Microwave-assisted drying

Abstract

AD is a common and well established industrial drying process. As idealised in Fig. 8.1, a typical drying curve of a foodstuff can be subdivided into three periods. After an initial phase (IP), a steady state is reached in which the drying rate (evaporated water over time) is constant. Therefore, this period is called "constant rate period" (CRP). In this period the product surface is still wet and the passing airflow transfers the heat for water evaporation to the surface. Simultaneously, the air is collecting the water vapour and is transporting it away from the product surface out of the dryer. This is a simultaneous heat and mass transfer. To keep the surface wet, enough water has to flow from the inside of the food to its surface. In the constant rate period this water flow is mainly capillary-driven and the surface temperature of the product is theoretically constant. As the water evaporation consumes energy, the surface temperature is typically significantly lower than the air temperature. The temperature profile in Fig. 8.1 A is adopted from a hot air drying process of carrot slices and it shows that this constant temperature period is not necessarily exactly reflected in real data. The factors that determine and limit the drying rate in the CRP are: the state of the air (temperature and relative humidity) as well as air velocity. By changing any of these parameters the drying process may be accelerated (or delayed) considerably.

Effect of convective, infrared and microwave heating on drying rates, mass transfer characteristics, milling quality and microstructure of steam gelatinized Paddy

“…The quick energy absorption caused faster evaporation of water, creating an outward flux of rapidly escaping vapor. Similar results have been reported by Gaukel, Siebert, and Erle () and Wang and Sheng (). The h m was increased with an increase in power density, which might be due to the formation of internal cracks, and fissures in the starch granules as confirmed from SEM of the rice starch (Figure ).…”

“…The vapor inside the kernel was transported to the paddy surface causing deformation in the internal starch granules. The extent of formation of cracks and fissures was dependent on microwave power density, heat and moisture transport, high temperature, and pressure inside the starch granules (Gaukel et al, ; Wang & Sheng, ).…”

“…The Bi was found to be in the range of 0.340-2.791 during HAD, IRHD, and MWD (Table 3). The Bi greater than 0.1 allowed to use the Equation (8) Gaukel, Siebert, and Erle (2017) and Wang and Sheng (2006). The h m was increased with an increase in power density, which might be due to the formation of internal cracks, and fissures in the starch granules as confirmed from SEM of the rice starch ( Figure 9).…”

Evaluation of microwave toasting of corn flakes

“…In microwave treatments is usual to inform the level of output power and time applied or instead the value of output energy. The amount of absorbed energy is rarely reported, because it depends on several factors such as oven design, dielectric properties, size and shape of the product, and moisture content of the material, among others variables (Gaukel, Siebert, & Erle, ).…”

“…Taking the limit for t ! 0 in the Equation 1 the inverse relationship between k 1 and the initial drying rate (V i ) is obtained: (Gaukel, Siebert, & Erle, 2017).…”