The stages of canning
- cleaning of raw materials;
- size reduction;
- Metal Detection;
Cleaning of raw materials
All foods to be canned must be washed thoroughly to remove contaminants and inedible matter. For some products this is a one-step process. For others, such as stews, all separate ingredients must go through this process.Size reduction
Certain ingredients, such as vegetables, need to be sliced or diced in order to fit into cans. However, some varieties of carrot are grown especially so that they fit into the can whole.Blanching
Products which contain fruits and vegetables need these ingredients blanched (i.e. immersed in boiling water) before they can be packaged. This process helps with filling the can. Blanching must take place, quickly & cooled immediately to prevent any enzymic reactions such as discolouration from occurring.Metal Detection
Detection and removal of metal contamination (Ferrous, Non Ferrous & SS) from incoming raw materials and formulations prior to/in process of manufacturing of foods.Filling
The cans are filled automatically/Manually with a measured weight or volume of product. A solution of brine, savoury sauce, fruit juice or sugar syrup is usually added. A space is left at the top or the can will distort when sterilized. Food is also packaged in plastic containers. These are known as Retortable Pouches.Sealing
The cans are sealed, under vacuum, using a double seam on the can rim. A vacuum is applied to draw out the air at the top of the can and seal the lid. At this stage some product may seep out.Washing
Once sealed, cans are washed to remove any external particles, and are then ready to be sterilisedSterilisation
Batches of cans are placed in a retort, which works like a large pressure cooker. The time taken to sterilise the contents at boiling point would be relatively long. By canning under pressure less time is needed as the temperature rises to 121º C / 127º C.
The retort is sealed and steam is injected. This causes the temperature to rise and eventually results in air being driven out of the retort.
The type of product being canned is of importance at this stage. 'Solid pack' contents, e.g. canned mushrooms, need more time as the heat needs to penetrate the product by conduction. However 'liquid pack' contents, e.g. soups, need far less time as the liquid present helps transfer the heat by convection. This will dictate the pressure and time needed to sterilise the product throughout.
After the cans have been sterilised, they are cooled to prevent overcooking of the contents. This is achieved by spraying cool water over the cans and a gentle reduction in pressure. Any sudden drop in temperature would cause the cans to distort and damage the can seams. The cans are cooled properly to reduce the temperature further.Drying
The remaining heat from the can evaporates any water left on the surface. This is important to prevent rusting during storage and the risk of intake of dirty water if there is a seam defect.Labelling
Finally, cans are coded then labelled with a 'best before' date.