WO1991015971A1

WO1991015971A1 - Method and apparatus for expanding a foodstuff with microwaves

Info

Publication number: WO1991015971A1
Application number: PCT/GB1991/000618
Authority: WO
Inventors: Roger John Meredith
Original assignee: Apv Corporation Ltd
Priority date: 1990-04-21
Filing date: 1991-04-19
Publication date: 1991-10-31
Also published as: JPH04506754A; CA2062725A1; GB2252890A; AU7691991A; GB9127021D0; EP0482147A1; GB9009016D0

Abstract

The present invention relates to a method for expanding an expansible foodstuff, which method is characterised in that it comprises subjecting the foodstuff to an initial microwave pre-heating stage (3) at temperatures below that at which expansion of the foodstuff occurs; and passing the pre-heated foodstuff to a second microwave heating stage (8) where expansion of the foodstuff is achieved by heating the foodstuff to a temperature in excess that at which expansion of the foodstuff occurs; at least part of the reflected microwave energy from the second heating stage (8) being used to supply at least part of the microwave energy required for the pre-heating stage (3).

Description

__τ__ττ. Method and apparatus for expanding a foodstuff with lxli-n^: microwaves.

The present invention relates to a method and apparatus, notably to a method of dry frying foodstuffs using microwave energy and to a microwave oven for use therein.

BACKGROUND TO THE INVENTION

It has been proposed to manufacture cooked foodstuffs by a dry frying process in which a thermoplastic starch or similar product is rapidly expanded by the generation of steam within the product in the substantial absence of fat or oil. The steam is conveniently generated by passing the product through a microwave oven. However, the microwave oven has to be lightly loaded in order to achieve the required high rate of power absorption by the product necessary to expand the product. This results in a high level of reflectance of the microwave power from the product and this can damage the microwave generators.

One method of reducing the problems associated with dry frying of foodstuffs is described in British Patent No 2 193 619. In that method, circulators are used to divert a substantial proportion of the reflected power away from the magnetrons or other devices generating the microwave energy. Inherently, this results in a loss of power from the system and this loss can amount to from 30 to 50% of the power generated by the microwave generators.

We have now devised a method and apparatus by which the amount of the input power which is lost to the system can be reduced, thus enabling savings in operating costs and increased through puts to be achieved. SUMMARY OF THE INVENTION:

Accordingly, the present invention provides a method for expanding an expansible foodstuff, notably a starch based foodstuff, which method comprises subjecting the foodstuff to an initial microwave pre-heating stage at temperatures below that at which expansion of the foodstuff occurs, which is typically the temperature at which the expansion agent with the foodstuff volatilises, for example the boiling point of water within the product, usually 100°C where steam is used to expand the foodstuff; and passing the pre-heated foodstuff to a second microwave heating stage where expansion of the foodstuff is achieved by heating the foodstuff to a temperature in excess that at which expansion of the foodstuff occurs, typically in excess of 100°C; at least part of the reflected microwave energy from the second heating stage being used to supply at least part of the microwave energy required for the pre¬ heating stage.

By carrying out the pre-heating stage, the energy reflected in the second heating stage can be recovered as useful energy, instead of being dissipated to waste as in previous methods. Furthermore, the pre-heating stage can be carried out at a comparatively slow heating rate, since expansion of the foodstuff is to be avoided in this stage. The power density in the pre-heating stage can thus be reduced to a level at which the amount of energy reflected from the material being heated is reduced. Thus, the pre-heating stage can be carried out at typically 1/30 to 1/2 of the power level required to achieve expansion of the foodstuff in the second heating stage. This increases the power .efficiency of the overall process. The reduction in the power density in the pre-heating stage can be achieved by the use of a deeper bed of foodstuff particles than in a conventional high power density process and this may enable a higher through put of the foodstuff to be achieved.

The second heating stage is carried out using a high power density in the bed of products to ensure that the particles are heated rapidly to above their expansion temperature. The high energy reflectance which results from the use of such a high power density can be tolerated in the process of the invention, since the reflected energy is recovered at least in part in the pre-heating stage.

