MFR PAPER 1151

Vacuum Cooling and Thawing Fishery Products

JOSEPH H. CARVER

ABSTRACT-A standard pilot plant size retort was modified for purposes ofdeveloping empirical information on dehydrocooling fresh fish andvacuum/heat-thawing (VHT) frozen fishery products. Results indicated thateffective multipurpose vacuum/heat-processing systems can be constructedfrom readily available commercial equipment. Dehydrocooling tests indicatedthat small fish such as whiting can be cooled to O°C in about 1/2 hour, and VHTtests showed that frozen blocks of headed and gutted (H&G) whiting andshrimp can be thawed in from 1/2 to 1 hour. The described equipment is usableboth ashore and at sea. At sea, it is proposed that enough heat energy to powerthe equipment can be extracted from waste stack gases.

INTRODUCTION

Several years ago, two novel process­es were patented for processingfish-one for rapidly cooling and storingcooled fish (Beckmann, 1961) and theother for thawing frozen fish blocks(Bezanson et aI., 1973). Though the ap­parent purposes of these patents appearto be diametrically opposed, the effec­tiveness of both described processes isdependent on the use of water to trans­fer heat energy under vacuum condi­tions.

The first of these two patents is basedupon dehydrocooling and was issued toH. Beckmann (1961). Vacuum precool­ing, or dehydrocooling, is effectivelybeing used today for cooling leafy veg­etables. This process takes advantageof the physical penomenon wherein theboiling point of water decreases with adecrease in atmospheric or absolutepressure. At an absolute pressure of760mm (14.7 inches) of mercury or normalatmospheric pressure, water wi]] boil atlOooe (212°F); at 4.5 mm (0.18 inch) ofmercury, liquid water will boil at oDe

Joseph H. Carver is with theNortheast Utilization ResearchCenter, National Marine FisheriesService, NOAA, Gloucester, MA01930.

(32°F). According to Beckmann's pa­tent, dressed fish are placed in an air­tight container, and when loaded, the airis evacuated from the chamber to pro­mote a rapid evaporation of water fromthe surface of the fish. Since heat energyis required to change liquid water togaseous water, the heat necessary tocause this change of state is supplied bythe fish; and, thus, the fish loses heatand is cooled. The amount of heatenergy given up by evaporation is about596 calories per kilogram lost water atoDe or 1,073 British Thermal Units(BTU) per pound of lost water at 32°F'.By evacuating the chamber to an abso­lute pressure of about 4.5 mm of mer­cury, the fish within can be cooled tooDe. Once cooled, the fish can then bestored within the chamber at oDe bymaintaining the absolute pressure atabout 4.5 mm of mercury. In order tocompensate for the loss of water duringthe cooling and storage stages, the fishare periodically sprinkled with waterwhile under vacuum. This added watercan, by dehydrocooling, freeze onto thesurface of the fish. Thus, without theuse of ice and refrigeration equipment,Beckmann (1961) claims that fish can beadequately cooled and stored in the

'Clarke-Built (WiUiams). Ltd., Thetford. England.Vacuum-Heat Thawing. Personal communication.

15

fresh state. Other advantages claimedare prevention of oxidative rancidity,inactivation of aerobic spoilage bac­teria, and the bleeding of the fish is en­hanced which results in lighter-coloredflesh.

The second patent is concerned withthawing and is known as the VacuumHeat Thawing (VHT) process. It wasspecifically designed to factory-thawlarge blocks of frozen foodstuffs-i.e.,100 pound (45kg) blocks of frozendressed fish. The process can be con­sidered to be the opposite of dehydro­cooling and thus takes advantage of notonly the fact that water will boil or steamcondense at a lower temperature undervacuum conditions, but also that thecondensation of water imparts heat.This is called the heat of condensationand is the opposite of the heat of vapori­zation which, again, is considerable.According to the patent, blocks of fro­zen fish are placed in an airtight con­tainer and the chamber is evacuated to apredetermined absolute pressure whichwill allow for a desired thawing temper­ature. For example, at an absolute pres­sure of 17.5 mm of mercury, a tempera­ture of20oe could be maintained. At thepreset pressure, steam is then meteredinto the chamber and will condense onthe surface of the fish giving up its heatof condensation. Under these condi­tions a kilogram of condensing steamwill give up about 585 calories of heatenergy (1,053 BTU per pound of steamat 68°F). By careful control of the abso­lute pressure and steam, it is claimedthat the fish can be rapidly thawed with­out heat damage. Additional advan­tages claimed are reduction of oxidativechanges, inactivation of aerobic spoil­age bacteria, and reduction of weightloss due to thaw drip.

