CA1096449A - Microwave heating of foods - Google Patents

Abstract

Abstract of the Disclosure:-Frozen food in packs, generally of rectilinear configuration, are heated for consumption by being passed through a microwave energy field which generates heat which is substantially uniform across the pack width and which traverses the pack length due to movement relatively between the pack and microwave energy source. The relative movement overcomes thermal runaway since the energy is continuously being dragged away from the zones which have just been heated and where thermal runaway could otherwise occur,

Description

cX.1~6 The pre~ent :invention relates to the microwave heatlng o:E ~oods, partirularly for the preparation for consumpt~on o~ fro~en pre-paclced meals.
Institutlonalised catering, ~or e~ample ~actory canteen~
or ho~pital ~eal services, desirably require the mini~um of preparation time, combined with a reasonable quality o~ product, a ~air choice o~ alternative~ and economy.
With these aims in mind, the u~e o~ pre-prepared frozen packs o~ ~ood ln conjunction with micro-wave heating has been considered; however re-heating i9 general~y o~ the order o~ ten minutes or so and this u~ually constitutes an unacceptable delay, and also a wide ranging f:ree choice i9 impractical. Consequently in -factory canteens it is ~till the practice to have pre-heated quantities o~ food available from which to serve meals o~ restricted choice range and q~ality.
Considering the ~actory canteen situation, a queue o~
people should desirably be able to sequentially select a meal. pay for it and ta~e it away on a tray; and i~ a smooth running system this should be possible in tl~o to three minutes.
I~ there~ore the re-heating time in a microwave oven can be reduced from ten minutes to less than three minutes, the meal becomes capable o~ being re-heated as the individual goes through the process o~ selecti~g and paying ~or it -and then there is ~o longer the requirement for pre-heated quantities o~ food being available and a~ improved quality and greater selection become possible.

_ ' ~' ' ' ' ' ' ' :' -G~f~9c~.14 To achieve this rapicl heating would be di~:~icult in a conventional multi-mode microwave oven because merely increasing the power accentuates the so-Galled thermal runawa~ problem~ Thermal runaway is the ef~ect ~ which occurs when microwave energy is applied to a : frozen food where, as soon a9 part o~ the frozen food thaws and changes ~rom ice to liquid, this part assumes a greater dielectric loss factor than the remaining ice and selectively takes more of the power from the s~stem, so distorting the energy field and resulting in uneven heating.
The present invention aims to provide a rapid heating method where the prohlem of thermal runaway is minimised or reduced and accordin~ly provides a metho~ of heating a pac~ of ~rozen food for consumption, which pack is of : substantially uniform length and uniform widthS comprising effecting relative movement of the pack in its len~th direction past an outlet fed by a source of microwave ener~y, and causing said outlet to supply microwave energy to the pack under conditions such that substantially uni~orm heating occurs across the pack width while in the pac~ length direction heating i9 concentrated in a band which is shorter than tne pack len~.th, and which throu~h the relative moveme~t between the pack and said microwave 25 outlet sequentiall~ traverses the pack length.
~ he invention also provides an apparatus ~or heating frozen ~ood packs for consumption comprising a microw~ve '~' .~ .

