682
I N D U S T R I A L A N D E N G I N E E R I N G C HE M I S T R Y
the contents aside from any effect of the humidity in the air of the room. To remedy this difficulty, last season a method of humidifying the packing cases in flat bundles as received from the factory was installed in this plant. By means of suitable apparatus, air supersaturated by admixture of steam is blown through the corrugation channels between the walls of the strawboard for sufficient time to bring the moisture content of the strawboard into humidity equilibrium with the atmosphere. The cases are then set up and filled with packages. They are closed with gum tape. In storage it is found highly important to maintain constant temperature. Fluctuating temperatures of the fish cause crystals to grow larger a t the expense of the small crystals, the result being, in time, the same effect as would have been caused by slowfreezing so far as crystallization is concerned. Apparently, fluctuation has also caused a progressive increase in the state of aggregation of the protein molecules increasing the amount of separable juice and toughening the muscle fibers.
Vol. 24, No. 6
It should be noted that conclusions are commonly drawn from readings of fluctuating temperatures of the air in the storage. The only temperature fluctuation that has any effect on fish is that in the fish itself. It is surprising how long a time is required to change the temperature of the mass of fish in storage. If stored a t too high a temperature, weeks may elapse in a room with air temperature of -17.5' C. (0.5"F.) before the fish reach a temperature of -8' C. (17.6' F.). If the freshly frozen product is allowed to warm during the packing process and is sent to the cold room at a relatively warm temperature, this exceedingly slow abstraction of heat results in adverse changes of the kinds mentioned above. The product is shipped in silica gel or in diesel-driven compressor-refrigerated cars, and is stored a t suitable marketing points throughout the country in public cold storage warehouses at a temperature preferably from - 17" to -15" C. (1.4'to 5' F.). RECEIVED April 7, 1932.
Chemical Problems of the Quick-Freezing Industry DONALD K. TRESSLER, The Birdseye Laboratories, Gloucester, Mass.
T
HE freezing of vegetable or animal matter is not a simple physical phenomenon such as occurs when a pure liquid freezes. For years scientists have studied the freezing of meat and fish, but even now they do not understand all that occurs when these products are frozen and then thawed. Still less is known about what happens when fruits and vegetables are frozen and then thawed. The difference between the effects of slow-freezing and quick-freezing has already been described numerous times (1, 2, 6, 6). From those reviews it is seen that in general the main advantages of quick-freezing over slow-freezing are the following: 1. The ice cr stals formed are much smaller and therefore cause much less Lmage to the cells. 2. The freezing period being much shorter, less time is allowed for the diffusion of salts and the separation of water in the form of ice. 3. The product is quickly cooled below the temperature at which bacterial, mold, and yeast growth occur, thus preventing decomposition during freezing. 4. The product is quickly cooIed to a temperature a t which enz me action is of no ractical significance, and the original fresKness is thus retaine!. This quick chilling is of great importance in the case of sweet corn, peas, lima beans, asparagus, strawberries, raspberries, and other fruits and vegetables.
It is not the purpose of this paper to discuss these points, nor to expound upon the theories which have been brought forth to explain what happens when flesh is quick-frozen, nor to explain the reasons for the different effects that quickfreezing has on flesh and vegetable products, but rather to indicate the applications of chemistry in this fast-growing industry and how some of the commercial problems of the industry are being solved. PREVEKTION OF DESICCATIOK GLAZING. N o frozen product can be kept for long periods unless it is protected from desiccation. Frozen products of all sorts dry out very rapidly during cold storage unless loss of moisture is prevented by some sort of covering or wrapping material. Products desiccated in cold storage lose their natural flavor and aroma and when rehydrated do not return to their original condition. For many years it has been common practice to glaze whole fish and thus pre-
vent their drying out without packaging or wrapping. This is accomplished by dipping the frozen fish in cold water. Since the fish are below the freezing point of water, a coating of ice or glaze freezes on the surface. When the fish is stored, the loss of moisture occurs only from the glaze until the glaze is entirely lost. Then the fish must be reglazed. Obviously, the frequent reglazing requires much extra work. PACKAGING. While glazing may be a fairly satisfactory means of protecting whole fish against loss of moisture, it cannot be used to advantage to prevent loss of moisture from filleted fish, small cuts of meat, small fruits, vegetables, and many other products. Products of this sort must be wrapped in moisture-vaporproof paper or foil, and packaged. There is a difference between a waterproof paper and one that is moisture-vaporproof. A waterproof paper does not go to pieces while standing in water and, moreover, does not permit liquid water to soak through it. A moisturevaporproof paper does not permit moisture vapor to pass through it; in other words, it must not permit much air to diffuse through it. Obviously a paper may be waterproof without being moisture-vaporproof. While many products-for example, haddock Wets, lamb chops, and other thin cuts of meat-may be frozen in somewhat less time before packaging than afterwards, yet, as will be seen from the following considerations, packaging first and then freezing has the following advantage: If a flesh product, such as haddock fillets or lamb chops, is first frozen and then wrapped and packaged, it will require nearly twice the area of wrapping material and will occupy, when packaged, at least 50 per cent more volume and therefore requires a much larger package than would have been the case if the product had been wrapped and packaged prior to freezing; for, after freezing, flesh products are hard and unyielding and cannot be packed solidly into a container. Obviously, when food products are compactly packaged, much less surface is exposed to desiccation and oxidation. Another factor which should be considered is that the volume occupied by a package containing a given weight of a frozen product is important from the standpoint of heat insulation, for, assuming that the packages are alike otherwise, the amount of heat entering the package is directly proportional to the total area of the surfaces of the package.
