Canning and Fish Packaging Technology

Unit 4 - Thermal process for canned foods

4.7.1.Cold spot
During heating a can of food in a retort, temperature varies from point to point. When the cans are heated from outside, the portion of can contents nearer to the container wall get heated first and the central portions later. Thus there will be a temperature gradient in the canned food at any point time during heat processing. Therefore, a point or a small space can be imagined in each can which gets heated last (final). Such a point of maximum heat lag is known as cold spot and its position inside can be determined experimentally. Since other portions of canned food gets more heat treatment than the cold spot, it is safe to assume that if the cold spot is sterilized, the whole can is sterilized.

It has been experimentally found that for conduction heating packs, the cold spot is at the geometric centre of the pack and for convection heating packs, it is on the geometric axis of the can at a distance of ½” to 2” (depending on the can height) from the bottom of the can.