:apid Estimation of Citrus Peel Oil A New Turbidimetric Method EVERE'ITE M. BURDICK AND JAMES S. ALLEN' Tezsun t7itrus Exchange Research Laboratory, Weslaeo, Ter. A rapid determinrttion of citrus oils is needed for control purposes; the official method in use is 90 minutes long. A rapid procedure is described for the eolorimetric estimation of oils in raw and pmeessed OitNS juices. The oil is distilled with aoetone and isteam to a known volume, an aliquot is diluted, and turbidity is determined. A. standardization pmcedure makes possible correlation of results in terms of Ithe official method of the United States Department of Ageculture.
I
N T H E produoti ties of peel oilI are incorporated by high-speed mechanical extractors, althougl1 modern processing plants have several meth^A^ c . ".."+"",I:"the peel oil content of juices. These plants operate at such high rates that a rapid estimation of the peel oil is necessary for control purposes. I n 1941, Scott (2) presented a steam-distillation method for determining the amount of peel oil in citrus juices and proposed that the quantity of this oil he used as a measure of undesirable constituents incorporated by the extractors. Small amounts of essential peel oil am desirable, in that they add materidly t o both'aroms. and flavor. Between 0.002 and 0.005Y0recoverable oil appears to be best for grapefruit juice, an d somewhat higher concentrations for orange juice. Higher corLcentrations of ail are objectionable even in fresh juices and dc teriorate in stored juices, as evidenced by the development c turpentinelike flavors. The Production and Marketing Adminir tration, Department of Agriculture as well as industry in genera uses a modified Clevenger method (5) for grading purposes. Thi requires a large sample and about 90 minutes for a single detei mination. This is far too slow for control work. as modern DrO(;essing plants are able to handle well aver 100 tons of fruit before a sinele - determination can be ComDleted. Recently... Burdick a n Scott (1) developed s turbidimetric method for the rapid estime1.. . . .. _.. .. . . . . . . .. . ... . . ttou of peel 0x1. 'lnis metboa muvoIvea a simple amtillation of yl
sYUYLY.L.L.6
solutions in all cases upon dilution with water. Subsequent investigations using methanol, butanol, %propanol, and mixtures of these ~lcoholsshowed them t o be either too sensitive or not sensitive enough to variations in peel oil concentrations. This paper presents a rapid method for the estimation of peel oil, suitable for quantity control in the industrial production of canned citrus juices. Acetone is a satisfactory substitute for ethanol, making available to industrial laboratories a rapid turbidimetric method. It is possible t o eliminate the saltingdut agent, inasmuch as the acetoneoil distillate attains maximum turbidity within 1 minute upon dilution with water and remains stable at least 5 minutes. PRELIMINARY EXPERIMENTS
Turbidity measurements on solutiqns obtained by diluting methanol. butanol. 2-propanol. and acetonesolutions containing known
APPARATUS AND MATERIALS
The distillation apparatus shown in Figure 1consists of a 500ml. distilling flask, a standard Kjeldahl distilling trap, and condenser (preierahly West type), having its delivery end
540
ANALYTICAL CHEMISTRY
can then be used for future adjustments. Optical density or per cent transmittance measurements should be made within 2 minutes after preparation of the turbid solutions; however, the clear distillates remain stable for several days if kept tightly stoppered. Calculate the per cent recoverable oil by multiplying the optical density reading of the unknown sample by a factor determined experimentally, as explained in the standardization procedure.
Table I. Comparison of Results Obtained by Modified Clevenger Method with Acetone Distillation Turbidimetric Method % Recoverable Oil, Clevenger 2nd Av.
STANDARDIZATION AGAINST MODIFIED CLEVENGER METHOD
1st
By preparing a series of solutions containing known amounts of various citrus oils, diluting these with distilled water to make turbid solutions of known oil content, measuring the optical densities of these turbid solutions, and fmally plotting the optical densities against the oil concentrations, i t is possible to show a slight variation or deviation from Beer’s law. However, it is seen from Figure 2 that a straight line results over a considerable concentration range. This straight line does not go through the origin, probably because of the very slight solubility of peel oil in the dilute acetone, which partially explains the nonconformity with Beer’s law.
0.0010 0.0020 0.0010 0.0025 0.0020 0.0025 0.0025 0.0080 0.0110 0.0125 0.0140 0.0160 0.0150 0,0175 0.0175 0.0190 0.0210 0.0270
0.0012 0.0010 0,0025 0.0020 0.0025 0.0025 0,0030 0.0100 0.0100 0.0135 0.0140 0.0150 0.0165 0.0175 0.0200 0.0200 0,0220 0.0280
Optical Density (OD X 10) 1st 2nd Av.
