Air-drying behavior of Dwarf Cavendish and Gros Michel banana slices

摘要

Air-drying behavior of untreated, and sodium bisulphite and ascorbic/citric acid treated Dwarf Cavendish and Gros Michel banana slices was investigated between 40 and 70℃. Pretreatments and increasing temperature decreased the browning, and the color change in the untreated samples was acceptable. Pretreatments and temperature did not affect the shrinkage. Cavendish samples shrank more than Michel samples at the same conditions. The drying happened in two falling rate periods. Variety of the samples did not affect the drying behavior. Effect of the heat transfer on the drying compared to the mass transfer was assumed negligible due to modified Lewis number estimated greater than 60. The drying was evaluated using the diffusion equation for an infinite slab for a Fourier number greater than 0.1. In all conditions the equation fitted the process well. The effective moisture diffusivity (D) increased with increasing temperature between 40 and 70℃ in the untreated samples. It increased between 40 and 60℃, and decreased at 70℃ in the pretreated samples probably due to case hardening and starch gelatinization above 60℃. It was estimated to be of the order of 10~(-11) and 10~(-10) m~2/s. The effect of temperature on D was evaluated using the Arrhenius equation. Activation energy (E_a) did not change with the variety. It was higher for the untreated samples than for the pretreated samples during the first falling drying period (Ⅰ. FRP), and almost the same during the second falling drying period (Ⅱ. FRP) for all samples. Value of E_a increased with transition from the Ⅰ. FRP to the Ⅱ. FRP. The surface moisture transfer coefficient exhibited the same behavior as did D. It was estimated to be of the order of 10~(-7) and 10~(-6) m/s. The whole process was controlled by the internal moisture transfer as deduced from the good fit of the diffusion equation and a Biot number greater than 10.