CHAPTER 4

Tuna Canning and the Preservation of the Raw Material through Brine Refrigeration

S V E N LASSEN

Van Camp Laboratories, Terminal Island, California

I. Identification and Resources 207 II. Fishing Methods 2 09

III. The Development of the Tuna Fishing Industry 209 IV. The Preservation of Raw Tuna by Brine Refrigeration 211

A. The Brine Circulation System 213 B. The Bait Water Circulation System 215 C. The Ammonia Refrigeration System 215 D. The Fishing 218 E. Stowing and Chilling 219 F. Packing and "Topping Off" 219 G. Brining 219 H. Freezing 220 I. Drying Up and Holding in Dry Storage 220 J. Thawing of Brine-Frozen Tuna 221 K. Unloading 222 L. Quality Evaluation of Raw Tuna 222

V. The Butchering 225 VI. Precooking and Cooling 225

VII. Cleaning, Cutting, and Canning 229 VIII. Retorting 231

IX. Standards and Quality Specifications 235 X. Quality Control of Canned Tuna 242

XI. Concluding Remarks 243 References 243

I. Identification and Resources

Tuna is a generic term applied to a group of two or more families of fish which, in the zoological system, are classified under the order of Scomberformes. The tunas are of considerable importance because of the role they have played through the ages as a food for man. Archeo-logical evidence (Corwin, 1930) indicates that tuna was used as a food by early civilizations thousands of years before the herring, the cod, and the salmon attained similar status (Fig. 1).

Tunas are generally large and migratory in habits. They are carnivo­rous and voracious. They feed on planktonic crustaceans, argonauts, squids, jellyfish, etc. Generally speaking, it can be said that tuna feed

207

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on whatever small sea animals are most abundant and the easiest to ca tch in the waters they visit. W h i l e tuna thus seemingly prefer live food, they are not adverse to taking dead, or even salted, fish (Nakamura , 1 9 5 2 ) . T h e y will even b i te on artificial lures when tuna are moving in the surface waters. W h e n they take bait , they seem to at tack it at high speed, after which they dive deep, turning obliquely to the rear. T u n a are predominantly pelagic fish. The i r distribution in the oceans of the world is still incompletely known. I t is known, however, that they inhabit mainly the equatorial and temperate zones of the ocean.

FIG. 1. Butchering of tuna in the 6th century B.C. (From an ancient Greek wine pitcher in the State Museum of Berlin, Germany.)

T h e Pacific Ocean has for many years been the center of large-scale tuna-fishing operations. T h e Atlantic Ocean supports minor commercia l tuna-fishing activity along the east coast of the United States, and some­what larger operations along the European west coast from Norway to Spain. T h e South Atlantic Ocean, considered a potentially r ich tuna-fishing area, is as yet largely unexplored and unexploited (Mol teno , 1 9 4 8 ) . Smaller tuna-fishing operations are established in many areas of the Mediterranean Sea, the Car ibbean Sea , the Indian Ocean, and in waters bordering the Australian south and east coast (Anonymous, 1 9 5 6 ) .

Li t t le is known about the spawning and growth of tuna. According to a report on Japanese fisheries (Nat. Resources Sect. Rept. No. 104, 1 9 4 8 ) , certain species of tuna, including the commercial ly important

4 . T U N A CANNING AND PRESERVATION O F R A W M A T E R I A L 2 0 9

yellowfin, spawn in the waters surrounding Japan, Taiwan, and the Phil ippine Islands, bu t very li t t le is known of the larvae immediately after hatching. Similar studies have b e e n made on the distribution of tuna larvae in the eastern part of the Pacific Ocean . These studies indicate the presence of a t least three prominent spawning areas, one off the west coast of Mexico , another off the west coast of Centra l America , and a third bel ieved to b e in the vicinity of the Galapagos Islands. T h e s e studies, and others concerned with the geographical distribution of tuna and their population relationships have, during recent years, added much needed information of importance to the orderly commercia l utilization of the tuna resources of the eastern Pacific Ocean . T h e main contributors to these valuable investigations have b e e n the Inter-American Tropica l T u n a Commission ( 1 9 5 7 ) , the California F ish and Game Division, and the United States F i sh and Wildl i fe Service (Shapiro , 1948; Powell and Hildebrand, 1949; Graham, 1957; Schaefer, 1 9 5 6 ) .

II. Fishing Methods

T h e methods of fishing for tuna vary greatly in the different tuna-fishing countries. F ish ing with gill nets, encircl ing nets, or drift nets is prac t iced in many fishing areas. I n other areas, stationary fish nets ending in a trap are strung out at a right angle to the coastl ine to ca tch migrat ing tuna moving inshore along the coast. T h e most important commercia l fish­ing methods in use, however, are the pole-and-line fishing method, the long-line fishing method, and the purse-seining method. T h e pole-and-line method and purse seining are used for surface fishing, while the long-line fishing method, developed to a high degree of perfect ion b y the Japanese , is for below-surface fishing. F o r a more detai led description of tuna fishing gear, and methods of fishing, t he reader may consult the several publications on the subject b y the U. S. F i sh and Wildl i fe Service, in particular, Fishing Leaflet No. 297 ( 1 9 4 8 ) .

III. The Development of the Tuna Fishing Industry

Japan has played a major role in the development of the tuna-fishing industry. Unt i l 1912, J apan s tuna-fishing operations were on a relatively small scale l imited to coastal waters. W i t h the introduction of motor-driven vessels into their fishing fleet, in the years immediately prior to Wor ld W a r I , the Japanese were able to exploit more fully the r ich tuna-fishing areas surrounding their islands. W h e n the tuna ca tch in the coastal and offshore home waters of the Japanese Islands had reached its maximum in the early thirties, the Japanese began to explore the overseas tuna-fishing areas in the mid-Pacific Ocean, and beyond. In the meant ime, the Americans were developing a tuna-fishing

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industry along the coast of California, which eventually was to extend to the larger fishing areas in the eastern parts of the Pacific Ocean. Whi l e the Japanese tuna-fishing industry was to a large extent based upon supplying fresh or frozen tuna to the consumer, the American tuna in­dustry was based from its beginning upon the production of canned tuna.

T h e first tuna canning in the Uni ted States was started in 1903, when the sardine ca tch failed (Tress le r and Lemon, 1 9 5 1 ) . Dur ing the first year, 700 cases of tuna were produced ( 1 case = 4 8 ^ - l b . c a n s ) . T h e supply of tuna for the canning industry which developed in San Pedro and San Diego came from the coastal waters of California, which for many years were able to satisfy the growing demands of the local tuna-canning industry. As the years went on, and the demand for canned tuna expanded, the tuna-fishing boats extended their operations beyond the California coastal waters. This expansion extended the tuna-fishing activ­ities into the Pacific Ocean in a southward direction toward the equator and beyond. T h e extension of tuna-fishing activities to areas sometimes more than 2 ,000 miles from home port created some very difficult technological and economical problems. T h e s e problems involved, among others, the design and development of the right type and size of fishing vessel, which would b e suitable for long-distance fishing and able to stay at sea over a period of 3 to 4 months. A type of fishing vessel called the "tuna clipper" finally emerged as an answer to these problems. T h e tuna clipper soon b e c a m e very popular and, as a result, today dominates tuna-fishing operations in the eastern Pacific Ocean.

T h e tuna clippers are large Diesel-motor-propelled ships equipped with all modern navigational aids, ab le to hold from 100 to 500 tons of tuna in their fully refrigerated holds. T h e cost of this type of boat is high, and in order to amortize the investment over a reasonable period and make a reasonable profit, three to four trips a year with a full load of tuna are necessary. T h e average trip to the fishing grounds and back usually takes 6 0 - 7 0 days. T h e clippers are equipped only to fish with hook-and-line tackle and live bait . Some purse-seining boats are used for tuna fishing, but they account for only a minor part of the total ca tch (Schaefer , 1 9 5 6 ) .

T h e seasonal run of a lbacore and bluefin in California's coastal waters used to b e caught by fishermen in smaller vessels resorting to hook-and-line gear. These vessels, due to the short distance they usually operate from home port, often use crushed ice for preserving the fish, and in some instances, a combination of crushed ice and ammonia re­frigeration. S ince 1958 there has been a complete change in long-range fishing. T h e predominant method for tuna catching by U.S. clippers is b y purse seiners.

4. TUNA CANNING AND PRESERVATION O F R A W M A T E R I A L 2 1 1

T h e quali ty of manufactured product depends to a large extent upon the quality of the raw materials entering into its manufacture. This generalization applies, naturally, also to the manufacture of canned tuna. T h e many ways in which the tuna raw material may b e affected b y the t reatment it is subjected to on the tuna cl ipper while in transit would seem to b e sufficiently related to the manufacture of canned tuna to justify a discussion of these aspects of tuna canning in greater detail. Inasmuch as the tuna clippers, as stated above, br ing in b y far the largest part of the tuna raw material , the discussion will b e l imited to methods of preservation as used on board tuna clippers.

