2. PACKAGING MATERIALS FOR CANNED FISHERY PRODUCTS


2.1 Metal Containers

2.1.1 Tinplate
2.1.2 Aluminium
2.1.3 Can construction
2.1.4 .Double seam formation and inspection procedures

2.2 Plastics and Laminates
2.3 Glass

2.3.1 Sealing mechanisms
2.3.2 Inspection procedures


Containers for thermally processed canned fishery products have several functions in common, whether they are constructed of metal, glass, plastic laminates or composites of plastic and metal laminates. The functions of packaging materials can be summarized as follows:

  1. to hermetically seal the product in the container while delivering a thermal process which will render it "commercially sterile";
  1. to prevent recontamination of the product after processing. and during subsequent transport and storage; and
  1. to provide nutritional benefits and marketing convenience, through presentation of preserved fishery products all year round, often far from the source of supply, and in the majority of cases without the need to rely on refrigerated food chains.

2.1 Metal Containers

2.1.1 Tinplate

The most frequently used form of packaging for canned fishery products is tinplate which is fabricated into two and three piece cans of a wide variety of shapes and sizes. Tinplate consists of a base plate of low-carbon mild steel, onto each surface of which is electrolytically deposited a layer of tin. Base plate gauge varies, depending on the size of the cans which are to be manufactured and their intended application; however, it is usually between 0.15 and 0.30 mm thick. Nowadays, for the manufacture of extra light gauge plate, steel sheet is cold rolled twice prior to being tin coated, and in these cases is referred to as double reduced (DR) plate. Tin coating mass varies, according to end use and whether or not lacquers are to be applied; the thickness of the tin coating layers ranges from around 0.4 to 2.5 micron. Shown in Table 4 is the Designation, nominal coating mass and minimum average coating mass for electrolytically coated tin plate. Plate on which the tin coating mass is the same on each surface is known as equally coated plate; whereas plate with different tin coating masses on each surface is referred to as differentially coated plate. When specifying tin coating masses it is customary to quote, for each surface, the nominal mass of tin per square metre of plate. Following the standard nomenclature, the designation E05 means that on each surface, there is 2.8 g of tin per square metre of plate; while the designation D10/05 means that the tinplate is differentially, coated and has 5.6 g of tin per square metre of plate on one side, and 2.8 g of tin per square metre on the other surface.

Date

Product

Code

Container

Min
Initial
product
temp.

Retort
No

Scheduled Process

Time

Retort temperature

Signed by

Size

No/R

Time

Temp.

Steam on

Vent closed

Temp. up

Steam off

Actual
process

Mercury
in glass

Thermo-
graph

                                 
                                 
                                 
                                 
                                 
                                 
                                 
                                 
                                 
                                 
                                 
                                 
                                 
                                 
Reviewed................................................ Date........................

Figure 5

Figure 6 Retort thermograph showing record of 90 min process at 121.C

Tin is applied to provide sacrificial protection of the steel base -the tin layer gradually dissolves and passes into the surrounding solution, while the steel layer beneath remains protected. Recently the high cost of tin has made attractive the production of tin-free steel (TFS) in which the conventional tin and tin oxide layers are replaced by chromium and chrome oxide layers. Thus conventional tinplate and TFS consist of multi-layered structures; tinplate comprises an innermost steel layer on top of which there is in sequence. and on each surface of the plate. a tin/iron alloy layer, a free tin layer, a tin oxide layer and an oil lubricant layer, whereas TFS comprises a base steel layer on top of which there is sequentially and on each surface, a chromium layer, a chromium oxide layer and an oil lubricant. Plain TFS cannot be readily soldered, it lacks the corrosion resistance of conventional tinplate (since there is no sacrificial protection of the steel by an overlayer of tin), but it provides an excellent key surface onto which can be applied protective lacquers. Since the introduction of TFS, there has been development of a third system using neither tin nor chromium put nickel as a coating material for the steel base.

