UNIVERSITY OF CALIFORNIA COLLEGE OF AGRICULTURE AGRICULTURAL EXPERIMENT STATION BERKELEY, CALIFORNIA THE SWELLING OF CANNED PRUNES E. M. MRAK and P. H. RICHERT BULLETIN 508 February, 1931 UNIVERSITY OF CALIFORNIA PRINTING OFFICE BERKELEY, CALIFORNIA 1931 THE SWELLING OF CANNED PRUNES E. M. MRAKi and P. H. RICHERT2 Canned "ready-to-serve" 3 dried prunes have been produced in California for a number of years, The pack has been relatively small and has not increased materially because of losses from swelling of the cans by hydrogen gas formed from the action of prunes on the tin plate. Thus the Census of Manufacture (1)4 for 1927 reports a pack of only 148,000 cases of canned prunes for California in 1927. If the spoilage losses could be eliminated or materially reduced the possibilities for expanding the market for the product would be good. Canned ready-to-serve prunes are in favor for use in hotels and res- taurants, since the product is fresh, attractive in appearance and taste, requires no preparation, and is standardized in quality. The trend in American homes is to minimize the time devoted to the preparation of meals; consequently, the consumption of all canned foods has greatly increased. Dried prunes require considerable time and care in preparation and canned prunes should therefore prove popular for home use. It is believed that many families who do not at present use dried prunes would use canned prunes. NATURE OF SPOILING Hydrogen swelling of canned prunes is caused by chemical reac- tions between certain constituents of prunes and tin plate. Observa- tions and analyses made in the Fruit Products Laboratory, University of California, show that the tin is partially, and in extreme cases, wholly, removed from the inner surface of the can, and largely absorbed by the flesh of the prunes. Perforation, i.e., " pin-holing ' ' of enamel-lined cans occurs frequently, but so called coke and charcoal plate cans are seldom perforated. The color of the prunes generally bleaches and the flavor of the prunes and syrup is adversely affected. Often the prunes appear bloated with entrapped hydrogen gas when the can is opened. 1 Eesearch Assistant in Fruit Products. 2 Graduate Assistant in Ffuit Products. s ' ' Ready-to-serve ' ' prunes is the term applied to dried prunes canned in syrup. 4 Reference figures in parentheses refer to publications in the terminal bib- liography. 4 University of California — Experiment Station The exact nature of the changes that occur in canned prunes dur- ing" " hydrogen swelling" is not clearly understood. Christie (2 > 3) attributed hydrogen swelling to corrosion of the tin plate in the pres- ence of oxygen from the "air pockets" in the fruit, and advised thorough exhausting as a certain method of preventing swelling. Kohman and Sanborn (4) found that a few cubic centimeters of oxygen in an enameled can of apples disappeared within three weeks, but that thereafter corrosion continued at a much more rapid rate than in cans in which oxygen was not originally present. In the tests recorded here, cans containing the water extract of prunes have developed rapidly into hydrogen swells even when the liquid was heated until practically free of oxygen before canning. This does not prove that oxygen would not hasten corrosion. Although oxygen occluded in the fruit at the time of canning may be a factor in the formation of swells, it is probably not one of the most important factors. Culpepper and Caldwell (5) found that fruits containing antho- cyanin pigments corroded tin plate more rapidly than fruits contain- ing little or no anthocyanin pigments. Possibly the coloring matter of prunes may also hasten corrosion of the tin plate. The authors found that canned natural prune juice corroded much more rapidly than prune juice decolorized with vegetable charcoal, a finding that appears to support the Culpepper and Caldwell theory. Clough, Shostrom, and Clark (6) reported that sulfur-spray residue on gooseberries increased the rate of corrosion and hydrogen swelling. In tests with peaches, Culpepper and Moon (7) found that sulfur-spray residues accelerated the swelling of canned peaches. When small quantities of sulfur were added to prunes canned in our experiments swelling was accelerated, also. Lueck and Blair (8) have attributed corrosion to an anodic relation of tin to iron caused by hydrogen overvoltage. Kohman and San- bo rn (9) stated that the anodic relationship of tin to iron depends upon conditions, an important one of which is the relative concentration of ferrous and stannous ions in solution. The power of proteins to combine with tin salts, which is well known, (10) keeps