The Radiation Preservation of Fish

CHAPTER 10

The Radiation Preservation of Fish

B. COLEBY

Low Temperature Research Station, Cambridge, England A N D

J. M. S H E W A N

Torry Research Station, Aberdeen, Scotland

I. Introduction 419 II. Ionizing Radiations 420

A. Radiation Sources 420 III. Potential Uses of Radiation in Food Processing 421

IV. Applications to Fishery Products 423

A. Sterilization 423 B. Pasteurization 425 V. Public Health Considerations 430

VI. Economics of Radiation Processing 431

VII. Present Prospects 432 References 434

I. Introduction

T h e growing interest during r e c e n t years ( B r a s c h and H u b e r , 1947;

Coleby, 1 9 5 8 ; Hannan, 1 9 5 5 ; Proctor and Goldblith, 1 9 5 1 ; Shea, 1 9 5 8 ) in the possible use of ionizing radiations for t h e preservation of foods is partly d u e to the search for ways of utilization t h e radiation that is produced b y t h e operation of a t o m i c energy reactors, b u t also, a n d perhaps primarily, b e c a u s e of t h e attention that any n e w m e t h o d of food preservation will c o m m a n d . T h e proliferation of food spoilage microorganisms is readily prevented b y irradiation, and provided that a large enough quantity of radiation is absorbed, sterile foodstuffs are obtained. T h e temperature c h a n g e during irradiation n e e d not e x c e e d 1 0 ° C , so that the preservation of raw food is feasible. Unfortunately, the elimination of b a c t e r i a is a c c o m p a n i e d b y associated c h e m i c a l changes which, though small in extent, m a y h a v e adverse effects on the quality of m a n y foods. T h e y are usually characterized b y changes in the odor, flavor, texture, and color of t h e food: with some foods t h e changes are extreme, b u t others are affected only slightly. A further limitation to t h e process is that enzymes are usually only partially inactivated with sterilizing doses of radiation, so that supplementary

419

processing m a y b e necessary to prevent enzymic deterioration. Never­

theless, the potential advantages of radiation preservation, particularly the prospect of storing raw foods without refrigeration, are sufficient to warrant considerable efforts in exploration of the field.

II. Ionizing Radiations

T h e r e are several of these ionizing, or high-energy, radiations, having in common the property of producing ionized and excited atoms in the material in w h i c h they are absorbed. Not all are suitable for use in food processing. S o m e are emitted w h e n the nuclei of radioactive atoms disintegrate: α-, β-, and γ-rays; others are produced b y electrical machines: high-speed electrons and X-rays. T h e r e is considerable varia­

tion in their ability to penetrate matter, and this property is most important in determining their utility for irradiating food. T a b l e I

TABLE I

THE USEFUL PENETRATION IN WATER OF SOME HIGH ENERGY RADIATIONS, THE INTENSITY AT ANY POINT BEING NOT LESS THAN 6 0 % OF THE MAXIMUM INTENSITY

Irradiation from Irradiation from Types of radiation one side (cm.) opposing sides (cm.)

ß-Rays (Srso) 0 . 1 0 . 2 5

γ-Rays (Co«>) ^{1 0 4 0}

High-energy electrons

2 m.e.v. ^{0 . 7 1.7}

5 m.e.v. 1.8 4 . 4

1 0 m.e.v. 3 . 7 8 . 7

2 5 m.e.v. 8 2 0

X-Rays ( 1 0 m.e.v.) ^{1 3} 5 0

indicates the useful penetration obtained with some common radiations.

T h e γ-rays and X-rays are suitable for treating really large samples, while high-speed electrons also have useful penetration; ß-rays would b e of use only for thin samples or surface treatments. Consideration of availability and costs further limits the c h o i c e of radiation, and electrons a n d γ-rays are the only two w h i c h have b e e n seriously considered for use in food irradiation. T h e chemical ( H a y b i t t l e et al., 1 9 5 6 ) and micro­

biological ( G o l d b l i t h et al., 1 9 5 3 ) effects of these two radiations are similar, although slight differences in their quantitative effects have b e e n noticed.

A . R A D I A T I O N S O U R C E S

1. Radioactive Isotopes

I n the operation of a nuclear reactor, an appreciable fraction of the energy is released in the form of γ-radiation, some of w h i c h could b e

10. T H E RADIATION P R E S E R V A T I O N O F F I S H 4 2 1 m a d e available for irradiation purposes (Jefferson, 1 9 5 9 ) . Thus, the fuel rods from some types of reactor are intensely radioactive w h e n removed from the pile, and could b e used as a source; b u t b e c a u s e of the rapid decay in their activity it would b e preferable to separate some of the isotopes of long half-life—notably cesium-137—for use as radia­

tion sources. Again, reactors could b e operated specifically for the production of γ-radiation, e.g., b y using neutrons to m a k e radioactive cobalt-60 or indium-116. I n some types of reactors, the γ-radiation would b e available during operation, e.g., using the activity in the sodium of a sodium-cooled reactor, or b y using gaseous fission products from certain other types of reactor. Isotope sources such as cobalt-60 or cesium-137, however, appear at present to b e the most feasible for general use. I t should b e noted that these isotopes will remain in comparatively short supply for several years to c o m e .