June, 1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
683
PREVENTION OF OXIDATION Vegetable parchment paper, glassine paper (paraffined on both sides), and Moistureproof Cellophane are excellent FATTY FISH. Desiccation is only one of many problems wrapping materials for use in protecting frozen foods against confronting those storing frozen food products. Another desiccation during storage. Vegetable parchment paper difficulty is that of preventing oxidation and hydrolysis durcoated with paraffin on both sides is widely used for wrapping ing storage. While desiccation becomes objectionable only fish fillets. It does not disintegrate in water even when when an appreciable loss of moisture occurs, oxidation in some boiled in it; however, a t low temperatures the wax coating is cases may cause serious changes even though only small somewhat brittle and tends to crack. amounts of oxygen are absorbed, especially when hydrolysis Glassine paper, paraffined on both sides, makes a partially occurs simultaneously. For instance, fatty fish, such as satisfactory heat-sealed mapper for the' outside of small salmon and mackerel, contain large amounts of unsaturated cartons. Unfortunately it gradually loses strength when fatty oils which rapidly absorb oxygen from the air. Simulwet and therefore is not suitable for use in direct contact taneously these oils hydrolyze, forming free fatty acids, with moist products. Glassine papers which have a moisture- glycerol, and certain objectionable odorous by-products. The fishery technologist calls the vaporprobf c o a t i n g similar results of these actions "rusting," to Moistureproof C e l l o p h a n e because the fish turns dark on are superior to the o r d i n a r y account of the darkening of the waxed glassine papers, as they I n the establishment of any new industry, oil. Food technologists say that h e a t - s e a l b e t t e r a n d do not the fat of "rustyJJfish is rancid. numerous unforeseen chemical problems are lose strength materially when All are agreed that the fish is wet. encountered. The quick-freezing industry has no longer appetizing. Moisture-vaporproofed transbeen no exception, for extensive chemical reSolidly packing fatty fish in parent sheet cellulose, such as searches have had to be carried out in order to cartons tightly w r a p p e d with Moistureproof Cellophane solve the problems of desiccation, oxidation, moisture-vaporproof paper reand similar m a t e r i a l s , is an leakage on thawing, development of off-flavors, tards but does not prevent rustexcellent lining for small cold ing, as sufficient air remains and odors and enzyme action. Desiccation and waxed cartons suitable for use in a n d diffuses t h r o u g h t h e in packaging fruits, vegetables, oxidation have been controlled largely by comp a c k a g e s to cause t r o u b l e . and small c u t s of m e a t a n d pact packaging with moistureproof and moisfureFatty fish keep much better a t fish. I n packaging larger cuts vaporproof materials. The leakage of moisture very low storage temperatures of m e a t s , p o u l t r y , and fish f r o m thawed fish has been controlled by proper than at the usual 0" or 10" F. in l a r g e size p a c k a g e s , the (-17.8" or -12.2" C.), .and brining of the Jillets in a pure salt solution. individual cuts are more conon this a c c o u n t temperatures v e n i e n t l y wrapped in MoisObjectionable enzyme actions in vegetables are of -20" F. (-28.9" C.) or lower tureproof Cellophane prior to prevented by blanching preparatory to freezing. are recommended. packaging a n d freezing, Chemical changes in frosted fruits are prevented Vacuum-packing of fish in When packed in this way, the by packaging with sweetened juices or sugar enamel-lined tin cans prior to i n d i v i d u a l cuts can be easily sirups. freezing may prevent rusting, s e p a r a t e d from e a c h o t h e r but tinned products are oreven when frozen. Moisd i n a r i l y a s s u m e d by contureproof Cellophane possesses sumers to be sterile, or nearly so, t h e a d v a n t a g-e s of b e i n g nearly completely moisture-vaporproof, waterproof, grease- and therefore capable of storage a