Gelation behaviors of some special plant-sourced pectins: A review inspired by examples from traditional gel foods in China

Pectins are acidic hetero-polysaccharides present in plant primary cell walls. With strong and interesting gelation capacity, some plant-sourced pectins including Nicandra physalodes (Linn.) Gaertn. pectin (NPGP), Premna microphylla turcz pectin (PMTP), and creeping fig pectin (CFP), have been traditionally used in making gel foods in China. However, little is known about their gelation behaviors and the mechanisms involved. In this review, the extraction, structures, gelation behaviors and the underlying mechanisms of NPGP, PMTP and CFP are introduced. Moreover, the structure-property relation of major pectin types (i.e. commercial citrus and apple pectins) is also summarized for providing a deep understanding on pectin gelation. Both the intrinsic pectin structures and extrinsic environmental factors significantly affect the gelation of pectins. The interplay between these factors is also important, which sometimes can impart pectins with unexpected gelation behaviors. For NPGP, PMTP and CFP, all of them possess a relatively high galacturonic acid (GalA) content and a very low DM value, which allow them to behave like the typical “polyelectrolytes” and make them highly sensitive to the presence of cations. Monovalent cations (e.g. Na+ and/or H+) are able to induce the gelation of these pectins at appropriate conditions. Besides, NPGP, PMTP, and CFP can also form Ca2+-induced gels at ambient temperature with a moderate gelling rate, which is rarely reported for citrus and apple pectins. The underlying mechanism is still unclear at this moment. Therefore, the structure-property relation on pectin gelation needs to be further explored.