Tuna Processing

8/3/2019 Tuna Processing

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Management TraineeVery often, a trainee's assignments are rotated among thevarious departments in order to develop familiarity with the whole organization and itsfunctions.Trainees may also get classroom instruction in subjects may include lectures, guest speakers,projects, and oral presentationsThe main objective of canning is to obtain a shelf-stable product that can be stored insuitable containers for a considerable length of time (at least two years) without

undergoing food spoilage, while retaining desirable nutritional and sensory qualities.

To achieve satisfactory shelf-life the following conditions must be observed:

i. The contents of the cans must be commercially sterilized.

ii. The inside of the can must be resistant to and damaging effects from the contents and the

outside must be resistant to corrosion under reasonable storage conditions.

iii. The can ends must be sealed to prevent ingress of water and/or air or any form of

contamination.

4.2 Processing Operations

4.2.1 General

Fish should be chill stored at a temperature between 0 and 2 C, or sorted frozen in

freezers at temperatures below -28 C.

When frozen fish is used, it must be thawed before grading and/or dressing.

In the following descriptions of the processing stages for various products, operations which aresimilar are described in Chapter 4.2.2 "Description of canning tuna in brine", and subsequently

only mentioned.

4.2.2 Description of canning tuna in brine

The description is related to canning plant with a capacity of 20 tons whole raw fish

(bluefin or yellowfin tuna) per 8 hours. The overall yield is approximately 50-55% which

gives approximately 10 000 450 g (1 pound) cans or 20 000 225 g (1/2 pound) cans per 8

hours. See Figure 4 "Layout for tuna cannery".

Simplified flow sheets for canning tuna in brine, tuna flakes with vegetables and tuna. pet food

are shown in Figures 5, 6 and 7 respectively.

Sequential processing operations for canning tuna are described as follows:

a. Frozen tuna is thawed, preferably, by means of running water at a temperature of 10-15 C. Loss

during thawing is 0.5-1.0%.


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Holding of frozen tuna for long periods before thawing may lead to oxidation of fat

resulting in a yellow to orange discolouration on the surface of the cooked loins. Usuallythis surface discolouration can be removed when the fish is cleaned.

b. Longitudinal cuts are common with large sized tuna and the viscera are removed from the fish

on board fishing vessels prior to freezing. Bonito and skip jack are frozen with viscera. Oncethawed, the tuna is washed and inspected for spoilage. If tuna is not eviscerated on board

vessels this must be done in the plant. The splitting and evisceration procedure is the only

butchering operation performed on the tuna while it is in the raw condition. All other cleaning is

performed after the tuna has been cooked. Loss of weight is approximately 24-27%.


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c. The tuna is given a pre-cook by heating at a temperature in range of 102 to 104 C.This operation is necessary to make it possible to hand pick the light meat fromthe carcass and also to remove some of the oil from oily fish.

The fish is placed in baskets which are placed on racks. The racks of butchered fish arerolled into the cookers which are usually of rectangular cross section and made of

reinforced steel plate with a door, or doors, at one or both ends. The pre-cooking is abatch type operation.

The pre-cooking time for individual batches varies widely according to the size of tuna.For example, the cooking time may vary from 1 1/2 hours for small tuna to 8 to 10 hours,or more, for larger tuna. Loss of weight is approximately 22-26%.

Steam is admitted through a steam spreader on the floor of the cooker. Steam vent anddrain valves are provided to permit removal of air and condensate. Pre-cooking may alsobe carried out in boiling brine.

d. Tuna is cooled thoroughly to firm the flesh before the manual cleaning operation can beperformed. Loss of weight is approximately 3-5%.

e. After the pre-cooking and cooling operations, tuna is individually cleaned. The head isremoved and the fish is skinned and split into halves before removing the tail andbackbone. The loins are produced by splitting the halves of the fish along the medianline. Red meat is then removed from each loin; the blood and dark meat are scrapedaway and the loins, edible flakes and waste products are separated; of these portionsapproximately 15% is flake tuna.

f. The production of solid packs was formerly a hand-packing operation, but is now carriedout by machines. This machine produces a cylinder of tuna loins of uniform density from

which can be cut can-zised segments of uniform weight.

