The basic material used for most food cans in the UK is steel. The first step in the steel making process is to produce molten iron. This is done by mixing iron ore with coke and limestone and heating them at very high temperatures in a blast furnace. This molten iron is fed into a vessel, along with a percentage of recycled steel and other chemicals, after which oxygen is blown into the vessel, again at high temperatures, to produce steel.
Steel used to make food cans has been processed and rolled into huge coils which are then shipped to a tinplate works. There they are given a fine coating of either tin, or chromium oxide for tin-free steel. This is to protect the steel from oxidation.
Rolled coils of coated steel are then delivered to can-making factories. For acidic foods like tomatoes and rhubarb, a thin coating or lacquer is needed on the inside of the can. This is put on at the can-making factory before the can bodies are stamped out. Tin coating and tin free steel are non-toxic, safe and strong. More than 25 lacquers and 30 tin coatings have now been developed for use with different foods.
There are two ways to produce food cans, either using two or three pieces of metal. The traditional method – three piece can-making – involves cutting one metre square sheets of steel and, in most cases, coating them on the inside with lacquer to protect the food inside the can. Smaller rectangles of steel are then cut and curled into a cylinder and the edges welded together. The bottom end is then seamed on and the empty can is sent to the canner to be filled, sealed and cooked.
To make a two-piece can – the latest and more modern method – the can maker feeds 10 tonne coils of steel through an automatic cupping machine where small steel cups are formed. The cups are fed into a machine that draws (stretches) the cup to make it taller. This process is called the ‘draw and wall iron’ process. The result is a can body with thinner side walls than the original steel thickness.
The top of a two-piece can and the top and bottom of a three-piece can body have to be curled out to make a lip to which the lids, or ends as they are known in the canning industry, can be fitted. This process is known as ‘flanging’. The curled end of the can is treated with a compound which ensures an airtight seal between the end and the can body. The cans are also ‘beaded’ with concentric rings around the middle for strength during cooking and stacking.
There are more than 50 sizes of three-piece and 12 sizes of two-piece cans available in the UK. Look out for them on your supermarket shelves!
The empty cans are packed on pallets and delivered to canning factories, often situated in the heart of the country’s food growing areas, to be filled. The cans and ends are sent separately so that the ends can be put on after filling, ready for the cooking process.