Physicochemical Characterization of Freeze-Dried Low-Molecular-Weight Lychee Polyphenol/Cyclodextrin Systems with Enhanced Antioxidant Activity

Low-molecular-weight lychee polyphenol (Lyp), a standardized oligomeric polyphenol derived from Litchi chinensis fruit, possesses potent antioxidant activity but is susceptible to physicochemical instability, which may limit its practical application. In this study, freeze-dried systems of Lyp with α-, β-, and γ-cyclodextrins (CDs) were prepared to investigate the effects of cyclodextrins on the physicochemical properties and antioxidant activity of Lyp. Powder X-ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, near-infrared spectroscopy, and solution-state NMR were employed to characterize the prepared systems. Freeze-drying produced amorphous Lyp/CD systems accompanied by enhanced thermal stability and changes in the hydrogen-bonding environment compared with the corresponding physical mixtures. Solution-state 1H NMR and NOESY analyses suggested molecular association between Lyp constituents and cyclodextrins, with the βCD system exhibiting the most pronounced spectral changes. Consistent with these physicochemical findings, the freeze-dried βCD system showed the greatest enhancement of DPPH radical scavenging activity among the three cyclodextrins examined. These results suggest that βCD provides the most favorable molecular environment for Lyp, leading to improved physicochemical characteristics and antioxidant performance. The present findings demonstrate that freeze-drying combined with β-cyclodextrin is an effective strategy for improving the physicochemical performance and antioxidant functionality of low-molecular-weight lychee polyphenol.