Controlled low-temperature vacuum dehydration and tunnel drying: A comparative study

Controlled low-temperature vacuum dehydration (CLTVD) and tunnel drying (TD) processes were applied to mashed potatoes and the quality of the dried products was compared for both processes. Slabs of different thickness (0.26, 0.52 and 0.78 mm) were prepared. CLTVD was carried out at a low temperature avoiding freezing the product, at absolute pressure of 0.67, 1.00 and 1.33 kPa. For the TD experiments air temperatures were 40, 50 and 60° C. CLTVD and TD processes were compared using engineering and quality parameters (bulk density, water adsorption and total color difference). During the constant-rate drying period 40 to 70% of moisture was removed. The best quality product was generated at 0.26 mm, 0.67 kPa for CLTVD and 0.26 mm, 40° C for TD. For these treatments, the flux of mass was 7.8 3 10 25 and 3.3 3 102 4 kg m 22 s 21 for CLTVD and TD, respectively. The mass transfer coefficient was 6.2 3 10 26 kg m 22 s 21 ΔY 21 for CLTVD and 3.6 3 10 22 kg m 22 s 21 ΔY 21 for TD. The falling-rate drying period was divided into three sub-periods. The effective diffusivity coefficient ranged from 1.1 3 10 211 to 4.8 3 10 211 m 2 /s and from 4.2 3 102 11 to 9.6 3 10 211 m 2 /s for CLTVD and TD, respectively. A change in water transport mechanism was achieved and both processes were controlled by mass transfer; internal and external mass transfer resistances were higher for CLTVD than for TD. CLTVD yields a product with lower shrinkage and a color closer to the white standard than the products obtained by TD. CLTVD may be considered as a potential alternative for drying of temperature-sensitive materials.