THE AIR-DRYING BEHAVIOR OF FRESH AND OSMOTICALLY DEHYDRATED BANANA SLICES

Ripe banana, cut to 10 mm thick slabs were osmotically treated in suga r solutions of 35, 50 and 65 degrees Brix for 36 h. The initial moistu re content fell from a value of 3.13 kg H2O DM to 2.19, 1.63 and 1.16 kg H2O kg(-1) for treatment in the three solutions, respectively. Thes e slabs, with Total Soluble Solids (TSS) contents of 26, 34 and 39 deg rees Brix, respectively, as well as freshly cut but untreated slabs (1 5 degrees Brix) were air dried in a cabinet type tray drier to near eq uilibrium conditions at fixed temperatures from 40 to 80 degrees C and at a constant air speed of 0.62 m s(-1). Drying was found to occur in the falling rate period only for both banana types and two drying con stants K-1 and K-2 were established for a first and second falling rat e period of drying. Increasing the drying air temperature significantl y enhanced the drying rate and the K-values, except at 80 degrees C wh en the rates fell, possibly because of case hardening of the slabs. Re ducing the slab thickness also improved the drying rate, but increasin g the air speed to 1.03 m s(-1) did not have any profound effect. As t he sugar content of the banana slabs increased through the osmotic tre atment, drying rates fell. Calculated apparent moisture diffusivities at 60 degrees C ranged from 34.8 x 10(-10) m(2) s(-1) (fresh slab) to 8,8 x 10(-10) m(2) s(-1) for dried (39 degrees Brix) slabs. The moistu re diffusivity was significantly lowered as the moisture content dropp ed in drying and with increased levels of sugar. Previously osmosed an d then air dried banana slabs showed appealing colour and texture comp ared to the fresh banana.