Dehydration of carrots by a combination of freeze drying, microwave heating and air or vacuum drying

Abstract

Carrot slices were dried by combining freeze drying with a short microwave treatment and air or vacuum drying. Total drying time, and quality parameters including color, dimensions of slices and rehydration ratio were determined. Freeze drying for 2 h at a plate temperature of 30 °C followed by 1.5 h at 55 °C was sufficient to remove all water by sublimation and reach a product moisture of about 40%. The partially freeze dried product was microwave treated for 50 s and then 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 vacuum oven had some beneficial effect on color. A considerable saving in freeze drying time was achieved by combining freeze drying with microwave treatment followed by air drying. Total freeze drying time at 30 °C was 9.5 h as compared to a combination of 3.5–3.75 h of partial freeze drying followed by a short microwave treatment and 3.75 h air drying.