The relation between sublimation rate and volatile retention during the freeze-drying of coffee

The relationship between mass transfer of water and the retention of selected volatile compounds has been studied in coffee solutions undergoing freeze-drying. A statistical experimental design enabled the evaluation of the influence of five processing parameters pressure, heating temperature, freezing rate, initial solid contents, and thickness of ale sample) on ice sublimation rate and volatile retention. Five marker volatile compounds were added to coffee extract, which was previously stripped of all volatile compounds. The solution was then freeze-dried in trays, and the amount of each volatile retained was measured using gas chromatography (GC). Models that contained process variables in linear, quadratic, and interactive terms described the results. Temperature, pressure, and initial solid content were found to have a strong influence on sublimation rate and volatile retention. Temperature showed a direct and significant influence on the sublimation rate and volatile retention, On the other hand, an increase in solid content and pressure resulted in a decrease in sublimation rate as well as volatile retention. Thickness and freezing rate did not show a significant influence on either parameter. A more fundamental explanation to account for these trends is being developed using theories of heat and mass transfer.