2. Electrical Machines

T h e penetrating ability of high-speed electrons is a function of their energy, and the range 4 - 1 0 m.e.v. appears most suitable for food irradiation (Crowley-Milling, 1 9 5 7 ) . T h e p o w e r of the m a c h i n e deter­

mines the quantity of food w h i c h c a n b e irradiated. Several machines are currently manufactured which are suitable—the V a n de Graaff, the resonant transformer, and the linear accelerator. O f these machines, the linear accelerator is the most versatile, b e i n g c a p a b l e of high-power outputs in the required energy range. M a c h i n e s could readily b e constructed to satisfy any demands that might arise for radiation.

3. Shielding

W h a t e v e r the nature of the source, careful shielding is necessary to protect personnel e n g a g e d in operating radiation sources. C o n c r e t e of several feet in thicness is the usual m e t h o d of protection. Provision for m a i n t e n a n c e of the source—if necessary, b y remote control—must b e made. T h e s e precautions, while not difficult to apply, require considerable care in planning and some additional financial investment.

III. Potential Uses of Radiation in Food Processing

T h e unit of radiation is the ' rad," w h i c h corresponds to any energy absorption of 1 0 0 ergs per gram of the material. T h e older units, the

"roentgen" and the "rep" (roentgen-equivalent-physical) are of similar magnitude, and for m a n y purposes m a y b e regarded as synonymous.

Varying quantities of radiation are required to produce different micro­

biological effects, as shown in T a b l e I I . Sterilization, involving in- activation of bacterial spores, particularly those of Clostridium botulinum,

requires a dose of about 4,800,000 rads to give the d e g r e e of inactivation customary for "commercial" sterility ( S h e a , 1 9 5 8 ) . A dose of this magnitude would raise the temperature of the food b y only about 1 0 ° C , b u t the effect on t h e quality is often extreme. B e c a u s e of this limitation, there is considerable interest in the use of lower doses of radiation, which while not giving sterile products, usually result in increased storage life

TABLE I I

APPROXIMATE DOSES OF IONIZING RADIATION REQUIRED TO INACTIVATE SOME BIOLOGICAL ORGANISMS

Organism Dose (rads)

Spore-forming bacteria 2,000,000-5,000,000

Yeasts and molds 1,000,000

Vegetative bacteria 500,000

Reproduction of insects and parasites 50,000

Sprouting tissues of potatoes and root crops 10,000

of the product under chilled conditions. T h e combination of antibiotic treatment with irradiation seems particularly useful in this respect.

S u c h processes, involving doses up to about 1,000,000 rads, are commonly known as "pasteurization" processes, and with some foods c a n give products indistinguishable from the fresh commodity. I t is important to appreciate, however, that pathogenic microorganisms c a n survive these moderate doses of radiation, and foods treated in this w a y should b e stored under conditions w h i c h prevent dangerous proliferation, e.g., at less than 5 ° C , to prevent t h e formation of botulinum toxin ( I n g r a m , 1 9 5 9 ) . I t has also b e e n reported that after irradiation, the surviving microorganisms are m o r e easily inactivated b y heating ( K e m p e et al, 1 9 5 8 ) so that a combination of radiation plus h e a t m a y give sterile products with less d a m a g e than would o c c u r with either process alone.

M o d e r a t e amounts of radiation c a n also b e used for the elimination of harmful pathogenic organisms (e.g., salmonellae) ( B r o o k s et al., 1 9 5 9 ) or parasites (e.g., Trichinella spiralis) ( G o u l d et al, 1 9 5 3 ) . S m a l l doses m a y also find use for disinfestation and prevention o f sprouting in potatoes.

Although there is increasing emphasis on t h e use of small amounts of radiation to minimize quality changes, there are several general methods of reducing adverse effects w h i c h are effective at all dose levels. Preventing access b y oxygen during irradiation and subsequent storage usually has a beneficial effect. I n any case, prolonged storage of m a n y foods in the p r e s e n c e of oxygen would lead to oxidative rancidity, a process w h i c h is usually hastened b y irradiation ( L e a , 1 9 5 9 ) . Irradiation in the frozen condition c a n result in a substantial diminution

10. T H E RADIATION P R E S E R V A T I O N O F F I S H 4 2 3 in the intensity of off-odors and off-flavors p r o d u c e d ( C o l e b y , 1 9 5 9 ) . However, microorganisms are p r o t e c t e d to some extent b y irradiating in a frozen condition, so that a larger dose m a y b e necessary to achieve the same degree of inactivation. T h i s effect is particularly n o t i c e a b l e with treatment designed to eliminate vegetative b a c t e r i a ; b a c t e r i a l spores are not p r o t e c t e d greatly b y freezing ( H a n n a n , 1 9 5 5 ) . Y e t another protective treatment is to add so-called "free radical acceptors" b e f o r e irradiation. E x a m p l e s of such substances are ascorbic acid a n d anti­

oxidants: they h a v e the effect of minimizing adverse quality changes, presumably b y preferential reaction with t h e radical p r o d u c e d b y irradiation.

It will b e s o m e t i m e before enough experience has b e e n gained to know w h i c h c o m b i n a t i o n of treatments will give t h e b e s t products, and present research is mainly c o n c e r n e d with evaluating t h e potentialities of the field.

