Low Temperature Preservation of Seafoods: A Review

LOUIS J. RONSIVALLI and DANIEL W. BAKER II

IntroductionBecause of their ease of digestibility

and the nature of the microbial andsystemic enzymes that cause theirspoilage, seafoods are among themost perishable of foods (Bramsnaes,1957). The seafood spoilage ratedepends on the speed with which thechemical reactions that cause theirspoilage proceed. As is generally truefor chemical reactions, the speed withwhich fish spoilage proceeds dependson the temperature of the system. Therelationship between the rate of fishspoilage and temperatures has beenwidely observed and reported, and

ABSTRACT-Cooling seafoods isamong the most effective methods forpreserving their quality. From a choiceof refrigerants, we can cool them to justabove the point of freezing (chilling);when freezing is undesirable, we cancool seafoods to a state in which they arepartially frozen (superchilling) which ex­tends the shelf life by 100 percent or lessbut still does notfreeze them in the usualsense; or we can freeze them solid (freez­ing) and extend their shelf life formonths and even years, when the tem­perature is low enough. This paperdescribes the common refrigerants in­cluding ice, brine, ammonia, fluorocar­bons, cryogenic gases and liquids, chill­ed seawater, and refrigerated seawater.Conventional processes and equipmentfor freezing seafoods, including gas andliquid mechanical refrigeration systems,are described, as are less conventional ortheoretical systems, including dehy­drocooling and high-altitude freezing.

April 1981, 43(4)

shelf life prediction devices have beenconstructed from some of thesestudies (Spencer and Baines, 1964;James and Olley, 1971; Charm et al.,1972; and Ronsivalli et al., 1973).

Although we are addressing the di­rect relationship between the spoilageof fish and the reaction rates of theenzymes, we do not want to ignorethe fact that these enzymes are pro­duced from bacteria and that themetabolic and numerical growth ratesof bacteria are also enhanced by anincrease in temperature (withinlimits). For example, while thegeneration time for Pseudomonasjragi, a common fish spoilagebacterium is about 12 hours at 32 of(O°C), it is only about 2 hours at 55 of(12.9°C) (Duncan and Nickerson,1961). Obviously, the rate of produc­tion of bacterial enzymes is influencedby the metabolic rates and number ofbacteria. While the roles of bacterialenzymes and bacterial numbers in fishspoilage are important and relevant tothe subject of this paper, no furtherdiscussion will be made of these here.Instead, the reader is referred to Ron­sivalli and Charm (1975), where amore detailed discussion already ex­ists and additional references aregiven.

Louis J. Ronsivalli is Laboratory Director andDaniel W. Baker II is a mechanical engineer­ing technician at the Gloucester La boratory ,Northeast Fisheries Center, National MarineFisheries Service, NOAA, Emerson Avenue,Gloucester, MA 01930.

There is little doubt that the storagetemperature is the most importantvariable influencing the spoilage ofseafoods; and to optimize the preser­vative effect of lowering their tem­perature, it is helpful to under­stand the principles at work as well asthe relationship between the temper­ature of a given seafood and its shelflife. Much work has been done tocombine low temperature with othertreatments such as the use ofbacteriostats and bactericides (Wind­sor and Thomas, 1974), and somework has been done in vacuum (Lic­ciardello et al., 1967) and modified­gas packaging (Veranth and Robe,1979), but this discussion is limitedonly to the control of temperature.

For prolonging the quality of fishby simply controlling the temper­ature, the best results are obtainedwhen the temperature control is ap­plied immediately after the fish arecaught. This is because the quality offish is highest at point of catch and itundergoes irreversible quality losseswith time at rates that are directlyrelated to temperature.

Three categories of temperaturecontrol can be applied: Chilling,superchilling, and freezing. Thesecontrol quality in somewhat differentways. The first category of temper­ature control is chilling. It is in the wetrange of temperatures (where nofreezing of the fish is desired). Thechief control imposed by the temper­ature in this case is that it slows downthe rates of the reproduction, growth,and metabolism of spoilage bacteria

1

Icing fish.

and the reaction rates of the bacterialenzymes. It slows the rate of enzymicspoilage simply because, as we statedearlier, spoilage involves chemicalreactions whose rates are proportionalto the temperature.

Superchilling, the second category,is in the narrow temperature rangefrom 26.6° to 30.2 OF (from -3.0° to-l.0°C). In this range there is somefreezing of the water in the fishtissues, and this is noticeable when anattempt to bend the fish is made. Ifthe temperature is lower than thisrange, the fish will be frozen solid. Ifthe temperature is above this range,there will be no freezing at all. Theprinciple by which superchillingworks is simply that the loweredtemperature further slows the mi­crobial metabolism and spoilage reac­tion rates. The formation of some icealso creates pockets of immobility in­sofar as bacterial activity is concern­ed.

In the third category of tem­perature control, freezing (O°F or-17.8°C, or below), the product isfrozen solid since most of its water istransformed to ice, and bacteria arecompletely immobilized. This isprecisely the principle by which freez­ing protects fish quality. However,although freezing provides protectionfrom microbial spoilage, other spoil­age vectors such as oxygen and en­zyme activity have to be controlled.

Chilling

Like other perishable foods,seafoods retain their initial quality forlong periods when they are properlypackaged and held properly frozen.However, there is a relatively high de­mand for fresh l (unfrozen) seafoodswhich command a significantly higherprice at retail than frozen seafoods.The reason for the difference in pricebetween fresh and frozen seafoods isattributed to the widely accepted be­lief that the quality of fresh seafoodsis superior and much higher than the

'In this context. fresh signifies never havingbeen frozen.

quality of frozen seafoods. Whetheror not this notion is accurate, it doesexist. Because of the high value offresh seafoods and because of theirrelatively high rate of perishability,there is an economic reason to max­imize their shelf life and to minimizethe chance that their quality willdeteriorate to the point that they losetheir commercial value.

One of the best methods known forpreserving the quality of fresh sea­foods is to surround them with flakedor crushed ice, because this provides aquick way to bring their temperatureto just above freezing.

Use of Ice

The preservation of the quality ofseafoods by chilling them with ice waspracticed as early as 1838 aboard NewEngland trawlers. The principle em­ployed was not different from that ofthe old domestic ice box: The ice washeld in one compartment, the foodwas held in another, and both com­partments were enclosed in a cabinetwhich served to keep the system

separated from the environment. Infishing vessels, ice was kept in onepen, and fish were put in the otherpens. This practice, while better thancarrying no ice at all, was not effec­tive, and it was not until fishermenbegan to mix the ice with the fish thaticing aboard vessels made possible thelanding of high-quality fish. The reg­ular use of ice during overland ship­ments began from Boston to NewYork in 1858.

A major value of ice for preservingfresh seafoods is that it has a high la­tent heat of fusion 2 so that it iscapable of removing large amounts ofheat as it melts without changing itstemperature at 32 OF (O°C). Of course,once it melts, the heat of fusion willhave been absorbed, and its temper-

'The latent heat of fusion is the amount of heatrequired to change a given weight of a solid to aliquid without changing its temperature. In theEnglish system. the weight used is one pound(Btu). In the metric system. the weight used isone gram (calorie).

2 Marine Fisheries Review

ature will begin to increase as it is ex­posed to more heat.

During the transition from ice towater, 1 pound (454 g) of ice absorbs144 Btu (British thermal units). Since1 Btu (252 calories) is defined as theamount of heat required to raise thetemperature of 1 pound (454 g) of wa­ter by 1°F (0.56 0C), then the removalof 144 Btu (36.3 Kcal) from a 6 pound(2.7 kg) fish, which contains about 75percent water, will lower thetemperature of that fish 32 OF(17.8°C) from an ambienttemperature of 64°F (l7.8"C) to 32°F(O°C). The calculation involved is 144-;- (6 x 0.75).