In canned fishery products (and with other proteinaceous packs such as meat and corn), it is customary to use sulphur resistant (SR) lacquer systems to prevent the formation of unsightly, yet harmless, blue/black tin and iron sulphides on the plate. Due to the inclusion of white zinc oxide, SR lacquers have a milky appearance. The reason for the inclusion of the zinc is that it reacts with the sulphur compounds, released from the proteins during thermal processing, to form zinc sulphide precipitates which cannot readily be detected against the background of the opaque lacquer.. Another lacquer system finding use for meat and fish packs relies on the physical barrier provided by the inclusion 0f aluminium pigments in an epoxy-phenolic (epon) lacquer. These lacquers, often referred to as V-enamels, are common in pet food cans.

2.1.2 Aluminium

The dominant position of tin plate as the packaging material of choice for canned seafood products has been challenged with the development of aluminium alloys. Alloys frequently lack the chemical resistance of pure aluminium; however, because they possess greater hardness than that of the pure metal, , alloys are well suited to the construction of cans. The mechanical characteristics, lacking in pure aluminium yet required in the material for food cans, are obtained by the inclusion of small amounts of magnesium and manganese. Depending on the can size and the alloys used, the thickness of the aluminium in fish cans normally ranges from 0.21-0.25 mm. Care must be exercised when manufacturing "easy-open" ends to control the depth of the scores so as to avoid "cut through"; practically, this restricts the lower limit for the, thickness of plate which cap be used in ends. Aluminium alloys are widely used for the manufacture of dingley, club, hansa, and a variety of conical and straight sided round cans. Some of the important factors which account for the increasing popularity of aluminium for the construction of fish cans are summarized:

Because of their relatively large surface area and flexibility, many aluminium cans ends (e.g. , those on club and dingley cans) are prone to distortion during retorting and early in the cooling cycle (i.e. , when the pressure in the cans is greatest). In some cases this will cause peaking, and it is in order to avoid this that these cans are commonly processed in counterbalanced retorts operating with an overpressure.

Table 4 Tin coating mass for electrolytic tinplate sheet a/

Designation

Coating mass (g/m)

Nominal

Minm average

Equally coated tinplate  
E02

1.1/1.1

0.9/0.9

E05

2.8/2.8

2.5/2.5

E07

3.9/3.9

3.6/3.6

E10

5.6/5.6

5.2/5.2

E15

8.4/8.4

7.8/7.8

E20

11.2/11.2

10.1/10.1

E27

15.1/15.1

13.4/13.4

Deferentially coated tinplate  
D05/02

2.8/1.1

2.5/0.9

D10/05

5.6/2.8

5.2/2.5

D15/05

8.4/2.8

7.8/2.5

D15/10

8.4/5.6

7.8/5.2

D20/05

11.2/2.8

10.1/2.5

D20/10

11.2/5.6

10.1/5.2

a/ Figures quoted show the tin coating mass per square metre for each surface of the tin plate

2.1.3 Can construction

Metallic cans are available in a multitude of shapes and sizes to suit all types of canned fishery products. A selection of the range can be seen in Table 5, in which are shown the common two and three-piece cans used by the fish canning industry.

Three-piece cans are manufactured from a rectangular piece of tinplate (known as a body blank) which is formed ifitO a cylindrical shape and then joined along a vertical seam by either soldering or welding; to this section are added two ends, one by the can maker and the other. after filling by the canner –the former is referred to as the can maker's end (CME) artd the latter the canner's end (CE). The seam joining the can end and the b()d.y is known as the double seam and it is the formation of this seal which is critical if the col1tainer is to function correctly. Errors in "double seaming" can lead tb lbS$ of the hermetic seal and the possibility of post-process contamination, giving rise to canned food spoilage. Diagrams illustrating the sequence of rolling double seams, critical doub.1e seam morphology and criteria fot: assessing double seams are presented in section 2.1.4. Experience has shown that the majority of problems arising through faulty double seam formation are associated with errors in application of the canner's ends. This is attributable to the greater difficulty in applying can ends under commercial filling operations, when compared with completing the same operation in the can making plant.

