June 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
(11) Lloyd, L.E.,Petroleum Refiner, 29, No. 2, 135 (1950). (12) Lobo, W. E., Friend, L., Hashmall, F., and Zenz, F., Trans. Am. Inst. Chem. Engrs., 41, 693 (1945). (13) Myers, H., M.S. thesis, The Pennsylvania State College, 1948. (14) Myles, M., Wender, I., Orchin, M., and Feldman, J., IND.ENQ. CHEM.,43, 1452 (1951). (15) Reed, T. M., and Fenske, M. R., Ibid., 42, 654 (1950). (16) Robinson, E. S.,and Gilliland, E. R., “Elements of Fractional
Distillation,” 4th ed., New York, McGraw-Hill Co., 1950. (17) Sorel, E., “La Rectification de l’bloohol,” Paris, 1893.
1459
(18) Struck, R. T., Ph.D. thesis, The Pennsylvania State College, 1948. (19) Struck, R. T., and Kinney, C. R., IND.ENG.CHEM.,42,77(1950). (20) Timmermans, J., “Physico-Chemical Constants of Pure Organic Compounds,” New York, Elsevier Pub. Co., Inc., 1950. RECEIVED for review August 11, 1951. ACCEPTED February 6, 1952. Presented before the Division of Industrial and Engineering Chemistry at the 120th Meeting of the AMERICAN CHEMICAL SOCIETY, New York, September 1951.
E
Processing and Cooling of
; :gn
Canned Foods
ring
R O C W
development
SOME HEAT TRANSFER PROBLEMS
V. H. HEMLER, D. V. ALSTRAND, 0.F. ECKLUND,
AND
H. A. BENJAMIN
RESEARCH DEPARTMENT, A M E R I C A N C A N CO. M A Y W O O D , ILL.
T
HE discovery of the method of food preservation by heating
in a hermetically sealed container is credited t o a Frenchman, Nicholas Appert, who announced his findings less than 150 years ago. Since chemistry was only in its beginning and bacteriology was unknown, he was not able t o explain why his products did not spoil. He could not call upon the sciences, as understood today, and so he groped blindly as t o the times and temperatures required in the heating of the food products. The use of calcium chloride to raise the boiling point of water and later the invention of the pressure retort reduced spoilage in canned foods, but the causes were not understood until the science of bacteriology was applied t o canning industry problems by Prescott and Underwood in 1895 (6). These men showed that most spoilage was caused by imperfect sterilization and demonstrated the effect of higher processing temperatures on the destruction of spoilage organisms. It was originally supposed that the food in the hermetically sealed container must reach a given temperature if the food was to be made sterile. Bigelow et at. (4) in 1920 pointed out that the destruction of spoilage bacteria is dependent on a timetemperature relationship, that all temperatures in the lethal range contribute toward the sterilization of the food. The information collected on the heat resistance of organisms and on the rates of heat penetration into canned foods was analyzed mathematically by Ball (8) and culminated in a method of computing processes for canned foods which would be adequate t o destroy spoilage organisms of known heat resistance. The mechanism of heat transfer in canned foods by conduction, convection, and combinations of the two have been discussed by Jackson (8). Although not studied as extensively, the mechanism of heat transfer in vacuum-packed products was also discussed. Extended periods of exposure to heat naturally cause deterioration of the quality of canned foods. Any means of hastening the rate of heat transfer t o the slowest heating portion of the container will aid in improving the quality of the product. The use of agitation of the cans during processing to increase the rate of heating was first used in 1885 for the processing of evaporated milk. The use of agitation, either by batchwise-operated retorts containing a revolving reel or by continuous cookers, has been used for a number of years for some brine-packed vegetables and more recently for vacuum-packed products. Continuous cookers or spinner cookers which operate at atmospheric pressure are used t o shorten the processing time for fruits and acid vegetables,
When the rate of heat penetration into foods is rapid, better quality is obtained if the product is sterilized at higher temperatures for shorter periods of time. This principle is used t o greatest advantage in high-short sterilization, the most common form of which is the heating of homogeneous food products in heat exchangers with the heating time ranging from a few seconds t o a very few minutes. This method of processing has been covered in the literature and is used commercially for fruit juices and tomato juice, and also for chocolate milk and creamed soup, the latter two products requiring aseptic filling and closing conditions. After the heat process, the sealed cans must be rapidly and thoroughly cooled in order to check the deleterious action of the heat. Two methods of cooling are commonly used, air cooling and water cooling. Water cooling may be accomplished in the retort, in a cooling canal, or under sprays with or without agitation. Water cooling in the retort was studied quite extensively by Benjamin and Jackson (3) to determine the most efficient methods. Some studies on canal and spray cooling will be reported here. EFFECT OF HEAT DISTRIBUTION DURING PROCESS
With the heat penetration curve established and also the rate of changeof bacterial destruction with change in temperature known, the sterilizing effect can be computed by the method of Ball (8)in terms of equivalent minutes a t 250” F., which has been symbolized by the term F. If the change in rate of bacterial destruction occurs in multiples of 10 with a n 18” F. temperature change, which is the usual average figure, the symbol is Fo. Obviously, any condition which might tend t o change the heat penetration curve would,also tend t o change the effective sterilizing value of the process for the product in question. One condition resulting in a change in the heat penetration rate is caused by improper venting of the retort. The processing medium is then a mixture of air and steam rather than pure steam and the rate of heat transfer t o the can contents may be substantially different, With a n air-steam mixture there are likely to be variations in temperature throughout the retort; this condition has been referred t o as one of poor heat distribution. When there is little or no air mixed with the steam, temperatures throughout the retort are uniform and heat distribution is said t o be good. Numerous retort temperature distribution tests have furnished data on temperature spreads resulting from improper retort operation
1460
INDUSTRIAL AND ENGINEERING CHEMISTRY
( 7 , I $ ) , but little published information has been available for evaluating these data quantitatively in terms of the reduction in sterilizing values in the products being processed.
