|Fish Handling, Preservation and Processing in the Tropics: Part 2 (NRI)|
Canning is a relatively modern technology which enables man to preserve food in an edible condition under a wide range of storage conditions for long periods - from a few months to several years. Essentially, the process involves hermetically sealing the food in a container, heat 'sterilising' the sealed unit and cooling it to ambient temperature for subsequent storage.
Filling and sealing
Fish, being a physically delicate food and, therefore, easily damaged and fragmented by mechanical handling operations, are still largely packed into cans or other retortable containers by hand, with brine, edible oil or sauce which may be metered in mechanically. Often, the fish, after the usual heading, gutting, cleaning and trimming operations, are subjected to pre-processing operations such as salting, brining, drying, smoking, cooking or a combination of these. Such pre-processing operations have the advantages of:
(a) denaturing the proteins and thus rendering the fish muscle firmer and more capable of withstanding handling during the filling operation; and
(b) removing water from the fish making them less subject to shrinkage and unsightly aqueous exudation inside the can during heat treatment.
Canned fish is famous for the way it is packed so tightly within the container, leaving very little space for additional liquids.
Heat transfer through the fish is by conduction and, therefore, very slow; at a processing temperature of 121°C, it would take 6 hours to raise the centre temperature of a 145.5 mm (diameter) by 168 mm (height) can from 10 to 100°C by conduction alone. In this time, the fish nearest the walls of the container would be grossly overcooked. By comparison, if ail the heat could be transferred by convection, in the same size can under the same conditions, it would only take 20 minutes to achieve the same temperature rise at the can centre. Obviously, it is best to have the fish surrounded by liquid so that the distance through which heat is transferred by conduction is kept to a minimum. Most fish canners increase in-can heat transfer rates even further by processing the cans in a rotary retort. The movement of the headspace bubble during rotation forces an increase in liquid movement and, therefore, convection heat transfer. The fish are more evenly cooked throughout the can and, those nearest the can walls are less likely to be overcooked (See Figure 3).
The headspace (or ullage) is the space left in the top of the can to allow for expansion of the contents during the heating process. However, leaving air in this headspace causes considerable internal pressure during processing and leads to oxidation of the contents (surface discoloration and rancidity) and the container (corrosion) during subsequent storage. it is, therefore, necessary to seal the can under vacuum (See Figures 4 and 5).
The Figures show how the lid (or end) is attached to the body of the can in the two DOUBLE-SEAMING operations. It is vitally important that the side seam and the double seams are completely hermetic. The former uses solder (98 parts lead: 2 parts tin) to complete the seal whilst the latter is sealed by the melting and resetting of a plastic sealing compound on the inside curl of the end-piece (See Figure 5).
It is important that double seams are checked regularly both for visible faults and, by measurement for slackness, for the extent to which the body hook penetrates the sealing compound in the curl of the end-piece.
Not all fish which are sealed into cans are heat processed. Anchovies, for example, are packed in salt and then sealed in cans without any further processing: the very high salt content prevents subsequent growth of micro-organisms. However, the product can only be eaten in very small quantities in this form and is generally used as a condiment or flavouring in other dishes.
If heat sterilisation is to be the method of preservation, it is essential that the effect of severe heat treatment on the fish tissues is known.
Firstly, it is impossible to produce a high quality canned fish product from fish which are at an advanced stage of spoilage.
Secondly, as the temperature rises, the muscle proteins become increasingly denatured and progressively lose the water which is loosely bound in the undenatured protein network. The watery exudate is unsightly and may cause a sauce to curdle whilst the partially dehydrated fish muscle, although surrounded by liquid, has a dry 'woolly' feel in the mouth. Besides this denaturation, the severe heat treatment may cause some degradation of proteins to amino acids and other simple (but often malodorous) breakdown products which may also react with the metal of the can walls, producing unsightly black deposits.
The quality of oily fish is much less impaired by the severe heat process than that of non-oily fish, which generally yield a product only suitable for fish paste or pet food manufacture. This may merely be a physical effect of the oil in the muscle tissue acting as a barrier to water loss from the protein structures, so enabling canned oily fish to retain their succulence through the heat process.
