Those who can…

The invention of the canning process has been ascribed to the French chemist and confectioner Nicolas Appert in the early 1880s. He found that it was possible to prevent the deterioration of food sealed in a glass jar, when subjected to heat. Canning per se started in America in 1819 and in 1825 Kenset and Daggert were granted a U.S. patent on a tin plate canister, from which the word ‘can’ is derived.

Nowadays there are various types of can available as well as other hermetically sealed containers, such as retortable pouches or plastic trays. Glass containers combined with a hermetic closure are also widely used.

Whatever the container, canning can be defined as a heat preservation method in which foods are packed in hermetically sealed containers and made commercially sterile by the application of a pre-determined amount of lethal heat. ‘Commercially sterile’ means that the food is free of microorganisms capable of growing at the normal non-refrigerated conditions at which the food is likely to be held during distribution and storage.

Process

Conventional canning follows several different stages, namely:

• Raw materials reception and preparation
• Filling
• Exhausting
• Sealing
• Thermal processing
• Cooling

Raw materials reception and preparation

Raw materials are inspected on reception to ensure that they comply with specifications and have low level defects. Raw materials are often stored due to temporary oversupply or to spread the production throughout a longer period. Storage may also be necessary when products with different harvest dates are to be canned, for example peas, potatoes, carrots and beans. Raw meat, meat products, fish and shellfish are highly perishable and their quality deteriorates quickly at ambient temperature. It is therefore imperative that refrigerated or frozen products be stored at the appropriate temperatures.

The preparatory operations transform the raw materials into a form that can be packed efficiently into individual containers and these operations vary depending on the product. Fruit and vegetables are usually washed to remove dirt and foreign matter, peeled, graded and blanched.

Blanching helps to achieve the following:

• Remove intercellular air and other gases, which would otherwise be released during the heat-treatment
• Inhibit enzymatic reactions, which may occur if products are held at ambient temperature, without blanching prior to heat processing
• Soften products, which helps to minimise physical damage during filling and improve the weight/volume ratio
• Adjust moisture levels
• Provide additional cleaning

Meats are normally cleaned, boned, cut, brined and cured. Salting or brining can help to minimise the loss of moisture and the resulting toughening that can occur during heat treatment. It also allows a reduction in the severity of the thermal process required to achieve commercial sterility. A further reduction can be achieved by curing (the addition of curing salts such as nitrate and other compounds), which has the added advantage of fixing the colour to a desirable pink.

Fish and shellfish must be washed, eviscerated, cut and brined. Some fish, for example tuna, can be pre-cooked. However, pre-cooking of small fish such as sardines or herring reduces the moisture that could be liberated in the container following thermal processing, with subsequent loss of quality.

Filling

Containers and lids should be inspected for visible defects and must be cleaned before filling. Inverted containers can be washed by hot water spraying or rotary brushing; drained upside down and then, ideally, remain inverted until filling.

The design of filling machines will vary depending on whether the product and containers are filled by weight or volume. Accuracy is vital and the headspace (the free space between the surface of the food and the top of the container) should be checked to ensure it meets specifications. This is important in ensuring product consistency and an appropriate vacuum. It is also important to consider that the heat penetration during processing is affected by the ratio of solid to liquid material.

Exhausting

Exhausting removes any residual air and other gases from the headspace and food. This is necessary to achieve the following:

• Minimise the strain on the container seams due to liquid and gas expansion during thermal processing
• Facilitate heat transfer during thermal processing
• Create a vacuum and concave ends on the container following processing and cooling
• Remove oxygen, thus minimising the risk of oxidative corrosion of the container’s internal surface. Oxygen removal has the added advantage of preventing vitamin C oxidation

Four methods of exhausting are generally used: mechanical exhausting, hot filling, hot exhausting and steam flow closing.

Sealing

Lids are attached by double seaming. Here, a hermetic seal is formed by interlocking the edges (flanges) of both the lid and body of the can. The finished double seam consists of five thicknesses of plates interlocked or folded and pressed firmly together.

The protection provided by the seal integrity will affect the storage life of the canned foods.

