Paper No. : 04

Paper Title: Unit Operations in Food Processing

Module- 34: Minimal processing technologies-1:

Ohmic heating, radio frequency heating and pulsed

electric field heating

OHMIC HEATING

1. Introduction Ohmic heating (OH) is defined as a process wherein electric

current is passed through materials with the primary purpose ofheating them.

Ohmic heating(OH) is an advanced thermal processing methodwhere in the food material, which serves as an electrical resistor,is heated by passing electricity through it

Ohmic heating is also called electrical resistance heating, Jouleheating, or electro-heating, and may be used for a variety ofapplications in the food industry.

Foods that contain water and ionic salts are capable ofconducting electricity but they also have a resistance whichgenerates 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 resistance of a food is measured using a multimeter

connected to a conductivity cell.

The measured resistance is converted to conductivity using

σ (Sm-1) = Product conductivity.

R (ohm’s) = Measured resistance.

L (m) = Length of the cell.

A (m2) = Area of the cell.

The resistance in an ohmic heater depends on the specific

resistance of the product, and the geometry of the heater.

R (ohm’s) = Total resistance of the heater.

Rs (ohms.m-1) = Specific resistance of the product.

x (m) = Distance between the electrodes.

A (m2) = Area of the electrodes.

The resistance determines the current that is generated in the

product.

V (volts) = Voltage.

I (amps) = Current.

Every product has a critical current density and if this is exceeded,

there is likely to be arcing (or flash-over) in the heater. The current

density is found by

Id (amps.cm-2) = Current density.

The rate of heating and the power by is found to be

Assuming that heat losses are negligible, the temperature rise in a

heater is calculated by using

∆θ (ºC) = Temperature rise

σa(Sm-1)= Average conductivity throughout temperature rise

A (m2) = Tube cross-sectional area

x (m) =Distance between electrodes

m(kg s-1) = Mass flow rate.

Cp (J kg-1 ºC) =Specific heat capacity of the product.

Q= m.Cp.∆θ P= VI = I2R

2. Application of Ohmic Heating in Food Processing It depends on the rate of heat generation in the system.

Ohmic heating can be used for heating liquid foods containing

large particulates, such as soups, stews, and fruit slices in syrups

and sauces, and heat sensitive liquids.

This is useful for the treatment of proteinaceous foods, which tend

to denature and coagulate when thermally processed.

Juices can be treated to inactivate enzymes without affecting the

flavour.

The heating rate of particles in a fluid depends on

a) The relative conductivities of the system’s phases.

b) The relative volume of those phases.

Advantages of Ohmic Heating

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.

This over processing leads to a destruction of nutrients and

decreased flavour.

Ohmic heating processes the particles and surrounding liquid

simultaneously, preventing overcooking.

Microbial Inactivation

Microbial inactivation in relation to ohmic heating is primarily thermal in nature.

Microbial inactivation curves of ohmic heating are similar toconventional heating curves except for a difference in theslope, which can most likely be explained by the presence ofthe electric field.

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 tobuild up charges, forming disruptive pores

The dielectric strength of a cell membrane is related to theamount of lipids (acting as an insulator) present in themembrane itself.

Excessive exposure causes cell death due to the leakage of

intracellular components through the pores.

Electroporation Process of a Cell

Disadvantages to Ohmic Systems

The costs of commercial ohmic heating systems, includinginstallation, can be in excess of $9,000,000 USD, which is acostly investment for a manufacturing facility.

Another slight disadvantage relates to the electrical conductivityof a substance.

As the temperature of a system rises, the electrical conductivityalso increases due to the faster movement of electrons.

An ohmic heating system that has not been cleaned thoroughlyenough may result in electrical arcing due to protein deposits onthe electrodes.

RADIO-FREQUENCY

Introduction

Radio-frequency (RF) processing has been used invarious food industry processes.

Dielectric heating transfers energy directly to theproduct. Applications of RF possess advantages overother conventional techniques.

Dielectric heating

Dielectric heating lies in the electromagnetic spectrumin the range of frequencies from 300 kHz to 300GHz.

Radio-frequencies (RF) range from 300 kHz to300MHz and microwaves (MW) range from 300MHzto 300GHz.

In RF, An electric field is developed betweenelectrodes.

Differences between Radio-Frequency &Microwaves

• Four principal frequencies, 13.56, 27.12, 915, 2450MHz,should be adopted for a particular dielectric heatingapplication.

• Microwave is just a subset of the RF range.

• RF covers 3 Hz to 300 GHz while Microwaves occupiesthe higher frequencies at 300MHz to 3GHz.

• For large scale processing applications of materials, radiofrequency with its longer wavelength is less prone tostanding waves and resulting non-uniform heating.

