Food processing by pulsed electric fields. II. Biological aspects

Investigations on the biological effects of high voltage electric pulses primarily concern membrane permeabilization and microbial inactivation. Electroporation resulting from increased transmembrane potential is probably the main cause for membrane permeabilization. Equations are presented for this phenomenon. Results from several studies on the inactivation of microorganisms by pulsed electric fields are summarized in tables that relate the rate of inactivation to process conditions. The influence of three sets of parameters is discussed: 1) type and physiological state of the microorganism; 2) chemical composition and electrical resistivity of the microorganism-containing food or medium; 3) process conditions such as field intensity, duration and number of pulses, dissipated energy, final temperature, type of pulse and of treatment chamber. Data can be used to select conditions which produce a 5-6 log cycle inactivation for many yeasts or vegetative bacteria. Bacterial spores generally resist inactivation. Pulsed electric fields of relatively low intensity may be used to permeabilize larger cells from plant or animal tissues in order to facilitate the extraction of specific constituents or to increase the drying rate. Little is known concerning the possible chemical or physico-chemical modifications of food constituents by high voltage electric pulses. Some enzymes appear to be inactivated, even at low temperatures, while others are more resistant. The sensorial characteristics of a number of foods subjected to electric fields do not appear to be significantly altered. Potential food applications are numerous and mainly related to liquid foods (fruit juices, milk, sauces, liquid egg) or to pumpable food pastes (fruit or vegetable purees, minced meat, etc.). Both neutral and acidic foods are likely candidates. The main objectives of microbial inactivation by electric pulses are food sanitation and/or extension of chilled storage. The process should be nonthermal (maximum Delta T of similar to 30 degrees C) to preserve food freshness and quality. A low operating cost, estimated at 0.4-0.8 US cents per liter of food (capital investment not included) and continuous operation at high flow rate (>1000 L/h) represent significant industrial advantages for this new technology.