Moisture microwave-assisted drying

To date, only microwave hot-air dryers used principally in flnish drying processes to level-off the moisture content in pasta, crackers or chips, and microwave vacuum dryers custom-made to specific product requirements, are available commercially (Ratti, 2008). Good microwave-assisted drying equipment should have a uniform distribution of the microwave field, provide a movement of the material to be dried, keep the microwave field strength in the drying chamber lower than the dielectric breakdovm strength, and possess an economic dehumidification system. 

Moisture and temperature history relate to quality changes during microwave-assisted drying processes and have, therefore, been intensively investigated. Their... 

Compared to its conventional counterpart, moisture can be removed at higher rate during microwave-assisted drying, as shown in Fig. 9.12. 

Fig. 9.15 Variation of specific energy consumption and energy efficiency with moisture content during microwave-assisted drying.

Table 9.2 presents the SEC and drying efficiency values for different microwave-assisted drying processes of food materials, as determined by various research groups. The minimal theoretical value of SEC for a dryer is 2.3 MJ per kg of evaporated water, based on the maximal theoretical specific moisture extraction ratio (SMER) of 1.55 kg per kWh and the latent heat of water evaporation at 100 °C (Holtz et cd., 2009). These results are expected to depend heavily on the scale and design of dryers, as well as on the method used to measure energy consumption (Durance and Wang, 2002). 

Several groups have correlated the volume or size changes of food materials during microwave-assisted drying with the moisture content (Tab. 9.3). 

Li et al. (2010) investigated the microwave-assisted convective drying of apple slices with time-adjusted power in feedback temperature control. Three desirable temperatures of 75, 65, and 55 °C were chosen, and the corresponding maximum power requirements were set at 400, 300, and 240 W, respectively. The relationships of power versus time and of power versus moisture content (d.b.) can be described by the following equations, respectively ... 

Subsequent to freeze stabilization, wet books can be thawed and air dried with or without interleaving or they can be vacuum dried in heated chambers (5). The moisture in frozen books can be sublimed by freeze drying (10) or removed by a vacuum/thaw/outgassing process (8), with microwave energy (6, 14), with dielectric energy (6), or by solvent extraction with or without vacuum assistance (5). 

---