This would suggest that fishermenshould take about one-sixth as muchice as the weight of fish they expect tocatch when the ambient temperatureis about 64 OF (17.8 0C). However, wehave to take into account the fact thatthe ice continually cools the pen wallsas well as the air around it from themoment the ice is loaded on the ves­sel, and it is expected to keep remov­ing heat from the system as well as theheat generated by the fish, once thefish are added, until such time as thefish are landed. The amount of extraice needed for accommodating theheat which the system receives fromthe environment will vary dependingon time of year, amount of insula­tion, length of trip, etc. This is boundto be quite high. At any rate,fishermen should take enough ice tomaintain the fish temperature atabout 32 OF (O°C) at all times. Theamount of ice taken on a trip will bebest worked out for each vessel, but itwould probably be from one-fourthto one-half the weight of the expectedcatch.

Since the spoilage of fish starts justafter they die, and since the spoilagerate is largely dependent on the tem­perature, the sooner the fish can becooled the better. This is precisely thereason that the quality of fish ispreserved longer when there is an ade­quate amount of ice, and it is welldispersed among the fish. When theice is in small particles, such as flakes,it does a more effective job of coolingthan when it is in large pieces. This is

April 1981, 43(4)

because smaller ice particles givegreater contact between fish and ice,and the rate of heat removal dependson the size of the contact area. An­other advantage of small ice particlesis that it avoids damaging fish inthe lower part of the pens. Largepieces of ice can exert point forces(from the pressure developed in thelower part of the pens when they arefilled) and thereby damage fish.

Although this paper does not coverthe modification of ice with bac­teriostats, it is important to point outthat the ice must be sanitary. Ob­viously, both ice and water that comein contact with food must be ofpotable quality. The need to ensurethe quality of the ice is not emphasiz­ed merely to meet legal compliancewhich is conerned with public healthbut rather to minimize the sources ofbacteria and other agents of spoilage.

Use of Chilled Seawater

Chilled seawater (CSW) is discuss­ed here because, in essence, it is an ex­tension of the use of ice. That is, it isthe end point to which can be carriedthe principle of maximizing surfacecontact (the smaller the ice particles,the greater the cooling rate, and mole­cules of cold water can be consideredto behave as minute articles of ice).

In CSW, the fish are surroundedwith a mixture of ice and water,thereby achieving maximum contactbetween fish and coolant. Whenenough ice is added to the system, itwill bring the temperature of the wa­ter down to 32 OF (O°C), and it willcontinue to remove the heat from thewater, that the water, in turn, re­moves from the fish. The transfer ofheat from the fish to the water andfrom water to ice will continue untilthe system is brought to a state oftemperature equilibrium. Actually, auniform temperature may never real­ly be attained, because the system isdynamic, continually absorbing heatfrom the environment and from thebacteria in the fish. However, provid­ed there is sufficient ice in the system,a point will be reached where theaverage temperature of the systemcannot be reduced further.

CSW is effective because the watercomponent of the CSW establishesmaximum contact between the cool­ing medium and the fish. Therefore,the cooling rate of fish in CSW ishigher than that of fish in ice.

CSW has sufficient advantagesover ice alone to warrant its adoptionby commercial fishermen for coolingtheir catch (Hulme and Baker, 1977).These authors reported that CSWcooling is fast, bringing the fishtemperature down to 32 OF (O°C)within 4 hours and maintaining thetemperature quite uniformly. Tem­perature uniformity is enhanced bythe thorough mixing due to the actionof the sea, as one might expect. Thereis no need to declump ice at sea;however, seawater must be added tothe ice as soon as possible. Otherwise,the ice tends to clump before a slushcan be obtained. The fish are lessdamaged in CSW because of thebouyant effect of the water. Also, fishcan be unloaded very quickly withpumps.

In our own work (Baker andHulme, 1977), we observed that whit­ing, herring, and other fish may bescaled by the agitation to which theyare subjected. This may be an advan­tage if the fish are to be scaledanyway, as they would be in mostprocessing operations. On the otherhand, if the fish were destined for amarket that required the scales to re­main on the fish, then CSW holdingwould not be suitable.

There have been reports of the de­velopment of discoloration of fish, ofrancidity, and of salt absorption dur­ing their holding in refrigeratedseawater which would have to holdtrue for CSW (Roach and Tomlinson,1969; Peters, Carlson, and Baker,1%5). We did not encounter theseproblems, however. The one concernwith CSW is that the holds must be ofspecial construction to prevent thebuildup of large surge forces in heavyseas. This may be prevented by the in­stallation of perforated baffles (Bakerand Hulme, 1977).

The ice requirements used in ourstudies were governed by an establish­ed ratio of one part ice, two parts

3

seawater, and seven parts fish. There­fore, for every ton (0.9 t) of fish weexpected to catch, we put into thehold 286 pounds (131 kg) of ice. Atthe first opportunity, 571 pounds (259kg) (about 68 gallons or 257 liters) ofseawater were added. Accordingly, ifone had an insulated hold or com­partment with a capacity of 10 tons (9t), he would start out by putting 1 ton(0.9 t) of ice in the hold. He wouldadd 2 tons (1.8 t) of seawater as soonas possible (not harbor water, becauseit is not clean enough). Two tons (1.8t) of seawater are equal to about 480gallons (1,817 liters). Then the holdwould be filled to capacity with fish,resulting in a mixture of 1:2:7(ice:seawater:fish). It should be notedthat the reason why a relatively smallamount of ice was adequate in ourCSW work is because the holds of thevessel that we used were well insu­lated. Both insulation and ambienttemperatures have major influenceson ice requirements.

Refrigerated Seawater

In ordinary application ofrefrigerated seawater (RSW), theseawater is usually cooled by me­chanical refrigeration. Thus, RSW, asopposed to CSW, is not as limited inits role of removing heat, and there isa reasonable control of temperatureover a range that is not possible withCSW. In addition, it has all the ad­vantages described for CSW and,unlike ice and CSW, it can be used forsuperchilling fish (see next section).

RSW systems may vary, but the ba­sic components are a pump to bringseawater into the vessel and to cir­culate the RSW, a heat exchanger toremove heat from the seawater, amechanical refrigerator to dischargeheat from the system, a circulatorysystem for transporting the refrig­erant between the heat exchanger andthe refrigerator, and a sparging sys­tem for spraying the RSW over thecatch or a tank to contain the fish andRSW. Auxiliary equipment can, andin some cases may have to be added;i.e., a filtering system, a holding tankfor the chilled RSW, a system forcontrolling the sanitary quality of the

4

water, and a system for removing fishoil.

The designs of a RSW system andits individual components are impor­tant (Peters, Slavin, Carlson, andBaker, 1965). Critical among these isthe design of the heat exchanger. If itis not properly designed, the RSWmay freeze and cause the system tofail. Even when the heat exchanger isof appropriate design, the RSW couldconceivably freeze if its rate of flow istoo slow. The pump(s) has to be cor­rosion resistant and have a relativelyhigh capacity. The entire system mustbe designed, especially the circulatorysystem (pipes, fittings, valves, etc.), soas to prevent the growth of microbialcolonies which can be the source ofcontamination that can easily be car­ried to the fish by the RSW. The sys­tem must be easily cleaned, especiallythe tanks that are used to hold thefish.

The earliest commercial use ofRSW occurred in the early and middle1920's to cool menhaden. It has beenused to preserve the quality of sar­dines. In some cases, the brine hasbeen made by the addition of salt tofresh water, especially when there wasreason to believe that the availableseawater was not satisfactory becauseof contamination or other deterrent.In the 1950's, Canadians used RSWfor preserving the quality of salmonand halibut both on the vessel at seaand in trucks on shore (Roach et al.,1961). While RSW has many advan­tages, its use has by no means prolif­erated. It has advantages that result instabilizing the quality of fish forperiods of about 1 week. However,while RSW controls temperature ex­ceedingly well, contact with the fishfor periods longer than 1 week mayhave deleterious effects that result inundesirable changes in odor and fla­vor (e.g., rancidity) and in appearance(e.g., loss of pigment from skin).