EXPE R IY E N TA L PROCEDURE.I n order t o evaluate t,he effects of a n air pocket in a retort, heat penetration cans were placed inside and outside a closed chamber located in a retort. Thisclosed chamber was made of galvanized sheet iron in aylindrical shape, 16 inches in diameter a n d 12 inches high. It contained adjustable vents on t h e top and bottom, so that the rate of 1 2 3 4 5 6 7 air removal from the T I M E AFTER STEAM-UP, MINUTES chamber could be varied and the temFigure 1. Average Temperature perature lag thereLag versus Time (Code 13) by controlled. Area under curve, 26.8 degree-minull-9 T h e r m ocouples located inside and outside the test cans indicated their rate of heating as well as the temperature of the atmosphere inside and outside the closed chamber. I n making a test run t h e vents on the chamber were set to a predetermined position and the cans and thermocouples set up as for a regular heat penetration test as described by Ecklund (6). Time and temperature observations were made during a steam pressure process a t 250" F. T h e can temperature data were plotted on semilog paper, the heating factors obtained, and the sterilizing values calculated hy the method of Ball (9)* The average difference in temperature in degrees Fahrenheit between t,he inside and outside oE the galvanized sheet iron chamber was plotted against time in minutes after processing temperature was reached as is illustrated in Figure 1. The temperature lag inside the chamber was determined as degree-minutes by determining the area under this curve. RESULTS.The results of the tests are shown graphically in Figure 2, most of the points representing average differences in sterilizing value of two or more cans inside and outside the closed containers. All of the tests were conducted a t a processing temperature of 250" F. and the comeup time was varied from 1.5 t o 2 minutes. The results are also shoan in tahular form in Table I. Tests were made using No. 2 and KO. 10 cans of water and S o . 2 cans of 5% Bentonite suspension. It will be seen from Figure 2 that the results of the tests with KO. 2 and KO. 10 cans of water fall on the same curve, excepting code D, n i t h reductions in sterilizing value of Po 1.2 t o 4.5 resulting from temperature lags of about 5 t o 50 degree-minutes. respectivelg. Also, when the curve is extrapolated t o zero temperature lag, a reduction in sterilizing value of Fo = 0.9 is indicated. It xould appear that this indicated reduction in sterilizing value results from differences in
Vol. 44, No. 6
rate of heating exist,ing during the comeup period. Even if the temperature of the atmosphere inside and outside the closed chamber reached processing temperature a t the same time, it is probable that, the temperature inside the closed chamber lagged far behind t,he temperature out,side the chamber for most, of the comeup period because of the slower removal of air from the closeti chamber. The reduct,ion in sterilizing value of FO = 0.9 for zero temperature lag aft'er steamup is interpreted therefore to be the result of temperature lag before retort temperature was reached. It iTould appear just,ifiable, therefore, t o subtract 0.9 from the reductions in sterilizing value resulting from teinpernt,ure ltlg .:hfter comeup only. Point E on Figure 2 was the result of a large temperature lag of shout 8 minutes' duration, while point D was the result, of a smaller temperature lag of about 16 minutes' duratiori. The latter condition appears t o have the greater influence, The tests using N o . 2 cans of 5% Bentonite suspension were intide to evaluate the effect of retort temperature lag on a conduct.ion (slow) heat8ingproduct,. .4large temperature lag resulted in only a small reduction in sterilizing value. The temperature lags are reported in degreeminutes below processing temperature (260" F.) and the lo value are in equivalent. minutes at 250" F. ( F o ) . If the t,ests had been conducted a t 240' F. approximately the same numerical figures would have been obtained, hut the temperature lag in degree-minutes would ha.ve been relat'ive to 240" F. and the loss in sterilizing value would have been in equivalent minutes at 240 F. If a thermal death-time curve slope of 18" F. is assumed (as is common practice), a sterilizing value of 1 minute a t 250" F. is equivalent to 3.6 minutes a t 240" F. Thus the r e d u d o n in sterilizing value in equivalent minutes a t 250" F. resulting from a given lag in temperature after steamup when processing a t 240" I?. n-ould be l j 3 . 6 or about' 28% of that found for a retort temperat,ure of 260 O F. Similarly, if t'he retort temperature had been 260 O inptead of 250' F., the loss in sterilizing value in equivalent minutes a t 250" F. for n specific lag would have been 3.6 times that found a t 260" F.
Test KO.
A
Cans inside Closed Chamber
Temp. Lag a f t e r Stearnuy, DegreeMinut es
No Yes
6.7
A\.. Sterilizing Value of Procras. Fu
Av. Red!iction in Sterilizing Value. PO
16.26 15.06 1.2 H S O 16.82 Yes 12 8 15,23 1,59 C N O 16.62 Yes 9.4 16.3'2 1.3 D so 16.74 TPR 11.2 10.92 5.82 E ?io 16.69 Y e.. 5 12.18 1.51 F No 16.23 13.65 Yes 2.58 C No 16.60 Yes 26. 14 00 2.60 I1 so 16.49 Yes 26.8 14 19 2.30 P R O ~ E S S I57a N G BENTOXITT: IS N o . 2 C A Y S ,80 MISVTIXA T 250' I?, ~
PROCF:SSISG WATERIT 40. 10 C.4ss, 30 ~ I I N C T E .AT Y2 j 0 ° 1'.
