Fabrication of alcohol-free curcumin-loaded chitosan quaternary ammonium salt nanoparticles for single-cell nanocoating of probiotics to enhance survival under harsh conditions.

Probiotics are highly vulnerable to environmental stresses, which greatly reduces their functional activity. In this work, a pH-driven self-assembly strategy was employed to prepare curcumin-loaded chitosan quaternary ammonium salt nanoparticles (Hacc/Cur) for single-cell nanocoating of probiotics. The optimized Hacc/Cur displayed a spherical morphology with a small particle size of 170.13 nm, a high positive zeta-potential of 44.47 mV, a high encapsulation efficiency of 82.09%, and a high loading capacity of 43.26%. They also possessed excellent stability across a wide range of ionic strengths (0-800 mM), pH values (4.0-8.0), and during low-temperature storage. After lyophilization, the nanoparticles maintained good redispersibility and increased the solubility of curcumin by approximately 88-fold. The Hacc/Cur were further used for single-cell coating of Lactobacillus rhamnosus GG (LGG), which markedly improved the viability of LGG against heat stress, freeze-drying, antibiotic exposure, and simulated gastrointestinal (SGI) conditions. Hacc/Cur was assembled via hydrogen bonding and hydrophobic interactions, and adhered to LGG surfaces through electrostatic interactions and hydrogen bonding. Meanwhile, curcumin underwent a crystalline-to-amorphous phase transition during nanoparticle fabrication. Curcumin also achieved sustained release during SGI digestion, and the system showed improved antioxidant capacity. Overall, this study provides a promising strategy to develop green, alcohol-free multifunctional foods and stable probiotic formulations.