Lecture 17: Microwave, Radio Frequency, Ohmic Heating, PEF, PL Flashcards
how to microwaves compare to telecommunication range
MW are longer
how can MC be generated?
by oscillating electric fields
what are restrictions of MW?
- interference w/ radar communications
- only some frequencies are permitted
what types of MW interactions are used to heat foods?
- dipole rotation: flip flop rotation of dipole molecules, like water
- ionic polarization: back and forth vibration of ionic salts like NaCl
what factors does MW heating depend on?
- MW frequency
- thermo physical properties of foods
- dielectric properties
define the dielectric constant
ability of material to absorb MW
define the loss factor
ability of material to dissipate MW
what go the dielectric properties of MW heating depend on?
product composition
temp
MW properties
how is penetration of MW energy related to depth of product?
penetration decreases exponentially w/ depth
shorter frequency of MW = ____ penetration
higher/lower
higher
what does the MW penetration depth depend on?
state of the product (eg. ice vs water)
how is penetration depth related to dielectric constant?
inversely related
what are some applications of MW?
- heating: pasteurization and sterilization
- tempering, thawing
- drying
- MW osmotic drying
- vacuum MW drying
what are some advantages of MW processing?
- volumetric heating
- decreased processing time
- instant control and clean process
- selective E absorption by polar molecules
- can be combined with other E sources and chemicals
what are problems with WM processing?
- lack of penetration depth in large samples
- lack of flavour development and browning due to wet environment cooking
- non-uniform heating due to non-uniform field strength
what can MW blanching be used for?
to inactivate enzymes
how does MW blanching compare to conventionally bleached products?
MW products are higher quality
what is fouling? what effects can this cause?
occurs at contact surface due to over heating of proteins, CHO, etc…
can cause reduced heat transfer and formation of off flavours
why does fouling occur in conventional heat exchangers, but not in MW?
conventional heating: surface is hot to transfer heat to fluids
MW: heats food directly, not indirectly by the surface. Since the surface is cooler than the product, there is no fouling
what is MW pasteurization used for?
milk
orange juice
apple juice
what are benefits of MW pasteurization?
- processing temp can be lower than conventional pasteurization
- better quality retention
- incr destruction of microorganisms and enzymes
what are thermal effects of MW processing?
explained by interactive effects of time and temp combinations based on some models
what are non-thermal effects of MW processing?
occurs in temperatures not low enough to cause any inactivation
what are enhanced thermal effects of MW processing?
when the observed effect is greater than can be explained by time and temp combinations
compare MW destruction kinetics to conventional kinetics
conventional
- has instant come up and come down time
- significant inactivation during hold time
MW:
- mostly non-instantaneous heating
- no hold time
- complete inactivation test during come up and down periods
why is MW tempering and thawing a significant application?
due to large penetration depth for MW in ice
what is “runaway heating” in MW tempering?
when rapid heating under MW results in quick melting of surface ice
describe MW drying and osmo/MW
what is it ideal for?
MW: adds heat and vaporizes water
air: carries water vapour
ideal for finish drying and osmotic drying
describe the role of MW and vacuum in VMD process
MW: expels water due to rapid heating
vacuum: removes moisture
what are 3 common MW packaging materials?
- transparent materials
- reflectors
- receptors of suceptors
describe transparent MW packaging materials
transmits MW energy to heat the product directly
describe reflector MW packaging
completely reflect MWs, so they can produce product from exposure to MW
describe receptor or suceptor MW packaging
can convert MW energy to IR
receptors on inside of package can create an oven inside the package (eg. crisping fries, pop corn)
what is RF heating?
radio frequency heating
describe RF heating
- alternative to MW heating
- uses high penetrative power
what are advantages of RF heating?
- rapid
- uniform
- high penetration depth
- high efficiency
- short residence time
- no heat transfer surfaces
what is the main limitation of RF heating?
higher initial capital investment
what are non-food and food applications of RF heating?
non-food: lumber, paper, textiles, plastics
food: drying biscuits, snack foods
why is RF better for cooking meat products than MW?
MW is more common in domestic use
RF has
- higher penetrative power than MW and more homogenous heating
- better for aseptic processing and industrial applications
what is the principle of Ohmic heating?
- heat is generated via electric current passing through an electircally conducting food product
what are other names for ohmic heating?
direct resistance
joule effect
electroconductive
electoresistant
in ohmic heating, heating rate is proportional to…. (4)
electrical power
electrical voltage
electrical conductivity
salt content
in ohmic heating, how can you match electrical conductivity?
by matching salt content
what is ohmic heating ideal for?
asceptic processing
what are advantages of ohmic heating?
- rapid heating
- don’t need heat transfer surface
- no moving parts
- quiet operation
- easy process control
- better E conservation
what is a limitation of ohmic heating?
electrode corrosion due to stainless steel contact
how can you minimize corrosion in ohmic heating?
high frequencies
what does PEF stand for?
pulse electric field
what are the principle of PEF?
preserving fluid foods via high voltage electrical pulses between 2 electrodes w/ food in between them
compare PEF with ohmic heating
similar, except PEF has higher voltage and shorter cycles
what foods is PEF usually used in?
liquid foods
what three things effect microbial inactivation in PEF?
- electric field strength
- time of treatment
- temp of medium
in PEF, when pH decreases, how does this affect microbial destruction?
dec pH = incr microbial destruction
what are limitations of PEF?
- no inactivation of bacterial spores (need to combine with temp)
- good only for homogenous foods
- not good w/ viscous fluids or foods w/ electrical conductivity
describe pulsed light processing
- intense and short duration pulses of white light
- effective when light can access all important volumes and surfaces
- can decontaminate food, package and contact surfaces
