1218
INDUSTRIAL AND ENGINEERING CHEMlSTRY
which a certain state of the system will exist. They should be particularly useful in determining the direction of change or changes necessary when a certain state of the system is not satisfactory.
Vol. 24, No. 11
LITERATURE CITED (1) PooIe, J. n7., IND.ESG. CHEM.,21, io98 (1929). (2) Poole, J. W., and collaborators, Ibid., 23, 170 (1931). RECEIVED Xay 6, 1932.
Sterilization of Fruit Juices by Filtration.
I
D. C. CARPENTER, C. S. PEDERSON, AKD W. F. WALSH New York S t a t e Agricultural Experiment Station, Geneva, N. Y.
T
HE sterilization of fruit juices and beverages on a commercial scale has become increasingly important as the fruit juice industry has developed during the past decade. Pasteurization has been relied upon in the majority of cases for the killing of organisms and prevention of spoilage. I n many cases, however, the pasteurization of bottled or canned juices has resulted in the precipitation of unsightly deposits that render the product far from attractive to the consumer. I n attempting to minimize or avoid this precipitation, prepasteurization a t a temperature higher than that used in the final stages of the process, followed by filtration, has frequently been resorted to with varying degrees of success. Contradictorv claims have been made of the destructive
had a detrimental effect on the flavor. As to pasteurization in a closed container, theresults of Carpenter and Walsh (4) have shownthatwithawiderangeof individuals it was almost entirely a matter of guesswork to attempt to identify which Of two fruit juice samples, identical in other respects, had been pasteurized. It is obvious a t
This would assure the manufacturer a sparkling clear juice in a sterile condition, which would remain in that condition until the package was opened. Such procedure would obviate the losses by breakage during pasteurization in glass containers The possible advantages offered by the so-called cold sterilization of fruit juices and beverages led to the present investigation of the commercial feasibility of sterilization by means of filtration. The pioneer concern in the manufacture of commercial sterilizing filters is the SeitzWerke of Kreuznach, Germany, and it has been with the Seitz germ-proofing filter that the experiments, reported in part below, were undertaken to obtain data on conditions of operation from both a bacteriological and a physicochemical standpoint.
Sterilization of fruit juices by means Seitz germ-proojng Jiller is possible proper operating conditions. A higher Jiltration is pointed out to be desirable mercial operation. Remota1 of organisms from liquids by Jiltration is deJinitely not attributable to siel'e action O f the filter*
Fruit juices are usually quite cloudy from colloidal material and cell debris, and r e q u i r e a prefiltration through FilterCel or other filter aid in an o r d i n a r y filter press before being s u p p l i e d to the Seitz filter. O t h e r w i s e the filter disks become so .th slimy plant tissue m a t e r i a 1 that filtration will practically
cdated
November, 19S2
i Nu
u ST
ii I A 1.
A N L,
E N G I p: 12 1s II I N (;
(: 11 E M i S T R Y
1219
would no doubt be iiiipwtical and, for Sruic juices, u~~neces- such repetitioii as found uunecessary. About tifty runs sary. The bottles and croirns rwuld probi~blybe sterilized were made during the experiments on the Seitz filter with the by steam, and the filling machinery vould be capable of three juices tested-~namely, sweet cider, currant, and g r a p e handling 500 to 600 gallons (18% to 2274 liters) per hour. and approximately seventy samples were taken on the averIt is obvious that a Seitz filter of col~espondingdelivery age per run, so that the data here recorded reprevent only a small part of the entire data collected. capacit,ywould be required for plant operations. Duplicate organThe room i n ism c o u n t s were which the filtration made by the agar experiment,s were plate method using m a d e was conB media composed structed of m~terof 0.25 p e r c e n t proofed cement and yeast e x t r a c t , 0.5 contained the usual per cent p e p t o n e , pasteurizers, filter 1 per