Top Editors ... New horizons for dielectric heating

Title: New horizons for dielectric heating
Date: 16/05/2007
Autor: By Erik Esveld, Wageningen University

New hybrid microwave and RF technologies open the route for minimal processing of semi-solid foods

Microwave and radio frequency (RF) heating technologies aim to overcome some of the problems associated with conventional heat transfer. Their rapid volumetric heating can result in a drastic reduction of the process duration and floor space coupled with an increase in flexibility and controllability.
And since the basic technology has already been available for four decades, it is not surprising that installations are found up and running for diverse applications in the food industry, ranging from defrosting and tempering, to drying and baking.
A major concern with conventional cooking, pasteurisation and sterilisation of semi-solid foods is that it inevitably results in a suboptimal product quality due to the inherently prolonged heating time. A minimal time at the high temperature would result in a significantly better retention of the fresh product qualities. Since microwave and RF technologies are not hampered by 33 www.foodandbeverageinternational.com the limited thermal conductivity inside the product, they are the obvious candidates to minimise the thermal impact on the quality.
However, their application in minimal processing of solid foods such as ready meals and meat products is still fairly limited. There, the advantage of rapid heating is equally met with some inherent concerns which are not experienced with conventional thermal processes. These are notably insufficient temperature homogeneity and the lack of robustness.
The first problem is well known to anyone who has experience with the microwave at home. The lack of robustness is an issue only encountered by industrial applications.
Unlike the domestic microwave oven, which is designed around the ‘one size fits all’ principle, industrial installations are designed and tuned specifically for the intended product. The delicate optimal configuration is sensitive to any change that might shift the process performance out of specification and even trigger complete malfunction.
These challenges can fortunately be met if the rapid volumetric heat transfer of microwave and RF can be combined with the reliability of a conventional hot surrounding medium. The contribution from either the electromagnetic field or the hot surrounding should then be comparable in order of magnitude. The addition of hot air to a microwave or RF tunnel will only contribute to the heating of small or granular objects. For solid foods in the centimetre range, either steam or hot water immersion is needed to stabilise the temperature of the product’s outer core.
The spherical shape acts like a lens for the microwaves which result in a very pronounced centre heating of the right potato. The left potato is heated by steam which only affects the temperature of the outer zone. In general the combination forms an ideal match, where microwaves are used to boost the centre temperature while a conventional steam environment is used to accurately set the end temperature.
The operation of the microwave is not very much affected by the steam, but its performance is less critical because local variations in microwave power are compensated by the additional steam heating. Turning the picture around, the performance of a conventional steam process can be enhanced by the addition of a microwave boosting section. The point is to use the advanced technology only where it makes a big difference: that is, to speed up the internal heating and leave the rest to the more reliable and much cheaper steam process.

Going up in scale and down in frequency

Microwave and RF heating both achieve a similar effect, but differ considerably in technical implementation due to the different operation frequencies. The wavelength of microwaves (2450 or 915 MHz) in foods is in the order of a centimetre while that of RF waves (27 and 40 MHz) is about a metre. Fig 2 illustrates the differences between the two technologies.
Microwaves, with their comparable small wavelength, can easily resonate within the cavity and product. This greatly facilitates the coupling between the external field and the product, but the resulting standing wave pattern creates distinct local field concentrations or hotspots within the product. For a brick sized product, the thermographic image reveals that the power is focused near the corners. The temperature homogeneity observed for RF heating is much better because the wavelength is too large to form internal standing wave patterns. For regularly shaped and fairly large products, the application of RF is to be preferred if it were not hampered by some fundamental technical issues.
To understand their origin, the electromagnetic field aspects of the technology need to be studied.
The power will only be effectively transferred to the food if the electrode capacitor is brought in resonance with an external inductor at exactly the frequency of the generator.
The electric field is almost entirely concentrated in the air gap between the top electrode and the food. This is caused by huge differences in dielectric properties between the food and the air. As a result, the electric field strength in the air should be very high, resulting in the almost unavoidable risk of arcing. The robustness is further challenged by the very critical and sensitive tuning. Moreover, for many products the resulting temperature distribution is not homogeneous at all, since the field will still concentrate around the product edges due to the stark contrast in dielectric properties.
The new approach as a solution to these problems is to replace the air by water as surrounding medium, which is much closer in dielectric properties to moist foods. The trick is to use water with sufficiently low conductivity so that it is transparent for the RF energy.
Unlike for microwaves, pure water is not heated at radio frequency, which only affects the free charges (ions). The concept of water immersed RF heating therefore eliminates the cause of the technical problems mentioned.
The risk of arcing is absent, the tuning is much more robust and the temperature much better distributed. Obviously, a hot water surrounding should be used for cooking, pasteurisation and sterilisation applications, which creates the extra buffer for temperature stabilisation and control.
The photo (left) shows a water immersed RF heating prototype, running 27 MHz which has been constructed and tested at the Food Technology Centre of Wageningen University. For various vacuum packed food loads, it was possible to obtain excellent homogeneity of heating with an acceptable efficiency. Several projects are currently under development to demonstrate the technology at pilot scale for applications in semi-batch and continuous operation. The advancement by the use of hybrid heating makes it feasible to employ the RF heating beyond the current applications of tempering and drying and can make the long held promise for minimal processing of semi-solid foods come true.