Although RSW should have signifi­cant long term preservative effects,empirical data do not support thetheory. Carbon dioxide (C02) hasbeen added to RSW in recent exper­iments as an adjunct preservative. TheCO2 lowers the pH which has a

beneficial effect on the quality of thefish; but because it does lower the pH,it then enhances the corrosiveness ofRSW components to intolerable lev­els. The latter problem has been cir­cumvented by using components thatare coated with corrosion resistantmaterials. When this is done the RSWsystem containing CO2 has shown aneffective inhibition of bacterialgrowth and an increase of at least 1week in the shelf life of the fish(Barnett et al., 1971).

Superchilling

Superchilling, as used for preserv­ing seafoods, has been defined as thelowering of the temperature of theflesh to within the range from -3 ° to-1°C (26.6-30.2 oF) (Carlson, 1969).The process also has been labeled"supercooling," "light freezing,""partial freezing," and "very poorfreezing. "

Pure water freezes at O°C (32 oF),but its freezing point is depressedwhen it contains dissolved substances.The water in biological systems(plants and animals) contains varyingamounts of dissolved substances;therefore, the freezing of seafoods oc­curs below the freezing point of purewater. When the temperature of sea­foods is lowered, the physical changeto a hardened mass occurs graduallyat rates that are fastest in the begin­ning and slower as the temperaturedrops. The water in the seafood is notspontaneously frozen at any giventemperature. As the flesh temperatureis lowered, the first water moleculesare frozen at slightly below O°C(32 oF). Successively more is frozen asthe temperature continues to fall. Ac­cording to Power et al. (1969), as thetemperature of fish muscle is loweredto _1°, -2 0, -3 0, and -4 °C, (30.2°,28.1°, 26.6°, and 24.4 oF), the percentof water frozen is 19, 55, 70, and 76,respectively.

At first, the rate of freezing of thewater in fish is relatively rapid; and bythe time the temperature is lowered toonly -6°C (21.2 oF), about 80 percentof the water is frozen and the flesh isrigid, even though the remaining 20percent of the water is not frozen. At

Marine Fisheries Review

Superchilled haddock fillet.

this point, the rate of freezing issharply reduced, and a further de­crease of about 36°F (20°C) (down to-14.8°F or -26°C) will freeze onlyabout an additional 8 percent of thewater Oeaving about 12 percent of thewater in the system still in the liquidstate). It is not until the temperature islowered to about -67 OF (-55°C) thatall of the water will appear to be fro­zen. (While the foregoing data mayvary slightly, it accurately representsthe processes.) A typical example ofthis process is described in Charm(1971).

From this, we can see that super­chilling will involve the conversion ofat least some water to ice, the amountdepending on the equilibrium tem­perature to which the system is finallybrought. Provided that the temper­ature is not permitted to go below26.6 OF (-3°C), superchilled seafoodswill not become rigidly frozen. Thus,superchilling is accurately defined aspartial freezing.

April 1981, 43(4)

The first record of superchillingwas reported in about 1935 (Carlson,1969), and it involved the use of brine(at about 26.6 OF (-3°C» as therefrigerant, resulting in extended shelflives for whole fish. In the first majoruse of superchilling, mechanical re­frigeration was used to hold fishaboard fishing vessels at about 30 OF(-1.1 0c) (Ranken, 1963).

Reports on the preserving effective­ness of superchilling fish leave littledoubt as to the considerable increasein the shelf life of the product.However, according to Carlson(1969), certain disadvantages surfacedin subsequent evaluations of the processby research teams from England andlater by teams from Canada, TheFederal Republic of Germany, andthe United States. Degradation ofappearance and texture and excessivedrip loss were also found and con­firmed (Power and Morton, 1965).Some of the quality degradation wasattributed to the partial freezing thatactually occurs during superchilling.The recommendations that derivefrom these subsequent evaluations arethat superchilling is effective andpractical, provided that the tem­perature does not fall below the pointwhere freezing is discernible (about28.4 OF or -2°C) and that the time ofholding does not exceed 12 days. Theuse of seawater with no added salt willinsure that the temperature of the fishwill not be lowered too much becauseseawater freezes at about 28.4 OF(-2°C).

Freezing

The preserving of foods by freezinggoes back into antiquity, having beenused by such ethnic groups as Eski­mos and Indians in certain cold areas.Fish caught in the winter months incold climates were frozen and heldfrozen in the cold ambient air. Redmeats were also frozen and held innatural, freezing, ambient conditions.

The industrial freezing of foodswas introduced by Clarence Birdseyeduring the 1920's when he developeda process for freezing foods in smallpackages suitable for retailing. Hefound that the quality of a variety of

foods, including fish, fruits, and veg­etables could be preserved for monthsby freezing and low temperaturestorage. Subsequent development infreezing equipment and techniqueshas gradually evolved into the higWysophisticated frozen food and mar­keting distribution system available tous today. Important events in thedevelopment of the frozen food in­dustry and a summary of its past andpresent are described by Fennema(1976). Much of the refrigerationequipment and techniques used in thefrozen food industry has been con­nected with the preservation of fisheryproducts.

While fresh seafoods are more ac­ceptable to U.S. consumers and carrya higher retail value than frozenseafoods, this is an anomalous situa­tion because the production costs offrozen seafoods are higher than forfresh seafoods. High quality seafoodsare worth their extra production costsbecause they have a much longer shelflife than fresh seafoods, and afterpurchase they may be put into do­mestic frozen storage directly withoutthe need to package them and to ex­pend the energy required to freezethem.

Regardless of the comparative ac­ceptabilities and values of fresh andfrozen seafoods, much of the sea­foods consumed in this country arefrozen at one time or another beforethey reach the point where they areconsumed. This is because of the longtimes involved in the distribution andespecially holding of seafoods.

While the use of ice, CSW, orsuperchilling is adequate for preserv­ing the quality of seafoods for shortperiods, none of these processes op­erates at low enough temperatures re­quired to protect quality for longperiods. Seafoods may retain theirquality for many months if they areproperly packaged and held at suit­ably low temperatures (below 0 OF or-17.8°C). Numerous data dem­onstrate that many seafoods remainvirtually unchanged in their qualityfor periods longer than 1 year whenthey are held at _40° (_400 P = -40°C).Even at -20 0 P (-28.9°C), long shelf

5

Frozen halibut in cold storage.

lives have been reported for seafoods.The need to thaw frozen seafoods

prior to reprocessing in food plants orfor domestic use is one undesirableaspect of freezing. Thawing is timeconsuming and, in some cases, is as­sociated with loss of product quality.It normally takes longer to thaw foodthan to freeze it under similar heattransfer conditions. In other words, ittakes longer for the temperature of afood to go from _10°F (-23.5°C) to60°F (\5.7°C) than it takes thetemperature of the food to go from60 of to -10 of. This is because thethermal conductivity of ice is aboutfour times greater than that of water(Baumeister and Marks, 1966).

This difference in thermal proper­ties affects the surface of the foodwhich is frozen during most of thefreezing cycle and unfrozen duringmosi of the thaw cycle. Thus, duringfreezing, immobilization of surfacemicroorganisms occurs early in theprocess before much deterioration canoccur; conversely, in the thawing pro­cess the surface is thawed first andsurface microorganisms are providedwith good growing conditions fornearly the entire thawing period.

Foods packaged in small units de­frost in a few hours at room tem­perature and during this time are notsubject to an undesirable amount ofdecomposition due to bacterialgrowth. However, seafoods frozen inbulk (i.e., large fish blocks) may pre­sent a defrosting problem. Becausebulk-frozen foods take a long time todefrost and because the rate at whichthe food defrosts depends on thetemperature to which it is exposed,there may be a tendency to defrost thefood at relatively warm temperatures.When this is done, the surface of thefood is subject to microbial spoilagebefore the inner portions defrost.

Some methods have been devel­oped to alleviate this problem.Refrigerator defrosting (holding attemperatures of 35-40 of orl.7-4.5 0c) is probably the bestmethod of defrosting bulk-frozenfoods when no fast method is avail­able. This would apply to large wholefish since bacterial or mold growth

6

would be limited under these condi­tions. However, in industrial process­ing, where bulk-frozen products arethawed as an intermediate step in themanufacture of the company's line ofproducts, the refrigeration space re­quired may be so large as to discour­age this practice.