cent glucose, presses, carbonatiiig and 1 per ccnt agar, a n d bottling mastandardized to a chinery, etc., in& pH of 6.6 to 6.8. dent to tlie processP l a t e s were i n ing of beverage macubated for 2 days terials. A portion a t 32' C. T e s t of t h i s r o o m i s t u b e s a m p l e s , as s h o w n in F i g u r e well a s b o t t l e d 1. saniples of the juice, I n t h e experiwere incubated for ments, as soon as 2 w e e k s a t 32" C. t h e juice required With acid juices of for the d a y ' s r u n this charaeter cerhad been p r e f i l tain organisms will t e r e d , t h e entire grow on the plates cement-lined room t h a t will n o t dewas t h o r o u g h l y velop in the filtered s p r a y e d w i t h 0.5 juice; on the other oer cent sodium hand. the tube hypochlorite sol u tion. The filter disks were then placed in the Seitz filter and bottle saniples, because of their larger volumes, will and given a preliminary tightening into ~ilace\with the thumb show the presence of an occasional organism in the filnuts. and the filter and delivery assembly m r e given a trate when the plate sample shorn a zero count. In general, thorough steaming for at least 10 minutcs with a slow currcrrt the bottled sumplcs arc to be considered a more certain of dry steam; meanwhile, the thumb nuts \yere gradually criterion of keeping quality than the actual organism count. tightened. During this time interval sterile bottles and test tubes plugged and autoclaved at a pressure of 15 pounds per INELUENCE o r OI'EI~ATINU P~LESSIJRE ON STERILIZATION square inch (1 kg. per sq. em.) for 30 minutes, were numbered OF ClDEE and placed in position where they would be readily accessible Assuming that it is possible to reniove organisms comto receive the filtered juice. After complcting the stcriliaation of the filter itself, the hose delivering juice from the pletely by filtration, it does not necessarily follow that this pump i ~ a attached s to the inlet on tlie filter, and the filtration removal may be successfully accomplished a t any and all was begun. At intervals stated in the talrles, samples were operating pressures. Experiments were therefore made with taken for bacteria count, and bottles were collected immedi- varying pressures, increased by successive steps as the 6 h a ately thereafter for laboratory records and incubation. In tion progressed, to ascertain what operating pressures were many runs the bottled samples were taken in triplicate, but permissible. It is obvious that, in practice, commercial
h*MYL.I
1 2 3 4 i 0 7 8
5
?L
lo 11 12
After 12 gallons (45.30 literal 15.0 LO.% 0.U14 0.053 A f t e r 14 girlions (52.92 liters) .. ... ... After 15 aallons (56.70 liten) ... .4fter 15.5 gd10n8 (68.69l!te+ 2i:u 1.646 0:oiz 0:045 A l ~ e r15.76 p s l l r m (59.580 litern) .. ... 17 After 15.9 gallons (00.102 literr) .. . . . ... 18 Y Plus sip" indiosfes bacterial oontaminstion of iilirata; rninurj sign indi+cs 18" bacterial oontarninatiue. b Spoilage *itor 10 daye i n bottled ~ ~ r n p l8 e sand 9; ~ p o i l a g eaftor 2 days ID bottled ~ainplea10 t o 18.
13
14
15 16
...
...
19-16 20-14
... ...
32-87 104-68 28-24
4-13
++ +++ +
+ +++ ++
+ +++ ++
1220
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 24, No. 11
TABLE11. CAPACITY TEST ox SEITZFILTERDISKS (Conducted with four disks) SAMPLE
CONDITlOh. OF FILTRATE
1 2 3 4 5 6 7
Initial cider entering Seitz filter Start After 2.5 gallons ( 9.45 liters) After 5 gallons ( 18.90liters) After 7.5 gallons ( 28.35 liters) After 10 gallons ( 37.80 liters) After 12.5 gallons ( 47.25 liters) .4fter 15 gallons 56.70 liters) After 17.5 gallons 66.15 liters)
8
9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34
P U V PP R E S S ~ R E Lb./sq. in. K g . / a q . cm.
...
7.1 ...
... ... ... ...
...
...
... ,..
8.5 ,..
...
...
7.6 8.5
...
... ...
... After 57.5 gallons (217.35liters) After 60 gallons (226.80liters) After 62.5 gallons (236.25 liters) After 65 gallons (245.70 liters) After 67.5 gallons (255.15 liters) After 70 gallons (264.60Ijters) h f t e r 72.5 gallons (274.05liters) .1fter 75 gallons (283.50liters) .4fter 77.5 gallons (292.95liters) After 80 gallons (302,40liters)
0:499
...
... ...
