Properly prepared canned products will remain stable and safe to eat over long periods provided that they are stored and handled correctly. Cans cannot be stored indefinitely; some products have comparatively short shelf lives.
During the manufacture of canned product many extracting criteria must be met but two in particular are fundamental to the achievement of shelf stability.
- The cans must be hermetically sealed which requires the can components to be sound, correctly assembled and properly seamed together. Unless an hermetic seal is achieved, spoilage, after short periods of storage, will result in the majority of cases.
- The cans must be adequately processed, which requires careful control of the thermal process (i.e. heating, cooling and drying) to ensure that all bacteria or micro-organisms which would otherwise cause spoilage, are destroyed or denied access.
Bacterial spoilage may manifest itself by the distension (blown cans) caused by gases formed by the bacterial action, although surveyors should be aware that spoilage can occur without gas formation, depending on the organism involved. The distension will vary from a reversal of the concavity of the ends (which may be pressed back with the thumb) to heavily swollen cans with permanently distended ends and a reversal of the countersink area. Ultimately the cans may explode and surface contaminate, damage and consequently corrode other cans in the vicinity. Such associated damage should also be reported.
The type of spoilage and its significance to public health will depend to a large extent upon PH (degree of acidity). Bacterial spoilage of low acid foods (pH 4,5 or above), e.g. meat and fish products, vegetables etc. could lead to food poisoning. High acid foods such as canned fruit, tomatoes, tomato puree etc. with pH’s below 4,5 will spoil but not present a serious food poisoning risk. Expert examination of the cans and contents, preferably those which have not ‘blown’ but are showing signs of spoilage such as swelling, will usually enable the problem to be accurately diagnosed.
External physical damage to cans may result in loss of can integrity, with potential contamination resulting, even though no visible leakage, hole or aperture exists. Additionally, physical damage may result in external rusting due to damage to the surface coating or internal corrosion by aggressive contents on areas damaged internally by the external impact.
Rusting of cans may be caused by wetting with fresh or salt water, condensation due to rapid changes in ambient temperature, from the glue used to apply labels or from high levels of sulphate or chloride in the label or case packing material. Rusting may be light surface rust through to highly localized and deep seated pitting. Rusty body plate below a label which shows no corresponding rust staining usually suggests that the cans were rusty when labeled.
The implications of damage and rusting to the safety of the product is a matter for the expert judgement of a canning technologist, and such advice should be sought if there is any doubt. The length of satisfactory storage time depends on the nature of the product inside the can, the specification of the can used, and the external conditions to which the can is exposed.
Cans are highly susceptible to external bacterial contamination immediately after the heat process, when cans are still wet, even when a good seam has been formed. Only when cans are cool and dry is this risk eliminated. The presence of seam defects increases the chance of this contamination occurring particularly whilst the cans are wet. Seam defects may be evident only to an expert, and careful visual examination together with internal seam measurements may have to be carried out to enable proper identification.
Bad design or maintenance of automatic handling equipment can cause damage which may include perforation. In such cases bacterial spoilage will occur which will usually not be manifested by ‘blowing’ of the cans as the gases can escape, although occasionally this may happen due to the product itself sealing the hole.
Inadequate drying of cans before labeling and packing can leave residual water which could cause rusting, particularly when plastic films have been used to collate products on trays.
Damage caused in handling of trays, cases or pallets can result in the problems described already. Wetting of outer packing due to exposure to the elements, or leakage of covering can result in subsequent rusting in transit when the moisture is taken up by the atmosphere surrounding the cans.
Exposure to cold conditions can result in the freezing of contents in extremis, but can cause subsequent condensation and rusting problems when the product is stored or stowed in warmer conditions. Freezing of the contents of the can may also have an unacceptable but non-hazardous effect on the quality of the contents. Stacking of pallets to excessive heights can cause crushing of product at the base of the stacks, with consequent damage, leakage and spoilage problems.
Damage and rusting problems can occur for the same reasons as described above, with similar effects. Movement of the load due to improper or loose loading of containers can result in a variety of problems from mild abrasion of labels to denting and critical underseam damage. In severe cases high levels of mechanical damage may be sustained resulting in crushed, leaking and badly dented material. Shifting of the load in a container can result in the consignment being impossible to remove without damage to the product.
Wetting of product due to leaking containers or covering, or to flooding, will result in rusting, but the problem will be exacerbated by salt water which will accelerate corrosion. Immersion of product in contaminated water (such as bilge water) brings additional risks of product contamination, and product spoilage. The quality of seam formation, and the presence of seam defects are critical in this respect, and no submerged product should be considered acceptable for human consumption. Product above the flood level will be subject to risks of splashed contamination and rusting, and expert advice should be sought in disposal of such product.
High temperatures in transit can result in bacteriological spoilage of product by organisms which will only grow at temperatures above those normally experienced in the storage and distribution of canned products. Such organisms (called thermophiles) have an optimum growth temperature of 55°C, and many will not grow below 40°C These organisms are latent in commercially sterilized canned goods (unless they have been subjected to a thermophilic organism process), and only under these extreme abuse conditions will they cause problems. Again, expert advice based on laboratory examination should be sought.
Condensation can cause rusty tins and the labels to peel off or become moldy, rendering them unusable on supermarket displays. Minimizing all sources of moisture is the key to a successful out-turn.
The following items should be considered when planning shipments:
- Ensure container is clean and dry and particularly the floor. A plastic sheet fitted over the floor is recommended.
- Check quality of cargo packaging with special regard to moisture content.
- Loading environment, especially in winter conditions, should be controlled.
- All steps should be taken to avoid moisture being absorbed into the container during loading via footwear or through an unprotected loading bay.
- If cargo is on pallets the moisture in the timber should be considered. Experience has shown that pallets that have been left in an outside yard prior to stuffing, are a sure source of condensation problems.
- Passive vents on the inside of a GP container to be taped over and “Dry-Bag” desiccant bags placed on top of the stow and in way of the door. The number of bags is subject to the anticipated climatic conditions during transit.
- Avoid using moist timber dunnage to provide stow stability.
- Cartons should be packed using a locking stow to provide even weight distribution and stability in the stow.
- If additional support is required, introducing cardboard slipsheets or similar at different tier heights will suffice.
- A protected stow, aboard ship and at terminals, away from temperature extremes such as radiant heat, will assist in protecting the cargo from temperature gyrations. As a general rule this is only necessary where extreme temperatures are anticipated.
Expert advice should always be sought by those responsible for the decisions on recovery or refurbishment of distressed consignments of canned goods. Abrasive methods of removing rust provide only a very short term and essentially cosmetic remedy; rust will rapidly reappear under normal atmospheric conditions. Mechanical damage demands expert assessment; apparently superficial damage may, depending upon its proximity to can seams, have far more significance than more severe denting in less critical areas. Any recovery operation which involves wetting the cans or treating with solvents should only be undertaken after careful consideration of the possible (and potentially serious) consequences by those qualified to make such judgements.