**4. Microwave food process design**

In designing microwave food processes and packaging, various factors that affect microwave heating of foods should be taken into consideration if the effect of uneven heating associated with the use of microwaves is to be kept under control. These factors fall into two broad categories. The first one is thermo-physical properties of the food. The second is factors associated with the dielectric characteristics of food and the field intensity distributions provided by various microwave energy applicators and heating systems.

#### **4.1. Physical factors**

22 The Development and Application of Microwave Heating

*sp*.) and shepu (*Peucedanum graveolens*).

the product surface, hence making the outer skin dry

the microwave oven.

texture and similar

conventional oven. The browning reaction takes time to develop and the product is not heated long enough for this reaction to proceed to the point where brown pigments and flavour compounds are produced. It should be noted that there are a wide variety of products where the time and temperature of heating do not create an issue for flavour development. High moisture products that are going to be reheated work very well. While some flavour will be lost during the heating process, it does not vary significantly from conventional reheating. Vegetables, with their own inherent flavor, can easily be steamed in

The sensory properties of vacuum-microwave-dried and air-dried carrot slices, which were water blanched initially. The vacuum-microwave-dried carrot slices received the higher ratings for texture, odour and overall acceptability as compared to the air-dried carrot slices. The retention of volatile components responsible for flavour was more in hot air microwave drying compared to conventional hot air drying alone. The flavour strength of garlic dried by hot air alone is 3.27 mg/g dry matter whereas the flavour strength of the garlic dried by microwave drying is 4.06 mg/g dry matter. Effect of microwave drying on the shelf life and sensory attributes (appearance, colour, odour and overall quality) of coriander (*Coriander sativum*), mint (*Mentha spicata*), fenugreek (*Trigonella foenum-graceum*), amaranth (*Amaranthus* 

Amaranth had similar scores for fresh and dried ones; however, there was significant decrease for the sensory attributes of other greens. They concluded that microwave drying was highly suitable for amaranth, moderately suitable for shepu and fenugreek and less suitable for coriander and mint. Wheat samples were evaluated and the sensory characteristics of grain were assessed by the panel of 10 members. The sample produced a burnt or roasted odour when exposed for a long exposure time (180 s) but there was no

Due to high temperature and long drying time, volatile compounds are vapourised and are lost with water vapour, resulting in significant loss of characteristic flavour in dried products. Case-hardening is a common problem in dried fruits due to rapid drying. As drying proceeds, the rate of water evaporation is faster than the rate of water movement to

At air dryer temperatures, volatile flavour compounds are lost, structural changes such as case hardening may inhibit later rehydration, and extended drying times allow chemical and enzymatic reactions to degrade vitamins, flavour and colour compounds microwave dried frozen berries had a higher rehydration ratio. Microwave (MW) drying generated three unique flavou compounds (2-butanone, 2-methyl butanal, and 3-methyl butanal) while freeze-dried berries lost several, including the typical blueberry aroma, 1,8-cineole. Compared with hot-air dried berries, MW-dried cranberries have better colour, softer

The advantages of MW blanching (MB) over conventional heat blanching methods (water or steam) include in-depth heating without a temperature gradient, and rapid inactivation of

significant difference in the grain odour when long exposure times were avoided.

The thermo-physical factors that require serious consideration in the design of microwave food processes and packaging systems are:

#### *4.1.1. Size and shape of food*

The physical size and shape of foods affect the temperature distribution within the food. This results from the fact that the intensity of the wave decreases with depth as it penetrates the food. If the physical dimensions of the food are greater than twice the penetration depth of the wave, portions of the food nearer the surface can have very high temperatures while the mid-portions are still cold. On the other hand, if the dimensions of the food are much lower than the penetration depth of the wave, the center temperature can be far higher than the temperature at the surface. This situation normally results in "the focusing effect," which results from the combined intensity of the wave (in three space dimensions) being higher at the inner portions than the outer portions of the product.

Some shapes reflect more microwaves than others. In addition, some shapes prevent increasing amounts of the waves from leaving the material by reflecting them back into the interior. For most spherical and cylindrical foods, wave focusing occurs for product diameters between 20 and 60 mm. In rectangular foods, focusing causes the overheating of corners. Thus in package design, sharp corners are avoided and tube-shaped pans have been suggested (Giese 1992). Moreover, in foods with corners, packages are designed using

metals or aluminum foils to reflect microwave energy away from corners and thus selectively heat some portions more.

#### *4.1.2. Surface area*

In microwave heating, the product temperature rises above its ambient temperature due to volumetric heating. Higher product surface area therefore results in higher surface heat loss rate and more rapid surface cooling. During microwave heating, the highest temperature is not at the surface of the product (despite the higher intensity of power absorbed there) but somewhere in the interior.

#### *4.1.3. Specific heat*

How much a food product will heat given a specific amount of energy depends on its heat capacity. The implication of this for microwave heating is that different food products heated together have different temperature histories. To control this, some microwave food packages are sealed tight to allow heat transfer between hotter and colder foods, thus giving similar temperature history for different foods in the same package.
