fish.

Various abnormal textures are sometimes observed such as stringy, rubbery, jelly like, or tough. While these characteristics may be due to improper methods of cooking, they also may be due to improper handling.

ORGANOLEPTIC TESTS

Organoleptic tests or determination of quality through use of the senses of sight, touch, odor, and taste are the means most widely used to determine freshness of fish. Organoleptic tests can be divided into two categories; namely, the fundamental, and the accessory tests. The fundamental tests are those which measure factors directly responsible for the quality of the fish such as odor, flavor, appearance, and texture.

The accessory tests involve observations of characteristics which normally occur simultaneously with changes in quality but which are not directly involved. Such characteristics include the appearance of the eyes, gills, and viscera; and the amount and appearance of the slime, etc. These characteristics are not entirely reliable indices, but are convenient to determine. Thus, when purchasing fish in the round, the condition of the eyes and gills may be noted in order to predict the probable quality which cannot be determined more definitely until the fish has been dressed, or cooked and tasted.

Chemical and bacteriological tests have also been promulgated which can be used when time permits. These tests are almost invariably of the accessory type, since they usually measure substances which form simultaneously with fish spoilage but which are at most only partially responsible for spoilage. Such tests have the advantage of determining the condition independent of the personal judgment of the observer, and which can be reproduced with considerable accuracy. In technological investigations they are often invaluable. Their limitations are that they must always be carefully standardized against the fundamental organoleptic tests, and usually considerable time is needed before the results are obtained.

Whole Fish--Observations which can be made on whole fish are unfortunately limited to the accessory tests. Even tests for odor must be so classified since it is impossible to distinguish the odor of the flesh from that of the gills, slime, viscera, or other extraneous sources.

One of the most important observations which can be made upon whole fish is the presence and degree of rigor mortis. A fish is soft and flabby just after being removed from the water. Indentations made with the finger tips on the surface tend to remain. Soon after catching, the muscle proteins coagulate, and this causes a shortening of the muscle fibers and the development of the characteristic stiffness of rigor mortis. In extreme cases, the fish is stiffened into an arc and can hardly be straightened.

Fish which are in rigor mortis are almost certainly of excellent quality. Usually some decomposition is present in fish not in rigor mortis and other suitable tests will show the extent.

Anderson (1907)* lists conditions under which rigor mortis is late in appearing and lasts longer; consequently, the fish keep better:

1. Fish in season.

2. Fish in healthy and vigorous condition.

3. Fish which are killed at once on capture.

4. Fish which are not only killed but pitted at the same time; that is, have the brain and spinal cord destroyed.

5. Fish gutted immediately upon capture.

6. Fish handled as little as possible after capture.

7. Fish kept at low temperature as when iced or kept in cold storage.

All of these conditions not only influence the length of rigor mortis, but are also factors in obtaining fish of high quality. The presence of full rigor mortis is not only an indication that the fish are absolutely fresh, but also that good handling methods have been used. Furthermore, rigor mortis is accompanied by an increase in the acidity of the muscular tissue and a setting of the flesh to a gel-like consistency, both factors which discourage bacterial invasion.

The odor of whole fish is another important index, but one which must be interpreted with care in order to avoid erroneous conclusions. Fresh fish usually have a characteristic

* Anderson, A. G., 1907. On the decomposition of fish. Fish. Res. Bd. Scotland Rept. 26, pt. 3: 13-39.

fishy odor, somewhat similar to seaweed, and a fish which has just been taken from the water ordinarily has no other odor. Fish which are not eviscerated frequently give off odors suggestive of decomposition long before any spoilage of the flesh has taken place. This is usually caused by a rapid decomposition of the substances upon which the fish have been feeding. A pronounced hydrogen sulfide odor from this cause is sometimes present in fish which are less than six hours out of the water.

The gills and slime of fish usually give off more pronounced decomposition odors than the flesh, and this fact is useful in predicting incipient decomposition. A fish in which the gills and slime have a perfectly fresh odor is apt to be in good condition while if the odor is stale, the fish itself may still be in good condition but will not remain so for long.