IV. The Preservation of Raw Tuna by Brine Refrigeration

T h e refrigeration system used in the tuna clipper is the br ine im­mersion refrigeration system. This method of refrigeration dates b a c k to 1913 when Ot tesen was granted a patent involving the direct immersion of fish into eutect ic brine. A modification of this system was selected in 1938 ( L a n g et al., 1 9 4 5 ) as the system of choice after the percentage of tuna rejects from boats using ice for tuna preservation has assumed major proportions. This crisis was caused b y the demand for more tuna than could conveniently b e provided from the local offshore tuna-fishing area. As a result, the fishing boats had to seek fishing areas further away from home port. In so doing, the limit beyond which i ce boats could safely b e used, without impairment to the quali ty of the iced tuna, was exceeded. A drastic reduction in rejects took p lace immediately following the introduction of the br ine freezing system, and this system of preservation has remained essentially unchanged during the past twenty years. T h e use of br ine as a heat transfer medium be tween the tuna and the refrigerating coils offers many advantages which are well understood b y engineers. An advantage of the br ine system, equal ly important but perhaps less appreciated, is the inhibit ing quali ty which salt ( N a C l ) in solution has on microbial life when present in hypertonic concentrat ion (Tanne r , 1 9 3 3 ) . T h e age-old method of preserving fish and meats b y salting or brining is, of course, based on that fact. T h e s e obvious ad­vantages may, in some instances, b e counterbalanced b y the fact that br ine solutions in contact with fish, particularly, i f extended over longer periods of t ime at temperatures above the freezing point of the brine, impart to the fish tissue qualities which may b e undesirable. In this instance, there can b e a considerable penetrat ion of salt into the tuna muscle. This has a denaturing effect upon some of the muscle protein, while another part of the muscle content seems to dissolve. Fur thermore , the remaining muscle tissue combines with the salt in such a fashion that it cannot again b e removed in subsequent canning operations ( L e w i s and

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Saroff, 1957; Saroff, 1 9 5 7 ) . Final ly, br ine in contact with the tuna has a tendency to draw moisture out of the fish, thereby causing a temporary or permanent loss in weight. Tempera ture and p H of the brine, as well as its concentration, are factors which influence these changes to a marked degree. A further study of the influence of sodium chloride brines upon tuna is badly needed. T h e use of sodium chloride brines as a freezing medium in a tuna clipper, if not used at all t imes at or near the eutect ic temperature, can, therefore, not be considered the ideal method of preservation, but must b e looked upon as a compromise be tween the advantages of br ine refrigeration and its disadvantages, some of which have been outlined in the above. T h e s e disadvantages may fortu­nately b e reduced to a reasonable minimum b y taking ca re that the temperature of the br ine when in contact with tuna is at all t imes kept at as low a temperature as proper operational prac t ice will permit.

Any at tempt to give a satisfactory description and illustration of the br ine refrigeration system in a tuna clipper is complicated b y the fact that this system consists not only of a highly compartmental ized ammonia compression and expansion system, bu t also of a br ine circulation system integrated into it for the purpose of transmitting, efficiently and fast, the heat from the highly perishable tuna to the heat-absorbing ammonia in the refrigeration coils. T h e periodic use of the freezing tanks (we l l s ) in a clipper for live bait , and in some instances for fuel during the first part of the voyage would further complicate any at tempt to incorporate all the mechanica l features connected with these uses into one composite description and illustration. In view of this, and for the sake of clarity, the ammonia refrigeration system is descr ibed and illustrated separately from the br ine circulation system, and the ba i t circulation system also will b e treated as a separate entity. I t is hoped that the advantages gained b y such a representation will outweigh the obvious disadvantages of presenting interrelated systems as separate units. T h e descriptive and illustrative details to b e given have been adopted to fit a situation such as one would find it in an average-size cl ipper of 2 5 0 tons capacity. T h e over-all specifications of such a boat would b e roughly as follows.

T h e boa t is made of steel, 121 ft. long, 28-ft. beam, 13-ft. draft, and will carry a fully refrigerated load of 2 5 0 tons of tuna. T h e boa t is compartmental ized into ten wells, five on ei ther side of the boat , and three bai t boxes on the stern. T h e wells will average 800 cubic ft. or 6,000 U.S . gal. and hold about twenty tons of tuna in each well. T h e bai t boxes will average 633 cubic ft. or 4 ,740 U.S . gal., or about sixteen tons of tuna each. All wells and boxes are usually insulated with a 5-in. layer of fiber glass or other satisfactory insulation material .

4 . TUNA CANNING AND PRESERVATION O F R A W M A T E R I A L 2 1 3

A. T H E B R I N E C I R C U L A T I O N S Y S T E M

In the br ine circulating system, br ine or sea water may b e circulated through the wells. T h e br ine in the course of its circulation comes into contact with the ammonia coils lining the inside surfaces of the wells, and will b e chi l led b y the evaporating l iquid ammonia in the coils, thereby gradually causing a transfer of hea t from any tuna stored in the wells. T h e evaporating ammonia in the coils will flow to the ammonia compressor where , after an adiabat ic compression, it will b e freed of its heat, and liquefied in the condenser and returned to the receiver for further use.

T h e br ine circulation system, as seen in F i g . 2 , comprises a 4- in . diameter sea suction main line from the sea chest and a 4- in . drain dis­charge main line connected to each well and box. T h e main suction and main discharge lines are again interconnected, so that in case of a break­down of any of the circulating pumps, the transfer pump can take over. T h e individual br ine circulating pumps, one for each wel l and box (excep t the back b o x ) , are 2 % - i n . open impeller type driven by a 3 - H . P . , 2 2 0 - V , 3 -phase A . C . motor, and a rating of 2 5 0 gal. per minute. W i t h the proper valves connect ing the suction main and the discharge main closed, each well becomes a closed circuit taking suction from the bot tom of the well, be tween the bai t screen, and discharging into the top of the well through the gooseneck in the coaming. E i the r the suction main or the discharge main may b e used to transfer br ine from one wel l to another using any one of the twelve circulating pumps. Bo th the suction main and the discharge main will b e used to transfer br ine if the transfer br ine in the transfer is used. T h e transfer pump is usually a 4- in . diameter, 5 H . P . , 5 0 0 G . P . M . open impeller type.

As can b e seen b y inspecting F ig . 2 , the br ine circulation system is remarkably flexible. B y handling the proper valves, any pump in the system can take suction not only from the main, bu t from any well or b o x and transfer or discharge its content, as required. S o m e of the disadvantages of this system are its complexity, and the fact that many valves and much piping are needed for its operation. As the valves age and corrode, opening and closing b e c o m e a laborious, t ime-con­suming operation. Another, perhaps more important, disadvantage is that it is impossible to flush the system completely following the transfer, circulation, or discharge of the br ine.

An effort to modify and simplify the br ine circulation system has recently b e e n m a d e b y a shipbuilding concern. I n their recent ly com­missioned clippers, the br ine and bai t circulation system are combined into one. This eliminates much piping and many valves, thereby simpli-

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4. TUNA CANNING AND PRESERVATION O F R A W M A T E R I A L 215

fying the construction and helping to clear an already too-crowded shaft alley. Present day economics of tuna fishing makes these savings of part icular interest. T h e wells and boxes ( excep t the stern b o x ) have strung along their inside surfaces ly^-in. standard galvanized ammonia coils 8 in. on center .

B . T H E B A I T W A T E R C I R C U L A T I O N S Y S T E M

T h e bai t circulation system is designed to satisfy the oxygen and food requirements of the large amount of live bai t which present pole-and-line fishing methods make it necessary to carry on a tuna clipper. T h e vessel illustrated in F ig . 3 carries as much as 4 ,000 scoops of live bai t at one t ime. This is equal to 2 0 tons of live bai t . L i v e ba i t requires a large amount of oxygen, c lean water, and food. T h e amount of food obtained from offshore "blue water" is probably inadequate for ba i t obtained from an inshore "green water" habitat , so the satisfactory feeding of bait , while in captivity, may b e as great a problem as is the supplying of the bai t with a satisfactory amount of oxygen-containing ocean water. Under favorable conditions, the major part of the bait , so indispensable to the fishing methods employed on board tuna clippers, may b e kept alive until they are used for "chumming."

T h e large axial flow bai t pumps are calculated to provide sufficient water to completely change the water in less than 9 min. in all wells and boxes. However , freshly caught bai t requires the highest flow rates. After the bai t has been in the wells for a few days and "settled down" the rate of flow may b e cut down considerably. T h e abil i ty of some species of bai t fish to survive be t te r than others is thought to b e due largely to their ability to bet ter adapt themselves to abrupt and sometimes extreme temperature changes in the ocean water which may occur when the cl ipper in its search for tuna moves from one locali ty and temperature to another.

C . T H E A M M O N I A R E F R I G E R A T I O N S Y S T E M

T h e ammonia refrigeration system on a cl ipper ship ( F i g . 4 ) consists of the five elements indispensable to any ammonia refrigeration system, namely, ( 1 ) compressor, ( 2 ) condenser, ( 3 ) receiver, ( 4 ) expansion valve, and ( 5 ) evaporating unit. In the standard tuna cl ipper the refrigeration installation consists of three 6-in. X 6-in., 2-cylinder, ver­tical, single-acting compressors, turning at 3 6 0 r.p.m., giving a total refrigeration of 54 .3 tons at 25 lb . per square inch on the suction side, and 185 lb. per square inch condenser pressure. T h e condenser will have 1,100 square ft. of cooling surface, with water circulating through the condenser at 2 5 0 gal. per minute. T h e twin ammonia receivers hold

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4 . TUNA CANNING AND PRESERVATION O F R A W M A T E R I A L 2 1 7

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1,200 lb . of ammonia combined. F o r each well and box (excep t the stern box, which usually has no refrigeration) both automatically and manually operated expansion valves are provided. T h e evaporation system consists of 1%-in. standard galvanized ammonia coils, covering the inside surfaces of the wells. T h e coils are lined up 8 in. on center. On this basis, a 30-ton well has about 1,000 linear ft. of evaporator coils, which gives the well roughly 0.84 l inear ft. of coil per cubic foot of space in the well. Besides this, the refrigeration system is provided with back pressure regulators ( H u b b l e valves) on the three suction lines of the wells, pressure gauges, oil traps, safety controls, etc., as shown on sketch. Refrigeration for the ship's stores is provided by a separate 3-in. χ 3-in., two-cylinder, vertical, single-acting compressor which is r igged into the main ammonia system as shown. T h e wells and boxes are all insulated, usually with a 5-in. layer of fiber glass. This insulation has, in many instances, proved inadequate, and other types of insulation are now being tested.

