Method of Heat Penetration during Thermal Processing

Heat penetration

During processing of food, heat penetration is an important factor and it should be known to ensure that every particle of food within the container receive the required heat treatment. If the cans are heated from outside, larger the can, larger time it will take to heat the center portion of can to any desired temperature. However, there are several factor besides size and shape of can that affect the heat penetration into food within it.

Liquid food such as canned tomato juice can be readily set into convection heating motion after receiving heat by conduction from the wall of the can. On other hand, solid food such as canned beef hash is too viscous to circulate and so it will be completely heated by conduction through the can wall and through itself. A product containing a combination of free liquid and solid such as  can of peas within a sugar syrup will be intermediate and will rise in temperature from the combination of conduction and convection. Convection heating is more rapid than conduction heating.

Methods of heat Penetration

During thermal processing, contents of the can is heated by following methods;

  1. Rapid convection heating: Occurs mainly in case of liquid fruit products and vegetable juices, thin syrups etc.
  2. Slow convection heating: Occurs mainly in case of canned product having particulate food materials such as peas, corn etc. with syrup.
  3. Conduction: occurs in solidly packed food with high water content but little or no free liquid. Example; fruits and vegetable concentrates, heavy cream etc.
  4. Broken curve heating: Occurs in certain canned food that exhibit a change in heating characteristics representing a definite shift from convection to conduction heating during the process. Food containing starch or food from which starch is readily leached from solid during the process resulting increase in viscosity of the filler. Example; soups, noodles product, mixed vegetables etc.

Important terminologies used during heat processing of food

Come up time (CUT): In canning technology, “Zero time” is the time when the retort reaches processing temperature and not the time when heating is started. Similarly, “Come up time” is the period of time from placing the can in the retort and heating until temperature reaches processing temperature.

Cold point: All the points within a container being heated are not at same temperature. The zone of slowest heating or the point in the can or mass of food which is last to reach the final heating temperature is called “cold point” of the container. It is that zone which is most difficult to sterilize due to lack of heating.

Fig: Cold point on food particle on conduction heating and convection heating

In case of products heating by conduction, cold point is located at the geometrical center of the can. However in food that undergo convection heating, unless cans are agitated, the cold point lies below the dead center of the can on the vertical axis.

The thermal process calculation should be carried out at cold point. If the microorganism at cold point are killed, microorganism at other part is definitely supposed to be killed.

Cooling of cans: After processing, the cans are cooled rapidly to stop the cooking process and to prevent stock burning, especially in cans at the center stocks where they may remain hot for several days. Prolonged heating results in an inferior and uneven pack. In extreme cases, peaches and beans become dark in color, tomatoes turn brownish and becomes bitter in taste. In many vegetables, prolonged cooking due to improper cooling results in development of flat sour by heat resistant spore forming bacteria. Cooling is done by;

  1. Immersing or passing the hot cans in tank containing cold water
  2. Spraying with jet of cold water
  3. By exposing the cans to air in small lots when water supply is scarce.

Stock burning: After cooling, cans are stored in go-downs. Stock burning is the burning of product during sticking of the cans. If the cooling is not adequate (in case of air cooling), the product in can continues to heat up and the product will have burned taste or overcooked taste. This burning is called stock burning. Although the product will be edible, the flavor and physical properties may be changed.



About Author

Name : Pratiksha Shrestha

Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.