THE ?RE5ER^ or PIS ^ 3A . 'i Qass. Book. 337 , >^ ' DEPARTMENT OF COMMERCE BUREAU OF FISHERIES HUGH M. SMITH, CommiMionw PRINCIPLES INVOLVED IN THE PRESERVATION OF FISH BY SALT By HARDEN F. TAYLOR Chief Technologist U. S. Bureau of Fisheries APPENDIX n TO THE REPORT OF THE U. S. COMMISSIONER OF FISHERIES FOR 1922 Bur«au of Fisheries Document No. 919 PRICE, 5 CENTS Sold by the Superintendent of Documents, government Printing Office Washington, D, C. WASHINGTON GOVERNMENT PRINTING OFFICE 1922 DEPARTMENT OF COMMERCE J. BUREAU OF FISHERIES HUGH M. SMITH, Commissioner PRINCIPLES INVOLVED IN THE PRESERVATION OF FISH BY SALT By HARDEN F. TAYLOR Chief Technologist U. S. Bureau of Fisheries Appendix II to the Report of the U. s. Commissioner OF Fisheries for 1922 Bureau of Fisheries Document No. 919 PRICE, 5 CENTS Sold by the Superintendent of Documents, Government Printing Office Washington, D. C. washington government printing office 1922 LIDF^ARY OF CONQFIESS FEB 2319^2 DOOUMfcNTc Li.v.^ilON PRINCIPLES INVOLVED IN THE PRESERVATION OF FISH BY SALT. By Harden F. Taylor, Chief Technologist, U. 8. Bureau of Fisheries. Contribution from the Fisliery Products Laboratory, Washington, D. C. CONTENTS. Page. Introduction 1 How salt preserves 2 How salt extracts w^ater 3 Factors affecting permeability of fish 5 Flavors of salt fish 8 Dry salting and brine salting compared 9 Loss by fish of nutrients in brine 11 Influence of method of cleaning fish on salting 13 Improved method of salting fish especially for warm weather 15 Scotch-cured herring 16 Mild-cured salmon 16 Behavior of fat during salting process 16 Reddening of cod and haddock 18 Recovery of brine 19 Accessory chemical agents and other factors in salting 20 Conclusions 21 Summary 21 INTRODUCTION. The art of preserving fish by means of salt is of great antiquity. It was practiced by the Phoenicians and Greeks and was brought to a high degree of perfection by the Romans. Mixed with spices, salt was used for the preservation of food on the shores of the Mediter- ranean and the outlying country in the time of Christ, reference being made in the Sermon on the Mount to a salt which has lost its savor, meaning a salt in which the spices have lost their aroma by evapora- tion. In the centuries following the art continued, both in the Occi- dent and the Orient, to play an important part in world economy, v^hakespeare put in the mouth of his most wonderful character, Fal- staff, the words : " If I be not ashamed of my soldiers, I am a soused gurnet"- — a pickled gurnard, the gurnard being held in such light esteem that it was a term of contempt. ^Whether " sousing " or pick- ling made the fish doubly contemptible had better be left to the phi- lologists to determine. Less than 25 years after Shakespeare wrote that play the Plymouth Colony landed in America and brought with ' Appondix II to the Ueport of the TT. S. Commissioner of Fisheries for 1922. B. F. Doc. 019. » King Henry IV, pt. 1, Act IV, Scene II. 2 U. S. BUREAU OF FISHERIES. them the arts of sousing and pickling fish. The descendants of the Pilgrims are still pickling fish around Cape Cod and particularly at Gloucester. To a great many people it may seem that science has contributed little or nothing to the improvement of methods of preserving fish by salt. Perhaps this view is shared by a considerable number of people who are engaged in the business of salting fish. To them it may appear that salting fish is just salting fish, and "that's all there is to it." It may be admitted readily that science has not so per- vaded and dominated the fish-pickling industry as it has other an- cient arts, but it has contributed something and is capable of con- tributing a great deal more, and here lies the purpose of this paper. That purpose is to present the rationale of salting and pickling fish, so that the reasons for the various steps and modifications will be readily understood and appreciated, to the end that the art ma}' be practiced more intelligently and successfully. It is a further pur- pose of this paper, by showing what the few attempts made by science have done for the art, to convince and persuade those on whom the industry depends for its existence and progress that science can be expected to do a great deal more than it ever has done if it is energetically studied and applied. HOW SALT PRESERVES. Salt preserves by extracting water. Spoiling is a series of chem- ical activities for which water is necessary; remove the water and spoiling is arrested. The removal of water by means of salt is in some senses a truer dehydration than actual drying in air, for changes of an undesirable sort take place in air drying that are never cor- rected, while salting may be done in such a way that few changes other than removal of water are brought about. The statement that salt preserves by extracting water is to be taken strictly and liter- ally, for salt has no peculiar preserving or antiseptic quality, as many people seem to think. Things live, die, and putrefy in the sea, which is one-tenth saturated Avith salt. But by sufficient concentra- tion salt, an otherwise almost inert, harmless substance, becomes a pov>'erful preservative, merely because, if concentrated sufficiently, it extracts water. The process of transferring water from one place to another, as from the inside of a fish to the outside, under the influence of con- centrated solutions, is known to physicists and chemists as osmosis. This principle of ospiosis is of almost universal application in nature and is used by men in the arts, but a good understanding of it is not common. By osmosis our food is taken from the intestines to the blood without any communicating opening. By osmosis oxygen is taken