-

Figure 1,-Pllot plant equipment used lor dehydrocoollng and vacuum heal thawing tests,

STEAMBOILER

~L PNEUMATICVALVE

RETORT

II

inally, high density polyethylene pipewas used; however, when steam wasallowed to escape through this pipe, thepipe distorted and caused severaljointsto open and leak thus causing a loss invacuum efficiency. The plastic pipe wasthen replaced with iron pipe of the samesize to eliminate this problem. Also, a101.5 mm (4-inch) iron gas cock wasinstalled in the vacuum line for quicklyapplying or shutting off the vacuum.

The vacuum chamber originally usedconsisted of a vertical retort of670 liters(23.6 cubic feet) capacity. A retort ap­peared to be ideal for the purpose be­cause its heavy walls could withstandthe stresses resulting from high vacuumand because it is capable of beingsealed. However, the vertical retort wasawkward to load, and it was replaced bya smaller horizontal retort whose inter­nal dimensions were 45.6 cm (18 inches)in diameter and 78.7 cm (31 inches) inlength. The capacity of this retort wasabout 128 liters (about 4.56 cubic feet).The only modification made to the re­tort was to weld in along the top centerline a 101.5 mm ID (4-inch) vacuum out­let containing a gate valve for opening orsealing off the vacuum to the retort.This modification in no way affected theusage of the retort for normal steamprocessing operations. This retort wasalso equipped with pneumatic controlsfor steam and air regulation, a steamsparger along its bottom length, and awater inlet.

Steam for V HT and for the steam jetwas supplied by a 98,095-watt (10 boilerhorsepower) steam boiler. Since theefficiency of the steam jet depends upon

~GATE VALVE

r-;::::::::=~ n...............-----.~

STEAM GASEVACTOR~OCK~

I lKINNEYPUMP

which cross through an air gap into aventuri tube. As the water crosses theair gap, air and other gases are entrainedin the water and are carried through theventuri and expelled. For economy'ssake, the water used is recirculated andstored in a tank of about 850 liters (30cubic feet) capacity. A provision ismade to add fresh cool water in theevent that the stored water warms. Thisunit is capable of generating a vacuumequal to that of the vapor pressure ofthe recirculated water, i.e., about 30mm (1.2 inches) of mercury absolutepressure at a water temperature of 21°C(69.8°F).

For the second stage vacuum, asteam jet evactor was used. In opera­tion, the steam jet evactor operates onthe same principle as a water aspiratorexcept that this pump uses steam as amotive force. Additionally, the steampassing through the jet condenses in thewater of the first stage thus helping togenerate higher vacuums. The steamjetevactor is dependent upon the first stagein that it does not become efficient un­less the absolute pressure at the dis­charge end of the steam jet is lowered toabout 500 mm (19.7 inches) of mercury.With both vacuum sources in full opera­tion, using 0.046 kgf/mm 2(65 psi gauge)steam, this system was capable of at­taining absolute pressures as low as 0.3mm (0.012 inches) of mercury. This ab­solute pressure is equal to the vapor ofice at -29.5"C (_21°F) which was morethan adequate for the intended work.

These vacuum sources were con­nected to the retort using a 101.5 mminside diameter (l D) (4-inch) pipe. Orig-

'One psi = 7.03 x 10 • kilogram-force per squaremillimeter.'Reference to trade names does not imply en·dorsement by the National Marine Fisheries Ser­vice. NOAA.

DESCRIPTION OFTHE EQUIPMENT

The unit designed and constructedconsisted of a two-stage vacuumsource, a horizontal food processing re­tort, and a steam boiler as shown inFigure I. All components used werereadily available and usable both at seaand ashore.

