INDUSTRIAL AKD E N G I N E E R I S G CHEMISTRY
920
of the starch (ratio of starch to water, 1 to 5 ) ( 2 ) . Colorimetric determinations were made on the clear liquid from which the starch had been removed [as noted by Ripperton
YJSCOSITY.The maximum viscosity of this starch is higher than the maximum obtained on other sweet potato starches which have been examined under identical condii3 per cent paste was prepared and its viscosity tions ( 3 ) . ; determined with a Stormer viscometer in the manner described in a previous article (3). Figures representing the viscosities which were obtained are given in Table 11. The values are plotted on the curve in Figure 2 . The viscosity curve for corn starch is inserted merely for reference. TABLE 11.
VISCOSITY O F
TEJIP. TIME C.
060.
80-
99'don,N.
is97
l*O,V,N
(994
reon,*
~99'1
,o'm,N
CPPY
7 L ~ , = . C L i o A 7 U I~N ~ & 7,MZ
POTATO STARCH FIGURE2. VISCOSITYOF SWEET
( 2 ), measurements made with the quinhydrone electrode are somewhat lower than those made colorimetrically]. The ash and protein content are both exceptionally low.
DATAON SWEETPOTATO STARCH TABLEI. ANALYTICAL ASH %
AtoIBTuRE
%
'
P H
uinhydrone Chlorophenol eiertrode red
PROTEIN ( N X 6.25)
%
Vol. 23, No. 8
Xin.
3 PER CENT SWEET POTATO STARCH
PASTE
VISCOSITY
TEMP. TIME
Stormer units
C.
Min.
VISCOSITY
Stormer units
TEMP. TIME C.
Min.
VISCOGITY
Stormer units
COST OF CHEMICALS. Approximately 2 pounds of sodium hydroxide and 0.75 pound of sulfur dioxide were used in each run. Thus, the cost of chemicals should be somewhat under 0.1 cent per pound of dry starch. Although this chemical treatment adds an appreciable amount to the apparent cost of production, the yield is increased sufficiently so as actually to lower the cost of production per pound of starch. LITERATURECITED (1) Balch and Paine, ISD.ENG.CHEM.,23, 1205-13 (1931).
(2) Ripperton, Hawaii iigric. Expt. Sta., Bull. 63 (1931). (3) Thurber. IXD. ENG.CHEW,25, 565 (1933). RECEIVEDJanuary 21, 1933. Presented before the Diviaion of Sugar Chemistry a t t h e 84th Meeting of the American Chemical Society, Denver, Colo , August 22 t o 2 8 , 1932. This paper is Contribution 127 from the Carbohydrate Division, Bureau of Chemistry and Soils.
Increased Acidity Inhibits Corrosion Application t o Canning Prunes E. F. KOHNANAXD X. H. SANBORN,National Canners Association, Washington, D. C.
D
RIED prunes possess certain unique dietetic and nutritive characteristics that justify their occupying a definite place in our dietary. As in the case of all dehydrated products, however, the forethought that is necessary to have them ready for a given meal militates against their use. This would indicate the desirability of canning this fruit, for which dehydration would seem superfluous. Evidence in favor of canning the fresh prunes may be found in recently published data ( 3 ) which show that, while dehydration results in a loss of most of the vitamin C in prunes and a considerable portion of the vitamin A, this latter vitamin is unaffected in canning fresh prunes, and vitamin C is fairly well retained. DEHYDR-4TION ADDSTO FLAYOR I n recent years the Italian variety of prunes, grown largely in Oregon and Washington, has been canned in considerable amounts in the fresh state. The canned product has a brilliantly colored sirup, and the prune itself presents an attractive appearance. Its relative tartness is an advantage from the standpoint of flavor for canning it in the fresh state As will be understood from the contents of this paper, its relatively low p H tends to minimize its attack on the can. This variety of prune, however, is not produced in as large quantities as the French (Santa Clara) variety, grown in California. When the latter variety is canned fresh, the
color of the sirup is mediocre compared with that of the Italian variety and the prune itself is less attractive than the Italian variety. PvIoreover, the flavor is flat and inFipid. The French variety of prune needs to be dried to give it the most desirable flavor. Because of the desirability of having prunes ready to serve, there was a general movement among canners a few years ago to can prepared prunes-i. e., stewed dried prunes, ready t o serve. The losses encountered due to hydrogen swells and perforations, because of the excessive corrosive effect of the prunes on the can, were so great that the canning of this commodity has heen almost completely abandoned and it is now impossible to purchase it on the retail market. The losses are not quite as great in the larger sized cans, and certain governmental departments have continued to purchase them in this form. Because of their convenience, the Navy has purchased prunes in canned form only for a number of years in spite of the fact that large losses were encountered.