from the air into the blood without any leakage of blood. By the same principle the kidney tubules remove unde- sirable substances from the body while holding back all desirable substances. By osmosis the roots of plants select the necessary minerals from the soil. A weak sugar solution will readily ferment, but if made concentrated it destroys yeast and bacteria by osmosis and is therefore an excellent preservative of fruits. Salt is also a preservative by virtue of its concentration. Any other neutral min- PRESERVATION OF FISH BY SALT. 3 eral substance equally soluble would preserve in the same way that salt does, but salt happens to be the only one that the human palate and stomach will tolerate. HOW SALT EXTRACTS WATER. At the risk of appearing verbose the writer undertakes to elucidate the principles that govern osmosis, because osmosis is nearly the whole principle of salting fish. Without a knowledge of osmosis people may salt fish successfully by rule, but without such a knoAvledge it is quite impossible to understand the process. If a thin animal skin or membrane separates two liquids and if the liquids are alike and of the same concentration, nothing happens. But if they are unlike and of different concentration, one or the other or both of the liquids will pass through the skin to the other side. This passage through the skin or membrane is called osmosis. Just what components pass through the membrane, in what direc- tion, and how much depend on many circumstances. For the pur- poses of salting fish water is always the liquid, plus wliatev<^r is dissolved in the water. The dividing membrane is the skin of the fish and the membranous inclosures of the microscopic cells of which the substance of the fish is composed. We thus have water and salt outside, cell membrane between, and fish juice, or protoplasm, in- side, and we desire to Iniow what will happen and how we can in- fluence the process to suit our needs. The quantity and direction of flow through the skin or cell membrane will depend on (1) the nature of the dividing membrane, and (2) the nature and quantity of the substances dissolved in the water on each side. The nature of the dividing membrane will be considered first. Almost any substance can be made into a thin film or membrane. Such things as glass, tin-foil, and mica may be exceedingly thin, but are totally impermeable and therefore uninteresting in the pres- ent connection. But other membranes or films, such as parchment paper, gelatin films, animal bladders, and goldbeater's skins are permeable to a greater or smaller degree. Suppose pure water were on one side of a membrane and water containing dissolved salt on the other. If the membrane is perfectly permeable to all constitu- ents, water will pass through to the salt solution and salt will pass through to the water, and these movements will continue until the two sides are alike and then stop. It is always the tendency for the two liquids to come to equilibrium, and they would do so if the membrane were perfectly permeable. Nearly all membranes, how- ever, permit a freer flow of the solvent, in this case water, than they do of the solute (that which is dissolved) . in this case salt. If the membrane permits the water to flow but absolutely prevents passage of a dissolved substance, the membrane is said to be semi- permeable. In the example taken above, of pure water on one side and salt solution on the other, if the rjiembrane were semipermeable then the water would pass through to the salt solution, but the salt could not get through to the water. The level of the pure water would fall and that of the salt would rise. The difference in liquid level would exert a pressure called osmotic pressure. Ideally semi- permeable membranes are not realized in nature, though some of the 4 U. S. BUREAU OF FISHERIES. membranes in plants and animals approach ideal semipermeability while they are living. Ideal semipermeability with respect to par- ticular dissolved substances has been achieved and is found in living- organisms. It is to be remembered that in case of semipermeable membranes the solvent will flow from the less conicentrated to the more concen- trated side of the membrane, so that if we wish to extract water we need only to make the outside more concentrated than the inside. If we wish to add water, we make the outside less concentrated than the inside ; that is, we use pure water outside, as has sometimes been done unfairly to swell oysters and make them appear " fat." It is also to be remembered that the degree of permeability of mem- branes does not necessarily remain unalterable. The permeability of the membrane can very readily be changed, as will be seen later. There is reason for believing, for example, that the permeability of fish to salt increases after death — for stale fish strike through more quickly than fresh fish — and that permeability increases at tempera- tures near the freezing point of water. The tissues of fish consist mostly of cells. Each cell is a bag of semiliquid, like the white of egg. The surface of every cell either is or acts like a semipermeable membrane. If we surround the cell with water, the inside will be more concentrated than the outside and water will go in. If we surround the cell with strong salt solution, water will pass out to the salt. Some salt will also pass into the cell, which fact shows that the cell wall is not ideally semipermeable. But what of the protein wdthin the cell ? Why does it not come out while the salt is going in? In order to answer these questions it is necessary to pass from a consideration of the nature of the mem- brane in osmosis to a consideration of the nature of the dissolved substance. By a great