Multifunctional Chitosan/Mn(II) Complexes: Preparation, Catalytic Activity in Imine Synthesis and Aldol Reaction, and Effect on Milk Fermentation/Post-Acidification

Herein, we prepared nanoparticles of chitosan-manganese(II) complexes in different molar ratios (1:2, 1:1, and 2:1) and fully characterized them using dynamic and electrophoretic light scattering, X-ray diffraction, SEM, FTIR, and thermal analysis. Nanoparticles <i>Chitosan + Mn²⁺ (1:1)</i> have a high catalytic activity in the oxidative coupling of benzylamine, resulting in the imine formation and also in selective aldol reaction. <i>Chitosan + Mn²⁺ (1:1)</i> catalyze the reactions in the greenest solvents: water and water/ethanol mixture. Moreover, <i>Chitosan + Mn²⁺ (1:1)</i> is very easy to prepare and convenient to use. The catalyst is separated from the reaction mixture by a simple nanoporous filter or centrifugation and does not lose catalytic activity after at least ten uses. The chitosan-manganese(II) complexes reduce the milk fermentation time, demonstrating the effectiveness in accelerating the fermentation process by <i>Streptococcus thermophilus</i>. They also contribute to increasing the shelf life of fermented milk products by inhibiting the undesirable post-acidification process. We found that the optimal ratio of chitosan and Mn²⁺ to manifest the apogee of the desired effects (acceleration of milk fermentation and increase in the shelf life of the fermented product) is 1:2.