Energy utilization and microbial reduction in a new film drying system

Experiments were conducted with pureed pumpkin to evaluate energy efficiency and microbial reduction effect of a new thin-film Refractance Window ™ (RW) drying method. The RW drying system was designed based on a novel concept that uses hot water circulating beneath and in contact with a transparent plastic conveyor belt on which a thin film of puree is dried. In the energy study, water temperature, water circulation velocity, puree temperature, and puree moisture content were determined. In both pilot and commercial scale RW dryers with circulating water at 95C, drying of pumpkin puree from 80% to 5% moisture content (wb) was achieved in less than 5 min. The Refractance Window ™dryer demonstrated 52% to 70% energy efficiency. The pilot scale unit was used to evaluate the effect of RW drying on microbial reduction. At a circulating water temperature of 95C, RW drying of inoculated pumpkin purees resulted in at least 4.6, 6.1, 6.0, and 5.5 log reductions of total aerobic plate counts (APC), coliforms, Escherichia coli, and Listeria innocua, respectively.