Two-piece cans for fishery products are made by the draw and re-draw (DRD) process using aluminium or tinplate. While it is possible to have two-piece cans using both tinplate and aluminium (e.g., a tinplate body with an aluminium end), they have the disadvantage that bi-metallic corrosion may occur if the two exposed surfaces come into contact. DRD cans are made from circular blanks of pre-lacquered plate which are first drawn into shallow cups and then re-drawn, once or twice depending on the cans final dimensions, causing an elongation of the wall and a simultaneous reduction of diameter. One great benefit of two-piece cans is that they have no side seam, and only one double seam, thus reducing the risks of leakage arising from imperfect seam formulation.

Aluminium easy-open two-piece cans enjoy great popularity for tanned sardines where the convenience of the tear:-off end is well suited to the dimensions of the product; however the functional benefits of the system have to be weighed against the slightly greater costs of the aluminium container and in some cases the need to process in counter-balanced retorts to prevent the light gauge plate from deforming during thermal processing.

Since the mid 1970s tapered two-piece tinplate and aluminium have been available. Here the can body is drawn from a blank and transported while nested with other cans in tiers. The system offers savings in equipment, labour costs and space when compared with un-nested conventional three-piece tinplate cans; it also overcomes the need to complete fabrication in the cannery. as occurs with cans bodies that are despatched in the flat for later erection (reforming) and addition of the can maker's end, prior to normal filling and sealing.

2.1.4 Double seam formation and inspection procedures

The double seam is an hermetic seal formed by interlocking the can body and the can end during two rolling actions. The first action roll curls the edge of the can end up and under the flange of the can body and folds the metal into file thicknesses (seven at the Side-seam) while embedding the flange into the compound. During this operation the circumference about the edge of the can end is reduced causing the "extra" metal to wrinkle. The second action roll flattens and tightens the seam so that an hermetic seal is formed. This action causes the wrinkles (formed in the first operation) to be ironed out while the compound is forced into any gaps between the metal surfaces. Shown in Figures 7, 8, 9 are diagrams showing the various stages in the formation of a can double seam; in Figures 10 and 11 are a cross section of a double seam and the major attributes affecting seam quality.

As product safety depends upon maintenance of the hermetic seal, it is important that double seam formation be checked regularly during production, after all jams under the sealing machine, after adjustment to the machine, and after machine start-up following a long delay in production. Good manufacturing practice guidelines indicate that visual inspection of double seams should be at least every 30 min, while full tear down procedures should be followed for each sealing head at least every four hours. Can manufacturers and can seaming machine suppliers usually supply directions for seam formation and standards against which double seams are evaluated.

Table 5 Summary of selected two and three piece aluminium and tinplate cans showing nominal dimensions and capacity typical products and net and drained weights

Type of can

Material

Capacity
(ml)

Length
(mm)

Width a/
(mm)

Height
(mm)

Product

Net weight
(g)

Drained weight
(g)

2 piece  
1/4 Dingley

Aluminium or Tin b/

112

105

76

21

Sardines, small fish

106

85

l./4 Club

Aluminium or Tin b/

125

105

60

29

Sardines, small fish & tuna

l25

95

1/2 Hansa

Aluminium or Tin b/

200

148

81

25

Herrings

195

130

1/2 Oblong

Aluminium or Tin b/

212

155

61

30

Kippers

225

225

1/3 Oval

Tin plate

200

149

81

25

Mackerel

195

130

1/2 Oval

Tin plate

270

149

81

25

Mackerel

250

180

2 & 3 piece  
2 piece round

Aluminium

225

-

90

40

Shrimp

217

150

2 piece round

Aluminium

115

-

78

32

Shrimp

111

75

2 piece 1/2 round

Aluminium or Tin

245

-

90

44

Fish & vegetables, herring, tuna

230

c/

2 piece 1 round

Tin plate

490

-

120

49

Fish & vegetables, herring, tuna

460

c/

3 piece round

Tin plate

106

-

66

40

Tuna

100

78

3 piece round

Tin plate

212

-

84

46

Tuna

200

155

3 piece round

Tin plate

400

-

99

60

Tuna

377

292

3 piece round

Tin plate

4 250

-

2l8

123

Tuna

4 000

3 100

3 piece round

Tin plate

8 500

-

218

245

Tuna

8 000

6 200

3 piece round

Tin plate

450

-

74

118

Abalone

425

213

3 piece round

Tin plate

450

-

72

118

Codroe in brine

425

300

3 piece round

Tin plate

450

-

101

64

fish cakes

400

260

3 piece round

Tin plate

900

-

101

121

Fish balls

800

520

a/ Diameter of round cans shown as width
b/ Cans constructed with aluminium or tin plate
c/ Drained weight affected by proportion of vegetables in pack

