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Background to the Regeneration of Ready Meals using Microwave Heating in two parts – The Challenge and The Justification

Part 1 - The Challenge

After the first microwave ovens were introduced in the late 1940s, microwave heating came to be considered as a new basic principle of cookery that would need to be harnessed and exploited by the questing chef or caterer. It was soon realised that the microwave appliance was a potentially useful item of equipment that could have revolutionary advantages but also unconventional limitations in a busy commercial kitchen or service environment.

The challenge facing chefs was to develop an understanding of how it could replace, or complement the other well established basic principles of cookery. For instance, at first glance, its cooking results were seen to be directly comparable with poaching, steaming, boiling, stewing and some types of baking i.e. cooking without colouring. Then it was realised that it might also be possible to use it in a complementary role, such as to speed up the defrosting of frozen foods or shorten the usual cooking times of grilled, roasted, fried, braised or baked food.
So the first step was to fully appreciate how this new heating technique differed from conventional cooking techniques. Up until this time, all conventional cooking methods were categorised by

Contact Heating, either dry or wet in aqueous liquids or oil
Convection Heating
Radiant Heating
Condensation Heating

These methods all rely on heat being applied to the outside of the food and the interior receiving its heat by conduction. On the other hand, in microwave heating the food is cooked primarily by microwaves instantaneously penetrating the food to a depth that is dependent upon its composition. The ability of microwaves to do this and generate heat is dependent upon a set of factors i.e.

Density For medium density products the average depth of instantaneous but diminishing penetration by microwaves all around the product is 25mm (l inch). Yet variable compositions, such as meat, fat and gristle in a joint can heat unevenly.
Temperature The lower the starting temperature, the longer it will take to heat. For instance, water can heat up 25 times faster than it did as ice.
Volume The larger the quantity of food, the longer it will take to heat e.g. Each additional item may take up to 75% additional time.
Containers Their suitability is determined by their ability to transmit, reflect or absorb microwaves to the user’s needs. In some instances, the ideal shaped containers would need to be round, oval or elliptical with rounded corners.
Power Levels Higher power levels are often suitable for heating easy to heat food and pre-cooked products, whereas lower power levels are often more appropriate for harder to heat heavy or large items and defrosting tasks.


As we applied our knowledge of these factors that Influence microwave heating so as to achieve the best results within the short heating time, it was found that the end results could be further improved by employing a number of helpful handing techniques. These varied from different ways of arranging the food to the stirring, turning, covering and standing of it. Furthermore, it soon became evident that this focus on the microwave heating of food involved three inescapably entwined developing technologies - in food, packaging and equipment.

As our experience grew in these three areas, it became evident that this new basic principle in cookery has another attribute that is formidable, and distinguishes it from any other form of heating. That is the ability not only to cook prepared raw items but also to re-heat pre-cooked, prepared meals or snack items within seconds, in safety.

In the commercial foodservice industry it prompted caterers in large scale operations, such as hospitals, industrial canteens and fast food restaurants to consider ways in which they could use this equipment to provide a more cost effective and efficient food service. As it happened, the first impetus came from vending operators in the United States. They adapted the idea of compact, push button microwave ovens (a significant advance from the first floor standing 6 feet high models) being used to heat simple meals and snack items that could be purchased from their vending machines. Then hospital groups started using central production kitchens to produce and assemble frozen or chilled meals that could be regenerated in microwave oven – either in specially built service units or service trolleys at ward level.  By the 1970s, over 90% hospitals in Southern California were using variations of such systems. By that time too, many restaurant groups such as Burger King and Little Chef had adapted their service system to use microwave ovens.

Meanwhile, chefs were beginning to take a keener interest in microwave being used in haute cuisine applications. One of the landmarks in 1969 was when 400 prominent personalities from the hospitality industry were invited to a Microwave Gala Evening that was being held at the London Hilton in Park Lane.  On show in the large dining room was a bank of microwave ovens that were used to regenerate a choice of dishes that had been created by the chefs, then frozen or chilled for each course of the four course menu. At the event, a limited edition cookery book was launched that contained recipes for prepared foods from a selection of leading international chefs. Described by the media as an “Haute Cuisine by Microwave” event, it stimulated the interest in quick-to-regenerate ready prepared meals at a time when the first consumer microwave ovens were making their entry.

At this time a key stimulator of the consumer market to emerge was Japan. Soon after the first American patents had expired, several Japanese companies took the initiative. They had the courage to make heavy investments in manufacturing facilities for consumer microwave ovens. As it happens, traditional Japanese food is the easiest to heat by microwave, and many Japanese housewives were persuaded to change direct from using their charcoal cooker to the microwave oven. Japan also has a large number of very small restaurants (more than in the United States) to which the microwave oven had an obvious and immediate appeal in a semi-domestic role.

The arrival of the consumer microwave oven and its additional perceived potential as a regenerator of good quality prepared meals was a stimulant to packaging technology, which then developed at a remarkable rate.  The most important requirement for any container or packaging used for the microwave heating of food is that it should be fully or partially transparent to microwave. In other words, it will allow microwaves to pass through it and get to the food. Secondly, that it will withstand whatever its degree of microwave, or food, or liquor absorption without any significant deterioration. Thirdly, that it will stand up to the temperature of any heat or steam created by its intended food contents; and fourthly, that the microwave or thermal heat used during the cooking process will not cause it to burn. So to sum up, the suitability of any container or packaging is related to its transparency, its porosity, its thermal tolerance and its flammability.

Some containers, such as heavy earthenware dishes with shiny surfaces will partially absorb and reflect microwaves, slowing down their heating effect by as much as 30%. In the early years, foil was often used to shield various parts of food that one wanted to cook more slowly e.g. legs and wings of a chicken. This, and other intentional shielding techniques were really the forerunners of the most sophisticated techniques used for regenerating microwaveable foods today. For example, susceptor lined cartons, producing a microclimate of convection currents of hot, dry air over the surface of the food product. Another technique involves laminating aluminium  foil to a polymeric film, then selectively etching it, to leave a desired structure or antenna design; the end product becomes a set of tools which can partially and selectively shield food that might overheat during the short heating time, alter bulk heating properties and affect surface heating (browning, crisping). As a result, raw pastry, pizza, poultry and breaded products may be cooked or regenerated in their packaging.

Meanwhile, there has been a co-incidental development of containers and packaging made from various types of cold and heat resistant paper, plastic and film, shaped as trays, tubs or pouches to hold the food heated directly by microwaves. Some are ‘dual ovenable (able to withstand conventional heat levels and suitable for microwaves) whereas others may be deliberately arranged to absorb excess moisture or grease exuded by the foodstuff during the process.  Another innovation includes microwaveable aseptic packaging material that ensures product safety and integrity, as well as retaining original flavour, colour and texture for six months without the need for refrigeration or preservatives. Yet other examples are specially designed steaming bags, with inbuilt pressure valves, capable of steaming food evenly in the microwave oven within a matter of minutes, retaining healthy nutrients and flavours.

When microwave ovens were first introduced in the UK, their cost equalled that of motor cars, and they could only be sold or leased to the commercial catering industry. However, such was their success, and the volume sold that within twenty years they had paved the way for the introduction of the first consumer microwave ovens. During that twenty years there were many exciting developments, not only with microwave appliances but also the integration of complementary heating, scanning and computation techniques. We will take a brief look at these to preface our next part, which will focus on how microwave ovens have justified themselves today, and how exciting their prospects are for tomorrow.

Coming soon ….

... Ready meals active container heating guidelines
... Industrial microwave food production considerations
... Breakthrough in Even-Heating Microwave Technology

© GAMA Microwave Technology

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