Optimized Isolation of White Tea Infusion Micro-Nanoparticles and Stability Mechanism: A Composition–Structure–Stability Perspective

White tea infusion micro-nanoparticles (WTMPs) are important for colloidal stability, but the optimal strategy for their isolation and the mechanisms underlying their stability remain unclear. Here, a multi-indicator TOPSIS strategy was used to optimize ultrafiltration–centrifugation, and the best condition was identified as a 100 kDa membrane, 3000× g, and 20 min. The isolated WTMPs were not merely a concentrated form of white tea colloids (WTCs), but a selectively enriched fraction with remodeled composition, more regular morphology, more ordered intermolecular organization, and improved environmental stability. In particular, gallic acid (GA) was enriched, whereas caffeine (CAF) decreased markedly after isolation. Spectroscopy and molecular dynamics simulations further suggested that GA and CAF played different roles in the protein–polysaccharide network: GA was more favorable for cooperative interfacial stabilization, whereas CAF behaved more like a locally associated ligand. Overall, these results support a composition–structure–stability relationship for WTMPs and provide mechanistic insight into the selective enrichment of GA and the enhanced stability of the isolated micro-nanoparticle fraction.