Water Status and Flavor Evolution in Vacuum Freeze-Dried Mulberry: A Potential Flavor Transition Point

Vacuum freeze drying significantly influences the water status and aroma characteristics of mulberry. This study monitored the dynamic changes in water distribution, non-volatile components, and key aroma-active compounds during a 72 h drying process. The results showed that free water decreased from 85.3% to 5.96% at 12 h and then leveled off, marking the entry of the drying process into a slow moisture-change stage. This transition in water status coincided with marked changes in non-volatile and volatile components. After 12 h, the decreases in most free amino acids and reducing sugars, as well as the consumption of linoleic acid, slowed markedly after 12 h; the contents of fruity compounds (ethyl isovalerate, β-ionone) and hexanal decreased by more than 50% within the first 12 h and then stabilized, whereas the lipid oxidation product (E)-2-nonenal began to increase continuously after 12 h. Correlation analysis revealed that free water and bound water were positively correlated with fruity compounds and (E)-2-nonenal, respectively, while linoleic acid was negatively correlated with (E)-2-nonenal. Principal component analysis and hierarchical cluster analysis further supported the identification of 12 h as a potential turning point for both water status and flavor transformation. Collectively, these findings suggest a coupling relationship between water status and flavor evolution during drying, providing a basis for optimizing vacuum freeze-drying processes to better preserve the characteristic aroma of mulberry.