An experimental investigation on the cooling curve and drying behavior of static and spin-frozen samples in freeze–drying process

An experimental study was carried out to analyze the cooling curve and drying behavior of static and spin-frozen samples (distilled water and skimmed milk) in liquid immersion freeze–drying process. The freezing process was carried out in two modes: static freezing and spin freezing. Subsequently, the immersion drying process of static and spin-frozen samples was performed in a static condition. The liquid immersion method aids in heating the entire surface area of the shell containing the frozen sample, which subsequently improves the heat transfer rate and reduces the drying time. The comparison of the cooling curve of the static and spin freezing processes showed that the supercooling phenomenon has not occurred in the spin freezing process against the static one. A detailed analysis of the drying rate, drying time, moisture content retention, heat transfer coefficient, and heat load during the drying process of static and spin-frozen samples is presented in this study. The results showed that the spin-frozen sample exhibited a larger ice-vapor surface area, lower ice thickness, higher drying rate, lesser drying time, and higher heat transfer coefficient than the static-frozen sample. The product thickness attained via spin freezing is about 88% smaller than the static one. Consequently, the drying time of the spin-frozen sample is almost reduced by 80% as compared to the static case.