Non-thermal Processing of Food

Non thermal processing methods are increasing growing in demand due to its ability to act instantaneously and simultaneously in a food product regardless of size and shape.

Non thermal processing methods of food is effective in inactivating vegetative cells of bacteria, yeast, and molds without exposing foods to adverse effects of heat and thus extending their shelf life and causes minimal or no changes in their physical, nutritional, and sensory qualities.

Nonthermal processing is a value-added technique with diverse benefits, which includes-

  • increase shelf life of product.
  • improved taste by preserving amino acids.
  • maintain product’s taste and appearance keeping food safe.
  • Render foods free from pathogenic and spoilage organisms.

Types of Non thermal food processing

  • Low Temperature Preservation

Low temperatures preserve food by lowering microbial activity. But psychrophic bacteria (members of the genera Pseudomonas, Alcaligenes, Micrococcus and Flavobacterium) are known to grow even at commercial refrigeration temperatures (7°C). Slow freezing and quick freezing are used to preserve food products for long term. Freezing reduces the amount of microorganisms in foods but doesn’t kill all of them . In microorganisms, cell proteins undergo denaturation as concentrations of solutes increases in the unfrozen water in foods, and damage is caused by ice crystals.

  • Refrigeration

Refrigeration slows down the biological, chemical, and physical reactions thereby reducing the rate of growth and reproduction of microorganisms and increasing the shelf life of food. This principle is employed in refrigeration and freezing.

Refrigerators should be maintained to below 12°C to control the growth of microorganisms in foods. Refrigeration is used to: i) reduce spoilage of perishable foods, ii) Holding period between harvesting and processing is increased iii) storage life of commercially processed foods is extended. All foods aren’t benefited from cold temperatures. For example: bread goes stale and banana when refrigerated.

  • Freezing

Freezing makes water unavailable to microorganisms. The chemical and physical reactions leading to deterioration are slowed by freezing. Due to water evaporation white or grayish patches are formed into the packages air spaces called freezer burn that causes deterioration of taste and appearance. This occurs in vegetables, fruits, meat, poultry, and fish.

Blanching of uncooked fruits and vegetables must be done before freezing to prevent deteriorative microbial reactions.

During freezing, the water in food forms ice crystals and the rate of this phenomenon decides the quality of frozen foods:

  • Slow freezing

During slow freezing large ice crystals which puncture cell walls are formed results in shrunken appearance of thawed food due to release of cellular fluid. In slow freezing process the freezing is done for 3-72 h. This method is employed in home freezer and temperature is lowered to -15 to -29°C.

  • Rapid freezing

In rapid freezing numerous ice crystals of small size are formed and cell structure remains same. The temperature of food is lowered to about -20°C within 30 min which blocks or suppresses the metabolism.

Low temperature helps in retention of nutrients and prevention of microbial growth and unavailability of water are some advantages of freezing. Loss of some B Group vitamins and vitamin C due to blanching of vegetables prior to freezing and unintended thawing can reduce product quality are disadvantages of freezing.

  • Preservation by freeze drying

Freeze drying or lyophilization is commonly used for storing, transporting, and preserving bacterial cultures. Firstly, the food is deep frozen, then the water is drawn off by a vacuum pump in a machine. Then the dry product is sealed in foil and is reconstituted with water. In drying or dehydration removal of water from the food by controlled process occurs. This can be done by evaporation due to heating of the product, e.g., drying of fruits, osmotic dehydration, like in brining of fish and sublimation, or freeze drying e.g., in the drying of coffee.

The principle of sublimation is employed in freeze drying and lyophilization. In this process a solid change directly to a vapor without passing through the liquid phase.

  • Irradiation

Alpha, beta, and gamma radiating particles are used for the preservation of food. These radiations are of high energy content with a high frequency and can break molecules into oppositely charged units termed as ions, therefore, called ionizing radiations. These radiations are useful in killing bacteria, molds and insect pests, reducing the ripening and spoiling of fruits and at higher doses can induce sterility. As in this method product is not heated it is sometimes called cold pasteurization process. This method  is useful only for foods of high initial quality as spoiled food cannot be reverted to unspoiled state.