the stannous ions at a low concentration in the solution inside the can. Kohman and Sanborn, (4) and Kohman/ 11 ' 12) have furthermore indicated that anthocyanin pigments function as hydrogen acceptors at the cathode. Culpepper and Caldwell (5) stated: "The principal factors concerned in corrosion are oxygen, acidity, anthocyanin pigment, and tannin. Some of these factors stand in antagonistic relationship. High acidity Bul. 508] The Swelling of Canned Prunes 5 generally favors corrosion but depresses the formation of metal- anthocyanin compounds and may thus retard corrosion. High acidity represses oxidation of tannin and formation of hydrous stannous oxide." Experiments in the Fruit Products Laboratory have shown that when the pH value of canned prune juice was decreased below 3.6 by the addition of citric, tartaric, or malic acids, the rate of swelling was decreased. 5 Corrosion, however, of cans in which the pH value had been lowered was observed to be about the same as of those con- taining the natural juice. The rate of swelling was also decreased when the pH value of the juice was increased to about 6 by the addi- tion of NaHC0 3 . A decrease in pH value caused bleaching whereas an increase in pH caused darkening of the prunes. The writers are of the belief that swelling and corrosion in canned prunes cannot be attributed to any single factor. An analysis of canned prunes showed most of the oxidized tin to be in the solid flesh of the prunes and only a relatively small quantity in the syrup. Canned prunes to which a tin salt (Na 2 Sn0 3 ) had been added, cor- roded and swelled more slowly than cans not containing added tin salts. These observations indicate that the findings of Kohman and Sanborn (4) apply in part at least to the corrosion and swelling of canned prunes. The present writers have observed, also, that prune juice decolorized with "Eppinite" (vegetable charcoal) and boiled before canning to remove dissolved oxygen, caused swelling. This would indicate that anthocyanin pigments are not the only cause of corrosion and hydrogen formation. The preceding discussion has presented the principal theories of corrosion and hydrogen formation by canned prunes. PRESENT CANNING PROCEDURE Commercial canning procedure varies greatly with the different beliefs of the canners concerning spoilage. The most common prac- tice is as follows: Dried prunes are washed in cold water for a few minutes and are then blanched 6 in boiling water for 5 to 15 minutes after which they are passed over a shaker, in order to remove adhering water. The blanched prunes are then put into plain or enamel-lined s This has very recently been corroborated by: Kohman, E. F., and N. H. San- born. Effect of acid on corrosion of tin cans. Jour. Ind. Eng. Cheni. 22(6) : 615-617, 1930. 6 Blanching- as used in this publication consist of submerging dried prunes in heated water or steam. 6 University of California — Experiment Station tin cans and filled with 17.5 to 20 per cent cane sugar syrup. The filled cans are exhausted 3 to 15 minutes at 190° to 210° F in steam, sealed, and then cooked in open vats, agitating cookers or retorts, for 1 to 3 hours at 212°-250° F. Most canners cool the cans in cold water for about 3 minutes after cooking, but with some the cans are allowed to cool in air. Some canners ship immediately after canning whereas others store the canned prunes for some time before shipping. These variations in procedure illustrate the lack of understanding of the relation of certain factors to the formation of swelled cans. PRESENTATION OF DATA In experiments by A. W. Christie (2> 3) on the formation of hydro- gen swells by canned prunes, work was limited to the influence of the exhausting period. Our investigations were conducted in an effort to develop a satisfactory method for canning prunes, and to ascertain something concerning the fundamentals of corrosion and hydrogen formation. The ultimate purpose of our work has been to establish a reliable procedure for the canning of prunes so that the distribution and sale of prunes may be increased. Experimental Procedure. — In conducting the canning tests, a standard procedure was adopted. This procedure was followed closely and was varied only when necessary, according to points studied in individual tests. This standard procedure was as follows: French prunes of the grade known as "Sunsweet 70 to the pound" were washed in cold water and then blanched for 8 minutes in boiling water. They were then filled into cans, the weights shown in table 1 being used for the different sizes of cans. TABLE 1 Fill-in Weights for Various Sizes of Cans Used as Controls Size of can