Gelatin dissolution is more affected by the structure than the water mobility
Résumé
Pig skins gelatin results of the collagen solubilization. Gelatin is used, amongst other applications, in the pharmaceutical industry to make hard capsules. Raw gelatin, on delivery, contains about 8-12% of water [1] and is a partially-crystalline polymer being composed of an amorphous phase (coil structure with primary chains) and a crystal phase [2]. The latter consists of partially reformed triple-helices of collagen. The shelf life of hard capsules is evaluated through gelatin USP dissolution test after aging under high temperature and high humidity conditions. After aging, the dissolution rate of gelatin is variable: either unchanged or reduced. The decrease of gelatin dissolution rate is partially due to intra- and intermolecular crosslinks formation in the amorphous phase which stabilize that structure [3]. Water in gelatin plays a role of plasticizer in the amorphous phase and is either free or bound to the polymer chains. The state of water in gelatin has been studied but no relation with aging or dissolution rate has been established. The aim of the present study was to understand how aging affects the gelatin structure and its water mobility, and in addition, to identify structural differences according to gelatin dissolution rate.
Thus, sixty pig skin gelatins with correct and non-correct dissolution rate were analyzed using Differential Scanning Calorimetry (DSC). Proton Nuclear Magnetic Resonance (1H-NMR) spectroscopy was used on a subgroup of twelve gelatin films to measure the spin-spin relaxation time (T2). For both methods, samples were taken before and after aging.
Aging decreased the amount and the stability to heating of crystals phase and increased the amorphous phase. The water mobility was increased which can be explained by the denaturation of crystals releasing the trapped water from the triple-helices [4]. After aging, the gelatins which failed the dissolution test showed a higher amount of amorphous phase but the water mobility was not modified. These results showed that the structure plays a major role in the gelatin dissolution rate. In order to support this major role of structural characteristics of gelatin, the conformation of the single chains and multiple helices will be assessed using circular dichroism.