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cX.146 energy source, energy :Eeecl means for i'eedlng energy from said source to an outlet, and conveying means ~or conveying a pack corltain:in~ -~ro~en food pas-t salcl outlet, said outlet t saicl conveying ~eans ancl said energy feed ~eans being S so posltioned in relation to ~ne another that substantially uniform heating occurs across the pack width while in the pack length direction heating i9 concentrated in a band which is shorter than the pack length.
Concentration o~ the length direction heating into a restricted band coupled with relative move~ent between pac~ and energy source enables a high heat input into the pack to be achieved without encoun$ering significant thermal runaway problems.
~his is because the energy source (the microwave outlet) is continuously being moved away ~rom zones where thermal runaway would otherwise occur. ~hus any distortion of the field consequent on frozen material : thawing i~ ~ept at a minimum, and the only such e~ect is a slight dragging o~ the heating zone in the direction o~ pack movement (i.e. heating will be at a zone slightly in advance o~ the centre o~ the microwa~e outlet); and this dragging effect wilI be evened out as the heating zone sequentially traverses the wllole o~ the pack length and each integral zone o~ the ~ood paok will have received in total substantially the same ~mount of hea$ing.
~ It will be recognised that the dragging e-~fect will ;~ tend to cause uneven heating at the ~ront ~nd re~r ends ~ cX.14 o~ the ~ood pack. This can be overcomle conveniently by either havirlg dummy loads preceding and succeeding the pack as it progresses past the microwaYe outlet; or an in:Einite number o~ pac~s juxtaposed WithOllt any gap between the ~ront o~ the ~irst and the rear of the ne~t (e~ectively an in~initely long pack) would also solve the end ef~ect problem.
Mowever we haYe found that the simplest way o~ avoiding overheating o~ the ends is by switching the micro~Yave energy on and o~ in timed relationship with the pack movement.
In particular the energy should be switched on as the leading edge o-~ the pack has moved about halfway across the microwave outlet ana should be s~itched off as the trailing edge is hal~way across the ~icrowaYe outlet. In practice owing to slight ~ield distortion switch on should be just after the hal~way point and switch o~ ust before the relevant edge reaches halfway. ~ .
As an alternative to choosing the correct time o~
switching on and o~, which will be e~fected wheM a uni-directional ~ingle pass ta~es place, if the pack is to be moved ~ack and ~orth across the microwave outlet overheating o~
the ends can be avoided by moving the pack back and ~orth over a restricted pathway, i.e. by reversing the movement ~ust be~ore a complete pa~s has tak0n place.
In order to ensure that the total quantity o~ heat received at dif~erent points along the length of the pack is uni~orm, as well as taking into account the encl e~ect , .

~, , , ~ cX.1~6 problems referred to above, two o-ther requlrements have to be substantially met. rrhe first of these is that the movement past the l~icro~ave outlet should be at a pre-determined rate, and -the second is that the food to be heated should be suitably ~istributed within the pack in relation to its energy absorbing properties.
Speed of moYement and distribution of -~ood within the pack are, of course, related functions, and while in practice constant speed and even distribution are the most convenient ways of achieving even heating, other co-related values of these two functions are theoretically also po~sible (for example if a portion of food near the centre o~ the pac~ needs more heat~ the speed of movement could be slowed down at that stage)~
The manner in which the food is arranged ~ithin the pack to ensure even heating i9 generally based on trial and error and experience. For egample dense high water content-foQds, e.g. spinach puree, absorb more energy than lower water content particulate foods, e.g~ peas;
and non-uniform geometric shapes such as lamb ~utlets create similar problems. It then becomes a matter o~
arrangi~g such materials within the pack in such a way th~t `~ the effective absorption properties are as uniform as possible.
As-previously stated heating in the pac~ length ; direction is concentrated in a band which is shorter than the pack length~ Generally the intensity o-f heatin~ in c~.116 this direction wi:Ll increase to and then rececle ~rom a pea~ o~ intensity sin~soidally within a distance which may be a third to a half of the pack length, when a pnck o~ usually encountered dimensions is used (~ee example to be described later).
However in the pac~ ~idth direction heating should be substantially uni~orm. Thi~ is pre~erably achieved by radiation of microwave energy in single mode with the Electric Field polarised in tlle direction o-~ the pack width ~ro~ an outlet of substantially the; same width as that of the pack. While this is the preferred method, other methods o~ achieving equa1 heating across the pac~
width are also possible such as by use o~ the equipment described in my US Patent 3 110 79~.
~ith the single mode arrangement where the Electric Field is polarised in the direction o~ the pack width the intensity of the ef~ective Electric Field will theoretically be substantially constant across the transverse ~idth o~ the pack; while intensity will increase sinusoidally to a peak and then similarly subside in the direction of movement.
In regard to the width direction, a single rectangular ~: waveguide outlet using the most commonly used frequency, i.e. 2450 ~z, would onl~ encompass a particularly narrow pack width i.e. about B cm. There~ore to achieve a pack width which is commercially more ~cceptable, we have found it desirable to substanti~lly double the wave~uide '~ 1 ,. .
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~ cX.1~6 outlet l~.idth by using an applicato.r in the ~orm o~ a type poll1er divider supplied from a s:ingle power sou.rce . ~he same ef:eeot could be achieved by u sing t~o wavegulde outlets ne~t to each other9 combined wi$h o the.r known power dividers, and greate.r multiples are al90 pogsible, As a further measure to improve the uni~ormity o~
heating across the wid th o~ the ~ood pack we have ~ound that ~hen heating certain particularly dense, high water content, ~oods steps need to be taken to prevent overheating at the side edges o~ such a pack. By guiding the microwave energy via a pair o~ slot 9 one at each side of the applicator and corresponding to the edges of the pack, greater uniformity can be achieved provided these slots 16 are spaced hal~ a wa~elength apart This provides, in e~-~sct, two in phase sources spaced hal~ a wavelength ; apart.
The e~ect then is that in the region o~ eaoh slot there will be a degree o~ cancellation due to out of phase power ~rom the opposite 910t reducin~ the power intensity, while midway `between the two slots, the powers ~rom the two slots are in phase and will re-in~orce one ; another. ~hese :lots ca~l ~or example be achieved by use : of a thin conductive ba~le plate parallel to and spaced ~rom the pack base and cl~sing the central zone o~ the outlet o~ the Y-type power dividerO

