GB2193619A

GB2193619A - Micro-wave oven

Info

Publication number: GB2193619A
Application number: GB08717491A
Authority: GB
Inventors: Roger John Meredith; John Mitton
Original assignee: APV Magnetronics Ltd
Current assignee: APV Magnetronics Ltd
Priority date: 1986-07-25
Filing date: 1987-07-23
Publication date: 1988-02-10
Also published as: GB8717491D0; GB8618218D0; GB2193619B

Abstract

The invention relates to the product of snack food products formed by the expansion of pallets or small granules containing edible starch and water. A microwave oven is used, comprising a plurality of sources of microwave power, each source comprising a magnetron and an associated waveguide feed, the feeds being arranged in a variety of orientations to give mixed polarisation of power. Between each magnetron and its associated radiating element, there is provided means at each source to isolate the magnetron from power reflected from the oven at high levels of energy dissipation density. Preferably the isolating means comprise an isolator ferrite, for example a microwave ferrite circulator and there is also provided an absorber load which may be a water load or a reflectionless dry load.

Description

SPECIFICATION Edible product manufacture The invention is concerned with improvements in or relating to the manufacture of edible products, particularly but not exclusively, socalled snack foods.

It is well-known to manufacture edible food products by frying starch-based pellets in hot oil to create a product which has been expanded by generating steam within the pellet.

Where the pellets contain for example 1015% water and the starch content is in thermoplastic form the water boils internally and the steam pressure developed distends the structure creating voids which remain to form a rigid sponge on cooling.

This process is widely used to make snack foods but has the following disadvantages; (a) a limited shelf-life of the product because the oil picked up eventually becomes rancid giving an unpleasant flavour.

(b) many cooking oils are currently considered injurious to health, and an oil-free product is very desirable commercially.

(c) cooking oil is an expensive ingredient and creates troublesome working conditions, as well as being a fire hazard.

The avoidance of the use of oil obviously solves all the above problems, giving a very much longer shelf life which is of importance in the distributive context, particularly in hot climates. However there is great difficulty in achieving a rate of energy input high enough to give the required expansion using conventional methods of infra-red or hot-air heating because the essential requirement is to generate steam at a fast rate so as to build up a relatively high internal pressure to give expansion, whilst at the same time the steam is escaping through the surface of the pellet.In conventional heating this means that high air temperatures have to be used which tend to burn the surface of the pellets, a similar result occuring when a high heat-flux of infra-red is used It is also well-known that microwave energy is an efficient means of volumetric heating and may be considered as a choice for use in expansion of snack-food pellets. However, the process requires a very high energy dissipation density ( > 20 watts/cc.) within the pellet which in turn involves a high voltage stress ( > 200 volts/cm. at 2450 Mhz.) to be applied to the product. A further complication is that microwave fields are polarised so that preferential heating can occur along certain axes and care has to be taken to ensure a mixture of polarisations to achieve uniformity of expansion.

Where these considerations would lead to a design of microwave heating chamber of the multi-mode type, a plurality of magnetrons would be required to feed energy through waveguide feeds of mixed orientations so as to give the required mixed polarisation. Moreover to achieve the required power density the volume of oven space per Kw of injected power is in the order of three to ten times smaller than is used in conventional microwave ovens. Because the volume of product in the oven is also relatively small the oven may be highly resonant so that the load impedance offered to each magnetron is very variable and at times highly reflective.This last feature causes each magnetron to "see" a high level of reflected power, and also a large amount of power from the other magnetrons feeding the oven, resulting in: (a) Excessive internal heating in the magnetron causing a serious reduction in its life.

(b) Phase-locking of the magnetrons so that some of them synchronise together and operate on precisely the same frequency, thereby in part destroying the advantage of the provision of mixed polarisation, and reducing the effective mix of standing waves, which impairs the uniformity of heating with the oven.

The invention therefore seeks to minimise the above disadvantages and to render it possible to produce oil-free edible food products of the kind above referred to.

The invention provides apparatus for the production of food products from pellets or other small granules containing edible starch and water, comprising an oven comprising a plurality of sources of microwave power each source comprising a magnetron and an associated waveguide feed, said feeds being arranged in a variety of orientations to give mixed polarisation of power, wherein there is provided at each source between each magnetron and an associated radiating element thereof, means to isolate the magnetron from power reflected from the oven at high levels of energy dissipation density.

Preferably said means may comprise a microwave ferrite circulator which, together with an absorber load, is introduced into the feed circuit of each magnetron. However, any suitable isolator ferrite may be used.

It will be understood that the power which would reflect from the oven under very high loads will originate not only from the magnetron itself but from all the other magnetrons feeding the oven. In conventional practice it is unnecessary to fit such a device because the typical loads inside a multimode arrangement give substantially aperiodic loading so that the amount of reflected power is small, particularly from other magnetrons feeding the oven.

There will now be described an example of the invention. It will be understood that the description is given by way of example only and not by way of limitation.

In the example of apparatus according to the invention a microwave ferrite circulator is used in the feed circuit of each magnetron of a multi-mode arrangement. Microwave ferrite circulators are well-known devices which usu ally have three waveguide ports, which may be referred to as A, Bind C. The magnetron generator is connected to port A, the oven to Port B, and the absorber load to Port C. In operation the device circulates energy in the direction A B C with negligible loss, but pre sents a high attenuation to energy circulating in the reverse direction C B A. It will therefore be clear that energy from the magnetron will be transmitted to the oven whereas energy returning from the oven will be directed to the absorber load and not to the magnetron. The absorber load may be a substantially reflectionless load which can either be a conventional water load or a dry load of appropriate power rating.

However a suitable alternative which may be used is an assembly of two four-port 3 dB hybrid junctions interconnected by ferrite nonreciprocal phase shifters, or alternatively a simple two-port isolator for possible lowerpower applications.

Various modifications may be made within the scope of the invention as defined in the following claims.

Claims

1. Apparatus for the production of food products from pellets or other small granules containing edible starch and water; comprising an oven comprising a plurality of sources of microwave power each source comprising a magnetron and an associated waveguide feed, said feeds being arranged in a variety of orientations to give mixed polarisation of power, wherein there is provided at each source between each magnetron and an associated radiating element thereof, means to isolate the magnetron from power reflected from the oven at high levels of energy dissipation density.

2. Apparatus as claimed in claim 1 wherein the isolating means at each station includes an isolator ferrite.

3. Apparatus as claimed in claim 1, wherein the isolating means at each station includes a microwave ferrite circulator.

4. Apparatus as claimed in either one of claims 2 and 3, wherein the isolating means together with an absorber load is introduced into a feed circuit of each magnetron.

5. Apparatus as claimed in claim 4, wherein the absorber load is a water load.

6. Apparatus as claimed in claim 4, wherein the absorber load is a reflectionless dry load.

7. Apparatus according to claim 3, wherein the microwave ferrite circulator is provided with three waveguide ports, a first port being connected to the magnetron generator, a second of the ports to the oven and a third of the ports to an absorber load.

8. Apparatus as claimed in claim 1 wherein the isolating means comprises an assembly of two multi-part hybid junctions interconnected by ferrite non-reciprocal phase shifters.

9. Apparatus for the production of food products, arranged substantially as hereinbefore described with reference to the example.