For the purpose of determining the degree of heat treatment which is needed to preserve food within a can, three pH groupings are recognised:
(i) Acid foods at less than pH 4.5 cannot support the growth of heat-resistant spore forming pathogens like Clostridium botulinum. To effectively preserve such foods (e.g. most fruits and pickles), it is necessary only to destroy the relatively heat sensitive acid tolerant microorganisms which could otherwise grow and cause spoilage. A mild heat process (e.g. the coldest point in the can should receive a minimum process of 5 minutes at 100°C) only is required.
(ii) Medium-low acid foods between pH 4.5 and 5.3 will support the growth of pathogenic heat-resistant spore formers like C. botulinum and must, therefore, be processed to reduce the chance of such a spore surviving to virtual insignificance (e.g. the coldest point in the can should receive a minimum process of 10 minutes at 121°C).
(iii) Low acid foods with a pH greater than 5.3 will support the growth of organisms like C. botulinum, as well as the germination and growth of highly heat-resistant spores like those of Bacillus stearothermophilus which cause flat-sour spoilage. Fortunately, these organisms will only germinate and grow at temperatures greater than 37°C because, if it were deemed necessary to heat process to destroy them, the severity of the process would probably render the food inedible.
Fish are a low acid food and it should, therefore, be remembered that canned fish which have been processed to eliminate the chance of C. botulinum spore survival should be stored at temperatures below that at which possible surviving spores of B. stearothermophilus could germinate.
Figure 6 is a thermal death curve showing the number of surviving spores against time at a given process temperature. Figure 7 is a death rate curve and shows the log10 number of surviving spores against time at a given process temperature. These figures show that the destruction of bacterial spores at a given process temperature is not instantaneous but decreases logarithmically with the exposure time to that temperature. The time taken for the graph to traverse one log cycle (i.e. the time taken at a given temperature to reduce a particular bacterial spore population to one tenth of its original number) is called the Decimal Reduction Time, D(O) (O being the given temperature).
Figure 8 is a thermal death time curve and it shows the Decimal Reduction Time against the process temperature. From this figure, the temperature interval over which a ten-fold increase or decrease in the value of D(O) occurs is called the Z value.
Most fish canning heat processes are based on the elimination of C. botulinum spores, which is reasonable since this is the most heat-resistant pathogen which could grow in the canned fish. However, it can be seen from Table 1 that a process achieving 12 decimal reductions of C. botulinum spores (i.e. 12 x 0.2 = 2.4 minutes at 121°C) would only achieve approximately half a decimal reduction of B. stearothermophilus spores.
Obviously it is not possible to achieve a 'cold spot' temperature instantaneously. The temperature at the cold spot rises slowly throughout the process which may use a temperature below 121°C anyway. It is therefore necessary to know the 'lethality' of all temperatures with respect to the lethality of 1 minute at 121°C. For this we use the reciprocal of D called L value or 'lethal rate'.
So, in this example, it takes 1000 minutes at 91°C to achieve the same killing effect on bacteria as 1 minute at 121°C: or 1 minute at 91°C has 1/1000 the lethal effect of 1 minute at 121°C.
If the initial concentration of bacterial spores is N1, which must be reduced in number to an acceptable level, No, the quantity log N1/N0 is called the 'Order of Process Factor'
Thus, if Nf is the number of spores surviving after processing
log Nf/N1 = log N0/N1
to ensure commercial sterility.
This can be expressed as follows:
is the L value for the related temperature occurring during the process lasting from time 0 to time tf,
is the order of process factor for commercial sterility, and
is the decimal reduction time for the spoilage organism under consideration at the reference temperature.
L values are available from tables or may be calculated from
where T is the related temperature.
Once the temperature history of the process of a canned food has been plotted and the main spoilage organism identified and its Z value found, a graph of L versus time of processing may be plotted throughout the process. The area beneath this graph must exceed mD for commercial sterility. This value has been called the 'equivalent time' end is the F value.
Packs with pH 4.5 are generally processed to commercial sterility with reference to C. botulinum, the minimum order of process factor 'm' being taken as 12. Thus, mD(q) should be 12 x 0.3 = 3.6 minutes at 121°C at the cold spot.
However, in different foods, there are often spoilage organisms with more heat labile spores than C. botulinum (See Table 3).
Sterilisation equipment (See Figure 9)
To achieve processing temperatures above 100°C, condensing steam under pressure is used in most conventional systems, although other processing media include gas flames, steam and air mixtures and even hot fluidised sand.