IV. Applications to Fishery Products

A. S T E R I L I Z A T I O N

F i s h and other sea foods w e r e a m o n g t h e first foodstuffs to b e tested b y irradiation, and although t h e results w e r e promising, they are b y no means as spectacular as was thought at first. Although sterile samples can normally b e p r o d u c e d in t h e laboratory with doses of about 2 χ 1 06

rads, "commercial" sterility, particularly the necessity to ensure a high d e g r e e o f inactivation of Clostridium botulinum, requires a dose o f nearly 5 X 1 06 rads ( H a n n a n , 1 9 5 5 ) . T h i s treatment does not, of course, cook the fish, b e c a u s e , as stated above, t h e t e m p e r a t u r e rise is only about 1 0 ° C . Unfortunately, it does result in such alterations to t h e organoleptic properties of t h e fish as to render t h e m quite u n a c c e p t a b l e to most consumers. As will b e seen from T a b l e I I I , treatment even at 0.2 χ 1 06

rads produces undesirable a n d u n a c c e p t a b l e changes in the flavor and odor of some o f the m o r e c o m m o n c o m m e r c i a l species of fish.

T h e s e irradiation odors a n d flavors, w h i c h h a v e b e e n variously described as "metallic," "burnt-feather-like," and "rubbery," can, of course, b e d e t e c t e d after irradiation at levels m u c h lower than those given in T a b l e I I I .

I n addition to the defects just mentioned, at sterilizing doses, there develop during storage strong b i t t e r flavors, b r o w n a n d other dis- colorations, particularly in the white-fleshed species, a n d a toughening of the texture. T h e s e changes are all a c c e l e r a t e d as t h e storage temperature is raised in, say, the range from 0 to 1 5 ° C ; and, indeed, the w h o l e picture is somewhat reminiscent of that associated with t h e storage of dehydrated

fish ( C u t t i n g et al., 1 9 5 6 ) . If, as is generally believed, irradiated sea foods must b e of exceptionally high quality if they are to c o m p e t e successfully with products preserved b y other methods, then it seems logical to c o n c l u d e that these sterilized products are not likely to b e commercially a c c e p t a b l e .

TABLE I I I

THE MAXIMUM PERMISSIBLE IRRADIATION DOSES THAT CAN B E GIVEN TO VARIOUS FISH AND FISHERY PRODUCTS0

Irradiation dose⁰

Species of fish (Megareps)

Butterfish 0.50-0.70

Butterfish (blanched at 140°F. for 5 min.) 1.5

Cod 0.8 -1.5

Cod (blanched at 140°F. for 5 min.) 1.5

Haddock 0.60

Halibut 1.0 - 2 . 0

Lemon sole 0.5 -1.0

Ling 0.5 -1.0

Mackerel 1.0

Pollock 0.5 -0.70

Pollock (blanched at 140°F. for 5 min.) 0.80

Salmon < 0 . 5

Tuna 1.0

Whiting 0.20-0.25

Whiting (blanched at 140°F. for 5 min.) 0.80

Ocean perch 0.80

Kippered herring 1.0

Kippered sablefish 1.0

Kippered Sturgeon 1.0

Crab meat 1.0

Shrimp 1.0

Pacific oysters (blanched) 5.0

a From various sources.

0 Doses in excess of those given produce undesirable organoleptic changes, e.g., in flavor and odor and in appearance, e.g., bleaching of pigment in salmon.

An interesting use has b e e n made, however, of t h e sterilization technique, viz., to try to identify the specific contributions m a d e b y autolysis and bacterial activity to fish spoilage. T h i s depends on the fact that b a c t e r i a are m o r e easily destroyed b y irradiation than enzymes, and, h e n c e , b y subjecting fish to sterilizing doses it was hoped to leave the tissue enzymes intact. As a result of such work ( P r o c t o r et al., 1 9 5 0 ; Nickerson et al., 1 9 5 0 ) , it was concluded that in haddock autolysis plays little part in spoilage, whereas in m a c k e r e l enzymic action can b e quite considerable. I t must b e r e m e m b e r e d , however, that some enzymes m a y b e appreciably destroyed b y irradiation with even 2 χ 1 0⁶ rads,

10. T H E RADIATION P R E S E R V A T I O N O F F I S H 4 2 5 and irradiation b y itself can cause the exudation of fluids from m e a t products ( S c h w e i g e r t , 1 9 5 9 ) .

Β . P A S T E U R I Z A T I O N

W h i l e sterilization appears, therefore, to b e of little m o r e t h a n a c a d e m i c interest at present, t h e possibility of developing a satisfactory t e c h n i q u e of pasteurization cannot b e so dismissed. M o s t of t h e work now in progress is c o n c e r n e d with screening selected fish and fishery products to determine ( 1 ) those most suitable for this treatment, ( 2 ) what dosages, with and without e n z y m e inactivation, result in organ- oleptically a c c e p t a b l e products, and ( 3 ) t h e shelf life of such products w h e n stored at chill temperatures. I n addition to enzyme inactivation b y blanching, pasteurization is b e i n g c o m b i n e d with treatments such as the addition of antibiotics or c h e m i c a l protectors in attempts to improve the organoleptic properties and t h e storage life of the irradiated products. T h e b u l k of the work on sea foods is b e i n g done in the U n i t e d States b y the F i s h and W i l d l i f e Service, in collaboration with such institutes as the Massachusetts Institute o f T e c h n o l o g y , O r e g o n S t a t e University, and M a r y l a n d State University, or industrial laboratories;

m u c h of the work forms part of the U n i t e d States defense program, b u t sufficient material has already b e e n published to afford an over-all picture o f t h e progress m a d e .

