Review: Harnessing alginate for the encapsulation of extremely oxygen-sensitive probiotics

Next-generation probiotics (NGPs) have been a matter of great interest in recent years thanks to their therapeutic potential. However, some of them are extremely oxygen-sensitive (EOS), which poses major challenges for their production, processing, storage, and colonic delivery. Interestingly, alginate appears to be an excellent candidate for encapsulating EOS probiotics in terms of its oxygen barrier and release behavior. Despite the widespread use of alginate for encapsulation, the release mechanisms of alginate under gastrointestinal conditions remain unclear. In this context, this review first provides a comprehensive description of the gelation mechanisms and release behavior of the alginate gel matrix under gastrointestinal conditions. Alginate gelation mechanisms vary depending on the type of cation involved, including H⁺-induced gelation, or ionotropic, electrostatic, and covalent crosslinking. The release of the alginate matrix in gastrointestinal fluid happens due to its electrophoretic properties and ion-exchange phenomena, influenced by intestinal pH and the presence of cations throughout the digestive system. In addition, this review summarizes strategies for encapsulating NGPs using alginate. Although probiotic encapsulation strategies using alginate have been proposed in previous studies, the protection mechanisms behind these formulations require further elucidation, especially the impact of each component in the alginate encapsulating matrix on the probiotic viability.