Sa
24 36
NO
12.3
22 67 24 46
1 69
Yes 3 2 a Initial temperature, 80' P. b Initial temperature, 180' I.'.
23 31
1 13
0"
Yes K O
June 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
This indicates the relative effect of lag in temperature at 240°, 250°, and 260" F. It does not indicate that good heat distribution is not necessary at 240' F. For example, a temperature spread after steamup of 45 degree-minutes (which would not be unusual) at a processing temperature of 240" F. would result in a in a convection heating product reduction in sterilizing value (Fo) of more than 0.8. This would amount to more than a 20y0 reduction in sterilizing value in accepted processes for green beans or asparagus in No. 2 cans. A recent heat distribution test on a retort load of pork and beans showed a lag in temperature after comeup time, because of inadequate venting of 170 degree-minutes. This was considered serious in the processing of pork and beans, but i t might have been disastrous had the product been a rapid heating vegetable. Thus, it can be seen t h a t the importance of uniform temperature distribution after the start of the process varies with the nature of the product and the processing temperature, uniform temperature distribution being more important for rapid heating products and higb retort temperatures. This might also be stated by saying t h a t uniform temperature distribution is most important in short processes.
1461
was 200' F. or below, which usually occurred in about 60 to 75 minutes. The maximum retort-can pressure differential was from 0 t o 3 pounds. I n cooling the cans which had been processed at 230" F. using the first method, the retort pressure was maintained at 10 pounds. No pressure measurements were made during any of the tests run at a retort temperature of 230' F. 50
4
40
4 1
E m
* 230 r t!
FILLED WITH WATER
0 :
?g w EZO
LLEO WITH 5 % BENTONITE ILLEO WATER. PROCESSING 250OF. COME-UP TIME
a
c
E 0
5
10
COOLING PRESSURE
Another factor studied which has an influence on the heat units effective for killing bacteria is the retort pressure during the cooling cycle of No. 10 cans of a conduction heating product. This phenomenon seems t o be related to boiling of the can contents when the pressure is released, thereby creating turbulence and rapid mixing of the cold and hot portions of the food. With pressure maintained in the retort and provided the headspace of the cans is not excessive, the can pressures are likewise maintained and the boiling of the contents is prevented. The rate of temperature change is therefore governed by the laws of heat conduction which is a relatively slow process. EXPERIMENTAL PROCEDURE. I n order to ascertain the effect of various pressure cooling conditions, a series of tests was conducted using No. 10 cans of a conduction heating product. These pressure cooling variables were studied in conjunction with three different retort processing temperatures:
1. Maintain pressure in the retort from 4 to 8 pounds above processing pressure during the entire pressure cooling period-i.e., until temperatures at the can centers were below 200' F. 2. Drop t h e retort pressure during the cool at a rate of 0.5 pound per square inch per minute. 3. Drop the retort pressure as fast as possible without distortion of t h e can ends. Heat penetration tests were conducted using No. 10 (603 X 700) cans fitted with thermocouples of the Ecklund type (6) having the junction a t the geometric center of the can. The product used in most of the tests was fresh cream-style corn tran5ported from a nearby cannery. It was found t h a t the heating and cooling characteristics of a 7'% starch solution were very similar t o that of cream-style corn so this product was used for part of the testa. I n each test run, one can was fitted with a Seaman can pressure measuring device ( I I ) , so t h a t the cooling schedule could be regulated t o prevent permanent deformation of can parts. The cream-style corn or starch solution was heated in a steamjacketed kettle t o 185" to 190" F. and 7 pounds 2 ounces of corn or 6 pounds ounces of starch solution were filled into each can. The cans were closed and the retort process was started without delay. Processing schedules of 145 minutes a t 250" F., 180 minutes at 240' F., and 225 minutes at 230" F. were used. The three pressure cooling schedules described previously were used following each process time. The heating d a t a obtained were evaluated by the method of Ball ( 2 ) and also by the method of Bigelow et al. ( 4 ) as modified by Schultz and Olson ( I O ) in order to obtain the sterilizing value. When using the first method of pressure cooling after processin at 250" F., the retort pressure was raised t o 20 t o 22 ounds a n 8 the retort filled with water as quickly as possible. T l i s pressure was maintained until the temperature a t t8he center of the can
0
0
I
2
3
4
5
6
7
REDUCTION in Fo VALUE
Figure 2.
