Canning and Fish Packaging Technology

Unit 3 - Unit operations in canning

3.2.3.7.1. Heat /thermal exhausting
Heat expands solids, liquids and gases. Gases expand to a maximum extent due to heat. As the can and its contents are heated, the solids, liquids and more importantly gases expand and gases are expelled from the head space. At this stage if the can is seamed and then cooled, these materials will contract and create vacuum. Thermal exhausting can be done is two ways.

Heating the food before filling: Heated food is filled into cans and immediately sealed. This is also known as hot fill method adapted for jams, jellies, marmalades and fruit pulps.

Heating the food before sealing: The can filled with food and the filling medium and then passed through exhaust boxes. In the exhaust box, the can and into contents are heated by steam for a predetermined length of time. As soon as the cans come out of the exhaust boxes, they are immediately sealed.
The time duration of thermal exhausting depends on the can size, quantity of food can contains and the nature of food. However, vacuum developed inside the can by thermal exhaust method depends on

• Temperature of the can and its contents.
• Head space in the can.

It has been found that by covering the cans by the lid, better exhaust can be obtained. However, there is a possibility of lids dropping off the cans. Some canners loosely join the lids to the body and the method is known as clinching. Clinching is done by using the seamer having only first operation roller or it is similar to this operation of the seamer. There are several advantages of clinching which may be listed as follows.

• Results in better vacuum and hence increase in shelf life of the product.
• Prevents displacement of lid and spillage of contents.
• Prevents scalding or scorching of food on the top surface.
• Cans retain round shape and hence the double seaming takes place without any problem.