Drying kinetics of extruded, starch-based pellets with initial moisture content of 0.695 kg/kg d.b. were experimentally investigated in a microwave vacuum drying system. The influence of the initial sample amount (100, 200 and 300 g), microwave power (400, 600 and 800 W) and system pressure (20, 50 and 100 mbar) were examined in detail. An increase in microwave power and a decrease in initial sample amount significantly shortened the drying time. Among several thin layer models, the model of Balbay and Sahin best describes the experimental data for starch-based pellets. The activation energy was calculated to be 10.89 W/g and effective moisture diffusivity varied from 3.4 × 10 −9 to 1.2 × 10 −8 m 2 /s, depending on microwave power level and sample amount. PRACTICAL APPLICATIONThird generation snacks provide an alternative to fully prepared puffed snack foods. In contrast to conventional extrusion, nonexpanded pellets are formed via extrusion, then predried and then expanded to achieve a crispy and porous texture. In this alternative process, the extrusion can be performed at high moisture content, moderate temperature and shearing conditions. This is beneficial for the preservation of thermally and mechanically sensitive ingredients and allows the production of snacks with high nutritional quality. A special feature of the process investigated in this study is that the expansion is achieved by microwave-vacuum processing which allows an indirect expansion without a time-and energy-consuming predrying process. In addition, due to the processing under vacuum, the temperature inside the product can be kept at desired moderate values. The drying kinetics, drying rate and diffusion coefficient in connection with the volume expansion allow process design and product quality improvement. Rother et al. 2009;Yan et al. 2010) and grains (Wu and Schwartzberg 1992;Singh and Singh 1999;Hoke et al. 2005) or fish (Zhang et al. 2007). In this way, cells or other toplevel structures act as vessels enabling pressure build up inside the product leading to expansion. The other kind is microwave-induced expansion of starch-based intermediate products, e.g., doughs (Ressing et al.

The influence of the degree of gelatinization (DG) on microwave‐induced expansion of starch‐based, extruded pellets under vacuum was investigated. Expansion of extruded pellets with initial DG between 43.6 and 72.9% was investigated in a microwave vacuum drying system. With increasing DG, volume expansion showed a linear increase. The number of pores significantly increased at DG above 64.2%. A larger amount of water available might be the reason for the increased nucleation at large DG. The pore sizes decrease with increasing number of pores. If a larger number of bubbles is nucleated, a fewer amount of water can diffuse into every single bubble possibly resulting in lower water vapor pressure. No relationship between rheological and expansion properties of the matrix could be observed. We therefore assume that nucleation has the strongest effect on pore size distribution. Practical Applications In contrast to direct‐expanded extruded snacks, third‐generation snacks are generally produced by predrying nonexpanded, extruded pellets, followed by expansion to achieve a crispy and porous texture. By this procedure, extrusion can be performed at high moisture contents, moderate temperatures and moderate shearing conditions. This way, thermal‐ and mechanical‐sensitive ingredients can be preserved. A distinctive feature of the process investigated in this study is that the expansion is achieved by microwave vacuum processing, which allows an indirect expansion without the time‐ and energy‐consuming predrying process. This study shows that total volume and inner structure of the expanded pellets is influenced by the degree of gelatinization. With these information, producers can turn structure‐related properties such as appearance, crispiness and crunchiness to targeted values.