D . T H E F I S H I N G

W h e n a school of tuna is sighted, the fishermen take their positions in the especially designed metal racks a t tached to the outside of the hull of the clipper, and the "chummer" starts throwing overboard the live bait , which may consist of anchovies, small pilchards or anchovettas, etc., to stimulate the tuna to bi te . I n the meant ime, the fishermen start to fish with their pole and line gear provided with barbless hooks. T h e hooks are made in the form of a lure cal led a "squid" or "jig." As soon as the tunas strike the hooks they are pulled on board, and in releasing the tension on the pole line as the tuna hits the deck, the barbless hook disengages automatically and the fisherman immediately throws out his line ready for another strike. In this way fishing continues so long as the fish continue to strike or until there is a "rail full." A "rail full" describes the condition when the tuna have filled the stern, back of the men in the racks, flush to the caprail. At this point there is no choice but to dis­continue fishing operations, and usually water chutes are r igged to flume the fish from the stern into the designated well. In most instances the t ime the tuna spend on deck before they are put into the well is of short duration. The re are, however, situations where the tuna may have to spend several hours on deck before they can b e stored in the well.

T h e high temperature usually prevailing in the area where tuna are caught, and the fact that tuna, in spite of their presumed lack of a thermoregulatory body mechanism, often have, when landing on deck, a body temperature of up to 1 4 ° F . above the surrounding ocean temperature, may make the t ime on deck a very crit ical one. B y hosing down or watering

4 . TUNA CANNING AND PRESERVATION O F R A W M A T E R I A L 2 1 9

the caught tuna occasionally, the internal temperatures of the fish can b e lowered faster to that of the surrounding temperature. B y hosing down the tuna, the blood and vomit that are discharged from the tuna during the death struggle are also removed.

E . S T O W I N G AND C H I L L I N G

W h e n a well or box is expected to receive tuna within the next 2 4 hr., the well is filled with sea water, if not already full, and the ammonia coils in that well are cut into the refrigeration system, thereby lowering the temperature of the sea water. Sometimes, however, due to the unpredictable course that fishing often takes, the well is not ready when tuna strike; at other times, fishing is so slow that it takes 2 to 3 weeks before a well is completely filled. In such instances, some of the tuna in the well will have been kept for a long t ime at temperatures generally considered unfavorable for optimal preservation of quality. T o overcome difficulties of this type requires much skill and good judgment on the part of the tuna boa t engineer.

F . PACKING AND " T O P P I N G O F F "

W i t h the chil led sea water circulating in the well and the tuna be ing added as they are caught, the well finally arrives at a stage of fullness usually cal led "tails" or "tails up." At this stage no more fish can b e added until some (o r a l l ) of the sea water is pumped out of the well . General ly a well with "tails up" will hold about 2 0 to 2 5 % more fish when pumped dry. T h e pract ice of settling the tuna in the well , thereby permitt ing 2 0 - 2 5 % more fish to b e carried, is referred to as packing and "topping off" and has been used ever since br ine immersion freezing was introduced into the tuna fleet. A closer study of the "packing" and "topping off" pract ice revealed this pract ice to contain many objection­able features which will slow down fast, efficient, and uniform freezing of the tuna during the br ine freezing which follows.

G. B R I N I N G

After a well is packed, a grating is secured low in the hatch coaming to prevent the fish from floating when high-gravity br ine is added, or made up in the well . T h e amount of salt required to make up a br ine depends upon the lowest temperature to which the br ine is to b e taken during the freezing of the tuna in the well. I t is customary to add enough salt, progressively or in one lot, so that the freezing point of the br ine is 4 - 6 ° F . lower than the final temperature the br ine will have prior to its final discharge from the well. A rule-of-thumb measure of the amount of salt to use in br ining a well is to use one-half sack of salt ( o f 1 2 5 lb .

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n e t ) per est imated ton of tuna in the well . Sometimes the br ine used for the freezing of one well may b e reused for another well . This procedure, however, does not extend beyond the second well , because the br ine soon becomes "heavy" with organic matter . T h e most common pract ice of brining a well is to make up the br ine in the coaming of the well containing the packed fish. W h e n the circulating sea water in the well has been brought down to about 3 0 ° F . , the required amount o f salt is dumped into the coaming on top of the grating, and the circulating water from the gooseneck inlet washes the salt down into the fish and ultimately dissolves the salt. Some engineers prefer the progressive salting methods, whereby the salinity of the sea water circulating in the wel l is pro­gressively be ing increased b y addition of salt. T h e procedure followed in the brining of tuna is not very uniform, owing in part to very restricted space conditions, and the fact that the wells, besides serving to hold fish, must also serve as ba i t tanks and fuel tanks during a sea trip which, as mentioned above, often takes more than 2 months.

H . F R E E Z I N G

W h e n the br ine in a wel l has at tained the required strength, the actual br ine freezing begins. B y circulating the br ine in a downward direction the coils a t tached to the inside surface of the wel l absorb the heat of the br ine and of the tuna around which the br ine circulates. T h e t ime required to br ing a well down to the desired temperature, usually be tween 15 and 2 0 ° F . , may vary from one to several days, but , in any case, after the desired low temperature has b e e n established, the well is not "dried up" for at least 7 2 hr. S ince quick freezing is generally bel ieved to b e less injurious to the fish muscle than slow freezing ( L u s e n a and Cook, 1953, 1954; Lusena, 1 9 5 5 ) , the ra te at which the temperature interval be tween 3 0 ° F . and 2 0 ° F . is traversed is of im­portance. Dur ing this temperature interval, most of the hea t of crystal­lization of the i ce in the fish muscle is given off. Th is evolution of heat slows down the ra te of cooling of the br ine until most of the tuna has frozen, after which the temperature again suddenly resumes its down­ward trend. T o br ing the temperature of the tuna down through this interval fast can b e done only when the br ine is ab le to circulate freely around the tuna in the well and the ammonia coils. A t ight packing of the tuna in the well is, therefore, not conducive to fast freezing and should b e avoided b y all means.

I . D R Y I N G U P AND H O L D I N G IN D R Y S T O R A G E

Fol lowing the lowering of the tuna in the br ine to the desired freezing temperature, the br ine is pumped out of the well . Refrigeration is

4 . T U N A CANNING AND PRESERVATION O F R A W M A T E R I A L 2 2 1

usually stopped several hours before the br ine in the wel l is actually pumped out to allow any coils that may b e iced and random pockets of ice around the sides of the wall to thaw out. This makes it easier to thaw the well uniformly and unload the tuna later on upon arrival in port.

After the drainage period, the ammonia coils are put into service again. T h e amount of refrigeration needed to maintain the temperature in the wel l will depend mainly upon the condition of the insulation of the well. In case of good insulation, very little refrigeration is needed to maintain the temperature. A further lowering of the temperature of the frozen tuna which often takes p lace after the br ine has been pumped out is bo th costly and undesirable.

J . T H A W I N G O F B R I N E - F R O Z E N T U N A

Before the tuna in the wells o f a tuna cl ipper can b e unloaded, i t is necessary that they b e partially or fully thawed. This takes t ime, and that is why the tuna cl ipper engineer sometimes requires up to a week's not ice before he can have a well ready for unloading. Although there is a considerable variation in detail of thawing methods, the general principle favored now seems to b e as follows: first, the ammonia refriger­ation on any wel l to b e thawed is stopped. This , depending upon circumstances, is often done several days before thawing actually starts. Next, sea water is introduced into the wel l b y pumping the water from the sea chest through the well in an upward direction, allowing i t to spill over the open coaming, or let t ing it escape through the bai t circulation overboard discharge. I t is customary to start at least two wells at a t ime, one port wel l and the corresponding starboard well , and then proceed stepwise to the other wells as they are needed. After approximately an hour, flow from the sea chest is stopped and the br ine circulation pump takes over, reversing the flow through the well . Sal t is then added, in amounts equal to, or slightly in excess of, the amount originally used to freeze the tuna ( s ee Sect ion I V , G ) . I f the temperature of the frozen tuna has b e e n lowered appreciably after the drying up of the well , additional salt will b e needed for rethawing. As soon as the br ine circulation begins, an abrupt drop in temperature of the br ine occurs, due to the melt ing of ice in and around the tuna and the absorp­tion of hea t resulting from this change of state. W h e n thawing comes to an end, after several days, the temperature of the tuna should b e around 2 8 ° F . , and the br ine is pumped out. As ment ioned above, one reason for thawing the frozen tuna in a cl ipper wel l is to br ing the fish to a condition where i t can b e unloaded. Th is involves the separation of the individual fish, which, due to the freezing procedure, have been cemented together in a matr ix of ice , which sometimes even fills the

222 SVEN LASSEN

interstitial spaces be tween the fish. Another purpose of thawing is to bring the temperature of the tuna up to such a point that the fish upon landing will b e in a condit ion suitable for butcher ing and precooking.

