One of the sterilisation methods generally employed in industry is by autoclave, where containers are treated in packets (“batches”) typically at a temperature between 90° C. and 130° C. for several minutes, at rates of several thousands of containers per hour. However, sterilisation at these temperatures can substantially alter the properties of the treated product (colour, taste, odour, biophysical, biochemical and other qualities). In a conventional thermal sterilisation process, the rise in temperature is effected slowly and allows the microorganisms to adapt and to better resist the increase in temperature.
Processes aimed at reducing the temperature threshold necessary for sterilising an aqueous liquid by application of electric fields are described in U.S. Pat. No. 4,695,472 and EP 1 328 167. The process described in U.S. Pat. No. 4,695,472 however is concerned only with the sterilisation of a flux of liquid and can not be employed for sterilisation of bottles or other containers filled with liquid. The proposed amplitude of the electric field, applied to a bottle of about ten centimeters in diameter, would require very high voltages, difficult to generate and apply homogeneously.
In EP 1 328 167, a process for the sterilisation of bottles or other containers filled with liquid is described. It is proposed to limit the sterilisation threshold temperature TS by subjecting the product simultaneously to heating by electric field and to the action of ultrasound vibrations. This technology does however prove to be ineffective in practice, on the one hand because different microorganisms have different sensibilities to ultrasound vibrations, as a function of frequency and amplitude, on the other hand because the homogeneous application of ultrasound vibrations throughout the volume of the container is difficult to achieve.
Also, with known processes of sterilisation via electroporation, it is difficult to achieve a good uniformity of treatment of hermetic containers containing liquid, due to the rapidity of heating and the form of the containers, causing disparities in temperature and electric field in the volume of liquid to be sterilised. To compensate these disparities and to ensure reliable and irreversible destruction of microorganisms throughout the volume of liquid, the average temperature and/or the amplitude or the application time of the electric field could be increased. However, the consequence of this would be increased alteration of the properties of the liquid.
During heating, the pressure inside the container increases and can be accompanied by an irreversible deformation of the container, especially with respect to bottles or other containers made of plastic materials. The advantage of processes of sterilisation by electric field is a drop in temperature and sterilisation time relative to conventional thermal pasteurisation processes. Yet there is an advantage to lowering the temperature and treatment time still further to reduce effects due to the rise in internal pressure.
Devices for pressure compensation in the field of high-temperature sterilisation of containers are described in patents GB390768, U.S. Pat. No. 2,909,436, FR1436405 and FR2035678. In these systems, the internal pressure is compensated by the pressure of the liquid surrounding the container, determined by the height of the column of liquid in which the containers are immersed. This liquid also serves to heat the content of the container, making the sterilisation process relatively slow, with negative consequences on the alteration of the properties of the food in the container. Such processes are also not intended for, nor adapted to, the sterilisation of PET bottles or other containers made of plastic of which the resistance to creep decreases sharply at conventional thermal pasteurisation temperature.