Study on structural characterization and immunological activity of lychee pulp crude polysaccharides from different drying methods

Abstract The effects of hot-air drying (AD), freeze-drying (FD), and microwave vacuum freeze-drying (MFD) on the structural characteristics and immunomodulatory activities of crude polysaccharides from lychee pulp were evaluated in this study. Notably, all samples assessed in this work are crude polysaccharide extracts, which may contain residual proteins and phenolic compounds. While monosaccharide profiles were comparable among all treatment groups, significant losses of arabinose and galacturonic acid, along with pronounced degradation of pectic domains and backbone glycosidic linkages, were detected in samples subjected to AD. Higher molecular weight was retained in FD treated samples. In comparison, glycosidic linkages and molecular weight closest to those of freshly prepared polysaccharides were maintained in MFD treated samples. Analyses via spectroscopy, X-ray diffraction, and microscopy indicated that MFD treatment was associated with better preservation of morphological integrity relative to AD and FD treatments among the tested drying methods. In immunocompromised mouse models and RAW264.7 macrophages, polysaccharides prepared via MFD exhibited enhanced immunomodulatory phenotypes relative to samples from AD and FD treatments in the tested assay systems, as evidenced by improved immune organ indices, ameliorated tissue architecture, enhanced macrophage proliferation and phagocytic capacity, and upregulated expression of both pro- and anti-inflammatory cytokines. Overall, MFD treatment was associated with better retention of structural features and immunomodulatory phenotypes in the tested models relative to AD and FD treatments among the drying methods evaluated.