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The microwave‐induced expansion of starch‐based extruded pellets under vacuum was investigated in order to understand the underlying mechanisms. Microwave power (400, 600 and 800 W), system pressure (20, 50, 100 and 190 mbar) and initial sample mass (100, 200 and 300 g) were selected as influencing factors. During processing, the surface temperature of the matrix mainly depended on the moisture content, independent of the process parameters applied. A kinetic factor τ was calculated, which accounts for the time necessary to remove 1 g of water from the product. This factor is a reasonable measure for the kinetics of mass transfer. The higher the power absorption of the product, the smaller the values for τ and the faster the kinetics for heat and mass transfer. This can cause a higher supersaturation of the matrix before nucleation occurs. In turn, a higher number of bubbles can be nucleated, and the resulting pore areas are shifted to smaller values. While decreasing τ from 22.6 to 5.9 s/g H2O, volume expansion increases from 0.82 to 1.49 with an increased number of pores (1.7/mm2 to 3.4/mm2) created, even when the pore areas are comparably smaller. System pressure does not significantly alter the expansion because in the investigated range, neither nucleation nor bubble growth is changed. However, it is important to keep the system pressure below a certain value (190 mbar); otherwise, coagulation of the moist pellets occurs, thus rendering the process unstable. Practical Applications In contrast to directly expanded extruded snacks, third‐generation snacks are produced by predrying nonexpanded, extruded pellets, followed by expansion to achieve a crispy and porous texture. Using this process route, extrusion can be performed at high moisture content, moderate temperature and shearing condition. This is beneficial in terms of preserving thermally and mechanically sensitive ingredients. A special feature of the process investigated in this study is that the expansion is achieved using microwave vacuum process, which allows an indirect expansion without the time‐ and energy‐consuming predrying process. This study shows that the total volume and inner structure of the pellets is influenced by process parameters in the expansion step, thus allowing producers to turn structure‐related properties, such as crispiness and crunchiness, into target values.

Modular Drying Processor for Application of Combined Drying ProcessesA modular drying processor for the development of energy and time efficient combined drying processes was constructed.It allows the serial and/or parallel application of different drying methods such as freeze-, hot-air-, infrared-, contact-, microwave-or vacuum-drying without interruption between the processes. Depending on the drying method precise product structures could be achieved by using the example of carrot cubes. The serial combination of freeze-and microwave vacuum drying shortened the process time about 40 % while highest product quality was obtained.

The influence of sucrose content on microwave‐induced expansion of starch‐based, extruded pellets under vacuum was investigated. Expansion of extruded pellets with different sucrose contents (0, 10 and 20%) was investigated in a microwave vacuum‐drying system. With increasing sucrose content, volume expansion showed a nearly linear increase. Also, the number of pores increased with increasing sucrose content. A softening of the matrix might facilitate nucleation. Pore areas decrease with increasing number of pores. We therefore assume that nucleation has the strongest impact on pore area distribution. Mainly pillow‐shaped and round final products were found. The shape of the final products could be related to the glass transition temperature Tg of the intermediate products and to heating and drying characteristics of the pellets during microwave vacuum processing. Tg decreases with increasing sucrose contents. If expansion of the initially cylindrical pellets takes place within the rubbery state, round final products are formed. If a glassy outer layer is built due to evaporation cooling of the surface and high Tg of the matrix, the pellets rupture upon expansion and the pillow shape forms. Practical Applications In contrast to direct expanded extruded snacks, third‐generation snacks are generally produced by pre‐drying nonexpanded, extruded pellets, followed by a separated expansion process. By this process route, extrusion can be performed at high moisture contents, moderate temperatures and moderate shearing conditions. This is beneficial in terms of preserving thermal‐ and mechanical‐sensitive ingredients. A special feature of the process investigated in this study is that the expansion is achieved by microwave vacuum processing that allows an indirect expansion without a time‐ and energy‐consuming pre‐drying process. This study shows that total volume and inner structure of the expanded pellets is strongly influenced by the amount of sucrose added to the starch‐based matrix. With these information producers can turn structure‐related properties such as appearance, crispiness and crunchiness to target values.

Development of a Continuous Microwave Vacuum Drying Processor with a Hexagonal Cross-Section of the CavityA continuous microwave vacuum drying processor with a hexagonal cross-section of the cavity was constructed. Due to the geometry of the cavity and the continuous movement of the product, homogeneous microwave power absorption of the product can be favored. By the expansion and simultaneous dehydration of non-expanded, starch-based extrudates, the applicability of the constructed processor could be shown.