PER
MXN.PER DISK
Gal.
... 0:634
... ... ... 0.597 ...
...
0:534 0.597 ... ... ... . .
0.0278 0,0298 0,0142 o.oi2n O.OllT, 0.0109 0.0104 0.0104 0.0107 0.0104 0.0095 0.0094 0.0085 0.0095 o.0086 0.0083 0.0103 0.0104 0.0132 0.0118 0,0107 0.0101 0.0100 0,0095 0,0094 0.0089 0.0088 0.0086
,..
0,0080
, . .
0.0086 0.0086 0.0090 0.0084
... ...
...
filtration would be made a t the highest operating pressure, yielding a sterile product so as to give a maximum volume of output per day, and for this reason the determination of the critical pressure is important. As a check on the steam sterilization of the assembled Seitz filter and disks, recently boiled distilled water was filtered through the apparatus to show that the filter chamber was sterile before the entrance of the juice under experiment. I n Table I are recorded data of a typical run showing the influence of operating pressure on the sterility of filtered cider. The initial cider purposely had about twenty times as many organisms per cubic centimeter as would generally be encountered in reasonably sanitary commercial practice in cider plants, so as to make the test on the filter a severe one. It will be seen from the data that satisfactory results are obtained a t operating pressures up to 10.5 pounds per square inch (0.74 kg. per sq. cm.), and that beyond this point spoilage occurs regularly. Other experiments shorn that the maximum safe operating pressure for the sterile filtration of cider is close to 10 pounds per square inch (0.7 kg. per sq. cm.).
CAPACITY OF SEITZFILTER DISKS Having established that no greater than 10 pounds pump pressure was permissible if a sterile cider filtrate was to be obtained, tests were conducted to ascertain how long the filter disk would continue to give a sterile product when operated below the critical pressure. In Table I1 are recorded one set of results of this series of tests. Owing to the long duration of the run, the experiment was interrupted 6 hours between samples 18 and 19 so that the operators could rest, and the run was resumed again next day. I n the meantime, the outlet tube leading from the filter was plugged with sterile cotton. The organism counts in samples 5, 6, and 23 of this series indicate contamination of the filtrate, but isolation and examination of the organisms showed that they were species foreign to cider, such as micrococci and spore formers, and were therefore to be considered accidental contaminations.
ORGANISM COUNT P E R CC.
G R O w r H IN:
Incubated tube
Bottled sample
-
-
Liter
....
...
7.6
FILTRATE DELIVERED
o:io5 0.113 0.054 n n4.s 0.042 0.041 0.039 0.039 0.040 0.039 0,036 0.035 0.032 0.036 0.032 0.031 0.039 0.039 0,050 0.045 0.040 0.038 0.038 0.036 0,035 0.034 0.033 0.032 0.030 0.032 0,032 0.034 0.032
125,000 0-1 0-0 0-0
- _
n-1
0-1 0-0 0-0 0-0 0-0 0-0 0-0 0-0 0-0 0-0 0-0 0-0 0-0 0-0 0-0 0-0
0-0 1-2 0-0 0-0
0-0 0-0 0-0 0-0 0-0 0-0 0-0 0-0 0-0
+
-
-
-
I t has often been reported that bacteria “grow through” a filter because of their reproductive processes. This would be a serious drawback t o sterilization by filtration, and it could no doubt occur with the Seitz disks if their use was continued for seyeral days. There is no evidence in any of the experiments, the duration of which has several times exceeded 7 2 hours, that any organisms have passed through the filter either because of failure of the filter or of growing through of the organisms. The thickness of the filter disk itself no doubt operates against this factor’s becoming troublesome in commercial filtration. The sterile filtering qualities of the Seitz filter disks have not been exhausted in any case a t pressures below the critical operating pressure, although 20 gallons (75.8 liters) of cider per disk have been filtered. The filtration rate toward the end of the run is not as high as is desirable, but it appears inevitable that the filter disk will plug more and more until the rate of filtration is no longer commercially feasible. The average rate of delivery over the entire run shown in Table 11 was 0.0104 gallon (0.0393 liter) per minute per disk. This is a rate of 0.0154 gallon per square foot of filtering surface per minute. This figure could no doubt have been increased by 10 per cent with safety if the pump pressure a t the beginning of the experiment had been somewhat higher. I n the