The high power density in the second heating stage will usually require that a shallow bed of foodstuff particles be fed to the second heating stage. It is therefore preferred that means be provided between the pre-heating and second heating stages for spreading the bed of foodstuff particles so as to form' a substantially mono- particle deep bed to be fed to the second heating stage. Thus, the output from the pre-heating stage can be divided to feed a number of microwave ovens operated in parallel in the second stage.

However, it is preferred to carry the bed of foodstuff particles through the pre-heating stage on a first conveyor mechanism; and to discharge the foodstuff particles from the first conveyor onto a second conveyor for carriage through the second heating stage, which second conveyor has a linear speed of travel greater than that of the first conveyor. Typically, the second conveyor will travel at least 5 times as fast as the first conveyor to achieve sufficient thinning of the bed of particles on the second conveyor, although relative speeds of 15:1 or more may be used if desired. The optimum speeds of the two conveyors 'can readily be determined by simple trial nή error tes s having regard to the nature of the foodstuff particles and the power output of the microwave generators in each heating stage. If desired, mechanical or other means can be provided to aid uniform spreading of the foodstuff particles on the second conveyor, for example an^' oscillating discharge from the first conveyor and/or a doctor blade spreader at the input end of the second conveyor.

The microwave power required for the first, pre-heating stage is conveniently supplied in full by diverting the energy reflected from the foodstuff particles in the second heating stage to the first stage. However, if necessary supplementary power can be provided from a separate source and/or by diverting part of the input power to the second stage directly to the first stage. For convenience, the invention will be described hereinafter in terms of the use of the reflected energy from the second stage to provide all the power required in the pre-heating stage.

As indicated above, the microwave power for the second heating stage can be provided by one or more conventional high power output microwave generators, typically operating at from 800 watts to 20 K watts. Any suitable generator can be used and many are available commercially. If desired, a series of generators can be used to spread the power input over the bed of particles to be heated. If desired, the output from each the generators can be subjected to continuous phase shift using suitable techniques and equipment to break any phase coherence between the generators cyclically, notably where the output from a single magnetron is split to provide two or more inputs to the heating stage. We believe that this use of phase shifters to enhance the uniformity of the power distribution within a bed of material is novel and reduces the problems of localised hot or cold spots encountered with conventionally operated microwave generators.

Accordingly, from another aspect, the present invention provides a method for operating one or more microwave generators, which method comprises subjecting the output from one or more of the generators to phase shifting whereby the microwave power output is out of phase with the output from one or more of the other generators. In a specific variation of this method of the invention, the output from a microwave generator is split to provide two or more outputs and at least one of those outputs is subjected to phase shifting.

As indicated above, part of the power from the microwave generators used in the second heating stage can be fed directly to the pre-heating stage, or the output from one or more of the generators can be split, for example by using matched waveguide T-junction or other suitable means.

A substantial part of the input power fed to the foodstuff particles in the second heating stage is reflected and would, in earlier proposals, have been dissipated to waste. In the present invention, substantially all of this reflected energy is directed to the pre-heating stage using any suitable mechanism, for example a ferrite circulator, a hybrid junction inter-connected by ferrite non-reciprocal phase shifters or the like. The optimum mechanism for directing the reflected energy to the pre-heating stage can be selected using parameters known in the art and many suitable forms of mechanism are available commercially.

The reflected energy can be derived from the waveguides serving one or more of the microwave generators used in the second heating stage. It will usually be preferred that each generator used in the second stage serve as a source of reflected energy for the pre-heating stage.

The reflected energy from the second heating stage may provide more power than is required for the operation of the pre-heating stage and/or the amount of energy reflected from the foodstuff particles in the pre-heating stage may exceed what can be tolerated by the microwave generators. In this case it may be desirable to divert part or all of the energy reflected in the pre-heating stage to a water or other load which absorbs the excess power using the method and apparatus described in British Patent No 2 193 619.

In carrying out the method of the present invention, a bed of foodstuff particles, for example potato chip or starch based discs, in the un-expanded state are fed to a suitable conveyor feeding the pre-heating stage. The foodstuff can be in any suitable form and will usually contain water as the expansion agent. However, foodstuffs containing other expansion agents may be used in the method of the invention if desired. The foodstuff is one which is deformable, for example by virtue of having a thermoplastic property or by virtue of the initial water content, so that it will expand upon heating.