0.0011 0.0015 0.0018 0.0023 0.0023 0.0025 0.0028 0.0090 0.0105 0.0130 0.0140 0.0155 0.0158 0.0175 0.0188 0.0195 0.0215 0.0275
0.20 0.20 0.15 0.25 0.25 0.20 0.20 0.85 1.25 1.60 1.40 2.00 1.80 1.55 2.30 2.25 2.10 3.20
0.05 0.15 0.10 0.15 0.25 0.15 0.20 0.80 1.20 1.75 1.45 1.80 1.80 1.65 2.25 2.20 2.20 3.20
~
Total
0.1959
Table 11. Distillate,
MI.
2. 5!-
m z
Water, M1. 1 2 3 4 5 6 7 8 9
5
4-
W
* 3-
10
-I
3’% ReooverTurbidirnekc able,Oil (OD x 0.0906)
0.13 0.18 0.13 0.20 0.25 0.18 0.20 0.83 1.23 1.68 1.43 1.90 1.80 1.60 2.28 2.23 2.15 3.20
0.0012 0.0016 0.0012 0.0018 0.0023 0,0016 0.0018 0.0075 0.0111 0.0152 0.0130 0.0172 0.0163 0,0145 0.0207 0.0202 0.0195 0.0290
-
Deviation
+o ,0001 +o, 0001 -0.0006 -0.0005 0.0000 0.0009 -0.0010 -0.0015 +0.0006 +o ,0022 -0.0010 f0.0017 +0.0005 0.0030 +0.0019 +0.0007 -0.0020 +O. 0015
-
-
21.60
Effect of Dilution on Turbidity Optical Density
x
10 o. .os .~ 0.50 0.70 0.90 1.10 1.10 1.10
1.00 0.90 0.85
% Concn. of Oil
n om
0.029 0.025 0.022 0.020 0,018 0.017 0.015 0.014 0.013
Optical Density per Unit Concn. o..9_ 17.3 28.0 40.8 55.0 61.1 64.7 66.7 64.3 65.4
2- 2I-
: \-
0’
a .Oh
6 .d\
Figure 2. 1. 2.
,Os -0’4 .0’5 % C I T R U S OIL
I
Optical Densities of Turbid Citrus Oil Emulsions
Synthetic mix (steam-distilled citrus oil used) Distillate
Discrepancies between the turbidities produced by known concentrations of citrus oils and those found by the modified Clevenger method have been observed-neither the modified Clevenger nor the acetone distillation method recovers 100% of added peel oil. It is thus necessary to establish experimentally the relation between the turbidity obtained by the acetone turbidimetric method and the amount of recoverable peel oil as determined by the modified Clevenger method. This is readily done by making duplicate analyses by both the turbidimetric method and the modified Clevenger method on a series of juice samples containing varying amounts of recoverable oil. Table I shows the results of such a set of duplicate analyses by two analysts. The optical densities are proportional to the oil found by the Clevenger method, The proportionality factor, 0.0906, is calculated from the averages. Thus i t is possible to multiply the optical density by the factor 0.0906 to get the per cent recoverable oil that would have been found by the modified Clevenger method. Because the method is empirical, each analyst should standardize his equipment against the modified Clevenger method. EXPERIMENTAL
The relation between optical density and per cent peel oil is shown in Figure 2. Curve I was obtained from “synthetic distillates” designed to represent distillates that would be produced by the complete re-
covery of various amounts of peel oil from 100-ml. samples of juice, by dissolving 1 ml. of distilled grapefruit oil in enough acetone to make 100 ml. Aliquots containing 0.5, 1.0, 2.0, 3.0, 4.0, and 5.0 ml. of this solution were then mixed with enough acetone t o make a total of 25 ml. of acetone solution and made up to 50 ml. with distilled water. These samples correspond to distillates from the complete recovery of oil from 100-ml. samples of juice containing 0.005,0.010,0.020,0.030,0.040, and 0.050’% oil. The optical densities of turbid solutions made by mixing 10 ml. of distilled R-ater with 5-ml. aliquots of the above 50-ml. samples were then measured in the photoelectric colorimeter, using the blue filter. A blank containing the appropriate amounts of acetone and water was used to adjust the colorimeter to zero optical density. Curve 2 was obtained by distilling 100-ml. samples of water with 25 ml. of acetone and 0.005, 0.010, 0.020, 0.030, 0.040, and 0.050 ml. of distilled grapefruit oil, collecting 50 ml. of distillab from each, diluting 5 ml. of these distillates with 10 ml. of distilled water, and measuring the optical densities of the resulting turbid solutions. I t is readily seen from these results that only a proportional amount of oil is distilled. This is also true for other steam-distillation methods. Scott ( I ) recovered only 77% of added peel oil, whereas these results show a recovery of about 83%. The results given are typical of those obtained with distilled orange oils, diatilled lemon oils, cold pressed orange and grapefruit oils, and d-limonene. Effect of Dilution on Turbidity. To check the effect of dilution on the turbidity, a (‘synthetic distillate” was made by adding
Table 111. Effect of Wave Length on Absorption Filter Violet Blue Blue-green Yellow-green Orange Red
Wave Length
Optical Density X IO
370 420 490 530 580 650
1.65 2.90 1.60 1.45 1.15 0.95
V O L U M E 20, NO. 6, J U N E 1 9 4 8
541
I: d W 5 4
1
g21
O3-1
a
0 1
0 ; 0
I
I
.01
L
1
.02 .03 .04
1
I
.05
% CITRUS OIL
Effect of Wave Length on Absorption. The blue (420) filter, as would be expected, gave the highest absorption. Table I11 s h o d the effect of various filters on the optical density of a typical acetone-oil-water emulsion. Effect of Temperature on Turbidity. To determine the effect of temperature on the turbidity produced when the distillates are diluted with water, a series of acetone-oil-water solutions containing known amounts of oil was distilled and the distillates were collected. Before dilution with water the distillates were brought to the desired temperature in a constant-temperature bath, then diluted with water having the same temperature. Much difficulty was encountered in making the optical density measurements a t the low temperatures because of condensation on the colorimetric test tubes. The results of the optical density measurements are shown in Figure 3. Here again, the solubility of citrus oil in the acetone-water solution appears t o be the controlling factor. As expected, an increase in temperature causes a decrease in optical density. Slight variations due to temperature are insufficient to necessitate correction for routine analyses.
