Evaluation of drying technologies for retention of physical quality and antioxidants in asparagus (Asparagus officinalis, L.)

The objective of this research is to evaluate drying methods that have the potential of adding value to green asparagus especially for use as ingredient in instant foods or as a nutraceutical product. Five drying methods were used: namely, tray drying (TD), spouted bed (SB) drying, combined microwave and spouted bed drying (MWSB), Refractance Window (RW) drying and freeze-drying. Asparagus spears with diameters between 9 and 12 mm were blanched in 85degreesC water-bath for 3 min, sliced into 2-4 mm thickness (or pureed for RW drying), then dried to moisture content less than 0.1 db. MWSB drying produced asparagus particles with good rehydration and color characteristics, and was the fastest among the methods where heated air was used. When using MWSB drying, the power level of 2 W/g and 60degreesC heated air resulted in highest retention of total antioxidant activity (TAA). TAA of asparagus was enhanced after RW and freeze-drying, with the TAA values being significantly higher than for heated air-drying methods. In all cases, the tip portion of asparagus retained more TAA after drying than either middle or basal parts. The highest amount of ascorbic acid was retained in the product after RW drying, followed by freeze-drying, MWSB and SB drying. TD resulted in the least retention of ascorbic acid. (C) 2003 Swiss Society of Food Science and Technology. Published by Elsevier Science Ltd. All rights reserved.