Chunk packs are produced from loins which are cut on a moving belt by means ofreciprocating cutter blades. The cut loins are then filled into cans by tuna filler machines.

Flakes and grated tuna, which is produced from broken loins and flakes, are J packed inthe same way as chunk packs.

g. The open cans next pass the line where additives such as salt, vegetables and finallyeither water or oil are added. Oil should be added slowly over a sufficient stretch of theline to permit its thorough absorbtion by the tuna meat. When oil is not added anequivalent amount of water replaces it. The oil temperature is recommended to be 80

C-90 C.

h. Small cans may be closed, without a vacuum, and processed directly, whereas largerones must be vacuum sealed.

As the pressure in the can increases considerably during heat processing , the vacuumis necessary to minimize the pressure increase in order to reduce the chance ofdistortion (peaking) and damage to the double seam.


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After processing and cooling the formation of .the vacuum causes the ends of the can toassume a concave profile which is characteristic of vacuum packed and hermeticallysealed cans. The vacuum also reduces the residual oxygen content in the can andtherefore the extent of internal corrosion.

In order to form a vacuum, cans are seamed by using either vacuum seamers or an

exhaust system.

When using the exhausting method the lids of the cans are first clipped or clinched on tothe body in such a way as to allow free passage of gases and vapours out of the can.The can and contents are then heated by passage through an exhaust box. The lid isseamed to the can immediately it emerges from the exhaust box, so that when thecontents cool a vacuum is obtained. Thus the system relies on sealing the can while thecontents are hot and allowing product contraction to create the vacuum.

An alternate method of achieving a vacuum in sealed cans is by using vacuum seamers.These machines close the cans and while so doing draw the air out thus creating avacuum.

i. The double seaming method is usually used to seal metal containers. The seam iscreated in two operations. See Figure 8. "Seaming Operation -Double Seam (CAC/RCP10-1976) .

The can, with the lid (can end) placed or clinched on top, stands on a base platewhich is raised so that a chuck fits into the countersink part of the lid, holdingboth in position.

The can end which is lined with a plastisol sealing compound is crimped intoplace so that it forms the so-called "cover-hook" around the lip of the container

body.

The "cover-hook" and the enclosed lip of the container are folded down againstthe container and interlock about the "body-hook". Both hooks overlap to form astrong joint which acts as a hermetic seal.

The sealing compound renders the seam air tight (hermetic). Around its circumference thedouble seam consists of five layers of metal -three layers of the can end and two layers of thecan body, however at the intersection with the side seam there are seven layers of plate, theextra two being due to side seam overlap.

The seaming operation must be monitored throughout the processing and visual inspections

should be carried out at least every 30 minutes (Warne, 1993).

Good manufacturing practice indicates that the. overlap should be at least 45 % of the internalseam lenght to ensure that the seam will function correctly and resist to minor abuse.


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Figure 8 Seaming operation -double seam

j. The sealed cans are transferred by a conveyor through a can washer which cleans thecans in detergent and water before discharging them into retort baskets. The retortbaskets are transferred into the retort and the cans sterilized.

Table 7Examples of retorting temperatures and times for canned tuna

Nominal capacityof cans

Alternative Processing temp.(C)

conditions time(min)

1.8 kg (4 pound) I 116 230

II 121 190

450 g {l pound) I 116 95

II 121 80

225 g (l/2 pound ) I 116 75

II 121 45

112 g (l/4 pound) I 116 65

II 121 40

All canned fish products are sterilized at temperatures above 100 C. Sterilization takes place in

retorts, with or without water. Overpressure is between 2-3 kg/cm. Processing conditionsshown are suitable for those canneries , operating under conditions of good manufacturingpractice. Individual canneries may select different processing times and/or temperatures to suittheir manufacturing requirements.