Effect of Air Contamination in a Retort
Measured by degree-minute temperature lags on the process sterilizing value
With method 2 (0.5-pound dro per minute) the can pressure followed t h a t of the retort during t t e cool, the pressure in the can being about 2 pounds per square inch above the pressure in the retort. In using method 3 (fast pressure drop) the retort pressure was raised 1 or 2 pounds and the retort then filled with water as rapidly as possible. When the retort was full of water the pressure was dropped as quickly as possible without exceeding the elastic limit of t h e ends. It was found t h a t in these tests, the retort pressure could be dropped very rapidjy t o 5 pounds and reduced from 5 pounds t o 0 pressure in about 3 minutes. The heating data from the slowest heating can in each run were plotted on semilogarithmic paper as described by Ball ( I ), Jackson and Olson (9),and others, and the slope of the heating curve was drawn to f h = 198, the j value being variable from one can t o another. (The j value is a number representing the time lag before retort temperature minus can temperature and assumes straight-line characteristics when plotted against time on semilogarithmic paper.) The slope of the heating curve, j h , is the number of minutes required for the straight-line portion of the curve t o traverse one log cycle. An extrapolation of the straight line will intersect the temperature axis corrected for comeup time a t a point j I degrees below retort temperature, where I is the difference between retort temperature and initial temperature. The sterilizing value was calculated using the method of Ball and using the actual jZ value found rather than by assuming a constant initial temperature. The can center heating data were also used t o determine the sterilizing value by the method as described by Bigelow et al. ( 4 ) . The heating data obtained and the sterilizing values calculated by the two methods are shown in Table 11. RESULTS. The results obtained indicate that turbulence occurs in large size cans of conduction heating product when the retort pressure is dropped quickly. Maintaining pressure in the retort during the cool prevents turbulence and allows the can center temperature t o continue to rise after the heating period has ended, thus effecting a higher sterilizing value at the can center as determined by, the Bigelow method. This would not hold true if the headspace volume of the can exceeded the end flip volumetric displacement so greatly t h a t the internal pressure could not be maintained. Within the commercial range of fills such excessive headspaces are not likely t o be encountered.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1462
Vol. 44, No. 6
temperatures for thermophilic bacteria which could survive the thermal process, FO and may result in reduced shelf life by Bigelow Bigelow corpossible corrosive action of the product (4) reoted on container. Overcooling, on the other 4.8 4.8 hand, wastes water and may cause exter21..48 2.9 nal rusting of cans. Optimum, from the 1.8 above considerations, is an average prod4.7 4.3 uct temperature of 100' F. when the cans 2.9 2.1 1.9 1 9 are removed from the cooler. The cool4.0 3 5 ing studies which were conducted by this 2.3 2.3 laboratory were based on the attainment of this temperature. During the 1950 packing season, the cooling efficiencies and water consumption for some of the methods used commercially for cooling brine-packed peas and creamstyle corn were investigated. I n addition, the spinner cooling of 46-ounce apple juice cans using spray and immersion methods was investigated in the laboratory.
OF PRESSURE COOL~N SCHEDULES G ON STERILIZING VALUE TABLE 11. EFFECT
Retort F.
Temp.,
Init.
Cooling Conditions
Temp.,
F.
Ball
250
Hold press. a t 20 lb. Press. drop, 0.5 lb./ min. Fast press. drop
180
jI 128
182 175
139 128
j 1.83 21..0741
198 198
1 1 .. 3 8
240
Hold press. at 18 lb. Pres?. drop, 0.5 Ib./ mln. Fast press. drop
181
112
1.9
198
2.3
182 180
105 122
1.81
2.04
198 198
2.7 1.9
Hold press. a t 101b. drop
180 179
1.61 1.82
198 198
2.8 2.3
230
Fast press.
80.5 93
/h
(8)
198
1.8
The effect of different pressure cooling schedules is shown in Table 111, where the data are calculated from those tabulated in Table 11. Within the various retort temperature groups it will be noted that uniform Fo values were not obtained when the heating data were evaluated by the method of Ball. The cooling curve by the Ball method is assumed constant, so it would be expected that variations in cooling procedure would not affect the calculated F , value. The fact that the Fo values were different must be due to the different temperature patterns in the contents of the can a t the start of the process. These variations in temperature patterns would also be reflected in the Bigelow Fa values. It would not be fair to compare the Bigelow Fo values within the three retort temperature groups without making some allowance for these conditions. Since the difference between the Ball and the Bigelow Favalues was nil under the conditions of a fast pressure drop, the Ball calculated value for this condition was taken as a basis. The Ball FOvalue for the other conditions was either increased or decreased to be equal to that obtained with a fast pressure drop, and the Bigelow Fo value was changed the same amount. These corrected Bigelow FO values then represent a more accurate comparison of the effect of the different cooling methods than do the uncorrected values. Admittedly they may not be precisely accurate, but in view of the technical difficulty in obtaining precisely identical heat penetration curves between different cans, it appears to be a reasonable means of compensating for such an uncontrollable variable factor. It will be noted in Table I1 that when the pressure in the retort is allowed to drop as fast as possible without buckling the cans, the sterilizing value as determined by the Bigelow method is nearly equal to that determined by the method of Ball. When pressure is maintained in the retort during the entire cool, the Fo value found by the Bigelow method is markedly increased, the Bigelow value being almost 3 FOunits more than the value determined by the method of Ball when the processing temperature is 280 O F. This increase was only 1.2 FOunits when the processing temperature was 230" F. and 2.4 FOunits when the processing temperature was 240" F. With a 250" F. process, the temperature difference between the can center and the retort a t the end of the process is much greater than exists in a 230 O F. process, thus allowing the temperature a t the center of the can to continue t o rise more than is possible when processing a t 230 O F. COMMERCIAL COOLING PROCEDURES
The previous section has dealt with the effect of variations in pressure cooling schedules in still retorts. Studies in commercial practices of cooling in still retorts have been reported by Benjamin and Jackson (3). This section will deal with some of the other methods used commercially for cooling cans. One of the most important steps in the production of quality canned foods is a quick and thorough cool after the thermal process. Undercooling may cause "stack burn" or quality impairment of heat-sensitive products, may provide ideal incubation
TABLE 111. RELATIVE EFFECT OF PRESSURB COOLING SCHEDULES ON STERILIZIXG VALUES
Cooling Schedule Hold at 4 t o 8 lbs. over processing pressure Drop pressure lb./min. after retort is full of water Drop pressure as fast as possible without buckling
Relative Sterilizing Value -of Process, % ' at-230' F. 240° F. 250' F. 152
226
...
