Determination of Air in Citrus Juices H. J. LOEFFLER Food Research Division, Bureau of Agricultural Chemistry and Engineering, P. S. Department of .4griculture, Los Angeles. Calif.
D
due to the capillary unci the sealed joint must be determined by calibration. Microburet units fitted with the three-way capillary stopcocks are available commercially. E and G may be of soft glass and connected by rubber tubing a t I to the Pyrex unit,
URISG the course of an investigation on the bottling of citrus juices, it was necessary t o measure the degree
of deaeration of the juices prior t o pasteurization. T h e literature contains few references t o such determinations. Pulley and von Loesecke ( 2 ) describe a n apparatus for this purpose, but the length of time needed made i t unsatisfactory for the author's purposes. Accordingly a procedure lias been developed by which an air determination and differential analysis can be made in 10 t o 15 minutes.
P-A.
The method involves drawing about 100 cc. of juice into the evacuation chamber; evacuating the chamber twice to remove most of the air from the juice; and warming the sample by drawing in a needle-fine stream of hot boiled water, followed by two further evacuations of the warmed juice. After each evacuation, the air released is drawn over into buret G where it is measured. K h e n evacuation and total measurement are completed, colutions of potassium hydroxide and pyrogallic acid are d r a n n down the sides of G t o measure the carbon dioxide, oxpeen a n d incrt gases.
Apparatus for Measuring Gas The apparatus, shown in Figures 1 and 2 , consists of a mercuryfilled evacuation chamber, A , fitted with a drain to a leveling bulb, B, which can be supported in three positions. I n the high position the base of B is level v i t h the top of A ; in the intermediate position it is 40 cm. (16 inches) below the base of A; in the low position it is 82.5 cm. (33 inches) below the base of A . C is a capillary three-way, mercury-sealed stopcock, E is also a capillary three-way stopcock, while G is a 5-cc. microburet connected with the mercury-filled leveling bulb, H . The unit can be built quickly and cheaply. A and B are 1liter Kjeldahl flasks with short glass tubes fused into the base and connected by rubber pressure tubing. The stem of A is drawn down to a small tube and fused to the capillary outlet from C, the capillary tube having been first blown out to facilitate the sealing. A 5-cc. graduated pipet may be fused to the capillary tube from E to provide the microburet. The volume correction
All air must first he removed from the system. This is accomplished by raising B to the high position and opening C to connect A and D a n d sweep out any air through D, which remains filled with mercury. C and E are changed to connect A and G and sweep any air from I into G . E is then changed to connect G and J . H is raised to sweep out any air in G through J . E is then closed. Air bubbles trapped in the mercury in A or in the rubber tubing between A and C are removed by closing C and lowering B to the low position. The p r e s s u r e i n A becomes sufficiently low so that the mercury drains almost completely into B, and any entrapped air will rise into A . B is raised t o the interFIGITHE 2 mediate position until A refills: then to the high position; and C is opened to D to s\veep out any air. The process can be repeated if necessary to prove the presence of a zero blank, but if B is left a t the intermediate position between determinations no air remains trapped and this evacuation is unnecessary. The juice or liquid to be tested is so placed that D is below its surface. B is then placed in the intermediate position and C opened between A and D until about 100 cc. of liquid are drawn in. A calibration mark on A indicates the approximate position. C is then closed and B placed in the low position until the mercury drains from A into B . When only slightly over a liter of mercury is used and the bulbs are maintained at the positions stated, the mercury will not overflow from B; nor is it likely to go below the base of A . If it does, the addition of a little mercury to B will raise the column to the base of A. B is then raised to the intermediate po3ition so that the mercury refills A and the air removed during the evacuation collects in the capillary above.
'i FIGURE 1. GAS ASALYSISAPP.4RATUS 533
INDUSTRIAL AND ENGINEERING CHEMISTRY
534 ~~
TABLE I. EFFICIENCY OF DEAER.4TIOK Sample Code No.
No. of Deaerations
Air Content
cc.
10
1
2.98
16 19 24 30 35 37 39 46
1 2
3 2 1 1
1.57 0.68 0.65 0.87 0.44 0.47 1.53 0.78
52
3 2
0.29 1.14
57
2
0.46
61
3
0.26
3 2 ~
0 0 0
49
?2p
2 2
COa Cc. 1.79 0 85 0 24
01
cc.
Nn
Remarks
cc.