• Moisture levelling is more effective at radio frequency forwet planar materials in drying applications.

• If drying needs to be carried out under vacuum to reducethe boiling point, as could be the case with sometemperature-sensitive materials, microwave energy ispreferred since the likelihood of arcing is much smaller.

Heating mechanism of RF

Radio-frequency heating system the RF generator creates an

alternating electric field between two electrodes.

The material to be heated is conveyed between the electrodes,

where an alternating energy field causes polarization, where the

molecules in the material to continuously reorient them to face

opposite electrodes much like the way bar magnets move to face

opposite poles in an alternating magnetic field.

Friction resulting from this molecular movement causes the

material to rapidly heat throughout its entire mass.

Among the electromagnetic absorbers water is the major

absorber (higher the moisture content better the heating), in high

carbohydrate foods such as bakery products - the dissolved

sugars and in alcoholic beverages - alcohol.

Overall heating efficiency depends upon Radiation frequency,food composition, and size of the material, salt content,moisture content and temperature.

Fig2: Classical Signs of checking

– Checking is the phenomenon occurs when there is non-uniform heat distribution in the product.

– non-uniform moisture distribution develops ultimately itcreates the stress and the product may crack. But this can beavoided by using Radiofrequency drying technique.

Fig: Simple RF Heating set up

Material Properties

The process of heating through permanent and induced

polarization is called dielectric heating.

Polarization effect is a function of the radiation frequency, the

dielectric and electric properties of the material, the viscosity

of the medium and the size of the polar molecules.

Factors effecting Radio-Frequency Heating

Water Content: It absorber of electromagnetic waves.

Food composition: Carbohydrate, Protein, Fat etc.

Density: Foods of lower density that transfer at a given level.

Applications of Radio-Frequency Heating

There are basically two types of applications of radio-

frequency.

1) Radio-Frequency Heating Applications

2) Radio-frequency drying applications.

1) Radio-Frequency Heating Applications

If foods to be heated to high temperature short time (HTST)

treatments generally deliver products of a superior quality.

Electromagnetic energy, with its rapid heating potential, may

offer a competitive edge in agricaltural and food applications.

Thermal treatment of food products

RF cooking of meat products resulted in reduced cooking time,

lower juice losses, acceptable colour and texture and

competitive energy efficiency.

Sausage products heated well and had a good appearance without

release or loss of moisture and fat when tested at 27 MHz.

RF-thawing techniques in the fish-processing sector has been

developed.

A processing method for the RF treatment of fresh carrot sticks to

reduce their microbial load and their enzymatic activity while

ensuring their quality has been developed.

RF treated carrot sticks had better quality in terms of colour and

taste.

Seed treatments

When a seed is exposed to RF fields of high frequency and

intensity, its temperature will rise due to dielectric heating, its

germination will decline.

The RF heating of alfalfa seeds for reducing pathogens.

Product disinfestation or disinfection

RF heating to control product pests in a variety of agri-food

products such as cherries, walnuts, stored grains.

Treatments at 39 MHz and 2450 MHz to control rice weevils.

2) Radio-frequency drying applications

Food drying

– Well-known applications of RF energy to drying, especially in

wood, textile and post-baking drying.

It is used for post-bake drying of cookies, crackers and pasta.

Cookies & crackers, fresh out of the oven, have a non-

uniform moisture distribution which may yield to cracking

during handling.

RF heating can help even out the moisture distribution after

baking.

Agricultural product drying

High frequency (10–15 MHz) and field intensity, drying of

grain may be completed within 20–25 min, but seed quality

deteriorates.

Lower frequency (1–5 MHz) and field intensity, the seed

quality of the grain is preserved but the drying period is

increased to 40–60 min.

Wood drying

RF wood drying, dielectric constant for water is about 20

times more than that of a dry cell wall for the same frequency

range (10–30 MHz) and temperature.

Under RF, water heats at a much more rapid rate than wood.

Water is heated internally more than the surrounding cell wall

material thus eliminating the slow conduction from the surface

to the core of the lumber that occurs in conventional kiln

drying process.

Conclusion

The advantages and disadvantages of RF are the following.

1.In many heat transfer problems as water is preferentially

heated.

2. A considerable decrease in process time.

3. Acts as a moisture leveling process.

4. Good overall energy efficiency.

5. No surface over-drying or over heating.

6. Low maintenance costs.

The disadvantages of RF are

1.High initial capital cost of equipment.

2.Subject to the fluctuations of electrical costs.

3.Skilled labour is required for the tuning.

The success of an RF heating set up based on the 50 Ω system,

lies in its design and in the impedance matching between the

power generator and the applicator.