t ordinary room temperaproof, and in addition nearly completely transparent, but its tures. Housewives have been known to place cans containing use is limited by its high price. Ordinary transparent cellu- frozen foods on their kitchen shelves and then wonder why lose, such as Cellophane, cannot be used because it is not the products spoiled. For this reason there is much prejudice moisture-vaporproof, and because the sheets adhere to each against the use of tin cans as containers for frozen products. other both while the product is frozen and after it has been Other frozen products which tend to be discolored because thawed. of oxidation are cherries, peaches, pears, apples, mushrooms, Moisture-vaporproof sheet cellulose acetate, such as and parsnips. FRUITS.Overholser and Cruess (4) in 1923 concluded Eastman's Kodapak, appears to offer an exceptionally good protective wrapper for quick-frozen products. Its trans- that the substance which turns dark when apple tissue is parency is very good; its permeability is practically as low exposed to the air is a complex tannin containing a catechol as that of any other grades of moisture-vaporproof cellulose group. The oxidizing system concerned was determined materials; there is no separation of the moisture-vaporproof to be a peroxidase and an organic peroxide occurring in the coating when in contact with moisture, and it does not ap- fruit. Apples and other fruits can be protected against this obpear to become excessively brittle a t low temperatures. The main disadvantage is that its tensile strength at room tempera- jectionable oxidative discoloration by freezing in a sugar ture is considerably less than that of other types of moisture- sirup. When frozen under sirup, they remain of normal vaporproof cellulose sheets; but still i t appears to be strong color for long periods if stored at a low temperature, but when enough to be used safely for a protective wrapper. However, thawed they gradually discolor unless given some additional it lacks pliability, which is a feature much to be desired. treatment. It often is charged with considerable static electricity, which As an example of the difficulty encountered in handling is undesirable for machine handling and not good for any an easily oxidizable fruit product, the freezing of peaches may kind of handling. It is likely always to cost slightly more be given. The trade calls for a sliced peeled peach. Acthan the nitrocellulose-coated cellulose materials, owing to the cording to the present commercial processes, the peaches are fact that the basic material, which is cotton linters, costs con- peeled rapidly, then sliced quickly, packed immediately under siderably more than the wood pulp from which the other sirup in water-tight containers, and frozen. The peeling is material is made. accomplished by immersion of the whole peaches in a hot
V
A
INDUSTRIAL AND ENGINEERING CHEMISTRY
684
TABLE I.
IN
COMPARATIVE STORAQE
TESTSOF HADDOCK FILLETS5 TREATED WITH
DIAMOND CRYSTAL SALT TEMP.-26" TO -29' c. TEMP.-go TO -6O C. TIME Results of Results of STORAQE Leakage cooking tests Leakage cooking tests Month8 % % 1 2.78 Excellent quality 4.07 Excellent qualjty 2 1.58 Excellent quality 2.14 Excellent quality 3 1.09 Excellent qualjty 0.84 Excellent quality 4 2.27 Excellent qual!ty 3.15 Excellent quality 6 0.97 Excellent quality 2.52 Excellent quality
TEMP.-26' Leakage
P U R E AND IMPURE SALT
-29OC. Results of cooking tests
IVIZA SALI
TO
% Excellent quality Excellent quality Excellent quality Excellent quality Slight odor Slight salt-fishy odor and flavor Slight salt-fishy odor and flavor Salt-fishy odor and fiavor Salt-fishy odor and flavor; colored Salt-fishy odor and flavor: colored Salt-fishy odor and fiavor; colored a n d tough Salt-fishy odor and flavor; colored and tough
6
4.52
Excellent quality
4.73
Very slight odor
4.83
2.88
Excellent quality
2.13
Slight odor
3.66
8
1.01
Excellent quality
1,76
Slight odor
1.36
9
3.77
Very slight odor
2.99
2.91
10
0.79
Very slight odor
3.17
2.69
11
1.02
Very slight odor
3.56
Slight salt-fishy odor a n d flavor Slight salt-fishy odor and flavor Salt-fishy odor and flavor
12
2.45
Very slight odor
