flow diagrams, and photos of themain steps in processing catfishinto refrigerated (Fig.1) or frozen(Fig. 2) products are described inthis publication.

Catfish of the proper size are soldiced, frozen, or battered and bread-ed in the following forms: whole;dressed (deheaded, eviscerated andskinned); filleted; steaked; or asstrips or nuggets. Size control of fil-lets to within 1- to 2-ounce weightincrements is essential in marketingthe filleted product. Fish of theappropriate size that will yield thenumber of fillets needed areprocessed by hand at filleting tablesor by automatic filleting machines.The fillets are trimmed to yield thenugget and the shank fillet, thensized and either frozen or packed inice for shipment. Channel catfish“steaks” are prepared by cuttinglarge fish into cross-section pieces.The steaks are then individuallyquick frozen (IQF) or packed in ice.Other products are derived fromthese primary cuts.In order to make various productforms, several steps must be com-pleted in processing channel catfish(Ictalurus punctatus) into a mar-ketable product. These includereceiving and weighing the live fishat the processing plant; holdingthem alive until needed; then stun-ning, deheading, eviscerating, skin-ning, chilling, size grading, freezingor ice packing, packaging, ware-housing, icing, and shipping the fin-ished product. Brief descriptions,

VIPR

August 2001Revised

SRAC Publication No. 183

Processing Channel Catfish

Juan L. Silva1,2, Gale R. Ammerman1, and Stuart Dean 2

1Department of Food Science and Technology, Mississippi State University

2Food and Fiber Center, Mississippi StateUniversity

Figure 1. Flow diagram for processing ice-packedchannel catfish fillets (manual and automatic).

Live Whole Fish

Vats

Stunning

Deheader Band Saw

Filleter Cavity Cut

Skinner Eviscerate

Trim Skinningnugget

Sizing

A

Chill

whole dressedfish

Sizing

Ice

Cooler

Automated Lines Manual Line(s)

A

Receiving and stunningBefore they are purchased for pro-cessing, fish are evaluated for unde-sirable characteristics such as over-all appearance and off-flavors by

experienced “tastetesters.” Sample fish aretaken from the pond atleast three times: twoweeks before harvest,the day before harvest,and the day of harvest.If any fish sample isdeemed unsatisfactory,fish from that pond willnot be accepted until thecondition improves. Fishare loaded into aeratedwater tanks and trans-ported from the pond tothe processing facility.The fish are unloadedfrom the truck into bas-kets for weighing andthen placed in an aerat-ed holding vat or direct-ly onto a conveyor beltto the plant. In mostcases, fish enter the pro-cessing line directlyfrom the trucks and arekept in holding vatsonly long enough to sus-tain plant operation iffish delivery is delayed(Fig. 3). The fish areplaced in cages or car-ried on a belt that passesunder electrodes wherethey are stunned with anelectrical current for a

Chill

Ice

Cooler

few seconds. This makes them easi-er and safer to handle by workers(Fig. 4). The fish are moved into theprocessing plant on a distributionconveyor belt (Fig. 5). The dressingarea is divided from the receivingarea (outside) by a wall. The fish areconveyed and manually or auto-matically size graded for each fillet-ing operation. Fish other than cat-fish (sometimes called “trash fish”)are sometimes present in ponds andmay be inadvertently harvestedwith the catfish. These fish are dis-carded at this point. Oversized fish(fish larger than what can be han-dled by automatic filleting lines) aresent to the manual draining/fillet-ing line for processing.

Manual dressing

Deheading

From the distribution conveyor belt,the stunned catfish drop into aholding bin for each processing line.In general, fish are sorted into eachline automatically (by weight) ormanually to attain maximum yieldand efficiency in the automatedlines. The first line operator is thelay-up person, who positions eachcatfish for quick and efficient headremoval by the bandsaw operator(Fig. 6). The head is pushed into achute that routes it to a waste dis-posal conveyor belt below the bandsaw, and the carcass proceeds to theevisceration operation. A good bandsaw operator can process 40 to 50fish per minute.

