Food preservation methods compared
How food preservation methods work and compare: heat, cold, drying, salt, sugar, acid, fermentation, preservatives, MAP, irradiation and high pressure.
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Preserving food means slowing or stopping the three things that make it deteriorate: microbial growth, enzyme activity and chemical reactions such as oxidation and browning. People have dried, salted, smoked and fermented food for thousands of years; the nineteenth and twentieth centuries added canning, refrigeration, freezing and pasteurisation; and recent decades brought high-pressure processing, pulsed electric fields and other non-thermal technologies. Each method has strengths and costs for safety, quality, nutrition, energy and price.
Comparing preservation methods is a standard topic in food science and technology exams.
Methods that use heat
- Blanching: brief heating of vegetables to inactivate enzymes before freezing, canning or drying.
- Pasteurisation: mild heat to kill pathogens and reduce spoilage organisms; products need refrigeration (milk, juices) or other hurdles.
- Sterilisation and canning: heating sealed containers to achieve commercial sterility; low-acid foods (pH above 4.6) receive at least the "botulinum cook" (12 log reductions of Clostridium botulinum spores).
- UHT and aseptic packaging: very high temperature for seconds, then filling into sterile packages; better quality than in-container sterilisation.
Advantages: reliable, well understood, long shelf life for sterilised foods. Disadvantages: loss of heat-sensitive vitamins, colour and texture; energy use.
Methods that use cold
- Chilling (about 0 to 5 °C) slows growth and reactions; shelf life days to weeks.
- Freezing (−18 °C or colder) stops microbial growth and greatly slows chemical change; shelf life months. Quick freezing forms small ice crystals that damage texture less than slow freezing. Enzymes and oxidation still act slowly, so vegetables are blanched and products packed airtight.
Advantages: good retention of flavour and nutrients. Disadvantages: continuous energy for the cold chain; freezing does not kill most microbes.
Methods that remove or bind water
- Drying and dehydration: sun drying, hot-air, spray, drum and freeze drying lower water activity below the limits for microbial growth.
- Concentration: evaporating juices and milk.
- Salting and curing: salt binds water; nitrite in cured meats inhibits C. botulinum and fixes colour.
- Sugaring: jams, jellies, candied fruits and condensed milk.
Chemical and biological methods
- Acidification: pickling in vinegar; low pH limits pathogens.
- Fermentation: lactic acid, alcoholic and acetic fermentations make yoghurt, cheese, sauerkraut, kimchi, soy sauce, wine and vinegar, adding flavour and safety.
- Preservatives: permitted additives such as benzoates, sorbates, propionates (bread), sulphites, nitrites and natural antimicrobials like nisin, at legal limits.
- Smoking: drying, heat and antimicrobial smoke compounds; concerns about polycyclic aromatic hydrocarbons limit heavy smoking.
Atmosphere and packaging
- Vacuum packing and modified atmosphere packaging (MAP): removing oxygen or raising CO2 slows aerobic spoilage organisms and oxidation; must be combined with chilling for safety.
- Active and intelligent packaging: oxygen absorbers, ethylene absorbers, freshness indicators.
Non-thermal technologies
- Irradiation with gamma rays, electron beams or X-rays: controls insects, sprouting and microbes in spices, fruit, meat and other foods without heating; approved for many foods in many countries, though consumer acceptance varies.
- High-pressure processing (HPP): pressures of about 400 to 600 MPa inactivate vegetative microbes in packed juices, guacamole, seafood and ready meals while preserving fresh-like quality; spores survive, so products are usually chilled.
- Pulsed electric fields, UV light and cold plasma: mainly for liquids and surfaces.
Choosing a method
The right method depends on the food's acidity, water content and structure, the target shelf life, how it will be stored and sold, consumer expectations of freshness, energy costs and regulations. A fresh juice for a supermarket chiller might use HPP; the same juice for export without refrigeration needs heat and aseptic packing.
Comparing methods
| Method | Main target | Typical shelf life | Quality impact |
|---|---|---|---|
| Chilling | slows microbes and reactions | days–weeks | minimal |
| Freezing | stops microbial growth | months | small (texture) |
| Pasteurisation | pathogens | days–weeks chilled | small |
| Canning/sterilisation | all microbes including spores | 1–3 years | larger (cooked flavour, vitamins) |
| Drying | low water activity | months–years | changes texture and some nutrients |
| HPP | vegetative microbes | weeks chilled | very small |
Energy to freeze versus energy to dry
Freezing 1 kg of chicken (about 70 per cent water) from 5 °C to −18 °C:
- Cooling to the freezing point (about −1 °C), specific heat about 3.2 kJ/kg·°C: 3.2 × 6 = 19 kJ.
- Freezing the water: 0.70 × 334 kJ/kg (latent heat of fusion) ≈ 234 kJ.
- Cooling frozen chicken to −18 °C, specific heat about 1.7 kJ/kg·°C: 1.7 × 17 ≈ 29 kJ.
- Total ≈ 282 kJ, plus refrigeration inefficiency and storage energy over months.
Drying the same water away needs at least the latent heat of evaporation, about 2,400 kJ per kg of water: 0.7 kg × 2,400 ≈ 1,680 kJ, and practical dryers use roughly two to three times that. Drying costs more energy up front, but dried food then needs no energy to store, whereas frozen food needs a cold chain throughout its life.
Key points
- Preservation slows or stops microbes, enzymes and chemical reactions.
- Heat: blanching, pasteurisation, sterilisation (botulinum cook for low-acid foods), UHT.
- Cold: chilling slows, freezing stops growth; neither sterilises.
- Lowering water activity: drying, concentration, salt and sugar.
- Chemical and biological: acids, fermentation, preservatives, smoking.
- Packaging (vacuum, MAP) and non-thermal methods (irradiation, HPP, PEF, UV) extend shelf life with less quality loss.
- Most foods rely on several combined hurdles.
What the exams ask
- School-leaving and entrance papers: principles and examples of food preservation methods.
- Degree and research-fellowship entrances: mechanisms and limits of each method, comparisons of quality and shelf life, non-thermal technologies, and calculations of heat and energy.
- Food technology examinations: selecting preservation methods for products, process design and regulatory aspects.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Does freezing kill bacteria?
Mostly not. It stops their growth, and some die, but many survive and resume growing when thawed. Food must be handled safely after thawing.
Is irradiated food radioactive?
No. Approved doses do not make food radioactive; they disrupt the DNA of microbes and insects, as with other forms of ionising treatment of medical supplies.
Why are preservatives needed if food is refrigerated?
Some foods, such as bread or sauces once opened, spoil from moulds or yeasts even when cool. Preservatives at legal limits extend safe shelf life, though many products now use other hurdles instead.
Sources
- Fellows, P. J. Food Processing Technology: Principles and Practice, 4th edition. Woodhead Publishing, 2017.
- Rahman, M. S. (ed.) Handbook of Food Preservation, 3rd edition. CRC Press, 2020.
- Leistner, L. and Gorris, L. G. M. "Food preservation by hurdle technology." Trends in Food Science and Technology 6, 41–46 (1995).
- Farkas, J. and Mohácsi-Farkas, C. "History and future of food irradiation." Trends in Food Science and Technology 22, 121–126 (2011).
- Huang, H.-W. et al. "Current status and future trends of high-pressure processing in food industry." Food Control 72, 1–8 (2017).
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