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~ cX.~46 Altelnatively :i t :i9 possible to use a d~ectric insert dispo~ed in the waveguide outlet ad~acent and genexally parallel to the food pack path, and O~r low 109s ~actor and o~ a greater relative permittivity than air, which can var~
the matching to the ~ood pack and thereby be utilised to improve the uniformity o~ heating across the pac~ width.
The shape and disposition o~ su¢h a low-loss baf~le can then be chosen to tailor the intensity o~ heating as desired.
From the foregoing, it will be apparent that i~ the pre~erred arrangements a substantially rectangular parallelopiped shaped pack will be used of which the width dimension is selected in relation to the waveguide outlet width, while pack length - though not critical should be taken into acoount in arranging ~or a switching sequence or reciprocal movement to overcome lea~ing and lagging edge end e~fects.
The third dimension of the pac~, i.e. height, needs to be restricted to take into a~count the energy transmission capabilitr o~ the microwave source. I~
height is too great the top of the pac~ would not receive adequa-te energy, but there is no minimum requirement.
An additional ~actor limiting pack height comes in when considering the method of conveying the pack and of screening the system to prevent radiation outwards ~rom the equipment to provide adequate safety. Conveniently : .

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~ c~ 6 the radiating outlet ~or the micrGwave source opens into a screened rectangular cross-section tunnel along which the pack i9 caused -to move. ~his arrangement will generally be T~haped.
In order to inhibit propagation o~ radiation, when this is polarised with i~9 ~iel~ horizontal, :Erom travelling along the upper horizontal limbs of the ~ (the pack path-way), these limbs should be less than half a wavelength - in height. ~his then pUt9 a si~ilar limitation on the height of the pack - i.e. since the pack has to pass along within these upper limbs it must also be less than half a wavelength high.
An embodiment of the invention will now be described by way of e~ample with reference to the accompanying diagrammatic drawings in whioh Figure 1 i3 a perspecti~e ~-iew part cut away of a pack heating device;
Figure 2 is ~ side view showing the form of the Electric Field;
Figure 3 shown the ~ield disposition pictorially;
Figure 4 shows the sequential h&ating effect on a pack;
- Figure 5 shows an end view o~ the waveguide outlet ~ with one~orm of field compensating de~ice;
; 25 Figure 6 and ~ sho~ similar views to Figure 5 with di~erent ~orms o~ field compensating deviee; and .