Evisceration

The body cavity is opened by handwith a knife, and viscera are with-drawn through a suction tube (vac-uum eviscerator) (Fig. 7). Onedeheader will feed about six evis-ceration stations. Viscera are con-veyed to the offal collector, and theeviscerated carcass proceeds to theskinning operation.

Skinning

The membrane skinner has been thestandard industry machine for skin-ning channel catfish since its intro-duction. This machine has a rotat-ing roller with sharp “teeth” thatpresent the fish to a sharp bladeheld in place by spring pressure

From Cooler/Process

A

Vacuum Tumbleor

Injection

Ship Ice Pack Bread & Batter

Heat Set Bread

IQF

IQF

Water Glaze

Pack

Freezer

Ship

Figure 2. Flow diagram for processing frozen channel catfish fillets.

Figure 3. Fish are held in holding tanks (vats) before processing.

(Fig. 8). Very close tolerancesbetween the blade and roller teethmake it possible to remove only theskin as the fish is passed over theroller. This tolerance is important indetermining yield and fat content ofthe final product, since most fat is

located underneath the skin.Capacity is about 12 to 14 fish perminute per operator. Most proces-sors use two operators per machine,so the machine capacity is 24 to 28fish per minute.

Chilling

After deheading, evis-cerating and skinning,the whole dressed fishis lightly spray-washedand conveyed into thechill tank where it isimmersed in a mixtureof ice and water, orcold water (≤35 oF)refrigerated indirectlywith ammonia.Chilling is the processof lowering the temper-ature of the fish byimmersing them incold water for a setperiod of time. Fish areheld in the chill tankfor 10 to 30 minutes,depending on size, at atemperature of 38 oF orless (Fig. 9). Fish mustbe cooled rapidly andheld below 40 oF toattain low microbialload, good flavor, andmaximum shelf-life,and to ensure overallquality. Fish may gainup to 1 percent weightby water absorption atthis point, but most is

lost before packaging. It is extreme-ly important to control the micro-bial build-up in the chiller water,because this is directly related tothe safety and shelf-life of the fish.Because chiller water is usually notchanged until the end of the day, aturnover rate of about 1 gallon ofwater per 5 pounds of fish is rec-ommended. Some processors addup to 20 parts per million of chlo-rine to the chiller water or rinsewater. Water is added continuouslyto replace water uptake. Fat is alsoskimmed off. Some processorsplace this step before the skinningoperation and after the eviscerationstation.

Size grading

When fish leave the chill tank theyare conveyed into a grading or asizing station. Here they are sortedby weight. In small plants, gradingmay be a hand operation; in largerplants, mechanical or electronic siz-ing systems are used. These use aphotoelectric cell (a scale whichweighs the fish and sends an elec-tronic signal) to send a signal to thecorrect “station/ gate” to open andlet the fish out when it passesthrough (Fig. 10). These gradersusually size fish in 2-ounce inter-vals. These fish are then ice-packed,injected and frozen, or furtherprocessed as explained later.

Automatic filleting

The stunned fish are manually sort-ed by size or automatically sortedby weight as described earlier.Depending on the number of auto-matic lines, whole fish are sorted indifferent size/weight ranges so asto attain maximum yield and pro-ductivity. Each filleting line canprocess up to 60 fish (120 fillets) perminute. The fish are then placedvertically (or horizontally in somedeheaders) with the head up on anautomatic deheader (Fig. 11). Thisis a rotating machine with severalknives that cut off the head andpull most of the viscera with it. Thedeheaded fish, with some remain-ing viscera, are dropped onto aholding table. The head and visceraare sent to an offal room. Fish fromthe holding table are taken individ-ually by an operator and positionedonto guides (head first) to be fillet-ed. The filleting machine has two

Figure 4. Fish are electrically stunned before processing.

