Mild preservation by killing micro-organisms with a pulsating electric field
Preservation with the aid of a high-intensity pulsed electric field is a mild preservation method, enabling vegetative micro-organisms to be killed off and chemical and physical product properties remain intact. As the only method of preservation, this treatment is not sufficient, making the use of multiple hurdles is necessary.
This treatment can only be applied to conductive and pumpable food products and is particularly suitable for pasteurization of acidic, chilled products with a short shelf-life, such as fruit juices and yoghurt. The product should have a homogeneous composition in order to be able to realize a constant field strength. A short circuit may occur in products with solid particles or bubbles and non-homogeneous products. The formation of air bubbles in the product can be prevented by degassing or pumping the product under pressure.
Description of Technique
When administering a power pulse of more than 1 volt across the cell membrane of vegetative micro-organisms in the lipid bilayer a mechanical stress occurs due to accumulation of ions across the membrane. This results in micro pores in the membrane, so toxins are able to penetrate the cell, necessary nutrients may leak out and eventually the micro-organism dies.
The treatment consists of repeatedly establishing an electric field for a very short period, on a column of food product between a positive and negative electrode. The food product is given between 20 and 100 pulses of each 1 to 100 ms, which are created by a connecting wire between the electrodes with a resistance therein. Through this wire with resistance, in fact, immediately a current will run when the voltage is applied, so that the voltage difference also decreases again immediately. The pulse frequency is about 1 pulse / second.
The field strengths vary between 10 kV/cm to 40 kV/cm. At a higher field strength there is a risk of short circuiting. The required field strength depends on:
- The type of micro-organism or enzyme
- The amount of time
- The growth phase (rapid inactivation in logarithmic growth phase)
- The pH
- The temperature of the food to be treated
- The ionic strength in the product
Each micro-organism has a threshold field strength. That is, below that field strength, regardless of the number of pulses, no inactivation takes place.
Bacterial spores are not killed off by a pulsed electric field. It is possible to let these spores germinate first or with the aid of enzymes - as it were - to dissolve their wall. Thereafter, a treatment with a pulsed electric field can then still be applied. Only some types of enzymes can be inactivated using this method.
It is a continuous process with a small increase of the temperature. In addition, the power consumption is often lower than with conventional heating.