110
266 161
100
100
100
The results reported for peas, cream-style corn, and apple juice would no doubt be applicable to other similar products, such as green beans, pork and beans, and grapefruit juice, to cite one example, respectively, for each of the first named products. Theoretical water requirements shown by the tables accompanying these studies do not include the amount of -rater necessary t o cool retort crates. PEASIN 303 x 406 CANS. SHORTWIDE Can-AL. A two-crate m-ide canal, 40 feet long, was in operation a t one plant cooling 303 X 406 size cans of peas from one pea line. Cans were being closed at a rate of 120 to 140 per minute, scrambled into crates (approximately 18 cases of 24 cans per crate), and processed a t 240' F. N o change in the speed of the automatic track 11-ai necessary for this rate of production. Water entered the tank a t the top of the exit end and discharged through an overflo\T at the end where the crates enter the canal from the retort room. An air-activated temperature controller in the water inlet line a as set t o open when the hot bulb, located about 1 / 3 of the distance from the can exit and 20 inches beneath the surface, indicated a temperature of 100' F. Typical water temperatures in this particular canal were 93 O t o 103O F. a t the surface, 93 O to 98" F. in the middle layer, and 93" to 97" F. in the bottom layer. Once the controller started to operate, cans leaving the canal had about the same temperature throughout the entire day's run. Had more than one pea line been in operation, a lower controller setting would have been necessary to balance the shorter time in the canal. With a cooling time of 1 8 1 / 2 minutes and 51 O to 53" F. water inlet temperatures, the cans were cooled to 111O to 112" F. a t the crate centers and to 102" to 103" F. near the crate wall. The water consumed was measured to be 7.8 gallons per case on a daily average basis. The theoretical water required based on heat balance was calculated to be 7.25 gallons per case. PEASIN 303 X 408 CAIW. LOXGNARROW C A N ~ LA. second plant was cooling a small number of 211 X 400 (No. 1) cans together with a large majority of 303 x 408 cans of peas. The tank was L-shaped. One leg was 78 feet long and was equipped with automatic track. The second section was 10 feet long. The
June 1952
INDUSTRIAL A N D ENGINEERING CHEMISTRY
crates in the latter section were pulled along by hand after entering the canal to meet the automatic track. The 303 X 406 cans were stacked on end in four layers with approximately 16 cases per crate. The four rows were spaced with perforated separators. Processing temperature was 240' F. At top production, crates left the tank at a rate of 1 per minute. The water consumption for two systems of piping was studied at this plant. These hookups are described as follows:
1463
TABLE IV. CHROIFOLOGICAL TEMPERATURE DURING TYPICAL DAY'SCOOLING OPERATION (303 X 406 cream-style corn in long narrow canal) Water Consumption, Gal./Case Theoretioala Actualb
Av. Water Temp.,
Minutes in Av. Product NO. F. Canal Temp., F. Time 3 :36 p.m. 20 77 34 91-99 4:41 .. 1st 80 79 38 85-92 7:21 li:8 1st 82 200 36 94-100 2nd .. .. 3:57 p.m. 88 40 28 106-111 6 4:33 2nd .. 80 88 21'/n 112-118C 5:14 10:4 .. 2nd 85 110 291/n 103-105 2nd 7:23 200 12.7 81 33 90-100 13:4 9:45 2nd 13.5 249 79 34 96-98 a For product and water temperature shown. b Daily average. 0 A foroed shutdown in operations elsewhere in t h e plant caused a reduced flow of water to the canal a n d a build-up of crates at t h e entrance of t h e canal. This condition was unusual a n d does not indicate a lack of attention t o t h e cooling operation. T h e data a r e presented to show what product temperatures can result from too short a time or too high a cooling water temperature. Cans over 105' F.were air oooled before casing.
..
..
SYSTENA. The Piping for the water perforated pipe inlet consisted of along 4,,/6 of bottom of the tank and a bypass which could, if needed, discharge water onto the surface a t the center of the 78-foot section. For the water outlet there was a 2-inch overflow a t the apex of the L, in conjunction with three ports in the side of the tank-one in the center and one a t either end of the 78-foot section. The controller was self actuated, with a bulb a t the surface about 15 feet from the crate exit end. It had a low actuating temperature (unknown). SYSTEMB. The piping for the water inlet consisted of a single 2-inch pipe about 10 feet from the crate exit end. For the water outlet there was a single 2-inch overflow a t the ape.: of the L. The controller was hand controlled.