experience of the authors it does not appear probable that, on large volumes of cider, any greater average rate than 0.012 gallon per minute per disk (0.0178 gallon per minute per square foot) can be expected if the filtrate is to be sterile. STERILIZATIOS OF GRAPEJUICE BY FILTRATIOS In Table I11 one of the experiments on the sterilization of grape juice by filtration is recorded in which the pump pressure is varied. It is apparent from the data that grape juice is easily sterilized by the Seitz filter. For a given pressure the rate of flow is two to five times as fast as with cider. In fact, the rate of flow is so fast and the sterilization so thorough that in no experiment have head pressures of sufficient magnitude been obtained to give nonsterile grape juice. In certain other experiments not recorded here, a sterile filtrate has been
INDUSTRIAL AND ENGINEERING CHEMISTRY
1222
From Poiseuille’s law for the flow of liquids in capillaries is derived the expression: KnPD4T L = quantity of liquid passing through capillaries = number of capillaries = diameter of capillary = length of capillary = time = pressure = constant for liquid in question Q=-
where Q n
D L T P K
From this equation it is obvious that the amount of liquid passing through the smaller capillaries a t a given pressure is of small magnitude as compared with that passing the larger capillaries, even though there are ten times as many of the former as of the latter. In column 4 of Table IV the percentage of the filtrate carried by capillaries of the various sizes has been calculated, taking into account the number of capillaries of a given size per square centimeter of disk and with the assumption that all of the capillaries are of the same length From these data it is concluded that about 94 per cent of the filtrate passes through capillary pores that are 40p or larger in diameter. It is through these larger pores that organisms will most likely be swept by the filtering stream of liquid, if they are to pass through the filter a t all.
SIZEOF ORGANISMS The sizes of the more prevalent types of organisms common to cider and grape juice have been determined by actual measurement under the microscope. Yeasts represent a large majority of the organisms ordinarily present. The size measurements were made with a screw micrometer eyepiece on living organisms that had been isolated and gro1T-n in pure culture in a broth of yeast extract and peptone. Measurements were made on cultures 2 days old. In Table V are recorded size measurements, giving not only the average dimensions of the type of organism, but the extremes encountered in measuring hundreds of single organisms of a given type. TABLE V. SIZE OF ORGANISMSIN CIDERAND GRAPEJUICE ORQANISM Small yeast Large yeast Long s e a s t Round yeast Acetobacter sp.
4 v . LENGTH Av. D14x. Mzcrons Macrons 1.7 1.2 2.7 1.2 10.0 1.3 3.5 4.0 1.5 0.45
Faced with the foregoing facts, it is quite impossible to hold that the filtration of organisms from a fluid can be sieve action. Present results substantiate those of Bechhold ( 2 ) who recorded successful filtration when the pore diameter values were eight to fifteen times as large as the longest dimension of the organisms with which he worked.
COMPOSITION OF FILTER DISKSAND DISTRIBUTION OF ORGANISMS The Seitz filter disk has been examined as to chemical composition and found to be composed of 62.3 per cent paper pulp and 37.7 per cent asbestos. From the stratified appearance of the filter disk, it appears to be built up in layers. The layers from front to back of the filter disk each contain paper pulp and asbestos in about the same proportion as shown by Table VI, so that the distribution of the two materials is no doubt intended to be uniform. The type of asbestos present was identified by the refractive index method as chrysotile, with refractive index 1.54. The total thickness of the entire disk is about 3 mm. TABLEVI. ASBESTOSCOXTESTOF VARIOUSSECTIONS OF SEITZDISKB SECTION
CONTEXT SECTION
COBTENT
%
70
4 1 (front) 39.93 38.29 2 41.56 5 (oontaining cloth backing) 30.57 3 37.17 Mean asbestos content 37.5 Samp!es taken by ?pl!tting d:sk 15 ith razor. Sections 1 t o 5 running irom ?(,ugh :front, side t o clxh-backed (back, side in numerical order: 3 ~ m p l e e dried 12 LJur. a: 101-11l3~C.. and sibes: ,3 es:imated b y ash deterinination.
