FOOD PACKAGING: SHELF STABLE PACKS
Diagram
Aseptic and retort packaging: sterilise first, or sterilise in the pack
Aseptic packaging
Food and pack are sterilised separately, then the sterile food is filled into the sterile pack in a sterile zone and sealed.
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Retort packaging
Food is filled into the pack and sealed first, then the whole sealed pack is heated in a retort to sterilise it.
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The result
Both give shelf stable packs that keep without chilling, but aseptic suits liquids and gives a fresher taste, while retort suits solid foods in cans and pouches.
Figure 1: Sterile filling compared with heating the sealed pack. Drawn in our own style from Lee, Yam and Piergiovanni, Food Packaging Science and Technology, CRC Press, and Robertson, Food Packaging: Principles and Practice, 2nd ed., CRC Press, 2005.
Introduction
Aseptic packaging and retort packaging are the two main ways of making a shelf stable pack, a pack that keeps at room temperature without chilling. In aseptic packaging the food is sterilised on its own, the pack is sterilised on its own, and the sterile food is filled into the sterile pack inside a sterile zone. In retort packaging the food is filled into the pack and sealed first, and then the whole sealed pack is heated under pressure in a retort until the food inside is sterile.
The core difference is the order of heating and sealing: aseptic sterilises first and seals after, while retort seals first and sterilises after. That order decides which foods each method suits, how the food tastes, and what packs can be used.
Quick comparison
Table 1: Aseptic packaging compared with retort packaging
| Aspect | Aseptic packaging | Retort packaging |
|---|---|---|
| Order of steps | Sterilise food and pack separately, then fill and seal | Fill and seal first, then heat the whole pack |
| Heating | Food heated fast in a heat exchanger, then cooled | Sealed pack heated in a retort under pressure |
| Pack types | Cartons, bottles, pouches that can be sterilised empty | Cans, jars, retort pouches that take heat and pressure |
| Suits | Liquids: milk, juice, soups, sauces | Solids and liquids: canned vegetables, meat, ready meals |
| Taste and nutrients | Fresher taste, because heating is short | More cooked taste, because heating is longer |
| Shelf life | Long, at room temperature | Long, at room temperature |
| Sterile zone needed | Yes, filling must stay sterile | No, the seal protects the food during heating |
| Equipment cost | High, with sterile filling plant | High, with retorts, but simpler filling |
| After opening | Needs chilling, like any opened sterile pack | Needs chilling, like any opened sterile pack |
| Food example | Long life milk in a carton | Canned beans, retort pouch ready meal |
Scientific explanation
Definition
Aseptic packaging is packaging in which a commercially sterile product is filled into a sterile container in a sterile environment and sealed. Retort packaging is packaging in which the product is filled into a container, sealed, and then heat processed in a retort to make it commercially sterile.
Working principle
In aseptic processing the liquid food is heated fast to sterilising temperature, held for a short time, and cooled fast, so microbes are killed with little damage to flavour. The pack is sterilised separately, often with heat or with a chemical sterilant that is then removed, and filling takes place in a sterile zone so no microbes enter. In retort processing the sealed pack is heated under pressure, usually with steam, until the slowest heating point inside the food has been held at sterilising temperature long enough, and then it is cooled. The seal keeps microbes out after heating.
Apparatus
Aseptic lines use a heat exchanger for the food, a steriliser for the pack material, and a sterile filling chamber. Retort lines use a filling and sealing machine and a retort, which is a pressure vessel heated with steam or hot water.
Measurement procedure
The books name no single standard test that compares the two methods. In practice the heat process is checked by temperature probes at the slowest heating point, by recording time and temperature, and by checking the seal and the sterility of packs from the line.
Factors affecting readings
For aseptic filling, the result depends on keeping the filling zone sterile and on the pack being fully sterilised. For retort processing, the result depends on the size and shape of the pack, on the thickness of the food, and on the temperature and time in the retort. A larger pack heats more slowly at its centre, so it needs a longer process.
Formulas and calculations
The books read give no single formula for aseptic against retort packaging. The heat process in both is described by time and temperature: a higher temperature for a shorter time can give the same microbial kill as a lower temperature for a longer time, and the process is set so the slowest heating point is held long enough. No time temperature pair is given here, because the pair is set for each food and pack size.
Industrial applications
- Aseptic packaging: long life milk and cream in cartons, where fast heating keeps a fresh taste.
- Aseptic packaging: fruit juice and soups in cartons and bottles, where a liquid can be heated fast and filled sterile.
- Retort packaging: canned vegetables, beans and meat, where solid food is sealed and then heated in the can.
- Retort packaging: ready meals in retort pouches, where a full meal is sealed and sterilised in the pouch.
- Retort packaging: pet food and sauces in cans and jars, where the pack must take heat and pressure.
Advantages and limitations
- Aseptic packaging, advantage: short heating gives a fresher taste and better nutrient retention, and packs are light.
- Aseptic packaging, limitation: needs a sterile filling zone and costly plant, and it suits liquids better than solids.
- Retort packaging, advantage: suits solid and mixed foods, and the sealed pack protects the food during heating.
- Retort packaging, limitation: longer heating gives a more cooked taste, and the process is slower per pack.
FAQ
Does aseptic mean the food has no preservatives?
Aseptic packaging is about sterile filling, not about preservatives. The long life comes from sterilising and sealing, not from added chemicals.
Why does long life milk taste different from fresh milk?
Because it has been heated to sterilising temperature, even though the heating is short in an aseptic line.
Can solid food be aseptically packed?
It is much harder than for liquids, because solids cannot be heated and filled as evenly. Retort packaging suits solids better.
Does a retort pouch need chilling?
No, while it is sealed and undamaged. Once opened, it must be chilled like any opened food.
What is a retort?
A pressure vessel in which sealed packs are heated, usually with steam or hot water, to sterilise the food inside.
References and further reading
- Fellows, P., Food Processing Technology: Principles and Practice, Woodhead Publishing (FTN book on disk).
- Lee, D. S., Yam, K. L. and Piergiovanni, L., Food Packaging Science and Technology, CRC Press (Google Books page read: Retortable and Aseptic, p. 357).
- Robertson, G. L., Food Packaging: Principles and Practice, 2nd ed., CRC Press, 2005 (Google Books page read: Aseptic Packaging of Foods, p. 255).