DEHYDRATION IKCREASES CORROSIVEEFFECTON CAN An investigation has disclosed the fact that drying or dehydrating prunes greatly increascs their corroiive effect on the can, thus giving rise to undue losses from hydrogen swells and perforations. This is definitely brought out in experimental packs represented in Table I. A word as to what constitutes "losses" in this connection will give a clearer
I N D L S T R 1.4 L A N D E N G I S E E R I N G C H E R I I S T R Y
August, 1933
understanding of the accompanying tables. All properly canned foods are packed so that the finished product will exert sufficient vacuum to hold both ends of the can in a concave position under any condition of temperature and atmospheric pressure that they may encounter during distribution. As soon as one end of the can bulges, or if it becomes perforated, it is no longer merchantable and is then a loss. Canned fruits may form enough hydrogen by their action on the can to dertroy the vacuum and thus permit one end to bulge without any effect on the quality of the fruit being evident. Practically all losses in fruits in plain cans are due to this cause. The action of fruits in enanieled cans is localized, and frequently the can becomes perforated without sufficient hydrogen being formed to destroy the vacuum in the can. This is characteristic of low-acid fruits, while high-acid fruits in enameled cans generally cause losses by bulging the can end rather than perforating the (can. This is explained in detail elsewhere (2). TABLEI. LOSSESWITH FRESH.4ND
DEHYDRATED PRUNESa
(Packed September 26, 1929) FRESH 7 DEHYDRATED PLAIN CANS
ENAMELED CAN0
Unpitted
Unpitted
7
0
PLAIN
ENAMELED C A N 8
LyeCANS Lye-Dipped Dipped Unpitted Unpitted
%
%
%
Not Lye-Dipped UnPitted pitted
%
%
... ... Dec. 12 1929 40:5 17.3 7:7 15:4 Feb. 24: 1930 7.7 46.8 28.9 32.7 Mar. 28, 1930 4 4 . 3 5 4 . 1 10.6 3 4.6 A r 29, 1930 ... 11.5 56.8 59.8 34.6 g a y 28, 1930 ... 26.9 67.5 84.7 57.7 July 30, 1930 42.3 ... 70.3 92.3 73.1 Sept. 2, 1930 48.1 100.0 73.0 3.5 82.7 Oct. 3, 1930 5.2 ... 57.7 73.0 86.6 Nov. 4, 1930 59.7 ... 5.2 78.4 86.6 Dec. 8, 1930 6.3, 5 ... 78.4 90.4 6.9 Feb. 10, 1931 a When the prunes were not lye-dipped, they were splir n half to facilit a t e dehydration.
... ... ...
0
92 1
It is almost universally the case that losses are greater in enameled cans than in plain cans for any given fruit. A comparison of columns 1 and 2 of Table I shows that this is true with fresh prunes; a comparison of columns 3 and 4 suggests a like conclusion on this point with dehydrated prunes, with which the action in both types of cans was so rapid as to make them commercially impractical. .4 comparison of columns 5 and 6 indicates that the pits left in dehydrated prunes somewhat augment corrosion. A comparison of columns 4 and 6 indicates that the lye-dipping has a slight effect in increasing corrosion. All the above-mentioned factors affecting the corrosive nature of prunes in the can are relatively small compared with the effect of dehydration. This is brought out by comparing columns 1 and 3 in Table I, representing the effect in plain cans, and by comparing columns 2 and 4, representing the effect in enameled cans. Considering all the facts, it is apparent also that the major effect of dehydration is on the flesh of the fruit rather than on the pits. It has been found that dehydrated apples that have had no pretreatment other than slicing are more corrosive when canned in comparison with fresh apples. I n canning both prunes and apples, the precaution of having the same amount of original fruit per can was observed. The reason for the greatly augmented corrosiveness of these dehydrated fruits is still in part a matter of theoretical speculation. It is probable, however, that during dehydration the prunes take up more or less oxygen from the air, which enters into combination with the prunes in a manner that enables it still to have oxidative-i. e., corrosive-properties. The presence of oxygen in any electrolyte tends to make tin electropositive to iron. As has been previously pointed out (W),it is particularly desirable to have tin electronegative to iron in canned fruits to avoid excessive corrosion. It may be assumed that, during dehydration, prunes absorb oxygen which retains its property of tending to make tin electropositive to iron. The negative potential of both iron and tin was found to be greater in dehydrated than in fresh prunes, and, while tin is generally anodic to iron in canned fruits, in some lots of dried prunes it was found to be cathodic.
