Preparation, characterization and freeze-drying protective mechanisms of natural polysaccharides as emerging lyoprotectants: A review.

Natural polysaccharides have gained considerable attention in food, pharmaceutical and biomedical applications due to their inherent renewability, biodegradability, biocompatibility, structural and functional diversity. Recently, increasing evidence has demonstrated that polysaccharides can serve as effective freeze-drying protectants for microorganisms, cells, proteins and other bioactive substances. Therefore, preparation and characterization of natural polysaccharides, and clarifying the mechanism of their freeze-drying protective effects are of great significance. This review systematically summarizes the extraction, purification and structural characterization of natural polysaccharides from plant and microbial sources. It particularly emphasizes the freeze-drying protective activity of polysaccharides and the underlying mechanism. Various extraction techniques including acid/alkali extraction, ultrasonic-assisted extraction, microwave-assisted extraction, ultrahigh pressure extraction, subcritical water extraction and enzymatic extraction have been developed to improve polysaccharide yield and quality. Property characterization is mainly achieved through analyses of molecular weight, monosaccharide composition, glycosidic linkage and morphology. In addition, polysaccharides possess freeze-drying protective property mainly through inhibition of ice crystal growth and ice recrystallization, vitrification and water replacement hypothesis. This review may provide theoretical guidance for the development of efficient, sustainable and multifunctional natural polysaccharides as lyoprotectants.