Will Solid State RF Energy Change the Way We Cook and Eat?
Using solid-state devices as the primary energy source appears to be the most promising alternative across many different applications.
The 60-year reign of the magnetron as the primary energy source in a microwave oven is starting to come to an end. Capable of only operating at full power or off and nothing in-between and reliant on mechanical techniques such as a stirrer or a turntable in an attempt to spread energy within the microwave cavity, the magnetron has had a good run. There is another aspect of the magnetron that is annoying and that is that its power output degrades over time. Typically over three to four years the output power of a magnetron will reduce by as much as 30%. Such degradation applies whether it is an industrial or a domestic oven. Few people notice that the same food typically needs to be cooked for a bit longer as the oven gets older.
There are other aspects of a magnetron-powered microwave that have become increasingly frustrating over time, the most annoying of which is that food is not uniformly cooked, the results are areas of over-cooked and under-cooked or even raw food. It is for this reason that microwaves are often not used to cook raw meats, ending up being used to reheat or defrost food. Another consideration is that a magnetron can only deliver its full output power, so you can only control the amount of power by adjusting its time on/off duty cycle. For example, a 900 Watt oven when operating at 90 Watt might product 900 Watt for five seconds and then switch off for forty five seconds. For cooking larger items, it is important to allow enough time for thermal conduction to raise the core temperature to a safe and palatable level. Allowing enough time is as important for homogeneous heating with a magnetron as it is with conventional heating processes. Unfortunately, the formal test of a microwave’s energy capability, IEC 705, is out of date since it only evaluates the devices capability to heat a one-liter jug of water placed centrally in the oven’s cavity.
So what’s the alternative? Using solid-state devices as the primary energy source appears to be the most promising alternative across many different applications—industrial, catering and domestic. Firstly, and most notably, solid-state cooking ovens are capable of detecting general types of food placed in the cavity. Such future ovens will be able to tailor the frequency, phase and output power to suit cooking a specific food, or even multiple foods placed in the cavity. In turn this allows more delicate foods to be cooked and that it will be possible to retain both moisture and nutrition, making food more tasty and healthy. Power profiles can be developed for common foods and prepared meals. The power output of a solid-state oven can also be varied in a linear manner, providing a lot more control, this being accomplished either by a linear control or by using a pulse-width modulation technique. Unlike using a single magnetron, solid-state ovens use either two or four “antennas” or channels from which the energy is radiated. This approach allows the energy to be distributed in a more accurate pattern throughout the cavity by altering the phase of the energy on each channel. Magnetron tubes are 1kW only whereas solid state can be designed with lower power for table-top like designs. The white-goods designer has a lot more design freedom with solid-state technology.
Figure 1 illustrates the two differences between a conventional microwave oven and that of its solid state equivalent. Apart from the points mentioned above, it will be noted that there is no need for the generation of high voltage (4 kV) in a solid-state oven. Without so many mechanical bulky parts the solid-state oven will also be a lot lighter, and the use of solid-state devices means that the power output will not degrade over time. The lifetime of the magnetron used in industrial ovens is a key issue. If the magnetron breaks down restaurants need to wait for replacement, by comparison solid-state units are designed for 20 years of continuous operation.
It is the ability to monitor the amount of energy being reflected back from the food being cooked in the cavity that clearly differentiates a solid-state cooking process. Constantly monitoring this in the cooking process is optimal since the composition of food changes as it is being cooked. By changing the frequency, phase and power in an adaptive manner allows algorithms to be deployed. It turns out that different foods heat in different ways, depending on the frequency and phase of the radiation applied. This can be tested by putting two different food samples in a standard cavity, in this case with four antennas, and applying different frequencies, as shown in Figure 2, in which the red and white lines represent the ‘return loss’ for each port, a useful way to quantify the energy absorbed in the cavity on each port. The green lines show the combined ‘compound’ return loss.
In another example, in a test solid state oven, different profiles were developed for cooking pancake batter by using the spectrum firstly in an even way across all frequencies, and secondly by adaptively biasing towards only modes with very good cavity power retention. (See Figure 3.)
Figure 3 shows a thick pancake batter heated first using all frequencies evenly (left) and (right) using an adaptive hearting algorithm biased to use only the best modes for efficient cavity power retention. The algorithm biased towards efficient coupling of energy into the cavity and load heating the center of the batter most directly, causing the middle to rise faster than the edges. The algorithm that used all frequencies rose evenly.
Such feedback systems can also be used to compensate for the way in which many foods, such as beef burgers, change their absorption and moisture characteristics as they cook. Using a solid-state cooking approach also heats food much faster than using a traditional magnetron. Especially for commercial and industrial applications this is a key value proposition towards the use solid-state technology and in some trials conducted with customers it has shown to be roughly 30% faster.
With the ability to more accurately predict the cooking requirements, it is envisaged that solid-state cooking techniques will unleash a raft of new food service concepts. Prepared meal suppliers, whether for large-scale consumption or for retail, will be able to indicate on the packaging the exact cooking profile required. Taking this concept further, meal packaging could have an RFID device or QR barcode that the oven could read thereby transferring precise cooking information so that the resulting meal retains as much texture and taste as possible, and each cooked meal is the same. Naturally, the food service and fast-food industries are showing a lot of interest in the predicable, reliable and fast results that solid-state ovens can deliver.
Today’s microwave ovens leave a lot to be desired. Their ability to heat food depends on their age, the temperature of the magnetron, the type of food being heated, and the pattern of standing waves created by the interaction between the source, the food and the cavity.
Moving to solid-state RF heating, combined with many advanced techniques originally developed for the communications industry, will enable designers to overcome many of the magnetron’s shortcomings. They will also allow appliance designers to develop much more sophisticated, adaptive heating techniques and enable new (lower power) cooking devices that could change how we cook and what we eat.