A Kinney Liquid Jet3 vacuum pump,Model KU-120, was chosen as the firststage in the vacuum system. This wasbecause of the availability of water andbecause this type of vacuum pump issimply a large water aspirator capable ofhandling large volumes of water vaporas might be expected from VHT anddehydrocooling. In operation, a 10horsepower centrifugal pump driveswater under high pressure through aplate with holes to create a series of jets

About the time that these inventionswere made known, an engineering study(Lange 1946) indicated that with theproper heat exchangers, as much as 770kg (1,700 pounds) of0.07 kilogram-forceper square millimeter (kgf/mm2 ) (100pSi)2 steam could be generated from theheat lost in the exhaust stack of an aver­age New England trawler. This is morethan enough energy needed to power alarge vacuum unit. It appears then thatvacuum systems could be operatedaboard the trawlers for only the cost ofthe proper equipment and its installa­tion.

To the best of our knowledge, theseprocesses are not being commerciallyutilized by the fishing industry. Possiblereasons for this are that there is no firminformation available on these vacuumprocesses, and the apparent costs of theequipment and manpower needed todevelop the required information arebeyond the means of our hard-pressedfisheries. Thus, because of thesereasons and the benefits claimed by theinventors, the Northeast UtilizationResearch Center undertook work to ob­tain empirical information on the designand development of a steam-poweredpilot plant vacuum processing systemand on developing empirical data on thefeasibility ofdehydrocooling and VHT.

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Figure 2.-Typical retort evacuation IImeslllustraling the capability olthe equipmentto rapidly produce suitable vacuums lor dehydrocoollng and vacuum heat thawing.

the use of dry steam, a steam separatorwas installed in the steam line as close tothe steam jet as possible. This separatorremoves water condensate from steamand allows only dry steam to pass.

Once constructed, the equipmentwas tested for leaks and overallefficiency. Leaks were detected byfilling the system with a halogenated gasto a little above atmospheric pressureand using an electronic halogen leak de­tector. All leaks found were sealed withsilicone rubber gasket material. Somevery small leaks remained as evidencedby a gradual loss of vacuum in thechamber after it was sealed off andevacuated. However, these leaks weretoo small to affect the outcome of theproposed tests. Subsequent tests indi­cated that when using only the Kinneypump with 18.3°e (65°F) water, an abso­lute pressure of20 mm gauge (0.79 inch­es) was obtained in 32 seconds. Sincethe vapor pressure of water at thistemperature is [5.8 mm of mercury, theKinney pump was capable of delivering99.4 percent of the total vacuumtheoretically obtainable or (760 mm ­20 mm/760 mm - 15.8 mm) ( 100). Whenusing both vacuum systems, an absolutepressure of 4 mm (0.16 inches) of mer­cury was obtained in 72 seconds. Thiscorresponds to the vapor pressure ofwater at -1.7°e (28.9°F). When the sys­tem was tightly sealed, absolute pres­sures of about I mm of mercury (0.04inch) were readily attainable. This vac­uum would correspond to the vaporpressure of ice at - 17°e (1.4°F). Somecontrol over the degree of vacuum wasachieved by closing down the gatevalve, and thus the temperatures withinthe chamber could be roughly con­trolled. Typical evacuation times areshown in Figure 2 when using 0.018 to0.070 kgf/mm 2 (25 to 100 psi gauge)steam pressure through the steam jet.

were used in an effort to determine theeffect of individual size on rates of de­hydrocooling. Preliminary work indi­cated that for best results, the fishneeded to be spaced apart from eachother, that is, not touching each other,so that the whole surface area of the fishwas available for dehydrocooling. Ac­cordingly, fresh wet fish were spacedsingle layer on an expanded metalscreen. In the first three series reportedon, the fish were very wet so as to read­ily drip water; and in the last two series,the fish were wet but not dripping.Thermocouples were inserted into thefish at sites immediately below the sur­face of the skin and next to the back­bone at the thickest portion of the fish.These latter thermocouples were about13 mm (0.5-inch) deep in the small fish toabout 25 mm (I-inch) in the larger fish.This allowed the temperatures of the

