Canning (Thermal processing or Thermal Preservation)


  • Commercial canning is a method of food preservation in which, carefully selected and prepared foods contained in a permanently sealed containers are subjected to heat for a definite period of time and then cooled.
  • In most canning process, the effect of heating on spoilage organisms is to destroy them and the permanent sealing of the container preventing the re-infection of the food by further organisms.
  • Canning has been in vogue for about 200 years now, since the French Biochemist, Nicholas Appert, in 1809 discovered the method of hermetically (without any hole) sealing glass jars filled with various foods, after heating it in a water bath, to overcome the problem of preservation of meat in the armed forces,.
  • In 1810, an English man, Peter Durrand, conceived and patented the idea of using tin cans instead of glass containers.
  • William Underwood followed it in 1817.
  • Boston and Thomas Kensette began using tins as cans in 1819 in New York.
  • Further, many more advances have been made in canning.

Canned meat products

Canned meat products

Recent developments in canning include

  • Aseptic Canning
    • Recently a new method of canning, known as aseptic canning, has been developed, which involves the use of high temperature for shorter duration.
    • The food is sterilized at 120°C for 6 sec to 6 min, depending on the food, before it enters a sterilized can, which is then closed with a sterilized lid.
    • This method is said to improve the flavour and the vitamin content.
  • Aluminium Cans
    • More recently aluminium or coated aluminium has been used in the fabrication of cans.
    • While it has the advantage of lightness and freedom from sulphiding and rust, it buckles fairly easily. 

Efforts are being made to produce an alloy strong enough to withstand the stresses of processing, packaging and transport.

Retort Processing

  • Flexible pouches made from laminates of thermoplastic and aluminium foil are widely used in Japan and are now being adopted in Europe and the United States.
  • They will not, however, withstand the high internal pressure developed during processing and must therefore be sterilized in media (water or steam and air) capable of providing an external pressure sufficient to balance the internal one.

Developments in modes of sterilisation

  • For the thermal processing of the open or sanitary can, flame sterilization, e.g. the Tarax flame sterilizer developed in Australia, combined with rotation of the can, is now used for certain products. This system has the advantage of being relatively cheap and is capable of providing very efficient heat transfer in those products with some liquid.
  • Future forms of thermal processing may involve the use of microwave energy, hydrostatic sterilizers using high-efficiency steam and fluidized-bed systems.

Thermal Destruction of Micr0-organisms

  • The effect of canning is destruction of spoilage organisms and both temperature employed and length of heating time determine the efficacy of the thermal process employed in killing spoilage organisms.
  • When bacteria in a suspension are exposed to heat, the number of remaining alive follows a logarithmic course (survivor or thermal death rate curve) against the length of heating time at a constant temperature. Some of the important concepts in thermal destruction of micro-organisms is listed below.
  • The decimal reduction time (D value) is the time taken at a constant temperature to reduce the surviving bacteria in a suspension to 10% of their original number (or) in other words to destroy 90% 0f the surviving organisms.
  • Total sterility is never achieved and the effect of any thermal processing is measured against the activity of the spores of Cl. botulinum, the most heat resistant pathogenic form known.
  • In modern canning operation there must be sufficient heat process equal to 12 times the D (decimal reduction time) of Cl. botulinum spores.
  • By a factor of 1012 i.e. a heat process equal to 12 times the D (decimal reduction time) of Cl. botulinum spores.
  • Foods with a pH of less than 4.5, in which Cl. botulinum spores do not germinate, may be subjected to milder heat treatments.
  • Z value refers to the degrees Fahrenheit required for the thermal destruction curve to traverse one log cycle. This may be used to determine equivalent thermal processes at different temperatures.
  • If, for example, 3.5 minutes at 140°F is considered an adequate process and z = 8, either 0.35 minutes at 148°F or 35 minutes or 35 minutes at 132°F would be considered as an equivalent process.
  • F value refers to the equivalent time, in minutes, at 250°F of all heat considered with respect to its capacity to destroy spores or vegetative cells of a particular organism.
Last modified: Tuesday, 10 April 2012, 10:52 AM