Canning

A can of preserved food.

Canning is a method of preserving food in which the food is processed and sealed in an airtight container. The process was first developed as a French military discovery. The packaging prevents microorganisms from entering and proliferating inside.

To prevent the food from being spoiled before and during containment, quite a number of methods are used: pasteurization, boiling (and other applications of high temperature over a period of time), refrigeration, freezing, drying, vacuum treatment, antimicrobial agents that are natural to the recipe of the foodstuff being preserved, a sufficient dose of ionizing radiation, submersion in a strongly saline, acid, base, osmotically extreme (for example very sugary) or other microbe-challenging environments.

No such method is perfectly dependable as a preservative. For example, spore forming thermal resistant microorganisms, such as Clostridium botulinum (which causes botulism) can still survive.

From a public safety point of view, foods with low acidity (a pH more than 4.6) need sterilization under high temperature (116-130°C). Foods that must be pressure canned include most vegetables, meats, seafood, poultry, and dairy products. The only foods that may be safely canned in an ordinary boiling water bath are highly acidic ones with a pH below 4.6[1], such as fruits, pickled vegetables, or other foods to which acidic additives have been added.

Contents

History

"An army marches on its stomach." —Napoleon

During the first years of the Napoleonic Wars, the notable French newspaper Le Monde, prompted by the government, offered a hefty cash award of 12,000 francs to any inventor who could devise a cheap and effective method of preserving large amounts of food. The larger armies of the period required increased, regular supplies of quality food. Limited food availability was among the factors limiting military campaigns to the summer and fall months. In 1809, a French confectioner and brewer, Nicolas François Appert, observed that food cooked inside a jar did not spoil unless the seals leaked, and developed a method of sealing food in glass jars. The reason for lack of spoilage was unknown at the time, since it would be another 50 years before Louis Pasteur demonstrated the role of microbes in food spoilage. However, glass containers presented challenges for transportation.

Preserved food.
See also: Mason jar

Glass jars were largely replaced in commercial canneries with cylindrical tin or wrought-iron canisters (later shortened to "cans") following the work of Peter Durand (1810). Cans are cheaper and quicker to make, and much less fragile than glass jars. Glass jars have remained popular for some high-value products and in home canning. Tin-openers were not invented for another thirty years — at first, soldiers had to cut the cans open with bayonets or smash them open with rocks. The French Army began experimenting with issuing tinned foods to its soldiers, but the slow process of tinning foods and the even slower development and transport stages prevented the army from shipping large amounts across the French Empire, and the war ended before the process was perfected. Unfortunately for Appert, the factory which he had built with his prize money was razed in 1814 by Allied soldiers invading France.

1914 magazine advertisement for cookware with instructions for home canning.

Following the end of the Napoleonic Wars, the canning process was gradually employed in other European countries and in the US. Based on Appert's methods of food preservation, Peter Durand patented a process in the United Kingdom in 1810. He did not develop the process, selling his patent in 1811 to Bryan Donkin and John Hall, who were in business as Donkin Hall and Gamble, of Bermondsey [1]. Bryan Donkin developed the process of packaging food in sealed airtight cans, made of tinned wrought iron. Initially, the canning process was slow and labour-intensive, as each large can had to be hand-made, and took up to six hours to cook, making tinned food too expensive for ordinary people. The main market for the food at this stage was the Army and Navy. By 1817 Donkin recorded that he had sold £3000 worth of canned meat in six months. In 1824 Sir William Edward Parry took tinned beef and pea soup with him on his voyage to the Arctic in HMS Fury, during his search for a northwestern passage to India. In 1829 Admiral Sir James Ross also took canned food to the Arctic, as did Sir John Franklin in 1845 [2]. Some of his stores were found by the search expedition led by Captain (later Admiral Sir) Leopold McLintock in 1857. One of these cans was opened in 1939, and was edible and nutritious, though it was not analysed for contamination by the lead solder used in its manufacture.

Throughout the mid-nineteenth century, tinned food became a status symbol amongst middle-class households in Europe, becoming something of a frivolous novelty. Early methods of manufacture employed poisonous lead solder for sealing the tins, which had disastrous consequences for the 1845 Franklin expedition to the Arctic Ocean.

Increasing mechanisation of the canning process, coupled with a huge increase in urban populations across Europe, resulted in a rising demand for tinned food. A number of inventions and improvements followed, and by the 1860s smaller machine-made steel cans were possible, and the time to cook food in sealed cans had been reduced from around six hours to thirty minutes. Canned food also began to spread beyond Europe — porche established the first American canning factory in New York City in 1812, using improved tin-plated wrought-iron cans for preserving oysters, meats, fruits and vegetables. Demand for tinned food greatly increased during wars. Large-scale wars in the nineteenth century, such as the Crimean War, American Civil War, and Franco-Prussian War introduced increasing numbers of working-class men to tinned food, and allowed canning companies to expand their businesses to meet military demands for non-perishable food, allowing companies to manufacture in bulk and sell to wider civilian markets after wars ended. Urban populations in Victorian era Britain demanded ever-increasing quantities of cheap, varied, quality food that they could keep at home without having to go shopping daily. In response, companies such as Nestlé, Heinz, and others emerged to provide quality tinned food for sale to working class city-dwellers. In particular, Crosse and Blackwell took over the concern of Donkin Hall and Gamble. The late 19th century saw the range of tinned food available to urban populations greatly increase, as canners competed with each other using novel foodstuffs, highly decorated printed labels, and lower prices.