K. UNLOADING

Upon arrival at the dockside of the cannery, the thawed tuna are unloaded. T h e unloading procedure varies with the equipment available at the dockside. I n some instances, a meta l bucke t is lowered into the well, where it is filled b y the fishermen. I t is then hoisted up and b y a release mechanism made to discharge its content into a flume, which carries the tuna into the cannery. I n other instances, where fluming is impractical , wheeled buckets are lowered into the wells, where they are filled b y the fishermen and hoisted over the side onto the dock, where they are rolled over the weighing scale into the cannery. In some in­stances, the hoisting is done b y the clipper's own hoisting equipment; in others, an electrically operated hoist located on the dock does all the hoisting, guided b y an operator stationed at the ha tch opening. In any event, the first step in landing of the tuna is, naturally, to weigh it, after which it is carried by conveyor to the butcher ing tables.

L . Q U A L I T Y E V A L U A T I O N O F R A W T U N A

T h e raw thawed tuna, when unloaded from the tuna clipper, reflects in its state of freshness the care it has been given during the several weeks it usually has been in transit from the fishing grounds. T h e most common and pract ical method for the determination of the state of freshness of the tuna, is the organoleptic test. This is a test which relies for its execution exclusively upon the use of man's sensory faculties such as sense of sight, sense of smell, sense of touch, etc. Such a test fulfills one of the primary requirements at this point, namely, that the test b e practically instantaneous and fairly rel iable. In a subjective test such as this, absolute reliability is, of course, impossible. T h e organoleptic method of evaluation has, here as in other branches of the food industry, stood the test of t ime and is therefore, generally accepted b y the three parties involved here, namely, the tuna boa t owner, the state cannery inspector, and the tuna canner. Several chemical tests have b e e n de­veloped for the purpose of putting the quality evaluation of raw tuna an a purely object ive basis. These chemical methods are generally based upon the isolation and chemical determination of one or several of the decomposition products which result from the deterioration and spoilage of fish, and which are not present in appreciable amounts in fresh fish. One of the main requirements of such tests is that their use should enable the observer to detect spoilage long before the fish is in such a condition

4. TUNA CANNING AND PRESERVATION O F R A W M A T E R I A L 223

that spoilage is obvious b y the organoleptic test. O f these chemical tests, the most important seem to b e ( 1 ) volatile reducing substances ( V R S ) , ( 2 ) total volatile bases, ( 3 ) the t r imethylamine test, ( 4 ) total volatile acids.

W h i l e all of these chemica l methods are useful in the determination of incipient spoilage, they have found only a l imited use for quality evaluation in raw tuna, mainly due to the t ime element involved in their completion. In cases of disagreement on quali ty be tween the parties involved, the chemica l tests may, however, serve a useful purpose as corroborative evidence. T h e organoleptic method for quality evaluation in raw tuna remains, therefore, the primary method, and, in the hands of properly trained personnel, can give results of high agreement be tween individual observers.

Several at tempts have been made to grade the raw tuna into several classes, instead of be ing satisfied with only a rejection or acceptance pattern. None of these attempts, however, has been fully satisfactory. An at tempted organoleptic grade classification of raw tuna is given in T a b l e I .

In this classification effort, four different grades have b e e n recognized. In these grades, six different physical characterist ics, such as appearance of gills, of eyes and skin, smell, and degree of physical damage to the tuna, have b e e n used. All of these characterist ics can b e recognized almost instantaneously. A quality classification is, therefore, a mat ter of seconds only. I n dividing up the total quali ty range into four different grades, the system used in other branches of the food industry has been followed. I t is, of course, possible to assign a numerical value to each characteris t ic in each quali ty class, and thus initiate a numerical scoring system which may have some pract ical application.

In spite of the advantages that the use of such a system may have, the score obtained does not define quali ty very concisely. I t will b e noticed that the grade classification does not contain any reference to the type of degradation of quali ty which is associated with adverse colors in the fish tissue upon cooking, and which sometimes is cal led brown or green fish. Nei ther does the classification take into consideration any condition producing the so-called honeycombed fish. Another characteris t ic of thawed raw tuna that has proven difficult to incorporate into a classifi­cation is the adverse effect of salt penetration. Excess ive salt penetration into the tuna muscle makes the tuna unsuitable for certain types of pack. F inal ly there is the important question of size of tuna, which has a strong influence upon yield, texture, taste, etc. , of the canned product and which it has not been possible to include in T a b l e I nor in any quali ty grading chart known to the writer. I t is, therefore, obvious that the problem of

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224 SVEN LASSEN

4. TUNA CANNING AND PRESERVATION O F R A W M A T E R I A L 225

devising a quali ty grading chart which takes into account all, or even most, of the many factors that constitute quali ty is a very difficult task. T h e pigments responsible for the development of off-colors in tuna upon cooking have been the subject of much speculation in the past. Recen t work on this problem seems to have established that the pigment responsible for the characteris t ic pink color of canned tuna mea t is a hemochrome, and that this pink hemochrome pigment upon oxidation changes into the less desirable tan hemochrome. This would indicate that reducing conditions, i f maintained during canning operations, should produce a more attractive canned product of good color quali ty ( B r o w n and Tappel , 1 9 5 7 ) .

V. The Butchering

T h e butcher ing tab le is a long, wide table , in the middle of which runs a slow-moving, wide be l t conveyor, level with the table top, which carries the tuna down the length of the table . T h e butcher ing crew, standing b y the tables, slit the be l ly of the tuna open with one stroke of their butcher ing knives, while with the left hand they pull out the viscera. Then , with another stroke of the knife, they make an incision be low the gills of the tuna, thereby severing the visceral connect ion with the head, and with a third stroke of t he knife they sever the visceral connect ion with the vent. T h e viscera, which amount to from 3 to 8 % of the total weight of the tuna, is used for fish-meal manufacture. D u e to their high content of vitamins A and D , the tuna livers often are sepa­rated from the rest of the viscera for vitamin extraction. T h e Sta te Board of Heal th in California maintains a cannery inspection service, and inspectors or other representatives of this service are always present to see to it that no tuna passes the butcher ing table which will not fully comply with their standards of suitability for human food. After the visceral cavity has b e e n rinsed with water, the tuna are transferred to wire baskets and p laced in wheeled racks holding about 700 lb . of fish, after which they are rolled into the cookers.

VI. Precooking and Cooling

T h e cookers used for precooking of tuna are large, rectangular-shaped steel chambers provided with l ive steam inlets, outlets for condensate, and vents and relief valves. T h e relief valve is set so that the temperature in the cooker never exceeds 2 1 6 ° F . T h e capaci ty of each individual cooker may vary from 2 to 5 tons of bu tchered tuna, corresponding to 6 to 16 racks. After the cooker has b e e n filled, the door is closed and bol ted, and steam is let into the cooker. Dur ing cooking, the tuna suffers a consider­able loss in weight. R a w tuna, in the condition it is received at the

2 2 6 SVEN LASSEN

cannery, contains 6 8 - 7 6 % water. During precooking, and coincidental with the heat coagulation of the fish protein, the water content drops to lower values ranging from 6 5 to 7 1 % . T h e subsequent sterilization cook of the canned tuna in the retort further lowers the water content of the tuna muscle to values ranging from 60 to 6 7 % .

As the cooking proceeds, water, and water-soluble proteinaceous material such as gelatin, nitrogen-containing extractives, and other sub­stances are leached out of the fish and accumulate in the condensed steam which flows from the cooker continuously during the cooking operation. This condensate also contains a certain amount of oil. T h e steam which, during the cooking, escapes through the steam vents contains certain volatile substances that are characterist ic of raw fish odor ( a m i n e s ) . Under the influence of heat, the protein in the tuna muscle will coagulate and shrink away from the bony structure, thereby, making easier the subsequent cleaning and separation of the dorsal and ventral loins which are used for canning. T h e precooking of tuna is, therefore, a very important step in the over-all canning operation, as this step, perhaps more than any other, influences not only yield but quality. I t is unfortunate that no studies on the cooking of tuna have been published, and as a result the precooking of tuna is still done on an empirical basis. I t is known, however, that in order to obtain a good cook, the temperature of the tuna, as measured along the upper part of the spinal column, in the thickest part of the fish, must b e brought up to approximately 1 4 0 - 1 5 0 ° F . Fur ther cooking beyond this point is not only unnecessary bu t actually reduces both yield and flavor of the tuna meat . This reduction in yield and flavor is accompanied b y a corresponding drop in water content of the tuna muscle protein. F igure 5 shows the relationship be tween the moisture content of tuna muscle protein of precooked yellowfin tuna and the temperature at tained in the center of the tuna at the end of the precook period. Inasmuch as the temperature attained in the center of the tuna is directly related to the t ime of pre­cook, the moisture content-cooking t ime relationship may b e expressed by a graph of similar slope.

T o obtain a good cook, it is also important that the tuna b e graded for uniform size, within very narrow limits. This sometimes proves difficult when the size distribution in a load of tuna is wide. Another important point to observe in connection with precooking of tuna is that all the tuna must have the same temperature when entering the cooker. T u n a which has not been fully thawed will need much more heat before a temperature rise takes p lace in the tuna than will a fully thawed fish. T h e preferred size of tuna lies be tween 10 and 2 0 lb . Often, however, skipjack are received which weigh considerably less; at other times tuna weighing

4. TUNA CANNING AND PRESERVATION OF RAW MATERIAL 227 R

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228 SVEN LASSEN

from 5 0 to 100 lb . are received. Such large tuna do not give as fine a canned product as do the small tuna, and are, therefore, not used for "fancy" grade pack.