many experiments it has been found that some dissolved substances never pass through membranes under any circumstances, while others will pass through some membranes. It is found that those which never pass through are also those which on drying out do not crystallize but shrink to a tough mass. They are called col- loids. Examples of them are glue, albumen, gelatin, and soap. The smallest possible particle of these substances is comparatively large, too large, we naay imagine, to go through the texture of the mem- brane. They are not only large of molecule but complex in structure. The bulk of animal bodies consists of colloids called proteins, dis- solved in water. The other class of substances, those that may pass through membranes and which on drying out crystallize in regular geometrical shapes, are the crystalloids. Examples of this class are salt, sugar, and like substances.. It is not to be supposed, however, that all crystalloids will pass with equal facility through any given membrane. Nearly all membranes are in some measure selective of particular crystalloids. The ideal semipermeable membrane permits none to pass, but as membranes degenerate from ideal semipermea- bility to complete permeability they permit more and more of these dissolved things to pass through. The phenomena of osmosis having been briefly reviewed, one may readily perceive the importance of applying the principles to the salting of fish. Salt is brought in contact with the exterior of the PRESERVATION OF FISH BY SALT. 5 cell. It dissolves in some of the moisture, forming a saturated solu- tion. This solution is separated from the contents of the cell by a cell niembrane which is more or less semipermeable. Water passes out of the cell to the salt and the processes of decay are stopped be- cause of insufficiency of water. The membrane, not being absolutely semipermeable, permits some salt to enter and the fish remains salty. The contents left in the cell are proteins or the valuable food ele- ments of the fish which, being colloids, are not permitted by the cell membrane to pass out. Thus water is extracted, salt enters, and the fish is preserved. When the time comes to eat the fish the process is exactly reversed. The fish is bathed in pure water. The cell contents are more con- centrated than the exterior, so water passes in. The cell membrane is to some extent semipermeable, so the protein does not escape, but the salt does. This exchange is carried to a point where the meat is again plump and a sufficient quantity of salt has been removed. Thus by exposing the meat of fish to salt we have removed the water and caused some salt to enter the meat and have stored the fish. We have then by exposing the fish to water put water back in the cells and taken out the excess salt. The actual food material of the fish — the cell protein — is still where it was, for practical pur- poses unchanged. If every step has been scientifically correct we have at the end very nearly the fresh fish we had to start with. But there is the rub. At every turn it is possible to depart from the scientifically correct. The principles of osmosis here very briefly stated are the fundamentals of the art of salting fish. In all that follows there will be frequent occasion to refer to osmosis. FACTORS AFFECTING PERMEABILITY OF FISH. The preservation of fish by salt is practiced extensively in the cooler parts of the United States, but very little has been done south of Chesapeake Bay. The reason fish have not been salted in the warmer parts of the country is that the process has not been satis- factory. Repeated efi^orts to salt alewives on the St. Johns Eiver in Florida previous to 1920 uniformly resulted in failure. In 1918 re- search on this problem was undertaken under the immediate direction of the writer. The results of a part of this program were published.^ The hypotheses which guided this work were somewhat as follows : During the course of " striking through " the fish two things are happening — (1) the flesh is brealdng down by autolysis (a process to be explained later) and (2) the salt is penetrating the flesh. Salt arrests autohsis when it arrives, but considerable damage may be done before the salt has reached the innermost parts of the fish. Now, these two processes — salt penetration and autolysis — are running a race, so to say. If the salt penetrates to the innermost parts before autolysis has destro^^ed them, the salt wins the race and the fish is saved. If before the salt can get to tfie innermost parts they have been decomposed by autolysis to an intolerable degree, then autol3^sis wins and the fish spoils. High temperatures accelerate both proc- esses, but while accurate measurements have not been made we know ' Trossler, D. K. : Some Considerations Concerning the Salting; of Fish. Appendix IV. Report of the IJ. S. Commissioner of Fisheries for 1{>19, 55 pp. B. F. Doc. No. 8S4. Washington, 1920. 6 U. S. BUREAU OF FISHERIES. by practical experience and by experiment that at a sufficiently ele- vated temperature the fish will invariably spoil if blood be present. Now, to make certain that the race mentioned shall always be won by the salt, we may do one of two things, namely, retard the rate of decomposition or accelerate the penetration of salt. Worldng at a lower temperature is the only practicable means of retarding de- composition, but since we desire a method suitable for warm climates it is necessary to accelerate penetration of salt. How can the salt be caused to penetrate fish more rapidly? The physiologists have shown that in living animals compounds of calcium, barium, and magnesium have a marked effect in retarding or arresting penetration of membranes. By examination of numerous analyses of commercial brands of salt it was found that the salts of calcium and magnesium are those nearly always present as impurities. A few of these analyses are given herewith : Analysis of Various Salts fob Cubing Fish.