Ohmic Heating

Advanced Thermal Processing Technique :

Ohmic HeatingBy

Nitesh ([email protected])

www.facebook.com/nitesyadav

Introduction• Ohmic heating is an advanced thermal processing

method where in the food material, which serves as 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.

Discovery of Joule Heating• First experimented with in 1841

James Prescott Joule– Hence the name: ‘Joule Heating’

• What did he do?– Passed a current through a wire of fixed length.– Immersed in a bath of water of fixed volume/mass.– Observed the temperature in the water varied with

current, length of wire, and time.

Came up with the Relation

Where:Q = Amount of Energy produced (in

the form of heat) , joules1 joule = kg m /s2

I = Current , amperesR = Resistance , ohmst = Time , seconds

tRIQ 2

History• In the nineteenth century, several processes

were patented that used electrical current for heating flowable materials.

• In the early twentieth century, ‘electric’ pasteurization of milk was achieved by passing milk between parallel plates with a voltage difference between them, and six states in the United States had commercial electrical pasteurizers in operation. (Sastry , 1992)

• The Electricity Council of Great Britain has patented a continuous-flow ohmic heater and licensed the technology to APV Baker. (Skudder, 1988).

• Within the past two decades, new and improved materials and designs for ohmic heating have become available.

• Ohmic Heating has been developed into a commercial process by the APV Baker company, using a licensed design by EA Technology.

Principles of Ohmic Heating

• Ohmic heating is based on the passage of alternating electrical current (AC) through a body such as a liquid-particulate food system which serves as an electrical resistance in which heat is generated.

• The rate of heating is directly proportional to the electrical conductivity.

• If the product has more than one phase such as in the case of a mixture of liquid and particulates, the electrical conductivity of all the phases has to be considered.

• The electrical conductivity increases with rising temperature, suggesting that ohmic heating becomes more effective as temperature increases.

• In ohmic heating, microbes are thought to be thermally inactivated.

• A mild electroporation mechanism may occur during ohmic heating operating at low frequency (50–60 Hz) which allows electrical charges to build up and form pores across cell walls.

Conventional Heating Ohmic Heating

In cans or aseptic processing systems for particulate foods, significant product quality damage may occur due to slow conduction and convection heat transfer.

Volumetrically heats the entire mass of the food material, thus the resulting product is of far greater quality.

Cleaning requirement are more because of product fouling.

Cleaning requirements are comparatively less than those of traditional heat exchangers.

UHT processing not possible due to the lower thermal conductivity of solid foods, which slows heat penetration to the centre of the pieces.

UHT Processing is Possible in solid foods.

Conventional Heating Ohmic Heating

Over-cooking of foods occur at the outside of particles.

Heat sterilise particulate foods under UHT conditions without causing heat damage to the liquid carrier or over-cooking of the outside of particles.

Integrity of Particles is not maintained. Lack of agitation in the heater maintains the integrity of particles and it is possible to process large particles (up to 2.5 cm) that would be damaged in conventional equipment.

Takes longer time for heating of foods. Comparatively takes very less time.

Heat penetration into solid pieces of food by conventional heating

(Adapted from Fryer 1995)

Heat penetration into solid pieces of food by ohmic heating

(Adapted from Fryer 1995)

Effect of Ohmic Heating Parameters

• Foods that contain water and ionic salts are capable of conducting electricity but they also have a resistance which generates heat when an electric current is passed through them.

• Conductivity measurements are therefore made in product formulation, process control and quality assurance for all foods that are heated electrically.

Electrical conductivity

• Electrical resistance of a food falls by a factor of 2 to 3 over a temperature rise of 120ºC.

• It also vary in different directions (e.g. parallel to, or across, a cellular structure)

• It changes if the structure changes (e.g. gelatinisation of starch, cell rupture or air removal after blanching).

Resistance is converted to conductivity:

σ = (1/R) (L/A)

Where• σ (Sm-1) = Product conductivity• R (ohms) = Measured resistance• L (m) = Length of the cell• A (m2) = Area of the cell

• The resistance in ohmic heater depends on specific resistance of product & geometry of the heater.

R = (Rs x) / A

Where R (ohms) = Total resistance of the heaterRs (ohms m-1) = Specific resistance of the product

x (m) = Distance between the electrodesA (m2) = Area of the electrodes

Resistance in Ohmic Heater

• The resistance determines the current that is generated in the product

R = V/IWhere

V (volts) = voltage appliedI (amps) = current

• If the resistance is too high, the current will be too low at maximum voltage.

