato blocks and consisting of various particle sizes were prepared and dried by both methods (Figure 3). The particle sizes were
