28-09-2013, 12:57 PM
MICROWAVE HEATING UNIFORMITY OF MULTICOMPONENT PREPARED FOODS
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ABSTRACT
During microwave frequency heating many variables in the food, package, and the microwave
oven itself affect how the food is heated. Multicomponent foods in particular heat unevenly, causing
problems with both sensory and microbiological quality. It is difficult to quantify the influence of factors
affecting microwave heating uniformity of foods. This is due, at least partly, to the problems in
measuring temperature distribution and the interaction between oven and the food. The objective of this
study was to improve the quality of microwave heated multicomponent foods by better understanding of
the factors controlling heating uniformity and the consumer perception thereof.
Dielectric properties of some food ingredients and components were measured by a cavity
perturbation technique at temperatures up to 95oC. Food material used in the heating experiments were
chilled ready meals and hamburgers. Temperature was measured by a fibre optic system during
microwave heating and by thermocouples after heating. Infrared imaging was used to determine surface
temperatures of hamburgers. Water diffusion in a hamburger bun was examined by an NIR water
measurement system. The effect of salt content (two types of salt), geometry and placement of food
components, as well as the type of tray on heating uniformity of food were investigated. A consumer
panel was used to examine how temperature affected the pleasantness of meal components and how the
more or less uneven temperature distribution of microwave heated meals was perceived.
The measurement of the dielectric properties of starches showed that mainly water content
determined the dielectric properties and that, in practice, the variations between the different starches
were too small to be significant for recipe modifications. The temperature measurements showed that
chemical modifications had either no effect on temperature development or they had only some effects
combined with other factors although the dielectric properties varied considerably. The most important
factors in modifying the temperature distribution of a ready meal were placement and geometry of
components, and the type of tray. Heating uniformity was also studied in a vertically layered system
with components of widely different composition and dielectric properties, a hamburger bun. The results
showed that uneven heating is difficult to improve by recipe modifications alone. Modelling of the
hamburger confirmed that the food itself causes the uneven heating due to focusing and diffraction
phenomena.
INTRODUCTION
Microwave ovens have become a common kitchen appliance in developed countries: the
microwave oven penetration is already over 100% in the USA, Japan and Australia and over
80% in the Nordic countries and UK, totally with about 225 million ovens in the homes. The
number of household ovens has increased rapidly while the number of industrial applications
has remained fewer than 1000 globally. Nearly 25 million household microwave ovens are
produced annually (Ohlsson and Bengtsson, 2001). However, it has not substituted the
conventional oven, and microwaveable foods have not been a big success either. Even
research and development of microwave foods decreased towards the end of 1990s. The main
reason for the product failures have surely been poor product design due to poor understanding
of the principles of microwave heating (Shukla, 1998).
Dielectric properties of foods
The dielectric properties of some foods can be found in the literature (Tinga and Nelson, 1973;
Stuchly and Stuchly, 1980; Kent, 1987; Thuery, 1992; Datta et al., 1995) and in databases. The
most common food products have a loss factor of less than 25 and permittivity between 30 and
80, which implies a penetration depth of 0.8 to 1.5 cm. However, literature data is mostly
limited to food ingredients and their components. For complex foods, like lasagne, the
dielectric properties must be measured or estimated (Ohlsson, 1989; Buffler and Stanford,
1991; Calay et al., 1995). The dielectric properties of some foods as a function of temperature,
at 2.8 GHz, are presented in Figure 4 (Bengtsson and Risman, 1971) and Figure 5 presents a
“food map” at 2450 MHz and 20-25oC (Buffler and Stanford, 1991).
OTHER FACTORS AFFECTING MICROWAVE HEATING
Besides the electromagnetic properties many other factors affect how foods are heated by
microwaves. Among these properties are the thermal properties of foods, the initial
temperature of food, size and shape of the product, and size, shape, and position of components
within the food, the packaging, and the microwave oven (James, 1993). According to Buffler
and Stanford (1991) two major effects influence the heating patterns in microwave heated
foods: in some cases the pattern is dominated by the hot and cold spot phenomenon, and under
certain circumstances there is a combination of geometry and oven dependence.
Thermal and mechanical properties
After microwave energy has penetrated the product, two thermal properties (thermal
conductivity, k, and specific heat capacity, cp,) and two mechanical properties (density for
solid, and viscosity for liquid products) determine how the product will be heated (Buffler,
1993). The heating rate depends on the thermal properties while absorbed power on the
dielectric properties. Buffler and Stanford (1991) recommend therefore that in multicomponent
products, which may have widely differing dielectric and thermal properties, it is necessary to
balance both the thermal and dielectric properties in order to get more even temperature
distribution. Usually it is more fruitful to adjust specific heat capacity than the dielectric
properties. However, if this is done by for example altering the water content, this will in turn
affect the dielectric properties.