With system A, typical water temperatures in the canal were 82 O to 90" F. in the surface layer and 66" to 78" F. in the middle layer. With system B, the temperatures were 84' to 99' F. in the surface layer, 83 to 95' F. in the middle layer, and 83" to 90" F. in the bottom layer. The water consumption for piping system A was 10.3 to 14.0 gallons per case based on inlet water temperature of 53' F., a cooling time of 17 minutes, and a final product temperature of 78" t o 85" F. For system B, the figure was 7.5 gallons per case but the product temperature was 92" to 102" F. The theoretical figure based on heat balance for system B was calculated to be 9.2 gallons per case. It was not possible to calculate the theoretical figure for system A because the overflow proportions were not measurable. The installation of an air line along the bottom of the canal did not appear to affect water consumption. It did, however, reduce the temperature variation within the crate. PEAS I N 307 x 409 CANS. CONTINUOUS PRESSURE AND ATMOSPHERIC COOLER. No water meter could be utilized at this plant and therefore no exact water measurements could be determined. The cooler was divided into two parts, a pressure section and an open section. Water at 51" F. was supplied both sections. The cans were admitted t o the pressure section at the rate of 166 cans per minute and at a temperature of 260" F. After 10.2 minutes in the pressure section, their average temperaperature was 117' F. An additional 4.5 minutes in the atmospheric cooler reduced the temperature to 106" F. Exit water temperatures were 160" and 101" F. for the pressure and open cooler, respectively, and the theoretical water requirements were 4.55 and 0.76 gallons per case for each section, respectively. CREAM-STYLE CORNIN 303 X 406 CANS. LONQNARROW CANAL. A cream-style corn canner packing only 303 X 406 cans cooled the product in an L-shaped canal which also had a small extension perpendicular to the short leg for admitting water into the tank. The short leg was 27 feet long and the long leg was 68 feet. I n the shorter section crates were pulled along by hand, whereas in the longer section automatic track was in operation. Countercurrent flow was obtained by a 2-inch water inlet at the exit end and a 2-inch overflow in the section where the crates entered the canal. There were no dead pockets in this arrangement. The two canning lines produced about 320 cans per minute combined. Processing temperature was 245 O F. The canal water
Observation D a t a
Crate
Day 1st
inlet was hand controlled. At top production and gfter the tank was up to temperature, water a t the rate of 95 gallons per minute was introduced a t the surface. A typical canal water temperature pattern when cooling cream-style corn differs from that obtained from a convection cooling product. I n the latter case a 15" F. water temperature spread is common a t the surface for long tanks, while the surface temperature for this cream-style corn canal differed by only 2" or 3 " F. Typical canal water temperatures during the operation were 79 O to 81 O F, in the surface layer, and 76" to 79' F. in the bottom layer. The final product temperatures for various periods of the day's run and different lengths of the time in the canal are presented in Table IV. The canal temperatures are shown as an average of the readings taken simultaneously with the timing of the respective crates. The normal cooling times for this canal were in the range of 33 to 36 minutes with canal water temperatures averaging 79" to 85" F. Under these conditions cans were cooled to between 90" and 100" F. The average daily water consumption was 13.4 to 13.8 gallons per caae. APPLE JUICEI N 404 x 700 (46OUNCE) CANS. SPINNER COOLER.Spray- and immersion-type spinner cooling of 46ounce apple juice cans were investigated in the laboratory. The cooling equipment consisted of variable-speed, chain-driven rollers set in a metal box above which were two 1-inch pipes tapped for inserting spray nozzles. Experiments with the product included the effect of headspace, type of spray head, immersion, water pressure (for spray cooling), water temperature, and speed of rotation. The initial temperature was kept constant (195' F.). Apple juice under these conditions was found to have the same cooling characteristics as water. Preliminary tests with varying headspaces revealed that cans topped to gross headspaces of */4, and inch cooled essentially the same, whereas cans which were filled completely cooled much slower. For an example, with 2l/2 minutes of cooling time, 70" F. water temperature, 120 r.p.m., full cone spray head, and 15 pounds of water pressure, the cans were cooled to 111 to 131O F. when full and to 105" F. when filled with l/4 to 3/4 inch gross headspace. (Gross headspace is the distance from top of the can doubleseam t o the surface of the product.) The initial can temperature was 195' F. A t 15pounds of pressure this spray nozzle delivered 0.56 gallon of water per minute. There was some difference in rate of cooling among the supposedly full cans. This was probably a result of unavoidable spill during closure or of a difference in closing temperature. The results reported herein are for the slowest cooling of eight to ten cans per individual test. Table V recapitulates the pertinent data obtained during the course of the tests. A summary of the effect of various variables is as follows:
INDUSTRIAL AND E N G INEERING CHEMISTRY
1464 TABLE V.
COOLING TIMESFOR 4 6 - 0 v ~ cAPPLE ~ JUICE
(Initial temperature, 195' F., final can temperature, 100' F.) Water Water -Cooling Time, A h . , Spinning Full $'-inch Pressure, Delirered" Rate, cans headspace Lb./'Sq. In. Gal./Min.' R.P.M. SPRAY-TYPE SPISNER COOLERb
30
60
120
30
60
120
3.25 3 0 3.0 3.0 2.75 2.75 3.0 3.0 3.0 2.75 2,50 2 50 2.25
4.75
2.25
20 25 30 35 40 15 20 25 30 35
10.5
10.5 10.5
0.226 0.269 0.298 0.330 0 342 0.376 0,226 0.269 0,298 0.330 0.342 0.376
(50" F.) 0.50 (GOo F.) 0.50
i70" F.) 0.ao (80'
F.)
15 0.56 20 0.66 25 0.74 30 0.80 35 0.82 40 0.86 IVMERSIOS-TYPE SPISSER
Still 30 GO
4 5 4.25 4 0 3 75 3.75 3.75 4 5 4.23 3.75 3.5 3.25 3,25 3.75 3.0 3.0 2 75 2.75 2.75
6.0 6.0 6.0 5.75 5.75 5.75 6.0 5.75 5.75 5.75 5.5 5.5 40 5.75 15 5.75 20 5.5 25 5.5 30 5.25 35 5.25 40 SPRAY-TYPF SPINTERCOOLER^ 5.0 15 5.0 20 4.75 25 4.75 30 4.60 35 4.50 40 15 0.56 4.75 20 0.66 4.75 25 0.74 4.50 30 0.80 4.50 35 0.82 1.25 4.. 0 0 86 4.25 0.50 4.5 10.5 15
60 60 60
... ... ,..
5 75 7.0 4.5 4.5 4.25 4.25 4.25 4 0
3.0 '
4.0
2.25 2.25 2.25 2 0 2.0 2.0
CooLERd
2.25 2.75 3.25 4.0 L O 4.75 4.25
1.25 1.5
1.5 1.75 5,O 3.0 2.5
Water inlet temperature, 62' to 66" F., except where otherxise indicated. b H o i l o ~ vcone spray with atomizing nozzle with '/i-inch pipe connection a n d 0.086-inch orifice. c Full cone, hard spray nozzle with '/s-inch pipe connection and 0.082-inch orifice. d Cans immersed In water t o 3-lnch depth.