T h e Sta te of California F i sh and G a m e Code specifies that "No yellowfin tuna or bluefin tuna weighing less than 7^4 pounds may b e sold, purchased, or processed." F o r albacore, the code states that "No albacore weighing less than 7 pounds may b e sold, purchased, or processed." F o r skipjack, the minimum weight is 4 lb . T h e code permits these tuna to b e taken at all t imes of the year.

T h e cooking t ime for tuna varies with size. T a b l e I I gives a general idea of the cooking t ime used in some tuna canneries.

TABLE I I THE RELATIONSHIP BETWEEN AVERAGE COOKING TIME AND SIZE OF TUNA

Size (lb.) Cooking time (hr.) Size (lb.) Cooking time (hr.)

4 WA 35 8 50

12 2 70 554 20 90 ey2

25 3y2

T o the cooking t ime shown in T a b l e I I must b e added the so-called "coming-up time," which is the t ime it takes to br ing the temperature up to 2 1 6 ° F . in the cooker. This usually requires one-half hour to one hour, depending upon the size of the cooker, the temperature of the tuna when entering the cooker, and the average size ot the fish. W h e n the precooking has b e e n completed, the steam is turned off, the cooker door opened, and the racks of steam-cooked tuna are rolled out and allowed to cool. T h e cooling often takes 1 2 - 1 8 hr., and is usually carried out in cooling rooms provided with good air circulation and screened for protection against insect infestation. Dur ing the cooling period, the tuna undergo some very important changes. T h e weight of the cooked tuna is further reduced through evaporation from the hot fish. A general drying up of the surface area of the fish often takes place. T h e skin on the tuna, which during cooking has loosened from the muscle tissue and which at that point may b e peeled off, will, as a result of the drying during cooling, dehydrate and b e c o m e leathery and reat tach itself to the cooked tuna muscle. Some of the oil contained in the tuna, which during cooking has accumulated on the surface of the cooked tuna, may b e c o m e oxidized as a result of the temperatures prevailing during the cooling, and the air circulation which characterizes a cooling room. Final ly, there is the problem of preventing microbial life from develop-

1 /2

3 /

m 4/2

4 . TUNA CANNING AND PRESERVATION O F R A W M A T E R I A L 2 2 9

ing on the surface of the tuna while it is cooling prior to be ing canned. B y reducing the t ime be tween cooking and canning to a minimum, the influence of microbial life on the surface of the tuna may b e reduced to during cooling is much more difficult to overcome and has, as a mat ter inconsequential dimensions. T h e effect of dehydration and oxidation of fat, only been overcome in part by bet ter design of cooling rooms.

VII. Cleaning, Cutting, and Canning

T h e racks of cooled tuna are now rolled into the cleaning room, where a crew of cleaners (usually w o m e n ) , standing on both sides of the cleaning table , start the task of separating skin, bones, fins, b lood meat , etc., from the light loin meat . T h e cleaning tables are long tables, usually covered with stainless steel, in the middle of which an elevated con­veyor belt , also of stainless steel, is provided to receive the loin mea t and carry it to the cutt ing machine on its way to the canning equipment. Under the cleaning table runs a steel conveyor be l t which, through holes and chutes in the cleaning table , receives the offal from the tuna clean­ing, and transports it to a cart in which it is transported to the fish mea l plant.

T h e previous cooking and cooling of the tuna have firmed the muscle tissue to such an extent that the fish can b e handled b y the cleaners without crumbling. T h e dorsal, ventral, and pectoral fins are first re­moved, after which the skin is scraped off the flesh ( o f the tuna ) with a knife, without cutt ing too deeply into the muscle meat . Next the tuna is c leaved b y hand into four longitudinal parts, each containing one of the dorsal or ventral loin muscles which surround the spinal column of the fish from head to tail and which form the main substance of the tuna. As the tuna is broken into these four sections, the mea t falls readily away from the bony structure, thus aiding in a complete separation of fish muscle from the bones. T h e dark vascular b lood meat , which separates the dorsal and ventral loins and is in part embedded in them, is scraped off, and the c leaned loins are p laced on the conveyor be l t which carries them to the canning equipment . T h e procedure which is followed from here on depends upon the style of pack which is desired. T h e T u n a Standards that have recent ly b e e n set, which will b e discussed in more detail later, recognize four styles of pack. T h e s e are ( 1 ) solid pack, ( 2 ) chunk pack, ( 3 ) flake pack, and ( 4 ) grated pack. T h e solid pack may b e packed b y hand or b y machine . In case a hand pack is used, the tuna loin meat , upon leaving the cleaning tables, is passed through a guillotine­like cut t ing mach ine which cuts the loins into proper lengths to fit the depth of the can. T h e individual loin pieces which, generally speaking, have the shape of a sector, are p laced neatly, side b y side, in the can b y

230 SVEN LASSEN

the cannery worker until it is filled completely. After this, the can is p laced back on the moving conveyor line where the required amounts of salt, oil, or other flavoring ingredients are added automatically. F r o m there the cans move either into an exhaust box before they finally enter the closing machine, or, i f an exhaust box is dispensed with, through a closing machine provided with a steam-jet vacuum mechanism. T h e more expensive hand packing is fast be ing replaced b y the more pract ical machine pack. T h e introduction of the modern closing machine in which a vacuum is pulled on the filled can immediately before closing has largely el iminated the need for exhaust boxes.

I f machine packing is used for solid packs, the loin meat , on leaving the cleaning tables, is formed into a cylindrically shaped continuous ex­trusion, b y passing through a machine cal led the pack-shaper. At the point where the empty can passes the discharge end of the pack-shaper, the cylindrically shaped tuna loins are pushed into the can and cut b y a reciprocating knife, thereby filling the can with a solid plug of tuna meat. T h e can is subsequently passed through a tapping machine which, by means of a revolving, reciprocating plunger, presses the tuna meat further down in the can, thereby leaving room for the subsequent addi­tion of salt, oil, or other flavoring ingredients. F r o m there the can is sent to the vacuum closing machine.

F o r chunk pack, the tuna loin mea t is machine-cut into suitable pieces by a cutting device similar to the one just ment ioned and passed through a so-called pack-former, an ingenious machine which fills the cans loosely, and subsequently b y means of a tapping machine presses the content of the loosely filled cans down to proper volume before the cans return to the line where salt, oil, or other ingredients are added, then on to the conventional vacuum closing machine ( see further Anonymous, 1 9 3 9 ) .

F l ake pack is manufactured in a manner much l ike chunk pack. T h e size of the individual pieces of tuna meat is much smaller than chunk pack. T h e relative content of large and smaller pieces is regulated b y the standards covering this style of pack. T h e individual p iece of flake must, however, b e of such a size that "the muscular structure of the flesh is retained." T h e standards covering the manufacture of canned tuna will b e dealt with in more detail later in this chapter.

T h e grated pack is produced b y passing the tuna meat from the cleaning tables through a grating machine, in which the tuna mea t is chopped up and passed through a screen to give a product of uniform size. I t is packed into the cans b y passing it through the pack-former, which is equally well suited for filling the cans with chunk, flake, or grated tuna. Leaving the pack-former, the cans containing the grated tuna are passed through the tapping machine , then b a c k to the packing

4. TUNA CANNING AND PRESERVATION O F R A W M A T E R I A L 2 3 1

line to have salt, oil, and other optional flavoring ingredients added, after which they are closed in the steam-jet closing machine .

After leaving the closing machine , the tuna cans are usually passed through a machine or device in which the cans are c leaned of any oil or other organic mat ter which has stuck to the surface of the cans, before they are dumped into cylindrical, perforated metal baskets in which they are transported to the retorts.

T h e canning operations and the equipment descr ibed here are illus­trative of methods and equipment used in the large tuna-canning centers located on the Pacific Coast of the Uni ted States. I t is be l ieved that the methods and the canning equipment used in other countries for similar purposes do not vary essentially in principle from those used in the United States. In countries where labor costs are lower and skilled oper­ators harder to get, there will naturally b e less incentive to install the expensive and highly complicated packing machines, which in this coun­try have b e e n found so useful for high quali ty mass production; in that instance, hand-packing or semiautomatic packing is used ( see also Jarvis, 1944, 1 9 5 2 ) .

F o r a discussion of standards of identity, fill of container require-and other optional flavoring ingredients used in tuna canning operations, ments for canned tuna, and of quali ty specifications for salt, salad oil, the reader is referred to Sect ion I X of this chapter.

VIII. Retorting

T h e retorting or autoclaving which follows the filling and closing of the tuna can consists of heat ing the can and its contents to such a tem­perature that all microbia l life inside the can is destroyed. This is the most cri t ical step in the whole canning operation and has, therefore, been the subject of considerable study. T h e minimum time required to destroy the spores of the most heat-resistant organisms at various temperatures must naturally b e known for any size and type of pack before recom­mendations on retorting temperature and t ime can b e made. T h e funda­mental work by Bige low et al. ( 1 9 2 0 ) on heat penetrat ion in processing canned food, and the establishment b y Bige low ( 1 9 2 1 ) of the logarith­mic nature of the thermal death-t ime curve for microorganisms did much to advance safe canning pract ice . I n the study on the heat resistance of sixteen different botul inum strains, Weisse ( 1 9 2 1 ) found that under best conditions of survival, the most heat-resistant spores of Clostridium botu­linum were destroyed in 5 hr. at 1 0 0 ° C , within 4 0 min. at 1 0 5 ° C , and within 6 min. at 1 2 0 ° C . Es ty and Meyer ( 1 9 2 2 ) also made extensive studies on the heat resistance of Clostridium botulinum. This last work had a profound influence upon the formulation of the cannery code in

232 SVEN LASSEN

California and elsewhere. A very comprehensive study of the bacter i ­ology of the thermal processes for canned marine products has been reported by L a n g ( 1 9 3 5 ) .