With microwave energy, food canbe thawed rapidly and with virtuallyno quality loss. That is becausemicrowaves, by their unique char­acter, cause a temperature risethroughout the product almost simul­taneously. The microwave beam

penetrates foods with an alternatingcurrent. In alternating current, thecharge alternates between positive andnegative. Because water molecules arepolar (i.e., they have positive andnegative ends), they are put into atwisting motion due to the alternatingcurrent which attracts first the pos­itive end of each molecule then thenegative end at a rate of millions oftimes per second. The twisting actionof the water molecules creates con­siderable friction which generatesheat. Ice is not affected by micro­waves, but neighboring unfrozen wa-

Marine Fisheries Review

ter molecules (frozen foods containsome unfrozen water) generate the in­itial heat that melts adjacent ice torelease more water which acceleratesthe heating.

Since the heat generated in foodsby microwaves is quite rapid (about10 times more rapid than by baking),when uneven heating in a frozenproduct does occur, the temperaturedifferences within a food can becomegreat. This, however, happens onlyunder certain conditions, and it can bedealt with quite easily. For this condi­tion, and also when one wants to en­sure uniform temperature control,one solution is to apply the micro­wave energy in intermittent bursts. Bythis technique, the absorbed thermalenergy generated during a burst ofmicrowaves is allowed to be distrib­uted by conduction during the inter­vals between the bursts, thereby per­mitting the temperature of the food toincrease more uniformly albeit moreslowly. Modern developments, suchas wave guides, have improved thedistribution of microwave energy.

The particular advantage of usingmicrowave energy for thawing foodsis that deterioration by micro­organisms is not a factor. Thefeasibility and benefits of microwavethawing of frozen meats and fish havebeen adequately demonstrated, espe­cially for thawing frozen shrimpblocks (Bezanson et al., 1973). Indus­trial microwave ovens are now usedby both the meat and seafood in­dustries.

One potential solution to the prob­lems associated with thawing and thecell damage caused by ice crystals in­vestigated by Charm et al. (1977) isworthy of mention and recommendedfor further investigation. Basically,the method involves the lowering ofthe temperature to below freezing(26.6 of or -3°C) without forming anyice by imposing a pressure of 272 at­mospheres on the product. Lowertemperatures are possible.

One aspect of seafood preservationthat has variable importance is pack­aging. Packaging of seafood per­forms several basic functions (e.g.,protection from contamination). In

April 1981, 43(4)

addition, the package has to serve ad­ditional functions, the critical one be­ing gas impermeability (Nickersonand Ronsivalli, 1979). Preventing thefrozen seafood from having directcontact with oxygen is highly impor­tant.

The rate at which foods are frozenis just as important as the temperatureat which frozen foods are held and therange of fluctuation of the storagetemperature. When foods are allowedto freeze slowly, water molecules,even though they are slow moving,have time to migrate to seed-crystalsresulting in the formation of large icecrystals. When foods are made tofreeze rapidly, the sluggish watermolecules do not have enough time tomigrate to ice crystals but instead are"frozen in their tracks," so to speak,forming relatively small ice crystalsmade up of local water molecules.Rapid freezing may be effected by avariety of methods which include theuse of liquid and gaseous refrigerants,cold-air blast, and cold-plate contact.

liquid Refrigerants

Freezing is most rapid when thefood is brought into direct contactwith refrigerants (i.e., where thefoods are immersed directly in a liquidrefrigerant, sprayed with liquid re­frigerants, or exposed to cold gasesemanating from liquid refrigerants).This is because the removal of heat isproportional to the temperature dif­ferential at the food surface, and thedirect contact between food and re­frigerant tends to maintain thetemperature differential at the highestpossible value for the particularsystem.

Brine

Brine may be defined as a salt solu­tion. Both the salt and its concentra­tion may vary, depending on theintended application. The salt is gen­erally sodium chloride, and the sol­vent is water. The principle thatdissolved substances depress the freez­ing point of water makes brine an ef­fective medium for freezing foods.Thus, salt solutions have lower freez­ing points than pure water, and brine

can be made cold enough to freezefoods which are immersed in it whilethe brine itself remains fluid. Thefreezing point of brine is determinedby the concentration of the salt.

At salt concentrations up to 23.3percent, the higher the salt concentra­tion, the lower the freezing point.When the concentration of the saltreaches a value of 23.3 percent byweight, the limit of the trend is reach­ed at a temperature of -6 OF (-21.2 0c).This is the eutectic point for NaCl,and it is the lowest temperature that aNaCI solution will remain fluid. Anyfurther increase in NaCl concentra­tion tends to raise the freezing temper­ture of the brine.

When one wishes to avoid the useof sodium or when one wishes to de­press the freezing point of the brinebeyond the limit that can be reachedwith NaCl, then CaCl2 may be used.It can be seen from Table 1 that, whilethere is little difference in freezingpoint depressions between the twosalts up to concentrations of about 20percent, at 25 percent concentrationcalcium chloride lowers the freezingpoint of water to a level that cannotbe accomplished with sodium chlorideat any concentration. Table 1 alsoshows that calcium chloride can de­press the freezing point of water to aslow as -67.0 OF (-55.0°C), the eutecticpoint for calcium chloride. It can beseen that the difference between the

Table 1.-E"ect of NaCI and CeCI, concentretlonson the freezing point of water.

Freezing point of Freezing point ofaqueous solution aqueous solution

of NaCI of CaCI,Percentsalt OF °c OF °c

0 32.0 0 32.0 05 26.7 ·2.9 27.7 -2.410 20.2 -6.6 22.3 -5.415. 12.3 -10.9 13.5 -10.320. 2.3 -16.5 -0.4 -17.823.300' -6.0 -21.225. 16.1 -8.8 -21.0 -35.325.200 32.0 026.285' 32.2 0.126.308' 38.0 3.329.870' -67.0 -55.0

, Eutectic point for NaC!., Transition point for NaC!., Saturation point for NaC!.• Eutectic point for CaCI,.

7

Table 2.- Names and properties of important refrigerants used in the food industry.

Heat ofvaporization

Boiling point Freezing point at boiling point

Refrig· Chemicalerant Chemical composition formula 'F 'C 'F 'C Btullb calfg

R717 Ammonia NH, -28 -33.3 -107.9 ·77.7 589.0 327.0R12 Dichlorodiflouromethane CCI,F, ·21.6 -29.8 -252 ·157.8 71.04 39.47R22 Monochlorodiflouromethane CHCIF/, ·41.4 -40.8 ·256 -160 100.45 55.81R502 Mixture of 48.8 percent R22 CHCIF,fC,CIF, -49.8 ·45.4 76.46 42.48

and 51.2 percent R115R115 Monochloropentaflouroethane C,CIF, ·38.4 -39.1 -159 ·106.1 54.20 30.11R728 Nitrogen N, ·320.4 -195.8 -415.5 ·248.6 86.0 47.8R744 Carbon Dioxide CO, ·1092 -78.4 -69.9 -55.6 247.0 137.1

eutectic points for NaCI and CaClz isconsiderable. While the data of CaClzare not carried out, it should be notedthat further increases in CaClz tend toraise the temperature. Despite the ver­satility of calcium chloride, brine forcooling seafoods is produced fromsodium chloride. Calcium chloride isused only when very low temperaturesare needed to effect rapid cooling orwhen cooling bulky products.

From the middle 1910's to the mid­dle 1920's, immersion of fish in brinewas the only known method of quickfreezing. While new methods forfreezing have been developed sincethen, brine immersion freezing is stillan effective and useful process be­cause of its rapidity and because thefish do not lose water. However, thefish can absorb some salt which has acatalytic effect in oxidative deteriora­tion of the quality of the fish duringsubsequent storage. Nevertheless,brine freezing is still employed in avariety of situations, including someu.s. vessels; however, in U.S. land­based operation, it has been replacedby other freezing methods. Manyplants use brine to prechill fish.

