Which material is best for preserves?

For two centuries, tin cans and jars proved to be an excellent way of preserving foods safely over a long period of time. In the meantime, however, more and more food products are appearing on the market packed in other materials, especially plastics in bag or can format. What advantages do these materials offer? Are they just as safe as the traditional cans, or is the good old tin can era possibly coming to an end?

The process of sterilizing and preserving foods in an autoclave has undeniable advantages. Foods treated in this way keep well for years, they are protected from oxygen, and they do not need to be kept constantly cool during storage. This is a significant advantage over frozen products that only keep for 12 or 18 months and whose storage consumes a great amount of energy. Preserves are among the safest foods that we know. Whilst oily fish can even become rancid when frozen, this problem does not occur in tinned herrings, sardines or mackerel. Canned foods are also highly convenient in that they can normally be consumed immediately after opening. This is a definite advantage over frozen products which have to be thawed first.

Tinplate and aluminium cans are robust, tried and tested. They can be recycled, but are sometimes difficult to open.

There are not many product ideas that were developed over 200 years ago and have survived almost unchanged to the present day. Although "modern" canned foods do not have very much in common with the "original" invention of Nicolas Appert, which was more the shape of a bottling jar, despite all technological progress the principle behind preservation techniques works the same way today as it did then. Fish, fruit, vegetables or other foods are filled into a hermetically sealable container, if possible so that when closing the lid as little air as possible remains inside, and then heated to 100° C or more to kill bacteria and other germs. Once this has been done the contents are reliably preserved and protected from spoilage for a long time. In 1938 several cans of food were found in the Arctic, the leftovers from an expedition undertaken in 1820. Their contents had not spoiled and are said to have still been edible. Durability of more than 118 years is quite amazing for products that were produced in the days when the first cans were just coming onto the market and the technology for their production was still in its infancy.

Today in Germany alone, about 3.9 billion tinplate cans and 1.3 billion glass jars of preserves are placed onto the market every year. In the United States the beverage industry fills about 100 billion aluminium cans with beer and soft drinks each year. Not only are the huge quantities remarkable, but also the variety of materials from which these food containers can be produced.


In the beginning was the preserving jar

The process of preserving foods in sealable jars, for which the French chef and confectioner Nicolas Appert was awarded the sum of 12,000 gold francs by Napoleon in 1810 for "the art of preserving all animal and vegetable substances ... with all their freshness, flavour and individual attributes for several years" has lost none of its significance today. In Germany, the proportion of glass jars compared to tins or other types of preserves has even risen slightly in recent years. Fish, sausages and meat preserves only rose by 4.5%, but the market share of fruit, vegetables, pickles and sweet bread spreads grew last year by between 8.7 and 13.1%.

One advantage of the relatively light, thin pouch is its light weight, but the contents inside have little protection against mechanical damage.

The advantages of preserving foods in glass jars are obvious. The material glass is tasteless and resistant to all the salts and acids (vinegar) that are used in the food industry. Because of these properties glass jars are particularly well suited to semi-preserves such as salted fish and pickled products (Rollmops. Bismarck herring). In addition, glass is usually transparent and allows the consumer a visual assessment of the product in the jar. This may well be beneficial, but sometimes also a problem, depending on the state in which the product is presented and how it was processed. Whereas chunks and flakes of tuna, for example, are usually packed in cans, large attractive fillet pieces are often packed in glass jars that allow the consumer to see the outstanding quality of the contents. Another advantage of glass is that it can easily be recycled.

The market share of glass jars would probably be even bigger if glass did not also have some significant disadvantages. For example: transparent material is not always suited to photosensitive products that fade or decompose under the influence of light. Glass is also relatively heavy, which puts a burden on the CO2 balance during transport, and also fragile, meaning that glass containers have to be handled with great care. The fragility of the material must also, of course, be taken into account already during the production of bottled preserves, although the process more or less corresponds to that of other types of food preserves. Canned foods can be strongly cooled immediately after autoclaving to prevent overcooking which would have a negative effect on the consistency of the contents. This is often not possible in the case of glass containers because temperature changes of over 50 degrees put a strain on the glass and can cause it to break.

In France, where the preserve was "invented" over 200 years ago, well-known producers market their products in their own specialized shops.

Cost is another significant aspect: closing glass jars is a relatively expensive process. Twist-off metal lids are usually used today. The lid is held in position by the vacuum within the jar and by a slight turn which holds it firmly on the jar like a screw top. The safety of the seal depends on several factors. The upper edge of the jar must be clean and the rubber ring in the lid intact. The lid must be placed onto the jar perfectly horizontally and should snap into position correctly when twisted. Only then will the vacuum remain intact and this is the prerequisite for the perfect condition of the preserve. That is why there should an audible click when the twist-off lid of a jar of preserves is opened for the first time.

Cans have to be laminated

In the same year in which Napoleon bestowed on Appert the impressive prize for the successful testing over a number of years of his preservation process the tin can was invented. The British merchant Peter Durand had taken up the idea, only he did not put the food into jars, but sealed them airtight into metal containers. He sold the patent which he received for his invention in August 1810 to Bryan Donkin and John Hall who, three years later in Bermondsey opened the world’s first canning factory that used containers made of tinplated sheet iron to make food durable for a long time.

Tinplate, cold rolled thin steel sheet, whose surface is coated with tin by means of electrolysis to prevent corrosion, is still the preferred material for cans used in the food industry. The technology for producing tinplate is in the meantime so sophisticated that one gram of tin is enough to coat one square metre of steel plate with a 0.15 mm thin layer. For additional protection tin cans are often varnished on their interior or coated with a thin plastic film (full seal) because some foods that contain fruit acids can dissolve tin from the can metal. Although tin is vital for human beings and has to be absorbed with the diet the necessary intake is extremely low and can be covered easily. The World Health Organisation therefore recommends limiting the daily intake of tin from food to 2 milligrams per kilogram of body weight. Cans with an interior plastic coating release practically no tin into the food they contain.