T h e m a i n feature that has e m e r g e d is the remarkable extension of shelf life w h e n t h e irradiated products are held at chill temperatures ( 0 to 5 ° C . ) . F r o m T a b l e I I I , it will b e evident that, of the species already tested, halibut and c o d stand up best, from the organoleptic viewpoint, to irradiation; and, indeed, most of t h e published work on pasteurization has b e e n d o n e on this latter species. At irradiation levels a b o v e those given in T a b l e I I I , the individual species b e c o m e u n a c c e p t a b l e owing to the p r e s e n c e of o b j e c t i o n a b l e odors and flavors. I n fatty fish, such as herring, rancidity of t h e oil is an a d d e d hazard. T h i s c a n b e partially eliminated b y irradiation under vacuum, in an inert gas, or in t h e p r e s e n c e of antioxidants. I n other species, like salmon, the pigment is b l e a c h e d , or in the white-fleshed groups, brownish discolorations m a y appear. S o m e of t h e off-odors m a y also originate in t h e packaging materials used—plastics, viscose coatings, etc.

Intensive efforts are n o w b e i n g m a d e to o v e r c o m e these defects, usually b y the use of one or m o r e of t h e so-called "combination processes"

mentioned above. T h u s , as stated earlier, irradiation in the frozen state often results in a substantial diminution in t h e off-odors and -flavors;

this is well illustrated in T a b l e I V , w h i c h gives the taste panel scores for various species of fish irradiated raw, unfrozen, and frozen at levels

TABLE I V

SENSORY ASSESSMENT OF FISH IRRADIATED WITH CATHODE RAYS

0.5 X 106 0.5 X 106 3.5 X 106 3.5 X 106

0 Rads Rads Rads Rads Rads

Type of fish (control) (unfrozen) (frozen) (unfrozen) (frozen)

T« CO Τ C Τ C Τ C Τ C

Raw odor scores

Haddock 6.9 5.3 6.1 4.7 7.0 4.5 5.0 2.0 5.0 4.0

Cod 8.0 6.7 5.8 3.1 7.7 5.7 5.1 3.0 5.7 3.7

Ling 7.9 5.4 7.5 4.0 6.9 4.8 5.4 2.2 5.6 2.8

Lemon Sole 8.0 5.5 7.0 3.8 6.6 4.7 5.2 2.7 4.9 3.5 Mackerel 7.0 6.5 5.5 3.8 5.5 5.8 5.0 1.8 5.7 4.6 Herring 6.9 5.6 6.3 4.4 6.3 5.0 5.5 2.8 4.6 4.2 Kipper 7.3 5.7 6.3 4.3 6.7 3.3 4.6 2.5 5.0 5.0

Cooked flavor scores

Haddock 7.4 6.2 5.8 4.5 6.2 4.7 2.8 2.0 4.9 3.7

Cod 7.2 6.5 6.8 2.5 7.1 4.0 3.8 3.8 4.3 4.3

Ling 6.4 5.6 4.9 4.8 5.3 5.0 2.5 3.4 3.4 3.6

Lemon Sole 7.3 5.6 5.9 5.4 7.9 4.8 3.9 3.4 4.8 4.2 Mackerel 6.1 6.2 4.5 5.0 5.2 3.1 4.3 2.4 4.1 4.0 Herring 6.6 7.0 4.5 4.2 5.6 4.6 4.1 2.2 4.2 3.8 Kipper 7.1 4.8 5.7 3.8 6.4 5.4 4.6 3.6 4.9 4.8

a Ύ = Torry Research Station, panel scores for 4.5 and under — inedible, 10 = fresh.

0 C = Low Temperature Research Station, Cambridge, panel scores of 4.0 and under = inedible, 9 == highly acceptable.

of 0.5 Χ 1 06 and 3.5 Χ 1 06 rads. I t will b e seen that in nearly every case the frozen fish gave higher organoleptic scores, i.e., w e r e m o r e a c c e p t a b l e than t h e unfrozen fish. B l a n c h i n g ( T a b l e V ) has b e e n reported to allow t h e irradiation dose to b e increased considerably without impairing t h e organoleptic properties of the product ( B e n d e r et al, 1 9 5 8 ) . Again, b y t h e c o m b i n e d use of radiation and b r o a d spectrum antibiotics, such as oxy- and chlor tetracyclines, compounds known to b e active against spoilage b a c t e r i a , a lower radiation dose c a n

TABLE V

T H E E F F E C T OF BLANCHING ON THE IRRADIATION DOSAGE LIMITS«

Blanched ( 1 4 0 ° F .

Raw fish for 5 min.)

Species fillets fish fillets

Cod 0.93 χ 106 r ad s 1.86 X 106 r ad s

Pollock 0.40 X 106 rads 0.93 X 106 rads

Butterfish 0.70 χ 106 r ad s 1.86 χ 106 r ad s

Whiting 0.23 χ 106 r ad s 0.93 X 106 rads

a After Bender et al, 1958.

