Algal polysaccharides are versatile hydrocolloids widely used in both food and non-food applications. Their functional properties, particularly viscosity and gel formation, depend strongly on their molecular weight. While it is well known that isolation and processing can decrease molecular weight and thereby alter viscoelastic behavior, much less is understood about the long-term stability of these polysaccharides during storage as dry powders. Such information is essential for establishing best storage practices and for assessing the reliability of samples used many years after preparation.

In this study, 24 polysaccharide samples were stored as dry powders for 5 and 10 years at room temperature, +4 °C, -20 °C, and -80 °C. Molecular weights were measured by high-performance size-exclusion chromatography, and samples included common algal polysaccharides, such as agarans, carrageenans, and alginates, as well as pullulan, dextran, and pectin for comparison. Comprehensive characterization was performed by NMR spectroscopy, while mineral part composition and sulfate content were quantified by ion chromatography.

Our results show that long-term storage can substantially reduce the molecular weight of polysaccharides, particularly in samples containing low levels of stabilizing metal ions. In some cases, molecular weight decreased by more than 50% over 10 years. Storage at -20 °C significantly limited degradation, whereas samples stored at -80 °C exhibited no measurable decrease in molecular weight even after a decade. For commercial agaroses, the extent of degradation correlated strongly with sulfate content.

These findings highlight the critical importance of low-temperature storage for preserving the molecular integrity of polysaccharides and ensuring long-term sample stability. This insight is especially valuable for manufacturers and research laboratories, for whom the long-term functional reliability of polysaccharide-based materials is essential