2.45
Salt-fishy odor and flavor
4.92
6.34
TEMP.-9'
Leakage
TO -6' C. Results of cooking teats
%
3.51 3.31 1.12 4.28 2.33
7
Vol. 24, No. 6
5.25 4.73 3.86 3.79 1.57 4.32 5.23 4.53 6.69 3.34 6.38 11.21
Excellent quality Excellent quality Excellent quality Slight odor Slight salt-fishy odor and flavor Salt-fishy odor and flavor Salt-fishy odor and flavor Salt-fishy odor and fiavor Salt-fishy odor and fiavor: colored Very salt-fishy odor and flavor. colored Very salt-fishy odor and flavor; dry and colored Very salt-fishy odor and flavor; dry, colored and tough
Av. 2.09 2.79 3.62 5 08 a Strictly fresh, skinned haddock fillets of known source were di ped in 8.1% salt solution for 21 seconds, allowed to drain 1 minute, then wrapped in Moistureproof Cellophane, packaged in Peters type cartons, and quict-froaen immediately in a Birdseye belt froster.
lye solution. The excess of lye is washed off, and the peach protected somewhat against oxidation prior to slicing by immersion in a dilute citric acid solution. Citric acid is an antioxidant of some slight value, but a much better one is necessary if sliced peaches are to be protected against oxidation for long periods. Small amounts of this acid when added to the sugar sirup used on sliced peaches aid in retarding oxidation. The great difficulty with frozen sliced peaches is that the product may begin to darken even before i t is completely thawed and, an hour after thawing, may be brown and uninviting. I n fact, if a large package of frozen peaches is thawed, the surface layer may be brown long before the inner layers are thawed. For this reason peaches to be frozen are sometimes packed in very small paper cups containing only enough sliced peaches for one individual dessert. The packer recommends that the peaches be served in a partially thawed condition with sufficient cream to complete the thawing. VEGETABLES.I n preparing mushrooms for freezing, they are handled much as if they were to be canned. This vegetable darkens rapidly after picking, especially if exposed to light or heat or if it is handled roughly. For this reason mushrooms must be transported promptly to the packing plant and kept cool until they are blanched. This blanching, which consists of scalding with steam or hot water, inactivates a large portion of the enzyme content of the mushrooms. Further, enzyme actions may be retarded by cooling the blanched mushrooms in a citric acid or a sodium chloride solution.
in leakage during long continued storage is much less rapid when pure salt is used. Oxidation of the easily oxidizable substances in fish occurs during storage. It has been observed that rapid aging of fish may be effected by ultra-violet light rays. Ultra-violet light changes oxygen into ozone, which is a very powerful agent and will cause in a few minutes the oxidative reactions which require months for air to bring about a t the temperature of cold storage. The odors of freshly frozen fish which has been exposed to ultra-violet light for a few minutes and that of brined quick-frozen haddock fillets held in cold storage for long periods are the same. This seems to indicate that saltfishiness in cold storage fish is caused by the oxidation of some of the components of the fish. Haddock filets treated with brine prepared from Diamond Crystal salt, analyzing 99.81 per cent sodium chloride, and similar fillets dipped in brine prepared from an Iviza salt, containing only 97.86 per cent sodium chloride, which had hitherto been used in preparation of brine for treatment of fillets in the plant of the General Seafoods Corporation, were exposed to ultra-violet light. Those brined in impure salt brine soon developed the characteristic salt-fishy odor and taste, whereas those treated with the Diamond Crystal salt brine did not. Storage tests on large quantities of haddock fillets treated with pure and impure brines verified this result (Table I),and indicated that haddock fillets treated in a pure salt brine prior to freezing could be stored satisfactorily a t 0' F. (-17.8' C.) for three months longer than similar fillets brined with a solution made from an impure s a l t - e . g., Iviza or other solar salt.