TABLE VI EXTENSION OF SHELF-LIFE OF FISH AND FISHERY PRODUCTS* Species Irradiation treatment Extension of shelf-life in days over the untreated controls Cod, pollock, butterfish Pollock and butterfish Cod Cod Pacific cod fillets Haddock fillets Pacific cod fillets Cod fillets Crab meat Crab meat

Blanched, vacuum-packed in tins, at 0.5 X 106 reps Raw, vacuum-packed in tins at 0.25 X 106 reps Raw, vacuum-packed in tins at 0.25 X 106 reps As above, at 0.25 Χ 106 reps As above, at 0.50 χ 106 reps As above, at 0.75 Χ 106 reps Raw, in polythene, at 0.20 Χ 106 reps Raw, in cellophane, at 6 χ 105 reps Raw, at 0.25 X 106 reps Raw, at 0.75 X 10« reps Raw, in polythene at 0.25 Χ 106 reps Raw, at 0.50 χ 106 reps Cooked, at 0.40 to 8.5 X 106 reps 20 days stored at 1.5° C. 10 to 20 days stored at 1.5°C. About 70 days stored at 1.5° C. About 40 days stored at 0°C. About 40-50 days stored at 5°C. About 50-100 days stored at 0°C. About 40 days stored at 1.5°C. About 30 days stored at 1 to 4°C. About 40 days stored at 0°C. About 100 days stored at 0°C. About 20 days stored at 0°C. About 50 days stored at 0°C. About 40 days stored at 0°C. a From various sources.

10. THE RADIATION PRESERVATION OF FISH 427

b e used to achieve the same increase in shelf life. T h u s , in experiments with cod ( S h e w a n and Liston, 1 9 5 8 ) a dip for 3 0 min. in 2 0 p.p.m.

Chlortetracycline c o m b i n e d with irradiation at 0.25 χ 1 06 rads resulted in an extension of shelf life at 0 ° C . of about 14 to 1 5 days over t h e untreated controls.

M o r e recently, it has b e e n found that b y a "tempering" t e c h n i q u e , i.e., storage of the irradiated product at certain selected temperatures—

4 5 ° F . or above—for a short time followed b y storage at or n e a r freezing point, the objectionable odors often disappear or are greatly reduced. I t seems possible, therefore, b y the operation of several such "tricks" that a c c e p t a b l e sea foods m a y yet b e produced.

S o m e of the data obtained so far on the extension of the shelf life of various products are given in T a b l e V I . I t will b e noted that with cod fillets irradiated at levels of from 0.25 to 0.5 χ 1 06 reps the limit of edibility can b e extended b y about 2 0 to 1 0 0 days at 0 ° C . S o m e indication of the present uncertainty regarding the irradiation t e c h n i q u e can b e seen from the results with shellfish. Normally, these sea foods are looked upon as b e i n g even m o r e perishable in nature than fish, and several workers have found that products such as raw oysters and raw or peeled shrimps are hardly a c c e p t a b l e after irradiation even at pasteurizing levels. O n the other hand, it has b e e n found that with c r a b and shrimp, unlike salmon, the carotenoid pigments are quite resistant even to an exposure of 1.0 χ 1 06 rads. C r a b m e a t after "tem­

pering" can yield a highly a c c c e p t a b l e product. R a w oysters also failed to yield an a c c e p t a b l e product w h e n pasteurized, yet w h e n "shucked"

are reported to b e a c c e p t a b l e after 4.5 χ 1 06 rads.

As might b e expected, these extensions in shelf life are due in a large measure to the reductions in the b a c t e r i a l loads as a result of irradiation ( T a b l e V I I ) . However, detailed bacteriological analysis of the flora of fish before and after irradiation and during storage at 0 ° C . has shown that pasteurization not only reduces the load but also has a

TABLE V I I

REDUCTION IN THE BACTERIAL LOAD IN FISH MUSCLE IMMEDIATELY AFTER IRRADIATION

(Expressed as log number per gram of muscle)

Species of fish

Nonirradiated controls

After in

irradiation megareps

doses Species of of:

fish

Nonirradiated

controls 0.1 0.2 0.25 0.4 0.5 0.6 1.0

Haddock fillets 5.5 ^{— — —} 2.5 ^— 1.75 —

Cod fillets 6.5 4.0 2.0

Cod fillets 6.3 — — 2.7 — 2.0 — sterile

TABLE VIII QUALITATIVE ANALYSIS OF THE BACTERIAL FLORA OF IRRADIATED FISH DURING STORAGE AT 0°C. Dipped (aureomy- cin) and Dipped fish treated fish (aureomycin), % (0.25 megarads), % 6 days Bacterial flora 1 day 16 23 1 day 13 21 1 day 13 21 1 day 9 16 23 at 0°C. 13 20 from plates after days days after days days after days days after days days days after days days incubated at fillet­at at treat­at at treat­at at dipp­at at at treat­at at 20 and 0°C. ing 0°C. 0°C. ment 0°C. 0°C. ment 0°C. 0°C. ing 0°C. 0°C. 0°C. ment 0°C. 0°C. Total organisms examined 100 200 200 40 100 100 35 100 100 75 100 100 100 50 90 100 Pseudomonas 40 55 72 5 52 100 3 7 99 32 5 7 7 2 89 97 Achromohacter 30 40 13.5 40 47 — 63 93 — 35 16 9 6 2 11 — Flavobacterium 11 1 — 2.5 — — 8 — — 5 11 4 1

—

—

—

— — —

—

— —

—

Normal untreated Treated fish (0.5 Treated fish (0.25 fish, % megarads), % megarads), %

10. THE RADIATION PRESERVATION OF FISH 429

marked selective action on the various groups normally present. I n particular, the pseudomonads, w h i c h are known to b e very active in fish spoilage ( S h e w a n and Liston, 1 9 5 6 ) , are almost completely eliminated.