~ cX.1~6' Figure 8 shows an overall view o~ the rllicrowave system layout.
Re~erring to Figure 1, a conveyor s,ystem (shown only schematically) includes a hori~ontal metal scre~ning guide channel 1 ~or conveying a food pack 2 past a microwave applicator and vertically disposea waveguide 3. Other dispositions than hori~ontal and vertical are of course also ~easible, ~ut are less convenient.
The microwave applicator and waveguide i~ located so that the ~lectric Field ~E) is at right angles to the longitudinal conveying direction L and is as uni~orm as possible across a horizontal plane in the E direction shown. In the conveyor direction however the intensity rises to a peak a,nd then ~alls again as shown by graph G
(Figure 2). The height of the gulde channel 1 is less than half a wavelength long so ~s to inhibit transmission o~
horizont~lly polarised raaiabion alon~ this channel.
The microwave applioator and waveguide 3 consists essentially o~ a rectangular waveguide 4 o~ standard internal dimensions (86 mm x 43 mm~ ~eeding into a flared outlet section 4 and ~ed ~rom a magnetron supply.
Within the outlet section 5 is a conductive div~der plate 6 (shown dotted) attached at each end to the side walls within the section 5. The dimensio~s and arrangement wlthin the ~lared outlet thus ~orm a Y type divider, giving rise to a widened ~one o~ constant Electric Fiela in the direction transverse to the conveyor .

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~ cX.146 direction (in `act two OlltpUtS in phase which oonsequently behave as one), which corresponds to the pack width (see Figure 3).
Conveni~ntl~ the outlet wldth may be abowt 115 mm, instead of 43 mm o~ the ~tandard waveguide, ancl a pack o~ 110 mm width mar be accommodated; and the eql~-lpment i9 fabricated ~rom thin conductive sheeting, for example aluminium sheeting about 1 mm thick. The depth of the channel 1 and the height o~ the ~ood pack should be less than half a wavelength, e~g. about 55 mm and 35 mm respectively.
While theoretically a Y type power divider, per se, gives a constant intensity field in the transverse direction, in use some edge over-heating would tend to occur with certain dense, high water content food~, e.g.
spinach puree, at edge zones 7 (see Figure 5).
Re~erring to Figure 5, one method of overcoming this problem i9 by provision of a baf~le plate 8 attached to the top o~ the dlvider-pla$e and to the opposing parallel walls o~ the ~lare 5 and spaced from the path o~ the pack base 9 SO as to leave ~ slot 9 at each end, corresponding to the edge zones 7 of the food pack which would otherwise be over~eated.
The centres of the two slots 9 are spaced apart by a distance equal to approximatel~ a half wavelength o~
the generated energy. Then, in use, there will be a degree of cancellation at each of the edges, which thus ~G~9 cx. 1~6 reduces thc hea-ting at zones 7, while the twv slot sources will augment one another in a central zone, Figure 6 shows an alternative version ~here the divider plate 6 and transverse plate 8 are replaced by 5_ a~,wedge 22, performing a basically similar function in the same manner.
Figure 7 shows another version where the plate 8 ,is replaced by a bloc~ 23 of polypropylene 2 cm deep which equalised the transverse field in a different man~er. This provided the most uniform and e~ficient transfer of power in the width direction, Since the polypropylene is a low loss material having low loss factor and a higher relative permittivity than the equivalent volume of air (about 2.2 times), it aff'ects the matching of power into the pac~. ~hus by selecting its depth, shape and location the power into the pack can be tailoxed to provide the required uniformity. Moreover power can be tran~ferred to the pack more effectiv~ly with less reflection bac~ down the waveguide. This method of matohing is also to be pre~erred over the previously discussea horizontal baf~le system since it can also be used with hi~her multiples of flared outlet than the double outlet previously deæcribed.
In practice ~ood packs containing 176 gm o~ frozen food and measuring 110 mm x 140 mm x 35 mm were fed past the applicator and were heated from the deep frozen state (about -20C~ to a temperature for consumption in less .

