Ohmic Heating of Foods.pptx

8/14/2019 Ohmic Heating of Foods.pptx

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Ohmic Heating of Foods

By,Divya P SHarish S

IndumathiJeevithaKarthik


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Ohmic Heating- An Introduction Ohmic heating is an advanced thermal processing

method wherein the food material, which servesas an electrical resistor, is heated by passing

electricity through it. Electrical energy is dissipated into heat, which

results in rapid and uniform heating. Ohmic heating is also called electrical resistance

heating, Joule heating, or electro-heating, andmay be used for a variety of applications in thefood industry.


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Ohmic Heating (Cont.) The concept of ohmic heating in this way is not

new, but it has been developed into a commercialprocess during the last 15 years by the APV Bakercompany, using a licensed design by EATechnology.

The process can be used for UHT sterilisation offoods, and especially those that contain largeparticles (up to 2.5 cm) that are difficult tosterilise by other means.

It is now in commercial use in Europe, the USAand Japan.


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Principles Ohmic heating is based on the passage of alternating

electrical current through a body such as a liquid-particulate food system which serves as an electricalresistance in which heat is generated.

AC voltage is applied to the electrodes at both ends of theproduct body. The rate of heating is directly proportional to the square of

the electric field strength, E, and the electrical conductivity. The electric field strength can be varied by adjusting the

electrode gap or the applied voltage. The most important factor is the electrical conductivity of

the product and temperature dependence.


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Ohmic Heating Process In ohmic heating, main alternating current is passed directly

through a conductive food, which causes heat generation within thefood.

Due to internal heat generation, the heating is rapid and moreuniform than traditional systems used for heating foods where heat

must travel from the outside surface to the inside of the food. The rapid and uniform heating of a food is advantageous in

retaining many quality characteristics such as colour, flavour, andtexture.

The efficiency of ohmic heating is dependent upon how well theelectric current can pass through the food, as determined by itselectrical conductivity.

Therefore, the knowledge of electrical conductivity of foods isimportant in designing processes and equipment involving ohmicheating.


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Pre Treatment Pre-treatments of solid components include: pre -heating in the carrier liquid to equilibrate resistances blanching pasta for moisture absorption heating the carrier liquid to pre -gelatinise starch

heating to melt and expel fats stabilisation of sauces by homogenisation, especially dairy saucesor others that

contain fats and heat sensitive proteins blanching vegetables to expel air and/or to denature enzymes

enzymic marinades to soften texture and enhance flavour ofmeats soaking in acids or salts to alter the electrical resistance of

particles sauteing to improve appearance of meat particles


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Theory

Electrical resistance of a food is measuredusing a multi meter connected to aconductivity cell.

The measured resistance is converted toconductivity using:

=(1/R)(L/A)

where (S m1) product conductivity, R(ohms)measured resistance, L(m) length of the cell

and A(m 2)area of the cell.


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As an example of ohmic heating, we will considerheating a liquid food with Newtonian characteristicswhen pumped through an ohmic heater.

We assume that the flow conditions through thetubular-shaped heater are similar to plug flow, and aconstant voltage gradient exists along the heater.

In this setup, heat is generated within the liquid due toohmic heating, and heat loss from the fluid is in radialdirection to the outside, if the heater pipe isuninsulated.


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For this setup, conducting a heat balance weget,


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where | V| is voltage gradient along the heater pipelength,(V/m);

is the electrical conductivity at 0C; m is the slopeobtained from equation

C is the characteristic dimension or diameter of theheater pipe (m);

L is the length of heater pipe (m); U is the overall heat transfer coefficient based on the

inside area of the heater pipe,(W/m2

C); T is the temperature of the air surrounding the heater

(C).


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The initial condition is

Expanding the terms in Equation andrearranging, we get


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Where

Integrating Equation (4.189),we obtain


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Uses

Ohmic heating has been used to processvarious combinations of meats, vegetables,pasta and fruits when accompanied by asuitable carrier liquid.


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Advantages of Ohmic Heating the food is heated rapidly (1Cs 1) at the same rate throughout and the absence of

temperature gradients results in even heating of solids and liquids if theirresistances are the same

heat transfer coefficients do not limit the rate of heating temperatures sufficient for UHT processing can be achieved there are no hot surfaces for heat transfer, as in conventional heating, and

therefore no risk of surface fouling or burning of the product which results inreduced frequency of cleaning heat sensitive foods or food components are not damaged by localised over-

heating liquids containing particles can be processed and are not subject to shearing forces

that are found in, for example, scraped surface heat exchangers it is suitable for viscous liquids because heating is uniform and does not have the

problems associated with poor convection in these materials energy conversion efficiencies are very high (>90%) lower capital cost than microwave heating suitable for continuous processing.

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Ohmic Heating of Foods.pptx