fish to be continuously recorded at5-second intervals throughout each test.The fish were then loaded into the re­tort, the retort closed and quicklyevacuated to an absolute pressure of2-4mm (0.08-0.16 inch) of mercury. Thesepressures correspond to the vapor pres­sures of ice from -9.7°e to -1.6°e(14SF to 29. 1°F). Since the vacuumsystem had a tendency to reduce theabsolute pressure below 2 mm (0.08inch) of mercury, attempts were madeto regulate the degree of vacuum byclosing down the gate valve. Results ofthe dehydrocooling tests are summa­rized in Table I, and a typical dehy­drocooling curve is shown in Figure 3.

In the first three series, the surface ofthe fish cooled to ice temperature in 8minutes regardless of the size of the in­dividual fish and regardless of the start­ing temperature. However, as either the

DEHYDROCOOLI NG

Upon completion of the unit, dehy­drocooling and VHT tests were con­ducted. For the dehydrocooling tests,fresh headed and gutted (H&G) whitingwas used since this species is in goodsupply in this area. The individualweight of the fish used ranged from 100to 150 grams (0.22-0.33 pound), exceptin one series where larger, 600-700grams (1.3-1.5 pounds), H&G whiting

Figure 3.-Typical dehydrocoollng curve showing the rapidity of the process andrelationship between surface and internal temperatures during dehydrocooling.

17

Table 2.-Summary 01 vacuum heal thawing lests on whiting and shrimp using 0.06 kgl/mm2 sleam pressure.

TemperatureTOlal

Block Tolal thawingsize weighl Maximum at timel Percent

Sample (kg) (kg) Inilial center line (min) thawed

Glazed 4.54 18.2 -4.4 05 24 60

WhitingBlocks 4.54 18.2 -55 2.5 60 100

Glazed '2.27 7.25 -6.1 61 1 60Shrimp 2.27 2.27 -7.2 20.0 8 95Blocks 4.54 9.1 -9.4 15.5 13 80

2.27 15.9 -11.1 23.4 51 952.27 9.1 -5.5 15.5 31 902.27 11.1 -3.2 15.0 26 80

'Total time of thawing within the chamber.'Loosely packed and lightly glazed.

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Table 1.-Dehydrocooling headed and gUlled Whiting.

Total Size Start- Time needed tosample of ing dehydrocool fishweight fish temp. 10 O'C (min)(kg) (g) ('G) Surface Interior

1.5 100-150 21.0 8 156.8 100-150 17.0 8 2880 600-700 18.5 8 331.0 100-150 21.0 1 141.0 100-150 5.0 1 7

total mass of the fish increased from 1.5to 6.8 kg or as the size of the individualfish increased from 150 to 600 grams, therate of interior cooling decreased. Whenthe tray was loaded from just a few fishto a maximum number of small fish (6.8kg), the time to cool the interior of thefish about doubled, i.e., from 15 to 28minutes. This indicated then that theinterior cooling rate is dependent uponthe total number of fish, or load, beingdehydrocooled. With respect to indi­vidual fish size, the interior rate of cool­ing decreased as the size of the fish in­creased. Small 100-150 gram fish werecooled to ice temperature along thebackbone in 15 minutes, whereas 33 min­utes were needed to cool 600-700 gramfish (about twice as thick) to this tem­perature along the backbone. This is tobe expected inasmuch as the heat fromthe interior ofthe fish travels by conduc­tion, and fish flesh is a poor conductor ofheat.

In the last two series when no excesswater was available on the surface ofthe fish, the surface of these fish cooledto ice temperature in about a minute.The interior temperatures of thesefish cooled to ooe in 7-14 minutes, de­pending upon the starting temperature.These two series illustrate the rapidityof dehydrocooling at low absolute pres­sures when no excess water is available.However, these fish tended to becomerapidly dehydrated to a point wheretheir appearance was adversely af­fected. The damage caused by dehydra­tion could not be wholly eliminated bythe later addition of water. In the firstthree series, dehydration was first evi­denced by a slow increase in surfacetemperature. This was probably causedby subsurface heat warming the skinsurfaces after most of the water evapo­rated from the surface of the fish.Further dehydration was averted byspraying the fish with water while undervacuum. For this purpose, a waterspray system was built into the retort.

