OSMO-CONVECTIVE DRYING OF GRAPES |
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Authors: | S Grabowski AS Mujumdar HS Ramaswamy C Strumillo |
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Affiliation: |
a Department of Food Science, Macdonald Campus of McGill University, Ste Anne de Bellevue, Quebec, Canada
b Faculty of Process and Environmental Engineering, Lodz Technical University, Lodz, Poland
c Department of Chemical Engineering, McGill University, Montreal, PQ, Canada |
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Abstract: | 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 immersion in a fluidized bed of sugar. The mass ratio of grapes to sugar was 1:1. The flow rate of hot air (at 45 and 60°C) was sufficient to fluidize the sugar bed, while grapes placed on the screen, 3 cm above the drying air distributor, remained generally stationary.
Due to the simultaneous osmotic and convection drying effects, the drying time was reduced by factor ∼1.5 as compared to drying under the similar conditions without added sugar. A special pre-treatment of dipping of grapes in ethyl oleate (2% solution in 0.5% sodium hydroxide) at 80†deg; C for 30 s further reduced the drying time by factor 2 in both cases. The color of osmo-convective dried grapes were comparable to that of sulfur dioxide treated grapes. The texture of osmo-convective dried grapes was more pliable (softer) than convective dried samples. The major problem associated with die osmo-convective drying 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. |
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Keywords: | Grapes fluidized bed kinetics osmosis quality |
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