Canning and Fish Packaging Technology

Unit 3 - Unit operations in canning

3.2.5.1.Horizontal and Vertical retorting

In fish canning industry batch type still retorts are used. Prior to the start of heat processing it is important to wash the retort and check whether the thermometer and pressure gauge work properly. Steam should not leak through the gasket fixed to the retort at the door/closure of the retort. If the gasket is worn out, it has to be replaced. While stacking the cans in retorting baskets or carts, jumble stacking in better as the steam spreads all round the cans. Jumble stacking reduces the retort capacity but cans receive good heat process. After loading, the retort has be closed tightly, and then the steam valve of the retort has to be opened. It is important to remove all the air inside the retort by venting prior to the start of heat processing. When all the air is removed, the pressure indicated by the retort is only due to steam and each pressure unit (due to saturated steam) indicates a particular temperature. After the retort attains required pressure and time counting is started. The time taken by the retort to attain the desired temperature and pressure (after the opening of steam value) is known as come up time. This time depends on the size of the retort, design, speed with which steam enters the retort, number of cans stacked in the retort etc., When large size cold cans are heat processed with very quick come up time, because of sudden increase in pressure, the cans may get dented and dents may be permanent. The end of retort come up may be considered as the beginning of the heat process. The process time is pre-determined for each food and for each can size. Throughout the heat process duration, the temperature and pressure inside the retort has to be closely monitored. It is important to note that if the temperature of the retort drops for certain durations of time, there will be loss of heat process which cannot be made up by increasing the duration of the process because the killing effect of lower temperature is much less at lower temperature.

As soon as the process time is completed, steam valve is close and all the vents are opened and all the steam is let out. It is not good to reduce the pressure suddenly because the pressure inside the can will be more than the outside pressure and can ends will protrude outwards and take convex shape, double seam gets loosened and deformed. Such cans are likely to get contaminated with bacteria from cooling water. The retort should be opened only after the pressure is reduced to “O” and immediately the cans have to be cooled.