One of the problems with the use ofbrines is their tendency to corrodeequipment when their pH is allowedto fall below 7.0, becoming acidic,and when air enters the system. ThepH of brine generally falls in thepresence of air due to the carbondioxide contained in air which dis­solves in the brine to form carbonicacid which lowers the pH, and corro­sion is enhanced due to the presenceof oxygen in the air.

Both corrosion and salt absorptioncan be reduced without sacrificing thelowering of the freezing point by sub­stituting sugar for some of the salt,provided that the present of sugar isneither restricted nor undesirable.There seems to be no significant con­clusions regarding the use of salt!sugar brines except that they ac­complish to some degree the objectivefor which they are used.

Cryogenic Liquids

Cryogenic liquids can be brought tovery low temperatures without solidi-

fying. They may be used in direct con­tact with foods in place of brines.These include liquid nitrogen at-320°F (-196°C), liquid carbon diox­ide at -108 OF (-78°C), and Freon-12(dichlorodiflouromethane) at -21°F(-29°C). The number of refrigerantsthat can be used in direct contact withseafoods is limited because they arerequired by the Food and Drug Ad­ministration (FDA) to meet the samecriteria that apply to foods, and onlya few of these refrigerants can meetthe criteria.

FDA requirements are not the onlycriteria imposed on the use of liquidrefrigerants. Freon-12, although inuse by industry for direct contact withfoods under FDA sanction over aperiod of years is now under EPA(Environmental Protection Agency)scrutiny because of its perceiveddamage to the Earth's atmosphericozone layer which would lead toreduce protection from the Sun's in­frared radiation (Sernling, 1979). Thisconsideration is bound to affectfuture decisions as to choice ofrefrigerants. The EPA's scrutinydirected at Freon-12 includes otherhalocarbons even though they may beused in closed systems and do notcome in direct contact with foods.

While freezing food by variousmethods that involve direct contactbetween them and liquid refrigerantsis practiced widely, the holding offrozen foods is largely done in roomsthat are kept at freezing temperaturesby systems that use any of a variety offluid refrigerants in what are properly

described as mechanical refrigerationsystems. These will be described in thefollowing section. However, at thispoint, we will continue with the dis­cussion of the cryogenic liquids thatare used in these systems.

The refrigerants used either in di­rect contact with food or inmechanical refrigeration systems areclassified into three groups. Group 1refrigerants, which are neither toxicnor flammable, include carbon diox­ide, liquid nitrogen, and the fluo­rocarbons. Group 2 refrigerants aretoxic, flammable, or both. Ammonia,which is used in some of the largerindustrial installations, is a represen­tative of this group. Group 3 refrig­erants are highly flammable andexplosive. They include propane, eth­ane, methane, ethylene, and propyl­ene. This group has limited use andis used only where a flammability orexplosion hazard is already presentand their use does not add to thehazard. A description of some of theimportant refrigerants follows. In re­frigeration jargon, which is mainlyused to avoid contending with the un­wieldly names of many of the refrig­erants, they are given numbers withan "R," the R meaning refrigerant(fable 2).

R717. Ammonia, R717, is a veryeconomical and efficient refrigerantbecause of its low boiling point, -28 OF(-33°C), and high heat of vaporiza­tion (589 Btu/pound or 327 calor­ies/gram) at atmospheric pressure.Although toxic and flammable undercertain conditions, ammonia is still

8 Marine Fisheries Review

~Insulatedwall

SPACE TO BE COOLED(Refrigerator or freezer)

one of the best and most widely usedrefrigerants. It is used extensively inlarge commercial and industrial re­frigeration plants. In small andmedium-sized commercial plants, am­monia, which is a strong irritant ofeyes, throat, nose, and lungs, is beingreplaced by Freon 12, 22, and 502,which are Group 1 refrigerants andhave many advantageous physicalproperties, as well as being muchsafer.

R 12. Dichlorodiflouromethane,or R12, has a boiling point of -21°F(-29.4 0c) at atmospheric pressure anda latent heat of vaporization of 71Btu/pound (39.5 calories/gram).Presently, it is the most widely knownand widely used refrigerant. It is usedin many small commercial refrigera­tion plants and has a multitude of ap­plications, ranging from small house­hold refrigerators and air conditionersto large centrifugal units for normal­and low-temperature plants and hasbeen used to obtain temperatures aslow as -130°F (-90°C) although otherrefrigerants are better suited to main­tain temperatures in that range.

The disadvantage of R12 is thatunlike ammonia, it is not compatiblewith moisture and care must be exer­cised to remove all air and moisturewhich otherwise would cause exces­sively high head pressure and freezingof expansion valves.

R22. Monochlorodifluorometh­ane, or R22, has a boiling point of-41°F (-40.6°C) at atmosphericpressure and a latent heat of vaporiza­tion of 100.5 Btu/pound (55.8 calor­ies/gram). Its physical properties aresimilar to those of R12 except that ithas a lower boiling point, and itoperates at a higher discharge pres­sure than R12. It is used in place ofR12 for low temperature applications.

R22 but with discharge temperaturescomparable to R12. It has a boilingpoint of -50°F (-46°C) and a latentheat of vaporization of 76.5 Btu/pound (42.5 calories/gram). R502 is arecent addition to the refrigerant listbut is rapidly becoming recognizedand is replacing R22 in many applica­tions.

R728. R728, liquid nitrogen, hasa boiling point of -320°F (-196°C).The latent heat of vaporization of liq­uid nitrogen is 86 Btu/pound (47.8calories/gram). However, the cold va­por is capable of absorbing another80 Btu/pound (44.4 calories/gram) inwarming up to _40°. Consequently,there is a usable heat-removal capaci­ty of about 166 Btu/pound (92.2calories/gram).

R744. R744 is the refrigerantdesignation for carbon dioxide, alsoknown popularly as dry ice. R744 hasa boiling point of -109.2 of (-78.4°C)and a heat of vaporization of 247Btu/pound (137 calories/gram). Inwarming up to _40°, it will absorbanother 14 Btu/pound (7.8 calor­ies/gram) for a usable heat-removalcapacity of about 260 Btu/pound(144.3 calories/gram) which is 57 per­cent greater than liquid nitrogen. Theadvantage of liquid nitrogen over car­bon dioxide is that it has a much cold­er starting temperature.

Both R728 and R744 are suitablefor cryogenic freezing, and the choiceis mainly an economic one, dependingon availability and cost at the loca­tion.

j~ .

ExpanSionvalve

R502. R502 is a mixture of mono­chlorodifluoromethane (48.8 percent)and monochloropentafluoroethane(51.2 percent) (R22 and R1l5, respec­tively). It is especially well suited tolow temperature applications pro­viding considerable capacity gain over

April 1981, 43(4)

Figure I.-Basic elements of mechanical refrigeration.

9

Figure 2.-Closeup view of plate freezer chamber.

Mechanical SystemsUsing Liquid Refrigerants

A mechanical refrigeration systemconsists of an insulated area or room(the refrigerator) and a continuous,closed system consisting of a refrig­erant, expansion pipes or radiator­type evaporator located in the refrig­erator, a pump or compressor, and acondenser (Fig. I). The compressorand condenser are located outside therefrigerator. The refrigerant, such asammonia or one of the freons, flowsinto the expansion pipes as a liquid.Here it evaporates to a vapor and inchanging from the liquid to the vaporphase it absorbs heat through theevaporator. The vapor is pulled intothe compressor by the suction actionof the pump and is then compressedinto a smaller volume of hot gas. Thelatter action causes the gas to heat upand this heat must be taken out. Thisis done by passing the compressed gasthrough a system of pipes or radiators

usually cooled by water, or sometimesby forced air. Cooling the compressedgas liquefies it, whereupon it is thenreturned to the evaporator in the re­frigerator. The conversion of the gasto a liquid also produces heat which istransferred to the water or air of thecondenser. Special valves at both endsof the evaporator allow the requiredflow of liquid refrigerant in and ofvapor out of the expansion system inthe refrigerator.