The degree of odor of fish is markedly effected by temperature. Thus, fish which are near the temperature of melting ice may have an imperceptible odor which might be increased to a relatively stale one, if the temperature of the fish were raised to ordinary room temperature.

The odor of fish kept in finely-packed ice ordinarily progresses from fishy through one of sweetness, to staleness, and finally becomes putrid. The latter is caused by such substances as hydrogen sulfide, indole, etc.

The eyes of fresh fish have a bright transparent appearance which becomes cloudy, and often assumes a pink color when fish are stale. When fish are fresh, the eyes often protrude, and with increasing staleness, they tend to sink. Gills of fresh fish are commonly bright red, and the color gradually fades to a light pink, then to a yellow, and finally, to a brown or sometimes a green shade. These changes in the eyes and gills do not always occur simultaneously with the spoilage of the flesh so they are not entirely reliable and the absence of such changes is not proof of the freshness of the fish.

Immediately after a fish is caught, it often exhibits a striking pearlescent appearance, if held so that the light strikes the fish at the correct angle. This "rainbow" effect somewhat resembles the appearance of pearls or some sea shells. The pearlescent appearance wears away relatively rapidly, so that any fish showing this characteristic is almost certainly in excellent condition. The absence of the pearlescent colors is no evidence of decomposition, since with some species, the beforementioned appearance is less striking, or it may be completely lacking.

Each species of fish has certain characteristic markings and colors, As the fish becomes stale, these patterns and colors fade and become less pronounced. Also, during the spawning season, the appearance may become somewhat abnormal with unusual colors often being observed.

The slime on fresh fish is transparent, almost water white, and usually not enough is present to be conspicuous except by the slimy feeling. As spoilage progresses, the slime increases in amount, becomes turbid and finally thick and often of a yellow color. Presence of such slime is good evidence that the fish is not fresh but its absence is meaningless, since it can easily be removed.

Dressed Fish--During the dressing operation, a number of observations can be made which are not possible with round fish. If the fish has not been eviscerated, the condition of the viscera may show some evidence of decomposition. Each organ, such as the heart or liver, is intact and easily recognizable. When extensive decomposition has occurred, the viscera may be an almost homogeneous mass in which one organ cannot be distinguished from another.

If the odor of the flesh is observed when the fish is first cut, and then again a short time later, the first odor is sometimes much stronger than the second. The first odor may represent an accumulation of gases, either from the flesh or from the contents of the viscera. The skin of fish is almost impervious to gases, so that a very small evolution in the body cavity over a period of time may produce an odor which will lead to a wrong conclusion unless the odor is again noted a few minutes later.

If

The flesh between the viscera and the backbone of fresh fish is firm and sound. the fish is handled in the round, the enzymes from the digestive system start to decompose

the flesh in this region. Shortly after death, the flesh adjacent to the bone begins to soften and eventually may become almost entirely broken down so that a small amount of handling, such as occurs during filleting, causes the bone to separate from the flesh completely. These changes are sometimes accompanied by the formation of a peculiar yellowish gloss on the flesh near the backbone. This gloss sometimes described as an "apple jelly appearance" is suggestive of extensive decomposition.

Frequently, fish which have been held for some time show a reddening of the flesh near the backbone, and in the more advanced stages of decomposition, the color changes to a brown, as the hemoglobin is changed to me themoglobin.

The texture of fresh fish is firm and very little liquid can be expressed even when very heavy pressure is applied. As decomposition proceeds, the texture softens and sometimes fluid is spontaneously liberated. At the same time, the appearance of the flesh changes from a transluscent sheen to a dull luster. The color may also fade if the original shade is other than white, and, in the latter event, it may become somewhat yellow.