Weight, grams Weight, ounces 90 210 240 360 1680 3 No. 1 tall 7 No. 2 tall 8 No.2H 12 No. 10 56 The cans were completely filled with 20 per cent cane sugar syrup and exhausted at 210° F for 20 minutes. This period was later changed to 10 minutes at 210° when it was observed that the shorter Bul. 508] The Swelling of Canned Prunes exhaust was effective. The hot cans were sealed with lids having "Canco" compound gaskets. The sealed cans were cooked in boiling water for 1 hour and then cooled in running cold water for 3 minutes. The cans were stored in a room at an average temperature of about 80° F. Most of the cans were stored in the vertical position, a few being stored in the horizontal position with the side seams up. They were examined at regular intervals. Summaries of observations on the various experiments are given graphically. In order to conserve space, the entire curves for each experiment are not given except in figure 1. In the other graphs, the 100 5 ^A \ = > o Ld _J _i at to w Z < °50 1 /A f/^A IL o L 2 9 4 Date Canned 1- June 1927 UJ Z-July 19 27 O Si 3-Januaey i9ze Tsy -3 4- May i 928 < *S y^-A 50 100 150 2.00 150 300 350 400 450 Number, of days Fig. 1. — Comparison of the rates of swelling of control cans of prunes canned at different seasons. curves represent the time required for 50 per cent of the cans to swell in each test of the various experiments, For the purpose of plotting, the 50 per cent spoilage point in each test was arbitrarily chosen, it being well beyond the point that would determine the prac- ticability of each particular method of canning. Furthermore, this method of graphic presentation, summarizing as it does, allows of more direct comparison of the methods used. Reproducibility of Data. — Figure 1 shows the rates of swelling of cans in several tests in which the prunes were canned by the standard procedure in two different seasons, 1927 and 1928. Although the curves do not coincide exactly, the variation in most instances is less than 10 per cent. Experimental error due to uncontrollable varia- 8 University of California — Experiment Station tions in composition of the prunes, in the kinds of cans used and in storage conditions, etc., was probably responsible for the failure to obtain more exact reproducibility in the control tests. In a single experiment in which several tests were made at about the same time and under similar conditions, the data were more reproducible than when they were made at different times, probably through failure to duplicate conditions exactly. Comparison of Various Kinds of Containers. — In commercial prac- tice, prunes have been canned in the following types of containers: glass jars, single and re-enameled cans, and in plate, coke and char- coal plate cans. Laboratory tests showed that charcoal plate cans were the most resistant to corrosion and swelling. These cans are more costly than coke plate or enameled cans, but the slower rate of swelling should compensate for the added cost. Plain tin cans formed swells at a slower rate than enameled and re-enameled cans. The comparatively rapid swelling of the enameled cans was caused by con- centration of corrosion on small exposed areas of plate where the enamel failed to cover the tin. For this reason most of this corrosion occurred along the side seams and countersinks on the ends of the cans where considerable tin was exposed. Ready-to-serve prunes packed in glass jars sealed with lacquered lids, and stored upside down, have held perfectly for more than two years. This refutes the theory that hydrogen swelling is caused by germination of the prune pits. Comparison of Various Sizes of Containers. — Small containers of canned prunes swelled faster than larger ones because, per given volume, the area of the inside surface is greater than in large cans. In other words the difference in ratio of areas of the inner surface to the volume of various sizes of cans is an important factor in the formation of hydrogen swells. In a laboratory test small tin con- tainers were sealed inside No. 10 cans containing ready-to-serve prunes in order to increase the ratio of surface area to volume. Swelling was much faster in these cans than in the controls. However, the rates did not entirely correspond, mathematically, to the ratio of sur- face to volume, probably because of variations in other factors. Comparison of Different Varieties and Grades of Prunes. — Canned French prunes were superior to Sugar, Imperial, Robe de Sergeant, and Burton prunes in flavor and appearance. Most of the prunes grown in California are of the French variety. The quality of canned prunes naturally varied with the quality