SPECIALPROBLEMS FISH. The value of quick-freezing was first demonstrated for freezing fish. It was found that quick-frozen fish on thawing lost much less by leakage or drip than did slow-frozen fish. However, in the case of haddock some leakage occurs no matter how quickly the fish has been frozen. Chemical studies of means of preventing this leakage have indicated that it can be reduced to an almost negligible amount by slightly brining the fish prior to freezing. If a solar or other impure salt is used for the brine, the leakage occurring on thawing gradually increases during cold storage. The higher the storage temperature, the more rapidly this increase in leakage occurs. On the other hand, it has been shown recently ( 7 ) that, if the brine used is prepared from pure salt, the leakage occurring immediately on the thawing of the quick-frozen fish is less than when impure salts are used. Further, the increase
PREPARATION OF VEGETABLES FOR FREEZING When the quick-freezing of vegetables was proposed, it was assumed that they could be handled just as fish and meat had been. The difficulties were not discovered in the preliminary laboratory work as the samples were stored at -20' F. (-28.9' C.). The first commercial trials seemed to indicate that frosted vegetables could not be stored long except a t very low temperatures (-20" F. or lower) or "haylike" flavors would develop. Spinach was the one product which retained its natural flavor for long periods a t 0' F. (- 17.8' C.), and curiously spinach was the only vegetable which had been blanched (scalded) prior to freezing. Following this lead, other vegetables were blanched and quick-frozen, and found to retain their fresh flavors even when stored for long periods a t 0' F. Chemical studies have indicated that many vegetable
June, 1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
enzymes continue to effect chemical actions a t temperatures of 0" F. (-17.8' C.) and lower. These reactions are both oxidative and hydrolytic in nature. Some of the enzymes, as, for example, catalase, are easily inactivated by heating, g., tyrosinase-require heating for longer whereas others-. periods and at higher temperatures. Moreover, it has been demonstrated in The Birdseye Laboratories that, when the enzymes of vegetables are inactivated by heating these foods prior to quick-freezing, the frozen products keep satisfactorily in a 0" F. storage. Other methods of inactivating the enzymes of vegetables have been tried. Certain chemicals-e. g., citric acid, sodium chloride, and sulfur dioxide-when used in sufficiently concentrated solutions, will effectively inactivate certain enzymes, but such agents ordinarily cannot be used, as they are likely to impart an undesirable odor or flavor to the product. Until the past season, much of the work of preparing fruits and vegetables for quick-freezing was done by hand. Then, when commercial operations began to be conducted on a large scale, it became evident that mechanical methods would have to be employed if the cost of production were to be kept down. It has been found that greater care has to be used in the selection and handling of products for quick-freezing than is the case with products intended for canning. For instance, lima beans intended for quick-freezing may be vined provided the beans are promptly blanched after vining. The beans must be packaged and quick-frozen soon after blanching Kohman ( 3 ) has demonstrated that the vining operation bruises lima beans and peas and consequently accelerates enzyme actions. If these vegetables are cooked immediately after vining, their flavor is the same as that of the hand-shelled product. However, if the vined peas or lima beans are permitted to stand without blanching, the accelerated enzyme actions rapidly develop off-flavors in the vegetables. VARIETY STUDIES Some of the major problems of the quick-freezing industry are, strictly speaking, neither chemical nor physical. In developing the industry, it has been necessary to carry out a large number of studies of the different varieties of fruits and vegetables. Such work is perhaps more of a horticultural or agricultural nature than chemical. Certain varieties of a fruit or vegetable may not be affected deleteriously by quick-freezing and subsequent cold storage, whereas others do not seem to stand this treatment well. For instance, the Cuthbert raspberry, when quick-frozen, stands up and retains its delicious flavor and aroma. On the other hand, the June variety of this berry loses much of its flavor. Strawberries of high sugar content and high acidity seem to stand storage better than those lower in sugar and acid. This laboratory has found that the Klondike berry, sliced and packed with sugar, retains its flavor and color better than any other of the many varieties of strawberries examined. Blakemore and Senator Dunlap berries also make a desirable product when packed in this way. During the past year a large number of varieties of winter apples have been quick-frozen, and the products tested to determine their pie qualities after storage. Of the varieties examined, the following give a satisfactory product: Baldwin, Winesap, Northern Spy, Stark, Jonathan, Spitzenberg, Rome Beauty, and Rhode Island Greening. All of the common varieties of lima beans have been quickfrozen and examined after cold storage for long periods. The Henderson bush variety seems to be the best of the smallseeded bush varieties. The Fordhook apparently retains its flavor better than any of the common large-seeded varieties. Spinach is the one vegetable which seems to give a satisfactory quick-frozen product regardless of the variety chosen.
