Dielectric, Ohmic, & Infrared Heating

what is ohmic heating?

a process where an alternating electric current is passed through a food, and the electrical resistance of the food causes the power to be translated directly into heat

what is the specific electrical resistance?

a resistance within a food that generates heat when an electric current is passed through it

what do the wave guides do?

aluminum tubes that transmit microwaves to the chamber

what is the dielectric loss tangent?

an indicator of how well a material can be penetrated by an electromagnetic field and how it converts the electrical energy to heat; tan sigma

what is runaway heating?

as food heats, MW absorption increases --> increases the rate of temperature increase --> increases rate of MW absorption, etc.

what are the main applications of IR industry? what types of foods?

drying low-moisture foods; breadcrumbs, flours, grains, pasta products, tea

what is infrared energy?

electromagnetic radiation that is emitted by hot objects

what does the magnetron do?

generates microwaves (electromagnetic waves)

what effect do salt and moisture contents in food have to do with MW absorption?

high salt and moisture contents increase the efficiency of MW absorption and decrease of depth penetration

if the dielectric loss factor is ______ (higher/lower) the more energy is absorbed by the food

higher the loss factor, more energy absorbed

what's the relationship between electrical conductivity and electrical resistance?

inverse

what does the stirrer do?

it distributes waves inside of the oven chamber; like a rotating fan

the _________ (lower/higher) e" and the _______ (lower/higher) the frequency, the _______ (lesser/greater) the penetration depth

lower e" and lower frequency = greater penetration depth

what are the two types of dielectric heating?

microwave and radio frequency

do foods better conduct electricity if they have more or less water and ionic salts?

more

do microwaves heat food from the inside out?

no

do higher temperatures produce longer or shorter wavelengths?

shorter which have a greater depth of penetration

do electromagnetic waves penetrate deeper in foods that contain small or large amounts of water?

small amounts of water

what is the wavelength of infrared radiation determined by?

temperature of the source

what is the dielectric loss factor (e'')?

the ability of a material to dissipate electrical energy as heat; the amount of energy absorbed by foods from electromagnetic waves

what is the dielectric constant (e')?

the ability of a material to store electrical energy

what is the penetration depth?

the distance from the surface where power of the wave has decreased to 37% of its power at the surface

why is MW penetration different when water turns to ice?

the molecules are less free to move or absorb energy from the alternating electric field; ice has a lower loss factor

do more concentrated solutions heat faster or slower? why?

they heat more quickly due to ionic polarization (more collisions of the charged ions moving at an accelerated rate)

what is infrared heating?

when infrared energy is absorbed by a food and the radiation gives up its energy in order to heat the materials

does a greater penetration depth equate to increased heating?

yes, but the dielectric properties are not constant (they vary with temperature) ~as food gets hotter, the rate will increase

what is the frequency range of microwave energy for food processing applications?

~2450 MHz for a home oven ~915 MHz for industrial systems

what is baking (related to dielectric heating)?

~MW & RF ~^heaters are located at the exit of tunnel ovens to ~~reduce moisture content ~~complete the baking without further changes in color

what is dehydration (related to dielectric heating)?

~MW & RF ~low rates of heat transfer due to low thermal conductivity of foods ~reduces product shrinkage ~prevents damage to food surface ~eliminates case hardening

what is tempering (related to dielectric heating)?

~MW & RF ~temperature is raised from around -20C to -3C ~widely used for meat/fish products that are more easily boned at temperatures just below the freezing point

what is thawing (related to dielectric heating)?

~MW & RF ~used to rapidly thaw small portions of food and for melting fats ~however, doesn't work well with large blocks

what are susceptors?

~an advancement in MW technology ~composed of ceramic or aluminum flakes ~aids in baking & crust formation ~ex. Hot Pockets

what is convection MW?

~an advancement in MW technology ~it combines the traditional MW with a convection oven ~rapid heating and surface browning/crisping

what is the effect of IR radiation on foods?

~changes the flavor and color of foods due to Maillard reactions and protein denaturation ~seals moisture and flavor/aroma compounds in the interior of the food

what factors affect penetration depth?

~determined by loss factor and frequency of the waves ~e'' is in turn affected by other factors (moisture content of food, presence of salts, etc.)

what are dielectric properties?

~dielectric constant ~dielectric loss factor

what are characteristics of dielectric heating?

~direct methods of heating ~non-ionizing energy ~electromagnetic energy is transmitted as waves ~waves penetrate food and is converted to heat

what are the advantages to ohmic heating?

~food is heated rapidly throughout the bulk of the food ~no hot surfaces for heat transfer --> heat transfer coefficients don't limit rate of heating ~liquids w/ particles aren't subject to shearing forces ~lower capital cost than MW heating

what is ionic polarization? how does it create heat? what is a major factor that affects the amount of heating?

~ions move at an accelerated pace ~kinetic energy is converted to thermal energy ~the distribution of ionic molecules affects the amount of heating

what are disadvantages of MW heating?

~non-uniform heating ~"runaway" heating ~potential of electrical arcing due to presence of metals ~superheated liquids

what is radio frequency heating?

~operates at lower frequencies than MW ~food is passed between electrodes in which the RF voltage is applied ~the voltage changes the orientation of water dipoles in a similar way to MW

for ohmic heating, what size/density should the particles be?

~pliable and small ~density should be about the same as the carrier liquid

what are advantages of both MW & RF heating?

~rapid heating ~food surface doesn't overheat; minimizes heat damage & surface browning ~equipment is small and compact, clean in operation, and suited to automatic control

what are the advantages of IR heating?

~reduction in roasting/baking time ~savings in energy compared with traditional processes

what does the heat of transfer for IR heating depend on?

~surface temperature of the heating and receiving materials ~surface properties of the two materials ~shapes of the emitting and receiving bodies

what are the most important industrial applications of MW & RF heating?

~thawing ~tempering ~drying ~baking

what are disadvantages of RF heating?

~thickness of food is restricted by distance between the capacitor plates

what kinds of foods can be heated with ohmic heating?

~those with about 60% solids ~fruit slices in syrups ~soups/stews ~heat sensitive liquids

what is dipole rotation? how does it create heat?

~when dipoles attempt to orient themselves to an applied electric field ~the constant reorientation of dipoles creates frictional heat ~~increases in water temperature can heat the surrounding components by conduction and/or convection

what are the 2 methods of producing and transmitting power to RF applicators?

1) conventional RF where the applicator is part of the RF generation circuit 2) 50 ohm technology, where the RF generator is separated from the applicator and connected using high-power coaxial cable

what are the two types of microwave heating?

1) dipole rotation 2) ionic polarization

what are 3 factors that limit the widespread commercial use of ohmic heating?

1) irregular and complex heating patterns & difficulties in predicting heating characteristics 2) lack of data on critical factors that affect rate of heating 3) lack of accurate temperature-monitoring techniques to locate cold spots during process

what are the components of a microwave?

1) magnetron 2) wave guides 3) stirrer 4) oven chamber

what is the frequency range of microwave energy?

300 MHz to 300 GHz