This is particularly evident in T a b l e V I I I , w h i c h gives some results of experiments with cod ( S h e w a n and Liston, 1 9 5 8 ) . I t will b e noted that after irradiation the p e r c e n t a g e of Pseudomonas spp. fell from about 4 0 % in the untreated controls to about 5 % after irradiation, and that the main group present was Achromobacter. I t will also b e seen ( T a b l e V I I I ) that the antibiotic treatment b y itself reduced the relative proportion of Pseudomonas spp. present; and w h e n this treatment was combined with irradiation, virtually all microorganisms w e r e eliminated except yeasts.

During storage at 0 ° C . ( T a b l e V I I I ) t h e Pseudomonas spp. gradually reasserted themselves, so that b y t h e twenty-first day, they constituted over 9 0 % of the flora in the irradiated fish and over 7 0 % in the controls.

It seems safe to conclude, therefore, that t h e e n h a n c e d storage life of irradiated fish products must b e related to the elimination or suppression of the active spoilage types belonging mainly to the Pseudomonas group.

An interesting feature of t h e cod experiment just quoted, and one already noted b y other workers, is that the usual c h e m i c a l tests for assessing spoilage—the estimation of total volatile bases and trimethyl­

amine—do not show a good correlation with the organoleptic character­

istics; and this emphasizes t h e fact, recorded some time ago b y T a r r ( 1 9 3 8 ) , that the relationship b e t w e e n t h e a b o v e c h e m i c a l indices and the sensory characteristics of fish during normal spoilage in i c e is somewhat fortuitous.

V. Public Health Considerations

So far, t h e irradiation of fish has b e e n considered only in terms of quality changes in the product, b u t it is c o n c e i v a b l e that irradiation could also alter the food in a m a n n e r harmful to t h e consumer. Any n e w method of food processing must satisfy t h e requirements that no toxic materials are produced in t h e food, and that the food is not unduly impaired nutritionally. I n these respects, radiation processing is b e i n g studied most intensively. I n the U n i t e d States, under a cooperative program, the Office of t h e Surgeon G e n e r a l of the Army and t h e F o o d and D r u g Administration are conducting a prolonged series of animal feeding trials (Kraybill, 1 9 5 9 ) . B y t h e early nineteen sixties these should indicate whether a variety of irradiated foods are wholesome. E x p e r i ­ ments in other countries are less advanced, b u t some feeding trials have b e e n m a d e in the U n i t e d Kingdom ( H o m e and Hickman, 1 9 5 9 ) and

10. T H E RADIATION P R E S E R V A T I O N O F F I S H 4 3 1 the Soviet Union, a n d there are indications that other E u r o p e a n countries are b e c o m i n g actively interested in the problem.

Irradiation, like most other methods of processing, leads to some destruction of vitamins, b u t on b a l a n c e , these losses are no greater than those occurring during h e a t processing. S o m e vitamins (e.g., B i and E ) are relatively m o r e sensitive to irradiation than others. I f irradiated foods w e r e t h e sole vitamin source in the diet, then some vitamin supplementation would b e necessary. Gross destruction of nutrients does not appear to b e a serious problem, particularly since most tests designed to study this aspect h a v e b e e n with foods w h i c h have r e c e i v e d large doses of radiation. T h e production o f small quantities of harmful materials is a hazard w h i c h is extremely difficult to refute, b u t long- term feeding tests with a variety of animals over several generations have not given any indications of toxic products to date.

O f the experimental results w h i c h have b e e n published, a few relate specifically to fish items ( M i l l e r et al, 1 9 6 0 , 1 9 6 1 ; Shewan, 1 9 6 1 ) . E v i ­ d e n c e a c c u m u l a t e d with other foods, however, would suggest that radia­

tion processing should offer no hazards to the health of the consumer.

A natural question is w h e t h e r there is any risk of induced radio­

activity in t h e foods after irradiation. W i t h γ-rays from radioactive isotopes, t h e question does not arise, b u t strict monitoring would b e necessary to d e t e c t any accidental contamination w h i c h m i g h t possibly occur through leakage of the isotope from t h e source. T h e use of electrons of very high energies implies a certain hazard, w h i c h has b e e n the subject of theoretical a n d experimental study. B e l o w 10 m.e.v.

there is no problem, b u t since t h e penetration of such rays is limited, energies are technologically desirable. Studies with electrons of 2 5 m.e.v.

energy ( O v a d i a et al, 1 9 5 9 ) indicate that the amount of i n d u c e d radio­

activity ( i n c a r b o n and c h l o r i n e ) is b e l o w permissible levels, a n d would present no hazard to a consumer if the food h a d b e e n stored for a m i n i m u m period of a week.

VI. Economics of Radiation Processing

T h e costs of irradiating foods using various types of sources have b e e n calculated, b u t in the a b s e n c e of actual operating experience, some of t h e estimates must b e regarded as provisional. T h e d a t a in T a b l e I X are from various sources ( B e e l e y , 1 9 5 8 ; Crowley-Milling, 1 9 5 9 ; Murray, 1 9 5 9 ) . W h i l e it is feasible to think in terms of about one cent to irradiate one l b . of food with 1,000,000 rads, it must b e r e m e m b e r e d that these estimates do not include any transport, packaging, or storage charges that would arise. I t will b e seen that estimates b y workers in

t h e U n i t e d K i n g d o m tend to give rather higher figures than U n i t e d States workers. E v e n allowing for the different basis for calculations, t h e r e would seem to b e some genuine difference of opinion. S o m e reductions in t h e costs of linear accelerators m i g h t b e e x p e c t e d with increased experience of construction and operation.