The simplest and most common retorts today are horizontal, or vertical, batch retorts.

The following general description. applies to processing in batch retorts using saturated steamas the heating medium.


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After the retort is loaded the door or lid is closed and the seal is checked to confirm that all thelugs are fastened securely. The temperature recorder is checked to ensure that it is workingcorrectly. Following this the vents and bleeders are opened and the drain and overflow areclosed {unless the over flow is used for venting).

The retort is now ready for operation during which the following operational procedures should

be adopted:

Steam is admitted by gradually opening the controller and the steam by passlines.

When the correct venting temperature is reached (>100 C) and/or the specified,vent time has elapsed, the vents are closed. It is bad practice to vent less thanthe recommended time; nor should reliance be placed on agreement betweenthe mercury thermometer and pressure gauge readings as a criterion forcomplete air elimination, as this is not necessarily a true indication of the requiredcondition. If the pressure gauge is reading high while the temperature is readinglow, there is still air in the retort and venting should be continued until agreementbetween pressure reading and the corresponding retort temperature is reached.

Gradually close the bypass as the retort approaches the processing temperature.This will prevent a sudden drop in temperature as the steam supply is cut whenthe retorting temperature is reached.

When the retort has reached the processing temperature, check the temperatureindicated on the mercury and recording thermometers. While it is not serious ifthe thermograph indicates a temperature slightly lower (say 1 C) than themercury thermometer, it is most important that it never reads higher. At all timesthe mercury thermometer should be used as the reference, for indicating trueretort temperature.

At the start of the process, record on the production records the time, themercury thermometer reading, the pressure, and the temperature indicated bythe recording thermometer.

Keep a record of the come-up time to make certain it has been long enough to .allow sufficient venting.

Maintain the retort temperature at the recommended processing temperature. Throughout the process, check that the specified temperature is being

maintained. Leave all bleeders wide open during the entire process. When the recommended processing time has elapsed, turn off the steam and

immediately start the cooling cycle.

k. When processing medium sized or larger cans (say greater than 250 g) in retorts usingsteam it may be necessary to cool the cans under pressure so that the ends do not peakduring cooling. Steam may be used to maintain the pressure but compressed air is more

usual. The cooling time depends on the processing temperature, the temperature of thewater used for cooling, the can size and the nature of the pack (i.e., liquid to solid ratio).

l. If necessary the cans should be washed before temporary storage, however under noconditions should the processed cans be manually handled while wet.

4.2.3 Description of the processing operations of an automatic canning linefor skipjack


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The descriptions are related to a plant with a capacity of 10 tons whole fish per 8 hours. Theoverall yield is 50-55% and approximately 9 000-10 000 450 g (1 pound) cans are produced per8 hours. See Figure 9 "Layout por skipjack cannery". The weight of the fish to be processed isapproximately 2 to 5 kg per fish.

a. From the chill room the whole fresh skipjack is brought to a bulk elevator which

transports the fish to a gutting machine which consists of a semi-automatic machine inwhich the fish is placed with its belly upermost. A rotating knife opens the fish andremoves the viscera.

The fish is conveyed to a rotating nylon brush under which it is cleaned by water spraynozzles.

After evisceration the tuna is conveyed to an inspection table where the final cleaning iscompleted manually; or where those fish, too large for the gutting machine, can bebutchered. At the discharge of the gutting machine the offal is directed to a rotating filterdrum for separations of water; the retained offal, is pneumatically pumped to containers.


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Figure 9 Layout for skipjack cannery


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b. Inspectors examine the fish, while it passes on the conveyor, to ensure that is hasbeen satisfactorily cleaned.

c. Fish are automatically size-graded to assist in feed to the rotor cooker andregulation of cooking time.

d. Rotory cookers are frequently built with counter flow and discharge belts, and acapacity of approximately 1.3 tons per hour with variable cooking times of up to 2hours.

