It is well know that many canned goods such as canned food. canned pet food, and the like require to be sterilised after canning to prevent the growth of toxic bacteria in the can. This is a relatively energetic process and considerable efforts have been made to improve the energy efficiency of processes for the heat sterilising of canned goods. A further requirement of such processes is that, at least in the case of foods for human consumption, it is important to retain the flavour, texture and nutrient status of the contents of the can. Typically this requires the minimum heating possible consistent with the proper sterilisation of the food. A difficulty that then arises is that the rate at which heat can be transmitted to the canned product will affect the total heating time for the food. Many foods also have a tendency to burn onto the inside of the can if heated too quickly. The canner must then heat the can slowly to the sterilisation temperature which results in the contents of the can being maintained at all elevated temperature for considerably longer than is desirable for optimum taste, texture and nutrient status.
It has been proposed in U.S. Pat. No. 3,961,150 to heat canned goods by the application of heat to the can by the use of induction coils. The cans are described as being rotated in a reciprocating fashion beneath a flat induction coil. This arrangement was found in practice to be unsatisfactory. The time at which the contents of the can were maintained at an elevated temperature was excessive due to the problems of heat transfer rates, despite the reciprocating rotation of the cans.
French patent specification 2 501 631 describes an alternative arrangement in which cans are passed through the lumen of each of a plurality of helically wound induction coils. The cans are described as being rotated about their own axes between successive induction coils to allow homogenisation of the temperature of the contents of the cans. It is believed by the present inventors that in this design, where the cans are not rotated while they are actually being heated that either the rate of heating would have to be very low or there would be unacceptable burning of the contents against the can wall during the heating steps.
U.S. Pat. No. 4,256,922 describes the heating of loose foodstuffs on a rotating metallic tube surrounded by a helically wound coil. In this case the beat transfer problems are rather different due to the fact that the food is free to tumble and rotate relative to the tube unconstrained by a surrounding can.
The present invention is directed to an alternative method of heating canned product which offers an alternative to the known processes described above.