**4.1. Application in food industries**

Due to the very large number of microwave ovens in households, the food related industry not only uses microwaves for processing but also develops products and product properties especially for microwave heating. This way of product enhancement is called product engineering or formulation.

### *4.1.1. Baking and cooking*

Detailed references to the baking process of bread, cakes, pastry etc. by the help of microwaves on industrial scale can be found. An enhanced throughput is achieved by an acceleration of the baking where the additional space needs for microwave power generators are negligible. Microwaves in baking are used in combination with conventional or infrared surface baking; this avoids the problem of the lack of crust formation and surface browning. An advantage of the combined process is the possible use of European soft wheat with high alpha-amylase and low protein content.

In contrast to conventional baking microwave heating inactivates this enzyme fast enough (due to a fast and uniform temperature rise in the whole product) to prevent the starch from extensive breakdown, and develops sufficient CO2 and steam to produce a highly porous (Decareau, 1986). One difficulty to be overcome was a microwavable baking pan, which is sufficiently heat resistant and not too expensive for commercial use. By 1982 patents had been issued overcoming this problem by using metal baking pans in microwave ovens (Schiffmann et al., 1981 and Schiffmann, 1982).

The main use of microwaves in the baking industry today is the microwave finishing, when the low heat conductivity lead to considerable higher baking times in the conventional process. A different process that also can be accelerated by application of microwave heating is (pre-) cooking. It has been established for (pre)cooking of poultry (Helmar et al., 2007), meat patties and bacon. Microwaves are the main energy source, to render the fat and coagulate the proteins by an increased temperature. In the same time the surface water is removed by a convective air flow. Another advantage of this technique is the valuable byproduct namely rendered fat of high quality, which is used as food flavoring (Schiffmann, 1986).

### *4.1.2. Thawing and tempering*

4 The Development and Application of Microwave Heating

These advantages often yield an increased production.

with high alpha-amylase and low protein content.

**4. Microwave applications** 

engineering or formulation.

*4.1.1. Baking and cooking* 

**4.1. Application in food industries** 

intelligence.

**3. Uses, advantages and disadvantages of microwave heating applications** 

Today's uses range from these well known applications over pasteurization and sterilization to combined processes like microwave vacuum drying. The rather slow spread of food industrial microwave applications has a number of reasons: there is the conservatism of the food industry (Decareau, 1985) and its relatively low research budget. Linked to this, there are difficulties in moderating the problems of microwave heating applications. One of the main problems is that, in order to get good results, they need a high input of engineering

Different from conventional heating systems, where satisfactory results can be achieved easily by intuition, good microwave application results often do need a lot of knowledge or experience to understand and moderate effects like uneven heating or the thermal runaway. Another disadvantage of microwave heating as opposed to conventional heating is the need for electrical energy, which is its most expensive form. Nevertheless, microwave heating has a number of quantitative and qualitative advantages over conventional heating techniques that make its adoption a serious proposition. One main advantage is the place where the heat is generated, namely the product itself. Because of this, the effect of small heat conductivities or heat transfer coefficients does not play such an important role. Therefore, larger pieces can be heated in a shorter time and with a more even temperature distribution.

Due to the very large number of microwave ovens in households, the food related industry not only uses microwaves for processing but also develops products and product properties especially for microwave heating. This way of product enhancement is called product

Detailed references to the baking process of bread, cakes, pastry etc. by the help of microwaves on industrial scale can be found. An enhanced throughput is achieved by an acceleration of the baking where the additional space needs for microwave power generators are negligible. Microwaves in baking are used in combination with conventional or infrared surface baking; this avoids the problem of the lack of crust formation and surface browning. An advantage of the combined process is the possible use of European soft wheat