Cooked Fish--Since odors are more intense at high temperatures, it is not surprising that more intense odors are usually observed during the cooking of fish. Cooked fish, if fresh, has a pleasing flavor, characteristic for each species, and no aftertaste after eating. Fish of lower quality may have a variety of off-flavors. In some cases, the fish may lack some of the characteristic flavors without evidence of decomposition, and in this case, it is described as flat or tasteless. When only a small amount of decomposition is present, no difference in flavor may be noted, except possibly that there is a strong fishy or a slight "off" flavor. When more extensive decomposition has taken place, the "off" flavor may be described as acrid, bitter, stale, or even putrid. With oily fish, rancidity may lead to a persisting aftertaste, which, while perhaps not entirely disagreeable, is at least annoying.

Abnormal textures are also frequently noted in cooked fish. may be very soft, crumbling into small pieces when cut with a fork. may be tough, rubbery, or stringy.

In some cases, the fish
In other instances, it

Methods of Making Organoleptic Tests--In commercial marketing of fish, producers and wholesalers gain considerable experience over a period of time in judging the quality of fish by organoleptic means. The homemaker who only occasionally buys fish may have difficulty in accurately appraising quality, but by making the organoleptic tests outlined in the preceding sections anyone should soon be able to distinguish between the various stages of freshness with little or no difficulty.

In some types of examination as in the case of food inspection or in laboratory testing in connection with development of new products, or new methods of handling fish, it is sometimes necessary to be able to distinguish minute differences in quality. In such cases, it is often helpful to utilize observations of a large number of persons, and when this is done it is important that each observer make his examination independently and without knowledge of the findings of others. Some persons are unable to distinguish fine differences in odors or flavors, and if they attempt to judge the condition of fish, a rather erroneous appraisal is obtained. Under these circumstances, observers should be selected who have demonstrated their ability to obtain consistent results. Then, if an average is made of the observations of several persons, a very accurate estimate of the condition of the fish is obtained.

Estimation of Keeping Quality by Organoleptic Tests--Appraisal of keeping quality of fish by organoleptic tests is much more difficult than the mere estimation of the quality at the time of examination. Future keeping quality of fish depends not only upon its present condition, but also upon past and future handling conditions. Although some idea of the handling conditions prior to examination may be obtained through careful interpretation of the organoleptic data, usually no certain prediction as to future storage conditions of the fish can be made except in the case of the ultimate consumer who will have possession of the fish continuously until it is eaten.

Fish which are already partially stale will certainly not keep in edible condition for a much longer time. It is ordinarily the fish which still seem to be in relatively good condition about which knowledge of future keeping quality is desired. Such fish may

have been freshly caught, or they may have been kept well-iced for a considerable time. In the former case, assuming good future handling conditions will be used, the fish would be expected to keep for some time, while the keeping quality of the latter fish would be strictly limited. Accordingly, in predicting future keeping quality any indications that the fish had been stored for a considerable period are especially to be noted. For example, fish which have been iced for long periods often exhibit faded surface colors due to leaching. Another indication is a peculiar sweet odor which develops in fish held iced for extensive periods. This sweet odor is not considered especially undesirable, but since it is usually not present in strictly fresh fish it may be taken as a warning of imminent spoilage.

Any indication that the fish have been roughly handled, such as loose scales, fork holes, bruises, or general softness of the fish, should be considered evidence of poor past handling conditions and probably poor future keeping quality. Fish that have been bruised are especially susceptible to autolytic changes which may lead eventually to sudden bacterial putrefaction. Finally, fish taken near the end of the spawning season keep less well than those taken at other times.

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DRY ICE REFRIGERATION OF FRESH FISH FILLETS

By E. F. Kapalka and R. H. Flowers*

to use solid carbon Owing to the diffi

During the past several years, various attempts have been made dioxide (dry ice) as a refrigerant in packing fresh fish for shipment. culty of controlling the temperature and preventing freezing, the method has not been generally used for unfrozen fish. However, recent restrictions placed on the use of tinplate in the manufacture of fillet boxes for shipping fish fillets packed in ice has created a need for further study of this method of refrigeration.