Demand for tinned food skyrocketed during World War I, as military commanders sought vast quantities of cheap, high-calorie food to feed their millions of soldiers, which could be transported safely, survive trench conditions, and not spoil in transport. Throughout the war, soldiers generally subsisted on low-quality tinned foodstuffs, such as the British "Bully Beef" (cheap corned beef), pork and beans and Maconochies Irish Stew, but by 1916 widespread boredom with cheap tinned food amongst soldiers resulted in militaries purchasing better-quality food to improve morale, and the complete meals in a tin began to appear. In 1917 the French Army began issuing tinned French cuisine, such as coq au vin, whilst the Italian Army experimented with tinned ravioli and spaghetti bolognese. Shortages of tinned food in the British Army in 1917 led to the government issuing cigarettes and amphetamines to soldiers to suppress their appetites. After the war, companies that had supplied military tinned food improved the quality of their goods for civilian sale.

Today, tin-coated steel is the material most commonly used. Laminate vacuum pouches are also used for canning, such as used in MREs.

Double seams

Modern double seams provide an airtight seal to the tin can. This airtight nature is crucial to keeping bacteria out of the can and keeping its contents sealed inside. Thus, double seamed cans are also known as Sanitary Cans. Developed in 1900 in Europe, this sort of can was made of the traditional cylindrical body made with tin plate. The two ends (lids) were attached using what is now called a double seam. A can thus sealed is impervious to the contamination by creating two tight continuous folds between the can’s cylindrical body and the lids. This eliminated the need for solder and allowed improvements in manufacturing speed, reducing cost.

Double seaming uses rollers to shape the can, lid and the final double seam. To make a sanitary can and lid suitable for double seaming, manufacture begins with a sheet of coated tin plate. To create the can body rectangles are cut and curled around a die and welded together creating a cylinder with a side seam.

Rollers are then used to flare out one or both ends of the cylinder to create a quarter circle flange around the circumference. Precision is required to ensure that the welded sides are perfectly aligned, as any misalignment will cause inconsistent flange shape, compromising its integrity.

A circle is then cut from the sheet using a die cutter. The circle is shaped in a stamping press to create a downward countersink to fit snugly in to the can body. The result can be compared to an upside down and very flat top hat. The outer edge is then curled down and around approximately 140 degrees using rollers to create the end curl.

The result is a steel tube with a flanged edge, and a countersunk steel disc with a curled edge. A rubber compound is put inside the curl.

Seaming

See also: Can seamer

The body and end are brought together in a seamer and held in place by the base plate and chuck, respectively. The base plate provides a sure footing for the can body during the seaming operation and the chuck fits snugly in to the end (lid). The result is the countersink of the end sits inside the top of the can body just below the flange. The end curl protrudes slightly beyond the flange.

First operation

Once brought together in the seamer, the seaming head presses a first operation roller against the end curl. The end curl is pressed against the flange curling it in toward the body and under the flange. The flange is also bent downward, and the end and body are now loosely joined together. The first operation roller is then retracted. At this point five thicknesses of steel exist in the seam. From the outside in they are; a) End, b) Flange, c) End Curl, d) Body, e) Countersink. This is the first seam. All the parts of the seam are now aligned and ready for the final stage.

Second operation

The seaming head then engages the second operation roller against the partly formed seam. The second operation presses all five steel components together tightly to form the final seal. The five layers in the final seam are then called; a) End, b) Body Hook, c) Cover Hook, d) Body, e) Countersink. All sanitary cans require a filling medium within the seam as metal to metal contact, otherwise such an arrangement would not maintain its hermetic seal. In most cases a rubberized compound is placed inside the end curl radius, forming the critical contact point between the end and the body.

Probably the most important innovation since the introduction of double seams is the welded side seam. Prior to the welded side seam the can body was folded and/or soldered together, leaving a relatively thick side seam. The thick side seam required that the side seam end juncture at the end curl to have more metal to curl around before closing in behind the Body Hook or flange, with a greater opportunity for error.

Seamer setup and quality assurance

Many different parts during the seaming process are critical in ensuring that a can is airtight and vacuum sealed. The dangers of a can that is not hermetically sealed are contamination by foreign objects (bacteria or fungicide sprays), or that the can could leak or spoil.

The most important part is the seamer setup. This process is usually performed by an experienced technician. Amongst the parts that need setup are seamer rolls and chucks which have to be set in their exact position (using a feeler gauge or a [clearance gauge]). The lifter pressure and position, roll and chuck designs, tooling wear, and bearing wear all contribute to a good [double seam].

Quality control usually involves taking full cans from the line - one per seamer head, at least once or twice per shift, and performing a teardown operation (wrinkle/tightness), mechanical tests (external thickness, seamer length/height and countersink) as well as cutting the seam open with a twin blade saw and measuring with a double seam inspection system. The combination of these measurements will determine the seam's quality.

Incorrect setups can be non-intuitive. For example, due to the [springback] effect, a seam can appear loose, when in reality it was closed too tight and has opened up like a spring. For this reason, experienced operators and good seamer setup are critical to ensure that double seams are properly closed.

See also

Incidents and accidents related to tinned foods

Famous canned foods

References

  1. ↑ A brief account of Bryan Donkin FRS and the company he founded 150 years ago. Bryan Donkin Company, Chesterfield, 1953
  2. ↑ Oxford Dictionary of National Biography

External links