Moisture sorption and desorption properties of gelatin, HPMC and pullulan hard capsules

The moisture sorption and desorption properties of hard capsules have a great influence on the quality of capsule products. However, studies on them have rarely been reported. Herein, we studied the moisture sorption and desorption properties of three kinds of hard capsules (gelatin, hydroxypropyl methyl cellulose (HPMC) and pullulan capsules) in terms of hygroscopicity, crystallinity, thermal behaviors and so on. It is found that HPMC capsules have weaker moisture sorption ability and moisture keeping ability than pullulan or gelatin capsules with lower moisture sorption rates, equilibrium moisture contents, moisture keeping rates and higher critical relative humidity. In comparison with gelatin capsules, pullulan capsules have weaker moisture sorption ability and comparable moisture keeping ability. HPMC or pullulan capsules can more effectively protect high, moderate and low hygroscopic capsule contents (chitosan, potato starch or ethyl cellulose) from outside moisture absorption. The diffraction peaks of the moisture equilibrated gelatin, HPMC and pullulan capsules are much smaller than those of their dried ones. The dried and the moisture equilibrated gelatin, HPMC or pullulan capsules all have smooth surface morphology. HPMC or pullulan capsules can be an attractive alternative to animal gelatin capsules due to their appropriate moisture sorption and desorption properties.