PEF processing of fruits and vegetables: Big advantages

Mild food preservation, improved extraction processes, higher concentrations of bioactive compounds, enhanced drying processes: pulsed electric field (PEF) treatment is a versatile technology. Moreover, PEF is a mild process that leaves valuable compounds unaffected and often leads to product and process improvement and energy reduction. What are potential applications of this technology in fruit and vegetable processing?

PEF treatment is an excellent technique for mild preservation of liquid foods and beverages. By permeabilizing cell membranes of micro-organisms at relatively low temperatures, PEF enables a targeted shelf life increase, retaining product quality and freshness. Micro-organisms are killed as a result of a series of microsecond, high voltage pulses, whereas the pulsed electric field has no effect on, for example, vitamins and proteins.
PEF processing improves the shelf life of fresh fruit and vegetable juices, smoothies and other fruit or vegetable based food products, such as purees and compotes. For preservation of liquids a field strength of 10-20 kV/cm and energy delivery of 50-120 kJ/kg is appropriate.
The shelf life of cold pasteurized drinks and foods is comparable to that of thermally pasteurized foods. A challenge here is that enzymes are not fully inactivated after PEF treatment. Cooled storage is necessary. Sterilization by a combined heat and PEF treatment is also possible. Challenges here are the effect on enzymes, spores and aseptic filling.
Pulsemaster has developed industrial scale equipment – named Conditioner – with treatment capacities up to 5,000 l/h (1320 US gal./h) for microbial inactivation. On a commercial scale total costs of 10 Euro/ton (= 1 Eurocent per liter or kg) have to be expected. In US Dollar this is approximately 4.92 US Dollarcent per gallon.

Plant structure modification

A completely different application of pulsed electric field processing or electroporation is cell disintegration of plant tissue, which enhances mass transport processes and improves product quality for a broad spectrum of applications. The extraction of oils, juices and bioactive components is improved and extraction yields are increases. Cutting and peeling processes are enhanced as well and drying is accelerated.
Pulsemaster’s Conditioner range for cell disintegration purposes has treatment capacities up to 50,000 kg/h (110,000 lb/h). On a commercial scale total costs of 1 Euro/ton (= 0.1 Eurocent per liter or kg) for treatment of plant cells have to be expected. In US Dollar this is approximately 0.056 US Dollarcent per lb.

Examples

Tomato peeling offers a good example. The physical treatment with pulsed electric fields at ambient temperature softens tomatoes and improves peeling efficiency. It can replace the standard thermal treatment needed before potato peeling. Compared to traditional heating PEF treatment requires less energy and reduces energy costs.
Another interesting application is lycopene extraction. Lycopene is a carotenoid pigment responsible for the red color in tomatoes. In this application PEF technology increases the extraction yield.
Targeted structural modification also allows use of new raw materials. Tough and inconsistent produce such as sweet potato, turnip and beet root become easily processable with PEF.
Disintegrating (softening) vegetable tissue before drying with a pulsed electric fields improves mass transport of water. This can be used in dried sweet pepper manufacturing. The field strength of the treatment may range from 1.0 to 3.0 kV/cm and the energy delivery is 5 to 10 kJ/kg.

Stress response

Stress induction by electroporation is a very promising and emerging research area. As a result of the PEF treatment the formation of secondary metabolites in plants is induced. In broccoli this may lead to higher concentrations of health promoting glucosinolates. In this application the field strength is rather low: 0.1 to 0.5 kV/cm. The energy delivery is 0.1 to 1.0 kJ/kg.