FOOD PROCESSING AND PRESERVATION: STERILISATION
Diagram
Heat the sealed pack, or sterilise food and pack apart
Retort processing
The food is sealed in its pack first, then the whole sealed pack is heated in a retort to sterilise it.
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Aseptic processing
The food and the pack are sterilised separately, then the sterile food is filled into the sterile pack in a sterile room.
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The key point
Both give a shelf stable food, but the order of sealing and heating is opposite.
Figure 1: Retort heats the sealed pack; aseptic sterilises food and pack separately and then fills. Drawn in our own style from Sun, Handbook of Food Safety Engineering, chapters on aseptic processing and packaging and on conventional processing systems, and Lee, Yam and Piergiovanni, Food Packaging Science and Technology.
Introduction
Retort and aseptic processing both sterilise food for a long shelf life, but they do it in opposite orders. In retort processing the food is sealed in its pack first, and then the whole sealed pack is heated in a retort until the food inside is sterile. In aseptic processing the food and the pack are sterilised separately, and then the sterile food is filled into the sterile pack in a sterile environment and sealed. That difference in order changes the heat the food receives, the quality of the food, and the kind of pack that can be used.
Sun covers aseptic processing and packaging and conventional processing systems, and Lee, Yam and Piergiovanni cover retortable and aseptic packs. This article follows those books.
Quick comparison
Table 1: Retort processing compared with aseptic processing
| Aspect | Retort processing | Aseptic processing |
|---|---|---|
| Order | Seal first, then heat the sealed pack | Sterilise food and pack separately, then fill and seal |
| Where heating happens | In the retort, with the pack inside | In a heat exchanger, before filling |
| Heat the food receives | Longer heating, because heat must pass through the pack and the food | Shorter heating at high temperature, because the food flows thinly through the heater |
| Food quality | More cooked flavour and softer texture from the longer heat | Fresher flavour, colour and nutrients, from the shorter heat |
| Pack | Must survive the retort heat: cans, glass jars, retort pouches | Must be sterilised separately, often by heat or chemical, then filled sterile |
| Shelf life | Long, shelf stable at room temperature | Long, shelf stable at room temperature |
| Speed | Slower, batch or semi continuous | Fast, continuous flow |
| Equipment | Retort, a pressure vessel that heats sealed packs | Steriliser, aseptic filler, and a sterile filling room |
| Risk if it fails | Underprocessed packs can spoil or become unsafe | A break in sterility during filling can contaminate the whole batch |
| Typical foods | Canned vegetables, canned meat, ready meals in pouches | Long life milk, juices, soups, sauces in cartons |
Scientific explanation
Definition
Retort processing is the sterilisation of food after it has been sealed in its container, by heating the sealed container in a retort, a pressure vessel, until the food inside is sterile. Aseptic processing is the sterilisation of the food and its package separately, followed by filling the sterile food into the sterile package in a sterile environment and sealing it. Both give a shelf stable food that keeps at room temperature, but the order of heating and sealing is opposite.
Working principle
In retort processing, heat passes from the retort through the wall of the pack and into the food, and the coldest point in the pack must reach the sterilising temperature and hold it long enough to kill spores. Because heat moves slowly through a full pack, the heating time is long, and the food near the wall is overcooked while the centre catches up. In aseptic processing, the food flows as a thin stream through a heat exchanger, so it heats and cools fast, and the pack is sterilised on its own line. The sterile food is then filled into the sterile pack without ever meeting unsterile air, so the heat damage is much less.
Apparatus
The books used here name the retort as the apparatus for retort processing, and the heat exchanger, aseptic filler and sterile filling room for aseptic 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: retort processing is checked by temperature at the coldest point of the pack and by the time held, and aseptic processing is checked by the temperature and time in the heat exchanger and by the sterility of the filling room. The books used here give no single standard laboratory procedure for telling the two apart after the pack is sealed, and no worked calculation with numbers is given.
Factors affecting readings
Pack size and food thickness change retort time, because heat must reach the centre. Flow rate and food viscosity change aseptic heating, because they change how fast the food moves through the exchanger. The acidity of the food changes the sterilising temperature needed 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
- Retort processing is used for canned vegetables, canned meat and ready meals in retort pouches.
- Aseptic processing is used for long life milk, fruit juices and soups in cartons.
- Retort processing is used where a solid pack, such as a can, must be sterilised whole.
- Aseptic processing is used for liquids and pumpable foods that flow through a heat exchanger.
- Both are used to make shelf stable foods that need no refrigeration.
Advantages and limitations
- Retort processing: advantage: simple and proven, and the sealed pack is sterilised whole, so filling sterility is not a risk. Limitation: long heating overcooks the food and softens its texture.
- Aseptic processing: advantage: short heating keeps flavour, colour and nutrients fresher. Limitation: needs a sterile filling room, and one break in sterility can contaminate the batch.
- Retort processing: advantage: works for solid foods in cans and pouches. Limitation: slow, and the pack must survive the retort heat.
FAQ
Which one gives better quality food?
Aseptic processing usually gives fresher flavour and colour, because the food is heated for a much shorter time.
Can any pack be used for aseptic processing?
No. The pack must be sterilised separately and filled in a sterile room, so cartons and formed packs are used.
Is retort food the same as canned food?
Retort processing is the heating step of canning: the food is sealed in the can and then heated in the retort.
Do both need refrigeration?
No. Both give shelf stable food that keeps at room temperature until opened.
References and further reading
- Da-Wen Sun (ed.), Handbook of Food Safety Engineering, Wiley, 2011 (Google Books page read)
- Dong Sun Lee, Kit L. Yam and Luciano Piergiovanni, Food Packaging Science and Technology, CRC Press, 2008 (Google Books page read).
- Gordon L. Robertson, Food Packaging: Principles and Practice, 2nd ed., CRC Press, 2005 (Google Books page read).
