Phenomenological modeling of drying by starting-accessibility and water internal diffusion (SAWID); case of swell-drying of Tunisian thyme

Abstract

This study deals with the use of the innovative swell-drying process in the case of two different proveniences of thyme (Thymus capitatus); KEF and BEJA. This process is based on two successive stages: texturing by instant controlled pressure drop (DIC) and drying performed by hot airflow. DIC consists of subjecting the vegetable matrix to a high temperature/high pressure for a few seconds followed by an almost instantaneous pressure drop toward a vacuum of about 5 kPa. It normally causes an adequate autovaporization within the product, a controlled expansion, and a cooling of the vegetable matrix. This new structure allows the final drying step to reach much lower water content. In the two considered cases of thyme, the product structure (thickness of the leaf), the functional behavior, and drying technology aptitudes were studied. The impact of DIC conditions of total treatment time and a number of cycles was statistically analyzed using a two-factor, five-level central composite design of experiments (DoE), through the response surface methodology (RSM). DIC treatment allowed achieving four times better drying efficiency. The range of negligible external resistance (NER) allowed defining the two coupled steps of superficial starting accessibility and water internal diffusion (SAWID) phenomenological drying model. Swell-drying implied the effective diffusivity to increase up to 10 times and the starting accessibility up to twice better than the conventional hot air drying for a relative expansion ratio of almost 2. The highest DIC treatment parameters were measured to be 1 cycle/60 s for D eff and 7 cycles/20 s for δW s for both thyme proveniences (KEF and BEJA).