TABLE I X

ESTIMATED COSTS OF RADIATION PROCESSING OF FOODS (EXCLUDING PACKAGING, STORAGE AND TRANSPORT CHARGES)

Estimated cost (cents

Radiation type Source per megarad-lb)a Reference

γ-Rays ^{CS1 3 7} 0.15 Beeley, 1958

Gaseous fission

products 0.14 Beeley, 1958

Co®° 0.2-0.75 Murray, 1959

CS1 3 7 0.7-2.8 Murray, 1959

Electrons 2 Linear accelerators

18 m.e.v., 50 kw. 0.1 Beeley, 1958 1 Linear

accelerator

6 m.e.v., 5 kw. 1 Crowley-Milling, 1959

a The discrepancies may be partially explained by the widely differing assump­

tions made by the various authors in computing these costs. Reference should be made to the original publications.

B e e l e y concludes that for low-dose treatments, isotope sources would b e cheaper, while for high-dose treatments, linear accelerators would b e t h e c h e a p e s t source. I n any case, it seems that if a worth-while radiation process for fish products w e r e developed, the costs of irradiation would p r o b a b l y not b e a limiting factor.

VII. Present Prospects

I t would appear t h a t the general application of radiation sterilization procedures is not possible at present, b e c a u s e of t h e o b j e c t i o n a b l e quality changes p r o d u c e d in most fish products b y high doses of radiation.

L o w e r doses, however, confer an a p p r e c i a b l e extension of storage life of fish held under refrigeration, and these pasteurization procedures m a y have considerable applications.

Assuming that the irradiation of fish will b e a process a c c e p t a b l e to public health authorities, then apart from costs, the most important factor influencing t h e general a c c e p t a n c e of radiation processing will b e t h e quality of t h e product. E x a m i n a t i o n of t h e published literature indicates the n e e d for very careful appraisal of this aspect. T h u s , although t h e data in T a b l e I I I show that various fish are a c c e p t a b l e

10. T H E RADIATION P R E S E R V A T I O N O F F I S H 4 3 3 after m o d e r a t e doses of radiation, there m a y b e a considerable difference b e t w e e n an " a c c e p t a b l e " product and the fresh fish. C o m m e r c i a l establish­

ments do not generally show any inclination to m a r k e t an inferior commodity even if it does h a v e superior storage properties.

E x a m i n a t i o n should also b e m a d e of the criteria used in measuring t h e extension in storage life of fish "pasteurized" with radiation. I n m a n y cases, the reported shelf life refers to the time n e e d e d for the onset of microbial spoilage, b u t there is evidence with m e a t products ( C o l e b y , 1 9 5 9 ) and fish ( S h e w a n and Liston, 1 9 5 8 ) that t h e quality o f radiation-pasteurized commodities m a y deteriorate long before microbial spoilage b e c o m e s a limiting factor. T h u s , the effective gain in shelf life m a y b e smaller than is sometimes reported.

It should also b e r e m e m b e r e d that quality assessments have normally b e e n m a d e using taste panel techniques. Only consumer trials c a n give a realistic appraisal of t h e acceptability of irradiated fish. W h i l e consumer panels often confirm the findings of taste panels, they are sometimes less discriminating ( C o l e b y , 1 9 5 9 ) . T h e difficulties confronting taste panels are illustrated b y some of the discrepancies in T a b l e I V , w h i c h arose from the facts that the panel at the T o r r y R e s e a r c h Station was a c c u s t o m e d to tasting fish b u t unused to irradiation flavors, whereas the panel at the L o w T e m p e r a t u r e R e s e a r c h Station was inexperienced with fish, b u t familiar with assessing irradiation flavors.

Notwithstanding these reservations, the use o f radiation pasteuriza­

tion processes could have a considerable i m p a c t on fishing industries (Anonymous, 1 9 6 0 ) . I t is scarcely y e t feasible to visualize the installation of radiation sources in fishing vessels, b u t inland distribution could b e greatly facilitated. T h e extension in shelf life of pasteurized sea foods has already aroused a considerable amount of interest in t h e United States. Provided that care is taken to minimize some of the m o r e dis­

a g r e e a b l e odors a n d flavors p r o d u c e d either in the fish itself or in the packaging material, then the fishing industry will have at its c o m m a n d a most valuable n e w m e t h o d for the short-term preservation of its products. I n particular, it might help to solve some of the difficulties now b e i n g encountered b y t h e trade in t h e marketing of p r e p a c k a g e d unfrozen fish, e.g., for self-service stores. I f an increase in shelf life of only a few days is required, then a very low radiation dose could b e used, and quality deterioration m i g h t b e very slight during these short periods. S u c h processes could revolutionize the present methods of marketing fish to inland populations far from the ports in countries like the U n i t e d States and C a n a d a , b y making available to them unfrozen fish and other fishery products of a quality e q u a l to that now b e i n g enjoyed b y the consumer at the coast.

R E F E R E N C E S

Anonymous. (1960). Marketing feasibility study of radiation processed fishery products. U. S. Fish Wildlife Serv.

Beeley, R. J. (1958). Radiation processing of foods. Nuclear Sei. and Eng. 3, 660-693.

Bender, M., Miyauchi, D. T., and Carver, J. H. (1958). Progress report on the radiation pasteurization and sterilization of sea food. Quick Frozen Foods 20, 150-151.

Brasch, Α., and Huber, W. (1947). Ultrashort application time of penetrating electrons. A tool for sterilization and preservation of food in the raw state.

Science 105, 112.

