Strategies for Enhancing Expansion in Starch-Based Microcellular Foams Produced by Supercritical Fluid Extrusion

Supercritical fluid extrusion (SCFX) is a recent technological development for production of expanded starch-based foams in which formation of a microcellular structure is achieved by injection of supercritical CO2 into the melt. The high effective diffusivity of CO2 in the porous matrix favors escape of the gas to the environment, reducing the amount available for diffusion into the bubbles, thus posing an important challenge. This study utilized two approaches to address this problem: (a) increasing the nucleation rate and thus the final bubble density in the foam, and (b) reducing the melt temperature. The former was achieved by decreasing the nozzle diameter in order to achieve a higher pressure drop rate as the starch-CO2 melt flows through the nozzle. The second approach was evaluated by introducing a cooling zone prior to the entry of the melt into the nozzle. Bubble density increased more than fourfold when the nozzle radius was decreased from 3.00 to 1.50 mm. A higher bubble density led to a greater barrier or resistance to diffusion of CO2 to the environment, and increased expansion ratio by as much as 160%. Cooling of the melt resulted in a decrease in diffusion coefficient of CO2 in the starch melt, and thus reduced CO2 loss to the environment. The expansion ratio increased by 34% as the melt temperature decreased from 60 to 40°C. The above-mentioned strategies can be useful in controlling and enhancing expansion, which ultimately determines the textural attributes of the expanded food product.