Structural stability and release properties of emulsion-alginate beads under gastrointestinal conditions

Summary

Ca-alginate beads are promising vehicles for targeted release, amongst others for oil. The release pattern is largely determined by the structural stability of such hydrogels (i.e., shrinking, swelling, or even disintegration), albeit that these processes are poorly understood. We designed a range of alginate hydrogel beads with different alginates (M/G ratio 1.2 or 2.1) and oil content (20–54%), and monitored their behaviour under simulated gastrointestinal conditions (INFOGEST protocol).

Low M/G ratio emulsion-alginate gels were mechanically stronger than their counterparts prepared with high M/G ratio alginate. Beads prepared with high M/G ratio alginate swelled and disintegrated under gastric and intestinal conditions, with a high oil load contributing to reduced structural stability. This could be mitigated by using a higher Ca2+ concentration (10 mM), confirming that calcium plays a key role in structural stability.

The actual release of fatty acids was co-determined by the properties of the hydrogel and their disintegration, which was further enhanced by extended oil digestion. Depending on the release pattern required to achieve a target, both mechanisms can be considered. For release in the more distal gastrointestinal (GI) tract, the structure needs to be maintained for longer, which is possible through the use of low M/G ratio alginate in combination with higher Ca2+ concentration (10 mM). These insights contribute to the development of alginate-based vehicles that can deliver their payload target to the small intestine or colon.