With several such layers comprising the filter disk, it \vas of interest to find which layer was holding the organisms. Sections of several disks that had been used on filter runs were dissected and dispersed in sterile water, 1.5 per cent urea, and 5 per cent sugar solutions, and the number of organisms present determined by the usual plating method. In order that the dispersed samples might be reduced to a common basis for comparison, aliquots were evaporated to dryness and weighed so that a fairly accurate weight of the filter associated with the respective organism counts was obtained. TABLEVII. DISTRIBCTIOS O F ORGANISMS FILTERD I S K S ~
E X T R E M EISN hlEAsURElnENT8 R E C O R D E D Length Diam. Mzcrons Macrons DISPERSION 1.0-2.5 1.0-1.5 DISK MEDIUM 1.5-4 1.0-1.5 4.5-16 1 . O-1 . 5 1 Water 0 : 8-i. 4 0:35-0.55
Table V shows that in no case does the extreme dimension of even the “long” yeast cell approach the magnitude of the diameter of the capillaries in the filter disk that are transporting over 90 per cent of the filtrate. In the small, large, and long yeast cells having practically the same diameter, the efficiency of removal of organisms by the filter is entirely independent of the length of organism. Under the proper operating pressures, organisms are removed completely in passing through the filter, no matter whether the diameter of the capillary they pass through is five or thirty times as large as the greatest dimension of the organism. With some of the organisms there is a tendency for the formation of long chains and clumps of cells. These chains, it might be supposed, would most certainly be caught in passing through an irregular capillary of the filter. While this is perhaps worth consideration in the case of gravityfed filters, the formation of chains and clumps of organisms has little or no bearing on pressure filtration where any existing chains or clumps will be broken up thoroughly when the fluid passes through the pump before it reaches the filter.
Vol. 24, No. 11
WEIQHTOF SEC- DISK USED TION IN C O U N T Gram 1 0.121 2 0.200 3 n.172 4 5
THROUGH
SEITZ
C.4LCD. ORGANISM C O U N T PER
ORQANISM COUNT Thousands 587,000 2.900
GRAM DISK Thousands 4,850,000 14.500
5% sugar s o h .
1 0.198 1,186,000 5,990,000 2 0.110 9,200 83,600 3 0.185 4,500 24,300 4 0.152 18 118 2,500 16,150 5 0.155 a T h e filter disks used in these studies were from a capacity run on cider in which t h e disks finally became so plugged t h a t t h e filtration r a t e dropped t o less t h a n 100 cc. per hour. T h e filtrate was a t all times sterile. Section designated a s 1 denotes intake side of filter dlsk. 2
I n Table VI1 are recorded typical results on the distribution of organisms through Seitz filter disks. Some difficulty was experienced in dispersing the sectioned disk in a medium that would permit accurate sampling for the organism count. The tendency is for the disk material to mat together and not disperse evenly throughout the medium. I n spite of
November, 1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
this difficulty the data show that practically all the organisms are caught on the intake side of the disk, and relatively few organisms are carried any appreciable depth into the capillaries. I n certain other experiments, in which two different organisms (for instance, yeast and acetobacter) were present in the juice, any. segregation of the organisms in different portions of the disk has been looked for, but no segregation due either to size or other properties has been demonstrable. It is obvious that the intake side of the disk completely plugs with organisms and colloidal material long before the filter layers deeper in the disk are incapacitated. Because of this fact, a thinner filter disk would no doubt give as satisfactory service and a t the same time reduce the cost of the filtering operation. COI~CLUSIOKS It is quite possible to sterilize fruit juices by filtration through the commercial-size Seitz germ-proofing filter and to obtain a sparkling clear product if proper operating conditions are observed. These conditions are as follows: (1) sterilization of the assembled filter and disks by steam, (2).sterilization of containers receiving the filtered juice, ( 3 )
1223
sterilization of caps or seals, (4) proper attention to pump pressure, and (5) general sanitary room conditions. The rate of filtration through the Seitz filter disk is somewhat slower than is desirable for American commercial practice, and improvement along this line is to be sought. ACKNOWLEDGMENT The writers acknowledge their indebtedness to Philipp Wirth, Inc., of New York, for placing a t their disposal an allmetal Seitz germ-proofing filter, size 30/8, for these experiments. LITERATURE CITED Bartell and Carpenter, J . Phgi. Chem., 27, 252 (1923). Bechhold, Z. Hug. Infektionskrankh., 112, 413 (1931). Bigelow and Bartell, J . Am. Chem. Soc., 31, 1194 (1909). Carpenter and Walsh, N. Y . State Expt. Sta., Tech. Bull. 202 (1932). (5) Kohman, Eddy, and co-workers, IND.ENG. CHEM.,15, 273 (1923); 16, 52, 1261 (1924); 22, 1015 (1930); Natl. Canners hssoc., Bull. 19L (1924). RECEIVED July 15, 1932.