^ Substances present. Turks Island salt. Trapani , Italian salt. Iviza, Spanish salt. Diamond Flake, domestic salt. Leslie Velvet Grain, Califor- nia salt. Sodium chloride Calcium chloride Calcium sulphate Magnesium chloride . Magnesium sulphate . Sand, etc Per cent. 96.52 Per cent. 95.82 .32 Per cent. 98.05 .49 Per cent. 99.78 Per cent. 99.96 1.53 1.20 1.19 1.75 .15 .37 .00 .00 .00 .067 .00 .010 .022 1 These figures represent analysis of single samples of each brand taken in the market and are not aver- ages of numerous samples. Not only is some variation in manufacture unavoidable, but the chemical djtermination of such small quantities of impurities is subject to small errors. Therefore it should not be expected that any purchased lot of salt would conform exactly to the composition shown here. The figures represent in a general way the degree of purity that can be expected. By appropriate methods of measuring the rate of penetration of salt into fish it was found that if absolutely pure salt is used a A^ery rapid penetration is obtained, but that even small additions (from I to 5 per cent) of these salts of calcium and magnesium cause a very pronounced retardation of penetration. For example, by appropriate methods of analysis it was found that pure salt pene- trated as deeply in less than five and one-half days as did salt con- taining 1 per cent calcium chloride in nearly seven days. Similarly, a salt containing 4.7 per cent magnesium chloride penetrated no far- ther in five days than pure salt did in three. In order to bring about a much more rapid penetration of the tissues then, we have but to obtain a salt free from these impurities. The time gained by the use of pure salt enables fish to be salted at a much higher temperature and yet not spoil. Fish were salted in an incubator room in Wash- ington at a temperature of 90° F. at first, rising to 100° F. — the hottest summer Aveather. No unpleasant odor developed, and the fish upon being cooked and eaten were pronounced excellent. There was a further and somewhat unexpected difference between the effects of pure and impure salts. The flesh of the fish salted by impure salt is white, opaque, or chalky in appearance and much harder or firmer in consistency ; that of fish salted Avith pure salt is translucent and somewhat yellowish and much softer. While the former Avhite. firm fish is the customary quality demanded in com- PRESERVATIOlSr OF FISH BY SALT. 7 merce, there are strong reasons for believing the softer and yellowish fish produced in pure salt to be superior. There is reason for be- lieving that the whitening of the fish in impure salt is explained by the fact that the calcium coagulates the protein, just as heat by coagu- lating egg white causes it to be white and firm. But where there is no calcium in the salt the protein retains its natural translucency and yellowish color. The calcium in impure salt is retained by the fish, a matter that will be discussed later under the subdivision on flavor of salted fish. While no investigations appear to have been made on the influence of temperature on the permeability of fish flesh, investigations have been made on a great variety of other living things, so tliat it is prob- ably safe to generalize cautiously regarding such influences on fish. Osmotic pressure varies, approximately, as absolute temperature.* That is, if Ave double absolute temperature osmotic pressure is doubled, other factors being held constant. The range from 32 to 100° F, within which fish salting is usually done is, on the absolute scale, rather narrow (491.4 to 559.4° A.), so the maximum variation due to this cause would be about 14 per cent. It is, however, a com- mon experience in pickling fish that the warmer the temperature the more rapid the striking through, a difference too great to be accounted for by temperature variations of osmotic pressure. The cell mem- brane^ itself must change. Whether any more free permeability caused by warm temperature is permanent after the fish is chilled again is not known, but the question would be Avell worth investi- gating. Cold, when in the neighborhood of freezing, also promotes permeability, as has been proved by various experiments. It is quite possible that fish chilled to a point near freezing (as in the mild cur- ing of salmon) would strike through much more quickly than fish at the customary warmer temperatures. This matter also should be investigated. Stale fish — that is, fish whose cell membranes have " died " — are more permeable than fresh fish. Some fish Avere held in the labora- tory all day at a temperature of about 75° F. and toward night were salted in pure salt and put in an incubator at 100° F. By the next day they were struck through. The combination of stale fish, high temperature, and pure salt brought about extraordinarily rapid penetration. At this point mention should be made of another effect of salt upon the protein constituents of fish. Strong solutions of salt pre- cipitate certain protein substances, different substances falling out successively from a mixture of dissolved proteins as the concentra- tion of salt is increased. The nature of the proteins is not altered by this precipitation, for upon replacement of the salt solution with fresh water the proteins redissolve and appear to V^e restored to their original condition. Salt thus causes a temporary precipita- tion or fixation of proteins in fish, to ascertain extent hardening the tissues and reducing the likelihood of changing. Not only does quite pure salt penetrate the fish more rapidly, but when the time comes to cook the fish it is found to soak out more rapidly also. Practical experiments in the experimental kitchen of the Bureau of * Absolute temperature is based on absolute zero, the point of no beat, or absolute cold which i.s —273° C. or —4");). 