In contrast to conventional baking microwave heating inactivates this enzyme fast enough (due to a fast and uniform temperature rise in the whole product) to prevent the starch from extensive breakdown, and develops sufficient CO2 and steam to produce a highly porous (Decareau, 1986). One difficulty to be overcome was a microwavable baking pan, which is Thawing and tempering have received much less attention in the literature than most other food processing operations. In commercial practice there are relatively few controlled thawing systems. Frozen meat, fish, vegetables, fruit, butter and juice concentrate are common raw materials for many food-manufacturing operations. Frozen meat, as supplied to the industry, ranges in size and shape from complete hindquarters of beef to small breasts of lamb and poultry portions, although the majority of the material is `boned-out' and packed in boxes approximately 15 cm thick weighing between 20 and 40 kg. Fish is normally in plate frozen slabs; fruit and vegetables in boxes, bags or tubs; and juice in large barrels. Few processes can handle the frozen material and it is usually either thawed or tempered before further processing.

Thawing is usually regarded as complete when all the material has reached 0 0C and no free ice is present. This is the minimum temperature at which the meat can be boned or other products cut or separated by hand. Lower temperatures (e.g. -5 to -2 0C) are acceptable for product that is destined for mechanical chopping, but such material is `tempered' rather than thawed. The two processes should not be confused because tempering only constitutes the initial phase of a complete thawing process. Thawing is often considered as simply the reversal of the freezing process.

However, inherent in thawing is a major problem that does not occur in the freezing operation. The majority of the bacteria that cause spoilage or food poisoning are found on the surfaces of food. During the freezing operation, surface temperatures are reduced rapidly and bacterial multiplication is severely limited, with bacteria becoming completely dormant below -10 0C. In the thawing operation these same surface areas are the first to rise in temperature and bacterial multiplication can recommence. On large objects subjected to long uncontrolled thawing cycles, surface spoilage can occur before the centre regions have fully thawed.

Conventional thawing and tempering systems supply heat to the surface and then rely on conduction to transfer that heat into the centre of the product. A few, including microwave,

use electromagnetic radiation to generate heat within the food. In selecting a thawing or tempering system for industrial use a balance must be struck between thawing time, appearance and bacteriological condition of the product, processing problems such as effluent disposal, and the capital and operating costs of the respective systems. Of these factors, thawing time is the principal criterion that often governs selection of the system. Appearance, bacteriological condition and weight loss are important if the material is to be sold in the thawed condition but are less so if it is for processing. The main detrimental effect of freezing and thawing meat is the large increase in the amount of proteinaceous fluid (drip) released on final cutting, yet the influence of thawing rate on drip production is not clear.

James and James (2002) reported that studies have shown that there was no significant effect of thawing rate on the volume of drip in beef or pork. Several authors concluded that fast thawing rates would produce increased drip, while others showed the opposite. Thawing times from -8 to 0 0C of less than 1 minute or greater than 2000 minutes led to increased drip loss (James et al., 2002). The results are therefore conflicting and provide no useful design data for optimizing a thawing system. With fish, fruit and vegetables ice formation during freezing breaks up cell structure and fluids are reduced during thawing. In microwave tempering processes the heating uniformity and the control of the end temperature are very important, since a localized melting would be coupled to a thermal runaway effect.

#### *4.1.3. Drying*

The benefits of microwave drying we should first have a quick look at the much more conventional method of air drying. As shown in Fig.1, a typical drying curve of a foodstuff can be subdivided into three phases. The first period is one of constant drying rate per unit of surface area. During this period the surface is kept wet by the constant capillary-driven flow of water from within the particle. The factors that determine and limit the rate of drying in the so-called `constant rate period' all describe the state of the air: temperature and relative humidity as well as air velocity (Erle, 2000).

**Figure 1.** Typical drying curve for air drying

In drying the main cause for the application of microwaves is the acceleration of the processes, which are (without using microwaves) limited by low thermal conductivities, especially in products of low moisture content. Correspondingly sensorial and nutritional damage caused by long drying times or high surface temperatures can be prevented. The possible avoidance of case hardening, due to more homogeneous drying without large moisture gradients is another advantage. Two cases of microwave drying are possible, drying at atmospheric pressure and that with applied vacuum conditions.