0 . 2 7 0 9.3 Very rapid deaeration
0 0 74 0 045 0 40
Slow deaeration
Rapid deaeration >Ion. deaeration Slow deaeration
is raised to the high position; C is opened to connect with I ; H is placed slightly below I ; and E is opened cautiously to pass just the released air from A into I . If a small amount of juice comes with the air into I , itwill not cause trouble. C is then closed also, B is lowered, and the evacuation and removal of air are repeated. B is then lowered only to the intermediate position. One hundred cubic centimeters of distilled water are heated to boiling in an Erlenmeyer flask which is then lifted around D. C is just cracked, to draw the boiling water in on the juice in a needle-fine stream. C is turned off, so that no air can be draim in after the hot water. It is advisable t o leave a few cubic centimeters of water in the flask. B is placed in the lorn position and kept there until boiling ceases in A , which usually takes less than 10 minutes. B is again raised and the released air is drawn into G . B is lowered for one more evacuation, but no more than traces of air are usually released. If a measurable quantity is obtained, the air is passed into G and the sample is evacuated once more as an added precaution. B is then raised to the high position and the juice sample is discharged through D into a graduated cylinder for measurement. The volume of juice used is the difference between this quantity and the amount of hot water added. With B still in the high position, C is changed to connect with G, and E is opened to I , so that mercury from A will sweep the air from I into G . E is closed; the levels in H and G are balanced; and the quantity of stir in G is determined. J is dipped below the surface of a solution of 35 per cent potassium hydroxide, H is lowered slightly, and E is opened to draw 0.5 to 1 cc. of the basic solution in throu h G. The solution flows in a film down the sides of G, absorbing a f t h e carbon dioxide in the sample almost instantaneously. After a few seconds for draining, the levels are balanced and the quantities of gases remaining are determined. The difference is carbon dioxide. H is again lowered and 0.5 to 1 cc. of alkaline pyrogallol (1 gram of pyrogallic acid to 24 cc. of the potassium hydroxide solution) is drawn in on the sample. The decrease in volume measures the oxygen content; about a minute is required for this reaction. The residual gas is primarily nitrogen. The gas plus absorbing solutions is discharged through J, the buret is rinsed with weak nitric acid, and the apparatus is ready for the next determination. The gas volumes are corrected from room t.emperature and pressure to standard conditions and listed as “cc. of gas per 100 cc. of juice”. An unwieldy feature of the method is the lifting of the liter flask filled with mercury from the low to the intermediate position. When this is difficult, a ratchet and gear or small pulley lift can be used. €3
Discussion of Method The use of capillary tubes and a microburet makes possible the measurement of a small amount of gas and consequently t h e use of a relatively small sample of juice. The moderate sized samples, combined with the rapid direct heating, high degree of evacuation, and agitation from the mercury flow facilitate the rapid removal of both dissolved and occluded air. A 50-cc. sample is satisfactory for nondeaerated juice.
VOL. 12, NO. 9
Since the volume of gas is small, a rapid quantitative absorption is achieved by dropping the absorbing solutions on the sample. This would not be possible with larger quantities of gas. This procedure is much faster and simpler than the use of separate absorption pipets. If the diameter of the buret is too small, the reagents will not flow down the sides but mill trap the gas below.. The sample is drawn in from the bottom of the juice container, so i t is never in contact with air. Correspondingly, the juice sample is measured after deaeration rather than before to avoid incorporation of air during transfer t o and from a graduated cylinder. The adjustment of a hollow spike and packing gland on D will make possible the measurement of gas in the head space of cans and bottled where the quantity of gas is very small. Where the cans or bottles have been closed under high vacuum with a small head space, the gases dissolved in the juices will not only be larger quantities but more important ones. The author has been measuring the dissolved gases in bottled orange juice by removing the cap and immediately withdrawing a sample through D from the bottom of the bottle. The changes in the character and amount of dissolved gases in bottled orange juice during storage will be reported at a later date. Only one stopcock, C, is exposed to a large difference in pressure. KOleaks occur with the use of a mercury seal. This is demonstrated by zero blanks without a sample or when a sample of hot boiled water is used. This shows also t h a t no air is brought into the system Tvhen the hot water is added to the juice. The capillary bore is sufficiently large (1 to 2 mm.), so t h a t even partially strained juice will not clog. The tTvo halves of the apparatus are semi-independent. Thus while one sample is being analyzed in G, the next sample can be undergoing its first evacuation in A . The needle-fine stream of hot water shoots into the juice sample a t a high velocity, causing violent agitation and preventing any local overheating t h a t might cause combination between the juice and the oxygen still prescnt. The vapor pressure of water has been neglected in the calculation of results. However, the maximum error would be 0.02 cc. pcr cc. of gas. The water vapor would be measured either as carbon dioxide or inert gases; in either case, this quantity is insignificant.
Results This method has been used as a routine test of the degree of deaeration of orange juice before pasteurization. An inverted dome type of deaerator as suggested by Mottern and von Loesecke (1) was used. I n most cases, only the total air content was determined. All gas quantities are given in cubic centimeters per 100 cc. of juice at standard temperature and pressure. The results in Table I show that sufficient deaeration is rarely obtained by a single passage through the deacrator; two or three passages are essential. The efficiency of air removal appears dependent also on the rate of passage through the deaerator. The few differential analyses included indicate t h a t carbon dioxide is present in comparatively large quantities in the fresh juice, apparently arising from respiration in the oranges themselves rather than from incorporation during the reaming and screening.
Literature Cited (1) Mottern, H . H . , and von Loesecke, H . W.,Fruit Products J . , 12, No. 11, 325 (1933). (2) Pulley, G . X., and von Loesecke, H. W., IND.Esc. C H E ~ I .31, , 1275 (1939). RloPxEsENTe collaborative work between The Glass Container Association Fellow and the Fruit and Vegetable Chemistry Laboratory, Loa Angelen, Calif. Food Research Division Contribution No. 496.