Thermal processing

The aim of thermal processing is to achieve a microbiologically stable (commercially sterile) product. Heat penetration studies or other equivalent procedures should be carried out to determine the optimum process.

The thermal resistance of microorganisms decreases as the pH of the medium is lowered. Most bacteria, including Clostridium Botulinum, will not grow at pH < 4.6. Therefore, less severe processing is required to achieve commercial sterility for acid foods (pH < 4.6) compared with low acid foods (pH > 4.6).

Low acid foods can be processed at temperatures above 100°C under pressure (sterilisation), whilst acid foods can be processed at temperatures below 100°C (pasteurisation). In pasteurisation, the food should achieve a minimum of 85°C throughout.

The lethal effect of heat on bacteria is a function of the time and temperature of heating and the bacterial population of the product. The optimal process temperature/time combination depends on a number of factors including pH, product specification (composition, weight, particulates size etc.), presence (and level) of preservatives, container size and type, product water activity, headspace and intended storage conditions.

The target microorganisms considered when developing thermal processing conditions are often product specific. Thermally resistant microorganisms presenting health hazards and/or spoilage microorganisms are considered; the latter includes Clostridium Sporogenes, Bacillus Stearothermophilus or Clostridium Saccharolyticum. Spores of Clostridium Botulinum are usually selected as the health hazard target and it is generally agreed that the minimum safe thermal process for a low acid food will result in a 12-log reduction in the number of Clostridium Botulinum spores. This requires a thermal process of 121°C for 3 minutes, i.e. Fo = 3 (often referred to as ‘Botulinum cook’).

Cooling

Cooling must be carried out as quickly as possible after thermal processing, in order to avoid seam strain and prevent over-cooking. Cooling is usually carried out in clean, chlorinated water. After cooling, the containers are labelled, placed into cartons or shrink-wrapped and stored until distribution. It is important that the cans are completely dry to avoid rusting.

Canning equipment

Heat processing can be achieved either by indirect heating (using saturated steam or hot air) or direct flame heating (applicable to cans only). The method most frequently selected is indirect heating using saturated steam.

Processing can be carried out either in batch retorts or continuously in pressure cooker-coolers where foods are sterilised under pressure at temperatures above the boiling point of water (usually 115-127°C).

There are two basic types of batch retorts: static, where there is no movement of the containers during heating and cooling, and rotary, where the containers are agitated (end-over end rotation, reciprocating or rocking action).

Similarly, there are static and rotary options for the continuous cooker-coolers, which are often used for large scale production plants. In the case of static cooker-coolers, the containers are carried through the pre-heating, sterilising and cooling sections by means of a roller track or chain conveyor. In the case of a rotary cooker-cooler, the containers are carried in a spiral track permanently attached to the inner wall of the shell. Pressure locks allow transfer between the three sections (that are at different pressures) in both the static and the rotary cooker-coolers.

Conclusion

Canning is a highly effective means of food preservation and packaging. Whilst the t echnology has moved on, the essential principles remain the same.

There are a number of different processing options available to the canner and the best option for any given food will depend on the specific circumstances. However, even when technically feasible, canning may not be the best method for food preservation. Facilities, such as the Process Research Centre (PRC), which has a selection of canning equipment available alongside a much larger range of equipment for processing all types of food, can help food manufacturers assess the best processing options for their own food products.

References

Food canning Technology (1997). Edited by Larousse, J. and Brown, B. Wiley-VCH, Inc., New York.

Hersom, A.C., Hulland, E.D. (1980). Canned Foods. Thermal processing and microbiology, Churchill Livingstone, Edinburgh, London and New York.

Sprenger, R.A. (1999). Hygiene for management, Highfield Publications, Doncaster.

Brennan, J.G., Butters, J.R., Cowell, N.D., Lilley, A.E.V. (1990). Food Engineering Operations, Elsevier Science Publishers Ltd., London and New York.

Jackson, J.M, Shinn, B.M. (1979). Fundamentals of food canning technology, The Avi Publishing Company, Inc., Westport, Connecticut.