4° F. If we use degrees the same size as Fahrenheit 3 degrees, then (»° F. is 4ii9.4 absolute; ,50° F. Is 4o9. 4 + 50=500.4 ab,soIute, etc. 77635°— 22 2 8 U. S. BUREAU OF FISHERIES. Fisheries indicate that fish preserved in very pure salt soak out in from a third to a half the time required by fish preserved in crude salt. What is the practical lesson of this work? It shows that by the judicious selection of salt, not on the basis of its cheapness but on the basis of composition, one can produce a salt fish of almost any desired quality'. If salting is to be done in very warm weather it will be necessary to use the purest grade of salt to secure very rapid penetration. In this way a soft, yellowish fish of excellent quality is obtained. Where weather is cool enough to permit, a salt contain- ing more calcium and magnesium may be used, in which case a whiter and firmer fish will be produced. Can these very pure salts be obtained commercially? Several brands of salt of the highest degree of purity are available both on the east and west coasts and at a cost not much above the price of cruder salt. In many cases the single item of fish saved that might otherwise spoil will repay the extra cost of pure salt, to say noth- ing of the improvement in quality of the salt fish. FLAVORS OF SALT FISH. The calcium and magnesium are taken up by the protein in the cells and held, not coming out when the fish is soaked. Now, these impurities, particularly calcium, have an acrid taste and greatly accentuate the " saltiness " of salt. Pure salt is not so " salty " to the taste as crude salt. If the calcium is held by the tissues at the time of soaking out while the salt is removed, then after soaking there is a much greater amount of calcium present in proportion to the amount of sodium than there was in the original salt and a corre- spondingly more acrid " salty " taste. It is therefore necessary to soak out fish much longer or until they are " flat " if they have been cured with crude salt, while with pure salt they may be soaked out until they suit the taste, after which they retain their original flavor. Certain improvements in the flavor of fish have been noted after they have been salted by improved methods. The fish variously known as mud shad or gizzard shad {Doroso77ia cepedianum) is plentiful in certain parts of the country but is held in very low esteem because of its muddy, unpleasant flavor. After being washed free from blood and salted in pure salt this unpleasant flavor disap- peared and the fish compared favorably with fish commonly more esteemed. The muddy taste of the carp and other fish from muddy ponds and streams is believed by some to be caused by species of Oscillatoria, a blue-green alga growing in the slime of the fish; by others it is believed to be humic acid derived from the mud. Per- haps the two views could be entirely reconciled, but the actual chemical compound or compounds responsible for the unpleasant flavor seems to be removed by the brine. It is not difficult to understand how the alteration of taste may be brought about by salting. The main bulk of the fish, pure protein and pure fat, is believed to be tasteless and odorless. The substances which give rise to taste are free fatty acids (decomposition products from fats), amino acids (decomposition products of proteins), highly odoriferous tiiethylamines, and various waste materials classed by the chemist as purines. The absolute quantities and also the relative proportions of these materials vary froni^ species to species of fish, PRESERVATION OF FISH BY SALT. 9 and they even change in the same individual fish as staleness de- velops. Now, most of these odoriferous substances are soluble in water or brine, and after the salting process woukl be found in the brine. They are not replaced when the fish is soaked out. It might therefore be anticipated, as has actually been found, that the fresh fish, disagreeable because of the presence of strong substances, are rendered sweet by the removal thereof in the salting process. If this lead were followed in detail, it is quite possible that salting would turn out to be the best method of utilizing fishes that are of a rather poor edible quality when in the fresh condition. This aspect of the matter deserves particular attention of the canners. Many species of fish of great abundance might in time be profitably packed if the flavor were inviting. With highly improved technique in salting, the undesirable flavors might be removed by curing and soaking out before canning. This process would be unthinkable on the basis of the customary salting methods where there is in the end an excessive saltiness or flatness of flavor, but the mild, sweet fish prepared by improved technique and pure salt is a much more promising possibility for canning. DRY SALTING AND BRINE SALTING COMPARED. The next question taken up in the investigations referred to was that of the relative merits of the application of the salt to fish in the dry state and as a concentrated brine. In the Chesapeake Bay region the herring are usualh'^ pickled in brine. By a strict compari- son of the two methods it was found that there is developed a smaller quantity of the products of decomposition — the amino acids — when the salt is applied dry. Not only this, but it was also found that salt applied in the dry condition penetrates the fish more rapidly. Among the products of protein decomposition are amino acids. A determination of amino acid nitrogen was taken as a measure of decomposition — the more of the amino acid nitrogen present the greater the amount of decomposition. This being true, the following table, summarized from Tressler's results, will show the superiority of dry salt over strong brine for preserving fish. Amounts of Amino Acid Xitrocjex Formed Per Kilogram of Fish at Diffesent Temperatures. Method of salting. Tem- pera- ture. Amount of amino acid nitrogen per kilogram offish after — 19 hours. 