Figure 3. Effect of Temperature on Optical Densities of Citrus Oil, Acetone, and Water Emulsions 0.040 ml. of distilled grapefruit oil t o 50 ml. of acetone and making
up t o 100 ml. with water. Turbid solutions were then made by diluting 5-ml. aliquots of this synthetic distillate with various amounts of water. Table I1 shows the optical densities of these turbid solutions, the per cent concentration of oil, and the optical densities per unit concentration. These results indicate that a maximum optical density was reached when only 5 mi. of water were used to dilute 5 ml. of distillate, but to ensure near maximum turbidity per unit concentration of oil it is advisable to use 10 ml. of water as diluent. By adequate dilution it is possible to obtain all readings in a zone of near constant optical density per unit Concentration.
SUMMARY
A rapid method for the estimation of peel oil in citrus juices, based on distillation of the sample with acetone, gives consistent and reproducible results comparable to those obtained with the time-consuming modified Clevenger method. Complete determinations can be made in about 7 minutes on a sample of only 100 ml. LITERATURE CITED
(1) Burdick, E. M., and Scott, W.C., unpublished. (2) Scott, W. C., J . Assoc. Oficial Agr. Chem., 24, 165-70 (1941). (3) U.S. Dept. Agriculture, U. S. Standards for Grades of Canned Grapefruit Juice, Production and Marketing Administration. RECEIVEDOctober 31. 1947.
Polarographic and Amperometric Determination of Barium I. 31. KOLTHOFF AND H. P. GREGOR, School of Chemistry, L'niuersity of Minnesota, Minneapolis, M i n n . The polarographic waves of barium in lithium chloride, calcium chloride, and magnesium chloride were studied. Calcium chloride was found to be the best supporting electrolyte for the measurement of the diffusion current of the barium ion, no maximum in the wave being observed in the presence of calcium. With magnesium chloride, diffusion currents were high and a special correction was necessary. The amperometric titration of 0.001 M barium with chromate cannot be carried out conveniently in aqueous medium. In 20,30,and 5070 ethanol the titration can be carried out rapidly with an accuracy of the order of 370.
P
OLAROGRAPHIC studies on barium were first made by Heyrovsk9 and Berezicky (I), who found a well defined wave with: a pronounced maximum in 0.1 N lithium chloride. The amperometric determination of barium with sulfate was also performed, but poor accuracy was found. The polarographic behavior of barium in aqueous and ethanolwater mixtures using tetraethylammonium iodide as supporting electrolyte was studied by Zlotowski and Kolthoff ( 6 ) . The half-wave potential in water us. the saturated calomel electrode was found t o be -1.94 volts, and the waves satisfied the Ilkoviii equation. POLAROGRAPHIC WAVES OF BARIUM IN LITHIUM, CALCIUM, AND MAGNESIUM CHLORIDES
The current-voltage curves were measured with a manual apparatus. The eonventional H-shaped cell was used and all
cathode potentials were measured against the saturated calomel electrode. The experiments were carried out in a thermostat a t 25" C. Analytical reagent chemicals were used throughout and the ethanol was redistilled. The capillary, unless otherwise specified, had an initial drop time of 3.6 seconds in 0.1 Ar lithuim chloride with a disconnected electric circuit. The weight of mercury flowing from the capillary per second under these conditions was 1.002 mg. per second. The height of the mercury column was 20 cm. Current-voltage curves of barium in 0.1 M lithium chloride show a pronounced maximum which cannot be suppressed by thymol, methyl red, methylene blue, or gelatin. These maxima extend over but a limited voltage range. Although no distinct diffusion current region is observed (see Figure 1) the diffusion current (corrected for the residual current) measured a t the minimum was found to be proportional to the concentration as shown in Table I.