The cooker is divided into 16 cells into which the fish is fed. The bottom of thecooker is separated by a heavy mesh plate, and under this are placed heatexchangers for indirect steam heating.

After cooking the fish are transported on a continuous belt to the counter-flowcooler (a stainless vessel with freshwater supply) and then on to the cleaningstations.

e. Typical cleaning arrangements consist of:

One working table for cleaning the tuna after cooling One rotating table fitted with holders/cups for loins One discharge elevator for transportation of loin portions from the rotating

table to the packing machine One offal conveyor monted beneath the cleaning station to collect and "

discharge offal.

The operations are as follows:

The 1st operator removes the fish from the conveyor and transfers it to a working.table where the skin and the head are removed.

The 2nd operator opens the fish, removes the backbone and divides the fish into 4loins. Simultaneously the majority of dark meat is removed and single loins areplaced in the cups on the rotating table.

The 3rd and 4th operators clean the loins, removing the remaining dark meat. Thecleaned loins are then removed automatically from their cups to a rubberbeltconveyor which transports them to an overlying conveyor.

f. The loins then pass to an automatic scale for continuous weighing which makes itpossible to monitor production yields. After weighing the loins pass to an Iaccumulation station prior to passing to a pack-shaper which packs the loins into

cans.

The filled cans pass to an oil/brine filling machine before the vacuum seaming machine.

Description of the subsequent operations and their effects on yield are similar to thosedescribed for canning tuna in brine.

4.2.4 Description of canning sardines in oil


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The method for canning sardines in oil is often called the raw packaging method which contrastswith the method in which the sardines are thermally treated before packing into cans.

The processing plant related to the description has a capacity of 15 tons raw fish per 8 hours.The overall yields is approximately 50% of the weight of raw fish. See Figure 10 "Layout forsardines in oil, cannery" which shows a plant of this capacity.

.2 Machines for Canning Tuna

5.2.1 Pre-cookers

The most common pre-cookers are live-steam cookers. fitted with condensate drains.vents and safety valves. The pre-cookers operate on a batch system. with doors at eachend (so that fish may be rolled in and out on a flow-through basis). The fish are loadedinto galvanized iron baskets. and the baskets are placed on racks which are rolled into

the cookers for steaming.

Other preparatory stages taking place before filling are completed manually. and in manycanneries. filling is also a manual operation. There are. however. fully automatic filling machinessuitable for packing tuna in all pack styles in round and oval cans.

Figure 28 Side elevation of continuous flash cooker for pre-cooking sardines; (diagramCourtesy of Trio Mask in Industry A/S)

5.2.2 Filling machines


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Machines are available for filling chunk and grated (shredded) tuna which operate at speeds ofbetween 80 and 350 cpm with cans ranging from 112 to 445 9 (approx.). There are a number ofmanufacturers with various operating procedures, but one manufacturer (Carruthers EquipmentCo., USA) has several machines for automatic tuna filling. In one machine (the Pack-Former)fish is discharged into filler bowls from where it is transferred into a series of piston pocketspositioned around the circumference of the machine. As the filling heads complete a revolution,

the fish is compressed into a cylindrically shaped slug in the pocket, in which form it is pushedout the bottom of the piston and is trimmed to the correct length, so that the weight of the packin each can is controlled. The fish is then fed into the can which has been located below. Amachine (a Carruthers Nu-Pak) operates on a similar principle, at speeds ranging from 200 to600 cpm, with 225 g cans (and smaller).

Solid style tuna loins are packed fully automatically by a machine (a Carruthers Pak.-Shaper)which handles cans ranging in size from 112 g to 1.8 kg (approx.) at speeds from 30 to 130cpm. The machines are fed with solid loins which are transferred to a forming hoop in which theflesh is molded into the desired shape and then cut off cleanly to produce segments of therequired length (and therefore weight).

Equipment used for the remainder of the tuna canning process is described in the followingsection.

