FOOD PROCESSING AND PRESERVATION: NON-THERMAL PROCESSING
Diagram
Pressure or heat to kill microbes
High pressure processing
Very high hydrostatic pressure, at low or room temperature, inactivates vegetative microbes. The food is already packed.
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Thermal processing
Heat inactivates microbes. Pasteurisation kills vegetative cells; sterilisation also kills spores, but heat changes flavour and nutrients.
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The key point
HPP is a cold kill by pressure. Thermal processing is a kill by heat.
Figure 1: High pressure processing kills microbes with pressure at low temperature; thermal processing kills them with heat. Drawn in our own style from Hendrickx and Knorr, Ultra High Pressure Treatment of Foods, and Ohlsson and Bengtsson, Minimal Processing Technologies in the Food Industries, chapters on thermal and non-thermal methods.
Introduction
High pressure processing and thermal processing both kill microbes to preserve food, but they use different forces. High pressure processing, or HPP, puts already packed food under very high hydrostatic pressure at low or room temperature, and the pressure inactivates vegetative microbes. Thermal processing uses heat: pasteurisation kills vegetative cells, and sterilisation also kills spores. The pressure route leaves the food closer to fresh, while the heat route changes flavour, colour and nutrients more.
Hendrickx and Knorr describe HPP as an attractive alternative to conventional methods of thermal processing, and Ohlsson and Bengtsson set thermal methods against non-thermal methods. This article follows those books.
Quick comparison
Table 1: High pressure processing compared with thermal processing
| Aspect | High pressure processing | Thermal processing |
|---|---|---|
| Killing force | Very high hydrostatic pressure | Heat |
| Temperature | Low or room temperature | High, from pasteurisation heat up to sterilisation heat |
| State of food | Already packed, then pressurised in water | Heated in the pack, or heated and then packed |
| Vegetative microbes | Inactivated by pressure | Killed by heat |
| Bacterial spores | Survive pressure alone, so HPP does not sterilise | Killed only by sterilisation heat, not by pasteurisation |
| Flavour | Stays close to fresh, because little heat is used | Cooked flavour, because heat changes the food |
| Nutrients and colour | Better kept, because little heat is used | Some loss from heat, more with longer or hotter treatment |
| Texture | Little change in most foods, because pressure acts evenly | Softens food, because heat breaks down structure |
| Shelf life | Chilled shelf life, longer than fresh but not shelf stable | Pasteurised food keeps chilled; sterilised food keeps at room temperature |
| Typical foods | Juices, guacamole, ready meals, sliced meat, seafood | Milk, canned food, sauces, soups, ready meals |
Scientific explanation
Definition
High pressure processing is the treatment of already packed food with very high hydrostatic pressure at low or room temperature, which inactivates vegetative microorganisms without heating the food. Thermal processing is the treatment of food with heat to inactivate microorganisms: pasteurisation uses moderate heat to kill vegetative cells, and sterilisation uses higher heat for longer to kill spores as well. Hendrickx and Knorr call HPP an attractive alternative to conventional thermal processing, and that is the frame of this article.
Working principle
In HPP, the packed food is put in a vessel filled with water, and the water is pressurised. Pressure acts at once and evenly through the whole pack, so there is no cold spot, and it damages microbial cell membranes and proteins, so vegetative cells die. Bacterial spores resist pressure, so HPP alone does not sterilise. In thermal processing, heat moves into the food from the outside, so the centre heats last, and the heat denatures microbial proteins and kills the cells. Spores need sterilisation heat to die, and that heat also cooks the food.
Apparatus
The books used here name the high pressure vessel as the apparatus for HPP, and the heat exchanger, retort or pasteuriser for thermal processing. 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: pressure and holding time for HPP, and temperature and holding time for thermal processing. 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
Pressure level and holding time change how many microbes HPP kills. Temperature and holding time change how many microbes heat kills, and acidity changes the heat needed. Food composition, water activity and the starting microbial load change the result in 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
- HPP is used for cold pressed juices, guacamole, salsa and ready meals that must taste fresh.
- Thermal processing is used for milk, canned foods, sauces and soups.
- HPP is used for sliced cooked meat and seafood to extend chilled shelf life.
- Thermal sterilisation is used for shelf stable canned food that keeps at room temperature.
- Both are used to make food safe without adding preservatives.
Advantages and limitations
- HPP: advantage: keeps fresh flavour, colour and nutrients, because little heat is used. Limitation: does not kill bacterial spores, so it cannot make a shelf stable low acid food.
- Thermal processing: advantage: sterilisation kills spores and gives a shelf stable food. Limitation: heat changes flavour, colour, texture and nutrients.
- HPP: advantage: pressure acts evenly, so there is no cold spot. Limitation: needs costly high pressure equipment and works in batches.
FAQ
Does HPP cook the food?
No. HPP works at low or room temperature, so the food stays close to fresh.
Can HPP replace canning?
No. HPP does not kill bacterial spores, so it cannot make a shelf stable low acid food the way sterilisation can.
Why does HPP food still need chilling?
Because spores and some microbes survive, so the food keeps chilled, not at room temperature.
Is HPP food safe?
Yes, when the pressure and holding time are set to inactivate the vegetative microbes of concern, as the books describe.
References and further reading
- Marc Hendrickx and Dietrich Knorr (eds.), Ultra High Pressure Treatment of Foods, Springer, 2012 (Google Books page read).
- Thomas Ohlsson and Nils Bengtsson, Minimal Processing Technologies in the Food Industries, Elsevier, 2002 (Google Books page read).
- Da-Wen Sun (ed.), Handbook of Food Safety Engineering, Wiley, 2011 (Google Books page read)