Stream :Pollution by Irrigation Residues C.
S. SCOFIELD, United
States Department of Agriculture, Washington, D. C.
T
HE water supplies for many of our irrigated areas are obtained by diverting from stream channels all or a portion of the stream flow. The irrigation water is spread over the land where part of it is absorbed by crop plants or lost by evaporation, the remainder being returned to the stream channel as drainage. This return drainage contains the major portion of the dissolved salts carried by the irrigation water, with the result that successive (diversions from a stream cause a progressive increase in the salinity of its water in the downstream direction. COSDITIOXSI I ~THE RIO GRANDEREGION Observations on the Rio Grande in the vicinity of El Paso, Texas, during 1931 show that a t four points below Elephant Butte Reservoir the average salt content of the stream expressed as parts per million was as follows: At Leasburg 610, a t El Paso 956, a t Fabens 1500, a t Fort Quitman 2206. The annual discharge of the Rio Grande a t Leasburg is, approsimately 750,000 acre-feet of water containing approximately 650,000 tons of dissolved salts. At Fort Quitman the annual discharge is approximately 200,000 acre-feet of water containing approximately 650,000 tons of salt. These figures show that, while the irrigated lands in the vicinity of El Paso consume annually about 550,000 acre-feet of water, the total salt burden of the stream through that section is not diminished. This is manifestly as it should be if we have in view the sustained productivity of these irrigated lands. Were the salt balance conditions of this area such that the annual salt outflow was substantially less than the salt inflow, it would be evident that there was occurring an accumulation of salt in the irrigated land to its ultimate detriment for crop production. The investigation here reported has shown that, while the annual tonnage of salts leaving the project a t Fort Quitman is substantially the same as that entering the project a t Leasburg, the relative proportions of the salt constituents are very different a t the two points. The analyses included three
anions-bicarbonate, chloride, and sulfate--and two cationscalcium and magnesium-with the alkali bases determined by difference. With respect to these constituents the annual summaries shorn that for bicarbonates the inflow was 100,000 tons, with the outflow 37,000 tons; for chlorides the inflow was 70,000, the outflow was 234,000 tons; and for sulfates the inflow was 240,000 and the outflow 138,000 tons. Thus for the anions, the bicarbonates and sulfates were retained in the project lands, whereas there was a very large net r e lease of chlorides. With respect to the cations, the inflow of calcium was 80,000 tons, the outflow 50,000 tons; for magnesium the inflow was 16,000, the outflow 12,000 tons; and for the alkali bases (chiefly sodium), the inflow was 111,000 and the outflow 165,000 tons. Thus it appears that the salts of low solubility (chiefly calcium carbonate and calcium sulfate) were precipitated in the irrigated soil, whereas a substantial quantity of sodium chloride was removed from the soil and carried away in the drainage water. This represents a favorable salt balance for the irrigated land, but is also a striking example of stream pollution by irrigation residues. The salt concentration of the Rio Grancle as it passes Fort Quitnian is too high to be safe for irrigation use. Fortunately the streams joining the river below that point, chiefly from the hIexican side, contribute large volumes of water of low salinity, so that the mater as diverted a t Roma and below for use in the area above Brownsville is not too saline for safe use where adequate drainage is provided. CONDITIONS
I N THE
COLORADO BASIN
The Colorado is a larger stream than the Rio Grande with a much larger salt burden. It has no important tributaries below the Grand Canyon. Observations reported by Howard for the Grand Canyon gaging station show that for the five years ending in 1930 the mean annual discharge of the river a t that point has been 15,700,000 acre-feet of water, carrying 12,000,000 tons of dissolved solids. It is believed that a substantial part of this salt burden is contributed by the drainage
