Mechanisms of retention of organic volatiles in freeze‐dried systems *

Abstract
Summary: Retention of organic volatiles was studied in freeze‐dried systems containing various carbohydrates ranging in molecular weight from glucose to dextrans. On the basis of the freeze‐drying experiments and studies on sorption and desorption after drying, a mechanism for the retention and/or loss of volatiles during drying was postulated. The mechanism involves formation of ‘microregions’in the dry cake in which the volatile is entrapped. The space between the microregions is freely passable by the volatile. The total amount of volatile retained under given conditions depends on the moisture profile during drying, and on the effective vapour pressure of the volatile in the system.The present paper considers the following molecular aspects of the formation of microregions in freeze‐drying of solids: orientation of the molecules of the solid with respect to volatile molecules prior to or during freeze‐drying; effects of size and polarity of the volatile on orientation of surrounding carbohydrate molecules; effects of mobility of these molecules under conditions of drying; molecular and environmental factors which control the moisture level at which the microregions become sealed, and subsequently impermeable to the volatile.