The air drying behaviour of fresh and osmotically dehydrated banana slices

Abstract

Ripe banana, cut to 10mm thick slabs were osmotically treated in sugar solutions of 35, 50 and 65° Brix for 36h. The initial moisture content fell from a value of 3.13kg H2O DM to 2.19, 1.63 and 1.16kg H2O kg−1 for treatment in the three solutions, respectively. These slabs, with Total Soluble Solids (TSS) contents of 26, 34 and 39° Brix, respectively, as well as freshly cut but untreated slabs (15° Brix) were air dried in a cabinet type tray drier to near equilibrium conditions at fixed temperatures from 40 to 80°C and at a constant air speed of 0.62m s−1. Drying was found to occur in the falling rate period only for both banana types and two drying constants K1 and K2 were established for a first and second falling rate period of drying. Increasing the drying air temperature significantly enhanced the drying rate and the K‐values, except at 80°C when the rates fell, possibly because of case hardening of the slabs. Reducing the slab thickness also improved the drying rate, but increasing the air speed to 1.03m s−1 did not have any profound effect. As the sugar content of the banana slabs increased through the osmotic treatment, drying rates fell. Calculated apparent moisture diffusivities at 60°C ranged from 34.8× 10−10 m2 s−1 (fresh slab) to 8.8×10−10 m2 s−1 for dried (39° Brix) slabs. The moisture diffusivity was significantly lowered as the moisture content dropped in drying and with increased levels of sugar. Previously osmosed and then air dried banana slabs showed appealing colour and texture compared to the fresh banana.