There are a number of ways inwhich refrigeration may be applied tothe insulated area which is to be cool­ed. Expansion pipes where the refrig­erant is evaporated may be locatedalong the walls of the freezer. In thiscase natural circulation of air (thecold air being heavier) may be de­pended upon to refrigerate areas with­in the room away from the expansionpipes, or some type of forced air cir­culation may be used. In some in­stances radiation-type evaporationunits are used. A fan which blows air

through the radiator fins provides cir­culation of cold air throughout thefreezer.

A variety of methods are associatedwith the use of liquid refrigerants.The following discussion is limited toonly some of the systems that are incurrent use.

Plate Freezing. In plate freezing(Fig. 2), layers of the packaged prod­uct are sandwiched between metalplates. The refrigerant (a fluorocar­bon such as R12) is allowed to expandwithin the plates to provide temper­atures of -28 OF (-33.3°C) or below,and the plates are brought closer to­gether mechanically so that full con­tact is made with the packaged prod­uct. In this manner the temperature ofall parts of the product is brought tooOF (-17.8 0c) or below within aperiod of 1.5-4 hours (depending up­on the thickness of the product). Thepackages are then removed, put intocases, and stored.

Continuous operating plate freezersare now in use. In one such system thefreezer is loaded at the front and un­loaded at the rear after completion ofthe freezing cycle. This is doneautomatically and continually. In an­other continuous system, the pack­ages are fed automatically on beltswhich place them in front of eightlevels of refrigerated plates. Thepackages are slid into the spaces be­tween the plates and the plates closedto provide contact. As freezing pro­ceeds, the packages are advanced by asystem such that with each opening ofthe plates the packages are advancedby one row with a new set of packagesentering the front row. By the timethe packages reach the far side of theplates, they are completely frozen,and they are pushed out of the freezerand unloaded to be cased and stored.

The vertical plate freezer was devel­oped mainly for freezing fish at sea. Itis usually used in sodium chloridebrine freezing wells and consists of anumber of vertical plates formingpartitions in a container with an opentop. The product is simply droppedinto the brine from the top. This type

10 Marine Fisheries Review

of freezer is widely use by the tuna in­dustry. (Calcium cWoride can be usedwhen faster cooling times are desired.)

Immersion Freezing. Productseither packaged or unpackaged can befrozen by direct immersion in cryo­genic liquids. The products may becarried through the refrigerant by asubmerged conveyer. When the prod­uct to be frozen is large (e.g., wholelarge tuna and swordfish), it may sim­ply be immersed in a tank of refrig­erant. There are not many choices forfreezing large fish because of therelatively long freezing times required.Here, direct immersion effects rapidfreezing.

Product/ ,rConveyor

a::=::::::::::=====::=:::=========~:::D..--;;~--Refrigerant collection pan

Insulated wall

pump

Figure 3.-Continuous liquid-refrigerant freezer.

Spray Freezing. The freezing offoods by direct sprays of cryogenic li­quids (nitrogen and carbon dioxide) iswidely used. In this process individualfood portions are placed on a movingstainless steel mesh belt in an insu­lated tunnel where they are sprayedwith liquid refrigerant (Fig. 3). Excessrefrigerant is recovered, mtered, andrecycled. The food leaves the freezerin the frozen state and is thereafterpackaged, cased, and stored. Thismethod provides very fast freezingand is being used especially for somemarine products such as the variousforms of frozen shrimp. When the liq­uid refrigerant is evaporated, the stillcold vapors are used to precool andtemper the product entering the freez­er. The very high freezing rates assoc­iated with liquid nitrogen freezingresults in improved texture, particu­larly in the case of certain fruits andvegetables.

In the case of carbon dioxide (C02)

freezing, in order to utilize the liquidCOH it must first undergo a change ofstate to freeze the product at or nearatmospheric pressure. Since liquidCO2 cannot exist at pressures of lessthan 69.9 psial (4.9 kg/cm2

) when it

]Pounds per square inch absolute. This meansthat when the pressure is measured with apressure gauge, the value of the prevailingbarometric pressure must be added to the gaugepressure to obtain psia.

April 1981, 43(4)

expands from its storage pressure toatmospheric pressure, both gaseousCO2 and solid dry ice are formed; anddepending on the design of the equip­ment, either the production of CO2

gas or CO2 snow can be maximized.The CO2snow at -108 OF (-78 0q thencomes in contact with the productand, combined with the gases, effectsthe freezing process.

The liquid freon (LF) system is thenewest system on the market and usesspecially purified dicWorodifluoro­methane (RI2). The product is carriedinto the unit by a conveyer and drop­ped into a moving stream of R12 on apan to separate and crust-freeze foodparticles as they are distributed andmoved from the drop zone of thefreeze belt. The freeze belt carries thefood under sprays of refrigerant tocomplete the freezing process. A thirdconveyer then carries the food out ofthe freezer. R12, which has been va­porized as a result of heat extractionfrom the food, is recovered and reliq­uefied by contact with a condenserlocated above the freeze conveyer.Condensed refrigerant is collected in asump and recycled to the spray noz­zles. The system is very efficientbecause only minimal amounts of re­frigerant are lost, about 0.5-0.7 kg per45.5 kg (1-1.5 pounds per 100 pounds)of processed food. Most of the lossesare residual amounts left on the food,and these evaporate very rapidly.

Refrigerated Air

Although air is fundamentally apoor conductor of heat, the fact thatits density changes as its temperaturechanges permits its use as a contactrefrigerant, albeit relatively slowly.Cold storage warehouses can lowerthe temperature of a food that is athigher temperature than the air withinthe cold room by conduction at theinterface between the food (or itspackage or overwrap) and the airwithin the room which is put into mo­tion as its specific gravity changes.That is, air made cold by the evap­orator (or expansion pipes) is mademore dense and tends to migrate tothe floor of the cold room forcing thewarmer air to migrate upward. Thus,the food surfaces are continually ex­posed to cooler, moving air moleculesthat acquire heat from the food byconduction, then are lifted by thebuoyant force of the denser air mole­cules away from the food whereuponother cold molecules repeat the cycle.When foods are not protected bypackaging or an ice glaze that is im­permeable to water vapor, or other­wise prevents loss of moisture, thecold air which is also relatively drywill condense water molecules andtend to dehydrate the food as well ascool it. The water carried by the air isthen condensed on the evaporatorcoils where is can be seen as "frost."

JJ

Figure 4.-Jacketed freezer, cross-sectional view.

SIDE VIEW

Bank of fans

ffiffiffiffiffi~/

]•. ;f 7)/

oduct in ca~rs holding productrts in trays

IIII

(.II caII

II

I

Figure 5.-Tunnel blast freezer.

Fan I

I vCooling coilsl

I

Arrows show airstreams

END VIEW

r:: --.~'f~

..-

I\'

Sharp Freezers

Sharp freezers employing the re­frigerated air principle, were used inearly installations and are still general­ly used on the Pacific coast and inAlaska. They are essentially coldstorage rooms that cool the foods byair convection. In some of the earlysharp freezers, shelves made of pipegrids or metal plates containing the re­frigerating medium were installed onwhich the product to be frozen wasplaced. There is no rapidly circulatingair so freezing is relatively slow depen­ding on the size and shape of theproduct and the manner in which itwas distributed on the shelves.

Jacketed Freezers

The jacketed freezer (Fig. 4) isdesigned so cold air circulates throughan enclosed jacket completely sur­rounding the product storage space. Itis simply a room within a room andallows storage at near 100 percentrelative humidity and at a constanttemperature. These conditions greatlyreduce the weight loss of unpackagedfoods and frost formation insidepackaged frozen foods. The jacketedprinciple, although a good one, hasnot been accepted by the industry norused to any great degree owing toprohibitive building costs.