685
Commercially the Broad Leaf, grown in Oregon, is the variety which has been packed. The large-seeded market garden varieties of green peas make a much more desirable quick-frozen product than do the small-seeded paler peas commonly grown for canning. To be suitable for quick-freezing, snap beans must be absolutely free from strings and have no fiber in the side walls. Canners can use more mature beans which have fiber in the side walls, for the long cooking and high temperature of the process makes such beans tender. Golden Bantam sweet corn is the best variety yet found for freezing. To produce a high grade frosted corn, it must be harvested while the kernels are still very tender, sweet, and full of milk. More mature starchy corn kernels have tough skins; these may be tendered by the long cooking of the canning process, but are objectionable in the frosted product.
FUTURE RESEARCH Just as new varieties of fruits and vegetables had to be bred in order to have products entirely satisfactory for canning, so the quick-freezer will have to call on the plant breeder if all of his problems are to be completely solved. A study of every known important variety of strawberries has not disclosed a single one which will not leak after thawing. It seems to be the plant breeder's problem to produce a new variety which will solve this problem. As indicated in the early part of this paper, relatively little is known concerning the physical, chemical, and colloidal effects of the freezing of the tissue of fruits and vegetables. Before it will be possible to reduce the leakage from certain types and varieties of fruits, it will be necessary to understand more about what happens when such fruit is quick-frozen. Here is an excellent opportunity for some difficult colloidal research. Little is known concerning the nature of the enzymic reactions occurring in fruits and vegetables during long continued cold storage. As has been indicated, the enzymic actions of vegetables have been largely eliminated by blanching. It is not practical to treat fruits in this manner, as such treatments will change the flavor of the products and give the fruit a cooked flavor. If more were known concerning the nature of the enzymic reactions and chemical changes occurring in fruit during frozen storage, then it might be possible to inactivate or control the action of enzymes by means other than heat. This chemical knowledge is necessary if certain fruit products are to be kept in a frozen condition for long periods. Large-scale researches will have to be conducted in order to adapt the mechanical methods of handling fruits and vegetables now commonly employed by the canner to the preparation of products for quick-freezing. Fatty fish must be kept at very low temperatures in order to prevent the oxidation of and development of rancidity of the fat, and even then they cannot be stored for long periods. Chemical means of retarding the development of rancidity are needed. As the demand for fish grows and the yield of our coastal waters and nearby banks decreases, it will become necessary to go farther and farther from port to get sufficient fish. Soon the vessels will have to operate so far away that it will be necessary to fillet and freeze the fish on shipboard and store in a refrigerated hold. Fish handled in this manner can be handled in better condition than those now caught relatively close to port. Such a commercial procedure will bring up new problems, especially of a chemical engineering and an engineering nature. A large quantity of waste will be available which may be utilized for the production of fish meal, fish flour, and oil if a satisfactory by-product plant can be erected and operated on shipboard.
INDUSTRIAL AND ENGINEERING CHEMISTRY
686
From the above brief consideration of the problems which have been partially solved by the chemists and those on which little headway has been made, it can be seen that, although quick-freezing has progressed a long way commercially, a great deal still needs to be done by the research worker if all of the problems are to be satisfactorily solved.
(3) Kohman, E. F.. Paper presented before the Pea Section a t the (4)
(5)
(6)
LITERATURE CITED (1) Birdseye, C., Food Ind., 3, 213 (1931). (2) Birdseye, C., and Fitzgerald, G. A., IND.ENQ. CHEM.,24, 676 (1932).
Vol. 24, No. 6
(7)
25th Annual Convention of the Nat. Cannera Assoc., Chicago, Ill.. Januarv 25 to 29., 1932. ~ ~ Overholser, E. L.,and Cruess, w. V., Calif. Agr. Exp. Sta., Tech. Paper 7 (1923). Plank, R., Ehrenbaum. E.. and Reuter, K.. “Die Konservierunn von Fischen durch das Gefrierverfshrung,” Zentral Einkaufgesellschaft, Berlin, 1916. Taylor, H. F., Bur. Fisheries, Document 1016 (1927). Tressler, D. K., and Murray, W. T., Fishing Gaz., 49, No. 2, 24-6 (1932).
RECEIVEDApril 7, 1932. [ E N DOF SYMPOSIUM]
Indium Available in Commercial Quantities WILLIAMS. MURRAY,805 Watson Place, Utica, N. Y.
