OSMO-CONVECTIVE DRYING OF GRAPES

In order to reduce browning of grapes during drying, a special drying method was developed and evaluated using a laboratory scale fluidized bed dryer. Fresh Thompson seedless grapes were initially dried by imme rsion in a fluidized bed of sugar. The mass ratio of grapes to sugar w as 1:1. The flow rate of hot air (at 45 and 60-degrees-C) was sufficie nt to fluidize the sugar bed, while grapes placed on the screen, 3 cm above the drying air distributor, remained generally stationary. Due t o the simultaneous osmotic and convection drying effects, the drying t ime was reduced by factor approximately 1.5 as compared to drying unde r the similar conditions without added sugar. A special pre-treatment of dipping of grapes in ethyl oleate (2% solution in 0.5% sodium hydro xide) at 80-degrees-C for 30 s further reduced the drying time by fact or 2 in both cases. The color of osmo-convective dried grapes were com parable to that of sulfur dioxide treated grapes. The texture of osmo- convective dried grapes was more pliable (softer) than convective drie d samples. The major problem associated with the osmo-convective dryin g of grapes on a sugar bed was the stickiness, caused by sugar, on the fruit surface. This was reduced by partially substituting sugar with semolina (maintaining a 1:1 ratio) to create fluidized bed.