Figure 5. Distribution belt from stunning to primaryprocessing.

lets are sorted in 2-ounce incre-ments from less than 3 ounces tomore than 17 ounces. Fillets forfresh market are dropped in taggedboxes with a plastic liner and icedto a 1:2 fish to ice ratio by weight.The remainder are placed ontagged plastic totes and held in acooler until frozen or otherwiseprocessed.

Freezing

Before freezing, channel catfishproducts are treated (injected ortumbled) with a polyphosphatesolution that acts as an antioxidantand prevents excessive water lossduring freezing.The most important part of main-taining excellent quality frozen fishis ensuring that they are processedand frozen rapidly, and held at 0 oFor below until used (Fig. 2). Thetemperature at the core of the fishmust be reduced from 32 to 15 oF in30 minutes or less for the fish to beconsidered quick-frozen, and for itto retain its original quality. Surfacefreezing alone is not sufficientbecause the fish will thaw in stor-age before refreezing, which leadsto textural breakdown, oxidationand shorter shelf-life. The channelcatfish are IQF (Fig. 14) in a cryo-genic tunnel or mechanical spiralfreezer. Carbon dioxide, liquidnitrogen, or conventional mechani-cal (ammonia) freezing systems arebeing used in various plants tofreeze channel catfish. The choice offreezing media and machinery ismainly a question of economics.Depending on the source, produc-tion rates of more than 2,000pounds per hour are handled bymechanical freezing. The fish orfish pieces are evenly distributedmanually on the conveyor belt tothe freezer. The variable speed beltis regulated so that the fish remainin the freezing chamber for therequired time and are completelyfrozen when they exit the tunnel.

Packaging

Frozen

When whole frozen fish leave thefreezer, they are conveyed througha water bath or sprayer. A coatingof ice (glaze) is formed over thefish. This is the first step in packag-ing. The IQF, glazed, whole fish or

rotating knives in series that cut thefillets from the frame (Fig. 12). Thefillets drop skin down onto a con-veyor belt, while the frame andremaining viscera drop onto theoffal line. These are carried to theoffal station by a conveyor belt,water flumes or vacuum. The filletsare conveyed to the skinner wherethey are pressed against two rotat-ing knives that remove the skin.The fillets are sprayed with waterthat may contain up to 50 ppmtotal chlorine to reduce bacterialload; then they are conveyed to thetrimming table. Fillets are trimmed

manually with a filleting knife toseparate the nugget and bones fromthe finished fillet, called the“shank” (boneless) fillet (Fig. 13).Trimmers are sometimes paid byproduction and yield, by weighingfillets in and shank fillets, nuggetsand trimmings out. This is a labor-intensive operation, requiring 12 to16 trimmers per filleting line. Theshank fillets are then placed on aconveyor belt, cooled in a rotatingchiller with ice water or refrigerat-ed water and conveyed to the grad-ing station. Here they are sortedautomatically by weight. Shank fil-

Figure 6. Deheading operation with a band saw (manual).

Figure 7. Evisceration (vacuum) stations to remove viscera manually before skin-ning.

fillets are sized and packaged incardboard shipping cases lined withpolyethylene bags. The wholefrozen fish are divided by weight(in 2-ounce increments) and packedin 15-pound boxes. Frozen fillets arepacked in 15-pound boxes, with fil-lets divided into lots within a 1- to2-ounce range (Fig. 15). They areweighed and may pass through ametal detector (a critical controlpoint in HACCP plans) beforewarehousing and shipping.

Ice-packed

Whole iced fish are divided into thesame size categories as wholefrozen fish and packed in ice in 50-pound shipping boxes that contain30 pounds of fish and 20 pounds ofice. Steaks are packed in 15-poundshipping cartons.

WarehousingFrozen channel catfish are held at 0 oF (or below if required by statelaw) in a frozen storage warehouseuntil shipped. The iced product isusually packed and shipped within48 hours in refrigerated trucks. It isheld at the processing plant inrefrigerated storage at 30 to 38 oFuntil shipped.