Blast Freezers

Blast freezers are generally roomsor tunnels in which cold air is circu­lated by one or more fans through anevaporator and around the product tobe frozen. Air blast freezers aregenerally preferred when unwrappedproducts of irregular size are involved(i.e., large fish in the round). Theblast freezer may be of the batch type,semicontinuous or continuous, de­pending on whether the product issupported on racks, trucks, or mov­ing belts (Fig. 5). Some of the latestdesigns are very efficient and space re­quirements are minimal due to a verti­cal spiral configuration of the con­tinuous conveyer which moves theproduct throught the freezer. Theseare known as belt freezers but, alongwith tunnel freezers, are still basicallyof the air blast variety.

12 Marine Fisheries Review

Temperature (OF)

- 50'----'------'-----'--8L---.l'0-'-'-2-----'-14-'-'-6----'1823

Time (min)

59

3.0

35

« 10

15

""., ~t

.'. ~...........--.__• AI l

--e__e_ 32 J

_~'-c1.l-_-7:410:--_-;C28!;-;,.9;-(0(-1;-';1~,""'8---:_6~,.7----,J4.41.5Qlo -40 -20 0 20 4

1

0 0

Figure 6.-Dehydrocooling curvefor whiting (Carver, 1975).

Figure 7.-Ambient tem­peratures at altitudes above10,000 feet (3,048 m).

'Mention of trade names or commercial firmsdoes not imply endorsement by the NationalMarine Fisheries Service, NOAA.

an area of need where other means oftemperature controlled transportationis unavailable.

By this proposed method, freshlanded fish are placed in special con­tainers and loaded in specially design­ed aircraft and flown to inland air­fields located more than 2 hours offlying time from coastal areas - suf­ficient time to freeze the product. Theproposal, initiated by an NMFS tech­nologist, was evaluated by theManager of New Products Investiga­tions, the Boeing Company", and giv­en as a problem to the company'sengineering staff. Convinced of thepotential of the idea, the Boeinggroup produced a design for a proto-

Use of Jet Aircraftat High Altitude

It is widely known that thetemperature at altitudes used by com­mercial jet-powered aircraft is verylow (Fig. 7). The use of jet altitudefreezing was proposed as a means oftransferring seafoods from the coastalareas of large underdeveloped areaslike India to the interior of the coun­try as a solution to transport proteinfoods from an area of abundance to

the condensation rate while simultan­eously accelerating the evaporationrate. During this rate imbalance, theheat lost exceeds the heat gained, andthere is a net cooling effect. If thevacuum is maintained, the cooling ef­fect can be substantial, and foods canbe cooled to below freezingtemperatures relatively quickly.

Although dehydrocooling also candehydrate the product, this can benullified by the addition of water tothe system through an internal watersparger. Employing a water sparger toprevent dehydration, Carver (1975)found that he could cool headed andgutted whiting from a temperature ofabout 59"F to 32"F (15"C to O"C) inabout 18 minutes (Fig. 6).

Beckman (1%1) was granted a pa­tent for "conserving fresh fish"aboard a vessel. By the Beckman pro­cess, the fish are eviscerated, washed,and placed into cylindrical tankswhich are connected to a steam jetvacuum pump located in the engineroom, which reportedly can reducethe pressure within each tank to about2 mm of mercury. The tanks resemblevertical retorts and can be loaded andunloaded by baskets filled with fishand which fit into the tank. Each tankis designed to reduce heat gain andcontains a water injection system forpreventing dehydration of the prod­uct. The temperature of the productis brought down to 30.2°F (-1°C) andmaintained at that temperature untilbrought to land. The advantages oflow temperature holding and the elim­ination of oxygen, especially withtreatment occurring just after the fishare caught, are obvious.

Dehydrocooling

Dehydrocooling is the lowering ofthe temperature as a result of removalof water by evaporation. It is a prac­tical method in current use for someapplications, especially for coolingleafy vegetables. In practice, the pro­cess is a simple one involving the con­trolled evaporation of moisture fromthe surfaces of products to be cooled.

Water and substances containing itmaintain a water-vapor pressure (thatdepends on temperature) above themin a state of equilibrium. Althoughthe system is a dynamic one (Le.,water molecules are continually beingvaporized while vaporized moleculesare continually being condensed), theequilibrium is maintained for anygiven temperature. This is because thenumber of molecules being vaporizedequals the number being condensed.In this condition, the heat lost byevaporation is regained by condensa­tion.

This equality always exists untilthere is a change in conditions such asa change in temperature or pressure.A drastic disturbance to the systemoccurs when a vacuum is created in achamber containing water or a sub­stance containing water. The vacuumcreates a reduction in vapor pressurewhich in turn creates a sudden drop in

Fluidized Bed Freezers

When particles of fairly unifonnshape and size are subjected to an up­ward air stream, they are said to be­come fluidized. By this principle,food products of small unifonn size,such as scallops and krill, can be fro­zen. Depending on the characteristicsof the product particles and the airvelocity, they will float in the airstream, each one separated from theother, surrounded by air and free tomove. Under these conditions, themass of particles behaves like a fluid.The product can then be frozen andsimultaneously conveyed by air with­out the need of a mechanical convey­er. The advantage over belt freezing isthat the product is truly individuallyquick frozen (I.Q.F.) and even appliesto foods that tend to agglomerate.

April 1981, 43(4) 13

type system using a modified Boeing727 (Fig. 8) and a special container(Fig. 9). Theoretical problems such aspossible aerodynamic interferencefrom the need to pass the outside airthrough the aircraft and inefficientheat removal by the rarefied atmo­sphere at jet altitudes were considered,analyzed, and discounted. Althoughwe have not tested this concept, nordo we have any information thatanyone else has, theoretically, it hasbeen deemed entirely feasible, andthere is no known impediment to itsuse. With the rising costs for energy tofreeze foods, this concept has poten­tial for foods that are destined forlong distance transportation; becausein these cases, the heat removal isdone at little or no cost.

FISH CONTAINERS

D

(Courtesy, The Boeing Compony)

Figure 8.-Modified Boeing 727 for freezing fish at high altitudes.

Summary

----(Courtesy, The Boeinq Compony)

Figure 9.-Container for freezing fish at high altitudes.

ually involves mechanical refrigera­tion, and, in this case, the producttemperature can be lowered below32°F (O°C) to the superchill range.

FISH CONTAINER

of ice:weight of water:weight of ex­pected catch) has been used success­fully with an insulated vessel.

Refrigerated seawater (RSW) us-

While there are a number of factorsthat affect the rate at which seafoodsspoil, temperature control remains asa major one in the control of theirquality. This paper has reviewed theprinciples, the methods, and the re­frigerants used to chill seafoods (32"For O°C), superchill them (26.6 to3O.2"F or -3 to -IOC), or freeze them(O°F or -17.8°C or below).

For chilling seafoods, ice, with itshigh latent heat of fusion, is effectiveand widely used. When ice is in smallparticles (i.e., flaked), cooling ratesare high and damage to the product isminimized. The amount of ice requir­ed varies with each situation, but itshould be enough to maintain theproduct at 32"F (OOC) as long asnecessary. In some instances, vesselsrequire about one-fourth the weightof the expected catch or less. In othercases, the requirement could be ashigh as half the weight of the expectedcatch. Ambient temperature, lengthof trip, and degree of insulation areamong the major factors that affectthe weight of ice required.

Chilled seawater, a mixture of iceand water, chills fish more quicklythan ice alone, and by its buoyant ef­fect, prevents damage to the fish. Therequirement, again, varies with eachsituation, but a ratio of 1:2:7 (weight

14 Marine Fisheries Review

This system provides a better andmore varied control, but it requirescapital equipment and maintenance,and provisions have to be made to in­hibit the corrosive effects of theseawater on the RSW components.

Because the seafood spoilage rate isdirectly related to the temperature,superchilling, which is in the rangefrom 26.6 to 30.2°F (from -3 to -1°C)provides the product with a longershelf life than chilling (32°F or O°C).