5.3 General Fish Processing Machinery

5.3.1 Brining machines

Brining machines are sometimes coupled with washing machines, so that the two operationsoccur simultaneously. In continuous applications, the machine is usually a rotating perforateddrum partially immersed in a brine bath and through which the fish pass at a predeterminedrate. In less sophisticated operations. brining can be a batch process in which the fish are

loaded into perforated drums which rotate and, because of the tumbling action, gently transportthe fish through the salt solution. Whether using automatic, semi-automatic or batch equipment,it is important that, the salt concentration be maintained at the desired level -this means thatperiodically the effects of gradual dilution must be monitored and salt added. The material usedfor construction of the equipment must resist the corrosive effects of the salt.

5.3.2 Exhaust boxes

The exhaust box is used to heat the contents of cans, so that they may be sealed hot,thus ensuring that, after cooling, a vacuum has formed in the container. Exhausting alsodrives entrapped air from the pack. Exhaust boxes may i take many shapes and forms,depending on the requirements of the cannery; basically they consist of a tunnel through

which the open and filled cans pass while being exposed to atmospheric steam. Theyrequire a feed and a discharge mechanism, and a conveying system for transporting thecans from one end to the other. Recent models are frequently constructed with stainlesssteel, however many canneries still find painted mild steel systems adequate .

5.3.3 Sealing machines

When selecting can sealing machines. fish canners must consider the following factors:


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the size and shape of the container, the anticipated speed and volume of production, the level of skill required to maintain the machine in good working order. the cost and availability of spare parts, and the ease of "changeover" when the machine settings have to be converted to

accommodate cans of more than one shape and/or size.

In order to cater for the diverse requirements of fish canners, there is a wide range of machinesfrom which manufacturers can choose a model to suit their operations. Since many sealingmachines have features in common, the following is confined to a general description of themajor categories which are readily available.

The simplest of machines are required by those packers who run their lines at speeds of from 8to 25 cpm using hand operated or semi-automatic single-head equipment with motorized drives.For those with a low output (i.e., < 20 cpm), hand operated models are ideal -as with seasonalproduction or in those plants which are required to prepare test packs.

Single head seaming machines may be fitted with steam-flow closing or mechanical evacuationapparatus as a replacement for, or as an adjunct to, hot filling or exhausting. When mechanicalvacuum closing is required the operator places the container (with the can end sitting in placeon top of the can) in a chamber, which is then closed and evacuated by opening a line leadingto a vacuum pump. When the desired vacuum is obtained in the chamber, the sealing operationis initiated by depressing a foot pedal which lifts the can up to the chuck on the sealing headand into position for double seam rolling. The first and second action rollers are sequentiallybrought into action while the can is rotated by the spinning seaming head. At the completion ofthe seaming operation the sealing chamber is opened to the atmosphere and the hermeticallysealed container is removed. Machines of the type described can frequently have the facility forsteam flow closing, in which case steam is injected across the headspace of the container(while it is positioned in the sealing chamber) immediately prior to double seaming.

Fully automatic in-line single-head steam flow closing machines which operate in the range of70-90 cpm are available; while for canneries operating at higher speeds there is a variety ofmultiple-head machines from which to choose. Of the latter, three, four and six spindlemachines are common and can be selected to cover seaming speeds of from 200 to 600 cpm,depending on can sizes and production capacity.

Machines for sealing glass containers generally do not operate at the speeds of can closingequipment, however, they can be fitted for steam-flow closing or mechanical evacuation. Fullyautomatic steam flow closing machines are available to apply caps at around 400 to 500 cpm(depending on container size), while semi- automatic machines can be operated at around 15cpm. As with cans, vacuums in glass jars may be also obtained by hot filling, or by addition ofhot brine, or by exhausting.

Laminated packaging materials are sealed by the fusion of the two facing layers of theinnermost ply. The material is heated while clamped between jaws of the sealing machine forsufficient time for the two layers (usually polyethylene or polypropylene) to fuse and form anhermetic seal. One of the greatest difficulties faced by users of laminated packaging materials isthat of ensuring effective seal formation. Under all circumstances the sealing surface must beclean and free of particulate matter, which can present difficulties when packing fish products,as it is not always possible to prevent flakes of flesh from contaminating the sealing surfaces.