On the basis of carefully conducted studies on heat penetrat ion and distribution in packs of a given style and fill, and the establishment of the thermal death t ime for spores of a heat-resistant organism (usually Clostridium botulinum) in a pack, it is possible to calculate the t ime and temperature at which a given can size containing a given style of pack must b e heated to obtain total destruction of microbial life inside the can. I t is on the basis of such measurements that the California Depar t ­ment of Publ ic Health, Bureau of F o o d and Drug Inspection, Cannery Inspection Sect ion issues instructions regarding the temperature and the t ime at which it must b e maintained during retorting (Natl. Canners' Assoc., 1 9 3 1 ) . F o r further details on process calculations of and general bacteriological and chemical examination of canned food, the reader is referred to Cameron and Es ty ( 1 9 2 6 ) and Townsend et al. ( 1 9 5 4 ) .

T h e retorts used in the tuna canning industry are, with a few excep­tions, batch-type retorts, consisting of long, horizontal, cylindrical pres­sure vessels, made of j4 hi-? or thicker steel plate, with ends dished for safety. At one end is a hinged door of the same diameter as the shell. T o secure a safe and airtight closure during retorting, the door is usually secured by a large number of eye-bolts, a t tached to the shell, which may b e swung into the recesses in the per imeter of the door, and secured b y nuts. In some retorts, the so-called spider closure mechanism is used for securing the retort door.

T h e retort is provided with a t rack which allows the cylindrical baskets filled with cans from the canning machines to b e rolled into the retort. In a 4-ft.-diameter retort, the cylindrical baskets on wheels will have a 42-in. diameter so that at least a 3-in. c learance is left be tween the perforated meta l basket and the wall of the retort. W i t h a 32-in. length of basket, a 24-ft.-long retort will b e able to hold eight baskets of cans and still have ample room to spare for the important steam and water circulation. T h e retort has steam, water, and compressed air con­nections, and is provided with air vents, bleeders, a water and steam distribution system, relief valves, pressure gauges, temperature measur­ing and recording instruments, etc.

In order to give the publ ic maximum protection against the danger of inadequate retorting, the California Sta te Depar tment of Publ ic Heal th has, as already mentioned, imposed upon the canners of California very strict specifications, with regard to the t ime and temperature at which canned tuna must b e held in the retort. I t also has issued specifications with regard to the design of the retorts to b e used. These recommended

4 . t u n a c a n n i n g a n d p r e s e r v a t i o n o f r a w m a t e r i a l 233

specifications on retort design pertain mainly to such mechanica l features as the methods of introduction of steam and water into the retort, the number of vents, bleeders , thermometers , etc. , all of which have for their purpose to secure proper and uniform heating, cooling, and venting of the retorts. Detai ls of these specifications may b e found under the head­ing "Retort Equ ipmen t and Operation," Sections 1 2 7 2 5 - 1 2 7 8 5 of Article 8, enti t led "Cannery Inspect ion Regulat ions," Subchapter 2, Chapter 5, Ti t le 17 (Pub l i c Heal th of California Administrative C o d e ) . T h e follow­ing is a general description of the procedure followed during retorting operations in a tuna cannery.

After the retort has b e e n loaded, the retort door is closed and bolted, and a check is made to see that all vents and bleeders are open, and drain and overflow closed. S team is then admit ted gradually through the steam control valve as well as through its by-pass valve, into the perfo­rated steam spreader which runs through the entire length along the inside bo t tom of the retort. W h e n the temperature has come up close to the processing temperature, the by-pass valve is gradually closed and the temperature is brought up to the final processing temperature b y the steam control valve alone. Having finally arrived at the processing tem­perature, the recording thermometer and pressure gauge are checked, and time, temperature, and pressure are entered in the production record. T h e t ime it takes to br ing a retort up to the required processing temperature is the "lag" or "come-up t ime." Dur ing the processing period, the tem­perature must b e kept constant, and frequent temperature checks must b e made to make certain that no fluctuation in temperature occurs. I t is important to see to it that all b leeders are kept open during the entire processing period. W h e n the processing t ime has come to an end, the steam is turned off, and all b leeders are closed, after which compressed air is introduced into the retort, increasing the pressure about 2 lb . per square inch above the processing pressure which existed in the retort during processing. W a t e r is now introduced into the retort through the perforated top inlet pipe which runs inside the retort along its entire length. A constant pressure is maintained in the retort during the intro­duction of the cooling water b y manipulat ing the compressed air valve. As the water level in the retort rises, the pressure in the retort should b e closely watched and kept constant. W h e n the water level is near the top of the retort, the overflow or drain is opened slightly and then opened rapidly the moment the retort is filled with water, to avoid pressure fluctuations which may damage the cans in the retort. Cooling of the cans b y the water is now cont inued for some t ime, during which the pressure in the retort should b e kept up until the cans have cooled down so that a lower pressure in the retort will not subject the can lids to

234 SVEN LASSEN

any excessive strain. T h e final step is now to close all valves, except the drain valve, open all vents and bleeders, thereby releasing all pressure in the retort, after which the door may b e unbolted. T h e water cooling should b e continued so that the retorted cans when removed from the retort do not have a temperature of above 100° to 1 1 0 ° F . T h e baskets containing the retorted cans are now rolled out of the retort and placed in a cooling room where they remain until released b y the State Board of Health, Canning Inspection Service, upon submission of evidence b y the canner that the cans involved have been retorted at the temperature and for the length of t ime prescribed. Sect ion 12470, Article 8, of the California Administrative Code (Pub l i c Hea l th ) Cannery Inspect ion Regulation, details the nature of this evidence as follows: " E a c h l icensed retort oper­ator shall keep a record of the cooks as required b y the State Board of Publ ic Health.

" a ) T h e original and duplicate of the production record must b e kept by filling in accurately in complete detail the form approved by the Depar tment of Publ ic Health. E a c h entry in the record must b e made by the operator at the t ime the specific retort operation is observed and not copied afterwards. I t must b e in legible handwriting and b e signed by the operator or operators.

" b ) Charts of recording thermometer must show full t ime and temperature as required, otherwise the product will b e restrained.

" c ) E a c h production record and recording thermometer chart shall b e stamped, initialed and numbered by a State Cannery Inspector before use and must b e accounted for.

"d) T h e cook or ba tch number and size of cans involved must b e recorded b y the canner in each respective curve of all temperature charts.

" e ) Production records and charts must b e scrutinized and checked b y a State Cannery Inspector before product is released for shipment."

All cans must naturally b e coded. T h e regulations covering coding of tuna cans are contained in Sect ion 12475 of the above mentioned Cannery Inspection Regulation, as follows:

" E a c h plant must submit and have approved a code to appear legibly on the cover of each container. This code will show the plant where packed, year packed, the product contained therein, ba t ch number or day code. I t is understood b y the packer that where a day code is used, the entire day's output shall b e considered as one ba tch in case of question."

After the retorted cans have been released b y the State Cannery Inspector, the cans are brought back into the canning line b y emptying the baskets into a hopper to which is a t tached a so-called "unscrambler,"

4. TUNA CANNING AND PRESERVATION O F R A W M A T E R I A L 2 3 5

an ingenious device which, b y passing the cans over a dome or letting them slide down a tray reciprocat ing at a right angle to the can motion, arranges the cans in such a way that they can b e fed b a c k into the conveyor line. On this line the cans are passed to the label ing machines , after which they are packed in corrugated cardboard cartons and sent to the warehouse.

IX. Standards and Quality Specifications

T u n a has been canned in commercia l quantities in the Uni ted States since 1903. T h e tremendous growth which the tuna industry has under­gone since then has been due to the increasing popularity of canned tuna, since tuna in any other form, such as fresh or frozen tuna, is used only in negligible amounts b y the consuming U. S. public. T h e solid-pack style of canned tuna has dominated the market during most of these years. This pack was, to begin with, made mostly from albacore tuna. T h e disappearance for several years of a lbacore from the tuna-fishing areas along the Pacific Coast of the Uni ted States, forced the tuna canners to look elsewhere for tuna, and to base their rapidly expanding production on yellowfin tuna, bluefin tuna, and other species. Skipjack seems at the moment to b e replacing the yellowfin tuna as the largest contributor of raw material to the tuna-canning industry.

T h e gradual development of several different styles of pack, such as chunk style, flaked, and grated style, made from the many different species of tuna and tuna-like species, created problems among canners as well as among the regulatory authorities with regard to the identity of tuna raw material , fill of container, etc., which, it was thought, could bes t b e solved b y the establishment of tuna standards. T h e U. S. F o o d and Drug Administration, in cooperation with the canners, therefore, began in 1949 to work on standards of identity, definitions and standards of fill of container for the tuna industry. After several years of work, agreement was finally reached on details of the standards, so that the F o o d and Drug Administration could issue an order establishing "a definition and standard of identity, and standard of fill of container for canned tuna." T h e text of the Standards was published on February 13, 1958. T h e s e Standards, which normally will b e effective one year after publication, limit the species which m a y b e packed and designated as tuna to eleven, whose scientific and common names are given in T a b l e I I I .

