DEHYDRATION OF CARROTS BY A COMBINATION OF FREEZE-DRYING, MICROWAVE-HEATING AND AIR OR VACUUM DRYING

Carrot slices were dried by combining freeze drying with a short micro wave treatment and air or vacuum drying. Total dying time, and quality , parameters including color; dimensions of slices and rehydration rat io were determined. Freeze drying for 2 h at a plate temperature of 30 degrees C followed by 1.5 h at 55 degrees C was sufficient to remove all water by sublimation and reach a product moisture of about 40%. Th e partially freeze dried product was microwave treated for 50 s and th en dried to 5% moisture in vacuum or forced air. The color; dimensions and rehydration ratio of the partially freeze dried microwave treated and air dried product were similar to same quality parameters of the product freeze dried to the final moisture content. Final drying in va cuum oven had some beneficial effect on color: A considerable saving i n freeze drying time was achieved by combining freeze drying with micr owave treatment followed by air drying. Total freeze drying rime at 30 degrees C was 9.5 h as compared to a combination of 3.5-3.75 h of par tial freeze drying followed by a short microwave treatment and 3.75 h air drying. (C) 1998 Elsevier Science Limited. All rights reserved.