Figure 7 Cross-section showing the positioning of the parts of the can body and loose end which will form the double seam (Courtesy of Standards Association of Australia.)

Figure 8 Cross section of the seam after the first operation (Courtesy of Standards Association of Australia.)

Figure 9 Cross section of the seam after the second operation (Courtesy of Standards Association of Australia.)

Figure 10 Cross-section of a double seam away from the side seam (Courtesy of Standards Association of Australia.)

Figure 11 Cross-section of a double seam showing some of the attributes that influence seam quality (Courtesy of Standards Association of Australia.)

As a guide the major quality criteria for assessing double seam quality are summarized:

  1. External inspection: much information as to the quality of a double seam can be obtained by a visual and tactile examination of the rolled seam. For skilled operators it is often not necessary to strip a double seam and measure the component in order to determine whether the sealing machine is rolling seam which comply with the requirements of good manufacturing practice. Conversely, an alert machine operator can pick up drift in performance before double seam criteria fall below acceptable limits. when conducting these assessments it is necessary to check for the following defects:

Also shown in Figure 13 are the locations where double Seam measurements of the stripped seam components of circular cans should be conducted, and in Figure 15 are shown the positions for component measurements on rectangular cans.

  1. Tear down inspection: a complete analysis of the double seam form and dimensions; should be completed at least every four hours of continuous production for each seaming head. At times when there are difficulties with seam formation, these tests should be completed more frequently until satisfactory performance is demonstrated. The double seam attributes to be assessed include:

In parentheses are shown the Australian recommended specifications for round cans of 74 mm diameter; however. as these values change for cans of different sizes and shapes. manufacturers ought consult their can suppliers to determine the satisfactory compliance criteria for use with their cans. Shown in Figures 16 and 17 are schematic diagrams for double seam sections of an end hook showing the juncture rating and the tightness rating. respectively. A cross section of a partially stripped seam in which the pressure ridge is visible is shown in Figure 18.

Figure 12 Double seam Showing spur, a drop at the juncture and an incompletely rolled region known as a skidder (Courtesy of Standards Association of Australia.)

Figure 13 Positions designated 1, 2 and 3 are the points at which to measure the double seam components. Also shown is a skidder resulting from incomplete rolling of the seam (Courtesy of Standards Association of Australia.)

Figure 14 Cross section of a seam showing cut-over. a fractured cut-over and a cut seam (Courtesy of Standards Association of Australia.)

Figure 15 positions for measuring the double seam components on ,rectangular cans. The Tangent points are indicated by the letter T (Courtesy of Standards Association of Australia.)

Figure 16 Section showing the juncture rating. which is equal to the percentage of the end hook which is available for overlap; in the example the juncture rating is 80% (Courtesy of Standards Association of Australia)

Figure 17 Section of an end hook showing increasing degrees of wrinkle from left to right. The tightness of the different parts of the seam is shown by the figures which indicate the percentage of the end hook length which is not wrinkled (Courtesy of Standards Association of Australia)

Figure 18 Section of a partly stripped seam showing the pressure ridge on the inside of the can body (Courtesy of Standards association of Australia)

2.2 Plastics and Laminates

With the development of plastic and plastic and aluminium foil, flexible, semi-rigid and rigid laminated packaging materials, has come a range of systems suitable for in-container sterilization of fishery products. Of these, the best known is the retortable pouch, which because of its flat profile and correspondingly high surface area to volume ratio (relative to that of cans), heats more rapidly than conventional cans. However, despite certain of their advantage,; (e.g., greater retention of heat labile nutrients and other quality, benefits arising due to rapid heat transfer to the thermal centres of retort pouch packs; the favourable costs of transportation, and the ease of opening and heating contents) they have not replaced, to the extent that was anticipated, conventional packaging materials for heat sterilized fishery products.