, cX.146 than three minutes. A subst~nt,ially u-ni~'orm he~ating with the absence or mlnimum o~ thermal runaway wa~
observecl. The sys-tem was coupled to a m~gnetron giving M nominal 2 Kilowatts output power via a conventional matching device which produces an effective power transfer of about 1~ ~W into the foodpack.
~ he overall set up of the microwave system is shown schematically in Figure 8. The applicator is connected to a wave guide section collta}ning an adjustable st~b 1 for matching. ~he next section is a circulator 15 (with three ports); one port is connected to the magnetron; the second port goes to the ~lared outlet applicator and the third port i9 connected to a water load 17, incorporating a probe 18 connected to a crystal detector 19 and a microammeter 20. The circulator directs all the power from the magnetron forward to the applicator, and also divert~ any power reflected ~rom $he applicator into the dummy load 17~ thereb~ protecting the magnetron. The cry~tal detector monitors the :~ .
reflect0d power which is minimised by ad~ustment of the matching stub. An oscillatory feeding mechanism 21 i9 provided.
Setting u-p the matching is a -compromise. ~here is a big dif~erence between the impedance of the food material in the frozen and thawed conditions, but the fully frozen condition lasts such a short time that it is preferred to set up cX.146 the matching ~or the unfro~en condit-lon. In the un~rozen condition the match varies somewhat with the type of food and, to a small extent, with its temperature.
We have found by experience that ~ s~ti~actory compromi~e is to adjust the matching stub to give minlmum re~lected power (crystal current)-when 200 ml o~ water in a carton of the si~e re~erred to above i~ stationary a~d centrally over the flare. Under this condition the e~fective microwave power was measured by recording the temperature rise o~ the water in 20 seconds. (At perfect match -zero cry~tal current - 1.6 to 1.7 kW was obtained from the microwave power pac~ in use.~
Overheating of the end edges o~ the pack can occur due to the field lagging as the pack enters the heating zone. This was taken accu~nt of by restricting the length o~ oscillating travel acro~s the wavegulde outlet.
The length of the oscillating travel was investigate~ -:~ :
by observing the heating pattern as the length was altered, When the travsl was too short the leading and trailing edges were too eold, and when too lnng the edges were overheated. ~he optimum travel was 3~ cm either side of the central position, i.e. a total travel o~ 7 cm of the pack o~ which the base is 11 cm long. ~he point~
to which the ends o~ the pack move and t~en change dire¢tion to move back are indicated by the lines 12 ~nd 13 of Figure 1. Thus, viewing Figure 1, an oscillating pac~
moves to the le~t until its right hand edge is at line 13 _ 15 :.
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, ~ cX.146 and then mo~es bac~ to the right until it9 le~t hand eclge is at line 12 and subsequently oscillates between these positions.
A number o~ di~ferent methods o~ olperating the ~lare was possible. Using a single ~lare t]he best method was to move the ~oodpac~ back and ~orth across the flare ~outh about twelve times at a speed of 150 cm per minute, the travel across the waveguide having been restricted to avoid end edge ovexheating, as previously discussed.
Satis~ctory heated packs were achieved by this met.hod ~:
in about one minute.
For a continuous flow system it was preferahle to use several spaced flares arranged sequentially in the path o~ the foodpac~ and with correspcnding switching arrangements to ensure against end edge overheating.
Using two ~lares, a ~ood pack speed o~ 10 cm per min~te gave a heating time of one minute ~rom each ~lare, and this achieved the desired temperature. With this continuous flow operation the points ~or ~witching the pac~ on were similarly locatea to the lines 12 and 13 o~ Figure 1, switch-on occurring when the leading edge reaches the line 12 and switch-of~ occurring when the trailing edge o~ the pack reaches the line 13.
Triggering o-~ the switches can be effected in any convenient manner such as by micro-switches or light beams.

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6~ c~. 146 ITs-ing a sirlgle flare wiSh a slower ~peed (5 cm per minute approximately) was often satis~ac-tory to reach the desired temperature in two minutes, but with some packed products this mode of operation introduced a degree o~ unevenness to the heatin~ effect.
~ he description i9 written in terms o~ a transmission frequency o~ 2450 ~z which at the present time i9 the normal microwave heating frequency. However it will be understsod that other microwave heating frequencies are equally permis~ible provided the waveguide and pack geometry are adjusted 1~ accordance with the principles previously discussed.

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Claims (21)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of heating a pack of frozen food for consumption, which pack is of substantially uniform length and uniform width, comprising effecting relative movement of the pack in its length direction past an outlet fed by a source of microwave energy, and causing said outlet to supply microwave energy to the pack under conditions such that substantially uniform heating occurs across the pack width while in the pack length direction heating is concentrated in a band which is shorter than the pack length, and which through the relative movement between the pack and said microwave outlet, sequentially traverses the pack length.

