Saturday, June 12, 2010

Factors Affecting Time of Thermal Process

Factors Affecting Time of Thermal Process
The thermal process time necessary for preservation of a chosen food depends on the rate of heat penetrating into a food.

Since every part of the food in a container or can must receive the same adequate heat treatment to prevent its spoilage, the process time work depended on the part which receives heat most slowly.

The food part near the center of the container usually receives heat most slowly. Heat penetration in food is mainly due to conduction as well as convection, which may operate simultaneously or successively.

Conduction occurs in solid particles of foods and is slow while heat transfer by convection occurs in liquids.

Some foods such as sugar syrup, thick soup and tomato juice undergo changes in their consistency during heating.

A variety of factors influence the time required for thermal processing for food. These include:

  1. The material of the container – glass having a slower rate compared to that of a metal.
  2. The size and shape of the container – smaller and slim cylindrical cans attain the desired temperature faster
  3. Initial temperature of the food - a higher initial temperature provides the lethal range for the microorganisms for a longer time and its average temperature during heating is higher than the food in the can with a lower initial temperature. A higher temperature is necessary for foods which heat slowly, e.g. meat, pumpkin etc.
  4. Retort temperature a higher retort temperature heats the food rapidly.
  5. Consistency of the food – nature, sizes and shapes pieces decide the rate of heat penetration.

    The pieces of food may retain the identity as in peas, plums, beets, asparagus and whole grain corn with smaller pieces requiring less time.

    Pieces may become mushy or change into a viscous mass as in the case of pumpkin, sweet potatoes, squash and cream-style corn, which get heated slowly as conduction is the main mechanism of heat transfer.

    Pieces may form layers as in the case of spinach and asparagus. In addition, the consistency of the foods depends on the different types of sauce starch salt or sugar that may be present.

    For example tomato sauce on baked beans slows down heat penetration more compared to plain sauce. Starch concentrations up to 6% decreases heat penetration, but at higher concentration, has no additional effect.

    Sodium chloride does not influence heat penetration, as its concentration is usually low. On the other hand increasing of sugar decreases heat penetration. However, this effect is counteracted by the decrease in viscosity of sugar solutions with increasing temperature.
  6. Rotation and agitation of the container during heat processing hasten heat penetration, particularly if the food is a fluid, but may cause undesirable physical damage to some solid foods.


Cooling of the canned foods also is influenced by the same factors as the heating process. Rapid, artificial cooling is usually adopted to effect a greater control on the cooling rate, because slow cooling may cause overcooking of the food and also allow the growth of thermophilic organisms.
Factors Affecting Time of Thermal Process

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