F o r further description and identification of each species, the follow­ing references may b e consulted: Godsil and Holmberg ( 1 9 5 0 ) , Godsil and Byers ( 1 9 4 4 ) , Godsil ( 1 9 5 4 ) , and Kishinouye ( 1 9 2 3 ) .

This list of species of fish contained in the Standards which may b e

236 SVEN LASSEN

canned as tuna and so designated are generally acceptab le to the canners and to the fishing industry as a whole. Unfortunately, it has as yet b e e n impossible to identify b y chemical or other positive tests a canned sample of any one of the above species from another or, what is perhaps more important, to differentiate be tween a canned sample of any one of the above species and a can of the several other tuna-like species which might find their way into the tuna market.

TABLE I I I TUNA SPECIES ACCEPTABLE UNDER FEDERAL STANDARDS

Thunnus thynnus Bluefin tuna Thunnus maccoyi Southern bluefin tuna Thunnus orientalis Oriental tuna Thunnus thynnus orientalis Albacore Thunnus ohesus Big-eyed tuna Thunnus alhacares Yellovvfin tuna Neothunnus rarus Northern bluefin tuna Euthynnus pelamis Skipjack Euthynnus alletteratus Little tunny Euthynnus Uneatus Black skipjack Euthynnus yaito Kawakana

T h e new T u n a Standards specify concisely what part of the tuna may b e used for canning b y stating: "The optional form of processed tuna consists of loins and other striated muscular tissue of t he fish. T h e loin is the longitudinal quarter of the great lateral muscle freed from skin, scales, visible blood clots, bones, gills, viscera, and from the non-striated part of such muscle, which par t (known anatomically as the median superficial m u s c l e ) , is highly vascular, dark in color, because of retained blood, and granular in form." T h e median superficial muscle referred to in the above, is the so-called dark mea t or b lood meat , which is typical of many fish, and which is imbedded in the muscular structure be tween the dorsal and ventral loin of the tunas.

T h e four different types of pack style referred to under methods of manufacture in the tentat ive T u n a Standards are descr ibed and defined as follows:

" 1 ) Solid or solid pack consists of loins freed from any surface tissue discolored b y diffused hemolyzed blood, cut in transverse segments to which no free fragments are added. I n containers of one-pound or less of net contents, such segments are cut in lengths suitable for packing in one layer. In containers more than one-pound net content, such segments may b e cut in lengths suitable for packing in one or more layers of equal thickness. Segments are p laced in the can with the planes of their traverse cut ends parallel to the ends of the can. A p iece of a segment may b e

4 . TUNA CANNING AND PRESERVATION O F R A W M A T E R I A L 237

added if necessary to fill a container. T h e proportion of free flakes broken from loins in the canning operation shall not exceed 1 8 % .

" 2 ) Chunks or chunk style consists of a mixture of pieces of tuna in which the original muscle structure is retained. T h e pieces may vary in size, bu t not less than 5 0 % of the weight of the pressed content of a container is retained on a half-inch mesh screen.

" 3 ) F l a k e or flakes consist of a mixture of pieces of tuna in which more than 5 0 % of the weight of the pressed content of a container will pass through a half-inch mesh screen, bu t in which the muscle structure of the flesh is retained.

" 4 ) Grated consists of a mixture of particles of tuna that have been reduced to uniform size, that will pass through a half-inch mesh screen, and in which the particles are discrete and do not comprise a paste."

T o descr ibe and define a style of pack so that it incorporates all essential features of the pack, and which does not permit of any am­biguity in interpretation of wording, is difficult. T h e above description of style of pack seems to have overcome this difficulty rather well.

Canned tuna is also subject to color specifications. T h e color designation is obta ined b y comparing, in a simple color comparator using filtered light, the reflectance value of the surface of the tuna meat prepared, under specified conditions, with that of mat te surface neutral reflectance standards of specified Munsel l value.

T h e color designations for canned tuna are white, light, dark, and blended. W h i t e is a designation which may b e used only for a lbacore, provided the reflectance value is more than 6.3 Munsel l units. T h e term "white meat" for tuna has always, in the mind of the purchasing public, been associated with a lbacore, hence this restriction.

Light. This color or shade designation may b e used on canned tuna if its reflectance value is not be low a Munsel l value of 5.3. Most tuna canned in the Uni ted States, and abroad, would pass this requirement. I t sometimes occurs that some lots of canned yellowfin tuna, and even skipjack, will have a reflectance value above 6.3 Munsel l units, which would make it as light as whi te meat . In such instances, the designation will still have to b e light, as the term "white," as indicated, is reserved for a lbacore only.

Dark. This is a color or shade designation which applies to all canned tuna with a Munsel l value be low 5.3. T u n a of this designation often has a so-called "mahogany brown" color and is often identified with large-size tuna fish. T h e U. S. consumer of tuna has a decided preference for l ight or whi te mea t tuna; dark tuna, which is often also of coarse texture, has, therefore, only a l imited market .

Blended. This is a designation with a l imited use in that it may b e

238 SVEN LASSEN

applied only to canned tuna flakes consisting of a mixture of tuna flakes of which not less than 2 0 % b y weight meets the color standard for ei ther white tuna or l ight tuna and the remainder of which fall within the color standard for dark tuna.

T h e packing media which are permit ted under the Standard for canned tuna are as follows: ( 1 ) any edible vegetable oil other than olive oil, or any mixture of such oils not containing olive oil; ( 2 ) olive oil; ( 3 ) water. W h i l e no further specifications in regard to quality of oils are incorporated in the T u n a Standard, they should obviously b e of such quality that they satisfy all state and federal pure food laws. Soybean and cottonseed salad oils are the most popular oils used in tuna canning, and when used should b e tested for identity, color, free fatty acids content, rancidity, moisture content, cloud point, smoke point, e tc .

Wheneve r possible, salad oil to b e used should first b e packed in an experimental pack and tested for flavor reversion. T h e greater care exercised within recent years b y the oil mills in the selection and treat­ment of their cotton seeds and soybeans, and improvements in oil refining techniques, have reduced flavor reversions to a comparatively rare occurrence.

Olive oil, which is used to a l imited extent, particularly for the so-called "Tonno" solid pack, is often imported from the Mediterranean area. Some very good olive oil is also produced in California and is used in the tuna-canning industry. B u t whatever the origin, to b e acceptable , the olive oil must b e cold-pressed oil, must b e of good color, and have a fairly low free fatty acids content. T h e typical olive oil flavor, which is favored b y many, is present to a variable degree in olive oil from different localities. This flavor can b e developed to some extent b y the way the olives are processed. Therefore , much pract ical experience is required in selecting the right olive oil for tuna canning and it cannot usually b e decided upon b y the chemical analytical report alone.

T h e Standards for canned tuna permit the seasoning or flavoring with one or more of the following ingredients: ( 1 ) salt; ( 2 ) purified mono-sodium glutamate; ( 3 ) hydrolyzed protein; ( 4 ) hydrolyzed protein with reduced monosodium glutamate; ( 5 ) spices or spice oils or spice extracts; ( 6 ) vegetable broths; ( 7 ) garlic.

Salt is needed as a flavoring or seasoning ingredient in all tuna packs except in the so-called "dietetic" low sodium pack. F lavor acceptabi l i ty tests have revealed that a salt content of roughly 1.5% is most acceptab le to the consumer. Unfortunately, the original salt content of the tuna mea t that is packed often varies within a wide range. This forces the tuna packer to carry out frequent salt determinations on the cooked tuna meat (o r on the raw tuna ) so that proper adjustment can b e made in

4. TUNA CANNING AND PRESERVATION O F R A W M A T E R I A L 2 3 9

the dosage of the automatic salt addition machine on the canning line. T h e variations in method of t reatment during br ine freezing and thawing which the tuna is subject to while on board the tuna clippers influence its salt content. Under favorable conditions the over-all salt content of the tuna loin mea t may b e be low 1%. T h e salt which is added to each can on the canning line as it passes the automatic salt dispenser should b e fine-grained, free-flowing, dry salt. A canne r s salt produced b y vacuum concentrat ion of br ine is acceptable . I t must b e low in impurities and microorganisms. Among the inorganic impurities, magnesium and iron are particularly object ionable , iron because of a possible discolor­ation that it may cause in the canned tuna meat, and magnesium because of the possibility that it may aid in the formation of magnesium ammonium phosphate, the so-called "struvite," a harmless but neverthe­less most undesirable crystalline formation which sometimes develops in canned tuna, salmon, shrimp, lobster, and crab. T h e reason for the occasional development of "struvite" in canned tuna is still unknown.

T h e addition of purified monosodium glutamate as a seasoning agent is permit ted under the T u n a Standards. Monosodium glutamate has been known as an intensifier of flavors of food. I t is the monosodium salt of the L-form of glutamic acid which has this extraordinary property; the isomeric D-form possesses none of the flavor-intensifying properties ( M a n ­ning and Buchanan , 1 9 4 8 ) . T h e addition of monosodium glutamate to a can of tuna in amounts of 0.15 to 0.25% of the weight of the tuna meat brings out satisfactory flavor intensification. I ts present use in the tuna canning industry is, however, l imited.