67 hours. 5 days. 7 days. 9 days. Condition at end of salting period. Dry salted. . Brine salted Dry salted.. Brine salted Dry salted. . Brine salted Dry salted.. Brine salted Dry salted. . Brine salted Dry salted.. Brine salted ' F. 63 63 70 70 75.5 75.5 80 80 87 87 93 93 Grams. 0.078 .08i( .084 .077 .102 .074 .086 .076 .097 .065 .080 Gra ins. 0. 083 .129 .086 .165 .092 .186 .086 .189 .089 .244 .105 .238 Grams. 0.085 .135 .098 .158 . 099 .179 .119 .210 .159 .266 .151 .320 Grami. 0. 085 .183 .097 .190 .104 .228 .141 .300 .195 .377 .193 .465 Gram^. 0.119 .234 .126 .292 .134 .316 .158 .383 .208 .510 .236 Good. Do. Do. Do. Fair. Do. Do. Spoiled. Do. Do. Do. Do. 10 U. S. BUREAU OF FISHERIES. It is seen that the brine-salted fisli consistently uncler<;o a greater decomposition than those salted with dry salt, as shown by the abundance of decomposition products, amino acids. The average excess of amino acid nitrogen in the six lots pickled in brine over the six lots in dry salt is 51 per cent, a very material difference. It Avill be noticed in the last column of the table that spoiling of fish ]jickled in brine takes place at a lower temperature than it does in dry salt. Fish were satisfactorily salted in dry salt at 80° F., but at this temperature fish pickled in brine spoiled. To complete the evidence in favor of using dry salt, the following table from the, same paper shows the rate of ])enetration of salt into squeteague when applied dry in comparison with brine: Penetration of Salt. Section of fish. Percentage chlorine in dry sample after— 1 day. 4 days. 7 days. 10 days. Dry salted Outer layer, from surface to a depth of J centimeter. Inner layer, from J to 1 centi- meter below surface. Outer layer, as above 9.8 2.6 8.4 1.8 16.2 11.0 15.3 8.3 19.6 16.0 17.3 12.2 19.5 Do 18.7 17.8 Do Inner laver. as above.. . 15.7 What is the reason for the superiority of dry salt over strong brine or pickle, especially since the dry salt very shortly forms its own pickle ? In answer to this question it is necessary to refer to the principles of osmosis. It was shown that the flow of water is from the less concentrated to the more concentrated. The relative con- centrations govern the direction of flow and also the rate or quantity of flow. Salt is going into the fish and water coming out. If brine is used, it is losing some of its salt which penetrates the fish and is being diluted with water which is coming out. This process rapidly brings the contents of the cells into equilibrium with the brine ; that is, with the film of brine immediately in contact with the fish. Stir- ring as usually done may cause a momentary increase of penetration by removing the film of dilute brine adjacent to the fish, but we may imagine that a new dilute, film forms again very rapidly. If instead of brine dry salt is placed in contact with the fish very material dif- ferences are at once apparent. Part of the salt dissolving in the free moisture forms strong brine, which begins its extraction of water from the fish. The water coming from the fish is not able to dilute the adjacent brine, because some of the excess of dr}^ salt present iromediately dissolves, and thus assures saturated brine at all times. It should also be obvious that since the very purpose of using salt on fish is to extract water the addition of water at the beginning simply supplies just so much water to the salt and satisfies the affinity of salt for water to that extent. The water should come from the fish and not elsewhere. To put into words the conclusions from this section of the paper, when salt is applied dry to the fish there is a more rapid penetration of salt, less decomposition of fish, and it is possible to preserve fish PRESERVATION OF FISH BY SALT. 11 at a higher temperature. The superiority of dry salt over brine resides in the fact that the brine in contact with the fish is not per- mitted to be dihited if salt is present in crystalline condition. LOSS BY FISH OF NUTRIENTS IN BRINE. The liquid that comes from fish during the salting process is not pure water, as every fisherman knows, but contains a quantity of material derived from the fish. Most of the nitrogenous matter found in brine represents just so much good food gone to waste and just so many pounds of fish that might have fetched a good price gone overboard. The quantity of protein that escapes into the brine is highly variable, for reasons that will appear later. That some idea may be had of the magnitude of the loss of fish substance in brine the following figures are given. These figures were obtained in the course of investigation on the recovery of valuable materials from old brine : fx)ss BY Fish of Nutrient Materials in Brine. Brine. Grams dry protein per liter of brine. Avoirdu- pois ounces per gallon. Rockfish brine from Alaska 29.30 34.80 73.30 3.9 Herring brine from Gloucester 9.8 Cod brine from Gloucester 4.6 Since all the nitrogen in the brine was calculated as protein, these figures are undoubtedly too high ; but the bulk of the nitrogen is cer- tainly of protein origin, so the figures may be taken to illustrate the point made. If we assume fresh