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.

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.