6 The Development and Application of Microwave Heating

*4.1.3. Drying* 

use electromagnetic radiation to generate heat within the food. In selecting a thawing or tempering system for industrial use a balance must be struck between thawing time, appearance and bacteriological condition of the product, processing problems such as effluent disposal, and the capital and operating costs of the respective systems. Of these factors, thawing time is the principal criterion that often governs selection of the system. Appearance, bacteriological condition and weight loss are important if the material is to be sold in the thawed condition but are less so if it is for processing. The main detrimental effect of freezing and thawing meat is the large increase in the amount of proteinaceous fluid (drip) released on final cutting, yet the influence of thawing rate on drip production is not clear.

James and James (2002) reported that studies have shown that there was no significant effect of thawing rate on the volume of drip in beef or pork. Several authors concluded that fast thawing rates would produce increased drip, while others showed the opposite. Thawing times from -8 to 0 0C of less than 1 minute or greater than 2000 minutes led to increased drip loss (James et al., 2002). The results are therefore conflicting and provide no useful design data for optimizing a thawing system. With fish, fruit and vegetables ice formation during freezing breaks up cell structure and fluids are reduced during thawing. In microwave tempering processes the heating uniformity and the control of the end temperature are very

The benefits of microwave drying we should first have a quick look at the much more conventional method of air drying. As shown in Fig.1, a typical drying curve of a foodstuff can be subdivided into three phases. The first period is one of constant drying rate per unit of surface area. During this period the surface is kept wet by the constant capillary-driven flow of water from within the particle. The factors that determine and limit the rate of drying in the so-called `constant rate period' all describe the state of the air: temperature and

important, since a localized melting would be coupled to a thermal runaway effect.

relative humidity as well as air velocity (Erle, 2000).

**Figure 1.** Typical drying curve for air drying

Combined microwave-air-dryers are more widespread in the food industry, and can be classified into a serial or a parallel combination of the both methods. Applied examples for a serial hot air and microwave dehydration are pasta drying and the production of dried onions (Metaxas et al., 1983) whereas only intermittently successful in the 1960s and 1970s was the finish drying of potato chips. The combination of microwave and vacuum drying also has a certain potential. Microwave assisted freeze drying is well studied, but no commercial industrial application can be found, due to high costs and a small market for freeze dried food products (Knutson et al., 1987). Microwave vacuum drying with pressures above the triple point of water has more commercial potential has microwave vacuum drying with pressures above the triple point of water.

Microwave energy overcomes the problem of very high heat transfer and conduction resistances, leading to higher drying rates. These high drying rates correspond also to lower shrinkage and to the retention of water insoluble as shown in Figure 4. In parsley, for example, most of essential oils are present as a separate phase with high boiling temperature. For fast drying conditions (high microwave energy input) only the small amount of volatile essential oils that is dissolved is lost, whereas there is not enough time to resolve the remaining oil in the separated phase (Erle, 2000).

In contrast the retention of water soluble aromas, as in apples, is not as advantageous, since the microwave energy generates many vapour bubbles, so that the volatile aromas have a large surface to evaporate. Nevertheless, the low pressures limit the product temperatures to lower values, as long as a certain amount of free water is present and this helps to retain temperature sensitive substances like vitamins, colours etc. So, in some cases the high quality of the products could make also this relative expensive process economical.

Microwave vacuum dehydration is used for the concentration or even powder production of fruit juices and drying of grains in short times without germination .Newly and successfully applied is the combination of pre-air-drying, intermittent microwave vacuum drying (called puffing) and post-air-drying. It is predominantly used to produce dried fruits and vegetables, with improved rehydration properties (Räuber, 2000). After the form is stabilized by case hardening due to conventional air-drying, the microwave vacuum process opens the cell structures (puffing) due to the fast vapourization of water and an open pore structure is generated. The subsequent post-drying reduces the water content to the required value.