Headspace was found t o be the most significant I~EADSPACE. factor affecting the cooling of apple juice in a spinner cooler. Tests conducted wit,h 62" to 66" F. cooling water temperatures showed that full ca.ns (4 to 6 minutes of cooling time) cooled 11/4 to 23j4 minutes slower under sprays t,han cans which were topped to 1/4-inch gross headspace (2 t'o 41/2 minutes of cooling time). This was also apparent when cooling by immersion, in which instance full cans were cooled in 23/4 to 43/4 minutes, while cans having 1/4-inch gross headspace took 11/2 to 3 minutes, t'he difference in cooling time between full and cans topped to l/4-inch being 11/* to l a / , minutes. TYPEOF SPRAYNOZZLE.Actually no true comparison could be made between t'he hollow cone atomizing nozzle and t'he full cone hard spray-type nozzle which were tested, since a t identical water pressures these nozzles delivered different amounts of mat,er. To obtain the same water delivery n-ith the two nozzles, special pumps providing water pressures up to 150 pounds would be necessary. These mere not' available for these tests. Under conditions of the test, however, the atomizing nozzle was more economical of water while cans under the full cone spray cooled, on the average, 11/4minutes faster. I m r E R s r o w COOLINGVERSUS SPRAYCOOLISG. Immersion cooling at low speeds of rotation had no advantage over spraying with the full cone nozzle. However, a t 120 r.p.m. faster cooling was obtained hy immersion (23/4and ll/? minutes' cooling time for full and '/d-inch topped cans, respectively) than that under the full cone sprays (4 and 2 minutes' cooling time, respectively).
Vol. 44, No. 6
WATER PRESSURE o s SPRAYCOOLING (AMOUNTOF \$'ATER DELIVERED). Under full cone sprays a decrease of 1/2 minute in cooling time was effected by increasing water pressure from 15 to 40 pounds. The rate of cooling, of course, is a function of the water delivered a t t'hese pressures. The volume of water discharged a t the different pressures is included in Table V. Under the atomizing nozzle full cans only were similarly affected, while cans which were topped to 4!' inch showed appreciable decrease in cooling time, depending on the speed of rotation. \vTiATER TEMPERATURE. The effect of increasing the cooling water temperature IO" F. was rather insignificant, especially with cans having some headspace. Not until a 30" F. temperatui~e rise had been introduced did the cooling time vary to any extont, the greatest difference being noted in full cans. This factor wouitl probably become more influential as the water temperature and desired final can temperature approached the same value ( looo
F.). SPEEDOF ROTATION.Doubling the speed of rotation for spray
cooling had relatively little effect as far as reducing the cooling time. Quadrupling the speed was similarly insignificant except in the case of cans topped to '/? inch and cooled under hollow cone sprays. In the latter case a reduction of 1 minute in cooling time %-asaccomplished. Doubling the speed of rotation of cans cooled by immersion from 30 to 60 r.p.m. had little effect, while the influence of the change from 60 to 120 r.p.m. or 30 to 120 r.p.m. was progressive. The maximum effect was achieved by changing t'he speed of rotation of full cans from 30 to 120 r.p.m. A 2-minute decrease in cooling time resulted in this case. DISCUSSION.Using countercurrent piping, peas in 303 X 406 cans were found to be cooled in canals in 17 to 18l/2 minutes with an expenditure of 7.5 t,o 7.8 gallons of water per case. Water consumption in the S o . 2 can size (307 X 409) would be about 9 gallons per case, assuming that the larger size can would require 20% more LT-ater t'han the 303 X 406 can. This water usage represents a considerable saving over that required in vertical or horizontal retorts. For the system of cooling in vertical ret,orts found to be the most econ3niical of i n t e r by Benjamin and Jackson (5)the saving would be 3 gallons per case. However, for the methods ordinarily used t,his saving could run as high as 12 gallons per case. \Tat.er consuniption in horizontal retorts was calculated t o be 12 to 15 gallons per case above that used for canal cooling. The savings obtained by continuous retorts would be even more than the above listed values based on an approximate consumption of 6 gallone per case. Cream-style corn in 303 X 406 cans was observed to cool in one canal with 13.4 t o 13.8 gallons of water per case being expended. Cooling times were in the range of 33 to 38 minutes for cooling below 100' F. If the S o . 2 size can were to require 20% more than the smaller can, about 17 gallons of water per case would bc used. This figure is 14 to 26 gallons ley4 than that required for cooling in vertical retorts and 12 to 26 gallons less than that in horizontal retorts. Many conditions affect the spinner cooling of 404 X 700 (46ounce) apple juice cans, especially the factor of headspace. At cooling Jyater temperatures in the range 62" to 66" F., completely full cans can be cooled by immersion in as little time as 2 3 / 4 minutes or by spraying in 4 minutes. Cans having '/,-inch gross headspace can be cooled by immersion in 11/2 minutes or by sprays in 2 minutes. The TTater consumption for spray cooling would be about 20 gallons per case of 12 cans. S o consumption data are available for immersion cooling. SUM i l Z A R Y
Heat distribution in a retort as affected by air contamination has a substantial effect on the sterilizing value of the process on those products which heat rapidly and which are given relatively short process times. For those products which heat by conduction heating, lags up to 40 degree-minutes have only a slight effect on the process sterilizing value. Heating lags beyond this value were not tested. The pressure maintained in a retort during the cooling cycle for KO, 10 cans of a conduction heating product (cream-style corn.
