FOOD PROCESSING AND PRESERVATION: NON-THERMAL PROCESSING
Diagram
Electric pulses or pressure, both without heat
Pulsed electric field
Short, high voltage electric pulses punch tiny holes in microbial cell membranes, so the cells die. It suits pumpable liquids.
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High pressure processing
Very high hydrostatic pressure, at low temperature, damages microbial cells. It suits already packed foods.
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The key point
Both are non-thermal methods, but one uses electric pulses and the other uses pressure.
Figure 1: Pulsed electric field uses short electric pulses; high pressure processing uses hydrostatic pressure. Drawn in our own style from Rahman, Handbook of Food Preservation, chapter 33 on pulsed electric fields and chapter 34 on high pressure treatment.
Introduction
Pulsed electric field and high pressure processing are both non-thermal preservation methods: both kill microbes without cooking the food. Pulsed electric field, or PEF, sends short, high voltage electric pulses through a pumpable liquid, and the pulses punch tiny holes in microbial cell membranes, so the cells die. High pressure processing, or HPP, puts already packed food under very high hydrostatic pressure at low temperature, and the pressure damages microbial cells. The difference is the force used, and the kind of food each one suits.
Rahman gives a chapter to each method. This article follows that book.
Quick comparison
Table 1: Pulsed electric field compared with high pressure processing
| Aspect | Pulsed electric field | High pressure processing |
|---|---|---|
| Force used | Short, high voltage electric pulses | Very high hydrostatic pressure |
| How microbes die | Electroporation: pulses punch holes in cell membranes | Pressure damages cell membranes and proteins |
| Temperature | Low, near room temperature | Low or room temperature |
| Food form | Pumpable liquids that flow through a treatment chamber | Already packed foods, solid or liquid, in a pressure vessel |
| Treatment | Continuous flow, as the liquid passes between electrodes | Batch, as the packed food sits in the vessel under pressure |
| Vegetative microbes | Inactivated by the pulses | Inactivated by the pressure |
| Bacterial spores | Survive, so PEF does not sterilise | Survive pressure alone, so HPP does not sterilise |
| Flavour and nutrients | Stay close to fresh, because little heat is used | Stay close to fresh, because little heat is used |
| Speed | Very fast, pulses last microseconds | Slower, the vessel must be loaded, pressurised, held and unloaded |
| Typical foods | Fruit juices, liquid egg, milk, soups | Juices, guacamole, ready meals, sliced meat, seafood |
Scientific explanation
Definition
Pulsed electric field treatment is the preservation of a pumpable liquid food by short, high voltage electric pulses, which punch tiny holes in microbial cell membranes, a process called electroporation, so the cells die. High pressure processing is the preservation of already packed food by very high hydrostatic pressure at low temperature, which damages microbial cells. Both are non-thermal methods, and Rahman gives a chapter to each.
Working principle
In PEF, the liquid flows between two electrodes, and a pulse of high voltage lasts only microseconds. The pulse builds a charge across each microbial cell membrane until the membrane breaks down and holes form, so the cell leaks and dies. The liquid itself barely warms, because the pulses are so short. In HPP, the packed food sits in water in a pressure vessel, and the water is pressurised. Pressure acts at once and evenly through the pack, and it damages microbial membranes and proteins. Both leave flavour, colour and nutrients close to fresh, because neither cooks the food.
Apparatus
The books used here name the pulse generator and treatment chamber with electrodes for PEF, and the high pressure vessel for HPP. No laboratory apparatus for telling the two apart is named, because the distinction is in the process, not in a test.
Measurement procedure
The distinction is made from the process record: field strength, pulse number and pulse width for PEF, and pressure and holding time for HPP. The books used here give no single standard laboratory procedure for telling the two apart after treatment, and no worked calculation with numbers is given.
Factors affecting readings
For PEF, field strength, pulse number, and the conductivity of the liquid change the kill, and the liquid must flow evenly past the electrodes. For HPP, pressure level and holding time change the kill. In both cases, the starting microbial load and the food composition change the result, and spores survive both methods.
Formulas and calculations
The books used here give no formula for this comparison, so no formula or worked calculation is given.
Industrial applications
- PEF is used for fruit juices and liquid egg, where a fresh taste is wanted.
- HPP is used for juices, guacamole and ready meals in packs.
- PEF is used for pumpable liquids in continuous flow lines.
- HPP is used for sliced cooked meat and seafood to extend chilled shelf life.
- Both are used to make food safer without heat and without preservatives.
Advantages and limitations
- PEF: advantage: continuous flow and very fast, with fresh flavour kept. Limitation: only suits pumpable liquids, and does not kill spores.
- HPP: advantage: treats food already packed, solid or liquid, with even pressure. Limitation: batch process, slower, and does not kill spores.
- PEF: advantage: low energy use per litre, because pulses are short. Limitation: needs a conductive liquid and a special pulse generator.
FAQ
Are PEF and HPP both non-thermal?
Yes. Both kill microbes without cooking the food, so flavour and nutrients stay close to fresh.
Can either one sterilise food?
No. Bacterial spores survive both methods, so neither gives a shelf stable low acid food.
Why does PEF only suit liquids?
The liquid must flow between electrodes so the pulses reach every part of it. Solids cannot flow through the chamber.
Why is HPP done in batches?
The packed food must be loaded into the pressure vessel, pressurised, held, and unloaded, which is a batch cycle.