• If the resistance is too low, the maximum limiting current will be reached at a low voltage and the heating power will be too low.

• The rate of heating is found using equation:Q = m.Cp.Δt

• The power byP = V I P = R I2

Temperature rise in a ohmic Heater• Assuming that heat losses are negligible

Δθ =

Whereθ (ºC): temperature riseσa (Sm-1): average product conductivity throughout

temperature riseA (m2): tube cross-sectional area,x (m): distance between electrodesm (kg s-1): mass flow ratecp (J kg-1 ºC-1): specific heat capacity of product.

Product Suitable For Ohmic Heating Heat sensitive liquids Soups Juices (to inactivate enzymes without affecting the

flavor) Stews fruit slices in syrups Sauces Meats Sea foods Proteinaceous foods which tend to denature and

coagulate when thermally processed. e. g. liquid egg

Advantages of Ohmic Heating

Microbial Inactivation

• The principal mechanisms of microbial inactivation in ohmic heating are thermal in nature.

• An experiment using Bacillus subtilis revealed that a two stage ohmic treatment (ohmic heating, holding period, ohmic heating) resulted in accelerated death rates (Cho et al., 1999).

• Another experiment involving Saccharomyces cerevisae showed that ohmic heating enhanced the leakage of intracellular components as compared with the conventional method of boiling in water (Lee and Yoon, 1999).

Electroporation

• At low frequencies (50-60 Hz) and high field strengths (>100V/cm) most commonly associated with ohmic heating, the naturally porous cell walls can allow the cell membrane to build up charges, forming disruptive pores (Cho et al., 1996).

• Electroporation occurs because the cell membrane has a specific dielectric strength, which can be exceeded by the electric field.

• The pores formed can vary in size depending on the strength of the electric field, and can reseal after a short period of time.

• Electroporation is highly damaging to a cell and would enhance the lethal effects of thermal abuse already present from the ohmic heating.

Diagram of Bacterial Cell

Electroporation Process of a Cell

Other Advantages of Ohmic Heating

• The food is heated rapidly (1ºC/s) at the same rate throughout.

• Absence of temperature gradients results in even heating of solids and liquids if their resistances are the same.

• Heating takes place volumetrically and the product does not experience a large temperature gradient within itself as it heats.

• No risk of surface fouling or burning of the product which results in reduced frequency of cleaning.

• Heat transfer coefficients do not limit the rate of heating.

• Heat sensitive foods or food components are not damaged by localised over-heating.

• Energy conversion efficiencies are very high (>90%)

• It is suitable for viscous liquids. i.e. heating is uniform and good convection.

• Suitable for continuous processing.

• Relatively low maintenance cost (no moving parts).

• Ease of process control with instant switch-on and shut-down.

• Heating food materials by internal heat generation without the limitation of conventional heat.

• Temperatures sufficient for UHT processing can be achieved.

• A quiet environmentally friendly system.

Ohmic Heating Process and Equipment

Equipment design

• Inert electrode materials and control equipment accurate enough to keep the temperature within the necessary range.

• Currently, commercially available designs include electrodes that are located at various positions

• Distribution of electric field strength.

Electrode

• Previous designs attempted to use a range of electrode materials from graphite to aluminum or stainless steel.

• Use of a food-compatible electrode electrical material and the correct current density eliminates contamination problems.

Control• Emphasize on the controlling of ohmic heating

process.

• In continuous processing, problems can result if a single electrode pair is used to heat food material through a large change in temperature; substantial changes in liquid conductivity and thus in heating rate, may result along the length of the electrode.

• Multiple sets of electrodes are easier to control.

• A typical commercial ohmic heating system for liquid-particulate mixture is the APV Baker ‘ohmic heating’ process.

• The process was originally developed by the UK Electricity Council Research Centre, and was then licensed to APV Baker who have developed it into a commercial system.

Commercial Equipment

• Food is pumped through a vertical pipe containing a series of cylindrical electrodes connected to a 50Hz three-phase supply.

• Electric current thus flows through the food in the pipes connecting the electrodes.

• The food material is rapidly heated to sterilization temperature, then passes to a holding section and finally to an aseptic packaging plant.