Ultraviolet radiation is effective for reducing surface contamination on several foods. This short wavelength light has been utilized in the cold storage units of meat processing plants.

Irradiation dose reduce microbial spoilage (1.5 – 3 kGy) and eliminate pathogenic microbes (3-7 kGy) to enhance shelf life of meat, poultry and sea foods under refrigeration. It also reduces number of microorganisms in spices to enhance hygienic quality (10 kGy).

Advantages of Nonthermal Processing

  • Nonthermal processing methods are effective at ambient or sub lethal temperatures.
  • Minimum energy is used.
  • Desired qualities and nutritional parameters of food products are maintained.
  • It Improves the shelf life of food products.

Disadvantages of Nonthermal Processing

  • Nonthermal processing is an expensive method.
  •  Nonthermal processing techniques are still under research and development.
  • Commercial units availability is less.
  • Effectivity of this process depends on electrical conductivity of food.
  •  Not useful for solid foods.


High PressurePasteurization:600 MPa, ambient temperature (batch process)Sterilization: 600 MPa, 121.1 C rapid, homogenous heating and cooling due to adiabatic heating (batch process)  No formation of undesired compoundsNutritional value largely maintainedRetention of freshness Physical modificationDesirable flavor compounds generated by heat aren’t formed.Inactivation of vegetative micro-organisms and spores.Inactivation of enzymes, prions and viruses (p, T dependent)Treatment of packaged food Cold storage (4°C) or storage at ambient temperature.
Pulsed electric fieldCell disintegration:1-5 kV/cm1-10 kJ/kgNonthermal pasteurization:25-40 kV/cm50-200 kJ/kgShort processing times Continuous operationNo formation of undesired compoundsGentle processing, retention of freshnessCell disintegrationImprovements of mass transfer processesPhysical modificationSpores are not inactivatedDesirable flavor compounds generated by heat aren’t formedInactivation of anti-nutritional factors not achievedNo inactivation of enzymesIntensity dependent occurrence of thermal and electrical side effects. Refrigerated storage of products required.
Membrane Processing SafetyNo formation of undesired compounds.Desirable flavor compounds generated by heat aren’t formed.Inactivation of anti-nutritional factors not achieved.Only applicable for liquids.Expensive for complete products.Helps in separation of ingredients e.g., lactoferrin)
 UV RadiationUV radiant exposure, at least 400 J/m2Intense and short duration pulses of broad spectrum are required (ultraviolet to the near infrared region).No chemical is used Non heat related method. Long term exposure of UV radiation can be harmful to the industry workers.Uv radiation can be used for disinfection of water supplies and food contact surfaces.
Oscillating magnetic field (OMF)Food products are first sealed in plastic bags and then subjected to 1–100 OMF pulses (5–500 kHz, 0–50ᵒC, 25–100 ms)Exposure to magnetic fields causes inhibition in reproduction of microorganisms and also retard their growth.Theories of effect of magnetic field on microbial inactivation is still under research..Examples of food preserved with OMF milk, orange juice and yoghurt.
Non-thermal Processing of Food


Nonthermal technologies have the potential for producing high quality and safe food products. The use  of  UV-C  light  treatment proved  to  be  effective at  reducing  microbial  loads  of pathogens on fresh fruits and vegetables.

The emerging processing techniques are mostly employed to the liquid packed foods when compared to solid foods. Non thermal methods of food preservation are usually used for bulk quantities of foods; therefore, they are mainly used in the large-scale industries.

Despite being the cost of equipments used in the non-thermal processing is high when compared to equipments used in thermal processing but after minimizing the investment cost it can also be employed in small scale industries.

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5. Introduction to Food Engineering Book, 4th Edition, R.P. Singh and D.R. Heldman, Academic Press, NY, 2009


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