Recent developmentsand new technologiesThe catfish processing industry isstill very young. It has evolvedfrom a mostly manual batch opera-tion into a semi-automatic, semi-continuous operation. This has ledto increased production and quality.However, there is much room forimprovement in productivity, effi-ciency and yield, quality and chilllife, and safety. One of the advan-tages of catfish is the freshness ofthe product. Theoretically, a fishmay enter the plant and be readyfor distribution in 2 to 4 hours.Catfish also can have a long shelflife. However, there are still prob-lems with manual trimming, lower-ing temperature quickly, handling,and product flow. Some innovativetechnologies are becoming availableto increase production, yield, quali-ty, and safety of the product.One system recently developedtakes the fish, after receiving andstunning, into a hopper feeding

Figure 8. Mechanical skinning operation of whole fish.

Figure 9. Channel catfish in a chill tank.

Figure 10. Automatic size graders used for dressed and filleted catfish.

deheader/eviscerator unit thatremoves most of the viscera, thepelvic fins, and head. A secondaryevisceration system removes theremaining viscera and most of thetail fin. This primary process couldbe segregated from the filletingprocess. The fish is then conveyedto an air-agitated, counter-flow,two-stage chiller. The prechillercleans and removes most of theblood, while the chiller cools theheaded and gutted product. This isthen placed in a filleting machinethat removes the dorsal bone. Thefillets are then sent to a trimmingline (less trimming needed) and onto icing/grading or other processes.The resulting frame also yields abetter quality mince (less blood andviscera). This process is designed toflow better, produce a better qualityproduct, and perhaps lower theincidence of bacteria. The rapidcooling and better evisceratingimprove filleting and trimmingprocesses, yield and quality.Another technology combinesmachine vision with a cutting-edge(water pressure) to produce uni-formly cut strips and other prod-ucts. The fillet is presented to acamera that processes the imageand sends a signal to the cutter toproduce the most efficient, uniformcut.Processes such as prechilling fishbefore processing have beenattempted. However, catfish aresubtropical/temperate fish, and ifchilled immediately after harvest orreceiving, may produce a pink, softand exudative muscle.Grading and quality control soft-ware and hardware are now avail-able from many vendors. This tech-nology allows the production andquality assurance departments tofollow operations continuously andmake adjustments rapidly. Suchsoftware also can be used to moni-tor critical control points in aHazard Analysis and CriticalControl Points (HACCP) system.

Requirements andregulationsFederal, state, local and, if export-ing, international regulations mustbe followed from the design to theproduction stages. These regula-tions cover Good ManufacturingPractices, GMPs (Title 21, Code ofFigure 13. Manual trimming of fillets from automated lines.

Figure 11. Automatic deheading station.

Figure 12. Automatic filleting machine.

Federal Regulations, part 110,www.fda.org), as well as environ-mental and occupational safety.Some of the agencies that overseecatfish processing operations arethe U.S. Food and DrugAdministration (FDA); theEnvironmental Protection Agency(EPA) and its state equivalent; statehealth and/or agriculture depart-ments; the Occupational Safety andHealth Administration (OSHA);and others. Recent regulationsrequire seafood processing plants toconduct a hazard analysis andimplement a Hazard Analysis andCritical Control Points (HACCP)plan (Title 23, Code of FederalRegulations, part 123, www.fda.org),including Sanitation StandardOperating Procedures (SSOPs).

Figure 14. Fillets are individually quick frozen (IQF), glazed and packaged.

Figure 15. Frozen fillets are placed in plastic-lined boxes, weighed, and passedthrough a metal detector (if needed).

This publication was supported in part by a USDA-CSREES Grant No. 98-38500-5865 from the United StatesDepartment of Agriculture, Southern Aquaculture Center. Publication is an update of SRAC Publication No. 183by Gale R. Ammerman.