**5. Microwave drying applied in the food industry**

Microwave drying is not common in the food industry. There are many reasons for its limited use: the technical problems described above were not well- understood in the past. This has led to some failures, which have surely discouraged other potential users. Schiffmann (2001) has listed a number of formerly successful applications that have been discontinued. Among these are the finish drying of potato chips, pasta drying, snack drying, and the finish drying of biscuits and crackers. It is apparently not always the microwave process itself but rather changes in the circumstances of production that make competing technologies more successful.

In spite of these difficulties, there are some current applications. Schiffmann (2001) mentions cereal cooking and drying with a production rate of nearly 1 ton/h. Pasta drying with microwaves is carried out in Italy. Microwave- vacuum drying is being used for meat extract and, at least for a number of years, for the production of a powder made from orange juice concentrate.

The combination of air drying and microwave-vacuum puffing is being used in Germany and Poland for fruits and vegetables. As the food industry does not disclose all its production processes, we cannot expect this list to be complete. Hauri (1989) has provided values for the necessary investment and the specific energy requirements of five different drying methods (Table 1). Based on the same throughput, the investment needed for microwave-vacuum drying is rather high, while the energy figures are more favorable than for air drying.


**Table 1.** Comparison of five different drying method

## **6. Pasteurization and sterilization**

8 The Development and Application of Microwave Heating

stress during processing.

molecule in water is essential.

and aroma are coupled.

juice concentrate.

**5. Microwave drying applied in the food industry** 

*4.1.4. Quality of microwave-dried food products* 

In general, the quality is somewhere between air-dried and freeze-dried products. The reduction of drying times can be quite beneficial for the colour and the aroma. Venkatesh and Raghavan (2004) dried rosemary in a household microwave oven with good aroma retention. Krokida and Maroulis (1999) measured colour and porosity of microwave-dried apples, bananas, and carrots. Khraisheh et al. (2004) compared air-dried and microwavedried potatoes and found a reduction of shrinkage and improved rehydration for the latter. Venkatesh et al (2004) reported on chicken products, seafood, and vegetables of good quality. He used air at 10±20 0C to cool the product during microwave drying. Quality can often be improved further by the use of vacuum. This reduces thermal as well as oxidative

For instance, Yongsawatdigul and Gunasekaran (1996) showed that colour and texture of microwave-vacuum-dried cranberries were better than those of air-dried samples. If we look specifically at the retention of aroma, it becomes necessary to distinguish between two basic cases. In most foods the aroma molecules are present in very small amounts, so that they are likely to be dissolved in the water phase. In this situation, the volatility of the aroma

Considering the fact that we perceive aroma -as opposed to taste - with our noses, it is quite clear that aroma molecules are normally volatile; otherwise they would stay in the food during eating and not contribute to the aroma. In other words, if there is an interface between a water phase (i.e. a food) and a gas phase, the aroma molecules tend to choose the gas phase. In air drying, the surface where the aroma molecules can escape is mainly the outer surface of the particles. This is also where the water molecules evaporate. So the surface of the food particle will be depleted of aroma, but the losses cannot be higher than those that come with the capillary water flow from within. As a result, the losses of water

Microwave drying is not common in the food industry. There are many reasons for its limited use: the technical problems described above were not well- understood in the past. This has led to some failures, which have surely discouraged other potential users. Schiffmann (2001) has listed a number of formerly successful applications that have been discontinued. Among these are the finish drying of potato chips, pasta drying, snack drying, and the finish drying of biscuits and crackers. It is apparently not always the microwave process itself but rather changes in the circumstances of production that make competing technologies more successful.

In spite of these difficulties, there are some current applications. Schiffmann (2001) mentions cereal cooking and drying with a production rate of nearly 1 ton/h. Pasta drying with microwaves is carried out in Italy. Microwave- vacuum drying is being used for meat extract and, at least for a number of years, for the production of a powder made from orange Studies of microwave assisted pasteurization and sterilization have been motivated by the fast and effective microwave heating of many foods containing water or salts. A detailed review can be found in (Rosenberg et al., 1987). Although, physically non-thermal effects on molecules are very improbable, early works seemed to show just these effects. But in most cases the results claimed could not be reproduced, or they lacked an exact temperature distribution determination. The improbability of non-thermal effects becomes clear, when the quantum energy of photons of microwaves, of a thermal radiator and the energy of molecular bonds are compared. The quantum energy of a photon of f = 2.45 GHz is defined by E = *h f* ≈ 1\*10-5 eV, the typical energy of a photon radiated from a body of 25°C ≈ 298 K equals E = *k T* ≈ 0.26 eV and the energy of molecule bonds are in the eV-range.

Since the collection of energy with time for bound electrons are forbidden by quantum mechanics, only multi-photon processes, which are very unlikely, could yield chemical changes. Recently Lishchuk also showed that even a deviation of the energy distribution of water molecules from the conventional Boltzmann distribution cannot be proved (Lishchuk et al., 2001).

More thinkable is the induction of voltages and currents within living cell material, where eventual consequences are still in discussion (Sienkiewicz, 1998). Due to the unquestioned thermal effects of microwaves, they can be used for pasteurization and sterilization. Studied applications of microwave pasteurization or sterilization cover pre-packed food like yoghurt or pouch-packed meals as well as continuous pasteurization of fluids like milk (Helmar et al., 2007). Due to the corresponding product properties either conveyor belt systems or continuous resonator systems are invented.

The possibly high and nearly homogeneous heating rates, also in solid foods (heat generation within the food) and the corresponding short process times, which helps preserving a very high quality yield advantages of microwave compared to conventional techniques. The crucial point in both processes is the control and the knowledge of the lowest temperatures within the product, where the destruction of microorganisms has the slowest rate. Due to the difficult measurement or calculation of temperature profiles it is still very seldom industrially used.
