Unripe Banana Flour Produced by Air-Drying Assisted with Ultrasound - Description of the Mechanisms Involved to Enhance the Mass Transfer in Two Approaches

In this research the application of ultrasound, prior to air-drying, describing the phenomena in two approaches: unripe banana slices immersed in water (hydrated) and vacuum packaged (non-hydrated) were investigated. The results showed two falling rate periods during the air-drying. For the second rate period, an increase of water effective diffusivity due to the "sponge effect" (direct effect of ultrasound) and the microscopic channels formation (indirect effect) was observed. Scanning electron microscopy images showed that untreated dried slices were less porous, characterized by small cavities and high density, while ultrasound pretreated slices resulted in a porous structure with more free volume between cells. An increase in porosity decreased the resistance for diffusion, influencing positively the water effective diffusivity. Moreover, ultrasound produced partial disorder of the crystalline structure within the starch granules, reducing the amount of energy required for gelatinization. Moreover, reduction of resistant starch content was not observed.