Microwave and ohmic heating Flashcards
impact of microwaving on microrganisms is similar to?
traditional methods
how is heat generated by microwaving
the molecules become excited and movement causes friction which generates heat
describe thermal gradients of traditional vs microwave
traditional have a large heat gradient as food is heated from outside in which varies the nutritional and textural properties of food
-microwave is more homogenous as food is heated from within the product
define microwaves
very short wave lengths of electromagnetic energy
frequency of microwave ovens
2450 MHZ (home)
900mhz (commercial)
type of electric fields microwaves produces
rapid oscillating fields
-2 waves 90 degrees between them
-1 electric, 1 magnetic
how do the waves generate friction
placing a dipole product between the waves results in product changing in orientation to match the fields which generates movement and friction
explain the dipole in water
oxygen is negative, hydrogen is positive
other polar molecules in food include…
protein and carbohydrates
what determines the dielectric constant of a food
number of dipoles and changes induced by the electric field
what is the dielectric constant
measure of the ability of a material to store electromagnetic radiation
what is the dielectric loss factor
measure of the abiliity of a material to dissipate electrical energy into heat
what is penetration depth
ability of wavelength to penetrate food
what is a limitation to having a food than can convert electrical to heat energy really fast
it stops penetration so cooking may be uneven and cold spots can occur
explain how cold spots occur
when overlapping of the waves occurs
what is the amount of electromagnetic energy depnedant on
loss factor of material
types of heat transfer in food
convection and conduction
will a product with a low loss factor be heated
no,
glass, plastic adn paper are transparent to microwaves
what type of material reflect microwaves
metals
lower wave power generates _____ loss factor
higher
3 components to a microwave
microwave generator (magnetron)
wave guides
metal chamber
purpose of microwave generator/magnetron
convert electricity into electromagnetic waves
purpose of wave guides
aluminium tubes that guide the waves into the chamber
purpose of metal chamber
used in batch operations, to place the food product into
main problem with when energhy transfer rates are too high
non-uniform heating
what else can result in uneven heating
variation in moisture and composition of food
effect of dissolved salt in food
hinders microwaves
what can microwaves be used ot limit
browning
case hardening
sensory damage
how can microwaves be used in baked products
prevent furthur colour change but ensure baking by having them at the end of tunnel ovens
how can microwaves improve pasta processing
reduce drying time from 8h to 90 mins
reduce bacterial counts by 15x
advantages of microwaves for conventional thawing
faster
less storage space required
no drip loss
lower labour requirements
more hygenic
better control
major advantage to ohmic heating
heating takes place volmetrically and the product does not experiance a large temp gradient during heating
advantage of ohmic heating over microwave
depth of oenetration is not limtied and the extent of heating is determined by the electrical conductivity throughout tge product and resistance time
how long does it take to heat to sterilisation and cool a product using ohmic heating
less than 90 sec to get to 140C and 15 mins to cool
quality benefits of ohmic heating
better nutritonal retenion
less softening of delicate particulates
principles of ohmic heating
a food of electrical conductivty is placed between two electrodes with a field strenght betweeen them resulting in energy generation
what is the critical property that affects energy generation rate
s: electrical conductivity
what does the electrical conductivity of solid foods depend on
temperature and voltage gradient
describe ohmic heating process for viscous foods
enter continous-flow ohmic heating system through a feed pump hopper
twin-pisotn positive displacement metering pump minimises particle damage
product flows past a series o felectrodes in heating column to reach sterilising temp
product enters holding tube
then flows into tubular coolers andholding tanks before being aseptically filled
ohmic heating processes (4)
pastuerisation
aseptic processing
pre-heating
hygenic production
objectives of each heating processes
pasteurisation: inactivate vegetative microorgansism/ extend shelf life
Asepetic processing: inactivate microbial spores, commercially sterile
pre-heating: heat during processing stage
hygenic production: heating and inactivation of vegetative microorganisms
typical food application for each heating process
aseptic processing: pumpable particulate (ready made sauces etc)
pasteurisation: pumpable particulate for fruit and high acid foods
pre heating: in-can sterilisation products
hygienic production: ready meals
typical food products produced by ohmic heating
high acid: pasta sauce, fruit compote, pie fillings
low acid: tortellini in sauce, vege curries
things to consider about particulate of processing product
size, shape, concentration, density conductivity, specific heat capacity
size of particulates and things to consider
less than 25mm, flexible and elongate
-needs to clear electrodes, size to eat
particulate concentration considerations
20-70%, higher conc can be processed if particualtes are pliable, small and varied shape
-lower conc require more viscous solution to maintain dispersion
particulate density considerations
only an issue if significantly different from one another
carrier medium considerations
needs to be able keep particulates in suspension
-changes due things like gelatinisation can result in uneven heating
what are ohmic heating rates dependant on
electrical conductivity of food
process and installation cost comparison for ohmic heating, freezing and retorting for low acid
installation
freezing: 4.4M
retorting: 3.2 M
ohmic: 19.2M
10 year cost
freezing: 1.12/kg
retorting:1.44/kg
ohmic: 0.94/kg
process and installation cost comparison for ohmic heating, freezing and retorting for high acid
installation
freezing: 1.1M
retorting: 3.9 M
ohmic: 9.6M
10 year cost
freezing: 0.36/kg
retorting:0.36/kg
ohmic: 0.63/kg
is ohmic better for high acids foods
cost: not better
quality: way better
advantages ohmic heating
fresh tasting: shorter process time
not hot surfaces that can damage product
high particulate solutions can be processed
particulates remain firm