of dry prunes. When poor-grade prunes were packed, the appear- Bul. 508] The Swelling of Canned Prunes 9 ance and flavor were inferior. Corrosion and hydrogen swelling, however, did not depend on the quality of the prunes or the district in which they had been grown. Comparison of Sun-dried and Dehydrated Prunes. — When canned sun-dried and dehydrated prunes were compared, the rates of swell- ing of the two lots were about the same, indicating that dehydration did not inactivate the corroding element in prunes. Keeping Quality of Canned Fresh Prunes. — Fresh prunes were canned in No. 2 cans in accordance with the procedure described by Mrak and Cruess (13) in California Agricultural Station Bulletin 483. These cans formed swells only after a storage period of about IV2 years. The rate of swelling in this instance may have been retarded by the use of No.. 2 instead of 8-ounce cans. Effect of Blanching. — The method of blanching before canning was found to affect the rate of formation of hydrogen swells. One purpose of blanching is to cleanse the prunes. It also increases the weight and count per pound. Blanched prunes are also darker, glossier in appearance, have a better texture, and require less cook- ing than unblanched prunes. Three methods of blanching were compared : the use of boiling water, of live steam, and of steam under pressure. In the water blanch, the prunes were dipped in boiling water for varying lengths of time, drained for a short time and then canned. In steam blanch- ing the prunes were placed in a rectangular metal tank and subjected to steam at 212° F. In blanching in steam under pressure the prunes were placed in a small retort and subjected to steam under pressures varying from 5 to 20 pounds. After blanching the prunes were canned in accordance with the standard procedure. The rate of formation of hydrogen swells of canned prunes pre- viously blanched in water for periods varying from to 90 minutes tended to increase with increase in time of blanch, but not in direct proportion to the time (fig. 2). The cause for the increase in swelling rate with blanching time was not determined. It may have been due to increased cell permeability or to changes in the compounds influ- encing corrosion. Since swelling is accelerated with increase in time of blanch it is advisable to blanch the prunes for only short periods of time. The time of blanching should be varied with the size and conditions of the prunes but should not be less than 5 minutes as a rule. Although the formation of swells was retarded when prunes were blanched less than 5 minutes such a short blanch was found undesirable because 10 University of California — Experiment Station the prunes did not absorb the desired quantity of water and remained shrivelled after canning. Prunes blanched in steam behaved much like those blanched in water in that the rate of swelling increased with the length of time of blanching; although the rates of swelling of prunes blanched 40, 60 and 90 minutes were all about the same. Those blanched 0, 5, 10 and 20 minutes swelled at rates more closely proportional to the length of time of blanch. From figure 2 it may be seen that cans of unblanched prunes swelled at a faster rate than those with prunes 600 I'PLAIN STEAM 6LANCH 2'BoiLING WATER. BLANCH 00 > Q Hi z> < >\l z e ) IOO ■-* > O 10 ZO 30 40 SO CO 70 60 90 Time: of blanch in minutes Fig. 2. — Effect of the time of blanching on the time required for 50 per cent of the cans to swell. Both curves start with the point for unblanched prunes. blanched 5 and 10 minutes in steam. Swelling of cans filled with prunes blanched 5 and 10 minutes in steam was slower than of those containing prunes blanched 2% and 5 minutes in water. The smaller amount of water absorbed during the short period of steam blanching was probably responsible for this difference. Prunes blanched in steam at atmospheric pressure absorbed water slowly and remained shrivelled when canned. Because of this defect steam blanching is unsuitable. Several lots of prunes were blanched in steam at 2%, 5 and 10- pound pressures for 2y 2 , 5 and 10 minute periods. The rate of swell- ing increased with the time and pressure of the blanch. These results were not included in figure 2 ; the variations caused by dif- ferences in pressure could not be shown in that graph. Bul, 508] The Swelling of Canned Prunes 11 These tests indicate that it is of no advantage to use blanch with steam under pressure, for it did not retard the formation of hydrogen swells, and the prunes remained shrivelled. Effect of Weight of Fruit Per Can. — The rate of spoiling did not vary greatly when practicable fill-in weights were used. The maximum rate of swelling occurred when 1 ounce of prunes was added to each can. When a greater or lesser weight of prunes was added to each can, the rate of swelling was less than when 1 ounce was used (fig. 3). ig- I 2. 