fish to be 75 per cent water and 25 per cent dry protein and express the results in customary units, the figures show the equivalent amount of food-fish flash dissolved in brine to be 15.6, 39.2, and 18.4 ounces, respectively, or from 1 to 2| pounds to the gallon of brine. Bitting ^ calculated the losses in the curing of codfish as follows : Loss of weight in dressing, 40 per cent ; loss in salting, 40 per cent of what remained after dressing ; drying on flakes, 9 per cent of the salted fish. The 40 per cent of the dressed fish contains besides water much protein or valuable nitrogenous food. It would certainly seem to be worth our while to examine into the causes of this loss and to prevent or salvage it if possible. How does this protein get out of the fish? It was said above that protein is a colloid and that colloids do not diffuse through mem- branes. A small amount must come from the blood and from the cut surface on the fish, but most of it will probably be found to'come from the interior cells by a process not yet investigated. We do know something directly about autolj^sis, however, the great enemy of the fish dealer, which liquefies the contente of fish flesh, and we have every reason to believe that if autolysis were stopped the losses of protein into brine would be reduced to a minimum. What is autolysis and how does it do its damage? ^Bitting, A. W. : Preparation of Cod and Otlier Salt Fish for Market. IT. S. Depart- ment of Agriculture, Bureau of Chemistry, Bulletin No. 133, 63 p. Washington, 1911. 12 U. S. BUREAU OF FISHERIES. Protein, the colloid, can not pass through an osmotic membrane, but proteins can be decomposed into simpler substances which readily dissolve and pass through. The agency which breaks down protein into these simpler substances is called an enzyme, and protein must always be so liquefied or digested by enzymes before it can be ab- sorbed through membranes; hence the necessity of digestion in the stomach of animals preparatory to absorption of food through the intestines. Now, animals, including fish, require a certain amount of new protein to support the body activities, Avhich, failing, the animal would immediately perish. But the hazards in the existence of any animal often make it obligatory to do without food for a shorter or longer period. If the stomach became empty because of temporary shortage of food or an injured mouth, the animal would die unless special provision were made to supply protein from some other source. But nature has provided a means whereby the proteins in the less im- portant parts of the body can be used for the time being to support the activities of the absolutely necessary vital parts. The stored pro- tein is within cells and could not possibly be carried by the blood stream to the point of need unless it could get out. So there is in each cell stored along with the protein some enzyme ready in case of threatened starvation to break the protein down into simpler sub- stances which penetrate outward into the blood for transportation to the point of need. Fish may thus live for a time at the expense of their own bodies. These enzymes, present in every part of the fish, while almost an absolute necessity to the living fish, become the greatest enemy of the dead fish, for they soften and liquefy the cell contents, cause unpleasant tastes and odors, and permit the contents to escape from, the cell into brine. The proteins could not escape as long as they were proteins, but when they are broken down by autolysis into sim- pler substances the latter rapidly diffuse into the brine and are lost. This at least is the hypothesis, supported by some facts. What factors promote autolysis and what factors oppose it? Warm temperatures promote it directly. A temperature sufficiently high to destroy the enzyme stops it. Low temperatures retard it directly. If cells are ruptured, as they often are hj rough handling of the fish, autolysis rapidly decomposes the protein, and for this reason every bruise received by the fish during capture and subsequent han- dling results in the loss of so much protein during salting. A bruise on a fish has about the same effect as does a bruise on an apple, pro- moting rapid decomposition. Perhaps if the bruised fish turned brown, as the bruised apple does, the fisherman and packer would be more easeful in the handling of their fish. Factors that increase permeability of membranes seem to promote autolysis. Low temperatures seem to increase the permeability of the cells, so that fish that have been chilled decompose more rapidly on being warmed than fish that have never been chilled, though as long as the fish remain on ice the low temperature may prevent the enzymes from doing their work. It is as if increased permeability increases the escape of the enzymes, and that once escaped they play havoc if temperature conditions are allowed to become favorable. The optimum temperature for autolytic activity is about humaii body 1 PRESEKYATIOiSr OF FISH BY SALT. 13 temperature. 