This additional water not only pre­vented surface dehydration, but alsoaided in further dehydrocooling. Oncethe fish was cooled, the sprayed-onwater froze to form a protective iceglaze over the nonfrozen fish. Thus, itappears that though excess water slowsdown the dehydrocooling process byincreasing the load to be cooled and byadding some heat energy, some excesswater is needed to prevent damage bydehydration.

During the dehydrocooling process,it was noted that the ambient tempera­ture within the chamber cooled to about3°e (3rF) and remained at this tem­perature even though the fish was at O°c.The temperature of the retort shell didnot change, thus indicating that therewas little or no heat being exchangedthrough the "atmosphere" within theretort. Further, ice-glazed fish werestored in the chilled state for periods of5-6 hours by maintaining an absolutepressure of 4-6 mm. An examination ofthe treated fish indicated that some ofthe ice glaze was lost to evaporation butthere were no other adverse effects as aresult of dehydrocooling and the shortstorage period.

These results then indicate that whit­ing, and probably other fish of similarsize, can be rapidly cooled to ooe bydehydrocooling under conditions thatarrest oxidative changes and the out­growth of spoilage microorganisms.There is an indication that once cooled,the fish can be stored under a vacuumwithout additional refrigeration. How­ever, since this process depends uponthe removal of heat by the vaporizationof water, care has to be exercised toprevent dehydration damage to the fish.This can be done by the addition ofwater during the dehydrocooling and

storage periods. The frequency and theamount of additional water would prob­ably be dependent upon the capacity ofthe system and the degree to which thesystem is loaded.

VACUUM HEAT THAWING

Vacuum heat thawing tests wereconducted on commercially preparedfrozen blocks of H&G whiting andshrimp. Initial tests were conducted inthe aforementioned vertical retortwhere larger sized samples could behandled but the process could not beeasily controlled. In these tests 4.54-kg(IO-pound) blocks of H&G whiting andeither 2.27-kg (5-pound) or 4.54-kg(IO-pound) blocks of shrimp wereloaded onto an expanded metal traywithin the retort. Thermocouples wereinserted near the center midline and justbeneath the outside sUlface of theblocks for continuous temperaturemonitoring. The retort was then closedand evacuated to an absolute pressureof about 20 mm. Once the pressure wasreached the vacuum line was shut offand steam at 0.06 kgf/mm 2 (85 psi gauge)was allowed to expand into the chamberuntil an absolute pressure of about 50mm (1.95 inches) of mercury wasreached_ At this pressure, the surfacetemperature of the blocks rapidly in­creased to about 38°e (100°F). In orderto avoid overheating the samples, thevacuum was reapplied to dehydrocoolthe sUlfaces to about 22°e (72°F). Oncecooled, the cycle was repeated continu­ously until the thermocouples along themidline of the blocks indicated that theblocks had reached a temperature ofooe (32°F) or higher. Thereupon. theretort was opened and the sampleschecked for degree of thaw. If not

Table 3.-Summary of vacuum heat thawing tests on Whiling using various steam pressures and application ofsteam.

'The 0.45-kg sample (1-pound) size consisted of 3·5 H&G whiting frozen together in a block and the 2.27-kg samples(5-pound) consisted of 8-12 H&G Whiling frozen in a block. The 0.227-kg sample (0.5-pound) size consisted of 6 ormore H&G fish individually frozen and weighing about 0.227 kg (0.5 pound) each.'Full vacuum system used. Absolute pressures of 4.5 mm were used lor dehydrocooling.'One psi~7.03 x 10 --4 kgllmm'.

Sample Steam Initial Maximum Thawsize pressure Sleam temperature temperature time(kg)' (kgf/mm')' application (0G) (0G) (min)

0.45 0.0175 continuous -17.8 367 150.45 0.0352 cycled -17.3 36.7 262.27 0.0175 continuous -17.8 33.9 272.27 0.0175 cycled -17.8 36.7 452.27 0.0352 cycled -15.0 37.8 572.27 0.0703 cycled -12.8 36.7 582.27 00175 cycled' -7.8 37.3 740.227 0.0175 continuous -7.8 31.1 8·90.227 0.0352 pulsed -17.3 29.4 17

reasonably thawed, the samples werereturned to the retort and the processrepeated.

