Retention of volatile compounds in freeze‐drying slabs of malto‐dextrin

Summary A study was made of the effect of a number of process variables on the retention of some volatile compounds during freeze‐drying of ‘model’liquid foods. As a model liquid food, aqueous solutions of malto‐dextrin were chosen. To these solutions acetone, methanol, n‐propanol and n‐pentanol were added in low concentrations. The retention of volatiles strongly increased with increasing molecular size of the volatiles but was independent of the relative volatility. For constant dissolved solids concentration and constant temperature at the sublimation front in drying slabs or granules, process conditions leading to a maximum drying rate always resulted in maximum volatile retention. Retention increased with decreasing freezing rate when solids concentration, sublimation front temperature, and drying rate were constant. Retention always increased with decreasing temperature of the sublimation front. In drying granules, retention exhibited a maximum with respect to particle diameter, and increased with decreasing layer thickness. In the dissolved solids concentration range between 5 and 20% (w/w) the retention strongly increased with increasing concentration, whereas above 20% the retention became independent of initial solids concentration. The observed effects of process variables on volatile retention can be explained qualitatively with the selective diffusivity concept.