98° F. The autolytic enzymes act under a slightly acid condition. In neutral or alkaline medium they act very little, if at all. It has been noticed by A^arious investigators that autolysis does not begin until two to four hours after death. During rigor mortis there is a decided development of acid that may very materially pro- mote autolysis. It may therefore be that salting fish immediately after capture would strike through the fish before autolysis gains any headway. It may be possible, also, to take advantage of the removal of soluble products by brine in the salvaging of fish on the point of spoiling. Fish that have been held a long time are soft and of a disagi^eeable odor, because autolysis and possibly some bac- teria have decomposed the tissues to some extent. One might reasonably expect research to show that if rapid pene- tration is secured by means of pure salt the amino acids and other sour or disagreeable substances in stale fish resulting from autolysis would be removed by changing brine a few times, leaving the fish in a condition quite wholesome and fit for food. It is, of course, not intended here to encourage the practice of holding fish until they are bad and then salting them, but it is recognized that it is in the public interest neither to destroy food that can be used nor to mar- ket fish unfit for food, and it is recognized as legitimate and desirable to develop a means of saving fish whenever they have, through the unavoidable exigencies of the fishing business, come near to spoiling. It w^ould not be profitable to present this complicated subject any further here. Enough has been said to show that the loss in salting fish by solution of protein in brine is very great. Some discussion has been j^resented which will serve to show that losses of this kind are preventable, to point out the probable direction in which the remedy for this great loss will be found, and also, we hope, to assist in convincing the skeptics that scientific work on this aspect of the salting process would be worth while. It is of the greatest impor- tance that research work be undertaken for the purpose of discover- ing the conditions under which the cell proteins are digested and pass out and for ascertaining the conditions under which these proc- esses may be arrested. Specifically, such questions as follow should be answered: Once the permeability of cells has been increased by abnormally high or low temperature, does this increased per- meability persist after a normal temperature has been restored? When autolysis is set in action bj^ a bruise, do autohi:ic enzymes affect only the part bruised or do they escape and attack the unin- jured cells, destroying them also? To what extent does the acid of rigor mortis accelerate autolysis, and can this acceleration be pre- vented by early application of salt? To what extent is loss of solu- ble material in brine due to rough handling and to what extent to other factors? Can advantage safely be taken of the removal of products of protein decomposition by brine to salvage fish that are on the point of spoiling? „ INFLUENCE OF METHOD OF CLEANING FISH ON SALTING. In the various processes of salting or pickling fish the fish receive no preliminary treatment, or they may be gibbed, beheaded, split through belly, split through back, or cleaned perfectly by being cut 11 U. S. BUREAU OF FISHERIES. open, scraped, and washed before the salt is applied. By what cri- teria can we judg:e the merits of these various methods? The best way to answer this question is: Other conditions being held con- stant, which method or methods of cleaning result in least decom- position during the salting process? A series of trials was made by cleaning the fish by the various methods and salting them by the same process and determining the amounts of amino acid nitrogen developed. Two sets complete were tried, one consisting of one sample each cleaned by the various methods and held at a temperature of 79° F. during the salting process; another set similar to the preceding but held at 88° F. during the salting process. Both temperatures are high for salting fish, and the test is correspondingly severe. The results are shown in the following table, which is abbreviated from the paper by Tressler : Development of Amitvo Acid Nitrogen in Fish Cleaned in Various Ways. [Fish salted four hours after capture, with Diamond Flake salt containing 99.78 percent sodium chloride;' salting period, nine days.] Method of cleaning. Amino acid nitrogen Average formed tempera- during ture of salting salting. period per kilo of fresh fish. . op_ Grams. 79 0.77 79 .63 79 .68 79 .37 88 1.12 88 .76 88 .82 88 .47 Condition of fish at end of period. No cleaning, salted round Pipped : Head cut off, abdominal cavity split open, viscera, except nult and roe, removed. Cleaned perfectly, nult and roe removed, kidney and mem- branes scraped, and all blood washed out. No cleaning, salted roimd Pipped Head cut off, abdominal cavity split open, viscera, except milt and roe, removed. Cleaned perfectly, milt and roe removed, kidney and mem- branes scraped, and aU blood washed out. Badly spoiled, bloated. Spoiled. Do. Excellent condition. Badly spoiled, bloated. Badly spoiled. Excellent condition. Since amino acid nitrogen indicates decomposition, the conclu- sions from this table are entirely obvious. Only those fish were successfully salted at temperatures of 79 and 88° F. which had been thoroughly cleaned and from which all blood had been removed. While these high temperatures were chosen for the test because severe tests bring out diiferences in a more striking way, the differences will still exist even at low^er temperatures and manifest themselves in the poorer or better quality of product. Now, it may be either the blood or flesh, or both, in which the decomposition takes place. Since the perfectly clean fish decompose only slightly, it may be that only the blood decomposed in such cases as those given in the table, and that the decomposed blood pervading the otherwise sound tissue gave the appearance and odor of decomposition to the whole fish. On the other hand, it is possible that the enzymes in the blood when present operate to decompose not only the blood proteins but the tissue pro- teins also. However, this may be, the indisputable fact remains that if fish are to be salted in very warm weather it is absolutely PRESERVATIOiSr OF FISH BY SALT. 15 obli