Results of these initial YHT tests aregiven in Table 2. These results indicatethat though the apparent temperaturewithin the block was above ooe, theblocks were not necessarily thawed.This was because though the blocks ap­peared to be solid, pockets ofair and icewere later found among the fish andshrimp.

In the case of whiting, few enclosedpockets of air or ice were present and itwas found that by increasing the tem­perature to a few degrees above freezingnear the center of the block, the fishwere thawed to the point where they

could be easily separated and filleted.About I hour was needed to completely

thaw the 4.54-kg (lO-pound) blocks ofwhiting. With respect to shrimp, manypockets of air and ice were presentthroughout the blocks. The steam ap-

parently was able to enter the air pock­ets and transfer its heat to the shrimptissue. Thus, very loosely packed and

lightly glazed shrimp blocks became 60percent thawed when treated to YHTconditions for I minute. When theshrimp blocks were tightly packed and

more heavily glazed, the VHT time wassignificantly increased. Under the con­ditions of these tests, this thaw time var­ied primarily because the amount ofglaze and pockets of ice among the

blocks varied. When the temperatureof areas along the center line of theseshrimp blocks reached 6-lOoe, manypockets of ice and frozen shrimp stillremained. However, it was noted thatwhen the internal temperature of theseshrimp blocks reached 15-23.4°e, theblocks were 80-95 percent thawed with­out heat damage. The thaw time forthese shrimp blocks then ranged from 8to 51 minutes, depending on the amount

of glaze, the size of the blocks, and thesize of the load in the chamber. It wasalso observed that these shrimp wouldprobably thaw faster if some provision

was made to remove the thawed shrimpon the surface of the block from thestill-frozen shrimp in the interior of theblock. As the outside of the blocksthawed, the thawed shrimp tended toform an insulated mat, thus obstructingthe flow of heat into the block's interior.

Upon installation of the horizontal re-

tort, more detailed VHT tests weremade on H&G whiting. Since thecapacity of the horizontal retort wasless than that of the vertical retort, smaU­er sized samples were used throughoutthese tests. Generally, the procedurefor YHT was the same as that previ­ously described except that, as noted, insome tests the pneumatic controls onthe steam lines were used to metersteam into the retort during VHT. Bycareful throttling of the gate valve invacuum piping and by recalibrating theautomatic pneumatic controls, temper­atures of 29± loe (85± 1.8°F) could bemaintained throughout the YHT testperiods. Under these conditions, steamwas continuously metered into thechamber.

Resuits of these tests are given inTable 3. It is evident from these resultsthat VHT can be accomplished muchfaster by a continuous application ofsteam than by a cycled application. Thisis because with the latter, the surfacesof the products heat up much faster witha full flow of steam causing a need toremove excess heat by dehydrocooling,an extra step in the procedure. Thus,with a continuous flow of steam to main­tain a temperature about 36°e (97°F),the thawing times were nearly halved,i.e., 8 to 27 minutes for 0.227- to 2.27-kgsamples with a continuous flow of steamas compared with 17 to 45 minutes forsimilar size samples using cycled steam.These data also indicate that VHT wasfaster with lower steam pressures thanwith higher steam pressures, i.e., for asample size of2.27 kg, 45 minutes wereneeded to thaw the fish when using0.0175 kgf/mm2 (25 psi gauge), as com­pared to 58 minutes when using 0.0703

19

kgf/mm2 (100 psi gauge) steam. Thiswas because when using higher pres­sure steam the pressure was moredifficult to control, the tendency foroverheating increased, and there was aneed for longer dehydrocooling time.The greater efficiency of the lowersteam pressures also tends to indicatethat the major source of heat energy forthawing came from the heat of conden­sation. Lastly, when dehydrocoolingwith an absolute pressure of 4.5 mm(0.018 inches) of mercury, excessivecooling of the surface of the samplesoccurred, thus increasing the timeneeded to thaw the samples. For exam­ple, when using the full vacuum systemfor dehydrocooling at an absolute pres­sure of 4.5 mm (0.018 inches) of mer-

cury between cycles of steam injection,74 minutes were needed to VHT a2.27-kg sample when using 0.0175kgf/mm 2 steam. Under the same condi­tions, except that an absolute pressureof 18 mm (0.71 inches) was used, thesamples were thawed in 45 minutes.

