Introduction :

Ohmic heating has different names and each names shows its principle of heating – Joule heating OR Electric resistance heating OR Direct electric resistance heating OR Electroheating OR Electroconductive heating. In ohmic heating process, alternating electric current is passed through food material; heat is internally generated within the material owing to its resistance to the applied electrical current. Unlike conventional heating where heat transfer occurs from a heated surface to the product interior by means of convection and conduction, Ohmic heating has inside–outside heat transfer pattern.

 


Design

 


Advantages of ohmic heating

  1. It can heat particulate foods and Liquid–particle mixtures.  
    ii. By ohmic heating, high temperatures can be rapidly achieved. For e.g. temperatures for ultra high temperature (UHT) processing
  2. As there are no hot surfaces for heat transfer, there is low risk of product damage due to burning
  3. It has high energy conversion efficiency
  4. It requires relatively low capital cost.


Factors affecting ohmic heating

  1. Electrical conductivity of food and food mixture which in turn depends on food components: ionic components (salt), acid, and moisture mobility increase electrical conductivity, while fats, lipids, and alcohol decrease it
  2. Fluid viscosity : higher viscosity fluids shows faster ohmic heating than lower viscosity fluids
  3. In case of liquid + solid mixture, the property difference of the two components also affects ohmic heating
  4. Density and specific heat of the food product


Reference:

Handbook of food preservation 2nd ed. I M. Shafiur Rahman I 2007 I Taylor & Francis Group, LLC I Page: 741-750

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Comment by Sanjeev Sharma on March 15, 2012 at 10:15pm
Comment by Nitin Rana on August 22, 2011 at 10:02am

Advantages of Ohmic Heating

In general, ohmic heating systems are advantageous

  • Due to an optimization of investment (increased efficiency),
  • Instant shutdown of the system, and
  • Reduced maintenance costs because of the lack of moving parts.

In order to comply with governmental regulations concerning microbial lethality in food products, heating methods are applied at the coldest point of a system, which is generally the center of the largest particle. In conventional heating, the time it takes to increase the temperature at this cold point may overprocess the remaining particles and the surrounding liquid. This overprocessing leads to a destruction of nutrients and decreased flavor. Ohmic heating processes the particles and surrounding liquid simultaneously, preventing overcooking.

 

 

 

Disadvantages of Ohmic heating 

  • The costs of commercial ohmic heating systems, including installation, can be in excess of $9,000,000 USD, which is a costly investment for a manufacturing facility.
  • A disadvantage related to the type of food that can be processed lies in the presence of fat globules. A food that has fat globules can be troublesome to effectively heat ohmically, as it is non-conductive due to lack of water and salt .Any pathogenic bacteria that may be present in these globules may receive less heat treatment than the rest of the substance.
  • Another slight disadvantage relates to the electrical conductivity of a substance. As the temperature of a system rises, the electrical conductivity also increases due to the faster movement of electrons. Thus, this creates the possibility of ‘runaway’ heating. An ohmic heating system that has not been cleaned thoroughly enough may result in electrical arcing due to protein deposits on the electrodes.

 

Applications of ohmic heating

Applications for ohmic heating in the food manufacturing industry include:

 

  • heating liquid foods such as soups, stews, and fruits in syrup;
  • heat sensitive liquids processing;
  • juices treated to inactivate proteins (such as pineapple or papaya);
  • blanching;
  • thawing;
  • starch gelatinization;
  • sterilization;
  • peeling of fruits (eliminating the need for lye-a harmful corrosive chemical);
  • dehydration;
  • extraction;
  • fermentation and
  • processing protein-rich foods which tend to denature and coagulate when thermally processed

Used to process many varieties of foods including: baby food, artichokes, carrots, mushrooms, ketchup, fruit nectars, fruit juice, peppers, cauliflower, tomato paste, sausages, and fruit purée.

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