In the course of investigations by the Fish and Wildlife Service on non-metallic containers for packaging fish, experiments were conducted to test fibre fillet boxes refrigerated with dry ice. The fillet boxes were packed in corrugated fibre containers and were insulated from the dry ice by recently invented pads designed for this purpose. The studies were made at the Service's Technological Laboratory, College Park, Maryland, in cooperation with the Merkle Corporation and the Container Corporation of America, both of Philadelphia, Pennsylvania. The first mentioned firm furnished the Merkle insulating pads and the second furnished the fibre fillet and corrugated boxes.

Description of Package--A shipping unit holding two 20-pound packages of fillets was chosen as being the most satisfactory from the standpoint of trade practice and ease in handling. Initial experiments were made to determine the proper sizes of the shipping container and the fillet boxes, the most effective distribution of the dry ice, and the proper amount of insulation to prevent the fillets from freezing.

The fillet boxes were of the telescopic type, and constructed of lightly-waxed fibreboard 1/16 inch in thickness. The inside dimensions of these boxes were 214 by 10 by 2 inches. A typical 20-pound tin fillet box measures approximately 15 by 10% by 4 inches. The longer and more shallow fibre fillet box was selected in order to obtain more rapid cooling of the fillets and more uniform temperatures. The size was such that 20 pounds of fresh haddock or cod fillets completely filled the box.

A corrugated container or "ice box" was used for each shipping unit to hold the dry ice and to prevent the movement of the contents within the shipping container. The inside measurements of the "ice box" were 21-9/16 by 10-5/8 by 4 inches, and the board was 3/16 inch thick. The "ice box" was prepared in advance, so that it could be placed as a unit between the two fillet boxes in the shipping container. Two slabs of dry ice, wrapped in ordinary single thickness wrapping paper, or in a few thicknesses of newspaper, were placed inside the "ice box" horizontally sandwiched between two Merkle insulating pads. By placing the dry ice between the packages of fish fillets a more symmetrical arrangement was possible. Satisfactory refrigeration would then be maintained if the packages were accidentally inverted,

* Technologists, Fishery Technological Laboratory, College Park, Md.

In the early tests, a flexible 14-ply Merkle insulation pad was used below the dry ice, and a 6-ply flexible pad over it when the "ice box" was prepared. Reasonably good control of the refrigerating action of the dry ice was secured. In later experiments, the insulation under the dry ice was changed to a rigid 6-ply Merkle pad, 14 inches thick, and that over the dry ice to a flexible 2-ply Merkle pad. This insulation combination was used in the "ice box" for all subsequent tests.

The recommended shipping container was constructed of A and B fluted, corrugated paperboard, 5/16 inch in thickness. The inside dimensions, 22-3/8 by 11 by 12 inches, were such that there was a small space between the walls and the fillet boxes, which was filled by carbon dioxide gas from the dry ice, which in turn acted as insulation. The container was so cut that the top and bottom side flaps fitted snugly end to end, giving the maximum structural strength and best conditions for thorough sealing.

Method of Packing--In packing the shipping container, a 4-ply Merkle pad, 25-3/8 by 11-3/8 inches, was placed in the bottom. The length of the pad was sufficient to give a l inch overlap turned up at each end, making it possible to reduce thermal leakage by covering the seams and cracks in the bottom of the shipping container with an insulating material.

A fillet box holding 20 pounds of fresh fish fillets, precooled to 38° to 40° F. for moderate outside temperatures or lower for very warm outside temperatures, was set on the insulating pad which overlapped each end of the box. The "ice box", which was previously assembled with a suitable amount of dry ice, was placed above the fillet box, followed by the second fillet box. A 4-ply pad similar to that used for the bottom of the container was placed over the top fillet box, and the overlapping ends were bent down around the ends of the box. The shipping container was then thoroughly sealed with heavy three-inch gummed tape.

Experimental Results--A package containing 40 pounds of fresh cod fillets was held in a room in which the temperature was maintained at approximately 70°F. continuously during the test. The temperature was obtained on a recording resistance thermometer by means of thermocouples placed in various parts of the package. The package was not disturbed at any time during the test.