FOOD PROCESSING AND PRESERVATION: PRESERVATION METHODS
Diagram
Energy rays or heat to cut microbes
Irradiation
Ionising energy, such as gamma rays or electron beams, damages microbial DNA so microbes cannot grow. It is a cold process: the food barely warms.
↓
Thermal pasteurization
Heat kills vegetative microbes. The food is heated to a set temperature and held, then cooled. It kills vegetative cells but not spores.
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The key point
One uses ionising energy without heat, the other uses heat without ionising energy.
Figure 1: Irradiation uses ionising energy at low temperature; thermal pasteurization uses heat. Drawn in our own style from Rahman, Handbook of Food Preservation, chapter 32 on irradiation preservation of foods and chapter 23 on pasteurization and food preservation. The FTN Post-Harvest Technology notes, Unit 8, cover radiation preservation.
Introduction
Irradiation and thermal pasteurization both reduce microbes in food, but they use different energy. Irradiation uses ionising energy, such as gamma rays or an electron beam, which damages the DNA of microbes so they cannot grow, and the food barely warms. Thermal pasteurization uses heat: the food is heated to a set temperature, held, and then cooled, which kills vegetative microbes but not bacterial spores. One is a cold process, the other is a heat process, and that decides what each one can and cannot do.
Rahman gives a chapter to each method, and the FTN Post-Harvest Technology notes cover radiation preservation in Unit 8. This article follows those sources.
Quick comparison
Table 1: Irradiation compared with thermal pasteurization
| Aspect | Irradiation | Thermal pasteurization |
|---|---|---|
| Energy used | Ionising energy: gamma rays, electron beams, X-rays | Heat |
| Temperature rise | Very little: a cold process | Large: the food is heated and then cooled |
| How microbes die | DNA damage stops them growing and dividing | Heat denatures their proteins and kills vegetative cells |
| Vegetative microbes | Reduced or killed, depending on dose | Killed by the heat and holding time |
| Bacterial spores | Resist low doses; high doses are needed to kill them | Survive pasteurisation, which is not sterilisation |
| Effect on flavour | Little change at the doses used for food | Cooked flavour from heating |
| Effect on nutrients | Little change at the doses used for food | Some loss from heat, more with hotter or longer treatment |
| Packaging | Can treat food already packed, because rays pass through packs | Usually heats the food in the pack, or heats and then packs |
| Other uses | Also stops sprouting in potatoes and onions, and kills insects | Also inactivates enzymes that spoil flavour and colour |
| Typical foods | Spices, dried herbs, meat, poultry, fruit | Milk, juice, beer, liquid egg, sauces |
Scientific explanation
Definition
Irradiation is the preservation of food by ionising energy, such as gamma rays, electron beams or X-rays, which damages the DNA of microorganisms so they cannot grow or divide. It is a cold process, because the food barely warms. Thermal pasteurization is the preservation of food by heat: the food is heated to a set temperature, held for a set time, and then cooled, which kills vegetative microorganisms but does not kill bacterial spores. Rahman gives a chapter to each method.
Working principle
In irradiation, the rays pass through the food and the pack, and they break chemical bonds in microbial DNA, so the microbe cannot copy itself and dies or cannot grow. The dose decides the effect: a low dose stops sprouting and kills insects, a higher dose cuts microbes, and a still higher dose is needed for spores. In thermal pasteurization, heat denatures microbial proteins and breaks their cell structures, so vegetative cells die. The temperature and the holding time decide the kill, and spores survive because their coats protect them from moderate heat.
Apparatus
The books used here name the irradiator, with its gamma source or electron beam machine, for irradiation, and the heat exchanger or pasteuriser for thermal pasteurization. No laboratory apparatus for telling the two apart is named, because the distinction is in the treatment, not in a test.
Measurement procedure
The distinction is made from the process record: absorbed dose for irradiation, and temperature and holding time for pasteurization. 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 irradiation, the dose, the food composition and the packaging change the result, and spores need a higher dose than vegetative cells. For pasteurization, temperature, holding time, acidity and the starting microbial load change the kill. In both cases, the food must be handled cleanly after treatment, or it will be contaminated again.
Formulas and calculations
The books used here give no formula for this comparison, so no formula or worked calculation is given.
Industrial applications
- Irradiation is used for spices and dried herbs, where heat would drive off flavour.
- Thermal pasteurization is used for milk, juice and liquid egg.
- Irradiation is used to stop sprouting in potatoes and onions and to kill insects in grain.
- Thermal pasteurization is used for beer, sauces and ready meals.
- Both are used to make food safer without adding preservatives.
Advantages and limitations
- Irradiation: advantage: a cold process, so flavour and nutrients change little, and it treats food already packed. Limitation: needs a special irradiator, and some shoppers reject irradiated food.
- Thermal pasteurization: advantage: simple, proven, and widely accepted. Limitation: heat changes flavour and nutrients, and it does not kill spores.
- Irradiation: advantage: also stops sprouting and kills insects. Limitation: high doses are needed to kill spores.
FAQ
Does irradiation make food radioactive?
No. The energy passes through the food and does not leave radioactivity in it.
Is pasteurization the same as sterilization?
No. Pasteurization kills vegetative cells but not spores, while sterilization also kills spores.
Why irradiate spices?
Heat would drive off their flavour, while irradiation cuts microbes without heating them.
Does pasteurized milk still need chilling?
Yes. Spores and some microbes survive, so pasteurized milk keeps chilled, not at room temperature.
References and further reading
- FTN Post-Harvest Technology notes, Unit 8 (radiation preservation).
- M. Shafiur Rahman (ed.), Handbook of Food Preservation, 2nd ed., CRC Press, 2007 (Google Books page read)
