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SOP: Determination of the Cold Point of a Container

GIFSQ — Global Institute for Food Safety & Quality

Principle

When a can is processed, all points within it are not at the same temperature. The zone of slowest heating — the cold point — is the zone most difficult to sterilize. When heating is by convection the cold point lies on the vertical axis near the bottom of the container; when heating is by conduction it approaches the geometrical centre on the vertical axis. The exact cold point is found by fixing thermocouples at different heights in separate cans and identifying the slowest-heating one.

Apparatus required

  • Thermocouple assemblies (continuous or non-projecting plug-in type), copper-constantan or nichrome-constantan wires
  • Retort with stuffing box for the lead wires
  • Potentiometer (calibrated in °F or mV, capable of measuring to 0.5 °F)
  • Semi-log paper for plotting heat penetration data

Procedure

Fix thermocouples at different heights in separate cans, starting at 0.5 in. from the bottom in the first can, 0.75 in. from the bottom in the second can, and so on

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Place the test material in the cans, seal and retort

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Record the temperature at intervals of 1 min and plot the heat penetration data on semi-log paper, which gives a straight-line relationship between time and temperature with minor deviations

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Note the can in which the heating is the slowest

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Make all subsequent heat penetration determinations by fixing the thermocouple at the same height as in the can in which the heating was the slowest

Precautions

  • In canned fruits and vegetables, keep the fill-in weight of prepared material and of syrup or brine uniform — variation in filling weight alters the heating characteristics.
  • In liquid packs like tomato puree, thick soups and other viscous fluid products, heating is predominantly by conduction — place the thermocouple at the geometrical centre of the can.
  • In agitation cookers, where heat distribution is uniform for liquid and semi-liquid packs, the thermocouple may be placed at the geometrical centre of the can.
  • In general (convection heating), the slowest heating point will be on the cylindrical axis of the can at about one-tenth the height of the can from the bottom.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.

Board, P.W., Determination of Thermal Processes for Canned Foods, CSIRO, Division of Food Preservation, Homebush, Australia, Circular No. 7 (1965) — cited in the book.

Jackson, J.M. & F.C.W. Olson, Food Res., 5, 409 (1940) — cited in the book.