Brooks, J., Hannan, R. S., and Hobbs, B. C. (1959). Irradiation of egg and egg products. Intern. J. Appl. Radiation Isotopes 6, 149-152.

Coleby, B. (1958). Processing of foods with ionising radiations. Nature 181, 877-879.

Coleby, B. (1959). The effects of irradiation on the quality of meat and poultry.

Intern. J. Appl. Radiation Isotopes 6, 115-121.

Crowley-Milling, M. C. (1957). Electrical machines as sources of radiation.

Symposium on Food Irradiation, Cambridge, Engl. 1957.

Crowley-Milling, M. C. (1959). The economics of machine sources of radiation.

Intern. J. Appl. Radiation Isotopes 6, 207-210.

Cutting, C. L., Reay, G. Α., and Shewan, J. M. (1956). Dehydration of fish.

Gt. Brit., Dept. Sei. Ind. Research, Food Invest., Spec. Rept. No. 62.

Goldblith, S. Α., Proctor, Β. E., Davidson, S., Kan. B., Bates, C. J., Oberle, Ε. M., Karel, M., and Lang, D. A. (1953). Relative bactericidal efficiencies of three types of high energy ionizing radiations. Food Research 18, 657-677.

Gould, S. E., van Dyke, J. G., and Gomberg, H. J. (1953). Effects of X-rays on Trichina larvae. Am. J. Pathol. 29, 323-337.

Hannan, R. S. (1955). Scientific and technological problems involved in using ionizing radiations for the preservation of food. Gt. Brit., Dept. Sei. Ind. Research, Food Invest., Spec. Rept. No. 61.

Haybittle, J. L., Saunders, R. D., and Swallow, A. J. (1956). X-and γ-irradiation of ferrous sulphate in dilute aqueous solutions. /. Chem. Phys. 25, 1213-1217.

Home, T., and Hickman, J. R. (1959). Some notes on the wholesomeness of irradiated potatoes fed to pigs. Intern. J. Appl. Radiation Isotopes 6, 255-257.

Ingram, M. (1959). Combination processes. Intern. J. Appl. Radiation Isotopes 6, 105-109.

Jefferson, S. (1959). Nuclear sources of radiation energy. Intern. J. Appl. Radia­

tion Isotopes 6, 41-42.

Kempe, L. L., Graikoski, J. T., and Bonventre, P. F. (1958). Combined irradiation- heat processing of foods. II. Raw ground beef inoculated with spores of Clostridium botulinum. Appl. Microbiol. 6, 261-263.

Kraybill, H. F. (1959). Report on safety in the operation of radiation sources and use of irradiated foods. Intern. J. Appl. Radiation Isotopes 6, 233-254.

Lea, C. H. (1959). Chemical changes in stored foods when microbial spoilage is not limiting. Intern. J. Appl. Radiation Isotopes 6, 86-94.

Miller, S. Α., Licciardello, J . J . , Nickerson, J. T. R., and Goldblith, S. A. (1960).

A literature survey on the effects of ionizing radiations on sea foods with respect to wholesomeness aspects. Mass. Inst. Technol., Rept. No. 9656, 38 pp.

10. THE RADIATION PRESERVATION OF FISH 4 3 5 Miller, S. Α., Nickerson, J. T. R., and Goldblith, S. A. (1961). A literature survey

on the effects of ionizing radiations on sea foods with respect to wholesomeness aspects (continuation). Mass. Inst. Technol., Rept. No. NYO, 9933, 11 pp.

Murray, G. S. (1959). Economics of gamma radiation processing. Intern. J. Appl.

Radiation Isotopes 6, 211-215.

Nickerson, J. T. R., Goldblith, S. Α., and Procter, Β. E. (1950). A comparison of chemical changes in mackerel tissues treated by ionizing radiations. Food Technol. 4, 84-88.

Ovadia, J., Heinmetz, F., and Berschman, A. (1958). The problem of induced radioactivity in the use of high energy electrons (25 Mev) for sterilization and chemical processing. Proc. Intern. Conf. Peaceful Uses Atomic Energy 2nd Conf., Geneva, 1958 27, 418.

Proctor, Β. E., and Goldblith, S. A. (1951). Food processing with ionising radia­

tions. Food Technol. 5, 376-380.

Proctor, Β. E., Nickerson, J. T. R., and Goldblith, S. A. (1950). Storage of haddock. Refrig. Eng. 58, 375-379.

Schweigert, Β. S. (1959). The effects of radiation on proteins. Intern. /. Appl.

Radiation Isotopes 6, 76-77.

Shea, K. G. (1958). Food preservation by radiation as of 1958. A report to management. Food Technol. 12 ( 8 ) , 6-16.

Shewan, J. M. (1961). The influence of irradiation preservation on the nutritive value of fish and fishery products. In "Fish in Nutrition" ( E . Heen and R.

Kreuzer, eds.), pp. 207-219. Fishing News (Books), London, 1962.

Shewan, J. M., and Liston, J. (1956). Objective and subjective assessments of fish quality. Bull. inst. intern, froid, Annexe I, 137-146.

Shewan, J. M., and Liston, J . (1958). Experiments on the irradiation of fish with 4 Mev cathode rays and cobalt-60 gamma rays. Proc. Intern. Conf. Peaceful Uses Atomic Energy 2nd Conf., Geneva, 1958 27, 377.

Tarr, H. L. A. (1938). Trimethylamine formation in relation to the viable bacterial population of spoiling fish muscles. Nature 142, 107.