L
I K E many other elements, indium was discovered several years before sources and processes of recovery made it available for commercial use. Reich and Richter (4) are accredited with having discovered this element in 1863. Because of the indigo-blue lines which are in its spectrum, it.was named “indium.” It was one of those elements which was supposed to be widely distributed in several kinds of ores, but always in minute amounts. Later a zinc ore in Germany was reported to contain as high as 0.1 per cent. So far as the writer knows, no definite and serious effort was made to find lasting sources for this metal until a careful survey of all known ores was made. Because of the success of these efforts, indium can now be made available in substantial commercial amounts.
PROPERTIES OF INDIUM The properties of this element are fairly well known. It is a white lustrous metal, very soft and ductile, and slightly
heavier than zinc; it melts a t 155” C. and is said to boil at about 1450” C. It has very great surface stability at ordinary temperatures, but oxidizes and burns a t temperatures above its melting point, especially if finely divided. Indium is supposed to be trivalent in its stable compounds. Several of these are listed in the usual references. Especial attention has been given to its halogen compounds which are quite unstable. The cyanide is reported to be insoluble in water but soluble in cyanides of other elements. There are also listed the hydroxide, oxides, sulfide, and sulfite. It is believed that little is really known about indium and its compounds; the reason for this is that there have been heretofore but small amounts available for research, and therefore little interest was shown in such work. The physical properties are very concisely stated in a paper by Westbrook (6) from which the following is taken: Atomic weieht 114.8 3 (usually); also 2 and 1 Valence 115 Melting point, C. 1450 Boiling point, C. 7 .12 Specific gravity5 L.31 ‘27;3 Specific heat, joules per gram atom Electrical resistivity: A t 20’ C., ohms 9 x 10-6 29 X 10-6 At 155’C ohms Thermal expLnsion (Z/Ldl/dt) at 20’ C . 33 x 10-6 Hardness O Brinell 1 7.99 Tensile sirength (99.71% pure). tons per sq in. a According t o two different authorities.
-
It is believed by several that changes will be made in the above values accredited to its specific gravity, its hardness, and its tensile strength.
USESFOR INDIUM Considerable work has been done on the electrolytic deposition of the element. Thiele (5) in 1904 deposited indium from a bath slightly acidified with sulfuric acid. Dennis and Geer ( 1 ) in 1904 used formic acid. Kollock and Smith (8) in 1910 used gelatin as an addition agent in the presence of free sulfuric acid, acetic acid, or sodium acetate. Mathers (3) used fractional electrolysis as a means for purifying the element. Recent work has shown that all of the above baths are very unstable. Daniel Gray found that sugars or carbohydrates added to a cyanide bath would stabilize it. Westbrook (6) reviewed the work of all except Gray and finally turned to a sulfuric acid bath containing sodium citrate. Fortunately Gray perfected a bath which is entirely stable. Kot only can indium be plated from this bath, but it can be plated simultaneously with other elements. Work has been concentrated not only on the plating of indium but also on its combinations with other metals. I n every case the addition of indium to other elements has especially increased the surface stability and also the hardness of the combination until indium is in excess. Patented processes have been developed which make the recovery of this element in substantial amounts entirely practical. By careful and systematic search of all known ores, a district was found which has very rich indium ore deposits. The indium seems to be a replacement in the ore of this particular district. By concentration, the ore has been made to yield a very high percentage of its indium content. These deposits will furnish substantial amounts of this element. Assistance can be furnished to those who wish to work with this metal, not only in the pure state but also in the form of many of its salts. Already several uses have been found for this element. It would seem to have use in the automotive, electrical, jewelry, and dental fields. There are undoubtedly many other places in which this element may be used. It would be desirable to know what it will do in medicine, radio, thermometry, and many other lines of work. Indium is no longer in the gram class, but is surely an ounce of metal. LITERATURE CITED (1) Dennis and Geer, J. Am. Chem. SOC.,26, 438 (1904). (2) Kollock and Smith, Zhid., 32. 1248 (1910). (3) Mathers, F. C., Ihid., 29, 485 (1907). (4) Reich and Richter, J . prakt. Chem., 89, 441; 90, 179; 92, 480 (1863). (5) Thiele, 2. anorg. Chem., 39, 119; 40. 280 (1904). (6) Westbrook, L. R., Trans. Am. Electrochem. SOC.,57, 289 (1930). RECEIVEDApril 18, 1832.