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The solution to the problem is to clean the seal area before passing the package to the sealingmachine, however, this further retards what is in many cases an already slow sealing operation.

5.3.4 Retorting systems

For a detailed description of recommended retorts and retort fittings reference should be madeto the following publications:

FAO/WHO. 1977. Codex Alimentarius Commission, Recommended international code ofpractice for canned fish. Rome, FAO/WHO, Joint FAO/WHO Food StandardsProgramme, CAC/RCP 10-1976: 42 p. Issued also in French and Arabic

FAO/WHO. 1983. Codex Alimentarius Commission. Recommended international code ofpractice for low-acid and acidified low-acid canned foods. Rome, FAO/WHO. JointFAO/WHO Food Standards Programme, CAC/RCP 23-1979: 50 p.

The main types of retorts used in the manufacture of low-acid canned foods include thefollowing:

a. Batch retorts heated with saturated steam. These may be either vertical or horizontaland are by far the most common retorts used by fish canners. Simplified drawings ofthese types of retorts are shown in Figures 29 and 30; in Figure 31 is shown a lessfrequently used batch system for processing cans in saturated steam. The latter systemis referred to as a crates. Brief descriptions of these systems are found in sections 3.6.1,5.3.5 and 5.3.6.

b. Batch retorts heated with water under pressure. These retorts are vertical or horizontaland are most frequently used for processing glass containers which cannot beprocessed in pure steam because of the risks of thermal shock breakage. They are alsowidely used for sterilization of products packed in aluminium cans with score-line easy

open ends. Simplified drawings of these types of retorts are shown in Figures 32 and 33;operational guidelines are given in section 3.6.2 and features of the system aredescribed in section 5.3.7.


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Figure 31 Crateless retort -operating sequence (Courtesy of FMC Corporation)

c. Continuous retorts (other than hydrostatic retorts). Containers are passed through amechanical inlet port into a pressurized chamber containing steam where they areprocessed before passing through an outlet port and. depending on the make of theretort. into either another pressurized shell. or an open water reservoir. for cooling. Themotion of the cans through the retort causes some forced agitation which aids the rate ofheat transfer to the SHP of the container.

d. Hydrostatic retorts. A simplified drawing of this type of retort is shown in Figure 34 andthe system is described in section 5.3.8.

e. Retorts heated by a mixture of steam and air. The containers are processed underpressure in a system which relies on forced circulation (by a fan or a blower) for thecontinuous mixing of the steam with the air. Inadequate mixing can result in theformation of cold spots which could lead to under-processing spoilage. As with waterfilled retorts. this system is suitable for retortable pouches which require acounterbalancing overpressure to prevent their rupture.


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Code to symbols:

A Water lineB Steam lineC Temperature controlD Overflow lineEl Drain lineE2 ScreensF Check valves

J PetcocksL Steam spreaderM Temperature control probeN Reference thermometerO Water spreaderP Safety valveQ Vent

R Pressure gaugeT Pressure controlU Air lineV To pressure control instrumentW To temperature control instrumentX Wing nutsY1 Crate support

Y2 Crate guidesZ1 Constant flow orifice used during come-upZ2 Constant flow orifice valve used during cook

Figure 32 Vertical retort for processing glass containers

There is a comparatively rarely used retorting system whereby sterilization is achieved bydirectly heating cans with flames from gas burners positioned underneath containers which spin

past on guide rails. This system is suitable for packs which contain a high proportion of liquid,thus permitting rapid transfer of heat by convection, but it is not used commercially in fishcanning operations.

The most frequently used style of retort found in commercial fish canneries today, is the staticbatch system for processing cans in saturated steam. A description of the fittings for theseretorts is given in the following section; however, many of the other retorting systems referred toabove are similar with respect to fittings and methods of operation. The most significant


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difference between static retorts and continuous systems, is that the latter must have containertransfer mechanisms to regulate the movement of cans at a predetermined rate through theheating and cooling sections.