The prunes repre5ented in Table I were dehydrated in a hurricane drier similar to that used in industry. It is customary to lye-dip prunes for drying or dehydration to check the skin and permit more ready escape of the water vapor. This might be expected to raise the pH and thus increase corrosiveness ( 2 ) . To reduce the effect of the lye on the pH TABLE 111. EFFECTO F CITRIC ACID ON LOSSESWITH and thus permit any other effect of dehydration to be more DEHYDR~T PRUSES ED in evidence, the lye-dipped prunes represented in Table I (Packed November 5, 1929) were thoroughly washed after lye-dipping. In order to de--PLAIN CAKS-ENAMELED CANS--termine the effect of the lye-dipping, one lot of prunes was 0 0 0 25 0 5 1 . 5 0.0 0.25 0.5 1.5 % " % % % % % % % dehydrated without this treatment. To facilitate dehydraDATE PH PH P H P H PH PH PH EXAXINED 3.95 3.90 3.80 3.55 3pF5 3 . 9 0 3 . 8 0 3.55 tion without lye-dipping, the prunes were split in half. I n % % % % % % % % this condition they dried as readily as the lye-dipped prunes. A comparison of the taste of canned fresh prunes and canned prepared dehydrated prunes discloses the fact that dehydration affects the pits in some way which causes the constituents of the kernel to diffuse into the sirup of the fruit to a greater extent. This is possibly one reason why the dehydraa Percentage citric acid added on basis of dried prunes. tion process increases the corrosiveness of the prunes, since it has been found that cherry pits, when broken, accelerate I t was also shown that increasing the pH of any given corrosion in canned cherries. This effect is given in Table 11, fruit tends to render tin electropositive to iron and this has in which the increased losses resulting from crushed cherry an unfavorable effect in causing increased corrosion in the pits are recorded. For this reason one lot of split prunes was can. Since dehydrated prunes generally have a higher pH dehydrated without the pits. than the fresh prunes, this would tend to augment corrosion. Increasing the acidity or lowering the pH of fruits by the addiTABLE 11. EFFECT OF PITSON LOSSES WITH REDSOUR CHERRIES tion of citric acid, for example, has a tendency to render the (Packed June 21, 1926) tin electronegative to iron and thus lessens the hazards from DATEEXAMISED PITTED UNPITTED PITECRUSHED corrosion. The effect of citric acid with dehydrated prunes % % 94 .. is particularly marked. This is shown in detail in experiNov. 15, 1926 .. .. .. Dec. 20, 1926 .. .. .. mental packs recorded in Table 111. Jan. 17 1927 .. Feb. 14: 1927 March 15 1927 April 20, i927 June 3. 1927 Aug. 5 ; 1927 Oct. 24, 1927
..
4:0 6.0 14.0 34.0
2:0
S:O
10:2 14.3 51.0
24.0 50.0 72.0 86.0 96.0
..
ISCREASING ACIDITYDECREASES CORROSION Although the canning of prepared dried prunes has been almost completely abolished, except for special purchasers
922
INDUSTRIAL AND ENGINEERING CHEMISTRY
because of the losses incurred, it became particularly desirable to can strained or pureed prunes when the canning of strained or pureed vegetables came into vogue for infant feeding. The U. S. Department of Labor (1) recommends the feeding of prune pulp to infants beginning with the tenth month. Prunes, therefore, are necessary to make a complete line of infant foods. The general experience in the canning industry is that the losses due to corrosion are greater with small cans than with large. Infant foods are packed in small cans. The corrosive effect of prune pulp in small cans was so great that losses began to be pronounced within a few weeks after canning, thus making it an impractical commercial commodity; it was therefore difficult to distribute a complete line of infant foods. The addition of citric acid, while effective in lowering losses in canned dried pruner, is not suitable because the general buyer would not recognize citric acid, declared on the lahel, as a common constituent of prunes and of many other fruits. Lemon juice, however, as a source of citric acid, is quite as effective as citric acid itself if the slight buffer effect of lemon juice in preventing the lowering of the pH is discounted. The addition of lemon juice to prune pulp canned for infant feeding has now been practiced commercially for nearly two years with entire satisfaction. The Navy has purchased, on a semicommercial scale, canned prepared prunes acidified with citric acid, and hereafter intends to purchase only such prunes unless unforeseen events indicate othervise. It is not an uncommon household practice to add a slice of lemon to stewed prunes for purposes of flavor. The added citric acid or lemon juice improves the flavor of the prunes because they are somewhat insipid, owing to their low acidity. EFFECT O F ADDEDACIDEXPLAINED BY ELECTROCHEhtIC.4L THEORY OF CORROSION I n most instances within the range of pH commonly found in canned fruits, corrosion is in the order of the degree of acidity. Losses due to corrosion in canned fruits are in reverse order of the degree of acidity. This is entirely logical when the fundamental conditions are considered. I n previous publications (2) it was definitely demonstrated that, contrary to common belief, tin is electronegative to iron in canned fruits. Increasing the acidity intensifies this electronegative condition. It was further shown that this is a particularly wholesome condition to prevent excessive corrosion in canned fruits. If tin were electropositive to iron, the corrosion of the iron would be excessive and perforations premature. This electronegative condition of tin, particularly in a plain can where the area is enormous compared with the exposed iron, affords a strong inhibiting effect on corrosion of iron. Moreorer, the tin ion itself acts as an efficient inhibitor against iron corrosion. Both these factors are greatly minimized in enameled cans, since most of the area of tin is covered by the enamel and therefore the amounts of tin ion in solution are held to a minimum, Herein lies one reason for the greater losses with enameled cans as compared with plain cans. Tin has one property of great value-a tendency to form high over-voltages. This apparently plays a greater role in reducing corrosion in plain than in enameled cans. This overvoltage is so pronounced that it practically inhibits the formation of gaseous hydrogen when the product tends to attack the tin of the can rather than exposed iron. Consequently there is a high reduction potential created which permits the reduction of various fruit constituents that may act as hydrogen acceptors. I n the more acid fruits in which iron is strongly electropositive to tin, this is so pronounced that practically all the tin may be corroded off without any appreciable quantity of hydrogen being formed. When, however, the tin ir extensively removed by corrosion, iron corrosion takes place and hydrogen begins to form.