The pre-heating stage is operated so that the foodstuff particles reach a temperature just below, say 1 to 20°C below, the temperature at which they expand, for example by conversion of the water in the particles to steam. As indicated above, this pre-heating stage can be carried out at a comparatively slow rate of heating of the particles since it is not necessary to cause them to expand in this stage. The optimum power density in the bed of particles is thus much lower than is required in the second heating stage and can readily be determined by simple trial and error tests for any one product and design of oven, but will typically lie within the range 0.03 to 10 K watts per ■cubic metre of the bed of foodstuff particles. If desired,, some other form of heating, eg. a hot air stream can be used to assist the pre-heating of the particles. The pre-heated particles are typically discharged from the pre-heating stage at from 80 to 100°C onto a second conveyor travelling at from 5 to 15 times as fast as the first conveyor. This has the effect of spreading the particles as a thin bed on the second conveyor suitable for treatment at high power densities, for example greater than 1 Megawatt per cubic metre, eg 20 watts per cc, and voltages gradients greater than 200 volts per cm at 2450 MHz in the second stage.

In the second stage, the foodstuff particles are caused to expand by the rapid heating of the particles to above their expansion temperature. Typically, this will be to above 100°C, eg. about 105°C, where water is used as the expansion agent and the oven is operated at ambient pressure. However, the oven can be operated at sub- or super- atmospheric pressure if desired, since this may enable expansion to be carried out at more desirable temperature ranges having regard to the nature of the foodstuff to be expanded.

By virtue of the high power density used in the second heating stage, a high proportion of the input power, typically 30 to 50%, will be reflected from the bed of particles and will be diverted to the pre-heating stage as described above. In order to assist removal of the expansion agent from the second heating stage it may be desired to pass air through the oven and this may be preheated to 80 to 100°C to reduce heat losses within the oven.

DESCRIPTION OF THE DRAWINGS:

The invention will be illustrated with reference to three preferred embodiments thereof as shown in the accompanying drawings in which Figure 1 is a diagrammatic block diagram of one form of the method and apparatus of the invention; and Figures 2 and 3 show alternative forms of the method and apparatus of Figure 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS:

As shown in Figure 1, foodstuff particles are fed to a first conveyor 1 and pass through a microwave choke 2 to a first microwave oven 3 where they are pre-heated to 80 to 100°C by microwaves fed to the oven from the ferrite circulator 4 serving the waveguide of a magnetron 5 used to heat the foodstuff particles in a second heating stage.

The bed of pre-heated particles is discharged from oven 3 through microwave choke 6 and onto a faster moving second conveyor 7 which carries the particles through a second microwave oven 8 in which the particles are caused to expand. The power required for the second heating/expansion stage is provided by one or more conventional magnetrons operated at from 0.8 to 20 K watts. The power density absorbed by the bed of particles on conveyor 7 is typically in the range 1 to 5 M watt/cubic metre. As indicated above, a substantial proportion of the power directed onto the foodstuff particles in the second heating stage is reflected and is directed by the circulator 4 to the first, pre-heating stage.

In the alternative shown in Figure 2, the reflected power from oven 8 is directed to the pre-heating stage by the circulator 4 to provide the power required for the pre¬ heating stage. Where the power reflected from the foodstuff particles in the pre-heating stage is excessive, .a second circulator 4a is provided to divert that to a water or other load 9 where it can be absorbed. Alternatively, the power reflected from the foodstuff particles can be recovered in a pre-preheating stage similar to the pre-heating stage described above.

In the alternative shown in Figure 3, the output from the magnetron 5 is fed to a T-junction 10 where it is split between two circulators 11 to provide two power feeds into the oven 12 of the second heating stage. In order to enhance the power distribution within the bed of foodstuff particles, the output from either or both of the circulators 11 can be provided with an oscillating phase shifter 13 to break the phase coherence between the feeds to the oven 12. The phase shifters can be located anywhere in the path from the circulator to the oven.

The reflected power from the second heating stage is fed via the circulators 11 to two circulators 14 serving the pre-heating oven 15. If desired, the circulators 14 can serve water loads 16 to absorb excessive energy reflected from the foodstuff particles in the pre-heating stage.