Code to symbols:

A Water lineB Steam lineC Temperature controlD Overflow line .E Drain lineI El Screens

F Check valvesG Line from hot water storageH Suction line and manifoldI Circulating pumpJ PetcocksK Recirculating lineL Steam spreaderM Temperature control probe

N Reference thermometerO Water spreaderP Safety valveQ VentR Pressure gaugeS Inlet air control

T Pressure controlU Air linesV To pressure control instrumentW To temperature control instrumentZ Constant flow orifice valve

Figure 33 Horizontal retort for processing glass containers


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Figure 34 Hydrostatic retort (Courtesy of Churchill Livingstone)

5.3.5 Standard batch retorts for processing cans in steam

Irrespective of whether retorts for processing cans in steam are vertical, horizontal or crateless,they have a number of features in common. The major fittings are as follows:

Steam inlet. The steam enters through a perforated steam spreader pipe which provideseven distribution of the heating medium throughout the retort. The steam inlet ispositioned opposite the main vent: in standard vertical retorts the steam spreader isusually located at the base of the vessel, while in crateless retorts it is circular and at thetop; in horizontal retorts it extends the full length of the retort. In general the total cross-sectional area of the perforations in the spreader should be 1.5 to 2 times the smallestcross-sectional area of the steam inlet line.


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reaches operating temperature, the process commences (step 4). At the completion of thescheduled retorting time, the steam is turned off, and water and compressed air are pumpedinto the vessel and initial cooling commences (step 5). After partial cooling, cans are releasedinto the cushion water canal (step 6), which runs underneath the bank of retorts, and from therethey are automatically transferred, on conveyors, into the cooling water canal.

Although offering considerable labour savings and flexibility, it is important that care be takenwhile loading and unloading the cans. In the former case there is a danger that, if the retorts areoverloaded, or the cushion water level in the retort is too low, or if there are "floaters" (causedby insufficient removal of air prior to sealing the cans), incoming cans will damage the doubleseams of the uppermost layer of cans. Similarly, during unloading, seam damage can occur ifthe cans are permitted to drop out of the retorts in an uncontrolled manner. The risks to theseam are heightened at this stage if the cans are still hot, because the compound will be softand the cans under positive internal pressure, so that damage to the double seam area maycause momentary venting of the seal. Because of the potential danger to the hermetic sealsduring unloading, it is strongly recommended that the water level in the cushion water canal bemaintained above the level of the exit door (as shown in stages 5 and 6 of Figure 31). If thisprocedure is adopted, the cans gently float down and out of the retort, which means that their

double seams are not exposed to as much physical abuse as when they are dropped directlyinto the cushion water lying below the level of the exit.

5.3.7 Batch retorts for processing glass containers in water

The operating principles for processing glass under water in counterbalanced retorts have beendiscussed in section 3.6.2. The similarities in retort fittings for processing glass in water andcans in steam are evident when comparing Figures 29 and 30, with Figures 32 and 33,respectively. The main functional characteristics peculiar to systems for processing glass arethat:

water is introduced and mixed with the steam as it enters through spreaders, at the baseof the retort (thereby preventing thermal shock breakage); and counterbalancing air is required to transmit sufficient pressure through the water to

ensure that there is always a greater pressure in the retort than in the container. (it willbe recalled that this modification is to prevent the closures from being forced from thefinish of the glass during the thermal process).