3 4 5 6 7 Weight of f£uit pee. eight-ounce can in ounces -Effect of weight of fruit per can on the time required for 50 per cent of the cans to swell. The minimum weight of fruit that would eventually cause swelling was not determined, although swelling was very slow when less than % ounce was used in each can. Likewise, a sufficiently large weight to prevent swelling was not found. These differences in rates of swelling were probably caused by several factors. Kohman (14) has advanced the theory that such differences in swelling as those pre- sented above are caused by variations in the quantities of hydrogen acceptors, i.e., anthocyanin pigments, present in the can. In other words when one prune was canned in an 8-ounce can the can swelled rapidly because only a small amount of hydrogen was absorbed by the comparatively small amount of hydrogen acceptor in the can. When a number of prunes were in each can the cans swelled more 12 University of California — Experiment Station slowly because by adding" more prunes the quantity of hydrogen acceptor in the can was increased. The absorption of hydrogen was therefore increased and the rate of swelling decreased. This explanation, however, did not seem to apply to the canning test in which 6 ounces of fruit were used in each can ; although cans formed firm swells in many instances the tin plate was not heavily corroded. It was observed that the intensity of corrosion gives some indication of the production of hydrogen. Hence, in view of the fact that some of the cans were relatively lightly corroded the quan- tity of hydrogen gas produced was comparatively small. Although a large quantity of hydrogen acceptors were present in the large quantity of prunes, these cans swelled, indicating that other factors beside hydrogen acceptors influenced the unusual behavior. Difference in rates of diffusion of the corroded tin into the prunes and corroding material out of the prunes was probably a second important factor. The visible corrosion in 8-ounce cans containing less than % ounce of prunes was slight, being restricted to the seams on the sides of the cans and to the concentric panels on the ends. Corrosion was more apparent in the cans containing 3 and 4 ounces of prunes than in those containing more or less than these amounts. It was most appar- ent in cans containing 4 ounces; these were entirely detinned after several months of storage. The volume of gas produced in a moderate length of time was greatest in 8-ounce cans containing 3 and 4 ounces of prunes. The quantity of corrosive material in cans containing less fruit was appar- ently insufficient to produce a large volume of gas. TABLE 2 Eecommended Fill-in Weights for Various Sizes of Cans Size of container Net contents of prunes lJ^-2 ounces 3-4M ounces 7J^-8H ounces No. 1 tall No. 2 tall No. 2^ No. 10 3-3M pounds Although the rate of swelling was greatly decreased when 6 ounces of prunes were filled into an 8-ounce can this weight cannot be used because the prunes remain shrivelled and do not soften. Although the most rapid spoiling occurred when 3 and 4 ounces of prunes were Bul. 508] The Swelling of Canned Prunes 13 used in 8-ounce cans these are the most desirable weights to use in order to obtain quality and gain in size grade. Recommended fill-in weights for use in canning prunes are given in table 2. Effect of Kind of Sugar and Concentration of Syrup. — Prunes were canned in pure prune juice, and in various syrups containing 20 per cent of one of the following: sucrose, confectioners' glucose, ' * Nulomolene ' ' (invert syrup), and commercial dextrose. Swelling was found to be about the same for the first three sugars, slightly less when pure commercial dextrose was used, and much less when pure 1%, 2, 3, 4, 5, and 6 hours at 212° F in an open vat; other lots were cooked in a retort at 225° and at 238° for 20, 40 and 60 minutes and 10, 20, and 40 minutes respectively. The cans cooked more than 3 hours at 212° swelled much faster than those cooked for a shorter time. While the forma- tion of swells in cans cooked at 212° for time periods less than 3 hours was not entirely consistent, the tests indicated that cooking for 1 hour at 212° F was sufficient. Canned prunes cooked at temperatures above 212° swelled faster than those cooked at 212°. Effect of Cooling After Cooking. — Although cooling- of canned prunes has been advocated and considered an essential