Influence of food geometry and dielectric properties on heating performance

B. Wäppling-Raaholt    SIK – Swedish Institute for Food Biotechnology Sweden

T. Ohlsson    formerly at SIK – Swedish Institute for Food and Biotechnology, Sweden

2.1 Introduction

During microwave heating, several interacting variables related to food, packaging and the microwave oven itself will influence how the food will be heated. Multi-component foods often require tailor-made product development, aiming at avoiding uneven heating, which might otherwise cause problems with both sensory and microbiological qualities.

In this chapter an overview is given of the food-related phenomena which influence microwave heating performance, with a special focus on how food geometry and dielectric properties of foods affect the microwave heating characteristics. As food parameters, both geometry and size of different components, as well as recipe formulation and relative placement of different components in, for example, a tray, could be regarded. Examples are given for some selected scenarios. Owing to the limited space, this overview cannot be complete; instead some important information is given regarding the effects of food geometry and dielectric properties on microwave heat distribution. This chapter provides a starting point; the interested reader can find more information in the references.

Sections 2.2 and 2.3 describe how food geometry and dielectric properties among several other parameters could influence microwave heating performance by contributing to different microwave heating phenomena. Section 2.3 focuses on factors which influence microwave heating performance, and the related problem of non-uniform heating is described, exemplified and discussed. In Section 2.4, some remarks on methodology and applications of controlled microwave heating are given. Section 2.5 presents modelling-based tools for improving heating uniformity, where parameters such as food geometry might be one of the variables in the modelling and optimisation. References are also given to previous works. Finally a section on future trends is given, followed by sources of further information and advice (Sections 2.6 and 2.7).

2.2 Microwave heating distribution and uniformity

Electromagnetic waves with frequencies in the range from 300 MHz to 300 GHz are defined as microwaves, with the corresponding wavelengths between 1 m and 1 mm. There are several distinct frequency bands which have been allocated for industrial, scientific and medical (ISM) use. For food applications, the frequency is however often limited to the ISM band of 24501 ± 50 MHz. In North and South America, another recognised ISM frequency is used industrially, namely 915 ± 13 MHz. In European countries, the latter frequency band is not generally available. However, in the UK the frequency 896 ± 10MHz is generally available. Another frequency, 5.8 GHz has also been available for some time, with applications mainly for heating of materials in some specific cases (on the one hand, materials which do not couple well to lower microwave frequencies; on the other hand, materials of low volume such as fibres and thin sheets, but also lower quantities of, for example, powder and granulates). This latter frequency is, however, not so commonly used.

Microwave heating of foods has sometimes been connected with uneven heating, due to so-called ‘hot and cold spots’ which may be present in the food product after heating. The microwave heating profile in foods is determined by the thermo-physical properties of the food item (dielectric properties, but also thermal properties) as well as of the distribution of the absorbed microwave power in the food. The latter is, in turn, determined by several factors: the electric field inside the microwave cavity or applicator, the dielectric properties (complex permittivity) of the food item but also by the microwave frequency. Computational modelling-based design of the oven cavity (e.g. size and shape of the cavity or applicator) as well as the waveguide system, gives tremendous possibilities to control the electromagnetic field pattern. Heating uniformity is, however, also to a great extent influenced by size, geometry, position and composition of food as well as package during heating. This will be described further in Section 2.2.1. More information on the general principles for microwave heating can be found in the literature. Several authors have described the subject, e.g. Bengtsson and Risman (1971), Ohlsson (1983), Walker (1987), Buffler (1993), and Ohlsson and Bengtsson (2001).

2.2.1 Factors which influence the microwave heating uniformity

The microwave heating performance of foods is influenced by several factors, which interact in a complex way with each other. Among such factors which may affect the heating uniformity are, for example: the design of the oven cavity, the waveguide or applicator system, as well as food parameters like the geometry and size2 of the food item and the food packaging, the relative placement of food components, as well as the distance between neighbouring components. Furthermore, another significant variable which will affect the heating performance is the permittivity of the food, commonly called the dielectric properties.

2.2.2 Dielectric and thermal properties

The macroscopic interaction between an electromagnetic field and a material is expressed by the permittivity ε* and the permeability μ* of the material. The permittivity describes the electrical properties, while the permeability describes the magnetic properties of the material. However, since foods are non-magnetic, the magnetic part of the power contribution will not give rise to heating. The permittivity designates how a material will interact with microwaves by expressing to what extent and how the material will absorb microwave energy and convert it into heat, how strong the reflection and transmission phenomena will be, as well as how much the microwave wavelength will be reduced in the material.

The permittivity is a complex quantity. Thus, it consists of both a real part (the real permittivity ε′) and an imaginary part (the dielectric loss factor ε″). Basically, the permittivity describes the ability of a material to absorb, transmit and reflect electromagnetic energy. Several measurement techniques are available for determining the permittivity of materials. The choice of technique will be based on the different advantages and limitations of available methodologies. A relatively large amount of data are available on basic food components, especially at 2450 MHz. However, much less data are available for ready made recipe foods and formulated foods. Few predictive models are available by which dielectric properties can be calculated from proximate analysis, and these models have limited applicability (Mudgett, 1986). Predictive equations for dielectric properties of foods have been developed by several authors, where the influence of food composition, moisture content and temperature is often included.

The absorbed microwave power is related to the permittivity and to the electric field by the following equation:

V=2πfε0ε″E2

where Pv is the power which is absorbed in a given volume of the food material (W/m3 ), f is the frequency in Hz, E is electrical field strength in V/m inside the food, ε0 is the permittivity in free space (8.854 x 10- 12 F/m) and ε″ is the relative dielectric loss factor.

This equation gives an understanding of the influence of the dielectric loss factor and the field strength on power absorption. Furthermore, it illustrates the advantages of using very high frequencies for heating, e.g. when comparing microwave and high frequency dielectric heating at 27 MHz. At the higher frequency, considerably lower field strength is required for a given energy input. The dominating variable, the electric field strength, is unfortunately variable and very difficult to estimate or measure, when heating in a microwave oven....