• The use of multiple electrodes gives a much greater degree of control together with a uniform electric field in the pipe sections.

• The process allows food products containing particulates up to 25mm to be heated to sterilization temperatures up to 140ºC in less than 90 seconds.

• The whole process is automatically controlled using a programmable logic controller.

The 5 kW pilot scale ohmic heating system by APV Baker, Ltd. (Courtesy of APV Baker, Ltd.)

• Type of product • Electrical resistance • Change in resistance over the expected temperature

rise• Flow rate• Temperature rise (determines the power

requirement)• Heating rate required• Holding time required

Important Factors

Small Scale Ohmic Heater

Applications of Ohmic Heating

• Ohmic heating can be applied to a wide variety of foods, including liquids, solids, and fluid–solid mixtures.

• Ohmic heating is being used commercially to produce liquid egg product in the United States.

• It is also being used in the United Kingdom and Japan for the processing of whole fruits such as strawberries.

Ohmic Heating is now in commercial use in Europe, the USA and Japan for:

• Aseptic processing of high added-value ready meals, stored at ambient temperature.

• Pasteurisation of particulate foods for hot filling.

• Pre-heating products before canning.

• High added-value prepared meals, distributed at chill temperatures (Fryer, 1995).

Results

Conventional Heating Vs

Ohmic Heating

Pereira et. al. 2006

Bozkurt & Icier , 2009

Pereira et. al. 2006

Advanced Applications of

Ohmic Heating

Blanching

Mizrahi (1996) determined that ohmic heating was an effective method for blanching because the rapid, uniform heating exhibited by ohmic heating eliminated the need for dicing vegetables.

The quick process time and reduction in surface area (no dicing) reduced solute losses by an order of magnitude during blanching.

Lakkakula et al. (2004) showed significant lipase deactivation in rice bran during ohmic heating, with and without a corresponding temperature increase.

Sensoy and Sastry (2004) found that using ohmic heating during the blanching of mushrooms resulted in the shrinking of mushrooms at a lower temperature and with less water use as compared to conventional blanching.

Wang and Chu (2003) studied the effect of ohmic heating on the vacuum evaporation of orange juice and found that the evaporation rate could be increased as much as three times using ohmic heating, and resulted in enhanced product quality.

Conclude that ohmic heating has potential as a fast evaporation method and recommend further development in this area.

Evaporation

Wang and Sastry (2000) showed that ohmically treating sweet potato prior to dehydration accelerated the hot-air drying rate significantly compared to raw, conventionally treated, and microwaved samples.

Lima and Sastry (1999) found that the lower the frequency of AC used in ohmic heating, the faster the hot-air drying rate. Maximum drying benefits were seen when drying to initial or intermediate moisture contents.

Dehydration

Fermentation

Fermentation is the process of releasing energy from a carbohydrate without oxygen by producing alcohol or lactic acid.

An ohmic heating system may be particularly useful in this important process of food manufacturing, particularly the dairy industry, where fermentation by Lactobacillus acidophilus is necessary for the production of cheese and yogurt.

Cho et al. (1996) showed that ohmic heating of a fermentation vessel containing L. acidophilus reduced the lag period of the bacteria in the early stages of growth.

Utilizing the ohmic heating process in the early stages of fermentation may shorten the total processing time of a dairy product.

This speedier process would save untold amounts of time in overhead and labor costs.

Lima and Sastry (1999) and Wang and Sastry (2002) found that ohmically heating apple tissue prior to mechanical juice extraction significantly increased apple juice.

Lower the frequency of AC, the higher the extraction yield.

Lakkakula et al. (2004) used ohmic heating to significantly increase the extraction of rice bran oil from rice bran (with moisture addition), especially at low (1 Hz) frequency.

Extraction

Extracting Oil From Rice Bran

• Rice bran oil has outstanding nutritive, sensory and cooking characteristics, it is relatively expensive to produce.

• Ohmic heating could enhance the extraction of rice bran oil from rice bran, with the ultimate goal of making the production of rice bran oil economically feasible. (LSU Ag Center, 2002)

Disadvantages

• The costs of commercial ohmic heating systems is high.

• 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 (Rahman 1999).

• Any pathogenic bacteria that may be present in these globules may receive less heat treatment than the rest of the substance (Sastry 1992).