In Thailand and other supply countries tuna is preferably packed in flat round cans.

However, more recently, some of the substances that are used to coat food cans have come under criticism. The BADGE (bisphenol-A diglycidyl ether) that they contain can be passed on to the preserves in small amounts and this substance is suspected of being a possible carcinogen and mutagen. In the opinion of many experts this will not lead to any acute health hazard but the contamination is generally "undesirable." The problem of slow poisoning by substances which pass from the cans into the food they contain is not new. In 1845, Sir John Franklin and his companions set out on an Arctic expedition to search for the Northwest Passage. Within three years, during which time they lived almost exclusively from canned food, they all died of lead poisoning. The contents of the cans were edible and not in any way spoilt, but at that time cans were soldered with lead.


Opening the cans remains a problem

Today efforts are made to find better solutions to this problem and thus limit and control the resulting risks. One approach is to produce aluminium cans, which first emerged in the late 1950s and are now – hardly noticed by many consumers – taking over more and more of the market. Aluminium is lighter and cheaper than tin, and offers the same protection against corrosion. In addition, it is much more malleable than tinplate. This makes it possible, for example, to form cans from a single aluminium plate, in contrast to tinplate cans which always have to be shaped and folded in several labour-intensive steps. Aluminium cans often have a conical shape so that the blanks can be stacked together to save space.

Regardless of that the variety of shapes and sizes of cans is today immense. Not only are there preferences for certain formats in individual countries, but also preferences for certain shapes for individual products. Tuna, for example, is most often to be found in shallow cans, wild salmon usually in tall round cans, and herring products in flat oval cans. The most important advantage of all metal cans is probably their mechanical stability which makes them virtually indestructible. This is, however, also their biggest disadvantage and generations of inventors have put a lot of effort into developing an effective can opener. Ring-pull openers, a slotted key around which a pre-scored metal strip can be wound for opening sardine tins, and other devices have solved the problem to some extent, but countless fingernails are still broken every year while opening cans.

Both tinplate and aluminium cans have the advantage that they can be completely recycled and can be used repeatedly without loss of quality. In the U.S., about every second aluminium beverage can and almost two thirds of all tin cans are recycled. In Germany the recycling ratio for cans is even well above 90%.


Pouches have good barrier properties

About ten years ago pouches, which have been known in Asia for some time, became popular in North America as a packaging material for food preserves. Pouches are lighter than cans, their production requires less energy and their contents do not need to be heated for so long to preserve them. While canned tuna is usually cooked twice, the contents of the pouch are usually heated only once, which is reflected in a better product quality. After a suitable piece of fillet has been placed into the bag it just has to be closed under vacuum and then sterilized by heating.

If the plastic film of which the bag is made is not punctured, torn or otherwise mechanically damaged, the manufacturer guarantees a shelf-life as long as that given for conventional cans. That is usually a minimum of four years. The multilayered material from which the pouches are made already makes sure of this. In general, it consists of polyester film that is laminated with aluminium and sealed with polypropylene. This makes the material resistant to temperature and very resilient. Pouches have good barrier properties against light, gases, micro-organisms and odours. And the bags can be easily opened by tearing along a notch on the side or using scissors. However, the flexible material does not serve as sufficient protection of the pouch’s contents against pressure or similar external influences. If a customer presses or squeezes the bag to convince himself of the shape and consistency of its contents the product may be damaged. This is probably the reason why at present, it is mainly tuna and salmon, i.e. fish species with relatively firm flesh, that are preserved in pouches.

Before the cans are sealed they are filled up with brine so that there is no more air left in the interior.

Meanwhile pouches are available in a further developed form as stand-up pouches with a big base that allows for upright presentation and storage of the bags. Stand-up pouches are hardly used so far in the seafood sector and are mainly used for packing beverages, liquid soaps, detergents and the like, but it will probably not take long before the fish industry also discovers the potential of this packaging.


Plastic boxes of all shapes and colours

For several years tin cans and glass jars have had to face new competition. At FachPack 2007 the packaging specialist Weidenhammer presented a can made of ‘Perma Safe’ plastic which offers huge advantages. Perma Safe is lightweight, dimensionally stable and can be sterilized (it is even said to be able to withstand microwave), it can be formed to produce various shapes and can be printed on. With the Perma Safe packaging Weidenhammer is making use of the advantages of plastic materials for preserves for the first time. The plastic packaging is just as stable and robust as conventional sterilizable cans made of solid materials, but much easier to handle, more lightweight and more cost efficient. Perma Safe packaging which is made using injection moulding is virtually impermeable to oxygen and water vapour. Using the removable sealing film the packs can be opened very easily. Previous tests have shown that sterilized and pasteurized foods preserved in plastic containers have a shelf-life of at least one and a half years. This is not as long as traditional tinplate cans or glass jars but the plastic packaging offers considerable advantages in terms of weight and stackability, energy balance during processing, transportation and storage.

Like all packaging materials, plastic cans also have certain advantages and disadvantages. The ideal material for preserves that meets all requirements and is absolutely superior to all others does not exist. Depending on the product and intended use, one thus has to decide from case to case which packaging material is most suitable. Only one thing is certain: with each new material, with each additional packaging that comes onto the market, tinplate and glass, the "classic" materials for food containers, lose a bit more of their market share. That they might eventually disappear altogether, is currently not very likely however ...