Canning and Fish Packaging Technology

Unit 3 - Unit operations in canning

3.2.3.3.Vacuum inside the can and its measurement
In canning technology, vacuum inside a can is a relative term and it does not mean the absolute vacuum. Moreover, it is impossible to remove air or vapors from food cans completely. Hence ‘vacuum’ inside a food can is ‘incomplete vacuum’. The extent to which the pressure inside a food can has been reduced in comparison to the atmospheric pressure is considered as vacuum inside the can. This relationship is given by the following equation.

Vacuum inside the can (V) = Atmosphere Pressure (Pa) – Pressure inside the can (Pc)
V = Pa - Pc

From this formula, it is evident that can vacuum not only depends on the pressure exerted by air, gases and vapours inside the can but also on the atmospheric pressure. The units used in measurement of can vacuum is the same as the one that is used in measurement of pressure. The units used are kg per centimetre square (kg/cm2) or lb/inch2. The other commonly used units of pressure are in terms of centimetres of mercury column. Suppose pressure in the head space of can is 50cm and the atmospheric pressure at sea level is 76 cm, then vacuum inside the can is 76-50=26 cm at the sea level.