After the cans are sealed in the retort, steam is admitted and the temperature of the retort allowed to rise to 100°C and maintained at this temperature until all air is flushed out of the retort. Air pockets left in the retort can lead to localised under-processing, as air insulates any cans it may surround from the steam.
Pressure is applied by closing off the drain and steam exit valves whilst still allowing steam into the retort. Various petcocks are left open to allow any air, which may be admitted with the steam, to escape. Common processing temperatures are 115.5°C and 121°C. The pressure, and hence the retort temperature, is controlled by an automatic steam pressure control valve; this opens when the set pressure is exceeded and closes again when the pressure falls below that set.
The pressure in the retort is maintained after closing the steam inlet valve by admitting compressed air to the retort. If this were not done, the large pressure inside the can compared with the low pressure inside the retort would cause the cans to distort outwards ('peaking'), possibly damaging the integrity of the seams. As the retort pressure is being maintained with the compressed air, chlorinated cooling water is admitted to the retort.
The cooling water is chlorinated because, at this stage, the sealing compound in the double seams is still molten and the vacuum forming in the headspace due to condensing steam could pull drops of cooling water through the double seam. If this cooling water contains viable micro-organisms, this leakage may lead to 'leaker spoilage'. This type of spoilage is by far the most common that is implicated in food poisoning attributed to canned food. Cooling water is generally recirculated and dosed automatically with chlorine. A residence time of at least 20 minutes between dosing and utilisation for can cooling is necessary to allow the free residual chlorine responsible for the bactericidal effect to accumulate. The free residual chlorine content of the cooling water should be measured in water draining from the retort rather than in that entering the retort. Common chlorination levels lead to 5 - 20 ppm free residual chlorine in the drain water. Too high chlorination levels can lead to can corrosion problems.
As cooling proceeds, it becomes necessary to reduce compressed air pressure in the retort since the pressure inside the can falls with the temperature of its contents, eventually becoming a partial vacuum. If, then, the pressure outside the can far exceeds the pressure inside, the can may buckle inwards ('panelling') which could also damage the can seam.
When the cooling process has been completed (the can contents having reached a sufficiently low temperature and the retort pressure having been reduced to atmospheric), the retort is opened and the wet cans lifted out. It is essential that the wet cans are not handled at this stage: the danger of contaminating the can contents via a leaking seam still exists. Cans should be conveyed mechanically to a can drier along chlorinated runways before they are labelled and packed into cases or shrink-wrapped.
Other special problems related to fish canning
Fish proteins, and especially crustacean and shellfish proteins, are rich in sulphur amino acids which, on heat processing, release hydrogen sulphide. This can react with iron in the tinplate producing black ferrous sulphide ('sulphur staining'). To avoid these unsightly black deposits, a special lacquer incorporating zinc oxide or zinc carbonate is used to coat the internal can walls. The hydrogen sulphide released now reacts preferentially with the zinc oxide or carbonate producing white zinc sulphide which remains embedded in the lacquer so that an attractive internal appearance is maintained.
In some canned fish products, glass-like crystals of calcium struvite may form on storage and become the reason for many 'foreign body' complaints in canned fish. This phenomenon may be avoided by the addition of small amounts of citric acid to the product prior to filling and processing. Citric acid complexes available calcium ions, thus preventing them from forming calcium struvite.
In conclusion, it should be noted that, although fish may be canned to provide an excellent long shelf life product, which in the case of some canned oily fish products like sardines and pilchards is said to improve with keeping, setting up a commercial canning operation involves extremely high capital expenditure. Also in the case of fish canneries, the method of packing the fish in cans makes the operation highly labour intensive.
The canning line should be designed so that the retorts, can driers, and labelling and packing sections are well removed from the raw fish handling sections because of the danger of leaker contamination.
Quality inspection of raw materials, can seams and cooling water chlorination levels at regular intervals is essential but it is also normal quality control practice to hold back samples of cans from each retort batch for incubation tests - this means that canned products should carry some device which enables their production batch to be identified.
In general, it may be said that good quality canned fish products can only be made from good quality, clean fish. White fleshed fish tend not to make good canned products whatever their quality: the heat process makes a dry, discoloured product which falls apart.