Freezing occurs over a broad rangeof temperatures below 26.6°F (-3°C).However, freezing is a stepwise pro­cess, and it is not until the tempera­ture is lowered to O"F (-17.8'C) thatenough water is immobilized to effecta reasonable stabilization of productquality for to about 1 year. At highertemperatures, chemical reactions in­volving enzymes and oxygen willeventually degrade the product quali­ty. At lower temperatures the productquality will remain high for manymonths and even years.

Because salt lowers the freezingpoint of water, brine (usually a solu­tion of sodium cWoride) is used tofreeze large products like whole tunaby direct immersion. Short brine dipsare also used to chill fillets. Brines doimpart salt to the product, theamount depending on a number offactors, and they do tend to corrodeequipment.

Other freezing solutions that can beused for direct immersion of food in­clude liquid nitrogen, liquid carbondioxide, and a number of halocarbonssuch as Rl2 (dicWorodifluorometh­ane). The halocarbons, includingR12, R22 (monochlorodifluorometh­ane), and R502 a mixture of R22 andRl15 (monochloropentafluoroeth­ane), and ammonia are used in me­chanical refrigeration systems wherethere is no direct contact with thefood.

Conventional freezing techniquesand equipment described include platefreezing which involves sandwichingthe product between refrigeratedplates; immersion freezing which sim­ply involves the immersion of productin cryogenic liquids; spray freezingwhich occurs when cryogenic liquidsor gases are sprayed directly on theproduct carried by a conveyor belt;

April 1981, 43(4)

refrigerated air, a slow process occur­ring when a product is simply placedin a chamber that is held at freezingtemperatures (the sharp freezer is anexample); the jacketed freezer whichinvolves a double-walled chamber andcontrols humidity to 100 percent rel­ative humidity is another variation ofrefrigerated air; blast freezing whichinvolves refrigerated air that is drivenacross the product by powerful fans;and fluidized freezing which involvesthe suspension of product particlesthat are frozen as they are buoyed bycold air forced upwards. A conven­tional process in the agricultural in­dustry, dehydrocooling, has ademonstrated potential for freezingseafoods. A theoretical, but apparent­ly sound process, jet altitude freezing,has not yet been demonstrated but hasa scientific basis for its considerationand has withstood analytical critiquesregarding aerodynamic, economic,and technical feasibilities.

Literature CitedBaker, D. W., and S. E. Hulme. 1977. Mixed

species utilization. Mar. Fish. Rev. 39(3):1-3.

Barnett. H. J .• R. W. Nelson. P. J. Hunter.S. Bauer. and H. Groninger. 1971. Studieson the use of carbon dioxide dissolved inrefrigerated brine for the preservationof whole fish. Fish. Bull., U.S. 69:433-442.

Baumeister, T.• and L. S. Marks (editors).1966. Standard handbook for mechanicalengineers. 7th ed. McGraw Hill. N.Y.

Beckman, H. 1961. Method for conservingfresh fish. U.S. Patent 2.987,404. U.S.Pat. Off., Wash .• D.C.

Bezanson. A. F.• R. J. Learson. and W.Teich. 1973. Defrosting shrimp with micro­waves. Proc. Gulf Caribb. Fish. Inst.,25th Annu. Sess., p. 44-55.

Bramsnaes. F. 1957. Handling and chillingof fresh fish on vessels at sea. FAO Fish.Bull. 10(1):25-41. Food Agric. Organ.U.N .• Rome.

Carlson. C. J. 1969. Superchilling fish - areview. In R. Kreuzer (editor). Freezingand irradiation of fish. p. 101-103. Fish.News (Books) Ltd., Lond.

Carver, J. H. 1975. Vacuum cooling andthawing fishery products. Mar. Fish. Rev.37(7): 15-21.

Charm, S. E. 1971. The fundamentals offood engineering. 2nd ed. The Avi Pub!.Co., Inc.• Westport, Conn .• 692 p.

_-----:-----:-----:. R. J. Learson. L. J. Ronsivalli.and M. S. Schwartz. 1972. Organoleptictechnique predicts refrigeration shelf life offish. Food Techno!. 26(7):65-68.

____• H. E. Longmaid. III, and J. H.Carver. 1977. A simple system for extend­ing refrigerated nonfrozen preservation ofbiological material using pressure. Cryobi­ology 41 :625-636.

Duncan. D. W.• Jr.. and J. T. R. Nickerson.1961. Effect of environmental and physio­logical conditions on the exponential

growth phase of Pseudomonas frag;(ATCC 4973). In Proc. Low Temp. Micro­bio!. Symp.• p. 253-262. Campbell SoupCompany. Camden, N.J.

Fennema. O. 1976. The U.S. frozen food in­dustry: 1776-1976. Food Techno!. 30(6):56-61. 68.

Hulme. S. E .• and D. W. Baker. 1977. Chil­led seawater system for bulkholding seaherring. Mar. Fish. Rev. 39(3):4-9.

James. D. G.• and J. Olley 1971. Spoilage ofshark. Austr. Fish. 30(4):11-13.

Iicciardello. J. J .• L. J. Ronsivalli, and J. W.Slavin. 1967. Effect of oxygen tension onthe spoilage microflora of irradiated andnonirradiated haddock (Melanogrammusaeglefinus) fillets. J. App!. Bact. 30(1):239-245.

Nickerson. J. T. R.• and L. J. Ronsivalli.1979. High quality seafoods: the need andthe potential in the United States. Mar.Fish. Rev. 41(4):1-7.

Peters. J. A., C. J. Carlson. and D. W.Baker. II. 1965. Refrigerated seawateras a storage medium for fish. ASHRAE.April. p. 64-67.

---:=--,..,..,....~. J. W. Slavin. C. J. Carlson, andD. W. Baker. II. 1965. Storing groundfishin refrigerated sea water: Use of ultravioletradiation to control bacterial growth. InFish handling and preservation. pro­ceedings of a meeting on fish technology,September 14-17. 1964. in Scheveningen.p. 79-87. Organ. Econ. Coop. and Dev.Paris.

Power, H. E., and M. L. Morton. 1965. Ef­fect of superchilled storage on the qualityof gutted round cod. Fish. Res. BoardCan. New Ser. Circ. 23.

_..,.--,-_-:-::- • M. L. Morton, and R. E. Sin­clair. 1969. The chemical and physicalcharacteristics of cod stored at superchilltemperatures. In R. Kreuzer (editor),Freezing and irradiation of fish, p. 104­113. Fish. News (Books) Ltd., Lond.

Ranken, M.B. 1963. Some recent advances inrefrigeration machinery installations infishing vessels. Prog. Refrig. Sci. Techno!.,Int. Inst. Refrig., Comm. 8. 1773.

Roach, W. W., J. S. M. Harrison. and H. L.A. Tarr. 1961. Storage and transport offish in refrigerated sea water. Fish. Res.Board Can., Bull. 126.

---:-----:---:-_. and N. Tomlinson. 1969. Partialfreezing of Pacific salmon intended forcanning. In R. Kreuzer (editor), Freezingand irradiation of fish, p. 114-118. Fish.News (Books) Ltd., Lond.

Ronsivalli, L. J., and S. E. Charm. 1975.Spoilage and shelf life prediction of refrig­erated fish. Mar. Fish. Rev. 37(4):32-34.

---:-::-::-,--=:-' R. J. Learson. and S. E. Charm.1973. Slide rule for predicting shelf life ofcod. Mar. Fish. Rev. 35(7):34-36.

Semling, H. V. 1979. Liquid food freezantslace scrutiny. Food Process. 40(13): 18.

Slavin. J. W.• J. A. Peters. and S. R. Pot­tinger. 1958. Studies on a jacketedcold-storage room. Food Techno!' 12(11):602-609.

Spencer, R., and C. R. Baines. 1964. The ef­fect of temperature on the spoilage of wetwhite fish. Food Techno!. 18(5):175-179.

Veranth. M. F., and K. Robe. 1979. Co,­enriched atmosphere keeps fish fresh morethan twice as long. Food Process. 40(4):76-79.

Windsor, M. L.• and T. Thoma. 1974. Chem­ical preservation of industrial fish: Newpreservative mixtures. J. Sci. Food Agric.25 :993-1 005.

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