Interaction of novel hydrocolloids from indigenous New Zealand ferns with κ-carrageenan and low-acyl gellan gum

Abstract

Growing demand for sustainable hydrocolloids has spurred the exploration of polysaccharides from New Zealand's diverse fern flora. This study investigates interactions between polysaccharides extracted from Sphaeropteris medullaris (mamaku gum, MG) and Parablechnum novae-zelandiae (kiokio gum, KG), with κ-carrageenan (κC) and low-acyl gellan (LAG), selected from a preliminary screening of commercial hydrocolloids. MG and KG are non-gelling anionic glucuronomannans exhibiting unusual shear-thickening behaviour, but their interactions with other polysaccharides remain unexplored. Mixtures were characterised using rheology, TPA, FTIR, cryo-SEM, syneresis, water-holding capacity (WHC) and freeze–thaw stability across different blend ratios, temperatures, ionic strengths, and pH values. Crude and purified forms of MG and KG impacted differently in these mixtures; κC and LAG gels containing crude fern gums were significantly firmer, with functionality dominated by endogenous ions. Conversely, κC and LAG gels containing purified fern gums were softer, heterogeneous and displayed micro-phase separation, which was exacerbated at pH 3 and pH 9. These findings are consistent with a segregative interaction resulting in a phase-separated gel network. From a formulation perspective, purified fern gums were able to modulate gel microstructure and functionality independently of ionic effects, offering advantages for ion-sensitive formulations. The LAG-purified MG blend also maintained its WHC after one freeze-thaw cycle, highlighting potential for frozen products. Overall, the distinct behaviour of crude and purified fern gums offers an adaptable approach to texture modification in mixed polysaccharide systems.