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Vacuum freeze-drying is one of the best water removal methods, with final products of the highest quality. The solid-state of water during freeze-drying protects the primary structure and the product's shape with minimal volume reduction. As the leading quality problem of dehydrated green Asparagus, this experiment was to study the technique of improving the rehydration of dehydrated green Asparagus by adding a hydrophilic substance (Maltodextrin, sucrose, salt) and controlling two ways in the process of vacuum freeze-drying. The mixed solution was soaked at the rate of three different concentration ratios, i.e., 1 (10%), 2 (15%), and 3 (20%) for maltodextrin, sucrose, and salts, respectively, using the L9 orthogonal and two-factor comparison experiment. It was concluded that increasing the mass of the Asparagus samples decreased the convective heat transfer coefficient. The evolution of drying months in the range of 1.78 - 4.74 W/m2˚C was recorded for the mass of Asparagus samples. The results noted that to dry the Asparagus by vacuum freeze-dryer from 09:00 to 18:00 hour decreases the product's drying rate up to 0.011g.(H2O).g-1(d. m).cm-2.hr-1 and moisture level up to 8%. The study results noted that the pre-freezing condition was 23°C with the frozen time of 4 hours, which could remarkably improve the vacuum freeze-dried green asparagus rehydration. Finally, from the results, it was recommended that, from the actual production, to save energy, reduce costs; 23oC was better for the precooling temperature with the pre-freezing time was 4 hours for drying green Asparagus.
Keywords: Green Asparagus; Hydrophilic Substance; Rehydration; Vacuum Freeze-Drying