June 1952
1465
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
pumpkin, concentrated soup, etc.) has a substantial effect on the lethality manifested at the can center, especially at the higher processing temperatures. Maintaining pressure on the cans after shutting off the steam prevents the can contents in cans which contain a t least standard legal fills from boiling and mixing in the can. When boiling is permitted in the can by release of the external pressure, the cooling of the contents is relatively rapid with consequent reduction in sterilizing value effective at the can center. Data are presented giving the cooling times and water consumption of various methods of cooling cans outside of retorts for cream-style corn, brine-packed peas, and apple juice, which are representative of canned foods of various consistencies. ACKNOWLEDGMENT
Sincere appreciation is expressed to the Mayville Canning Co., Clyman Canning Co., Oconomowoc Canning Co., and Illinois Canning Co., whose assistance and cooperation made the cooling tests involving canal and continuous pressure coolers possible.
LITERATURE CITED
(1) Ball, C. O., Canner, 64, No. 5, 27 (Jan. 22, 1927). (2) Ball, C. O., Univ. of Calif., Public Health, 1, No. 2 , 15-245
(1928). (3) Benjamin, H. A., and Jackson, J. M., Canner, 88, No. 12, Pt. 2, 75 (Feb. 25, 1939). (4) Bigelow, W. D., Bohart, G. S., Richardson, A. C., and Ball, C. O., Natl. Canners Assoc. Research Lab., Bull. 16L (1920). (5) Bitting, A. W., “Appertizing, or the Art of Canning,” San Francisco, The Trade Pressroom, 1937. (6) Ecklund, 0. F., Food Technol., 3, No. 7, 231-33 (1949). (7) Ecklund, 0. F., andBenjamin,H. A,, FoodInds., 14, No. 3, 40-42 (March 1942). (8) Jackson, J. M., Proc. I n s t . Food Technol., 39 (1940). (9) Jackson, J. M., and Olson, F. C. W., Food Research, 5 , No. 4,409 (1940). (10) Schultz, 0. T., and Olson, F. C. W., Ibid., 2, No. 5, 399 (1940). (11) Seaman, M. L., American Can Co., General Research Laboratory, private communication. (12) Somers, I . I., Food Inds., 16, No. 2, 80-83 (February 1944). RECEIVEDfor review April 16, 1951. ACCEPTEDFebruary 4, 1952. Presented a s part of the Symposium on Chemical Engineering Aspects of Food Technology before the Divisions of Industrial a n d Engineering Chemistry and Agricultural a n d Food Chemistry at t h e 119th Meeting of t h e AMERICAN CHEMICAL SOCIETY, Boston, Mass., April 1951.
Separation of Gases by Means of Permeable Membranes
Engfinnedering Bocess development
PERMEABILITY OF PLASTIC MEMBRANES TO GASES
I
DAVID WILLIAM BRUBAKER
AND KARL KAMMERMEYER STATE UNIVERSITY OF I O W A , I O W A CITY, I O W A
U
K T I L the advent of commercial production of plastic films little attention had been paid to the gas permeation behavior of these materials. The extensive developments in the use of such films for packaging purposes introduced problems of gas and vapor permeation, and investigations were initiated during the past 12 years. While some work, especially on natural rubber films, was published by Graham ( I S ) as early as 1866, the majority of references are of very recent origin. The possibility of utilizing plastic films in the separation of gas mixtures is probably one of the most recent applications. While the present paper does not touch upon this subject specifically, the findings which are presented contribute t o the basic knowledge required for the study of gas separation by means of such membranes. It was the primary object of this research to obtain data on the rate of gas permeation through a number of selected plastic membranes a t different temperatures. I n addition, ‘some observations were made on the effect of varying amounts of plasticizer in the membrane upon the rate of gas permeation. The findings on the effect of varying amounts of plasticizer are presented to a limited extent only, as this work will be published separately in the near future. THEORY AND LITERATURE SURVEY
The process of diffusion in polymers may be conveniently divided into two parts: first, the flow of gases and vapors which are not readily condensable a t or near standard conditions (hydrogen, carbon dioxide, sulfur dioxide, and ammonia) in plastic organic compounds; and second, the flow of water and organic liquids as well as their vapors through membranes.
This division can be made because the process of gas diffusion obeys Fick’s law dC/&
=
Kd2C/dX2
(1)
fairly rigorously, while vapor diffusion does not usually follow this law. Simril and Hershberger (21, 22) found that the permeability rates of gases are about 10,000 times smaller than the permeation rates of vapors. The present work will only be concerned with the first type-that is, the flow of gases. As early as 1866 Graham ( I S ) studied the flow of gases through rubber. He regarded this permeation process as a sequence of solution, diffusion, and re-evaporation of the diffusing gas, a viewpoint which is held today. The fact that Fick’s law of diffusion applies within a plastic membrane was first pointed out by Wroblewski (28) in 1879 and has been verified since then by many investigators ( 2 , S, 5 ) . The pressure, area of membrane, thickness of membrane, and the temperature are the possible variables in the kinetics of permeation. If Henry’s law,
s
= kp
(2)
can be applied to the solubility, S , of the gas in the polymer substance, and Fick’s law, Equation 1, is taken to represent t h e diffusion process within the material, the permeation rate for steady state flow is directly proportional to the pressure difference and to the area of the membrane, and is inversely proportional t o the thickness of the membrane. The rate of transfer of a gas across a film of unit thickness per unit area per unit pressure difference is known as the permeability constant and characterizes the effectiveness of the film as a harrier. The permeability coefficient, P , follows from
