Impact of ultrasound pretreatment on banana chip production by convective drying using hot air: Consideration of shrinkage and variable effective mass diffusivity in the process description

In this study, we used experimental analysis and numerical simulation to describe and compare the convective drying (CD) of banana slices, without and with pretreatment with ultrasound (US) at 25 kHz. Banana slices, with an average initial thickness of about 6.60 x 10-3 m and an average diameter of about 3.01 x 10-2 m, were subjected to CD at temperatures of 60, 70, and 80 degrees C, with air speed of 1.5 m s-1. Considering such dimensions, the one-dimensional diffusion equation in Cartesian coordinates was numerically solved by the finite volume method, using a fully implicit formulation (direct problem). A solver with a numerical solution was developed for the direct problem. This solver was coupled to an optimizer to determine the parameters a and b of functions that relate the effective moisture diffusivity (D) with the local moisture ratio, and also to determine the convective mass transfer coefficient (h) (inverse problem), considering the shrinkage. The proposed model was used to describe the data obtained. The results revealed that the drying rate increased in samples that were subjected to pretreatment with US, for each drying temperature. The best-simulated results were those that related D to the local moisture ratio by means of a bcosh (aMR2) function. It could be observed that pretreatment with US allowed saving of time (and consequently energy) of 28.3%, 16.0%, and 12.4% at drying temperatures of 60, 70, and 80 degrees C, respectively.Practical applicationsThe main contribution of this paper is to present a solver and an optimizer to determine the mass transfer properties considering the shrinkage during convective drying of banana pieces pretreated with ultrasound by the inverse problem. With the information obtained for the mass transfer properties and the proposed numerical model with a hyperbolic cosine type function for the effective moisture diffusivity, it is possible to more accurately determine the time and, therefore, the energy savings of processing banana chips pre-treated with ultrasound. Convective drying of banana slices with US application is numerically simulated. The technique uses an optimization algorithm to determine D and h, rather than conventional slope-based methods. A cosine-type function was adequate to describe D's behavior. Simulated results are in agreement with experimental results. image