step in order to retard swelling, cooling did not substantiate this belief. Cans were cooled from to 16 minutes after cooking, but the rates of swell- ing- were about the same. Cooling- for at least 3 minutes in cold water, however, is advisable in order to prevent stack burning 7 while the cans are on the cooling- platform. In canning fresh fruits such as apricots and peaches it is neces- sary to pack large quantities of fruit in a few weeks in order to fill the orders for the ensuing" year. The large pack must then be stored until it is possible to distribute it during the year. Although this practice has proven very satisfactory for most fruits, it is unsatis- factory for ready-to-serve prunes because of loss from hydrogen swell- ing. Since dried prunes are always available it is possible to can ready-to-serve prunes only as orders are received. By this means the canned prunes may be shipped and distributed shortly after can- ning- and, normally, consumed before swelling occurs. However, should it be necessary to store canned prunes, the storage temperature should be as low as practicable in order to retard swelling. It has been observed that cans held at 30° and 40° F remained in good con- dition much longer than those stored at room temperatures. Kohman and Sanborn (16) have shown that swelling in canned berries and cherries varies with the storage temperature. 7 Stack burning is over-cooking caused by prolonged heating, resulting when cans are insufficiently cooled. Bul. 508] The Swelling of Canned Prunes 21 Rough handling that may dent the cans may also injure the tin coating and so expose the iron; this permits local cell action and hence increases the rate of corrosion. Furthermore, when cans are badly dented the vacuum and head space are usually decreased and swelling may be accelerated correspondingly. Kohman and San- born (16) found that when cans of cherries were dented, the rate of swelling was greatly increased. The present writers have observed that canned prunes behave similarly. Changes Occurring During Storage. — Tests were made in order to determine the changes occurring in concentration and volume of syrup in the can. Prunes were canned in distilled water and 10, 20, 39.5 and 50 per cent syrups. In each case, 100 cubic centimeters of water, or of the syrup, were added to a constant weight of prunes in each can. At frequent intervals during the storage period the volume and per cent of the syrup were noted (see table 5). The greatest change in concentration and volume of syrup occurred during the first 24 hours after canning. The magnitude of these changes decreased when the concentration of syrup added to the cans was increased. In table 5 the volume readings are accurate only to about 10 per cent because of unavoidable experimental error in obtaining the readings. TABLE 5 Changes Occurring During Storage in Concentration and Volume op Syrup in 8-ounce Cans When Various Concentrations op Syrup Were Used Per cent* immediately after cooking 1 day 2 days 3 days 4 days 12 days Syrup concentration Vol. cc Per cent Vol. CC Per cent Vol. CC Per cent Vol. CC Per cent Vol. CC Per cent 19 26.6 32 4 50 5 57.5 40 31 65 85 97 23 305 37.7 51.3 58.5 35 41 62 83 98 24.9 31.9 37.1 50 4 57.9 35 40 62 85 100 27.2 32.2 37.2 50 6 57.1 31 41 62 85 96 27.5 32 37.5 49.8 57.3 35 41 61 84 104 28 4 Sucrose 10 per cent Sucrose 20 per cent Sucrose 39. 5 per cent. Sucrose 50 per cent 31.6 36.5 49 4 57.6 Per cent* immediately after cooking 26 days 43 days 58 days 89 days 128 days Syrup concentration Vol. CC Per cent Vol. CC Per cent Vol. CC Per cent Vol. CC Per cent Vol. CC Per cent 19 26.6 32.4 50 5 57.5 26 36 50 80 105 28.5 32 9 38 8 50 56.8 24 33 55 90 102 28 33.2 37 48.5 57 17 30 44 80 103 28 33.5 38 49.8 56.4 36 30 46 76 100 27.2 32 36 2 50 57 32 32 46 72 104 28 8 Sucrose 10 per cent Sucrose 20 per cent Sucrose 39 . 5 per cent. Sucrose 50 per cent 35 36.4 50 1 57.2 ♦Volumes were not recorded at this time. 22 University of California — Experiment Station RECOMMENDED CANNING PROCEDURE The following procedure, based on the results of the investigations, is recommended for use in the commercial canning 1 of prunes: Sound French prunes of good quality should be washed in cold water and then blanched 5 to 10 minutes in boiling water. The blanched fruit should be filled into plain or charcoal plate cans in accordance with table 2 of this publication. Cane sugar syrup 20-30 per cent should then be added to the cans as directed below. In adding syrup, the canner should allow considerable head space. Depths of head space recommended are given in table 3. The cans should then be exhausted for 10 minutes at 210° F