With respect to YHT whiting at amaximum temperature without causingphysical damage to the fish, it was foundthat the skin of the fish began to cookwhen exposed to temperatures of43-46°e (J 10-1 15°F) for a few seconds.When this happened, the skin becamegummy and began to slough offfrom theflesh below. This was also true for otherthin-skinned species such as herring.Exploratory VHT tests indicated thathigher maximum thawing temperaturessuch as 55°e (130°F) could be used onother species, such as cod and oceanperch, without causing physical dam­age. These latter species have thickerskins which apparently offer greater

protection from the rigors of the pro­cess.

It is doubtful that this process couldbe used to temper fishery products, i.e.,to raise the temperature of a frozen prod­uct, without thawing, in order toachieve a degree of hardness suitablefor easy cutting with a saw. However,where there is a need to thaw the prod­uct prior to further processing, thedata show that the process is rapid andeffective. For example, experiencewithin the laboratory indicates that24-48 hours were needed to thaw 2.27­to 4.54-kg (5- to 10-pound) blocks offishor shrimp in air at 2-4°e (35.6-39.2°F)and about 1-3 hours in 18.3°e (65°F)water. By the VHT process, these prod­ucts can be thawed in as little as 10minutes to I hour, depending upon thecapacity of the equipment, the size ofthe load, steam pressures, the applica­tion of steam, and the presence of extrawater or glaze. It should be pointed outhere that by the use of microwaves'some hard-frozen products can be veryrapidly tempered or thawed, dependingupon need. For example, 50 pounds ofhard-frozen shrimp were thawed perhour per kilowatt energy to the pointwhere they could be separated andfurther processed.

CONCLUSIONS

These exploratory studies haveshown that vacuum processes could beapplied for dehydrocooling and forVHT fishery products. The compo­nents needed are simple in design andoperation. Moreover, an engineeringstudy has indicated that enough energycan be recovered from waste stack heatto power the needed equipment. Thus,the equipment could be designed for useat sea as well as ashore where power isreadily available.

Good results were obtained in dehy­drocooling of small fish such as H&Gwhiting and herring. The exterior ofthese fish cooled rapidly, that is, fromambient to freezing temperatures inonly 8 minutes. One-half hour wasneeded to cool to ooe (32°F) all the in­terior portions of fish of about 50 mm (2inches) at their thickest point. How-

'Moore. F. E., P.O. Box 196, Rye Beach, N.H.Personal communication.

ever, in order to rapidly cool, it wasnecessary to expose the whole surfaceof the fish to the vacuum. This indicatesfish cannot be rapidly cooled by the de­hydrocooling process unless these fishare spaced apart, i,e., in layers one fishdeep rather than in large masses as isnow the practice. The largerthe fish, themore important would be the need toseparate the fish in order that the heatfrom the interior portions can be re­moved. Because the cooling processdepends upon the evaporation of water,there is a danger of overly drying thesurface of the fish. The danger can bereadily avoided by sprinkling the sur­face of the fish with water while under avacuum, and again, this is best done onlayered fish. The additional water by itsevaporation aids in the overall coolingeffect. Once cooled, the added waterthat has not evaporated will freeze overthe surface of the fish to form a protec­tive ice glaze. If judiciously done, thisglaze can be formed over nonfrozen fishinasmuch as the freezing point of fishflesh is about -2°C: (28°F). There is nofurther need to keep the cooled fishspaced apart so that the fish can bestored in a massed form. If stored undera maintained absolute pressure of about4.5 rom of mercury in an airtight con­tainer, the fish will remain cool. Only aperiodic addition of water would beneeded to replace that water lost by con­tinuing evaporation. This container, ifproperly constructed, could also serveas a shipping container, thus also aidingin the transport of the fish.