The development of new applications of RF heating requires

targeted equipment design, specific fine tuning of the applicator

design, high tech tools and highly skilled technicians.

PULSED ELECTRIC FIELD PROCESSING

Introduction

Pulsed Electric Field (PEF) processing is a non-thermalmethod for food preservation that uses short bursts ofelectricity for microbial inactivation and causesminimal or no detrimental effect on food qualityattributes

PEF can be used for processing liquid and semi-liquidfood products.

PEF processing involves the application of pulses ofhigh voltage to foods placed between two electrodes.

Pulsed electric fields (PEF) is an emerging technologythat has been extensively studied for non-thermalfood processing.

Principle of PEF

The basic principle of the PEF technology is the application ofshort pulses of high electric fields with duration of micro tomilliseconds and intensity in the order of 10-80 kV/cm.

It is based on pulsed electrical currents delivered to aproduct placed between a set of electrodes.

The applied high voltage results in an electric field that causesmicrobial inactivation.

After the treatment, the food is packaged aseptically andstored under refrigeration.

Two mechanisms have been proposed to explain the microbialinactivation by PEF:

1. Electroporation.

2. Electrical breakdown.

1. Electroporation

Electroporation is the phenomenon in which a cell exposed to

high voltage electric field pulses temporarily destabilizes the lipid

bilayer and proteins of cell membranes.

The main effect of an electric field on a microorganism cell is to

increase membrane permeability due to membrane compression

and poration.

Large pores are obtained by increasing the intensity of the electric

field and reduce the ionic strength of the medium.

Figure1: Electroporation process

Swelling Cell lysis Inactive Cell

Electrical breakdown

The cell membrane is considered as a capacitor filled withdielectric material of low electrical conductance.

Accumulation of charges with opposite polarity on both sides ofthe membrane leads to a naturally occurring, perpendicular trans-membrane potential of about 10 mV.

Exposure to an external electrical field an additional potential isinduced by movement of charges along the electric field lines,resulting in a viscoelastic deformation of the cell membrane.

When the overall potential exceeds a critical value of about 1 V,depending upon the compressibility, the permittivity and theinitial thickness of the membrane, the electro compressive forcecauses a local dielectric rupture of the membrane inducing theformation of a pore, acting as a conductive channel.

The electric breakdown is reversible if the pores induced are smallin comparison to the membrane area.

Figure2: Dielectric breakdown

Increase of electric field strength and treatment intensity by

increasing pulse width will promote formation of large pores.

The reversible damage will turn into irreversible breakdown,

with mechanical destruction of the cell membrane and cell death.

+ -+ -

+ -+ -

+ -

Cell membrane

Pulse Electric Field System

A pulsed Electric Field processing system consists of a

high-voltage power source, an energy storage capacitor

bank, a charging current limiting resistor, a switch to

discharge energy from the capacitor across the food and a

treatment chamber.

An oscilloscope is used to observe the pulse waveform.

The power source, a high voltage DC generator, converts

voltage from a utility line (110 V) into high voltage AC, then

rectifies to a high voltage DC.

Energy from the power source is stored in the capacitor

and is discharged through the treatment chamber to generate

an electric field in the food material.

Source: http://www.slideshare.net/StellaMariem/pulsed-electric-field-processing-of-food

Fig : Flow chart of a PEF food processing system with basic component.

High-voltage and high-current probes are used to measure the

voltage and current delivered to the chamber.

Applications

PEF is a continuous processing method, which is not

suitable for solid food products that are not pumpable.

PEF is also applied to enhance extraction of sugars and

other cellular content from plant cells, such as sugar

beets.

PEF also found application in reducing the solid

volume (sludge) of wastewater.

PEF pasteurization kills microorganisms and

inactivates some enzymes and, unless the product is

acidic, it requires refrigerated storage.

PEF treatment would be advantageous PEF pasteurized

products currently are stored refrigerated.

Recent developments

The company Cool Wave Processing has developed a

second generation PEF technology.

They released this technology to the market under the alias

“Pure Pulse”.

Pure Pulse’s unique PEF setup allows the heat load on the

product to be greatly reduced.

United States, the first commercial scale continuous

PEF system is installed at The Ohio State University’s

Department of Food Science and Technology.

The cost for a PEF treatment is around € 0.04 per liter. The

investment cost for a PEF machine (30 kW, 1000liters per

hour) is approximately € 250,000.

Conclusion

Food preservation technologies are based on the

prevention of microbial growth and on inactivation of

microorganisms so as to increase their shelf life.

Pulse electric field processing technology holds a

promising prospect for preservation of foods. Research

of pulsed electric fields technology is ongoing

around the world.

Most of the research conducted up until now has been

in the laboratory and on a pilot plant scale level, and

has shown promising results.

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