In America, problems were encountered with early versions of the flexible retort pouches which typically consisted of a three-ply laminate comprising an outer polyester layer for strength, scuff resistance and printability), a central foil layer (for excellent barrier properties) and an inner polyethylene or polypropylene layer (for heat sealability). These difficulties arose primarily because of FDA`s concern regarding approval of the food contact surfaces used in flexible pouch manufactured and although they have been overcome, there still remain other disincentives arising from:

Semi-rigid (all plastic), pouches (trays), are now available and with some of these systems the problems of slow filling and sealing speeds have been overcome by using integrated form-fill computer controlled equipment. Depending on the heat treatment selected fish processors may choose trays manufactured to withstand pasteurization conditions (i.e., at <100 C) or sterilization conditions (i.e., at 110-122 C). Irrespective of the form of the laminated container and the temperature at which it is processed, the function is the same -it must provide a strong hermetic seal, and because of this the seals should normally be at least 3 mm wide and continuous. For heat sealing the sealing surfaces should be plane-parallel to each other and the temperature of the jaw should be uniform across the entire sealing area. Since the integrity of the heat seal is critical to the safety of the product, it should be tested routinely. Typical testing protocols include,

Whether considering retortable pouches which are flexible. semi-rigid or rigid. all offer the common attraction of providing a means to minimize the nutritional and sensory quality losses (which often are associated with traditional thermal processing in rigid containers), while simultaneously providing the opportunity to display visually appealing products. This is why developments with pouch packs are establishing a tradition of promoting a high quality image for fishery products.

2.3 Glass

With the exception of some fish pastes. glass is rarely used for fishery products which are preserved by heat alone; however. it is frequently chosen to package semi-preserved items such as salted fish. pickled herrings and caviars. The principles of processing in glass are substantially the same as for cans, but there are certain modifications which are necessary because of the sealing mechanisms used. and the thermal properties of glass, which make it vulnerable to rapid changes in temperature of more than 50 C.

2.3.1 Sealing mechanisms

Like cans, glass must be hermetically sealed to prevent product contamination after sealing and processing. Closures for glass container are made with either lacquered tin plate or aluminium into which has been placed a flowed-in plastisol lining compound (or a rubber ring with a pry-off cap) that acts as a sealant between the glass surface (called the "finish") and the cap. The closure is held in place by the vacuum in the container and/or the friction between the glass finish and the cap. The sealing surface of the glass may be across the top of the finish as with twist caps (Figure 19) or around the side of the finish as with pry-off caps (Figure 20) or around both the top and side seals as with push-on twist-off (PT) caps (Figure 21). It is important that the glass sealing surface be free of defects and protected from damage. as otherwise there is an unacceptable risk that the container will leak and draw in contaminants. It is because of the latter requirement that it is recognized as good manufacturing practice to ensure that the diameter across the finish of the jar is less than that of the diameter across the body of the container.

Figure 19 Cross section of twist cap applied to glass finish: top seal

Figure 20 Cross section of pry-off cap applied to glass finish top and side seal

Figure 21 Cross section of press-on twist-off (PT) cap applied to glass finish: top and side seal

This prevents the closure from suffering undue damage through striking the closures on adjacent containers as they move along conveyors. Fortunately, with most containers that have lost their hermetic seals prior to processing, the caps will fall off during retorting and thus alert operators to pack failure.

In addition to obvious loss of vacuum. other faults to be aware of when using glass include the following:

2.3.2. Inspection procedures

The frequency of inspecting for adequacy of seals with glass containers should be sufficient to ensure consistent formation of hermetic seals. As a guide, this means that intervals between non-destructive testing should be no more than 30 min. while destructive testing should take place at least every four hours. In addition to this. visual inspection should follow every occasion that the capper jams. The results of all closure examinations should be recorded on the Appropriate form.