Canning

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Canning is a method of preserving foods through sealing them and became possible with the invention of tinplate. This allowed fruit, vegetables and fish to be sterilised and stored for long periods. Before freezing was generally available, canning enabled people to get these important food groups during the winter period.

Bottling and canning are closely intertwined as methods of preserving foods, so their histories are similar - the differences in general are that bottling is used more regularly in domestic kitchens and canning is a more industrial process, because of the costs and equipment involved. In reality, the best advice for home canning is - Don't do it! Because there is always the fatal chance of leaving some Clostridium botulinum spores alive, which can give rise to botulism when eaten.

During canning, the foods are sealed in the cans before sterilisation. Fruit is sterilized at near boiling, and vegetables at well over boiling point in an autoclave for 20 - 45 minutes, which kills all bacterial spores.

History of Canning

Although foods had been cooked and sealed for many years, it was not until the early 19th century that the story of canning really began. In 1810, Nicholas Appert published his book L'Art de conserver which explained his method of bottling, which was endorsed by the French government. His book was soon available in Britain (1811) then America (1812).

Appert took food of any type - fruit, meats and vegetables - and placed them into a glass jar with a wide-mouth. All the foods Appert used were either liquids or packed in liquids. These glass jars were then closed with a cork stopper, sealed with a weird mix of cheese and lime and then wired down like a champagne cork. The bottles were wrapped in sacking, placed in a water bath and then heated to "cook" them. Appert worked out relevant cooking times for different types of food. Further, he tested their stability in tests carried out in 1806, including shipping the preserves across the equator.

At the same time, Bryan Donkin undertook similar science in the UK - nicking ideas unashamedly from others. In 1808, Donkin was awarded 5 guineas for his technique of bottling fruit, which he took from Appert without any acknowledgement. Donkin kept his corks moist and airtight by storing the jars on their sides. Soon afterwards, he then started to use tinned iron containers in place of glass jars; it is not clear whether he knew that this method had already been patented twice in the UK during 1810, but similar to his treatment of Appert he ignored that trivial barrier. In 1812, he began to manufacture canned foods, offering samples to the Royal Navy and several British explorers. By 1818, his company, Donkin, Hall & Gamble, was the naval supplier of canned foods. Indeed, two of their cans provided for an 1824 Arctic expedition were opened in 1938 and found to still be in decent condition.

However, because many cans were large in size - over 6 kg (14 lb) in weight) - that the heat could not diffuse to the middle of the food, so the centres were not always sufficiently cooked to be safe and some would spoil. The Royal Navy, therefore, ordered that no cans should be larger than 2.7 kg (6 lb) in size, and in 1856, they set up their own cannery in Deptford, UK.

The earliest canned foods were expensive items. However, with meat scarce and relatively costly in the Britain of the mid-19th century, importing canned beef and mutton from Australia became economic. In the 1870s, US canned meat, then South American meat, began to be imported. These meats were of a much lower quality than before, so were a cheap food available for the poor. They were coarse in texture and fatty; they were given crude nicknames, e.g. "Sweet Fanny Adams" after the victim of a notorious murder victim in 1867 who was hacked to pieces. Cans of pork and beans were manufactured for the fishing fleet in Portland, Maine, in 1875, and can be regarded as the forerunner of the classic baked beans.

Canning of tomatoes began in 1847-9 in Pennsylvania. A powerful marketing drive that sent samples to President Polk and Queen Victoria soon found public acceptance, with tinned tomatoes remaining a staple of every cook's storecupboard to this day. Peas were first canned at scale in Baltimore in the 1850s, moving to a staple from a luxury relatively quickly. Californian peaches were being canned from the 1860s, with Hawaiian pineapples appearing in 1892.

The first canned fish were sardines, which were being commercially canned in Nantes in the 1820s. Traditionally, sardines had been salted and/or smoked to delay their rapid decay if not preserved. Canning in oil gave a product that tasted more like fresh fish and many regard as a delicacy. Sardines on toast remains a popular quick and easy light dish.

Condensed milk was one of the first items to be made, being bottled by Appert for the French government in his initial trials, with others later doing likewise, often sweetening. The concept was both to preserve fresh milk and to reduce its bulk, so giving it shelf life and reducing transport costs in a time without cheap fossil fuels. The breakthrough came in the 1856 when Gail Borden of Brooklyn, New York, patented his vacuum evaporation process. Borden's process worked at temperatures below boiling point, which resulted in processed milk that tasted less burnt and more like real milk. Borden's condensed milk was initially sweetened, but soon unsweetened versions were available; the sugar is used as a preservative and unsweetened condensed milk must be heated to a much higher temperature to preserve it.

By the 1880s, canned foods were a staple for people's general diet. Cans looked much like they do today, and the tins were thin enough to be opened with a can opener. Sealing was still made by a cumbersome soldering process. Nowadays, automated canning clamps a lid on and creates a double seam that seals the cans rapidly and securely.

One of the important side-effects of canning was that it expanded the globalisation of the food supply chain. Previously only ambient, dry foods had been tradable over distances - spices and teas - but now meats, fish, fruit and vegetables could also be traded globally. When refrigeration and better preservation techniques became economic, these supply chains were further expanded.

Modern Canning Processes

Modern canning starts with an initial blanching of the foods at a temperature just below boiling point. This destroys any enzymes that might discolour the food or give "off" flavours, and it removes air, which helps in filling the can.

Filling is done a continuous production line. The can is packed with food and topped up with brine, sauce or syrup, but leaving a 6 mm (0.25 in) headspace for expansion. The lid is clamped on by the machine and a double seam soldered on. It is important that the filled food is hot at this stage, so that when the can is heated it does not expand significantly and so burst the contents. If necessary, the can is prewarmed with hot water or steam.

The filled can is then heated in an autoclave, which allows the contents to heated under pressure to well above boiling point, plus the pressure helps prevent the can from bursting. The actual cooking temperature and time depends on the food, especially its acidity. The key requirement here is to heat the foods to destroy any botulism spores - Clostridium botulinum does not grow in acid conditions, so acidic foods like canned fruit or tomatoes only need to heated to boiling point at the centre of can, but other foods must be cooked to 121oC (250oF) or more for at least 3 minutes to destroy the spores of C. botulinum. This is why many canned foods seem to be "overcooked" - because they are.

Further Reading

  • Davidson, A. (1999) The Oxford Companion to Food, Oxford University Press. ISBN 0192115790.
  • Stobart, T. (1981) The Cook's Encyclopedia: ingredients & processes, Harper & Row. ISBN 0060141271.