5.3.8 Hydrostatic retorts for processing cans in steam

In the diagram of the cross section of the hydrostatic retort shown in Figure 34 can be seen thecolumns of water in the inlet and outlet legs which balance the pressure in the steam dome andgive this style of retorts their name. As the height of the column controls the steam pressure, it

also controls the temperature in the steam dome. Cans are automatically loaded onto the chainwhich carries them through a preheating zone at the top of the inlet leg and down into thecolumn where they are heated by water which becomes progressively hotter the further into theleg they move. At the bottom of the inlet leg the cans emerge from the water seal and thentravel up into the steam dome (or steam chamber). In some hydrostatic retorts the cans havetwo passes through the dome (one up and the other down), while in others the cans havemultiple passes. In Figure 34 a two-pass system is illustrated. The severity of the processdepends upon the residence time that the cans are in the dome (which is controlled by chainspeed and chain length). and the temperature of the steam (which is controlled by the height of


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the water column). At the completion of the process the cans move back through the water seal,on the cooling side of the retort, and up into the cooling leg where they are exposed toprogressively colder water. At the top of the cooling leg the cans pass through an air coolingsection and then pass down a final cooling section where they are sprayed with cool water. Thecooling water canal shown in the illustration is omitted in some hydrostatic retorts.

Because of the high capital investment, the time taken to adjust the conveyor systems to handledifferent can sizes, and the time required to bring the retorts to operating temperature,hydrostatic cookers are best suited to long I production runs. When dual chain systems areused, it is possible to process cans I of different sizes simultaneously, for different times but atthe same temperature. While savings of floor space, gentle can handling, and gradual

changes in temperature and pressure, are attractive features of these retorts, the systems areexpensive to install and maintain, and the costs of breakdowns can be high.

Incubation Tests

Although they should not be used as the sole criterion of product safety, incubation tests canprovide valuable information as to the adequacy of the thermal process and also a means ofmonitoring (indirectly) the microbiological 1 quality of in-coming raw materials.

Should a process be of marginal severity, so that a measurable proportion of the population ofspore-forming thermophilic bacteria survive, it may be possible to detect changes in theincidence of spoilage after thermophilic incubation tests on the production samples which havebeen collected as part of routine quality control. It is difficult to predict the level of spoilage in thetrade which correlates with a known incidence of spoilage arising from incubation of testsamples; however, it was reported by Stumbo (1973) that a thermophilic spoilage level of 1%after thermophilic incubation was found in commercial practice to give rise to a spoilage rate of0.001% (i.e., 1 in 100 000 units) in the trade. Should a fish canner be able to collect sufficient

data to draw their own conclusions concerning the relation between spoilage induced bythermophilic incubation and trade spoilage, the value of these incubation tests becomes clear;particularly for those manufacturers whose products are expected to be marketed in warmclimates.

Under normal circumstances there would be little point in routinely monitoring spoilage arisingfrom mesophilic incubation as all the spores which might lead to growth under these conditionsshould have been eliminated by thermal processes in which target Fo values are 10 to 15 min.However, if there are incidences of mesophilic spoilage detected by these test measures, it isreasonable to conclude that there has been either a significant lapse in the microbiologicalquality of the raw materials, or a gross failure in the delivery of the scheduled thermal process(see Table 3 for a summary of factors that could lead to this phenomenon). In such

circumstances corrective remedial action should follow immediately and, suspect stock shouldbe isolated pending a detailed examination.

Incubation test may be carried out in the laboratory or with bulk samples. With the former, thevalidity of the results must; be verified before any conclusions as to the suitability of the testsample (and by implication, the suitability of the population from which the samples weredrawn). Factors to be considered when selecting testing procedures for laboratory incubationinclude:


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the purpose of the test and its statistical basis; the validity of the selection of incubation temperatures; the method of examination of incubated containers (e.g., not all spoilage will cause

blown cans); the sample size required to draw statistically significant conclusions; and the tolerable levels for accepting lots with given levels of defectives.

With bulk incubation the factors to be considered include:

the method can only provide the incidence of blown cans in the lot, under examination; the method can highlight changes in spoilage levels, and prompt management to find the

causes of any trends; and because of the heating lags in bringing cooled cans to incubation temperatures, it is

advantageous to commence incubation as soon as possible after the cans leave theretort, when they will still retain some of the heat from the process.

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