Vol. 25, No. 8
If no iron is present, these same fruits have no marked corrosive action on tin. Since exposed iron in the can is cathodic, it may be supposed that the hydrogen would collect on it and bubble off as gaseous hydrogen because of its lower overvoltage. However, the area of exposed iron is, relatively, extremely small, and it is not improbable that this exposed iron may change the potential of adjacent tin sufficiently to cause it to be cathodic to more distant areas of tin. Nascent hydrogen thus may be deposited largely on tin surrounding any exposed iron, although the cathodic iron is the actual cause of the tin corrosion. While it may seem unorthodox, therefore, to prevent corrosion by increasing acidity, it is evident that, when all the facts are considered, it is entirely in conformity with the principles of the electrochemical theory of corrosion. The application of these principles to the canning of prunes is a natural sequence to the data in earlier publications from this laboratory ( 2 ) . It appears from an article by Mrak and Richert (4) that they concurrently attempted an application but seem unfavorably impressed since they say: “Lemon juice retarded swelling but gave the fruit an objectionable flavor and color.” I n another publication ( 5 ) they report: “Decreasing the pH value of the sirup retarded the rate of swelling but not of corrosion and it affected the color of the prunes adversely.” Their statement that corrosion was not retarded is not clear from the data they present. I n regard to flavor, it may be said that lemon juice, like most citrus fruits, contains substances that under certain conditions lead to off-flavors commonly designated as terpene-like on storage. No difficulty of this kind results from the use of citric acid or certain concentrated lemon juices that are commercially available. SUMMARY The drying of the French or Santa Clara variety of prune, grown in greatest abundance, is necessary to produce the most desirable flavor. The forethought necessary to prepare a dehydrated product for any given meal is an obstacle to its use. Dehydration of prunes increases their tendency to attack the can when canned in sirup, and it has not been possible to can them ready to serve because of the losses thus incurred. Acidifying with citric acid corrects this condition and, because of the low acidity of the prunes, improves the flavor. Lemon juice, as a source of citric acid, makes it possible to accomplish this without the necessity of an unfavorable declaration on the label. LITERATURE CITED (1) Department of Labor, Children’s Bur., Publ. 8 (1932). ( 2 ) Kohman and Sanborn, IND.ENG.CHEM.,20, 76, 1373 (1928); 22, 615 (1930). ( 3 ) Morgan and Field, J. Biol. Chem., 82, 579 (1929) and 88, 9 (1930); J. Agr. Research, 42, 35 (1931); Eddy, Gurin, and Kohman, IXD.ENO.CHEM, 24, 457 (1932). (4) Mrak and Richert, Fruit Products J . , 10, 310 (1931). (5) Mrak and Richert, Univ. Calif. Agr. Expt. Sta., Bull. 508 (1931).
RECEIVED January 24, 1933
I n the correspondence on “Comparative EfCORRECTION. ficiencies of Amylo and Malt Processes for Production of Industrial .klcohol” by M. Neubauer, IND. ENG.CHEM.,25,712 (1933), tho following correction should be noted: Column 1, footnote a to the tabular material should read ‘ T s e d as malt in malt process.” Column 1, the sentence beginning on the fourth line under the tibular material should read: “On the basis of producing 100 liters of alcohol, the coal required, using saccharine raw materials, by the malt process, is 100 kg.; and from amylaceous raw materials, 130 kg. by the same process. Where the amylo process is employed with amylaceous raw materials, 230 kg. of coal are needed.”