Hydrolyzed proteins may b e added to canned tuna. Such hydrolyzates will usually contain considerable amounts of monosodium glutamate ( B l o c k and Boil ing, 1 9 4 5 ) and will, therefore, exhibit properties similar to monosodium glutamate. T h e other components of hydrolyzed protein may add a bouillon-like flavor to the tuna meat , which may appeal to some consumers. T h e hydrolyzed protein with reduced monosodium glutamate content will presumably have the bouillon-flavoring character­istics predominating over that which can b e obtained b y hydrolyzed protein.

Spice or spice oils, or vegetable broth and garlic are all seasoning or flavoring agents whose addition is permit ted under the T u n a Standards. T h e y produce special taste effects b y modifying the original flavor of the tuna mea t and, therefore, differ from the effect produced b y mono­sodium glutamate. Some of these seasoning or flavoring agents have found a favorable recept ion b y the consuming public, particularly among certain e thnic groups.

T h e standards for fill of containers for canned tuna are based upon a

2 4 0 SVEN L A S S E N

determination of the pressed weight of their content. Under specified conditions the pressed weight of a can is determined b y placing the drained content of can in a steel cylinder, inserting a plunger and b y means of a hydraulic press slowly exerting increasing pressure upon the tuna meat . B y increasing the pressure to 384 lb . per square inch of plunger face in contact with the can content, a certain amount of l iquid will b e pressed out. T h e press cake remaining in the cylinder when pressure finally is released is recovered and weighed. This method is simple and fast, and correlates fairly well with the amount of tuna meat which was originally put into the can during canning operations.

TABLE I V CAN SIZE, PRESSED WEIGHT RELATIONSHIP UNDER FEDERAL STANDARDS

Minimum value Minimum value for weights of for weights of pressed cake pressed cake

Can size (average of Can size (average of and form of 24 cans) and form of 24 cans)

tuna ingredient Averages tuna ingredient Averages

211 X 109 401 X 206 Solid 2.25 Solid 8.76 Chunks 1.98 Chunks 7.68 Flakes 1.98 Flakes 7.68 Grated 2.00 Grated 7.76

307 X 113 603 X 408 Solid 4.47 Solid 43.2 Chunks 3.92 Chunks 37.9 Flakes 3.92 Flakes 37.9 Grated 3.96 Grated 38.3

Some criticism of this method has been voiced, one be ing that only a small percentage of the total moisture content of the tuna meat is be ing removed b y pressing and that the moisture removed varies with the physical characterist ics and state of aggregation of the tuna meat , etc. I t has been suggested that a nitrogen determination of can content or a determination of acetone-insoluble solids would b e a simpler and more accurate method of evaluating the can s content of tuna.

T h e s e and other objections of a more or less substantial character have, however, not been able to obscure the fact that press weights, as determined b y the standard procedure, generally speaking, give a rel iable index of the amount of cooked tuna meat which was put into the can, and, therefore, provides the regulatory authorities with means to deter­mine whether provisions wi th regard to fill of container have been complied with. T h e minimum value for weight of pressed cake, as an average of 24 cans sampled, for the various can sizes, is given in T a b l e I V .

4. TUNA CANNING AND PRESERVATION O F R A W M A T E R I A L 241

TABLE V AVERAGE COMPOSITION OF CANNED TUNA

Style of pack Protein (%) Fat (%) Moisture (%)

Ash and carbohydrates

(%)

Medium packed Solid pack 29.9 9.0 58.2 2.9

Medium packed Chunk style 28.0 9.3 60.0 2.7

Medium packed Grated style 27.2 13.3 57.0 2.5

T h e nutritive value of fish is high ( M c L e s t e r , 1 9 4 4 ) . As a good protein food i t is h igh in essential amino acids. T h a t the high nutritive value of tuna is not affected adversely b y the canning process has b e e n demon­strated b y the work of Neilands et al. ( 1 9 4 7 ) and D u n n et al. ( 1 9 4 9 ) . Canned tuna, in common with other fish foods, is low in connect ive tissues and is highly digestible. Canned tuna contains important t race minerals high in iodine and fluorine. I t is a good source of several of the water-soluble vitamins of the B-complex, such as nicotinic acid, pyri-doxine, riboflavin, pantothenic acid, and biotin. I t is also a fairly good source of vi tamin D . An analysis of the average composit ion of some of the various styles of canned tuna is given in T a b l e V .

Wi th in recent years, various canned specialty products made from tuna have been appearing on the market (d ie te t ic tuna, tuna and noodles, e t c . ) . W h i l e the consumer response to these products at the moment seems to b e l imited, i t is probably too early to predic t the future potentialities of these products.

T h e final judgment on the suitability of the pressed weight as a pract ical and accura te index of fill of can will have to b e delayed until the tuna canning industry and the F o o d and D r u g Administration have used it long enough to fully appreciate its merits.

T h e can size designation used in the above and in industry generally is derived from its nominal dimensions. T h e first digit represents whole inches, the next two the extra fraction expressed as sixteenths of an inch. T h e diameter is c i ted first, followed b y the height ( C a n n e d F o o d Reference Manual , 1 9 3 9 ) .

T h e T u n a Standards also regulate the label ing permit ted for the various styles o f pack of canned tuna. Inasmuch as labeling, as such, does not involve any points of technological interest, this aspect of the T u n a Standards will not b e further discussed.

242 SVEN LASSEN

X. Quality Control of Canned Tuna

Quality control is necessary to produce a uniform and accep tab le canned tuna product. T h e quality control used in the tuna canning industry has for many years consisted of "cutting" a certain number of cans from each lot produced, and examining the content b y an orga­noleptic appraisal, and by measurements of drained weights, can vacuum, etc. T h e yearly industry-wide tuna cutting, which was initiated some years ago by the California tuna canners, enabled members of the industry to compare, and score, the quality of their own product with coded samples of those of their competitors. This gave the concept of quality and quality control a strong stimulus. T h e recent introduction of the T u n a Standards has further helped to give quality control the important p lace in the over-all tuna canning operations which it right­fully deserves. T h e result has been that quality and uniformity of product are now being measured on a rational basis. T h e variation in composition of the tuna raw material , due to variation in size of tuna, texture, color, salt content, state of dehydration, etc., makes it a major problem to obtain uniformity in canned pack. I t has, therefore, b e c o m e necessary to employ chemical , physical, and statistical methods of analysis of the canned product in order to exert proper quality control, and to regulate the flow of the tuna meat through the canning equipment so that the content of tuna in any can complies with federal requirements.

T h e cans necessary for the canning of tuna are usually supplied ready-made, with lids, b y the major can companies, in sizes which have b e c o m e standard for the tuna industry. T h e cans are now coated on the inside with a special enamel to preserve the natural characterist ics of the tuna meat . T h e introduction of the enamel-coated can, some ten years ago, has practically el iminated the occasional discoloration of tuna in some cans due to iron sulfide which resulted from contact be tween the steel base of the sometimes inadequately t in-coated can and the sulfur-containing amino acids of the tuna protein.

W h i l e the sanitary tin can, with or without enamel coating, has shown a remarkably wide range of application for all types of canned food products, the successful use in other countries of aluminum cans for fishery and other foods, and the recent lowering of the price of aluminum, may in the future see the aluminum can enter into competi t ion with the tin can in the field of tuna canning and other canned products. A strict quality control of the material from which the cans are made as well as of the cans themselves is maintained b y the can companies, and is, therefore, usually not included in the duties assigned to cannery quality control ( s e e Am. Can Co. Bull. No. 4800).

4. TUNA CANNING AND PRESERVATION O F R A W M A T E R I A L 243

XI. Concluding Remarks

In presenting a general description and discussion of the principles of tuna canning, and the preservation of the tuna raw material whi le in transit from the fishing areas to the shoreside cannery, the author has naturally been guided b y his long experience with the methods and technology as used b y the tuna processors on the Pacific Coast of the Uni ted States. T o describe and discuss these methods in preference to possible modifications thereof pract iced elsewhere, is justified b y the fact that the Pacific Coast tuna processors produce b y far the major part of the canned tuna consumed, not in the Uni ted States alone, bu t in the whole world. Thus , al though it is known that minor deviations from this method of tuna processing are pract iced in other countries and, to some extent, within the U. S. tuna industry, these deviations are, at the moment , not considered to b e of sufficient importance to b e discussed in the l imited space available here, nor do they alter in any essential way the general features of the tuna canning process which has b e e n outlined and presented in this chapter.

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Bigelow, W. D., Bohard, G. S., Richardson, A. C , and Ball, C. O. (1920) . Heat penetration in processing canned foods. Natl. Canners' Assoc., Research Lab., Bull. 16-L.

Block, R. J . , and Boiling, D. (1945) . "The Amino Acid Composition of Proteins and Foods," 396 pp. Charles C Thomas, Springfield, Illinois.

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Jarvis, N. D. (1944) . Principles and methods in the canning of fishery products. U.S. Fish Wildlife Serv., Research Rept. No. 7.

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Kishinouye, K. (1923) . Contribution to the comparative study of the so-called scombroid fishes. (In Japanese.) J. Coll. Agr., Imp. Univ. Tokyo 8 ( 3 ) , 294-475.

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Lang, O. W., Farber, L. Α., and Yerman, F. (1945) . Preservation, spoilage and methods for the detection of spoilage in fish and fishery products. Progr. Rept. George William Hooper Foundation, Univ. Calif. Med. Center, San Francisco, 52 pp.

Lewis, M. S., and Saroff, H. A. (1957) . Binding of ions to the muscle proteins, measurements on the binding of potassium-, and sodium ions to myosin A, myosin Β and actin. /. Am. Chem. Soc. 79, 2112-2117.

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