and sealed without cooling or refilling. The sealed cans may then be cooked in an agitating cooker or vat. One hour at 212° F is recommended for No. 2%, or smaller cans; and iy 2 hours for No. 10 cans. After cooking, the cans should be cooled about 3 minutes in cold water. It is inadvisable to store canned prunes for long periods. The safest procedure is to can and ship prunes only as orders are received. However, should it be necessary to store the cans for some time, the place of storage should be cool and dry. SUMMARY AND CONCLUSIONS 1. The marketing possibilities for canned prunes appear to be good but commercial production has not increased appreciably because of spoilage losses from swelling of the cans with hydrogen gas, formed by corrosion. 2. Corrosion and hydrogen swelling are apparently influenced by a number of factors both chemical and physical in nature. 3. Swelling of enamel-lined cans was more rapid than of coke- plate cans ; that of charcoal-plate cans was slower than that of coke- plate cans. 4. The procedure used in drying prunes apparently had no effect on the rate of swelling. Canned sun-dried and dehydrated prunes swelled at the same rate. Bul. 508] The Swelling of Canned Prunes 23 5. When the time or temperature of steam or water blanch was increased the rate of swelling increased. 6. The rate of swelling decreased when the concentration of syrup used was increased. However 30 per cent syrup was found to be the maximum sugar concentration that could be used because more con- centrated syrups caused the prunes to harden and shrivel. 7. The rate of swelling decreased when the depth of head space was increased. A comparatively large head space should be used as this provides additional space for collection of the hydrogen gas. 8. An exhaust in steam for 10 minutes at 210° F was found best. 9. The kind of gasket used had little effect on the rate of swelling; paper gaskets and rubber compound gaskets gave similar results. 10. Cooking for 1-1% hours in boiling water was found best for canned prunes. Pressure cooking increased the rate of spoiling. 11. Decreasing the pH value of the syrup retarded the rate of swelling but not of corrosion and it affected the color of the prunes adversely. 12. Factory tests confirmed all the major findings of laboratory experiments. 13. Canned prunes should be distributed very soon after packing. If storage is necessary the storage temperature should be as low as practicable in order to retard corrosion. ACKNOWLEDGMENTS The authors express their appreciation to the California Prune and Apricot Growers Association for their cooperation and assistance which has made these investigations possible. Helpful cooperation has also been received from the J. C. Ainsley Packing Company, Campbell, California; the Richmond Chase Pack- ing Company, and the American Can Company. 24 University of California — Experiment Station LITERATURE CITED i U. S. Bureau of Census. 1929. Census of manufacture. 1927. Canning and preserving. U. S. Department of Commerce. ^ Christie, A. W. 1924. Methods of canning prunes. Canning Age. 5:821-822. 3 Christie, A. W. 1926. Several methods of canning prunes. Western Canner and Packer. 18(3) : 10-12. 4 Kohman, E. F., and N. H. Sanborn. 1924. The nature of corrosion in canned fruits. Jour. Ind. Eng. Chem. 16:290-295. s Culpepper, C. W., and J. S. Caldwell. 1927. The behavior of the anthocyanin pigments in canning. Jour. Agr. Res. 35:107-132. e Clough, E. W., O. E. Shostrom, and E. D. Clark. 1924. Lime-sulfur spray on canned gooseberries. Canning Age. 5:531-534. 7 Culpepper, C. W., and H. H. Moon. 1929. The swelling of tin cans packed with peaches. Jour. Agr. Kes. 39:31-40. s Lueck, H. B., and H. T. Blair. 1828. Corrosion in the tin can. Trans. Amer. Electrochem. Soc. 54:257- 279. 9 Kohman, E. F., and N. H. Sanborn. 1928. Discussion. Trans. Amer. Electrochem. Soc. 54:279-287. io Goss, B. C. 1917. Absorption of tin by proteins and its relation to the solution of tin by canned foods. Jour. Ind. Eng. Chem. 9:144. ii Kohman, E. F. 1923. Oxygen and perforations in canned fruits. Jour. Ind. Eng. Chem. 15:527-528. 12 Kohman, E. F. 1924. A new hypothesis of perforation. Canning Age. 5:308-310. is Mrak, E. M., and W. V. Cruess. 1929. Utilization of surplus prunes. California Agr. Exp. Sta. Bui. 483:3-34. 14 Kohman, E. F. 1929. Amount of corrosion in cans explained. Fruit Products Jour, and Amer. Vinegar Ind. 8(7): 23. is Kohman, E. F. 1929. Research findings on corrosion and vitamin destruction. Canning Age. 9:227-231. i6 Kohman, E. F., and N. H. Sanborn. 1927. Storage temperatures for canned fruits. National Canners' Bui. 23L:1-16. 13wi-3,'31