Good results were obtained in V HTblocks of H&G whiting and shrimp.Normally 1-2 days are needed torefrigerator-thaw these products. Sev­eral hours are needed for thawing in18°C (65°F) water. Bacterial degrada­tion can occur during these thaw timesand especially in the case of water­thawing, the bacterial contaminationcan be rapidly spread by the water. WithVHT, these products can be completelythawed within I hour in a manner thatdoes not promote the spread ofbacterialcontamination. The thaw times givencould probably be reduced in some in­stances where the product is cookedduring later reprocessing. For example,since some breaded shrimp are partiallycooked prior to refreezing and sale,shrimp blocks for this purpose could be

20

V HT at higher temperatures and short­er periods of time. Any cooking dam­ages that might occur would probablybe of no consequence in cookedbreaded shrimp. It should be pointedout here that much has been done in de­veloping the technology on the use ofmicrowave energy for thawing and/ortempering frozen seafoods for fartherprocessing. For example, with one mi­crowave unit hard-frozen shrimp werecontinuously thawed for further pro­cessing at the rate of about 22.7 kg (50pounds) per hour per kilowatt energy.Where the microwave process is a con­tinuous process, V HT is a batch-typeprocess. Thus, to a processor who hasneed of rapidly thawing fishery prod­ucts, VHT appears to be an attractivepotential process.

Though no bacteriological work wasdone under the conditions of dehy­drocooling and VHT, it is quite con­ceivable that at least during the timethese processes are being applied, bac­terial degradation is arrested. It is be­lieved tha t most of the bacteria on fish­ery products are aerobic, i.e., they needair to grow and function. By the verynature of these processes, nearly allthe air that sustains these bacteria isremoved and thus they would cease tofunction.

It should be noted here that the basicequipment used for VHT is the same asthat used for dehydrocooling. Thus theequipment is quite versatile since it canbe used for VHT, dehydrocooling, re­torting cans of food, and as a pressurecooker for such products as crabs.There are indications that this basicequipment can be used for other pur­poses. Some suggested uses are tobriefly treat the surfaces offishery prod­ucts with high temperature steam in aneffort to destroy surface spoilage bac­teria. It is believed that most of thesebacteria live on the surface of the fishand most are psychrophiles or cold­loving. I t is further believed that thesebacteria can be easily destroyed by mildheat. Another suggested use is to de­stroy surface bacteria by the use ofsterilant gases.

Lastly, these results also indicate thatthese processes probably can be usedfor processing other foodstuffs. Sincefresh fish can be dehydrocooled, otherflesh foodstuffs could probably be

rapidly cooled. The crileria here wouldbe that the foodstuff be wel and offer alarge surface area with relation to prod­uct weight. For example, freshly de­feathered and eviscerated chickencould probably be cooled quite rapidlyboth inside and out by this process.Moreover, the process would promotebleeding, an added benefit. With re-

spect to YHT, this process might be ofsome benefit to those industries that re­process frozen foods. An example ofthis would be the meat industry wheremeal is frozen for later processing intohamburger. As with fish, the frozenmeat could probably be effectivelyYHT in a rapid manner which wouldlend to maintain the quality of the meat.

LITERATURE CITED

Beckmann, H. 196 J. Method for conservingfresh fish. U.S. Patent 2,987,404. U.S. PatentOff., Wash., D.C. 2023 J.

Bezanson, A., R. Learson, and W. Teich.1973. Defrosting shrimp with microwaves.Proc. GulfCaribb. Fish. Inst., 25th Annu. Sess.,p. 44·55.

Lange, N. A. 1946. Handbook of chemistry. 6thed. Handbook Publishers, Inc., Sandusky,Ohio.

MFR Paper 1151. From Marine Fisheries Review, Vol. 37, No.7, July1975. Copies of this paper, in limited numbers, are available from 083,Technical Information Division, Environmental Science InformationCenter, NOAA, WaShington, DC 20235. Copies of Marine Fisheries Revieware available from the Superintendent of Documents, U.S. GovernmentPrinting Office, WaShington, DC 20402 for $1.10 each.

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