Factors Influencing the Microwave‐Induced Expansion of Starch‐Based Extruded Pellets under Vacuum

Abstract

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.

Process Efficiency and Energy Consumption During the Extrusion of Potato and Multigrain Formulations

“…Potato ingredients in the form of native starch or modified potato products are used to improve the structure and texture when expanding the pellet extrudates (Kraus et al, 2014). Potato extrudates, which can be obtained from potato flakes and potato grits, have a similar expansion rate, but a completely different texture.…”

Microwave Foaming of Materials: An Emerging Field

“…Xi Peng et al [64], evaluated the change in a dielectric loss concerning microwave power. It was observed that at a higher microwave power range of 150 W/g-200 W/g, dielectric loss factor significantly jumped to over 0.3 even at a lower system temperature of 25 • C and remained at a steady-state up to temperature 160 • C. However, at a short microwave power range of 75 W/g to 150 W/g, the dielectric loss remained significantly low between 0.05 to 0.1 and increased when the process temperature was greater than 80 • C. In another similar study [65], microwave power's effect on the pore distribution was evaluated. The pore distribution results showed that there is a significant increase in the nucleation rate of vapor bubbles at high microwave power, which led to a significant increase in the number of pores within small areas.…”

Influence of Sucrose Content on Expansion of Extruded, Starch‐Based Pellets during Microwave Vacuum Processing

“…This temperature corresponds to a final moisture content of the pellets of approximately 5% (Kraus et al . ). Hence, when the microwave power input is stopped, the lowest T g,mixture is 100°C.…”

“…Every microwave vacuum-drying and expansion process in this study was stopped automatically when a surface temperature >120°C of the pellets was reached. This temperature corresponds to a final moisture content of the pellets of approximately 5% (Kraus et al 2014a). Hence, when the microwave power input is stopped, the lowest Tg,mixture is 100°C.…”

“…As discussed before, the differences in moisture sorption due to the addition of up to 20% sucrose are quite small. In addition, as shown in Kraus et al (2014a), the surface temperature development of the pellets during processing is mainly dependent on the moisture content of the matrix. Altogether, we assume only a very small change in water vapor pressure due to a change in water activity or temperature development during expansion upon the addition of sucrose to the matrix.…”