2. A method according to Claim 1 in which the microwave energy is propagated in single mode with the Electric Field polarised transversely to the direction of movement of the pack.

3. A method according to Claim 1 in which microwave energy transmission and pack movement are co-related to ensure even generation of heat at the beginning, intermediate zones and end of the pack length.

4. A method according to Claim 3 in which the pack is caused to perform an oscillatory movement back and forth across the microwave outlet, and said movement has an amplitude which ensures even generation of heat at the beginning, intermediate zones and end of the pack length.

5. A method according to Claim 3 in which the pack is caused to perform a continuous movement past the microwave outlet, and the microwave source is switched on and off in timed relationship with said continuous movement to ensure even generation of heat at the beginning, intermediate zones and end of the pack length.

6. A method according to Claim 2 in which a rectangular waveguide outlet is used which is flared to provide a width of energy field corresponding to the width of the pack and greater than a standard waveguide width.

7. A method according to Claim 6 in which at least one longitudinal baffle is used within the flared waveguide outlet to preserve single mode propagation conditions.

8. A method according to Claim 6 in which the flared outlet used is in the form of a Y type divider to ensure single mode propagation of the energy.

A method according to Claim 8 in which the Y typo divider has a baffle located parallel to the plane of the waveguide outlet and defining at its edges a pair of slots spaced half a wavelength apart so that energy emerging from said slots is subject to cancellation in the region of the slots and to re-enforcement halfway therebetween, whereby to improve the uniformity of heat generation across the pack width.

10. A method according to Claim 8 in which a dielectric insert having a relative permittivity greater than air and a low loss factor is disposed within the outlet, and has a shape and disposition so as to improve the uniformity of heat generation across the pack width.

11. A method according to Claim 1 in which the pack is caused to move within a metal tunnel less than half a wavelength high and parallel to the plane of the microwave outlet.

12. An apparatus for accomplishing a method for heating a pack of frozen food for consumption according to claim 1, comprising a microwave energy source, energy feed means for feeding energy from said source to an outlet, and conveying means for conveying a pack containing frozen food past said outlet, said outlet, said conveying means and said energy feed means being so positioned in relation to one another that substantially uniform heating occurs across the pack width while in the pack length direction, heating is concentrated in a band which is shorter than the pack length.

13. Apparatus according to Claim 12 in which the feed means is disposed so as to feed the pack in a direction at right angles to the direction of propagation from the microwave energy outlet and also at right angles to the plane of polarisation of single mode Electric Field propagation.

14. Apparatus according to Claim 13 including switching means for switching the microwave energy on and off at pre-determined times in the travel path of the pack to ensure even generation of heat within the pack along its length.

15. Apparatus according to Claim 13 in which the feed means is arrange to feed the pack in an oscillatory movement across the microwave energy outlet with an amplitude which ensures even generation of heat dwithin the pack along its length.

16. Apparatus according to Claim 13 in which the feed means is a rectangular waveguide outlet which is flared to provide a width of energy field corresponding to a pack of greater width than a standard waveguide width.

17. Apparatus according to Claim 16 comprising at least one longitudinal baffle located within the flared waveguide outlet to preserve single mode propagation conditions.

18. Apparatus according to Claim 17 in which the outlet is a Y type power divider.

19, Apparatus according to Claim 18 in which the outlet comprises a baffle located parallel to the plane of the waveguide outlet and defining at its edges a pair of slots spaced half a wavelength apart so that energy emerging from said slots is subject to cancellation in the region of the slots and to re-enforcement halfway therebetween, whereby to improve the uniformity of heat generation across the pack width.

20. Apparatus according to Claim 12 comprising a dielectric insert having a relative permittivity greater than air and a low loss factor disposed within the outlet, and having a shape and disposition which improves the uniformity of heat generation across the pack width.

21. Apparatus according to Claim 12 in which the feed means comprises a metal tunnel which is less than half a wavelength high and is parallel to the plane of the microwave energy outlet.