The invention also provides a microwave heating system which comprises a first microwave oven in series with a second oven, the second oven being provided with a source of microwave power and with means for directing microwave energy reflected from material being heated in that second oven to provide at least part of the microwave power required to pre-heat material fed to the first oven. Preferably, the second oven is provided with a ferrite circulator to direct reflected energy from the second oven to the first; and material to be heated in the two ovens is carried through the ovens by means of two conveyor means, the first conveyor serving the first oven and being adapted to travel at a slower linear rate of travel that the second .conveyor serving the second oven.

Typically, the microwave generators described above are operated in the frequency range 2000 to 3000 MHz, but this range can be extended to from 100 to 10,000 MHz if desired.

Claims

CLAIMS :

1. A method for expanding an expansible foodstuff, which method is characterised in that it comprises subjecting the foodstuff to an initial microwave pre-heating stage (3) at temperatures below that at which expansion of the foodstuff occurs; and passing the pre-heated foodstuff to a second microwave heating stage (8) where expansion of the foodstuff is achieved by heating the foodstuff to a temperature in excess that at which expansion of the foodstuff occurs; at least part of the reflected microwave energy from the second heating stage (8) being used to supply at least part of the microwave energy required for the pre-heating stage (3) .

2. A method as claimed in claim 1, characterised in that the foodstuff is a starch based foodstuff.

3. A method as claimed in either of claims 1 or 2, characterised in that the foodstuff contains water as the expansion agent and the foodstuff is heated to below 100°C in the pre-heating stage (3) and to above 100^{oC 1π the secoπd heatinα} ^sta9^{e (8)}-

4. A method as claimed in any one of the preceding claims, characterised in that the pre-heating stage (3) is carried out at a power density in the range 0.03 to 10 K watts per cubic metre of the foodstuff; and the second heating stage (8) is carried out at a power density greater than 1 Megawatt per cubic metre of the foodstuff.

5. A method as claimed in any one of the preceding claims, characterised in that the pre-heating stage is

' carried out by heating a bed of food particles on a first conveyor (1); and the second heating stage (8) is carried out on a second conveyor (7) which is travelling at a linear speed from 5 to 15 times that of the first conveyor (1) .

6. A method as claimed in any one of the preceding claims, characterised in that the microwave energy reflected from the second heating stage (8) is directed to the pre-heating stage (3) by means of a ferrite circulator

(4) or a hybrid junction interconnected by ferrite non- reciprocal phase shifters.

7. A method as claimed in any one of the preceding claims, characterised in that at least one microwave generator (5) is used in the second heating stage (8) and the output from at least one of the generators (5) is split (10) to provide two or more energy inputs to the second heating stage (8) .

8. A method as claimed in any one of the preceding claims, characterised in that at least two microwave generators (5) are used in the second heating stage (8) and the output from at least one of those microwave generators

(5) is subjected to phase shifting (10) whereby the output from that generator (50 is out of phase with the output from one or more of the other generators (50.

9. A microwave heating system suitable for use in the method of claim 1, which system is characterised in that it comprises a first microwave oven (3, 15) in series with a second oven (8, 12), the second oven (8, 12) being provided with a source of microwave power (5) and with means (4, 11) for directing microwave energy reflected from material being heated in that second oven (8, 12) so as to provide

•at least part of the microwave power required to pre-heat material fed to the first oven.

10. A. system as claimed in claim 9, characterised in that the second oven (8, 12) is provided with a ferrite circulator (4, 11) to direct reflected energy from the second oven (8, 12) to the first oven (3, 15); and material to be heated in the two ovens is carried through the ovens by means of two conveyor means (1 and 7), the first conveyor (1) serving the first oven (3, 15) and being adapted to travel at a slower linear rate of travel that the second conveyor (7) serving the second oven (8, 12) .

11. A method for operating one or more microwave generators (5) suitable for use in the method of claim 1, which method of operation is characterised in that it comprises subjecting the output from one or more of the generators (5) to phase shifting whereby the microwave power output of that generator (5) is out of phase with the output from one or more of the other generators (5) .

12. A method as claimed in claim 11, characterised in that the output from a microwave generator (5) is split to provide two or more outputs and at least one of those outputs is subjected to phase shifting.